Stanford Energy is brought to you by the Precourt Institute for Energy
By Mark Golden
Climate change solutions bring to mind solar and wind power, but those renewable technologies on their own probably will never be the energy source for making cement and steel.
To maintain modern economies and improve life in developing economies while deeply reducing global greenhouse gas emissions will require major advances in how we convert, store and transmit thermal energy, according to Stanford University professor of mechanical engineering, Arun Majumdar.
“About 90 percent of our current energy budget centers around heat conversion, transmission, storage and use,” Majumdar said as the speaker of MIT’s first International Colloquia on Thermal Innovations last week.
Credit: MIT Mechanical Engineering
Modern renewable technologies are the most inexpensive source of electric energy we have today, but solar and wind power are intermittent and account for a small percentage of the world’s energy, explained Majumdar, who is also the co-director with Sally Benson of Stanford's Precourt Institute for Energy. We need to increase this percentage, he said, but we cannot ignore decarbonizing heat or leveraging heat to store intermittent electricity from solar and wind.
His MIT talk, “Five Grand Challenges in Thermal Science and Engineering for Deep Decarbonization,” (video here), was followed by a discussion with Ravi Prasher, associate laboratory Director at Lawrence Berkeley National Laboratory, and Asegun Henry, associate professor of mechanical engineering at MIT. The Zoom-based event was organized by Gang Chen, MIT professor of mechanical engineering, and moderated by Chris Dames, professor of mechanical engineering at University of California-Berkeley.
Heat as energy storage
One grand challenge is to store excess wind and solar power as heat energy over multiple days and then convert it back into electricity when needed. The modern electric grid was not built for 70 percent or more of power coming from solar and wind. Even at much lower percentages, some places around the world—like California—often have much more electricity than they can use on sunny afternoons. Stanford’s recently transformed energy system stores excess renewable electricity as thermal energy in huge tanks of hot and cold water with uniquely large heat exchangers, Majumdar noted.
“For long-duration storage from 50 to 100 hours or more, thermal storage of renewable electricity is perhaps one of the most inexpensive and scalable solutions,” he said. “Even though the round-trip efficiency may be only 50 percent to 60 percent, the cost could be within the needed range of less than $10 per kilowatt-hour."
The key for thermal storage to store excess electricity cheaply is a medium that has high energy density, low cost and the ability to transfer heat at a high rate.
While many people are working on this and other big challenges, “we need R&D to get there,” Majumdar said.
Another grand challenge is generating the extreme heat needed in industrial processes, like making cement and steel. This could be accomplished by using heat and a catalyst to crack methane and produce hydrogen without the high greenhouse gas emissions of current hydrogen production. Instead, it would produce solid carbon. This needs R&D to make the process cost effective and scalable.
One grand challenge is on the opposite side of the thermal spectrum from heat: refrigeration. The goal is to invent refrigerants for both food and air conditioning without today’s leakage of hydrofluorocarbons, a set of extremely powerful greenhouse gases. Successful new refrigerants must be non-flammable, non-toxic and affordable and preferably drop-in solutions to today’s systems, said Majumdar.
“With the rise of refrigeration and cooling in emerging economies, this is a major challenge,” Majumdar explained.
In many developing economies the growing demand for air conditioning is about reducing humidity as much as temperature, added Prasher, so new refrigerants will have to accomplish this as well.
Buildings and heat transport
New building materials that can both conduct heat and block it—on demand—are needed to reduce energy for heating and cooling, which paradoxically occur to varying degrees simultaneously almost everywhere in modern economies. Some studies have shown that the ability to control thermal conductance in a building’s shell could save 10 percent to 40 percent of greenhouse gas emissions, said Majumdar, and much research is going into this.
A particularly large challenge is to develop the ability to transmit heat over long distances with little loss of energy. This is done with steam today, but that is not at the scale or distance needed to deliver heat where it is needed the way natural gas, for example, is piped around continents.
In the context of the coronavirus pandemic, “Let’s define what the new normal ought to be,” Majumdar exhorted, and “address the defining dual challenge of the 21st Century.”
Many enormous societal benefits have been borne during times of crisis, like the Great Depression and World War II.
“This is a moment in history,” he concluded. “It is our generational responsibility and opportunity to think big now.”
In the ensuing conversation based on questions from attendees, MIT’s Henry underscored the need for a sense of urgency in researching and developing climate solutions.
“We as a species are endangering ourselves with the infrastructure we have erected to improve our quality of life,” said Henry. “There are a few instances in history when scientists and engineers have come together and achieved something very remarkable in very short timeframes.”
“I hope our community will take this as an opportunity to take a similar perspective about climate change and other sustainability issues,” he added, “and make that our mission.”
Henry and the rest of the panel agreed, however, that meeting these grand challenges for deep decarbonization will require the effort not just of scientists and engineers, but of experts in economics, public policy and business.