The news media can be distracted by stories that seem more important than they are. The recent furor over the best energy resource mix for 2050 provides an example.

A debate between Stanford professor Mark Jacobson and former National Oceanic and Atmospheric Administration (NOAA) scientist Christopher Clack over whether the U.S. can be powered entirely by renewable resources in 2050 got a lot of attention. The New York Times headlined it “Fisticuffs.” The Washington Post called it a “bitter and personal feud.”

A 2015 paper written by a Jacobson-led team argued electrification of all U.S. energy sectors can be done by 2050 with almost 100% wind, water, and solar (WWS) resources, plus energy storage. That supply mix and demand response can keep the U.S. grid “stable at low cost,” it concluded.

But Clack, now CEO of renewable energy software firm Vibrant Clean Energy, argues that 100% renewables is a “valuable hypothetical aspiration” but should not be presented as a scientific article.

“Our work shows it is demonstrably false that there are only political barriers [to 100% renewables],” he said, “because there are also technical and economic barriers and more work is needed to get to very high renewables penetrations.”

Jacobson’s response to Clack's critique of his paper’s methodologies was uncompromising. “Clack's analysis is riddled with intentional misinformation and has no impact on the conclusions of our grid integration study,” he said.

The back-and-forth turned the exchange of scientific papers into a public feud over renewables, but Clack’s critique was never of Jacobson’s aspirational goal. It questioned the paper’s assumptions.

Media reports focused on the dispute about the resource mix and often overlooked how the energy system is evolving. But other researchers have begun to address the possibility of coupling together the many sectors of the economy that consume energy. They are thinking about new efficiencies from using electricity generated by renewables for heating and cooling and transportation.

Consultant Diane Moss, the founding Director of the Renewables 100 Policy Institute, said most experts she trusts agree technology advances will be instrumental in getting to renewables penetrations of 80% to 100% at scale.

"There is increasing agreement that getting to 100% renewables is technically feasible, but technology improvements and new approaches like cross-sectoral coupling will make that even more efficient, economical, and reliable," Moss said.

The Jacobson vision

Jacobson’s paper, “Low-cost solution to the grid reliability problem with 100% penetration of intermittent wind, water, and solar for all purposes,” addresses what it calls “the greatest concern facing the large-scale integration of wind, water, and solar into a power grid.”

It notes that other studies have addressed some of the grid reliability issues created by high penetrations of WWS but no paper has analyzed a 100% WWS system.

The 2015 paper uses LOADMATCH, a new modeling tool at the time, to demonstrate that a 100% penetration of WWS can be accomplished without the “high cost of avoiding load loss caused by WWS variability and uncertainty.”

It concludes that “low-cost, no-load-loss, non-unique solutions” would allow, by 2050 to 2055, “electrification of all U.S. energy sectors (electricity, transportation, heating/cooling, and industry).”

The tool incorporates detailed global weather data and six types of storage for heat, cold, and electricity. It assumes no need for natural gas, biofuels, nuclear power, or stationary batteries, but significant use of demand response (DR). It models 70% of industrial load and 85% of transportation load as flexible.

LOADMATCH assumes all residential, commercial-industrial, and transportation energy would come from electricity. Between 50% and 95% of cooling and 85% to 95% of heating would be met with thermal energy storage (TES), including underground TES (UTES). Non-flexible loads would be met with hydrogen.

“The 2050 delivered social (business plus health and climate) cost of all WWS including grid integration (electricity and heat generation, long-distance transmission, storage, and H2) to power all energy sectors was approximately $0.1137/kWh,” according to the paper. It ranged from $0.085/kWh to $0.154/kWh in 2013 dollars.

The price is higher than future conventional electricity cost of $0.106/kWh because it integrates transportation, heating/cooling, and industrial energy costs. Eliminating those sectors results in “a rough WWS electric system cost of approximately $0.106/kWh.

More importantly for the paper’s authors, it avoids the $0.170/kWh in the price of electricity attributable to the 2050 health and climate cost. “Whereas the 2050 business costs of WWS and conventional electricity are similar, the social (overall) cost of WWS is 40% that of conventional electricity.”

Jacobson and his co-authors envision a world where renewables and thermal storage combine to provide all of the nation's electricity. From the Jacobson paper

The Clack critique

“As an aspirational goal, it is good to push for higher penetrations of renewables,” Clack told Utility Dive. He and his paper's 21 contributing scientists intended only to “correct the science” and avoid a “backlash” from a public led to believe the energy transition would be “easy and cheap,” Clack said.

The Intergovernmental Panel on Climate Change, the National Renewable Energy Laboratory, and others agree “a diverse portfolio of clean energy technologies makes a transition to a low-carbon-emission energy system both more feasible and less costly,” Clack's paper, “Evaluation of a proposal for reliable low-cost grid power with 100% wind, water, and solar,” argues.

According to that paper, the Jacobson report “used invalid modeling tools, contained modeling errors, and made implausible and inadequately supported assumptions.”

Significant reduction of greenhouse gas emissions (GHGs) could be achieved with WWS, supporting technologies, and an optimally configured national high voltage direct current (HVDC) transmission system. And reliability is theoretically achievable.

But “achieving an 80% reduction in GHGs from the electricity sector at reasonable costs is extremely challenging,” the Clack critique asserts. Decarbonizing “the last 20%” and the rest of the economy will be “even more challenging” and limiting the technologies to WWS makes it even more difficult.

The Jacobson paper’s narrow generation options rely on “currently uncosted innovations” and it assumes “multiweek energy storage systems” that are not proven scalable “at a capacity twice that of the entire United States’ generating and storage capacity today.”

The Clack paper then details “modeling errors,” “implausible assumptions,” “insufficient power system modeling,” and “inadequate scrutiny of input climate model.”

“The paper became something of scientific gospel for policymakers and advocates,” Clack said. “We are correcting the science.”

Among the points most in need of correction is what the critique identifies as a severe overestimate of hydropower potential. It assumes a combined 2050 installed generation and pumped storage capacity could produce 1,300 GWs of power.

“Either the installed capacity must be higher or there’s an error in the modeling,” Clack said. “That’s the biggest mistake, but there are quite a few more.”

The Jacobson researchers promise the WWS mix, with storage and DR, will provide reliability “but they don’t actually model the grid in any way,” Clack said. They also assume a national HVDC transmission system, “but we found, in other work, that it is the hardest part of getting to high renewables penetrations and makes or breaks using renewables reliably.”

Clack also questions Jacobson’s assumptions about TES and UTES for heating and cooling. Even more significantly, “it boggles the mind how hydrogen will power the whole aviation and other heavy-duty transportation sectors,” he said. “That will be very expensive but it’s not costed at all in their model.”

In the Jacobson paper, all transportation, including aviation, and shipping, and all heating and cooling, depend on “two technologies that really don’t exist today at scale but will have to be at a scale in 33 years,” Clack said. “I don’t think we can do it without damaging the economy.”

The critique calls the Jacobson paper “a poorly executed exploration of an interesting hypothesis” because it is “unreliable” on the cost, technical reliability, and feasibility of the energy transition it describes.

But, Clack stressed, he supports thinking about high renewables penetrations. “If you told me we will have 80% renewables in 20 years, I could die a happy man.”

The Jacobson response

Jacobson’s line by line refutation of the critique argued the hydropower value is based on the LOADMATCH assumption that new turbines in existing reservoirs would increase the hydropower discharge rate.

The simulations show grid stability can be maintained “without increasing the hydropower discharge rate at all or with a hydropower discharge rate down to 700 GW (rather than 1300 GW),” Jacobson added.

The estimates of the cost of additional HVDC transmission in his paper are “as detailed as most other estimates but uncertain,” Jacobson argued. “We account for the uncertainty by presenting high and low results.”

The Clack critique “fails to demonstrate any important errors in our economic analysis” or show that “our estimates of the T&D system cost are significantly low,” he added.

UTES is not unproven, and in fact is “a form of district heating” used worldwide, Jacobson argued. With 60% of Denmark’s heat now coming from district heating, “the implication that this can’t be done on a large scale is false.” And it will sharply reduce the cost of heating because it is “1/300th that of battery storage and 1/30th that of water and ice storage per kWh stored.”

Finally, the scale-up of hydrogen in his model is significantly less than the scale-up of transmission proposed in a paper co-authored by Clack, Jacobson argued. It is also less than the nuclear or coal-carbon capture scale-up in other papers cited in the Clack critique, he added.

The cost assumptions for the hydrogen scale-up “are aggressive but reasonable,” according to Jacobson. The proposal is to convert short-haul aircraft by 2035 and long-haul aircraft by 2040. Because a four-passenger short-haul hydrogen fuel cell aircraft with a 1500 km range is already in service, the timeline for the scale-up “seems attainable.”

The critique of his scale up of hydrogen production in comparison to current U.S. demand for electricity is “irrelevant,” Jacobson added. “We propose to electrify all energy sectors and electricity is currently only 20% of all energy.”

Room for a new view

The advances that will allow a 100% renewables penetration are “difficult, if not impossible, to completely foresee today,” Moss said. A key strategy, now emerging in Europe and states where penetrations are growing fast, is cross-sectoral (a.k.a. intersectoral) coupling.

Janice Lin, the California Energy Storage Association Executive Director, agreed. “Limited supply diversity in Europe, and especially in Germany, has led to thinking about intersectoral coupling and about a much bigger energy circle.”

The idea has surprising applications. “Think about avoided costs,” Lin said. “If a consumer charges an electric vehicle instead of filling it at the gasoline pump, that is consuming more electricity but what costs is it avoiding in oil and GHGs? Shouldn’t the avoided costs framework be expanded to include those?”

In “Sector Coupling in a Simplified, Highly Renewable European Energy System,” University of Frankfurt researchers detailed the concept.

Coupling the electricity sector to heating and mobility enables wider decarbonization and adds power system flexibility. “Electric vehicles can change their charging pattern to benefit the system,” they argue. “Heat is much easier and cheaper to store than electricity, even over long times.”

The vision is to electrify all road and rail transport across 30 European countries. Because of the higher efficiency of electric motors, this would make final energy consumption 3.5 times lower, the researchers calculate.

All space and water heating “can be met by resistive heaters, gas boilers, and heat pumps,” they add. That would take advantage of the TES in “cheap hot water tanks.”

The researchers modeled scenarios with and without transmission connecting the countries. They found “optimal inter-connecting transmission” reduced system cost by 33%.

Overall, they concluded, the cost of a high renewables penetration system can be kept from rising if consumers invest in EVs and system planners “allow lots of onshore wind, international grid expansion, and sector-coupling flexibility.”

“Flexible sector coupling using grid-friendly EVs and long-term TES,” they add, “can reduce costs by 30%.”