Is 100% renewable energy the best goal to cut power sector emissions?
A new literature review says keeping some nuclear and CCS on the system could be more cost-effective
President Trump is expected this week to issue an executive order to roll back the Clean Power Plan, part of a slate of actions designed to undo former President Obama's climate and energy initiatives. That sentiment, however, has not stopped researchers from contending with the realities of climate change and how to prevent it.
One new literature review in particular raises some key issues for policymakers.
The research review, commissioned by the Energy Innovation Reform Project (EIRP), examines the best route to “deep decarbonization” of the power sector — nearly zero greenhouse gas emissions by mid-century. It argues 100% renewables is not the best way to get there.
“There are two branches of research on how to get deep decarbonization,” the research review co-author Jesse Jenkins told Utility Dive. “One looks at how to get to high renewables penetrations. The other looks at how to reduce GHGs in the power sector. The second group sees a lot more diverse resource mix.”
The review assumes that an 80% to 100% cut in carbon emissions from the electric utility sector is necessary to limit global climate change to 2°C this century. It argues the literature shows eliminating the last 10% to 30% of emissions needed for deep decarbonization is more cost-effective with a diverse energy mix.
That mix includes “a lot more wind and solar, more energy storage and demand response, but also what we call ‘dispatchable base resources,’” Jenkins said, including nuclear power, fossil fuel generation with carbon capture and storage (CCS), biomass, hydropower, and geothermal energy.
The EIRP review looks at work from a number of sources, including Stanford Professor Mark Jacobson, whose Solutions Project offers state and national roadmaps to 100% renewables by 2050.
Jacobson called the EIRP study “highly misleading” because “it is not what the international community believes.”
Nuclear and CCS will not necessarily reduce the costs of decarbonization, he told Utility Dive. “The United Nations International Panel on Climate Change (IPCC) found they may not be needed to get deep decarbonization and that nuclear in particular is expensive and risky,” he added.
While the current White House is unlikely to act on any plan for deep decarbonization, the findings of the EIRP review and the questions it raises could help inform policy decisions on the state level and for the post-Trump era.
The EIRP review
For the EIRP review, Jenkins and co-author Samuel Thernstrom reviewed 30 studies published since the release of the 2014 report from the United Nations Intergovernmental Panel on Climate Change, which itself contained an extensive literature review.
To stand a good chance of keeping global warming below 2°C, the report assumes the power sector will need to decarbonize faster than other parts of the economy, helping to electrify sectors like transportation and agriculture.
Across the studies surveyed, researchers found “no disagreement on the question of prioritizing the power sector in decarbonization scenarios,” according to the review.
Another key assumption, researchers wrote, is that getting to “deep decarbonization” of 80% or more will be more difficult than “comparatively modest emissions reductions (50%-70% or less).”
“Every step is increasingly challenging so it is important that we plan now for the final 20%, or we might find ourselves stuck along the way,” Jenkins said.
In their review, researchers found that a 100% renewable energy power mix “may be theoretically possible." But, they wrote, "it would be significantly more challenging and costly than pathways that employ a diverse portfolio of resources.”
In particular, including "dispatchable low-carbon resources in the portfolio, such as nuclear energy or fossil energy with carbon capture and storage (CCS), would significantly reduce the cost and technical challenges of deep decarbonization.”
The “main reason” these non-renewable resources are necessary for cost-effective deep decarbonization is the variability of renewable generation, Jenkins said. The two cost-effective backup options are a dispatchable base resource or energy storage with adequate duration.
A third option, “overbuilding” renewables, results in a lower system utilization rate and a higher cost per unit of energy, he said.
The advantage of dispatchable base resources is they can balance renewables' seasonal and long term variability more effectively than geographic diversity provided by new transmission or energy storage, Jenkins said. But unlike an overbuild of renewables, their high capacity factor keeps the overall cost down.
One paper cited in the EIRP review found a 100% renewables U.S. power system would cost “at least twice as much as an 80% renewables system, and 2.8-times the cost of a system with 20% renewables,” the EIRP review reports.
The paper, co-authored by Bethany Frew, found a 100% renewables California system “costs 2.1 to 2.8-times as much as an 80% renewable system, and 3 to 8-times more than a 20% renewable system,” the review adds.
The higher cost at higher levels of renewables is due to the need to meet demand when renewables generation drops off, Jenkins said. “You have to build several times the capacity to meet an increment of demand because the capacity factor is so low.”
Frew, Jacobson, and other 100% renewables studies assume new transmission connecting U.S. resources, but it only smooth short variations, Jenkins said. Storage only shifts supply and demand to meet peaks or balance diurnal variations.
“Without a fleet of reliable, dispatchable resources to step in when wind and solar output fade, scenarios with very high renewable energy shares must rely on very long duration seasonal energy storage,” the EIRP study argues. But, it adds, “the ten largest pumped hydro storage facilities in the U.S. are collectively capable of storing a total of just 43 minutes of U.S. energy consumption," and the studies typically assume long-duration storage technologies “that remain unproven at such large scales.”
Neither transmission nor storage is, therefore, a substitute for dispatchable base resources, Jenkins argued, but nuclear and CCS aren't the only options. Some regions, for instance, have abundant hydropower reservoirs and some have ample geothermal potential. The review argues primarily for nuclear power and CCS because they are the most widely deployable, Jenkins said.
Of the research reviewed, “every paper employing least-cost optimization techniques includes significant shares of dispatchable base resources in the decarbonized power portfolio,” the study reports. Only ones that “exclude those resources from consideration a priori” did not.
“That alone,” Jenkins argued, “shows they are not choosing the lowest cost mix but have preferences about which resources they want.”
Dispatchable baseload economics
The central questions around dispatchable baselaod generation involve cost and scale, but Jenkins said his own research shows CCS for enhanced oil recovery is effective and, with a price on emissions driving it, scalable to cost-effectiveness.
“In the long term, for deep decarbonization, we have to have a price on carbon or a value charged for carbon pollution that would encourage people to capture and permanently store it,” he said.
Along with CCS, new nuclear could prove cost-effective in scenarios with very high renewables penetration, Jenkins argued. But such projects currently in construction like Georgia Power's Vogtle nuclear facility and the VC Summer units are years behind schedule and billions over budget.
Jenkins acknowledged the financial and construction challenges facing the the Vogtle nuclear facility. When completed, its levelized cost of energy (LCOE) will likely be between $115/MWh and $120/MWh. That is, he admitted, significantly higher than Midwest wind or Southwestern utility-scale solar. But it is more competitive than New England offshore wind or rooftop solar.
Also, LCOE is not necessarily the most useful metric for comparing technologies, Jenkins said. “Anytime you push one type of resource too far, the marginal value of the next unit you’re building falls."
The LCOEs of solar and wind are lower, he said, but at very high penetrations of renewables, "the marginal value of additional units of nuclear is greater.”
“Nuclear, CCS, and other dispatchable base resources have challenges to scaling,” Jenkins admitted. Each will have advantages and disadvantages under specific circumstances.
“But," he said, "if you try to reach deep decarbonization without one of them, even with more and cheaper wind and solar and storage and transmission, the challenge is significant.”
A number of climate scientists cited by Jenkins in the EIRP took exceptions with conclusions of the literature review.
Frew, now an NREL researcher, said her paper's conclusions were reached independently from either Jacobson’s work or her work at NREL. She did not select between baseload and flexible technologies and did not consider the impact of markets, technology costs, or fuel prices.
Her work revealed, as the Jenkins study reports, “potential challenges in achieving a 100% renewable system, specifically in that last 20%,” Frew told Utility Dive. “It is technically feasible, but, at these very high penetrations, the diminishing capacity value of variable renewables and higher curtailment rates create additional challenges and associated costs.”
She departed from Jenkins' argument that dispatchable base resources are the single key. Many “operational or dynamic factors” could limit that technical feasibility. Of the options she studied, geographic aggregation through an amplified transmission system “was found to be the most effective at accommodating renewables,” she said.
Frew said her paper’s most important conclusion is that further research on supply side and demand side flexibility options will advance system operators’ ability to integrate very high renewables penetrations.
“Market design, in particular, is ripe for research,” she said. “We must know what services a future power system requires, and then design markets to properly signal those services.”
Stanford’s Jacobson developed computer models to demonstrate a strategically balanced grid system can integrate high renewables penetrations. His Solutions Project has designed 100% “wind, water, and solar” energy mixes for each state in the country and each nation of the world.
Jenkins is fundamentally wrong about nuclear generation, Jacobson told Utility Dive. “In the U.S., nuclear is absolutely not dispatchable.”
While nuclear power provides steady baseload generation, it is not as flexible as storage or gas plants. That, Jacobson said, is why the the IPCC concluded high penetrations of renewables “may not be ideally complemented by nuclear,” Jacobson said.
In addition, Jacobson said, the IPCC report’s Executive Summary concludes, “there is ‘robust evidence and high agreement’ that nuclear has meltdown, safety, weapons proliferation, and financial risks.”
The EIRP paper makes two comments about Jacobson’s grid integration study, he said. The first is that the storage capacity proposed is 2.5 times that of "current" U.S. installed generating capacity.
“That is irrelevant,” the Stanford researcher responded, “because we are not proposing to power the current U.S. power sector, which is only one-fifth of all energy.”
In fact, Jacobson said, his plan proposes electrification of all U.S. energy sectors by 2050, which would amount to five times today’s power sector generation.
“The storage we are proposing is smaller than the electricity demand in our scenario,” he said, “even though there will be a reduction in 2050 energy demand.”
The EIRP paper also claims that Jacobson’s 100% renewable energy proposal would require an amount of energy storage “equivalent to 37.8 billion Tesla Power Wall 2.0 home energy storage systems” or about 320 systems per U.S. household.
“That is plain wrong,” Jacobson said. “Our storage is equivalent to zero Powerwalls.”
His work shows how a 100% renewables penetration on the grid can be stable “with no stationary batteries at all,” Jacobson said. Instead, it models stabilization with “a combination of storage options that cost between 1/300th and 1/9th of Powerwalls, including low-cost electricity, heat, cold, and hydrogen storage.”
The EIRP review also assessed a study from the National Renewable Energy Laboratory (NREL) co-authored by Trieu Mai, a researcher at the Strategic Energy Analysis Center.
The EIRP review raises valid concerns, NREL’s Mai said, but the magnitude of the concerns he addresses” is not yet clear.
“Anybody would agree technology diversity has value,” he said. “It is just math. When you open the solution space to more options, the optimal solution will be lower cost. But it is unclear how much lower.”
NREL’s Mai was part of a team that modeled an 80% U.S. renewables penetration. The four-volume, 2014 study found a renewables mix, “in combination with a more flexible electric system, is more than adequate to supply 80% of the total U.S. electricity generation in 2050 while meeting electricity demand on an hourly basis in every region of the United States.”
Mai says he has not seen research proving the higher renewables plans will cost more. Solar and wind costs have dropped dramatically and technology advances make further declines likely, Mai said.
“The future competitiveness of technologies like nuclear and CCS is more uncertain. There have not been the same cost declines, though they may come.”
For now, the researchers agree, more research is needed to evaluate the most cost-effective resource mix for deep decarbonization. In the meantime, Mai said the power sector should continue to focus on adding the lowest-cost, lowest-emitting resources possible.
“Given this steep road and the numerous uncertainties, it’s probably more fruitful to focus on immediate growth of emissions-free generation rather than on the optimal solution for the final margin."