It's a common claim from advocates: We know we can create a 100 percent renewable grid, because Stanford Professor Mark Jacobson said we can.

Jacobson's peer-reviewed studies assert that it is possible to convert all energy use in the U.S. to wind, water and solar -- while maintaining grid reliability, saving money and creating jobs.

It will require a World War II-style mobilization, he notes. But it's possible.

But that conclusion is now being questioned in a big way.

On Monday, a battalion of fellow energy researchers published a rebuttal to Jacobson's plan in the same prestigious journal where his study first appeared. The 21 authors include some of the most prominent climate change and clean energy experts in the country, like Ken Caldeira of Stanford, Daniel Kammen of U.C. Berkeley, and Varun Sivaram of the Council on Foreign Relations.

The lead author is Christopher Clack, a former research scientist at the University of Colorado and current CEO of the grid modeling consultancy Vibrant Clean Energy.

The sheer number of co-authors suggests this is not a battle of egos. Their accumulated expertise has advanced the understanding of climate change and the system impacts of high amounts of renewable energy. They are not industry shills trying to undermine the advance of wind and solar; they are scientists who want to use evidence-based reasoning to optimize it.

And they deliver some pointed academic smack talk.

"The scenarios of [the Jacobson study] can, at best, be described as a poorly executed exploration of an interesting hypothesis," the authors assert.

The broader conflict is over the best way to achieve a low-carbon grid.

Jacobson opted for a constrained system that excludes all but a handful sources of energy. His work shows what could be technologically possible if society prioritizes the "right" things. However, because decarbonization is so hard, it requires a more diversified approach for success, say the group of researchers.

Jacobson's study has already encouraged some lawmakers to propose 100% renewable energy plans. The authors of the rebuttal say those policies are based on flawed science.

"If one reaches a new conclusion by not addressing factors considered by others, making a large set of unsupported assumptions, using simpler models that do not consider important features, and then performing an analysis that contains critical mistakes, the anomalous conclusion cannot be heralded as a new discovery," the authors write.

Jacobson remains unshaken.

"Virtually every sentence in the Clack article is false as evidenced by [my] line-by-line response," he wrote in an email Saturday, referring to a counter-rebuttal he had written. "There is not a single error in our paper."

Supersized hydropower

The Jacobson model uses wind and solar power as the primary energy providers for the U.S., supplemented by hydropower, pumped hydro storage, a few other storage technologies, and flexible load as needed to match total grid supply with demand.

If renewable generation exceeds load, the surplus goes into storage or hydrogen production. If renewable generation doesn't meet instantaneous demand, flexible loads get deferred for up to 8 hours. If generation can't satisfy inflexible load, the system draws on stored electricity and then from hydropower, "which is used only as a last resort."

The article, published in the Proceedings of the National Academy of Sciences in December 2015, says that, in the fully renewable system, "supply exactly matches load plus losses and changes in storage at all times." The modeling covers every 30-second increment from 2050 to 2055.

Data in the article contradicts the role that Jacobson says hydropower will play, the Clack study contends.

The authors point to supporting information from the Jacobson study that shows maximum output from hydroelectric facilities won't exceed 145.26 gigawatts, which itself is about 50 percent more than the existing hydro capacity today. That's actually the sum of hydro storage and hydropower, which operate differently.

In the Jacobson study, though, a chart covering several days of grid operation in January 2055 shows hydroelectric output exceeding 1,300 gigawatts. The method of last resort for grid balancing would have to perform at a capacity nearly 10 times greater than its stated maximum capacity -- more like 15 times, given that this is referring to hydropower and not storage.

(Image credit: Jacobson et al.)

The 1,300 gigawatts of hydro isn't an error; it's an assumption, Jacobson said in an interview.

His model maintains the same amount of hydro energy in a year, because that's tied to the volume of water available. In his vision, though, hydropower would be pulled off daily energy duty to store up large amounts of water for massive discharge on a few peaks each year. He assumes retrofits of higher capacity turbines on existing dams could make this possible.

The 100 percent model succeeds or fails based on its ability to meet power demand even when wind and solar can't supply it all. As currently constructed, hydropower backstops the whole system in moments when even stored electricity does not suffice. The feasibility of this roadmap hinges on how likely it is to massively increase the instantaneous discharge rate of U.S. hydro assets.

"There's no realistic scenario whereby you can expand the output of the U.S. hydropower system by a factor of 10," said David Victor, one of the 21 co-authors and director of the International Law and Regulation Laboratory at U.C. San Diego.

New hydro capacity has stalled out for the last 20 years or so, mostly due to regulations. The Department of Energy calculated that hydro and pumped storage could feasibly grow to 150 gigawatts by 2050, including upgrades to existing plants.

"Anything we have to do has to be done on a large scale," Jacobson said. "We’re talking about changing the entire energy infrastructure of the United States."

Grid reliability without grid modeling

Operating the grid requires more than producing enough power to meet demand in a given moment; it also has to be delivered. Transmission and distribution constraints play a significant role in determining whether generation can reach load centers, especially in the cases of wind and solar, which are often constructed far from dense load pockets.

Jacobson's grid reliability study does not model the spatial dimensions of the transmission system.

"As a result, their analysis ignores transmission capacity expansion, power flow and the logistics of transmission constraints," Clack and company write. "Similarly, those authors do not account for operating reserves, a fundamental constraint necessary for the electric grid."

The model also ignores requirements for frequency regulation to ensure grid reliability.

In his line-by-line response to these points, Jacobson writes: "This critique is wrong in critical respects and fails to demonstrate any important errors in our economic analysis."

In an interview, Jacobson added that models require tradeoffs. He included 30-second time resolution but no spatial resolution; other models focus more on spatial resolution with less granular time resolution.

The study includes an estimate of the cost of additional high-voltage DC transmission lines, he noted. But that left the other researchers unsatisfied with the broader impact to grid management.

"If you’re not even modeling the transmission system in any way, how can you say you’ve got a reliable grid?" Clack said in an interview.

Experimental storage trumps commercially ready options

Energy storage plays a critical role in absorbing surplus generation and discharging it at times of need. The storage used in the Jacobson plan, though, will look unfamiliar to observers of today's storage industry.

The young energy storage market is growing fast: GTM Research expects the U.S. market to be 22 times larger in megawatt-hour terms by 2022. Lithium-ion technology has dominated deployments for the last 10 quarters, capturing 96.5 percent of the market in Q1 of this year.

Lithium-ion storage plays no role in Jacobson's fully renewable energy system, other than to power electric vehicles. 

"Batteries for stationary power storage work well in this system too," the Jacobson study explains. "However, because they currently cost more than the other storage technologies used, they are prioritized lower and are found not to be necessary for a reliable system."

In its place, the model uses six other types of storage. Underground thermal energy storage (UTES), modeled on a government-funded pilot project called Drake Landing in Canada, will handle all storage for building air and water heating. This dwarfs the other types of storage, with a maximum deliverable capacity of 514.6 terawatt-hours. Chilled water storage and ice storage will handle cooling. 

Phase-change materials in concentrating solar power plants will store up to 13.26 terawatt-hours of electricity, and pumped hydro will hold 0.808. Hydrogen storage will supply transportation and high temperature processes.

Of those six, only pumped hydro has achieved widespread commercial use on the grid, but it has a clear limit to its growth potential.

Chilled storage has seen commercial deployments by companies like Calmac, Ice Energy and Viking Cold Solutions. These solutions use electricity to precool a liquid, which can then chill buildings at times when electricity is more expensive.

The reliance on UTES requires the technology to quickly transition from pilot scale to nearly every building heating system in the United States. Jacobson says that he believes the technology can scale.

"UTES has been demonstrated at the scale it needs to be deployed -- neighborhood and complex scale, and it has been tested in more extreme conditions (Canada seasonally) than it would be needed for in the United States," he writes in his line-by-line rebuttal.

Jacobson stressed in an interview that the technology itself works and can grow.

"When something is so simple, and it's cheap already, it has tons of potential to be commercialized and used on a large scale, particularly in new communities," he said.

The phase-change materials he pairs with CSP, similarly, have not passed from demonstration phase to commercial deployment; that technology is meant to carry the brunt of the electrical (as opposed to the thermal) storage in this system. 

Along with the risks inherent in relying on new technologies that haven't scaled to mass distribution, this also delays the timeline for implementation.

"Thirty-three years away in terms of energy isn't very long for installing things," Clack said. "We need to be installing things today, and we need to be installing things today that we know will be around for a decent amount of time."

Wind and solar are getting deployed all the time, but there's no clear pathway for converting most American buildings to underground thermal storage. That requires a massive supply chain, and companies that are ready to sell and install the devices.

No optimization

The Clack paper critiques too many other aspects of the Jacobson model to include them all here. (One notable contender: In calculating lifecycle greenhouse gas and mortality emissions for civilian nuclear energy, Jacobson factors in the effects of nuclear war, which is assumed to occur on a regular 30-year cycle.)

It's worth reiterating, though, that Jacobson's renewable roadmap is not, nor does it claim to be, an optimization study.

It did not survey all the options and select the best portfolio on the basis of speed, cost or some other metric. It runs a program to balance energy supply with demand every 30 seconds for a given configuration of renewable and storage assets. As Jacobson writes in his rebuttal, "It is a trial and error model."

He came to this after growing frustrated with the limitations of optimization models. Those models took a long time and couldn't include all the details he wanted to include.

"It's not a least-cost solution that we’ve come up with; it’s a low-cost solution," he said. "Our low-cost solutions are lower than the current grid costs."

This may be useful in signaling to the world that the math checks out; renewables and storage can be deployed at levels that, on paper, meet the total energy needs of the U.S. But it does not show that this is the best path. That's concerning to researchers like Clack.

"If you have these goals and you don't achieve them, there tends to be a very strong backlash," Clack said.

That backlash could come from a state legally enforcing the Jacobson plan, only to discover real-world technologies can't make it work. Additionally, it could limit the pursuit of other energy technologies that are important for decarbonization.

The passion behind the arguments illustrates how high the stakes have become. There's limited time left to chart a low-carbon energy pathway. The 21 authors countering Jacobson want to make sure we're paying close attention to the details.