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Journals & Magazines >IEEE Access >Volume: 10
ON THE HISTORY AND FUTURE OF 100% RENEWABLE ENERGY SYSTEMS RESEARCH
Publisher: IEEE
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Christian Breyer; Siavash Khalili; Dmitrii Bogdanov; Manish Ram; Ayobami Solomon
Oyewo; Arman Aghahosseini; Ashish Gulagi; A. A. Solomon; Dominik Keiner; Gabriel
Lopez; Poul Alberg Østergaard; Henrik Lund; Brian V. Mathiesen; Mark Z.
Jacobson; Marta Victoria; Sven Teske; Thomas Pregger; Vasilis Fthenakis; Marco
Raugei; Hannele Holttinen; Ugo Bardi; Auke Hoekstra; Benjamin K. Sovacool
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Abstract
Document Sections
* I.
Introduction
* II.
Definition of the Field of 100% Renewable Energy Systems Research
* III.
Milestones in the History of 100% Renewable Energy Systems Analyses
* IV.
Brief Bibliometric Overview of 100% Renewable Energy Systems Analyses
* V.
Overview of Global 100% Renewable Energy Systems Analyses
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Timeline of selected key milestones of 100% renewable energy systems research.
Abstract:Research on 100% renewable energy systems is a relatively recent
phenomenon. It was initiated in the mid-1970s, catalyzed by skyrocketing oil
prices. Since the mid-2000s,...View more
Metadata
Abstract:
Research on 100% renewable energy systems is a relatively recent phenomenon. It
was initiated in the mid-1970s, catalyzed by skyrocketing oil prices. Since the
mid-2000s, it has quickly evolved into a prominent research field encompassing
an expansive and growing number of research groups and organizations across the
world. The main conclusion of most of these studies is that 100% renewables is
feasible worldwide at low cost. Advanced concepts and methods now enable the
field to chart realistic as well as cost- or resource-optimized and efficient
transition pathways to a future without the use of fossil fuels. Such proposed
pathways in turn, have helped spur 100% renewable energy policy targets and
actions, leading to more research. In most transition pathways, solar energy and
wind power increasingly emerge as the central pillars of a sustainable energy
system combined with energy efficiency measures. Cost-optimization modeling and
greater resource availability tend to lead to higher solar photovoltaic shares,
while emphasis on energy supply diversification tends to point to higher wind
power contributions. Recent research has focused on the challenges and
opportunities regarding grid congestion, energy storage, sector coupling,
electrification of transport and industry implying power-to-X and hydrogen-to-X,
and the inclusion of natural and technical carbon dioxide removal (CDR)
approaches. The result is a holistic vision of the transition towards a
net-negative greenhouse gas emissions economy that can limit global warming to
1.5°C with a clearly defined carbon budget in a sustainable and cost-effective
manner based on 100% renewable energy-industry-CDR systems. Initially, the field
encountered very strong skepticism. Therefore, this paper also includes a
response to major critiques against 100% renewable energy systems, and also
discusses the institutional inertia that hampers adoption by the International
Energy Agency and the Intergovernmental Panel on Climate ...
(Show More)
Published in: IEEE Access ( Volume: 10)
Page(s): 78176 - 78218
Date of Publication: 25 July 2022
Electronic ISSN: 2169-3536
INSPEC Accession Number: 21927890
DOI: 10.1109/ACCESS.2022.3193402
Publisher: IEEE
Funding Agency:
Timeline of selected key milestones of 100% renewable energy systems research.
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Contents
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SECTION I.
INTRODUCTION
Looming threats of unabated climate change have propelled societal discussions
on the possibility of low-carbon or even carbon-negative sustainable energy
systems. The field of 100% renewable energy (RE) systems research proposes this
can be fully done using renewable sources not only for the electricity sector,
but for all energy and non-energy industry. Over time, the visions and scenarios
of science have taken root in politics and society. More and more countries are
setting net-zero emission targets, where all greenhouse gas (GHG) emitting and
absorbing sectors are combined. These analyses usually result in requiring the
energy system to be CO2-free, and in most countries, this means 100% RE supply.
Already, in 2011, Denmark set the target to reach 100% renewables across all
energy sectors by 2050 [1]. In 2016, 48 countries pledged at the COP 22 in
Marrakesh to reach 100% RE supply in the power sector at a minimum [2].
Additionally, more than 61 countries across the world have set 100% RE targets
for at least the power sector [3].
While many policy makers embraced 100% RE, recognition of this field’s academic
research has been slow. It took until 2018 for the Intergovernmental Panel on
Climate Change (IPCC) to acknowledge 100% RE research [4]. The International
Renewable Energy Agency (IRENA) has started approaching 100% RE for utilities
and countries [5]–[7]; however, its central energy transition scenario [8] does
not yet offer a 100% RE pathway. The International Energy Agency (IEA) has
developed a global Net Zero by 2050 scenario that only leads to a RE share of
67% (with 11% nuclear and the remaining supply coming from fossil fuels that is
partly combined with carbon capture and storage (CCS) [9]. However, in 2021 the
IEA also presented a first 100% RE country scenario [10]. By mid-2022, the
European Union has not published any 100% RE scenarios but did publish two
climate neutral scenarios in 2018 [11].
This review and perspective paper is intended to introduce 100% RE research and
its far-reaching potential to a wider audience. First, we will define the field
and look at the historic milestones and published literature with the
contributions of major research groups in the field. Then, we will describe the
present status of the field. Following that, the major criticisms on the results
and the resistance against 100% RE scenarios in major organizations are
discussed. The discussion also emphasizes how carbon dioxide removal (CDR) may
be added to create the net-negative system that is needed to stay below 1.5°C.
Finally, we end by describing research gaps and drawing conclusions.
SECTION II.
DEFINITION OF THE FIELD OF 100% RENEWABLE ENERGY SYSTEMS RESEARCH
To define 100% RE systems research, we first define the different aspects of
energy system analysis in general. We then specify what is covered by 100% RE
research. Energy system analyses can be structured as follows: energy sources;
energy conversion; energy storage; energy transport; and final energy fulfilling
energy services demand [12].
Sources of energy covered by 100% RE system research are: solar energy; wind
energy; hydropower; bioenergy; geothermal; and ocean energy (tidal, wave, ocean
current, ocean thermal). Research indicates that renewable electricity and
energy efficiency in combination with an energy system re-design will play a
dominant role in the transition due to its low-cost, high efficiency, wide
applicability, mature technologies, and vast access to renewable resources
[13]–[16]. In the past, bioenergy and hydropower were considered the most
important, whereas the strongest growth today is observed in solar and wind
energy [17], [18]. While solar and wind energy are also expected to dominate
100% RE system solutions on the global average [19], other renewable resources
could play a dominant role in individual countries or regions. Today, ten
countries supply near or more than 100% of their electricity from renewables,
mostly coming from hydropower [20].
Conversion means the energy sources can be stored, transported and used,
independent of the original form. Energy in its original form is called primary
energy. Energy in its final form as used at its final destination is called
final energy [21]. Electricity from solar photovoltaics (PV), wind power, and
hydropower is primary energy [21] only before transmission and distribution grid
losses, and electricity after grid losses is final energy for end-use. Modern
100% RE scenarios often make wide use of power-to-X (PtX) technologies, in
particular, power-to-heat [22] and power-to-hydrogen [23]–[26]. Where direct
hydrogen cannot yet be used, such as in the chemical industry or for
long-distance marine and aviation transportation, hydrogen can be further
converted to synthetic electricity-based fuels (e-fuels) as chemically bound RE
and such as e-methane [27], [28], Fischer-Tropsch fuels [29], [30], e-ammonia
[31], [32], and e-methanol [33], [34].
Technologies are available for all required energy conversions, but conversion
also leads to losses. For example: burning fossil fuels to produce electricity
usually leads to heat losses of over 50%, and in cars even 75% [35]. Another
example is transforming electricity into e-fuels (like hydrogen) and then back
to electricity which leads to the loss of over half the energy. Thus,
conversions should only be used when strictly necessary [13]–[15]. Energy
efficiency and waste heat recovery are important in district heating systems
[36], e-fuels [37], [38], and sector coupling [39]. Therefore, final energy use
should be prioritized as follows: use direct electricity wherever possible, for
instance highly efficient heat pumps and battery-electric vehicles, use low
temperature heat directly where possible, then add efficient hydrogen solutions
where required, and only use hydrogen-to-X conversions for e-fuels and
e-chemicals where other solutions are impossible.
Storage of energy is an important element of 100% RE systems, especially when
using large shares of variable sources like solar and wind [14], [40]–[42], and
it can take various forms [43]–[45]. Batteries can supply efficient short-term
storage, while e-fuels can provide long-term storage solutions. Other examples
are mechanical storage in pumped hydro energy storage [46], [47] and compressed
air energy storage [48], [49], and thermal energy in a range of storage media at
various temperature levels [43], [50]. Transport is available for all major
forms of energy including electricity, heat and chemical fuels. Electricity is
transported by power lines and has losses. Heat is transported using
heating/cooling networks. Chemical fuels are moved using pipelines, ships,
railways, or vehicles. Renewable electricity integration options have different
advantages and disadvantages and a clear focus on the ability to reduce fuel
consumptions in the entire supply chain can be recommended [51].
Final energy fulfilling demand will primarily be electricity and heat and cold
(used at various temperature levels) when discussing residential, commercial,
and industrial applications. Chemically bound fuels will be used in
long-distance transportation and steelmaking. Finally, non-energy feedstocks are
used by the chemical industry. Electricity will enable electrification of all
transport modes, the desalination of water supply [52], [53], and possibly
long-term CO2 storage for net-negative CO2 solutions enabling climate safety
[54].
The 100% RE energy system studies do not cover detailed power system
simulations, assessing the dynamics, security and reliability in detail. The
assumption has been that system operators will, as time passes, gradually manage
future power system operation at times with close to 100% inverter-based,
non-synchronous generation. This is still subject to ongoing research, as
discussed in Sections VI and IX. However, detailed regional and local grid
simulations have been done for more than a decade.
To summarize, 100% RE is a subfield of energy system analysis that assesses
solutions without the need for fossil fuels and nuclear energy, while using
bioenergy, hydropower, and geothermal energy within sustainable limits.
SECTION III.
MILESTONES IN THE HISTORY OF 100% RENEWABLE ENERGY SYSTEMS ANALYSES
An overview on selected key milestones of 100% RE systems analyses is presented
in this section and briefly summarized in Figure 1. The first 100% renewable
energy system analysis was published in 1975 by Sørensen [55] focusing on
Denmark as a case study in the prestigious journal Science. Remarkably, Science
has published only one article exploring 100% RE scenarios since. In 1976,
Lovins [56] published the second article on 100% renewables, but for the United
States, calling it “the soft energy path”, with the prescient sub-title: “The
road not taken?”. Lovins may have been inspired by Sørensen [55], as he was the
first scholar citing it [57]. Where Sørensen [55] carried out a quantitative
analytic study, Lovins [56] focused more on the framing, relevance and key
components. Both applied the approach of a vision-driven energy system
transition research, which is still up-to-date [58]. In 1996, Sørensen [59]
contributed another major milestone in the research field with the first global
academic analysis of a 100% RE system for the target year 2050. In 1993, a
report was published by the Stockholm Environment Institute for Greenpeace
International [60] on 100% RE for the target year 2100. Although this report
aimed to direct the IPCC on 100% RE, it took another 25 years to be acknowledged
[4].
FIGURE 1.
Timeline of selected key milestones of 100% RE systems research.
Show All
An additional 13 years passed until the second global 100% RE system analysis
emerged, authored by Jacobson and Delucchi [61] in 2009, prepared for the target
year 2030. More details of that study were published in 2011 [62], [63]. The
stark decrease in cost of RE [64], in particular wind power and solar PV, has
led to 100% RE being economically feasible and thus an interesting pathway to
study in detail. Whereas the Sørensen and Lovins papers included biomass,
biofuels, and biogas, Jacobson and Delucchi included no such fuels due to their
explicit goal of “consider(ing) only technologies that have near-zero emissions
of greenhouse gases and air pollutants over their entire lifecycle” [61]. Ref.
[62] is the most cited article in the field, and it has helped to overcome
belief systems and barriers across different fields on a global scale, and to
catalyze a global breakthrough of 100% RE. Updated research by Jacobson et al.
[13], [65]–[69] has overcome the previously identified limitations, and has
provided more detailed energy system results for almost all countries in the
world, as well as grid analyses of 20 or 24 representative regions encompassing
the countries. The first study examining specifically 100% renewable
transportation was published in 2005 [70]. It examined the air pollution and
climate impacts of transitioning all on-road vehicles in the U.S. to hydrogen
fuel cell vehicles powered by wind electrolysis.
Unfortunately, Sørensen, as one of the first pioneers in the research field and
with the first global 100% RE system analysis for mid-century and various other
methodological innovations, did not receive much recognition in the research
community at the time. A few reviews and related studies acknowledge his early
contributions [19], [41], [71], [72]. An outstanding methodological breakthrough
was contributed by Czisch in 2005 [73] with his dissertation describing the
first 100% RE multi-node simulation in hourly resolution based on historic
weather data for an investigated super-grid for one billion people in Europe,
Western Eurasia, North Africa, and the Middle East. A similar study with a
global perspective was published in 2004 [74], but appeared to be less noticed.
The landmark study of Czisch enabled various super-grid studies and supported
the Desertec Vision of those years, as described in more detail by Trieb et al.
[75], [76] and Breyer et al. [72]. In this context in 2005, the German Aerospace
Center (DLR) started early research on the development of a spatially and
temporally resolved cost-optimizing power system model called REMix. In contrast
to the existing models at that time, REMix methodically focused on the expansion
and operation of variable RE (VRE) technologies [77].
This focus also led to new insights into the interaction of VRE and existing
plants, resulting infrastructure requirements, and demonstrated that the
necessary load balancing in the system is technically and economically feasible
[78]. In 2010, Heide et al. [79] derived the first optimal balance of solar PV
and wind power for a 100% RE system for the case of Europe in hourly and high
spatial resolution and concluded that 45% solar PV and 55% wind power would be
an optimal mix. By using a stylized approach, known as weather-driven modeling,
Greiner and co-workers described the impact of assuming different wind and solar
combinations and heterogeneities among the European countries [80], [81] and
evaluated the impact of extending transmission links [82] and storage [83],
[84].
The most cited research team in the field is the group of Lund, Mathiesen, and
Østergaard from Aalborg University, who started in the field of 100% RE systems
research in 2004 [85] and contributed to a substantial expansion of the research
field with the freeware energy system analysis tool EnergyPLAN [86], [87], which
has been optimized for 100% RE system simulations in hourly resolution, sector
coupling, and overnight analyses. Several of the most cited articles in the
field are authored by this group, which facilitated a broad dissemination of the
concept of 100% RE to various research teams around the world. They also helped
to look beyond the power sector and electricity grids and started including heat
and transport in their model that facilitated insights leading to the smart
energy system concept [88]–[92]. This enabled detailed studies on the transition
of individual and coupled sectors of the same team [16], but also other teams,
such as for the heat sector [93]–[95], transport sector [96]–[98] and seawater
desalination [53].
Another building block that the field needed was the conversion of electricity
to chemical fuels, aka power-to-X, as previous research required substantial
shares of bioenergy (biomass, biofuels, biogas), often based on unsustainable
energy crops, or simplified hydrogen economy considerations. This conceptual
breakthrough was provided in 2009 by Sterner [27], who described a consistent
modern sector coupling view and the link of a 100% renewable electricity-based
system with renewable hydrocarbons, in particular e-methane. This required the
combination of the known processes of CO2 reduction using hydrogen [99]–[101], a
sustainable CO2 sourcing from a biomass source, point source, or from air [102],
and renewable electricity. In the analyses from the Lund, Mathiesen, and
Østergaard group since 2011 power-to-X for transport has been part of the
solutions [103], [104], while before the main options were electrification and
biofuels [105]. This conceptual innovation, also called power-to-gas, paved the
way for the broader power-to-X concept [106], as well as seasonal storage beyond
hydrogen, non-bioenergy-based solutions for the chemical industry [107], [108],
and drop-in solutions for long-distance aviation with e-kerosene jet fuel [109],
and for marine transportation [110], [111], including e-ammonia [31], [32] and
e-methanol [33]. This framing allowed the investigation of a cross-sectoral
comprehensive electrification, either directly, where possible, or indirectly.
It took another 12 years until the first hourly 100% RE system analysis
integrated the five central building blocks for a fully sustainable and scalable
energy-industry system for chemical compounds using: hydrogen, Fischer-Tropsch
based e-fuels, e-ammonia, e-methanol, and e-methane [39]. Overcoming the
limitations of hydrogen by adding CO2-to-X synthesis in energy system analyses
[14], [112] was conceptually initiated by Sterner. CO2-to-X is typically
discussed as carbon capture and utilization (CCU) [113]–[116], with CO2 sourced
from biomass, direct air capture or fossil fuels [108], and used for fuels
[109], [117], chemicals [107], [108], [117], and materials [118]. More than 100
academic 100% RE systems analyses are known using renewable electricity-based
CCU [117]. CCU is structurally different from CCS [54], [119] and it is a
central element of a zero CO2 emission and 100% RE system that includes
hydrocarbon-based fuels and feedstock. Unfortunately, by the end of 2021 not a
single Integrated Assessment Model (IAM) used for the IPCC, which include a
global representation of energy, economy, land and climate, is known to be able
to integrate these five fundamental building blocks of a sustainable
energy-industry system. The lack of these core elements might help explain why
IAMs struggle to construct 100% RE pathways.
A major milestone in broad societal outreach was contributed by Greenpeace and
the DLR with a series of reports and articles [120]–[123] highlighting the
merits of a 100% RE system. For the first time, the concept of 100% RE was made
accessible for a broad stakeholder basis beyond scientific circles across
disciplines and therefore generated more awareness amongst policy makers. These
studies also thoroughly explained 100% RE system options as a full transition
pathway in incremental time steps. The modeling framework has been further
developed [124], [125] and, although Greenpeace has discontinued its activities,
the long-term lead author Teske has continued in an academic capacity.
In addition to these research activities, the DLR is broadly investigating 100%
RE system analyses with its optimization model REMix [77], [78], [126]. The Open
Energy Modelling Initiative [127] was started in 2014 aiming to promote openness
and transparency in energy system modeling [128]. Many energy system models
(ESM) [129] exchange knowledge and best practices within this network. The
modeling framework Python for Power System Analysis (PyPSA) [130], together with
the instances for the European power system (PyPSA-Eur) [117], [131] and
sector-coupled system (PyPSA-Eur-Sec) [94], [132]–[134], set a new standard in
methodological progress in the 100% RE system analysis by combining high
modeling capabilities with full open science practices including an open license
that extends to the data, model and discussion of results. The PyPSA framework
is continuously expanded by Brown and co-workers and a steadily growing basis of
research groups use PyPSA for their analyses. PyPSA is currently regarded as
among the most advanced models for short-term energy system analyses according
to Prina et al. [135], and has been expanded in the meantime for long-term
pathway [133].
In research during the years 2017 and 2021, Breyer and Bogdanov established a
new standard in global-local transition studies toward 100% RE with the LUT
Energy System Transition Model (LUT-ESTM). It modelled the world in 145
individual regions in full hourly resolution with multi-node optimization,
various regional and country designs, and for an entire energy-industry system.
This modeling framework also includes comprehensive power-to-X sector coupling
with in total a set of about 120 technologies across all sectors and industries
[14], [39]. Earlier versions already contained a coupled power and heat sector
transition [136], power sector transition [137], [138] and power sector
overnight scenario [139]. The LUT-ESTM links to the first hourly global
0.45∘×0.45∘ mapping of a cost-optimized solar-wind-battery-e-methane-GT hybrid
energy system [140]. It also detailed insights for previously neglected regions
in the Global South [141] and identified new effects not observed before, such
as a battery-PtX effect [142] and a new pattern to mitigate the challenges of
the monsoon in India [143]. The LUT-ESTM is currently regarded as among the most
advanced models for long-term energy system transition analyses according to
Prina et al. [135]. The LUT-ESTM helped to reveal the true potential of solar
PV: it emerged as the dominating primary energy supply technology for the global
energy-industry-CDR system [54], [144]. In a way, this closes the circle: very
high solar PV shares of about 70% in total electricity supply were already shown
by Sørensen in the mid-1990s [59] and have since been confirmed by further
modeling teams [145], [146].
The evolving models enable the integration of more technologies, energy system
coupling, larger study areas with increased spatial and temporal resolution
[147], and the transmission grid [148]. Linking energy system models with more
detailed power system simulations for each synchronously operated system will be
needed to show the feasibility of operating the energy and power systems with
future wind and solar dominated resources [149], [150]. However, the history of
100% RE scenarios also has another perspective. For the pioneers, the first step
was often to demonstrate convincingly to national stakeholders that renewables
can at least partially replace fossil fuels, especially coal and nuclear power
plants with their high utilization rates.
In the following, the case of Germany is sketched. First publicly funded studies
of the 1980s showed visionary scenarios with RE shares of a maximum of 30% of
primary energy consumption in 2030, with nuclear and fossil energy still
dominating [23]. Until around 2000, progressive scenarios for Germany defined RE
as the possible main power generation source until 2050. However, these shares
hardly went beyond 60-65%, even after the phase-out of nuclear energy was
decided in 2000 [151]. Then, a series of so called “lead studies” were financed
by the German Ministry for the Environment with RE shares in the power sector up
to 80%, which, among others, showed the way for the energy concept 2010 of the
German government [152]. Even though the defined overall target of 80-95% GHG
emissions reduction by 2050 is mentioned there, the concrete targets and
subsequent studies have mostly focused on the minimum of 80% GHG emissions
reduction and 50% RE share in primary energy until well after 2015 [153].
Although a first national pathway with 100% RE by 2060 had been published in
2012 [154], which was followed by further 100% RE scenarios [155] or close to
100% RE scenarios [156], these studies have not yet played a significant role in
the political debate. Controversial discussions took place in the public debate
at that time especially regarding the costs of transformation and the economic
effects. It was not until new political pressure, including from the Fridays for
Future movement, supported by Scientists for Future [157], [158], that the
consequence of the signed Paris Agreement was more explicitly addressed in the
public and placed at the forefront of the political agenda. Since then, there
has been a long series of new studies concretely dealing with the design of a
100% RE scenario for Germany such as [97], [159]–[164], linking to earlier
studies preparing the ground [71], [155], [165], [166]. Due to the Russian war
in the Ukraine, politicians are currently even discussing ways to be largely
independent of fossil fuels, at least in terms of electricity supply, by 2035
[167], which is now the 100% RE target for electricity supply in Germany.
SECTION IV.
BRIEF BIBLIOMETRIC OVERVIEW OF 100% RENEWABLE ENERGY SYSTEMS ANALYSES
The field of 100% RE is very young and growing fast: most papers have been
published since 2018, and 2021 alone saw more publications than all the years
before 2015. By the end of 2021, 666 known peer-reviewed articles on 100% RE
systems, each analyzing a specific geographic scope have been published, plus 44
articles discussing generic questions and 38 articles reviewing the field of
100% RE system analyses, totaling 739 articles known in the field. These
articles do not include published reports in the field of 100% RE system
analyses focused on non-scientific target audiences such as industry, policy
makers and the general public. If these reports were included, the overall
number of publications in special interest and mainstream media would increase
significantly. The development of the peer-reviewed articles in the research
field since the mid-2000s is presented in Figure 2. The compound annual growth
rate (CAGR) of annually published articles between the year 2010 and 2020 was
27%, which indicates a strong growth of this research field.
FIGURE 2.
Development of peer-reviewed journal articles based on 100% RE system analyses
for concrete geographic entities. Only 12 articles are known from pre-2004. Data
are taken from Khalili [168].
Show All
The number of 100% RE system analysis articles published in 2021 forms a new
milestone with 146 articles in a single year, and an even accelerated
year-on-year growth rate of 52%. Additional analysis on the sectoral resolution
and journals that publish research papers, encompassing data up to the year
2018, can be found in Hansen et al. [19]. The five leading teams in the world,
according to the number of published articles, are Breyer/Bogdanov et al.,
Lund/Mathiesen/Østergaard et al., DLR/Teske et al., Greiner/Brown/Victoria et
al., and Jacobson et al., with 12%, 7%, 4%, 4%, and 4% of all known 100% RE
system analysis articles, respectively. Each of the five teams has published at
least 20 articles in the field. Their rank according to number of annual
citations, as of the year 2020, is Lund/Mathiesen/Østergaard et al.,
Breyer/Bogdanov et al., Jacobson et al., Greiner/Brown/Victoria et al., and
DLR/Teske et al. The two most used energy system models for 100% RE system
analyses are EnergyPLAN [86], [87] and the LUT-ESTM [169], with 70 and 60, known
articles respectively, as of the publication year until 2021. All other models
used for national energy systems or higher aggregations were used for less than
20 articles each on 100% RE system analyses. Following these five leading teams
with at least 20 articles in the field are six further teams with at least ten
articles on 100% RE system analyses, as well as the contribution of Sørensen for
whom his last article has been published posthumously in 2020. The six other
teams are Duic et al. [170]–[172], German Institute for Economic Research (DIW)
[163], [173], [174], Reiner Lemoine Institute (RLI) [140], [175], [176], Lenzen
et al. [177], [178], Johnsson et al. [179]–[181], and Blakers et al. [46],
[182], [183].
SECTION V.
OVERVIEW OF GLOBAL 100% RENEWABLE ENERGY SYSTEMS ANALYSES
This section focuses on an overview on global 100% RE system analyses as
presented in sections V.A and V.B. Of all 100% RE studies, however, only 8% are
global, 18% are regional and continental analyses, and 74% of studies are
investigations of national or sub-national 100% RE systems. The total number of
known articles underlying Figure 3 is 550 articles as of early July 2021 [168].
The most investigated countries are the United States (45 articles), Denmark (39
articles), Germany (35 articles), Australia (30 articles), China (17 articles),
the United Kingdom (14 articles), Finland (13 articles), Sweden (13 articles),
Japan (13 articles), Portugal (13 articles), Spain (11 articles), Croatia (11
articles), Italy (10 articles), and Greece (10 articles). These 14 countries
belong to the OECD plus China, and represent 63% of all national and
sub-national (states, cities, villages, islands) known 100% RE system analyses.
FIGURE 3.
100% RE system analyses per country. Global and regional studies are not
included. Data are taken from Khalili [168].
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Countries representing about 5 billion people are not yet well studied,
especially for Africa, the Middle East, Central Asia, South Asia and Southeast
Asia. It will be of highest importance for reaching ambitious climate and
sustainable development targets to close this research gap [19], as energy
transition agendas and measures are typically set on a national level.
Fortunately, 100% RE systems are probably eminently possible for these countries
since they often receive large amounts of solar energy and experience less
seasonal variations. In the following, insights of global 100% RE system studies
are presented and discussed, which allow us to consider trends for the entire
world.
A. GLOBAL 100% RENEWABLE ENERGY SYSTEM STUDIES AND MODELS USED
Several research teams have published global 100% RE system analyses as
summarized in Table 1. Only studies published in peer-reviewed journals are
considered. The studies represent the world from one single geographic entity up
to 145 ones. The use of multiple geographic entities in global analysis attempts
to draw conclusions for the entire world while reflecting regional differences
where applicable. Models and methods used by research teams differ, but they
consistently find that a global 100% RE system can be achieved by mid-century.
TABLE 1 Global 100% RE System Analyses. A Threshold of Minimum 95% Renewables
Share in at Least the Electricity Supply Was Used for Inclusion in the Table.
This Criterion Was Applied to Include the Near-100% RE System Analyses, But Also
to Ensure Appearance of Fossil Energy-Free Solution Structures. Abbreviation:
Simulation (Sim), Optimization (Opt), Power Sector (P), All Sectors (A),
Transition (T), Overnight (O), Total Primary Energy Demand (TPED)
A core differentiation of model types is according to simulation and
optimization [184]. A simulation model can be defined as a representation of a
system used to simulate and visualize the behavior of the system under a given
set of conditions. An optimization modeling approach uses several
decision-variables to minimize or maximize an objective function subject to
constraints.
The research results listed in Table 1 typically comprise the entire energy
system for the power, heat, and transport sectors, with industrial energy demand
typically being included as a component of other sectors. However, a clear
deficit and research gap exist, since a detailed description of the industry
sector, i.e. separated major industries such as cement, iron and steel,
chemicals, aluminum, pulp and paper, etc., is lacking in almost all cases.
Therefore, a full defossilization of the non-energy feedstock demand of the
industry sector has not been modelled in global 100% RE analysis.
The industry sector is described in detail in Pursiheimo et al. [145], though
the authors admit that TIMES, the model used, was not capable of applying full
power-to-X functionality for the industry sector, thus fossil hydrocarbon inputs
to the industry sector were still required by the model. Similarly, Teske et al.
[125] and Luderer et al. [146] mention that the chemical industry is still fully
based on fossil fuels. Analysis of a defossilized chemical industry, i.e.
phasing out fossil feedstock, though, suggest significant shares of CCU for
synthetic hydrocarbon feedstocks to industry, particularly for e-methanol [107],
[108], [185]. The latest version of the LUT-ESTM has the full functionality of a
100% renewable energy-industry system [14], but it has not yet been implemented
on a global level. The latest version of PyPSA-Eur-Sec also includes a detailed
modeling of the energy-industry interaction comprising also industrial feedstock
[134].
A new generation of energy system models has enabled the detailed analysis of
energy system transition options given specified constraints, e.g. climate
targets, societal preferences, energy resources availability, and energy
services. Cutting-edge energy system models show a high performance in temporal,
spatial, and technological resolution, and include sector coupling. Minimum
standards of model documentation are important to ensure transparency of methods
and data assumptions [135], [186]. According to Prina et al. [135], the leading
models meet the highest standards of describing entire transitions at hourly
resolution, with sector coupling, interconnected multi-regions, and a
technology-rich portfolio of energy system components. PyPSA, as one of the
promising open source tools, is best validated for energy transition analyses
for Europe [133], but it is not yet available on a global scale [187], and
therefore it does not appear in Table 1. The LUT-ESTM framework is
“global-local”, i.e. it can be implemented for energy system transition analyses
at various scales, from global to regional to local [14], [138]. It is currently
capable of analyzing a world divided into 145 individually modelled regions. The
modelling of Teske/DLR et al. [125] using Mesap/PlaNet (DLR-EM) uses 72 regions
and adds more detailed country versions [188]–[190].
Most global models subdivide the world into 12–24 regions as shown in Table 1,
which limits their ability to analyze important details. The studies of Jacobson
et al. [13], [66], [69] performed annual-average 100% RE analyses for 139, 143,
or 145 countries, then grouped those countries into 20 or 24 world regions,
respectively, for grid analyses at a time resolution of 30 seconds for multiple
years. Countries were grouped for grid analyses because currently, many
countries are interconnected, and interconnecting reduces costs relative to
isolating countries [68]. Sophisticated energy system models reveal that linking
least-cost solar PV electricity to low-cost batteries, low-cost electrolyzers,
CO2 direct air capture (DAC) technology, and hydrogen-based synthesis routes can
lead to a global average share of VRE of about 90% and 80% of electricity supply
and primary energy supply, respectively, as shown by Bogdanov et al. [14], and
Jacobson et al. [13], albeit without H2-to-X options. Sgouridis et al. [191],
Pursiheimo et al. [145], and Luderer et al. [146] reach VRE shares in total
primary energy demand (TPED) of 50-60%, i.e. substantially lower than Bogdanov
et al. [14] and Jacobson et al. [13], which is mainly due to both lower levels
of power-to-X functionality and higher assumed bioenergy availability in the
former models.
B. SOLAR PV AND WIND POWER IN GLOBAL 100% RENEWABLE ENERGY SCENARIOS
The role of solar PV and wind power may be the strongest differentiator among
the global 100% RE system analyses, which can be used as a starting point for
investigating conceptual differences in such studies. The following discussion
focuses on solar PV and wind power as the dominating sources of electricity and
energy in total in the investigated studies (Table 1, Figure 5), as 75% of all
studies find more than 80% of all electricity from these core pillars. This is
not intended to downplay the high value of the other RE sources, and aspects for
bioenergy and partly concentrating solar thermal power (CSP) are also discussed
in the following. Regionally, every single RE source can play a major role,
depending on local conditions.
FIGURE 4.
Solar PV and wind electricity generation [TWh/yr] in global 100% RE scenarios in
the year 2050. References are provided in Table 1.
Show All
FIGURE 5.
Shares of solar PV and wind electricity in global 100% RE scenarios in
electricity generation and in total primary energy demand in the year 2050.
References are provided in Table 1.
Show All
All known global 100% RE system scenarios published in peer-reviewed articles
that provided solar PV and wind electricity generation data were assessed
according to the shares of solar PV and wind power they project as a percentage
of total electricity supply (Table 1). In total, 17 studies were identified;
only the very first, by Sørensen [59], is from the 1990s, while all others were
published after the year 2008. The most cited study in the 100% RE research
field is the global study by Jacobson and Delucchi [62]. The results for
absolute solar PV and wind electricity generation are presented in Figure 4 and
the relative solar PV and wind power share in electricity generation and TPED
are shown in Figure 5. Most studies describe an energy transition from the
present until 2050 and for overall energy demand. Hourly modeling is becoming
increasingly standard amongst sophisticated models and is part of the methods
used in Jacobson et al., Teske/DLR et al., and Breyer/Bogdanov/Plessmann et al.;
all other models’ analyses suffer from a lack of hourly resolution.
A quarter of all studies show less than 20,000 TWh/yr of solar PV electricity,
and only three studies indicate more than 50,000 TWh/yr. The two studies with
the highest shares of PV have similar results: Pursiheimo et al. [145] arrive at
about 93,000 TWh/yr and Bogdanov et al. [14] at 104,000 TWh/yr by 2050. These
two studies use the lowest solar PV capital expenditures (capex) in 2050, with
246 €/kWp for fixed-tilted utility-scale power plants and corresponding capex
for rooftop PV. In Bogdanov et al. [14], [138] related capex for single-axis
tracking PV are also applied, as introduced by Afanasyeva et al. [194]. Bogdanov
et al. [14], [138], and Jacobson et al. [13], [67]–[69], [195], are the only
studies that consider solar PV tracking, leading to higher electricity yields
and lower electricity generation cost, which is a major trend in present
utility-scale PV power plants [196]. However, even in Bogdanov et al. [14], the
applied PV capex number does not reflect the latest cost trends, which indicate
about 30% lower capex in 2050 for utility-scale PV, i.e. 164 €/kWp as projected
by Vartiainen et al. [197]. Luderer et al. [146], though, have aligned their
solar PV capex projection to Vartiainen et al. [197].
Modeling with a techno-economic optimization approach will most likely lead to
even higher PV electricity supply, higher PV supply shares, and further
reductions in projected energy system cost in updated scenarios. The mentioned
development in solar PV electricity contribution is also reflected in the wind
electricity contribution, as three studies between 2011 and 2018 obtained values
of more than 40,000 TWh/yr [62], [66], [191], while, beyond 2018, all studies
remained below 40,000 TWh/yr. The cost-optimized studies with recent solar PV
cost find consensus values of 14-26% wind shares in electricity supply (Figure
5).
Interestingly, Sørensen [59] in 1996 had estimated a TPED share of 28% for solar
PV, while only three studies derived shares higher than 40%: Luderer et al.
[146] in 2021 with 42%, Pursiheimo et al. [145] in 2019 with 44% and Bogdanov et
al. [14] in 2021 with 69%. The main difference between the two former and the
latter studies is the stronger sector coupling in Bogdanov et al. and the fossil
hydrocarbons energy supply for industrial demand in Pursiheimo et al. and
Luderer et al. Further, the lower temporal resolution of TIMES used in
Pursiheimo et al. and REMIND-MAgPIE in Luderer et al. may have some impact on
power-to-X applications and sector coupling. There are several reasons for low
PV supply shares, and typically higher wind supply shares, in other scenarios.
The following discussion reflects reasons for higher or lower shares of main RE
technologies.
First, unreasonably high cost assumptions for solar PV automatically block
higher PV supply shares in cost-optimizing modeling. This is a major issue in
almost all scenarios created during the first half of the 2010s, when solar PV
capex projection were still very high in most cases. With the exceptions of
Pursiheimo et al. [145], Bogdanov et al. [14], and Luderer et al. [146], there
was a failure to anticipate the steep PV cost decline of the mid to late 2010s.
Relative cost differences can also impact the relative shares of solar PV and
wind power, which seems to be less a challenge with wind capex. Conversely,
financial assumptions for batteries have a strong impact as solar PV benefits
more from low-cost batteries compared to wind power. The three studies with the
highest solar PV shares consider low-cost batteries or respective system
integration costs.
Second, some scenarios assume relatively high bioenergy shares, such as in Deng
et al. [193]. High bioenergy use may be in serious conflict with sustainability
criteria, given that the global arable land is shrinking [198], ecosystems are
under massive pressure [199], the world population is growing [200], and more
food supply is required, while ongoing climate change impacts threaten even
current food production [201]–[203]. Creutzig et al. [204] conclude that no more
than 100 EJ/yr (about 27,800 TWh/yr) of bioenergy can be supplied sustainably.
Thus, overly large and possibly unsustainable bioenergy supply assumptions in
some models block indirect electrification opportunities that, absent bioenergy,
will otherwise be covered by solar PV. Teske/DLR et al. [122], [123], [125] and
Luderer et al. [146] have a substantial bioenergy share, but respect the 100
EJ/yr limit. However, scenarios without any bioenergy supply, as assumed in the
Jacobson et al. [13], [61]–[63], [65], [66], [69] studies do not lead to least
private cost solutions as recently shown in a comparison of model scenarios with
and without bioenergy [205], [206]. Even though Jacobson et al. do not find
least-cost solutions, they perform social cost analyses and find that both
annual private and social costs are consistently much lower than in
business-as-usual scenarios. However, cost optimization models have limits and
optimization for the use of resources such as metals and land-use that provide a
more holistic approach than the narrow focus on costs without considering
external costs. Such considerations are typically considered with applied
constraints. A model driven by cost inputs, which are estimated for the
calculated scenario period and the development that occurs within this time
period, e.g. between 2020 and 2050 rely on respective cost projections, is
subject to considerable uncertainties, especially for energy resources, such as
fossil fuels that do not play a role in 100% RE systems. In addition, the
projection of technology costs over decades implies uncertainties.
Third, some scenarios favor resource diversity over cost optimization for
reasons of political and societal robustness and broader considerations of
security of supply, though this may lead to higher costs. This is more often
applied in simulation type scenarios [184], in which specific shares for each of
the technologies included in the model can be defined. After the model scenario
is run, results are then checked for stable energy supply and costs within
applied constraints. Resource diversity is strongly emphasized in the various
Teske/DLR et al. scenarios as well as the Jacobson et al. scenarios, as high
shares of CSP plants are assumed. CSP in combination with thermal energy storage
(TES) is a technically feasible solution and enables a broader technological
diversity, but at a higher system cost compared to solar PV. Full year
optimization of CSP-TES compared to PV-battery systems based on latest cost
projections may lead to results adjustments in future studies. However, in the
case of coupled CSP-TES systems, advantages such as a high capacity factor of
over 90%, as well as lower life cycle emissions, the ability to provide process
heat for industry and water desalination, and the option of hybridization for
complementary use of biofuels or geothermal energy may be beneficial. Applying
outdated cost assumptions for solar PV and battery storage as in Kennedy et al.
[207] delivers conclusions on a TES-related value of CSP-TES that require
further investigation with latest cost assumptions. The CSP related factors can
play a major role locally in energy systems but are often not considered in
cost-optimizing models.
Fourth, distorted renewable energy resources assumptions can also lead to lower
PV supply shares, as in Löffler et al. [173]. This case is quite interesting,
since their PV capex are identical to Pursiheimo et al. [145] and Bogdanov et
al. [14], but the role of PV seems to be strongly underestimated due to an
artificially limited solar PV potential. This limitation strongly constrains the
PV capacity increase from 2035 onwards and thus leads to high additional wind
capacity installations. In almost all other scenarios for the years beyond 2035,
solar PV increasingly appropriates market share from wind power because the rate
of solar PV cost degression is greater, and solar PV electricity eventually
becomes cheaper than wind electricity.
Fifth, many scenarios suffer from incomplete power-to-X routes, a lack of
comprehensive sector coupling, and excessively high costs assumed for key
flexibility-providing technologies: batteries and electrolyzers. These are the
two most important VRE supporting technologies that strongly increase the VRE
supply share in scenarios by overcoming the day-night limitation of solar PV,
supporting strong electrification of practically all on-road transportation,
squeezing out biofuels for road vehicles, and enabling highly cost-attractive
power-to-hydrogen-to-X routes for almost all remaining energy segments that
cannot be directly electrified, including long-distance transportation, high
temperature industrial energy demand that may remain despite comprehensive
direct electrification, and hydrogen-based chemicals demand in industry. Thus,
low-cost batteries, low-cost electrolyzers, and established power-to-X routes
strongly increase the VRE share in covering the total primary energy demand.
Solar PV benefits more from low-cost electrolyzers than wind power, since
low-cost electricity is most efficiently matched with relatively inflexible
energy demand categories through the intermediaries of hydrogen storage and
electrolyzer-based power-to-X routes. More research on global 100% RE system
scenarios is required to further investigate a societally optimized balance of
resources and technologies, including power system studies with resource
adequacy.
Sixth, aside from the shares of solar PV and wind power, the absolute
contribution of the two most important VRE technologies differs, and their sum
differs substantially across the studies as displayed in Figure 4. This is
driven by three main factors within the respective studies. Different
assumptions on the development of energy services demand and thus final energy
demand have a strong impact on the overall VRE generation demand. This is
further pronounced by assumptions on energy efficiency development, which differ
across the studies. The assumed bioenergy utilization directly affects the need
for VRE generation as the degree of power-to-X is related to the supply share of
bioenergy. If strict sustainability limits for bioenergy are applied, or if
bioenergy supply is even blocked, the demand for VRE generation increases
significantly. Energy demand for the transport sector is discussed by Khalili et
al. [98] and for the heat sector by Keiner et al. [95], who highlight structural
differences of several of the studies also used for Figure 4 and in addition
compared to studies aiming for lower RE shares.
Resource-driven differences of technology shares are documented for higher solar
energy shares in the sunbelt, higher wind energy shares in the northern
hemisphere, higher hydropower shares in regions with excellent hydropower
resources, similarly for geothermal energy, and higher bioenergy shares in
regions of excellent bioenergy availability, typically related to a low
population density [13], [14], [65], [66], [124], [125], [138]. The remaining
ecological hydropower potential is estimated to 3290 TWh for costs at or below
100 USD/MWh [208], which indicates an increase potential of about 75% compared
to the hydropower generation of about 4350 TWh in 2020 [209]. Since the data
year of the ecological hydropower potential estimate, more than 400 TWh of
higher hydropower generation has been added. Given the enormous demand increase
for electricity generation, the remaining ecological hydropower potential can be
regarded as very limited and not substantially scalable. In addition, hydropower
generation has a substantially higher risk of negative climate change impacts
compared to wind power and in particular solar PV [210].
Finally, two strong arguments are in contradiction: full cost optimization that
leads to higher PV-battery shares versus broader resource diversity that would
lead to higher wind power and CSP shares, or trigger higher shares of geothermal
and ocean energy, resulting in higher energy costs in the system. The strong
system impact of the PV-battery-electrolyzer nexus is increasingly found in
energy system analyses on a global level [13], [14] and even more on a national
level, as for China [211], [212], India [213], and Africa [214], [215].
Depending on cost development, materials availability and local acceptance,
battery storage may compete with pumped hydro energy storage [169], [216] as the
pumped hydro energy storage potential could be much larger than most studies
considered so far [46]. As the cost and the operation profile of battery and
pumped hydro energy storage are very close, no relevant impact on system cost or
system structure may be expected. However, lower shares of battery storage and
higher shares of pumped hydro energy storage may be possible.
A major step ahead for the 100% RE system research may be achievable by means of
model intercomparisons, such as the one carried out for EnergyPLAN and the
LUT-ESTM [169]. Model intercomparisons could reveal undetected limitations and
thus further improve standards, as well as investigate the challenges already
identified here. In addition, cost comparisons of different transition pathways
generated using different input assumptions and constraints or technology cost
degression assumptions within the same model will allow researchers to further
clarify the cost impacts of given scenario constraints and options. A more
detailed analysis of the regional results generated with global models should
also consider any power system operational issues, long term resource adequacy
issues, sociotechnical, environmental and overall political and economic
aspects. These analyses should also examine the feasibility of the demonstrated
pathways in direct comparison with national studies. In recent years, more and
more co-benefits of 100% RE systems have been highlighted, such as reduced air
pollution [13], [217], a substantial reduction in energy-induced water stress
[218], a strong increase in jobs in the energy system [13], [219], higher levels
of energy security [220], first estimates of material requirements [221], and
stabilization and improvement in net energy [191], [222].
More efforts will be required for a solid description of the co-benefits and a
more comprehensive inclusion of the societal constraints framing and limiting
the energy transition [223] as well as the economy-wide impacts of RE [224]. It
will also be quite important to have the leading ESMs available as full open
science tools for a faster and broader uptake by newly joining research groups
and a more comprehensive stakeholder discourse.
High geographically-resolved global 100% RE system analyses can also help
overcome the strong imbalance of 100% RE studies for Europe, the United States,
and Australia and a dramatic lack of such studies for the Global South, as
already pointed out by Hansen et al. [19]. This also requires more openness of
scientific journals, as first of its kind studies should be favored by journals,
while marginal progress of intensively researched countries is regularly
published. Such imbalance requires critical reflection.
SECTION VI.
CRITICISM OF 100% RENEWABLE ENERGY SYSTEMS RESEARCH
Scientific progress implies challenging existing dogmas. 100% RE scenarios
challenge the dogma that fossil fuels and/or nuclear are unavoidable for a
stable energy system. This has triggered strong reactions with a crescendo in
2017 by Clack et al. [225], Trainer [226], and Heard et al. [227]. These, and
others like Jenkins et al. [228], have cast doubts on the technical feasibility
of 100% RE systems, their cost-competitivity, or, if affordable, the lack of
resources that they would require. However, in 2017, the field consisted of just
a few pioneers. Since then, the field has quickly grown with hundreds of
published papers by many different research groups across the world [168] (see
Figure 2 and Table 1 for an overview), and a consensus is starting to emerge
that many of those early criticisms do not hold when examined in detail. In
particular, Jacobson et al. [195], [229], [230], Aghahosseini et al. [231],
[232], and Sgouridis et al. [58] explicitly addressed Clack et al. [225]. In
response to Heard et al. [227], it was Brown et al. [233] who in 2018 provided
the first broad overview of 100% RE research and highlighted the technical
feasibility in detail, complemented by the response by Diesendorf and Elliston
[234]. Also, overall economic feasibility has been shown by several researchers
in various studies on the global level by Teske/DLR et al. [125], Jacobson et
al. [13], [65], [66], [69], Bogdanov et al. [14], [138], and comparable results
have been found for the leading 20 economies [235].
In 2021, Seibert and Rees [236] voiced new concerns on the feasibility of 100%
RE scenarios, and even claimed that “the pat notion of affordable clean energy
views the world through a narrow keyhole that is blind to innumerable economic,
ecological and social costs” and that the only way forward would be a drastic
curtailment of the global population to “one billion or so people”. Detailed
responses to these claims were provided by Diesendorf [237] and Fthenakis et al.
[238] as comprehensive reviews of the RE techno-economic evolution and history
of overcoming challenges in a fast growing field. We will now discuss the
different aspects of the various criticisms of 100% RE systems in more detail.
A. ENERGY RETURN ON INVESTMENT
A persistent stream of literature claims that a switch from fossil fuels to
renewables would be problematic or even impossible due to limitations in
fundamental energy economics [236], [239]–[241], based on metrics such as energy
return on investment (EROI). Authors making such claims often refer to
Georgescu-Roegen and his widely cited book from 1971 on entropy [242], which is
still prominent in economics. However, from a physics point of view, it should
be noted that Georgescu-Roegen’s attempt to apply the laws of thermodynamics was
fundamentally flawed [243]–[245], since he incorrectly characterized the earth
as a “closed” system, leading to predictions of economic collapse due to lack of
energy that ignored the constant influx of solar energy [246], [247]. The
concept of EROI was first proposed by Hall et al. [248] EROI = R/I is defined as
the ratio of R = the energy “returned” (i.e., delivered to the user) by a chain
of processes designed to exploit a primary energy resource flow (PE), to I = the
sum of the energy “investments” required to operate all such processes,
including manufacturing, maintenance and end-of-life disposal of all the
infrastructure. It is a concept embedded in biophysical economics and it gives a
fundamental insight into the practical viability of energy technologies from the
point of view of the end user. It should be noted, however, that EROI is not an
indicator of overall thermodynamic efficiency, which would instead be expressed
by the ratio η=R/(PE+I) . In other words, a process, or chain of processes, may
still be characterized by a high EROI even if it entails large thermodynamic
losses, provided that such losses are at the expense of the primary energy
resource being exploited, and do not entail a large increase in the energy
investments that are required per unit of output (i.e., R may even be ≪ PE, as
long as I≪R ). It has been often claimed that the EROI of RE technologies would
be too small in comparison to that of fossil fuels, thus creating a fundamental
limitation [236], [239]–[241]. This claim, though, is unsubstantiated for
several reasons.
Firstly, the realistic EROI of fossil fuels has been often overestimated by only
focusing on EROI values calculated at point of extraction. For instance, while
the EROI of crude oil at the well head may, in some cases, have been as high as
100 during its initial “golden age” [249], [250], detailed analyses have shown
that this value has been steadily declining over time as a consequence of
depletion [251]–[254]. Even more importantly, the many subsequent energy
investments required along the crude oil supply chain to process and deliver it
in the form of readily usable energy carriers have always reduced the resulting
EROI values at point of use to well below 10, irrespective of the initial EROI
at point of extraction [250], [255]. Similar, albeit perhaps not as drastic,
EROI reductions from point of extraction to point of use also affect all other
fossil fuels such as coal and gas. Furthermore, a substantial decline of fossil
oil and gas EROI is projected [252], [253], [255] for the decades to come [251],
[252]. The decline of the EROI of non-renewable resources is an unavoidable
effect of depletion, a phenomenon that has been dynamically modeled [254].
Secondly, many literature comparisons between the EROI of fossil fuels and those
of RE suffer from methodological inconsistencies that make their results
doubtful, as discussed by Raugei et al. [256], Diesendorf and Wiedmann [124],
[257], White and Kramer [222], Fthenakis et al. [238], and Diesendorf [237]. In
fact, in order to meaningfully compare the EROIs of fossil fuels to those of RE
technologies, the comparison must be framed using consistent system boundaries
[258]–[260]. This may be done either by calculating EROI as the ratio of
electricity output to energy investment, in which case the EROI of fossil fuels
at point of use is further reduced by a factor of 1/ηth (where ηth is the heat
rate of the thermal power plant), or by back-calculating the EROI “primary
energy equivalent” of RE technologies, by adopting a substitution logic whereby
each unit of electricity delivered is deemed equivalent to 1/ηLC units of
primary energy, where ηLC is the life-cycle energy conversion efficiency of the
grid mix into which the RE technology is embedded. The choice of the system
composition as optimally-designed may lead to different results depending on the
resource mix and corresponding location-dependent yields. Additionally, common
issues in EROI debates are the use of outdated data, neglect of the energy
learning [261], or even fundamental misconceptions [222], [257].
Thirdly, the EROIs of modern RE technologies, especially for solar PV and wind
electricity, have improved significantly in recent years, thanks to fast
technological improvements. Much discussion has been focused specially on solar
PV, and some recent studies have investigated the main reasons for the wide
range of EROI values reported in the literature for these technologies [262],
[263]. Recent studies have shown that the energy payback time (EPBT) of solar PV
has now reached values in the range of 0.5-2 years, depending on solar
irradiation levels and type of PV systems [261], [264]–[266]. This implies EROIs
in the range of 15–60 for a technical lifetime of 30 years, if the electricity
output is converted to primary energy equivalents, as explained above. The
ongoing PV system energy learning curve [261], showing efficiency and longevity
improvements, suggests additional EROI improvements in the future. For example,
insights by Peters et al. [267] indicate that PV modules could be operated for
50 years. Furthermore, recently enacted research funding from the US DoE is
focused on extending the lifetime of existing PV through improved encapsulation
and lower degradation [268]. A large meta-analysis of the published estimates
for the EROI of wind electricity up to the year 2010 [269] indicated an average
EROI of 20, if the electricity output is converted to primary energy
equivalents. Since then, more recent studies have pointed to even better net
energy performance, with average primary energy weighted EROIs ranging from 28
[270] to 34 [271], with maximum values up to 58 [271].
Other studies that evaluated global energy system transition options reported a
globally decreasing overall EROI trend, which supposedly risks falling below a
threshold that would be required to maintain a sustainable industrial economy
[241], [272]. However, the quantification of such a minimum EROI threshold is
problematic since it always implicitly rests on an assumed average efficiency
for the downstream processes where the various energy carriers are used
throughout the economy. However, one of the key benefits of a transition to a
100% RE system is precisely a shift away from inefficient thermal processes
across multiple sectors, thereby inherently reducing the requirement for high
EROIs at the point of use. Despite these methodological difficulties,
correctly-framed EROI studies are still useful in allowing for the development
of energy transition scenarios that are not based simply on the technical
feasibility of a 100% RE-based society, but which also question the specific
path that society needs to follow to carry out the transition before it is too
late.
A point that is often misunderstood in this latter debate is the one called
“energy cannibalism” [273]. This is an improper term, but it is sometimes used
to indicate the fact that the transition from a given resource of energy, e.g.,
fossil fuels, to another, e.g., renewables, requires the use of a certain
quantity of energy from the first resource to create the infrastructure for the
second one. Since, currently, the largest share of energy in the world’s mix is
sourced from fossil fuels, this gives rise to the incorrect claim that
“renewables cannot replace fossil fuels, since RE plants require fossil fuels to
be manufactured”. This issue has been framed as the concept of the “Sower’s way”
by Sgouridis et al. [191], because ancient farmers were faced with a similar
dilemma when they had to save part of each year’s harvest as seed for the
following year’s crops. In the present context, it means that a fraction of the
energy supply from fossil fuels needs to be used for the construction of the RE
infrastructure that will replace fossil fuels.
Studies that are based on the concept of EROI [191], [274], and the results
depend on various assumptions on how the EROI of different technologies will
increase with time as the result of technological progress or will decline as
the result of reduced site availability or the depletion of the mineral
resources needed for plants, including fossil fuels as energy sources.
Evidently, if the results were that the fraction of fossil energy invested is
larger than the energy currently supplied, the transition would not be feasible.
Instead, some initial studies [191], [274] indicate that the transition is
indeed possible, and that it can be fast enough to reduce the impact on climate
change below the limits set by the Paris Agreement, although doing so would
require larger investments than currently dedicated to RE. More research is
required to understand the link between various energy transition pathways and
the dominance of VRE in the energy system on the EROI. Such studies may be one
way of enabling the identification of the transition path that could conform to
diverse societal need as discussed above. The lack of more detailed EROI
analyses for the entire energy system transition constitutes a research gap that
needs to be closed soon.
B. DEALING WITH VARIABILITY AND STABILITY
Much of the resistance towards 100% RE systems in the literature seems to come
from the a-priori assumption that an energy system based on solar and wind is
impossible since these energy sources are variable. Critics of 100% RE systems
like to contrast solar and wind with ’firm’ energy sources like nuclear and
fossil fuels (often combined with CCS) that bring their own storage. This is the
key point made in some already mentioned reactions, such as those by Clack et
al. [225], Trainer [226], Heard et al. [227] Jenkins et al. [228], and Caldeira
et al. [275], [276]. However, while it is true that keeping a system with
variable sources stable is more complex, a range of strategies can be employed
that are often ignored or underutilized in critical studies: oversizing solar
and wind capacities; strengthening interconnections [68], [82], [132], [143],
[277], [278]; demand response [279], [172], e.g. smart electric vehicles
charging using delayed charging or delivering energy back to the electricity
grid via vehicle-to-grid [181], [280]–[282]; storage [40]–[43], [46], [83],
[140], [142], such as stationary batteries; sector coupling [16], [39],
[90]–[92], [97], [132], [216], e.g. optimizing the interaction between
electricity, heat, transport, and industry; power-to-X [39], [106], [134],
[176], e.g. producing hydrogen at moments when there is abundant energy; et
cetera. Using all these strategies effectively to mitigate variability is where
much of the cutting-edge development of 100% RE scenarios takes place.
With every iteration in the research and with every technological breakthrough
in these areas, 100% RE systems become increasingly viable. Even former critics
must admit that adding e-fuels through PtX makes 100% RE possible at costs
similar to fossil fuels. These critics are still questioning whether 100% RE is
the cheapest solution but no longer claim it would be unfeasible or
prohibitively expensive. Variability, especially short term, has many mitigation
options, and energy system studies are increasingly capturing these in their
100% RE scenarios. However, power system stability is usually overlooked as part
of energy-balancing studies, where the focus is on consumption-generation
matching on an hourly time scale. Wind and solar PV power plants are connected
to the grid by inverters, thus making them different from conventional,
synchronously connected power plants. The growing importance of
electricity-based systems has led system operators to analyze the challenges of
maintaining the reliability and stability of power systems dominated by
non-synchronous sources for generation in greater detail [149], [283]–[285].
Ongoing research is targeting ways to manage 100% inverter-based system
operations [286].
A 100% wind and solar PV inverter-based system operation has so far only been
seen on a smaller part of a larger synchronous system, or at small islands
[287]. As VRE will reach a 100% share of consumption at certain times (even if
their share is still much less on average), close-to-100% VRE operation should
be enabled as these events become more frequent. Currently, excess and not
otherwise usable wind power and solar PV is curtailed, not allowing
non-synchronous sources to exceed a given percentage at any instant. For
example, in the synchronous system of the island of Ireland, the so-called
non-synchronous system penetration was originally set at 50%, then raised to
60%, and is currently at 75% [288]. This enables a wind share of 40% without
extensive curtailment; however, to reach a higher renewable goal mostly
contributed by wind, the non-synchronous system penetration will need to be
increased to 90%. 100% inverter-based resources (IBRs) can be highly flexible
and controllable, with independent control over real and reactive current, and
they have an ability to shape the equipment’s response to various grid
conditions. New types of inverters, called grid-forming inverters, have
demonstrated the capability to provide the backbone for stable system operation
when no synchronous generators are online [289], [290].
This promising technology development together with evolving power system
modeling tools show possibilities to overcome the foreseen challenges
[291]–[293]. There could be opportunities to make IBRs behave in an even more
supportive manner than synchronous machines in some respects. However, the
changes are so profound that a fundamental rethinking of power systems is
required, including the definition of needed system services. One challenge is
that the control algorithms that dictate the response of IBRs to grid conditions
are not heterogeneous across various inverter designs and manufacturers, and
these can interact at both a local and system-wide level as well as with other
elements in the power system, such as high-voltage direct current transmission
terminals. This dramatically complicates the analysis of IBRs in the power
system and could lead to stability challenges [284], [286]. For 100% RE systems,
where solar PV and wind power dominate, more studies are needed to prove the
feasibility and assess the cost impact for grid-supporting resources, both at
wind and solar PV power plants and elsewhere. This and the challenge for
resource adequacy for weather-dominated, energy-constrained resources are
further discussed in section IX-E.
C. THE COSTS OF SOLAR PV AND WIND POWER
Some models and studies find that solar PV and wind would be too expensive,
especially if one adds measures that increase system flexibility to deal with
variability. Most often, though, this is because some of the model assumptions
result in an overestimation of the cost of wind power, solar PV and related
flexibility measures [294]. First, models that obtain high RE costs generally
lack the existing flexibility strategies described previously, including
dispatchable renewables, demand flexibility, sector coupling, transmission grid
expansion, and storage. Moreover, some models that lack detailed spatial and
temporal granularity include additional ‘integration cost’ for wind power and
solar PV that might overestimate the real integration cost and hamper the
penetration of VRE sources in the optimal solutions. When flexibility options
are properly included, large solar PV and wind power penetration are part of the
solutions [14], [145], [146], [277].
Second, some models overestimate the current cost of new technologies and
underestimate cost decreases. This limitation has been particularly severe for
solar PV as discussed by several authors [277], [294]–[296]. Additionally, most
energy models assume exogenous cost evolution for new technologies. In reality,
the cost of a technology depends on the cumulative installed capacity, through
the learning curve. Modelling endogenous learning in technologies is
computationally more difficult because the learning curve makes the model
non-linear, and some simplifications might be added. Moreover, it requires
estimating the learning rate based on historical data, which is particularly
challenging for immature technologies. When endogenous learning is included, the
penetration of wind power and solar PV typically increases and the cumulative
system cost decreases. Grubb et al. [297] even demonstrated that integrating
endogenous learning curves into the standard DICE model, which usually finds
that slow RE growth would be best, leads to remarkably fast and cheap transition
pathways because quickly adopting RE would rapidly reduce system cost and avoid
lock-in and sunk cost in fossil technologies. A similar observation was achieved
with the REMIND model, which has produced quite slow RE uptake [298], [299].
However, if realistic solar PV and VRE integration costs are applied, the model
switches to VRE-dominated solutions [146], [295].
D. RAW MATERIAL DEMAND FOR 100% RENEWABLE ENERGY SYSTEMS
As the previous criticisms are starting to become less and less tenable,
increasing attention is now shifting towards the more salient point of raw
materials needed for the transition towards a sustainable energy system.
Practically all research in this field finds critical limits for material
availability. This may be a major concern and should be addressed with more
consideration and analyses to truly test the material limits. Highly ambitious
energy system transition scenarios towards 100% RE systems have been used as a
basis for investigating material availability limits. Junne et al. [221] used
the scenarios of Jacobson et al. [13], Teske/DLR et al. [125] and Bogdanov et
al. [14] and identified criticalities for the four focused materials: lithium,
cobalt, neodymium, and dysprosium. A comprehensive overview on materials
criticality for the energy transition is provided by Lundaev et al. [300]. That
analysis identifies antimony, chromium, indium, manganese, molybdenum, nickel,
silver, zinc, and zirconium as minerals that can cause severe limitations to
energy transition without proper interventions, material substitutions, or
significant discovery of new resources. Their severity is because of the limited
number of known reserves/resources of these minerals compared to the expected
demand increase. For example, the nickel demand by 2040 could be more than 200%
of its 2020 demand due to its need in battery application for EVs and utility
services [301], [302]; however, the present reserve/resource could be depleted
in about four decades even at the rate of 2020 production [300].
Lithium extraction could reach material limits in the second half of this
century according to Greim et al. [303]. However, scenario combinations have
been identified by the same authors that enable transition scenarios without
conflicting with the lithium resource base, according to Bogdanov et al. [14]
and Khalili [168]. One option relies on extremely high collection and recycling
rates, close to 100%, eventually becoming mandatory, leading to an almost
circular economy for lithium batteries comparable to the present status for lead
acid batteries. A second option would be for the cost of lithium extraction from
ocean water to decline significantly. It is estimated that the oceans contain
6,000 times more lithium than on land, as it is the sixth most abundant
dissolved metal ion in the oceans [304], and new research by Li et al. [305],
Zhang et al. [306], Liu et al. [307], and Tang et al. [308] conclude that ocean
extraction could become relatively cheap. Another source of ocean-related
lithium extraction could be via brines of seawater desalination [309]. Finally,
lithium could be substituted, e.g., by Na-ion batteries that are gradually
getting closer to commercialization [310].
Cobalt demand may be managed by transitioning to cobalt-free lithium batteries
[311], [312]. Neodymium and dysprosium are primarily needed for permanent
magnets used in the motors and generators of vehicles and wind turbines. Their
availability requires further study, though these materials can be substituted
by ferrite-type magnets in wind turbines when their availability becomes
problematic [310]. For the case of electric vehicles, induction motors and
synchronous reluctance motors are well known alternative options [313].
Additional potentially critical materials are required for solar PV
technologies. For instance, silver is needed for the current generation of
silicon-based PV cells, and tellurium is used in CdTe PV cells. While it is
widely recognized that individual PV technologies would experience material
challenges for reaching very high levels of production, such sustainability
challenges do not appear before any technology reaches multi-GW annual
production and multi-TW cumulative production. For example, CdTe PV is
constrained by tellurium availability, but there is enough tellurium available
from copper anode slimes to support at least 4–5 times current production
capacity [314], which equals around 25–30 GW/yr, and cumulative TW-scale
production by 2050 [315]. Similar constraints apply to indium and gallium for
CIGS PV [316], [317]. The tellurium, indium, and gallium criticality may not be
dramatic, since more than 95% of the annual PV market consists of crystalline
silicon (c-Si) solar cells that do not use those materials [318].
If multi-TW annual manufacturing is achieved, silver supply will be not
sufficient for continuing to apply current c-Si PV metallization techniques
[319], [320]. The silver supply challenge may not be critical, though, as a
substitution with copper has already been investigated [319], [321] and PV cells
with the substitute technology are expected to be commercially introduced during
the 2020s [196]. Copper may be another material that requires more detailed
analyses, as a comprehensive electrification of the energy system will
inevitably lead to a surge in copper demand. So far, researchers analyzing
copper criticality have not yet identified copper limitations; however, most
have considered copper demand growth according to economic development and
population growth and collection-recycling rates of about 70% [322], [323]. If
additional demand from a more equitable energy supply development is included,
significant copper supply limitations are found, according to Elshkaki et al.
[324]. If copper constraints exist, aluminum, which is typically regarded as a
natural and practically unlimited substitute, could be used.
While comprehensive electrification of the global energy system has not yet been
considered in full, research by Kleijn et al. [325] strongly indicates that the
challenges will increase, with regard to both the long-term availability and the
potential impact of materials. Such challenges could become a short-term
bottleneck to the energy transition as mining projects have longer lead times,
often in the order of 10–20 years [326]. It is also noteworthy, though, that in
most cases, the scarce materials used in RE technology are in bulk form and can
be recycled with relative ease in comparison to materials used in dispersed
form. For instance, rare earth magnets can be easily separated from waste using
their strong magnetic field. Comprehensive analysis tools are necessary to
properly tackle potential challenges and more material criticalities may be
identified in the years to come. Nevertheless, it is clear that aiming for a
circular economy is indispensable [221], [327], [328]. All in all, there appears
to be reason for moderate optimism that material criticalities will not
represent an unsurmountable roadblock towards the transition to 100% RE systems.
However, it is also clear that it will be a formidable challenge to ensure the
timely availability of resources while simultaneously minimizing the negative
impacts of extraction on humans and the environment. This needs to be a focus of
upcoming research.
E. COMMUNITY DISRUPTION AND ENERGY INJUSTICE
A final critique sometimes levelled at RE systems is that they do not always
bring community co-benefits or promote equity or energy justice, and that they
may have their own negative externalities [329]. These can include toxic
materials used during manufacturing and installation, required integration with
other systems, land use and the loss of biodiversity, water use or consumption,
and dependence on rare earth mineral extraction that do have global
whole-systems geopolitical impacts [330], [331]. For example, hydropower dams
can provide clean baseload electricity but may require the relocation of
indigenous communities or the deforestation of tropical areas [332]. Wind power
plants rely on carbon intensive components such as concrete, fiberglass, and
steel with many manufacturing externalities spread across the supply chain
[333], especially in Asia. Patterns of solar PV adoption are not uniform, and
face demographic and social equity concerns given that those with solar PV tend
to live in higher-value homes, have higher credit scores, be more educated, live
in white neighborhoods, be older, and have steady jobs working in business and
finance-related occupations compared to the general population [334]. One study
examining diffusion patterns in the United Kingdom warned that increased solar
adoption risked transferring wealth between lower income and higher income
consumers, given that feed-in tariffs for solar PV are paid for by a levy on
energy bills by all consumers [335]. The access of low-income households to
solar PV rooftop systems can be ensured with the adequate design of policy
support mechanisms [336].
One meta-analysis of hundreds of academic studies published on the
sustainability of solar PV noted that many heavy metals embedded within solar PV
systems are hazardous for workers or the environment, especially lead, lithium,
tin, and cadmium, which can pose toxic risks during their manufacturing or
disposal [337]. Another noted the rising contribution of solar modules to global
stockpiles of electronic waste [338]. Atasu et al. [339] add that an additional
problem contributing to future stockpiles of waste is that rapid advances in
technology cause homeowners to sometimes switch or replace their solar systems
before the end of their useful lifetime to capitalize on better performing
systems. The authors refer to this as the “early retirement” problem with solar
involving the mass disposal of “no failure” panels. If one accounts for these
future waste streams, the levelized cost of energy for solar PV increases by a
factor of four, i.e., solar is four times more expensive than expected if one
includes the expected costs (and volumes) of waste. Similar problems with waste,
and solar “rebounds” where adopters increase energy consumption after installing
solar PV, have also been confirmed for Germany [340] and the UK [341]. Finally,
Ramirez-Tejeda et al. [342] critique unsustainable turbine blade disposal
practices, including landfilling in the United States. These aspects are
increasingly tackled by circular economy approaches.
Land use and community wellbeing emerge as a final concern connected to
mass-installations of RE systems. Looking at the siting and land politics of
solar PV power plants in India, injustices of process, planning, and
misrecognition in how such facilities are sited regardless of community concerns
are widespread [343]–[345]. Argenti and Knight [346] reveal how the development
of wind farms enabled enclosure via the appropriation and grabbing of farming
land, exclusion of local concerns from the planning process, encroachment of
environmentally sensitive sites with endangered fauna and flora, and the
entrenchment of inequality with no project benefits distributed to local
communities. Calzadilla and Mauger [347] show how global wind projects have
resulted in substantial increases in alcoholism and prostitution among both the
host communities and the camps full of job seekers. While this finding may be
more related to large construction projects than to a specific wind power
context, Shoeib et al. [348] find temporary increases in local rents and a boom
town feel as a result of US wind power development. Gorayeb et al. [349] also
catalogued how wind farms brought increases in child sex trafficking. Although
siting often focuses on impacts to “communities,” there are challenges in
defining what is meant by community and thus who “counts” in terms of
distributional justice, as wind power projects can have regional effects,
effects on Indigenous communities [350], and on communities of practice [351].
Nevertheless, many of these issues around justice, community acceptance, and
land use also occur with fossil fuels.
To be clear, in a comparative sense, RE is still less harmful than fossil fuels
in almost all contexts. Oil and gas systems in particular are known to pose more
serious and longer-lasting negative externalities including pollution, climate
change, and severe threats to some local communities [331], [352]–[355].
Moreover, while the characteristics of fossil fuel supply chains may have
unavoidable justice issues, especially related to their carbon content or
pollution flows, plentiful policy options and governance tools exist to make
wind, solar, and other low-carbon systems more just and equitable. In other
words, while fossil fuel injustices may be inevitable and unavoidable, those
facing 100% RE systems can be planned for, minimized, and at times even
eliminated. Figure 6 showcases policy mechanisms or measures cutting across raw
materials (e.g., better supply chain management), planning and policy (adherence
to proper informed consent), use (shared ownership models), and waste (extended
producer responsibility). Each of these measures would make RE technologies more
equitable, accountable, and just, helping to both contextualize and manage this
potential barrier. Unlike fossil fuels, the equity and justice issues facing RE
systems are avoidable and solvable, rather than inevitable.
FIGURE 6.
Policy mechanisms for more just RE transitions at multiple scales. Source:
Sovacool et al. [356] Note: N = frequency within an expert interview and
elicitation exercise.
Show All
SECTION VII.
DEFICITS IN 100% RENEWABLE ENERGY SYSTEMS DISCUSSION IN MAJOR ORGANIZATIONS
Large institutions are prone to ’institutional inertia’ and this is no different
in the energy transition [357], [358]. Similar to parts of the academic
community at large, they resist the challenge of 100% RE scenarios based on the
dogma that the world cannot do without fossil fuels and nuclear energy. Over the
years, two influential organizations have attracted especially heavy criticism
for underestimating VRE in general and solar PV specifically: the International
Energy Agency, and the Intergovernmental Panel on Climate Change, as pointed out
for instance by Philipps et al. [359], Breyer et al. [192], Creutzig et al.
[295], and Breyer and Jefferson [360].
A. INTERNATIONAL ENERGY AGENCY
The IEA is tasked with advising governments on their energy systems. While it is
probably the most important advisory body in the energy field, the IEA has
consistently failed to realistically project VRE in their flagship publication,
the World Energy Outlook (WEO). One example is that for twenty years it
projected that the yearly growth of solar PV installations would level off and
essentially come to a halt. At the same time, though, it also reported that
solar PV kept increasing by, on average, 43% per year. This was first put in
stark relief by Hoekstra [361] and can also be found in Breyer and Jefferson
[360].
Unfortunately, sustained criticism from various stakeholders seemed to have no
impact for many years. In 2019, this changed, helped by a massive critique
endorsed by dozens of stakeholders, including global financial giants such as
Allianz [362]. In the WEO 2020 [363], the IEA acknowledged that solar PV has
emerged as the world’s least-cost source of electricity and that it will remain
so for the foreseeable future in all major regions of the world. Notwithstanding
that verbal acknowledgement, solar PV shares in IEA scenarios remain low,
compared to various global scenarios [364]. Only the new Net Zero Emissions by
2050 (NZE) scenario seems somewhat realistic regarding solar PV. But even this
NZE scenario does not project the annual installation of solar PV and wind
turbines to grow after 2030, despite growing global energy demand [9], [209]. In
the NZE scenario, solar PV installations do not go beyond 630 GW/yr. For the
100% RE scenarios of Bogdanov et al. [14], about 2,000 GW/yr is needed in 2030
and 3,300 GW/yr in 2040. Similarly, Verlinden [319], based on Haegel et al.
[365], expects about 3,000 GW/yr from the early 2030s onwards. So even in the
most ambitious IEA scenario, solar PV is artificially capped. This indirectly
means more fossil CCS and unprecedented nuclear demand, while the system costs
increase dramatically [209]. Further societal discourse is apparently needed to
bring IEA scenarios closer to societal requirements. This can be eased by making
the data used in the IEA scenarios openly available, as called for by various
stakeholders [366], [367].
B. INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE
The IPCC is the world’s central advisory institution on the climate emergency.
Its reports are meant to summarize existing scientific insights on climate
change, and the mitigation options that are available to humanity. Therefore,
the IPCC might be expected to welcome the opportunities for rapid climate change
mitigation that 100% RE systems research offers. Yet, the first IPCC report that
mentioned the existence of 100% RE system scenarios at all was that on “Global
Warming of 1.5°C” in 2018 [4]. In that report, 100% RE system scenarios were not
discussed broadly; rather, their existence was only briefly mentioned. This was
43 years after the first 100% RE system article [55], 25 years after a dedicated
100% RE report of two leading international organizations [60], and 22 years
after the first global 100% RE system article [59]. By the end of 2017, at least
290 research articles discussing 100% RE systems were available, but they were
not included in the IPCC report. As Jaxa-Rozen and Trutnevyte [296] have
convincingly argued, more diversity is probably required, not just in terms of
scenarios, but also when it comes to the authors that are tasked with writing
these IPCC reports. The latter could potentially decrease institutional inertia
and enable new developments allowing 100% RE scenarios to reach stakeholders and
decision-makers faster and more comprehensively.
In early 2021, three studies using different methods [277], [294], [296]
concluded that the IPCC severely underestimated PV in practically all their
developed scenarios, and especially in the important IAM scenarios. This was
partly caused by the fact that very few scenarios used plausible assumptions for
the cost reductions of solar PV. The solar PV capex used in IAMs, documented in
Krey et al. [298], leads to 4–5 times higher cost in 2050 compared to up-to-date
projections of Vartiainen et al. [197]. Moreover, the IAM cost assumptions for
2050 are higher than reality in 2020. Strongly distorted scenario results are
the consequence, for instance presented in Eom et al. [299], confirmed by
Victoria et al. [277], Xiao et al. [294], and indicated in Jaxa-Rozen and
Trutnevyte [296]. Xiao et al. concluded: “In the worst case, transformation
efforts towards clean energy are delayed, in the false belief that they are too
expensive that may lead to misadjusted incentive systems.” Jaxa-Rozen and
Trutnevyte [296] wrote: “We […] recommend increasing the diversity of models and
scenario methods included in IPCC assessments to represent the multiple
perspectives present in the PV scenario literature.” Victoria et al. [277] found
that “the contribution of solar electricity to primary energy in 2050 averages
to 3.1%/6.8% in the IPCC 5thAR/SR1.5.” In other words, the first global 100% RE
system article in 1996 [59] indicated four times the TPED contribution of solar
PV than the average of IAMs used for the most recent IPCC report published over
20 years later. Victoria et al. [277] also reported some progress in some
publications using IAMs, though a major turn towards correcting PV cost data has
not yet been observed in IAM publications except for the recent Luderer et al.
[146] (as discussed below).
Victoria et al. [277] also point out that sector coupling and comprehensive
electrification for all energy demand is still missing from almost all IAMs and
their respective scenarios. An important example of such a lack of
electrification of demand is the IAMs treatment of electric vehicles. Research
into the ten leading IAMs for the IPCC found that they all assumed that electric
vehicles will remain more expensive until 2100 [368]. However, industry experts
expect total cost of ownership parity between 2020 and 2025 and retail price
parity between 2025 and 2030 [369]. Based on this, politicians in many countries
are starting to phase out all combustion vehicles with the EU currently drafting
a plan to ban new internal combustion cars by 2035 and combustion trucks by 2040
[370]. Furthermore, the UK, Netherlands, Sweden, Singapore, and Iceland are
planning a ban by 2030, and Norway even by 2025 [371], [372]. Nevertheless, IAMs
still assume EVs will be more expensive until 2100 and they compound the problem
with equally outdated assumptions on user acceptance.
At the end of 2021, Luderer et al. [146] sought to initiate a turning point.
They published a scenario based on the IAM REMIND-MAgPIE in which they applied
realistic costs for solar PV, fossil CCS, and nuclear energy using the PV
projections by Vartiainen et al. [197]. It led to the first known IAM scenario
which fulfils the criterion for inclusion in Table 1 as a 100% RE system
analysis. When these realistic assumptions where used, low-cost solar PV
disrupted nuclear energy and fossil CCS and led to a renewable electricity share
of 98% in 2050. Updated VRE integration cost functions led to VRE integration
cost of 10–20 USD/MWh in 2050, which is comparable to findings by Bogdanov et
al. [14] and Pursiheimo et al. [145]. Additionally, this scenario overcame the
previous overestimation of VRE integration cost, and led to realistic values for
very high shares of VRE, as pointed out by Brown and Reichenberg [373]. VRE
integration cost strongly varies across models, as not all transmission and grid
operations constraints are considered, resulting in respective uncertainties
[374].
Despite the application of realistic cost assumptions, Luderer et al. [375]
still neglected all H2-to-X routes, which unfortunately limits the VRE share in
TPED to less than 60%. Inclusion of these routes as done by Bogdanov et al. [14]
and partly Pursiheimo et al. [145], can lead to a VRE share in TPED of more than
80%. This might be rectified in future publications by the team of Luderer et
al., as their other research shows they are well aware of the value of e-fuels
and e-chemicals [376]. Ideally, they would integrate a 100% renewables-based
energy-industry transition with the five major e-fuels and e-chemicals
(hydrogen, e-kerosene/diesel/gasoline Fischer-Tropsch liquids, e-ammonia,
e-methanol, e-methane) as in Bogdanov et al. [39]. Such advances in Luderer et
al. [146] and Ueckerdt et al. [376] could trigger a structural advancement in
the entire IPCC. In parallel to this improvement, the IPCC could also consider
that its exclusive reliance on IAMs constitutes a potential risk. For that
reason, it could include 100% RE research directly, as recommended by Jaxa-Rozen
and Trutnevyte [296], Victoria et al. [277] and Xiao et al. [294].
SECTION VIII.
PROGRESSING 100% RENEWABLE ENERGY SYSTEMS RESEARCH TOWARDS NET-NEGATIVE CO2
EMISSIONS SYSTEMS
Carbon dioxide removal options are not yet consistently considered in 100% RE
systems research. The necessity to study net-negative CO2 emission scenarios and
a broader CDR portfolio that is integrated in long-term 100% RE scenarios is
outlined in section IX-A. Teske/DLR et al. [125] integrated natural climate
solutions (NCS) comprehensively, but their model lacked other CDR options such
as direct air captured carbon and storage (DACCS) and bioenergy carbon capture
and storage (BECCS) [377]. So far, only the LUT-ESTM has presented insights on
DACCS [54], [378] among the models used for 100% RE systems research, while
lacking the most important NCS. DACCS has been investigated with solar PV [54],
CSP [379], and geothermal energy [380], among others. Research exists projecting
CDR demand provided by DACCS in the order of 20–30 GtCO2/yr in the second half
of this century for ambitious climate targets [144], [381]. DACCS and BECCS are
not the only CDR technologies available, though, with other promising
technologies including more NCS, i.e. natural- and land-based solutions, such as
soil carbon sequestration, ecosystem restoration, afforestation and
reforestation, blue carbon and seagrass, and biochar [382].
Rigorous models connecting these CDR systems to 100% RE models are incredibly
rare, though. All 100% RE system research teams focus on true-zero CO2 emission
solutions for the energy system, a consequence of 100% RE system scenarios, but
they do not yet encompass technologies and pathways that could enable
net-negative CO2 emissions. Results by Sgouridis et al. [383] indicate that, at
present, focusing on expanding RE supply rather than on carbon capture is more
profitable in terms of advancing toward the energy transition but, in the long
run, active control of the atmospheric CO2 concentration may become a necessity.
Remarkably, Luderer et al. [146] present the first scenario created with a model
belonging to the IAMs that fulfills the minimum criterion of a 95% RE share by
the year 2050 and covers the entire energy system. This closed a research gap
within the climate energy research community that was addressed by Hansen et al.
[19] and Victoria et al. [277].
The next major development step in 100% RE research may be highly resolved
energy system models capable of incorporating a broader collection of CDR
options, and describing transition scenarios that trace all fossil fuels-based
industrial feedstock flows. This should include non-energy feedstock use, such
as fossil hydrocarbon demand in today’s chemical industry, and should consider
the full range of energy-industry-CDR options. It is becoming increasingly
important to describe 1.5°C climate target scenarios that fulfill the latest
insights of climate system science. Such insights indicate that the formerly
assumed remaining carbon budgets [4] must be corrected to lower values due to
negative climate feedback loops not yet correctly considered [384]. Recent
climate science research indicates that climate tipping points [385] may already
have been activated at present temperature levels, including dynamics in several
Earth systems that are likely irreversible at temperatures in the range of ca.
1.0°C to 1.5°C above the pre-industrial global average surface air temperature
level. These include progressive thawing of permafrost soils [386], melting of
Greenland ice [387], Western Antarctic Ice Shield instability [388], and coral
reef dieback [4]. This would mean that for a higher level of climate security,
substantially more ambitious climate targets must be the target. Such climate
security temperature target levels may be around 1.0°C, or even below, and in a
range of 280–350 ppm atmospheric CO2 concentration [389], [390], compared with
420 ppm CO2 concentration reached in the year 2021.
Thus, sophisticated energy system models must be upgraded so that they advance
the scope of analysis beyond zero emission energy systems. Generating scenarios
for a world with lower atmospheric CO2 levels than today will mean modeling
net-negative emission energy-industry-CDR systems, based on 100% RE supply.
SECTION IX.
DEVELOPMENT PERSPECTIVES FOR 100% RENEWABLE ENERGY SYSTEMS ANALYSES
Many methodological advances have been implemented in recent years in the
research on 100% RE system analyses. However, various gaps in methods, data and
research remain, which must be bridged to enable a comprehensive societal
discourse on the energy transition that lies ahead. Several of these major gaps
and aspects are discussed in the following sections.
A. ENERGY-INDUSTRY-CDR SYSTEMS SPANNING THE ENTIRE CENTURY
It is becoming increasingly apparent that 100% RE systems will emerge as the new
standard, since fossil CCS and nuclear energy represent more costly options, as
documented by the IEA [209] and recently in the IAM environment [146]. Fossil
hydrogen with CCS does not seem to be a silver bullet on the horizon either
[26], [391]. These trends highlight the need for 100% RE system analyses to
fully cover the energy-industry-CDR system. While ESMs are well developed for
the energy system, they still show substantial gaps for the industry
description, as only the LUT-ESTM is known to be capable to describe a full 100%
RE-based industry transition [39], while the latest version of PyPSA-Eur-Sec
also includes a detailed modeling of the transformation in the industry and
feedstock supply [134] and can be used for 100% renewable energy-industry
systems if fossil inputs are not allowed. Most ESMs, though, are not yet able to
fully describe the transition of energy-intensive industry, including green
steel [392]–[394], and a chemical industry without fossil feedstock [108],
[185], [395], [396].
Not a single ESM used for 100% RE system studies is able to take directly into
account the main CDR options of afforestation, reforestation, BECCS and DACCS.
Natural climate solutions for negative CO2 emissions are part of the scenarios
of Teske/DLR [124], [125], while the other ESMs used for 100% RE systems have
not yet implemented these attractive options. NCS and CDR options must be part
of any net-negative CO2 emission pathway discussion, which is an obligatory
discussion for any development beyond 2050 if the ambitious target of the Paris
Agreement of 1.5°C is to be taken seriously. Climate safety cannot stop at
1.5°C, given the severe distortion of the planetary climate system already
underway [397]; thus, more ambitious targets of substantially less than 1.5°C
require consideration, such as 1.0°C or 350 ppm of atmospheric CO2 levels [389],
[390] or less [144], for dedicated scientific advice on the option space and
respective societal discourse. This, in turn, leads to an expansion of 100% RE
system research beyond the often-adopted target year 2050, as net-negative CO2
emissions may be a major societal effort in the second half of the century. The
necessity of including CDR as a new energy sector using negative emission
technologies (NETs) is motivated by a rising availability of research addressing
the possibility that NETs will become urgently necessary for rebalancing the
Earth’s climate [398]. Especially for low-lying islands and coastal areas, this
topic is significantly pressing for the survival of whole nations considering
the long-term repercussions of climate change such as rising sea levels, even
after the year 2100 [399].
To represent this new energy sector in appropriate detail, comprehensive CDR/NET
technology portfolios must be developed. For such technology portfolios, an
assessment of technological and environmental limitations is indispensable
[377], [398], [400], [401]. The second half of this century will also be very
important for scaling the energy-industry-CDR system toward a truly sustainable
system [144], [212], since about 10 billion people will expect standards of
living comparable with the most developed countries as of today. This will
trigger a formidable additional energy demand that may lead to a doubling of
TPED at the end of the century compared to mid-century [402], leading to about
170 TW PV demand as the dominating source of energy as indicated by Goldschmidt
et al. [403] and Breyer et al. [212]. The consequences of an “energy for all”
strategy on the required energy resources, land-use and material demand for a
100% RE system for a truly sustainable civilization are still poorly understood
and not yet discussed.
B. SOFT COUPLING OF ENERGY SYSTEM MODELS AND INTEGRATED ASSESSMENT MODELS
The requirement to describe proper energy-industry-CDR systems leads to a
stronger coupling of ESMs to the insights of IAMs, especially those for emission
pathways and constraints, as well as land-use limitations. IAMs are inherently
unable to describe energy systems in the required high temporal and geo-spatial
resolution. The obvious solution is a stronger interaction and collaboration of
research teams specialized in ESMs with those with expertise in IAMs, as also
suggested in Hansen et al. [19] and Victoria et al. [277]. Such co-working could
integrate the best of both disciplines for the benefit of substantially
advancing the state-of-the-art in comprehensive transition pathway descriptions
and pathway comparisons.
C. MATERIAL CRITICALITY OF TRANSITION PATHWAYS
The transition from the present fossil fuels-based energy-industry system to a
solar-wind-based energy-industry-CDR system leads to new potential challenges.
Materials are essential to manufacture all the required components, and the
expansion of energy supply for reaching a sustainable energy system for high
standards of living for about 10 billion people is not yet well understood
[221], [403]. The discourse on critical materials tends to confuse the economics
of commodity cycles with geological scarcity and overlooks the vital aspect
that, unlike fossil fuels, most critical materials for renewable energy
technologies can be recycled [404]. Thus, circular economy will be a central
pillar for 100% RE systems, as clearly found for the case of lithium [303] and
indicated for solar PV [277], [403], [405]. However, a holistic analysis of
material criticality for 100% RE systems until 2050 and beyond represents a
substantial research gap. This also requires a feasible and meaningful concept
of criticality in terms of the likelihood and potential impact of shortages in
raw material supply [406].
D. IMPACT OF INTER-ANNUAL RESOURCE VARIATIONS ON 100% RENEWABLE ENERGY SYSTEMS
SOLUTIONS
The 100% RE system analyses use a variety of methods and datasets. The impact of
inter-annual resource variations and respective inter-annual storage demand is
not yet studied adequately, but will be important for ensuring energy system
security of supply for the known and foreseeable variations of the key renewable
resources, in particular solar, wind and hydro.
Resource variability can be grouped into two main categories: firstly, the
natural variability of resources as observed in recent decades; secondly, new
types of resource variations induced by climate change [210], [407]. Present
knowledge indicates a stronger variability for hydropower [408] and wind
resources and a rather marginal variation for solar resources. This correlates
well with an energy system mainly based on solar energy, as indicated by
Bogdanov et al. [14], Pursiheimo et al. [145], and Luderer et al. [146]. A
strategic energy reserve in the form of long-term and low-cost storage in
chemical compounds may be the prime solution for balancing inter-annual resource
variations, and detailed analyses should be able to deliver a quantification.
Re-visiting the resource adequacy paradigm, building balancing generation or
long-term storage in different forms should be complemented by opportunities in
new kinds of demand flexibility arising from defossilization, and the main
economic criteria for costs and risks [284]. International trade of e-fuels
[409] and an accompanying infrastructure to be built may ease a dispatch and
support in case of regionally limited inter-annual resource deficits. Detailed
global-local 100% RE systems analyses will be required for inter-annual resource
balancing investigations.
E. IMPACT OF VARIABLE RENEWABLES ON POWER SYSTEM RELIABILITY AND SECURITY
As the underlying nature of the power system is changing from one based largely
on a synchronous paradigm to one based on a non-synchronous paradigm, analytic
tools that help evaluate the operation of a power system with a large number of
IBRs are still to be developed [286]. Planning models increasingly need to take
the operational constraints into account [147]. IBR-dominated systems are
fundamentally different to current power systems in many ways, and the
differences need to be reflected in the design, analysis, operations, and
planning of power systems. There is a vast difference between a 75% penetration
of VRE supported by some synchronous generators and synchronous compensators and
a 100% VRE all-IBR system. The changes are so profound that a fundamental
rethinking of power systems is required.
One solution is to use a portion of wind and solar PV power plants as grid
forming, providing the capabilities needed for stable system operation. More
research and development and demonstrations are still needed regarding use of
new grid-forming technology in the power system. Where and when will the
grid-forming services need to be available? If installed as deeply embedded, at
medium and low voltages, would grid forming be effective for all the challenges?
Will a mix of synchronous condensers (SCs) and IBRs prove an economic solution
to adding system strength? Should these be large central units or smaller
decentralized units? How economic and practical is the use of large
decommissioned generators as SCs? [283] One of the challenges is how to manage
the power system when it is at times dominated by IBRs like wind power and solar
PV and batteries and, at other times (only hours apart), dominated by
synchronous machines, and all other possible combinations in between, both
spatially and temporarily.
IBRs can be highly flexible and controllable, with independent control over real
and reactive current, and they can shape the equipment’s response to various
grid conditions. Because of this, there could be opportunities to improve the
behavior of IBRs compared to synchronous generators in some respects.
Incremental tweaking and artificially forcing IBRs to function similarly to
synchronous machines is a short-term strategy that is limited and does not
leverage the true potential of IBRs. However, the control algorithms that
dictate the response of IBRs to grid conditions can interact at both a local and
system-wide level and with other elements in the power system, such as
high-voltage direct current transmission terminals. This dramatically
complicates the analysis of IBRs in the power system and could lead to stability
challenges [284], [286].
The experience of operating and planning systems with large amounts of variable
generation is accumulating, and research to tackle challenges of inverter-based,
non-synchronous generation is on the way. Energy transitions and digitalization
also bring new flexibility opportunities, both short and long term. However, no
study comprehensively addresses both the long-term and short-term challenges so
far. Linking the models to capture all constraints and potential cost impacts of
100% RE systems from power system operation will be needed. Some key issues and
recommendations can be identified across the challenges for planning, operation,
and system stability [150]:
* Modeling complexity: There will be an increased computational burden because
more variable IBRs details need to be captured, and more data are needed to
capture higher resolutions, both time resolution and distributed resources,
and larger areas, with extended time series to capture weather-dependent
events.
* Larger areas: The entire synchronous system is relevant for stability
studies. Sharing resources for balancing and adequacy purposes with
neighboring regions will be more beneficial.
* New technologies: All tools need to be modified to enable new types of
(flexible) demand and storage while facilitating further links through energy
system coupling.
* Modeling integration: There will be increased importance in integrated
planning and operation methodologies, tools, and data. Due to operational and
planning timescales, models need greater overlap. Flexibility needs and plant
capabilities must be incorporated into adequacy methods, and stability
concerns must be considered for network expansion planning and operating
future grids.
* Cost versus risk: The reliability interface needs to be revisited with the
evolution of flexibility and price-responsive loads to ensure that high-cost
increases are not imposed when modified reliability targets could yield
acceptable results.
* Looking forward, new paradigms for 100% IBR-dominated, asynchronous power
system operation can be found. This would profoundly impact the tools and
methods used, especially for stability.
F. DISTRICT HEATING AND COOLING IN TRANSITION SCENARIOS
Sector coupling, also referred to as smart energy systems, as argued by e.g.
Lund et al. [91] and Mathiesen et al. [16] offers the possibility of exploiting
synergies across energy sectors also using power-to-X. It has been developed a
combination of temporal and spatial modelling which enables a deeper
understanding of the possibilities in the heating sector, e.g. applied in the
Heat Roadmap Europe studies [410], [411]. Coupling with district heating, for
instance, holds more potential benefits in a 100% RE system context as it
enables using the vast amounts of waste heat present even in a future with
extensive electrification. Also, it offers to utilize waste heat from data
centers, power-to-X, solar thermal and geothermal energy [412]. Through the
exploitation of power-to-heat technologies and low-cost thermal energy storage
[40], coupling offers flexibility that can assist in the integration of VRE. In
future systems with further decarbonization and finally defossilization of the
transport and industry sectors partly using hydrogen or other e-fuels [14],
[39], [376], [409], [413], district heating systems may also serve as waste heat
sinks. Substantial sector coupling benefits have been identified for
hydrocarbon-based e-fuels with CO2 DAC, as the low-temperature heat demand of
DAC [102] can be mainly provided by recovered waste heat of e-fuels and
synthesis plants, such as from electrolyzers [37] and Fischer-Tropsch plants
[30]. Volumes, potential waste heat supply and heat demand may be huge, as the
global electrolyzer, CO2 DAC, and Fischer-Tropsch capacity is projected to about
11,000 GWel, 2300 MtCO2/yr, and 1700 GWFT, respectively, for a zero-emission
energy system by 2050 [14]. Including the e-chemicals feedstock demand, both
waste heat from electrolyzers and demand for CO2 DAC increases substantially
[39], [112].
In addition, heat losses from data centers [414], [415] and other activities may
provide a large quantity of thermal energy for district heating systems that
alternatively would be wasted, or which could even constitute local
environmental hazards. Presently, there is limited district heating outside the
EU, China, and the former Soviet Union [416], though US data is assumed to be
underestimated. Analyses have shown district heating prospects in China [417],
[418], Chile [419], and across Europe [410], [420]. For Denmark, two different
teams have investigated the optimal level and both, Münster et al. [421] using
Balmorel [422] and analyses based on EnergyPLAN [87], have found appropriate
shares in the 55–65 % range.
District heating is facing competition from individual solutions in certain
areas; however, while individual solutions in cases may be economically
attractive for users, the solutions are not necessarily optimal from a wider
systems perspective as indicated by e.g. the work on Europe [423].
Low-temperature district heating of the 4th or 5th generation [36], [424]
expands the utility of district heating as it improves the efficiency of heat
generation while lowering grid losses. Nevertheless, analyses have also stressed
the importance of local conditions as specific point sources of waste heat.
Additionally, issues regarding the raising of the temperature to appropriate
district heating levels need to be addressed [425]. The temporal and spatial
interaction of industrial waste heat and industrial and residential heat demand
requires more detailed consideration in 100% RE system analyses, also reflecting
overall system efficiency provided by synergies of sector coupling. Several
analyses of different solutions with extremely low temperature district heating
with local booster heat pumps do not seem to create a more energy efficient or
cost effective system, hence the limit tends to be temperatures where individual
installations can be avoided in the buildings [426]. While small building level
individual heat pumps are much more efficient than individual boilers, they are
not very efficient in integrating VRE [427]. District cooling may provide some
of the same sector coupling benefits to the energy system, and while district
heating is most relevant in temperate or cold climates, district cooling has
prospects from tropics to temperate climates. In areas where both are relevant,
synergies are even better [428].
G. RAISING GEO-SPATIAL RESOLUTION OF INTERCONNECTED ENERGY SYSTEM ANALYSES
The standards in geo-spatial resolution of global 100% RE system analyses are
insufficient for proper societal discourse. Most models used for global energy
system analyses aggregate the world into 10 to 24 individually modelled regions.
This is done strategically because grids are interconnected across political
boundaries. Europe, for example, is well-interconnected. LOADMATCH/GATOR-GCMOM
uses 24/143 regions, [R]E 24/7 uses 72, and the LUT-ESTM uses 145 regions (Table
1), which is still insufficient for connecting the global perspective to local
systems and to consistently address the interdependency between global paths and
local developments. Regular practices are national energy transition analyses on
a national level (Figure 3) with all relevant stakeholders in a bottom-up
approach. However, global-local interactions may be beyond that scope, and
regional and global scenarios require an improved understanding of limitations
caused by local restrictions.
A technical solution may be to substantially increase the number of regions. The
nomenclature of territorial units for statistics (NUTS) as developed for the
European Union [429], may be the right guidance to estimate what such a
resolution may mean on a global level. The NUTS1 level structures the EU-27 rim
into 87 well shaped regions, with the definition that all regions shall cover
not less than 3 million inhabitants, but also not more than 7 million, with an
average of 5.15 million per region for the present 448 million inhabitants. For
the current world population of about 7.9 billion this would translate to about
1530 regions.
Considering the NUTS2 structuring of EU-27, the 241 regions within the limits of
0.8 to 3 million inhabitants per region with an average of 1.86 million would
translate to about 4250 regions globally for the present. It may not be possible
to scale the existing models with 10 to 24 global regions in one step on a NUTS1
or even NUTS2 equivalent level; however, intermediate steps may enable that.
Experience for Europe clearly shows that a NUTS1 level can be managed, as shown
by Sassa and Trutnevyete [430], in demonstrating even a NUTS3 resolution for six
Central European countries in power sector overnight simulations applying a
soft-linked EXPANSE-PyPSA model. A similar experience with the LUT-ESTM for
countries such as Chile [216], Ghana [205], Cameroon [431], and Nepal and Bhutan
[432] clearly indicates that a NUTS1 equivalent level may be achievable
globally, as the average size per sub-region is about 2.9, 5.5, 3.8, 3.9 million
inhabitants for Chile, Ghana, Cameroon, and Nepal and Bhutan, respectively. Even
the NUTS2 level may be possible in the long-term, as indicated in the case of
Finland, which has been modelled into five NUTS2 regions as well as in seven
regions with the LUT-ESTM [433]. An intermediate step for models may be about
100 to 200 global regions, which has been demonstrated to be doable with the
LUT-ESTM using 145 regions. A next expansion step, then, may be the about 1500
regions in the NUTS1 equivalent resolution, or an intermediate step, with about
800 regions globally. A global resolution of about 800 regions would allow for
the first time to have the global megacities [434] individually modelled in
global-local context, and thus demonstrate how regions can be supplied despite
the lack of local energy resources. This has been shown for the first time on
the case of Delhi, the largest megacity mid-century in an interconnected energy
system analysis for the entire north Indian grid [435].
A true global-local energy system analysis framework would directly link the
global, continental, national and state-level and local energy transition
discourses in a single context, enabling various new insights.
H. OVERCOMING THE LACK OF 100% RENEWABLE ENERGY SYSTEMS ANALYSES FOR THE GLOBAL
SOUTH
The large majority of the known 100% RE system analyses are for the Global North
(and Australia and New Zealand), with 79% of all national and regional analyses
as documented in Hansen et al. [19] for the status of about 180 known articles
at the end of 2018 and visualized for 550 articles known by July 2021 [168] in
Figure 3. This has improved only marginally with 72% of about 666 known 100% RE
system analysis articles at the end of 2021. Many of the known 100% RE system
analyses for the Global South cover off-grid analyses of individual villages or
smaller regions or smaller islands [436], such as the very well investigated La
Réunion [437], Galapagos [438] or Canary Islands [126], so that the relative
share of country-level analyses are substantially smaller. We apply the
definition of the Global South according to Dados and Connell [439]. The known
studies for Global South countries, including countries from the northern
hemisphere if they are not yet on a high development level comparable to the
USA, Canada, EU, Japan, Korea and China, but excluding high-income countries
from the southern hemisphere, namely Australia, New Zealand, and Singapore,
reveal a substantial lack of used models, as tabulated for all ESMs with at
least ten known 100% RE system articles in Table 2, differentiated into global,
regional and countries, off-grid and small islands.
TABLE 2 Regional Scope of the Most Used Energy System Models With
Differentiation on Geographic Levels. ESMs are Considered, if at Least Ten
Articles on 100% RE System Analyses are Known. The Geographic Categories are
Global, Regional and Countries, and Off-Grid and Small Islands. The Total Number
of Known Articles Underlying This Table is 550 Articles as of Early July 2021
[168]. The Total per ESM is the Sum of Articles for the Categories “Global,”
“Smaller Geography,” and “Regions and Countries.” Definition of the Global South
According to Dados and Connell [439]. The LUT-ESTM Also Includes Earlier
Overnight Studies
The eight most used ESMs for 100% RE system analyses are summarized in Table 2,
with at least ten known 100% RE system analysis articles per ESM. The two
leading ESMs according to published articles are EnergyPLAN and the LUT-ESTM.
About 9% of these studies are for global considerations, while 28% are for
smaller geographic entities, in particular off-grid islands and city regions.
The remaining 64% of all articles cover the regional and national level of
countries, thereof 70% are for countries of the Global North, and only 30% for
countries of the Global South, where the majority of people live and where the
highest additional energy demand in the decades to come will arise.
Interestingly, the dominating share of all national studies for countries of the
Global South are carried out with the LUT-ESTM, with 84% of all 45 studies for
the Global South on national level. GENeSYS-MOD follows, covering 7% of the
national studies on regional and national level for the Global South, and all
other models are below 3%. A successful global transition toward 100% renewables
does require a detailed societal discourse all around the world, especially in
countries of the Global South, so that an effective leapfrogging can be enabled
that avoids stranded assets in a fossil infrastructure that is no longer needed
and establishes a low-cost energy supply, which is nowadays based on low-cost
wind and in particular solar energy.
I. OFF-GRID RENEWABLE ENERGY SUPPLY IMPLEMENTED IN COMPREHENSIVE ENERGY SYSTEM
ANALYSES
About 760 million people do not have access to electricity, and 2.6 billion do
not have access to sustainable cooking solutions [440]. Energy systems based on
100% RE and respecting sustainability criteria will lead to a massive
electrification across energy sectors, and will lead to a massive decline in the
role of bioenergy for energy services, especially for cooking in developing
countries [95], [215]. Even solar PV electricity-based cooking solutions are
thinkable [441], following the trend of electricity-based cooking [442].
However, not a single ESM is able to coherently model the energy transition
including off-grid solutions or a transition of off-grid and on-grid solutions
in a comprehensive energy system transition pathway with the interactions and
gradual phase-in and phase-out of existing solutions [443].
The integration of off-grid mini- and micro-grids is essential as they are
expected to play a vital role from transitioning as an energy access tool to
meeting aspirational energy growth [444]. HOMER is an ESM optimized for off-grid
electrification in a local micro-grid environment and used widely for respective
analyses [445], [446]; however, it is practically limited to electricity and not
used for national energy systems and interconnected multi-node analyses.
Further, solar home systems, which represent a major part of fast off-grid
electrification [447], are missing. ESMs addressing the off-grid aspects across
the energy sectors with interaction to the on-grid system are required to close
this methodological gap and thus enable a more coherent discourse with
stakeholders and policy makers for optimized electrification, sustainable energy
supply, and energy transition solutions.
J. SOCIETAL CONSTRAINTS FOR 100% RENEWABLE ENERGY SYSTEMS ANALYSES
As already underscored in section VI-E linking RE transitions to community
wellbeing and acceptance, it is increasingly understood that techno-economic
energy system transition analyses lack critical elements [223], [448]–[450] and
may fail the intended targets. ESMs try to implement various societal aspects
and constraints, such as air pollution [13], [217], water stress [218], jobs
[13], [219], critical materials [221], EROI [191], resource potential
limitations [451], and phase-in inertia in transition studies [14], while other
aspects remain untouched despite being of highest importance. Such aspects
include maximum area availability in societies, acceptance of specific
technologies such as wind power or power transmission lines, and critical
behavioral aspects that are still unknown, as for smart electric vehicles
charging and vehicle-to-grid operation, among many others.
It is also very likely that such aspects and associated societal risks for the
energy transition will differ from country to country. More research on
economy-wide impacts [224] and geopolitical consequences [452] of transitioning
toward 100% RE systems is required, which should then be linked to ESMs. New
methods must be developed to expand the features of ESMs to better cover
societal constraints and implement insights from social sciences. The
techno-economic models are powerful, but the right constraints must be set and
expanded. Otherwise, novel methods will be required to adequately integrate more
societal dimensions, especially concerning vulnerable groups, tradeoffs
concerning equity, or issues of policy, planning, and governance.
Furthermore, RE and sustainable technologies, and in particular solar energy,
wind power and the various storage and conversion technologies, have a higher
peace potential [453]. Energy security has various dimensions [454], which can
be engaged in many ways [220]. It had been found that energy security is
improved with storage technology [455], and an energy transition towards 100% RE
may improve key energy security dimensions [456], which strongly impacts an
overall resilience [457]. RE has already displayed many advantages over fossil
fuels in terms of international security and peace, mostly because renewable
resources are abundant, well distributed, and continuously replenished [404].
However, for concentrated forms of RE, in particular for hydropower, potential
conflicts require attention for beneficial solutions [458]. In terms of critical
materials and cybersecurity, renewable energy is thought to pose greater
security risks. However, technological developments and circular economy
approaches have the potential to address these needs and lead to more
decentralized resource availability compared to geocentric fossil fuels
exploitation. Moreover, there is an expectation that increased RE use may lead
to a variety of small-scale conflicts but will reduce the risk of large
geopolitical conflicts [452]. Further improved research is required linking 100%
RE system transition to energy security and consequences for peace and
stability.
K. OPEN SCIENCE AS THE NEW NORMAL FOR 100% RENEWABLE ENERGY SYSTEMS ANALYSES
All leading ESMs have been enabled with public funding, which leads to the
justified claim that the public shall have full access to the investment in an
open science environment. Open science comprises an open source of used modeling
tools, open data of inputs and results, and open access to publications, among
others. The ten most used ESMs for 100% RE system analyses fulfill such
requirements to various degrees, such as PyPSA and GENeSYS-MOD in full, and
TIMES partly, while all others are not yet available as an open-source tool.
There are plans to transfer the LUT-ESTM into an open-source environment.
Scientific and societal discourse is substantially facilitated by comprehensive
open science practices, while the transfer of knowledge to researchers of the
Global South can be substantially improved.
L. MODEL INTERCOMPARISION STUDIES OF 100% RENEWABLE ENERGY SYSTEMS ANALYSES
The critical aspect of model intercomparison in the field of 100% RE system
analyses is almost non-existent. Various reviews do exist which compare ESMs
[135], [459], [460], so that a minimum level of model overview is accessible.
However, real model intercomparison studies among ESMs are required for further
improving the models and closing model-specific gaps, limitations, and maybe
existing failures. ESMs can be validated using existing data on real systems,
but 100% RE systems in sector-coupled features do not yet exist, which may lead
to gaps and failures in their description. Such limitations could best be
identified and removed in direct model intercomparisons, which would serve the
purpose of model cross-validation. Such a cross-validation was done with the two
most used models for 100% RE system studies, and it helped to reveal limitations
[169], which could then be removed, at least in part.
Full ex-post model intercomparison will only be achievable if all research is
publicly available. This includes input data, more specific socio-economic data
and macroeconomic scenario assumptions. With fully transparent input data, it
will be possible to compare different ESMs with similar or equal input data. In
return, the results of the ESMs for such simulation or optimization runs will
enable an easier identification of differences and similarities between the
ESMs. An alternative are joint projects of different modeling teams with the
explicit goal of an in-depth model comparison, but these are associated with a
large effort regarding the harmonization of the model parameterization and the
synthesis of results [461]–[464].
The global ESMs could learn from respective model intercomparison efforts within
the IAMs [465]–[468], which helped substantially in standardizing specific
features and reporting structures and thus created higher relevance for policy
makers. In this case, IAMs used for the IPCC are one step ahead, by publishing
all assumptions and scenario variations for the used shared socio-economic
pathways (SSPs) [469]–[473] and providing a broad scientific literature
collection for transparency. Furthermore, a comprehensive database for all
numeric input data is available online [474].
M. PROVIDING PATHWAYS FOR REBALANCING THE EARTH’S CLIMATE WITHIN THE PLANET’S
LIMITS
Finally, all points raised in this section will have to be addressed
cumulatively. By doing so, ESMs will be able to provide valuable pathways
including NETs enabling net-negative CO2 emission energy-industry-CDR systems.
To this end, there are several challenges ahead. First, it is most important to
achieve a global 100% renewable energy-industry system by 2050 at the latest,
and ideally by 2035, in order to slow down the biggest threat to civilization
and most living beings on planet Earth: climate change. Second, pathways must be
investigated to ramp up CDR, especially in the second half of the century. This
is important to compensate unavoidable and remaining GHG emissions not related
to the energy system. However, this also opens the door for taking one step
further and using the options for net-negative CO2 emissions to rebalance the
global temperature below a 1.5°C increase. As estimated [144], about 1480 GtCO2
will have to be removed for rebalancing the CO2 concentration in Earth’s
atmosphere to 350 ppm, which may comply with a 1.0°C target. Such ambitious
goals will only be possible if the ESM and IAM research community act together
hand in hand, pushing each other to advancements via constructive criticism and
highest research standards.
SECTION X.
CONCLUSION
The research field of 100% renewable energy systems analyses was initiated in
the 1970s, started to attract attention in the mid-2000s, and has experienced
strong growth since circa 2009. Some methodological milestones have been the
ability to differentiate between local, national, and global perspectives to
address various stakeholders; the use of hourly temporal resolution; describing
not only the power sector but entire energy systems; and integral pathways that
deal with the variability of solar and wind through conversion and storage
combined with demand response and sector coupling.
Global 100% renewable energy systems analyses have been increasingly discussed
since 2009 and three main groups of studies can be identified: i) optimization
studies with low-cost solar PV leading to high shares of solar PV in electricity
supply of 70-80%; ii) higher solar PV cost assumptions partly in optimization
and often in simulation studies preferring substantial wind resource
utilization; iii) simulation studies implying a broader resource diversity often
assuming a higher bioenergy resource potential and less solar PV but more CSP
shares utilizing the solar resource potential. Several teams find more than 90%
of all electricity supply from solar PV and wind power in 100% renewable energy
systems analyses. However, only two teams find a solar PV and wind power supply
of at least 80% of the total primary energy demand, which is strongly driven by
a comprehensive power-to-X consideration in both teams and either a strict
sustainability limitation for bioenergy or even a full bioenergy ban. A further
finding is that a strict cost optimization with latest solar PV costs and
respective future projections leads to very high solar PV shares. Conversely, a
more diversified energy system, utilizing a broader variety of resources, may
also have value on its own, as the political, social, and economic risks of the
energy transition might be lower, however on the price of higher absolute cost.
After implementing the latest solar PV cost assumptions, reacting on continued
respective critiques, improving grid integration methods, and applying latest
insights on battery and electrolyzer cost, a first scenario using an integrated
assessment model joined the field of 100% renewable energy systems analyses.
The field of 100% renewable energy systems analyses has successfully emerged
from power sector analyses to describe the entire energy system, but the
industry sector is not yet well described by most energy system models. The
increasingly important new sector of carbon dioxide removal is also not yet
addressed by any of the leading energy system models. Moreover, potential
connections with injustice, social exclusion, community disagreement, and the
degradation of the environment are possible when RE systems are installed
without due consideration for equity, social acceptance, or good governance.
These issues need to be managed by strong industry practices supplemented with
robust policy enforcement.
As any new scientific field, 100% renewable energy systems research receives
continued critiques based on justified as well as unjustified claims, which is
part of a productive scientific discourse that has led to improved scientific
standards that help to expand the field and gain impact on a growing stakeholder
basis. Many studies have been carried out for energy return on investment on a
component basis, but there are not yet many on an entire energy system level.
Similarly, investigations on materials criticality will be most important for an
early reaction in addressing mitigation strategies on a component level. Major
international organizations, namely the IPCC, the IEA, and even the IRENA are
late followers for highly renewable energy systems solutions with a considerable
institutional inertia, while the time for adequate policy recommendations is
more pressing than ever. The latest progress for the IPCC and the IEA indicates
a shift in the right direction, although there is still a long way to go.
Within the field of 100% renewable energy systems analyses, various areas of
study require more attention and improvement in the years to come. This
especially includes full coverage of a coupled description of energy, industry
and carbon dioxide removal systems in an integrated framework spanning the
entire century. This shall lead to a stronger interaction of energy system
models and integrated assessment models. The consequences of 100% renewable
energy systems on materials criticality require substantially more attention so
that potential limitations can be identified early, and mitigation strategies
can be developed. Methodological improvements are required for inter-annual
energy resource variations as well as the geo-spatial resolution of
interconnected regions modelled on a global-local resolution. More detailed
power system studies are needed to prove the feasibility of an operation with
100% inverter-based resources at a non-synchronous mode of operation as well as
how to transfer those potential constraints to energy system models. The strong
imbalance of 100% renewable energy systems studies for the Global North vs. for
the Global South must be overcome so that a successful response to the climate
emergency can be enabled. Energy system models still ignore off-grid solutions
while hundreds of millions of people lack access to electrification and even
more to clean cooking solutions. Thus, comprehensive energy transition pathways
for developing countries must address off-grid and on-grid solutions within a
better framework. More emphasis must be put on societal constraints for
transition pathways toward 100% renewable energy solutions, especially issues of
justice as noted above, which also includes a consequent open science approach
in the field for improved societal discourse and faster diffusion of tools, data
and knowledge. Last but not least, diffusion of insights within the field must
be facilitated by initiating comprehensive model intercomparison studies of 100%
renewable energy systems analyses.
The main conclusion of the vast majority of 100% renewable energy systems
studies is that such systems can power all energy in all regions of the world at
low cost. As such, we do not need to rely on fossil fuels in the future. In the
early 2020s, the consensus has increasingly become that solar PV and wind power
will dominate the future energy system and new research increasingly shows that
100% renewable energy systems are not only feasible but also cost effective.
This gives us the key to a sustainable civilization and the long-lasting
prosperity of humankind.
ABBREVIATIONS
AbbreviationExpansion
BECCS
bioenergy carbon capture and storage
CAGR
compound annual growth rate
CAPEX
capital expenditures
CCS
carbon capture and storage
CCU
carbon capture and utilization
CDR
carbon dioxide removal
c-Si
crystalline silicon
CSP
concentrating solar thermal power
DAC
direct air capture
DACCS
direct air carbon capture and storage
DLR
German Aerospace Center
e-fuels
electricity-based fuels
EPBT
energy payback time
EROI
energy return on investment
ESM
energy system models
GHG
greenhouse gas
IAM
Integrated Assessment Model
IBRs
inverter-based resources
IEA
International Energy Agency
IPCC
Intergovernmental Panel on Climate Change
IRENA
International Renewable Energy Agency
LUT-ESTM
LUT Energy System Transition Model
NCS
natural climate solutions
NETs
negative emission technologies
NUTS
nomenclature of territorial units for statistics
NZE
Net Zero Emissions
PE
primary energy
PtX
Power-to-X
PV
solar photovoltaics
PyPSA
Python for Power System Analysis
RE
renewable energy
SSPs
shared socio-economic pathways
TES
thermal energy storage
TPED
total primary energy demand
VRE
variable RE
WEO
World Energy Outlook
*
*
*
Authors
Figures
References
Citations
Keywords
Metrics
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