Cache Energy in WSJ: The 2,000-Year-Old Cement Battery That Could Reduce Our Reliance on Fossil Fuel
By Christopher Mims
More than two millennia ago, not far from Pompeii, Roman builders hit upon a formula for concrete that allowed them to build bigger, stronger and more durable structures than ever.
Today, their science is the heart of a new kind of cement-based battery, one that stores heat instead of electricity. When combined with cheap, renewable sources of energy, this new take on old tech has the potential to displace much of the natural gas and other fossil fuels used for heating.
That’s big. About 20% of all global energy is used to produce heat for industry, and a further 10% is used to heat homes and water, according to the International Energy Agency.
The chemistry and engineering of this novel cement battery is deliberately simple, so as to make it scalable and cost-competitive. Take quicklime, otherwise known as calcium oxide, and just add water. The result is calcium hydroxide—ancient Roman cement.
This reaction releases a great deal of heat. It’s essentially the same thing that happens when you mix a bag of cement from the hardware store today. Roman engineers exploited that heat to create a fast-setting concrete, allowing them to build the Pantheon and other marvels.
But the reaction is also reversible: Add enough heat back to cement, and you can drive out the water and produce quicklime once more. When done right, it’s possible to recharge, discharge and recharge it again, many times. Just like a battery.
Water works
Since the 1970s, engineers speculated this might allow humans to store vast quantities of energy more or less indefinitely. Two problems: At the time, renewable energy cost too much to make it affordable, and adding water usually turns quicklime into an unwieldy goop.
A 10-person startup called Cache Energy, working out of a 10,000-square-foot facility in Champaign, Ill., says it has figured out how to make such a cement battery durable, efficient and affordable. The company’s approach is to form cement into tiny balls, each about the size of a kernel of corn. Its engineers add a binding agent—secret though widely available, they say—to keep the balls in shape during the discharge and recharge process.
Recharging the pellets requires heat, generated from electricity. When it’s time to discharge that stored energy, adding the right amount of water causes the pellets to release enough heat to generate temperatures up to 1,000 degrees Fahrenheit, says Cache’s founder, Arpit Dwivedi.
Cache Energy founder and CEO Arpit Dwivedi, in dark shirt. Cache Energy
Handling both the charge and recharge cycles is a reactor about 15 inches in diameter and 8 feet tall that can generate around 100 kilowatts of heat. The company also has a much larger model that generates 1 megawatt of heat.
And those tiny pellets can be stored in a grain silo—one for charged pellets, a second for discharged ones. Pellets are fed back into the reactor with a simple grain elevator, and the reactor itself has no moving parts.
If the technology can be scaled down, a home version might sit beside your water heater, allowing you to store electricity when it’s cheap and use it to heat your home in place of natural gas, Dwivedi says.
The science behind Cache’s system is sound, says Nishant Garg, a concrete expert and associate professor of civil engineering at University of Illinois Urbana-Champaign, who isn’t involved with the company.
But he sees the tiny startup facing challenges as it scales up. Chief among them is getting the right mix of calcium oxide and its proprietary binding agent, so the pellets effectively store energy while holding their shape over many charge-discharge cycles.
Cache stores thermal energy in cement pellets that can be held in ordinary grain silos. Cache Energy
In many parts of the U.S., there are times of day when renewable energy so far exceeds local demand that it costs little to nothing, says Jeffrey Rissman, an analyst at Energy Innovation, an energy and climate-policy think tank. Energy-storage technologies such as Cache are ideal for these areas.
But, says Rissman, Cache has at least a half-dozen startup competitors targeting the same initial market, industrial heat. Other technologies store heat differently, using solid materials inside insulated containers. (Cache’s pellets don’t need insulation.) It’s such early days that it’s unclear which kind of system will be best for which applications, he adds.
Cold, cold winters
Cache’s tech is already being tested by partners. At Whirlpool’s Kitchen-Aid factory in Ohio, the company has found the system performs “even better than expected,” says Scot Blommel, Whirlpool senior manager of global sustainability.
The Defense Department is looking into using Cache to heat military installations during extreme weather, emergencies or grid failures, says a spokesman for the U.S. Army Engineer Research and Development Center. And companies in Europe and Asia, wary of rising natural-gas prices, are also interested, says Dwivedi.
In Minnesota, Gov. Tim Walz recently signed a law mandating that utilities offer 100% carbon-free electricity by 2040. The leaders at the University of Minnesota Morris are testing Cache to determine whether the entire campus could eventually be heated by excess electricity generated by one of the university’s pair of giant, 1.65-megawatt wind turbines.
Heating UMN-Morris’s 38 buildings—around 1 million square feet—in the bitter Minnesota winter takes four times more energy, in the form of natural gas, than the entire campus’s electricity demand, says Troy Goodnough, the university’s director of sustainability. With fluctuating natural-gas prices, budgeting for that heat is difficult, he adds.
Goodnough’s team took delivery of Cache’s reactor and pellet storage system just last week, and has already begun evaluating how well it can heat a single building. “It’s doing great,” he adds.
Cache Energy's system is delivered inside a shipping container to the University of Minnesota Morris. Cache Energy
The university currently sells the excess electricity from its turbines to the local utility company, but it could use it to charge up Cache’s thermal cement batteries. Then, Goodnough says, the campus could achieve self-sufficiency at an all-in cost lower than natural gas.
Boiling point
Energy-poor countries and trillion-dollar AI titans alike seek cheap sources of power. The trick is putting storage technologies like Cache’s in places where there’s so much wind and sun that the local wind turbines and solar panels produce an overabundance of energy.
If North and South Dakota are the “Saudi Arabia of wind,” says Goodnough, Minnesota could be considered the “Qatar of wind.” Plains states have the potential to tap vast quantities of wind and solar power, which have lately become cheaper than fossil fuels on a per-kilowatt-hour basis.
Between battery storage for electricity and thermal storage for heat, Goodnough believes his university, and eventually entire states, could achieve energy independence.
“We store grain, we store corn, we store soybeans, we store propane,” he says. “Why would we not store electrons?”