Is 100% renewable energy the best goal to cut power sector emissions?

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.”

Researchers respond

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.