The September 2017 copy of the journal Renewable and Sustainable Energy Reviews included an article entitled “Burden of proof: A Comprehensive Review of the Feasibility of 100% Renewable-Electricity Systems,” by B.P. Heard, B.W. Brook, T.M.L. Wigley, and C.J.A. Bradshaw. The article referred to several published scenarios that claim a 100% renewable electricity system is achievable. However, the authors questioned whether that’s correct.
This review covered energy technologies classed as “renewable” (mainly hydroelectricity, biomass, wind, solar, wave and geothermal), often with the explicit exclusion of nuclear power and fossil fuels. They identified published scenarios that addressed the challenge of providing an electricity supply entirely from renewable sources.1 Heard and his colleagues applied the following screening criteria for their review:
- Scenarios had to be published after 2006 to cover literature representative of the current state of knowledge.
- Scenarios must propose electricity supply to be from at least 95% renewable sources.
- Scenarios must consider large-scale demand areas, such as the whole globe, whole nations, or covering extensive regions within large nations
- Scenarios must forecast to the year 2050 or earlier. If scenarios extended beyond 2050, but still allowed scores to be determined based on 2050 milestones, they included the scenario and scored it against the 2050 outcome.
The primary concern of this review was with evidence for the strict technical feasibility of proposed 100% renewable electricity systems. The review used viability as a subordinate concept to feasibility.
The definition of feasible was “possible within the constraints of the physical universe.” And, the demonstration of feasibility requires that evidence is presented so that a proposed system will work with current or near-current technology at a specified reliability. Here, feasible refers to the whole electricity system, not merely the individual items of technology, such as a solar panel or a wind turbine. Viable means that the system is not only feasible, but also realistic within the socio-economic constraints of society.
The concept of feasible requires four subsidiary criteria so that it can be workable when applied to a whole electricity network, including:
1. The electricity demand to which supply will be matched must be projected realistically over the future time interval of interest.
2. The proposed supply of electricity must be simulated/calculated to be able to meet the real-time demand for electricity for any given year, together with an additional backup margin, to within regulated reliability limits, in all plausible climatic conditions. As per Criterion 1, this reliability must be demonstrated as achievable for the full range of plausible future energy demand.
3. Any transmission requirements for newly installed capacity and/or growth in supply must be described and mapped to demonstrate delivery of generated electricity to the user network, so that supply meets both projected demand and reliability standards.
4. The proposed system must show how critical ancillary services will be provided to ensure power quality and the reliable operation of the network, including distribution requirements.
Ancillary Services
Ancillary services are a physical requirement of any electrical system; they have been necessary since the development of reticulated power. This review discussed two examples of ancillary-service requirements: frequency control and voltage control.
Today, the frequency of the alternating-current electrical system must be maintained close to the prescribed standard (typically 50 or 60 Hz within a normal operating band of ±0.1 Hz). In practice, the frequency varies due to changes in electrical load on the system. Changes in frequency arise from the small, instantaneous and ongoing variation in load that occurs due to consumer behavior (e.g., turning lights on and off), to larger changes in demand occurring in the normal course of a day.
Instantaneous frequency control is typically provided by the inertia of “synchronous” generators, where electricity is generated through turbines spinning in unison at close to the regulated standard. However, increased wind and solar penetration, with asynchronous generation of electricity, displaces traditional synchronous generators.
Voltage must be managed to within specified tolerances for insulation and safety equipment. Voltage management is affected by the expansion of generation that’s connected to an electrical-distribution network, known as “embedded generation.” The impact of embedded generation has been transformed by the rapid uptake of small-scale solar photovoltaic systems. As a consequence, voltage control at the distribution level has become a concern in places with high penetration of solar photovoltaics.
Projected 100% renewable electricity systems are incomplete in the absence of evidence that essential, regulated ancillary services will be maintained. This is particularly relevant for 100% renewable-supply systems that propose high reliance on asynchronous wind generation and embedded, asynchronous solar photovoltaic generation.
System Simulations
Of the 16 scenarios that provided simulations, only two simulated to intervals of <1 hour and only two tested against historically low renewable-energy conditions.2,3,4 Historical testing is useful in general, but such tests don’t address the high variability of output from renewable resources, let alone the attendant uncertainties associated with future climatic changes. Because of these issues, the system-simulation approaches applied so far mostly cannot demonstrate the feasibility and reliability of 100% renewable energy systems.Studies by Mason5 and colleagues reinforce the notion that integration of variable renewable-energy sources into existing grids can be cost-effective up to penetrations of around 20%, after which integration costs escalate rapidly. An upper threshold to economically rational amounts of wind-generation capacity is also found in simulations for the United Kingdom. Any further installed wind-generating capacity makes little difference in meeting electricity demand in times of low wind supply. While the cost-effective threshold for integration of variable renewable electricity will vary among grids, 100% renewable studies such as these reinforce that penetration thresholds exist, and that alternative dispatchable generation supplies are required to meet the balance of supply.
Observations
This review also found that reliability is usually only simulated to the hour or half-hour in modeled scenarios. A common assumption is that advances in storage technologies will resolve issues of reliability both at sub-hourly timescales and in situations of low availability of renewable resources that can occur seasonally. Yet in the 24 scenarios that were examined, 23 either already relied directly on expanded storage technology, or they described an implicit reliance on such technologies without simulation support. Despite these storage assumptions, only five of the 24 studies demonstrated sub-hourly reliability.
A high-penetration renewable scenario for California developed by Hart and Jacobson6 suggested that moving to 100% generation from renewables would require a lower bound storage capacity of 65% of the peak demand to decouple most real-time generation from real-time demand. The authors described this as a “significant paradigm shift in the electric power sector.” Achieving such a paradigm shift is an unresolved challenge, one that Hart and Jacobson claim will require a willingness to transform not only a region's generating fleet, but also the controls, regulations, and markets that dictate how that fleet is operated.
It’s reasonable to assume a greater range of cost-effective options in energy storage will be available in the future. Such solutions will undoubtedly assist in achieving reliability standards in systems with greater penetration of variable renewable generation. However, whether such breakthroughs will enable the (as yet unknown) scale of storage and associated paradigm shift required for 100% renewable remains unknown, and is largely unaddressed in the literature.
To bet the future on such breakthroughs is arguably risky, and it’s pertinent for policy makers to recall that dependence on storage is entirely an artefact of deliberately constraining the options for dispatchable low-carbon generation. In optimal systems for reliable, decarbonized electricity systems that have included generic, dispatchable zero-carbon generation as well as variable renewable generation, the supply provided by storage is just 2–10%.
Resource variability isn’t the only concern regarding hydroelectricity. The widespread potential disruption to rivers and associated habitats from hydroelectric dams are well-documented, particularly for the rivers and forests of the Amazon. Proposed hydroelectric developments in the Amazon will be major drivers of disruption to connectivity of habitat and deforestation. Proposed developments will also lead to displacement of indigenous populations.
Perhaps the reviews that are most concerning relate to the dependence of 100% renewable scenarios on biomass. The British scenario is a typical example: Even with the assumption of a 54% reduction in primary energy consumption, biomass requires 4.1 million hectares of land to be committed to the growing of grasses, short-rotation forestry, and coppice crops (17% of UK land area).
Lund and Mathiesen described how Denmark would need to reorganize farming from wheat to corn to produce the requisite biomass, in a scenario of 53% reduction in primary energy consumption from the baseline year.7 For Ireland, Connolly et al. calculated a biomass requirement that was 60% of the total potential biomass resource in Ireland.8
The demand-reduction assumptions in most of the scenarios that were considered, when combined with their dependence on hydroelectricity and biomass, suggest that 100% renewable electricity is likely to be achievable only in a low-energy, high-environmental-impact future. In this case, an increasing area of land would be recruited into the service of providing energy from diffuse sources. The realization of 100% renewable electricity (and energy more broadly) appears diametrically opposed to other critical sustainability issues, such as eradication of poverty, land conservation and reduced ecological footprints, reduction in air pollution, preservation of biodiversity, and social justice for indigenous people.
Remaining feasibility gaps lie in the largely ignored, yet essential requirements for expanded transmission and enhanced distribution systems, both to transport electricity from more sources over greater distances, and to maintain stable system operations.
Of the four proposed criteria, transmission networks could arguably be regarded as more a matter of viability than feasibility; the individual requirement of long-distance interconnection is well-known and understood. Rescoring all of the studies excluding this criterion (effectively granting all assumptions of a copperplate network), feasibility still isn’t met completely by any study.
The same grace can’t be granted for maintaining sufficient synchronous generation, voltage requirements, and ensuring robust system-restart capabilities in 100% renewable systems with high production from variable and asynchronous sources.
With direct use of batteries9 or modified wind turbines, maintaining stability could require interventions that include:
- Payments for minimum synchronous generation to remain online
- Development of new markets in ancillary services
- Network augmentation
- Mandated curtailing of supply from wind and photovoltaics in some supply situations.
Others have suggested that changes in market operations will be required to accommodate energy sources that are euphemistically described as “flexible.”
Conclusions
The assessment of studies proposing 100% renewable-electricity systems reveals that in all individual cases and across the aggregated evidence, the case for feasibility is inadequate for the formation of responsible policy directed at responding to climate change. Addressing the identified gaps will likely yield improved technologies and market structures that facilitate greater uptake of renewable energy. However, they might also show even more strongly that a broader mix of non-fossil energy technologies is necessary.
To date, efforts to assess the viability of 100% renewable systems, taking into account aspects such as financial cost, social acceptance, pace of rollout, land use, and materials consumption, have substantially underestimated the challenge of excising fossil fuels from our energy supplies. This desire to push the 100% renewable ideal without critical evaluation has ironically delayed the identification and implementation of effective and comprehensive decarbonization pathways. The review argued that the early exclusion of other forms of technology from plans to decarbonize the global electricity supply is unsupportable, and arguably reckless.
Specifically, the review stated:
“For the developing world, important progress in human development would be threatened under scenarios applying unrealistic assumptions regarding the scale of energy demand, assumptions that lack historical precedent and fall outside all mainstream forecasts. Other outcomes in sustainability, social justice, and social cohesion will also be threatened by pursuing maximal exploitation of high-impact sources like hydroelectricity and biomass, plus expanded transmission networks. The unsubstantiated premise that renewable energy systems alone can solve the challenge of climate change risks a repeat of the failure of decades past. The climate change problem is so severe that we cannot afford to eliminate a priori any carbon-free technologies.
“Our sobering results show that a 100% renewable electricity supply would, at the very least, demand a reinvention of the entire electricity supply-and-demand system to enable renewable supplies to approach the reliability of current systems. This would move humanity away from known, understood, and operationally successful systems into uncertain futures with many dependencies for success and unanswered challenges in basic feasibility.
“Uniting the alleviation of poverty with a successful climate-change response in our energy and electricity systems should be an international goal. This is likely to require revolutionary changes in the way we grow food, manage land, occupy homes and buildings, demand electricity, and otherwise live our lives. Such changes will require more, not less energy. It would be irresponsible to restrict our options to renewable energy technologies alone.”
The reality is that 100% renewable electricity systems don’t satisfy many of the characteristics of an urgent response to climate change:
- Highest certainty and lowest risk-of-failure pathways
- Safeguarding human-development outcomes
- Having the potential for high consensus and low resistance
- Giving the most benefit at the lowest cost
References
1. Australian Energy Market Operator Ltd. 100 per cent renewables study-modelling outcomes. New South Wales; 2013.
2. B. Elliston, M. Diesendorf, I. MacGill, Simulations of scenarios with 100% renewable electricity in the Australian National Electricity Market Energy Policy, 45 (2012), pp. 606-613.
3. M.Z. Jacobson, M.A. Delucchi, M.A. Cameron, B.A. Frew, “Low-cost solution to the grid reliability problem with 100% penetration of intermittent wind, water, and solar for all purposes,” PNAS, 112 (2015), pp. 1560-1565.
4. M.Z. Jacobson, M.A. Delucchi, A.R. Ingraffea, R.W. Howarth, G. Bazouin, B. Bridgeland, et al., “A roadmap for repowering California for all purposes with wind, water, and sunlight,” Energy, 73 (2014), pp. 875-889.
5. I.G. Mason, S.C. Page, A.G. Williamson, “A 100% renewable electricity generation system for New Zealand utilising hydro, wind, geothermal and biomass resources,” Energy Policy, 38 (2010), pp. 3973-3984.
6. E.K. Hart, M.Z. Jacobson, “The carbon abatement potential of high penetration intermittent renewables,” Energy Environ Sci, 5 (2012), p. 6592.
7. H. Lund, B.V. Mathiesen, “Energy system analysis of 100% renewable energy systems—the case of Denmark in years 2030 and 2050.”
8. D. Connolly, H. Lund, B.V. Mathiesen, M. Leahy, “The first step towards a 100% renewable energy-system for Ireland,” Appl Energy, 88 (2011), pp. 502-507.
9. J. Deign, “German firms turn batteries into power plants to aid grid control,” Energy Storage Update, London, UK (2015).
