Heat-based batteries are a surprisingly versatile tool

The first such application is at the State University of New York (SUNY) at Purchase campus, where Brenmiller worked with the New York Power Authority (NYPA) to install a bGen system alongside a combined heat and power (CHP) system at the university’s gym.

CHP systems, also known as cogeneration systems, are fairly common at university and corporate campuses, hospitals, and other sites that need both electricity and heat. By burning fossil fuels to generate power and capturing the excess heat that would otherwise be wasted, CHP systems can do both tasks at a lower cost than doing them separately.

But the demand for heat and for power from CHP systems doesn’t always match up exactly, said Steve Wilkie, director of research and development at NYPA. That’s the case at SUNY Purchase’s gym, which needs heat during the day when the building is occupied but doesn’t need it overnight.

“If we can’t use the heat because we have no use for the heat, we truly waste the heat,” he said. With the bGen system, ​“I’m storing heat in the middle of the night, when I don’t necessarily need to use it, and I’m shifting it to the middle of the day, when I do need it.”

The heat stored in the bGen system can also augment heat from the CHP system on particularly cold days, when ​“the demands for heat become greater than what we can recover from the exhaust itself,” he said.

Not all thermal energy storage systems are built to take in waste heat, Brenmiller noted. Some are designed to heat their storage media directly via electricity. Others rely on external electric heaters that blow superheated air through storage media, which tends to lead to greater energy loss than bGen’s integrated system, he said.

Determining whether the benefits thermal storage delivers to SUNY Purchase make up for the installation cost remains a work in progress, Wilkie said. Changing prices for fuel and grid electricity factor into that equation, as do the efficiency gains realized from being able to run the CHP system at a more stable and constant rate. What’s more, ​“new developments are always more expensive than they’d cost you two or three years down the road,” he said.

“Heat to power” — making power plants more efficient 

Another intriguing use case for thermal storage systems is on the power generation front — specifically, storing excess heat to boost electricity generation later.

Italian energy developer Enel Green Power is testing this use case with Brenmiller’s technology at a fossil-gas-fired power plant in Tuscany owned by its parent company, Enel. That project will pipe steam for the power plant’s turbines through a complex of bGen systems that can store up to about 24 megawatt-hours of heat at a temperature of about 550 degrees Celsius for at least five hours, and then discharge that stored heat to make steam to generate electricity later.

That stored-up energy can allow the plant to reduce how often it must ramp up or ramp down its own steam-generating capacity, which erodes the efficiency of the plant’s operations. It can also allow the plant to start up faster and alter its generation output more quickly than would otherwise be economically viable, which is useful to react to the ups and downs of renewable power generation.

“Flexibility and adequacy are two essential components of an efficient and reliable electricity system and they can be provided in an increasingly efficient way by storage systems,” Salvatore Bernabei, CEO of Enel Green Power, said in a 2022 statement marking the start of the project.

The same imperatives have led turbine manufacturers to improve the flexibility of their systems, as well as to integrate lithium-ion batteries with gas turbines in so-called hybrid power plants. These systems can significantly reduce fuel consumption, and thus carbon and air pollutant emissions, for the fast-ramping services that power plants are asked to provide at times when the balance of supply and demand on power grids is changing rapidly.

To be clear, conventional grid batteries are increasingly cost effective alternatives to gas-fired power plants on their own — and from a climate perspective, the direction the grid needs to move toward, along with adding more solar and wind. But in the near term, some amount of gas generation is here to stay, and making it more efficient by tacking on storage can have a meaningful impact on emissions.

And for larger gas plants, thermal energy storage may be a better option than lithium-ion batteries, Brenmiller said. Thermal storage systems take up less space per unit of energy stored than lithium-ion batteries do, he said. They can also deliver their stored energy without the efficiency losses that occur in converting electricity from the alternating current generated by power plants to the direct current that goes into batteries, and then back again to alternating current to flow to the grid.

NYPA has also explored adding a bGen system to one of its gas-fired power plants. That project, funded by the U.S. Department of Energy, didn’t receive follow-on funding, which led to its cancellation. But the early-stage research indicated that the concept was sound, Wilkie said.

That’s because the plant in question often has to shut down when demand for electricity on the grid wanes, he said. ​“There’s a cost associated with shutting it down — you’re adding wear and tear you wouldn’t have if you continue to operate consistently at a steady power rate,” he noted. ​“The thought was that that plant could continue to operate in those periods of time to generate and store heat that could be deployed” when electricity demand was higher.

Whether or not that would be cost-effective at commercial scale remains an open question, Wilkie said. ​“We felt encouraged by what we saw, but some of that information we just could not complete.”

Other ways to generate electricity from thermal batteries exist. Startup Antora Energy, which is targeting many of the same industrial heat and renewable energy storage targets as Brenmiller, Rondo, and other companies in the space, is also working on using specialized thermophotovoltaic (TPV) panels to convert thermal energy to electricity. Startup Fourth Power has set its sights explicitly on heat-to-electricity TPV, and boasts heat batteries made from advanced materials and using liquid metal to move heat.

These are more technologically complex approaches to turning heat to power, however, and may take longer to prove themselves at scale. Using thermal storage to reduce the emissions of gas power plants serving grids with rising amounts of wind and solar power could be a shorter-term solution.

The Enel project is still in its test stages, Brenmiller noted, which means it’s too early to calculate the benefits. But the experiment is in keeping with a much broader push by the energy industry to find ways to allow their power plants to ​“become more and more flexible to become more competitive with renewables.”