Fourth Power, Inc., a startup based in Pittsburgh, is building a thermal battery that stores surplus renewable electricity as extreme heat in carbon blocks, reaching temperatures of 2,400 degrees Celsius, roughly 4,350 degrees Fahrenheit. The company secured a Phase II Small Business Innovation Research award to stress-test the system’s most vulnerable components under those punishing conditions, with a stated cost target of $25 per kilowatt-hour of electricity. If the technology works at scale, it could offer grid operators a storage option that is dramatically cheaper than today’s lithium-ion packs and capable of dispatching power for hours without relying on any moving mechanical parts.
How Carbon Blocks Store Grid-Scale Energy
The core idea is straightforward: when solar or wind farms generate more electricity than the grid needs, that excess power heats blocks of carbon to white-hot temperatures. The energy stays locked in the blocks as thermal energy until demand rises, at which point the stored heat is converted back into electricity. What sets this approach apart from conventional batteries is the storage medium itself. Carbon and graphite can withstand extraordinary temperatures without degrading the way chemical electrolytes or molten salts do, and the raw materials are cheap and abundant.
The conversion step relies on thermophotovoltaic cells, or TPV cells, which generate electricity directly from the infrared and visible light radiated by the superheated blocks. Unlike a steam turbine, a TPV cell has no spinning components, no boiler, and no cooling tower. That simplicity could translate into lower maintenance costs and fewer failure points for utilities that need reliable, long-duration storage.
Federal Funding and the $25 Target
Fourth Power’s Phase II award from the National Science Foundation funds accelerated life testing of graphite fittings, seals, and pumps at temperatures up to 2,400 degrees Celsius and pressures up to 100 psi. The principal investigator listed on the grant is Arvin Ganesan. The explicit cost target embedded in the award is $25 per kilowatt-hour of electricity, a figure that, if achieved, would sit well below most published estimates for lithium-ion grid storage systems, which typically range above $100 per kilowatt-hour at the pack level.
That price point matters because the economics of renewable energy increasingly depend on storage. Solar and wind are now the cheapest sources of new electricity generation in many markets, but their intermittency means grid operators need a buffer. Lithium-ion batteries handle short bursts well, yet they become prohibitively expensive when sized for four, eight, or twelve hours of continuous discharge. A thermal system hitting $25 per kilowatt-hour could change that calculus entirely, making overnight or multi-day storage financially viable for the first time.
TPV Cells Break the 40 Percent Barrier
The feasibility of converting white-hot carbon back into usable electricity got a major boost from laboratory results published in Nature. Researchers demonstrated 41.1 percent TPV cell efficiency, plus or minus 1 percent, when the emitter temperature reached 2,400 degrees Celsius. The same study tested emitter temperatures ranging from 1,900 degrees Celsius to 2,400 degrees Celsius, confirming that efficiency climbs as the heat source gets hotter. That result directly supports the idea of storing energy at the extreme temperatures Fourth Power is targeting.
The National Renewable Energy Laboratory highlighted the significance of this threshold in its own reporting, noting that researchers captured light from heat at roughly 40 percent efficiency using thermophotovoltaics. At that conversion rate, a thermal battery can return a meaningful share of the energy it absorbs, narrowing the gap with electrochemical round-trip efficiencies. For context, conventional steam turbines used in power plants typically convert heat to electricity at 30 to 40 percent efficiency, but they require complex mechanical systems and water supplies that TPV cells avoid entirely.
The Liquid-Metal Engineering Problem
Most coverage of thermal batteries focuses on the headline temperature and the TPV cells, but the real engineering bottleneck sits between those two elements: moving heat from the storage blocks to the conversion surface. One promising approach uses liquid tin as a heat-transfer fluid, circulating it through the system at extreme temperatures. A peer-reviewed study published in Nature demonstrated continuous liquid-tin circulation at temperatures up to 1,673 kelvin, approximately 1,400 degrees Celsius, using specialized high-temperature pump and sealing approaches.
That result proved the basic physics works, but scaling it to commercial hardware is a different challenge. Liquid metals corrode conventional piping, and seals that function at 1,400 degrees Celsius tend to fail unpredictably over thousands of thermal cycles. This is precisely why Fourth Power’s SBIR grant focuses on accelerated life testing of graphite fittings, seals, and pumps rather than on the TPV cells themselves. The cells already work in the lab. The plumbing is the part that has to survive years of brutal thermal cycling in a real-world installation.
This distinction is often lost in discussions that treat thermal batteries as a solved problem awaiting only investment. The gap between a laboratory demonstration of liquid-metal pumping and a commercial system rated for 20 years of grid service is enormous. Seal degradation, thermal expansion mismatches, and creep in graphite components at 2,400 degrees Celsius are failure modes that do not show up in short-duration tests. The federal SBIR structure exists specifically to fund this kind of high-risk, pre-commercial validation work that private investors typically avoid.
Where Thermal Storage Fits Against Lithium-Ion
Lithium-ion batteries dominate grid storage today for good reason: they respond in milliseconds, their supply chains are mature, and costs have fallen steadily for more than a decade. Utilities rely on them for frequency regulation, short-duration peak shaving, and backup power. But lithium-ion systems are optimized for two to four hours of discharge. Stretching them to cover eight or more hours requires stacking additional battery modules, which drives up capital costs and amplifies concerns about fire safety, siting, and materials supply.
Thermal batteries like Fourth Power’s are not trying to replace lithium-ion in every application. Instead, they aim for the long-duration segment where cheap energy capacity matters more than ultra-fast response. A carbon-block system can, in principle, add storage hours by building a larger insulated tank and packing in more blocks, without needing proportionally more TPV cells. That design decouples power (the rate of discharge) from energy (the total amount stored), which could make it economical to provide ten or more hours of storage at a single site.
Round-trip efficiency remains a key trade-off. Even with TPV cells above 40 percent efficiency, a thermal system will likely return less electricity than a lithium-ion battery charged with the same input. But if the thermal option can be built at a fraction of the cost per kilowatt-hour, grid operators may accept some energy loss in exchange for lower capital expenditure and simpler materials. The fact that carbon, graphite, and common metals form the backbone of these systems also reduces exposure to critical-mineral supply risks.
Data, Privacy, and the Federal Pipeline
Behind Fourth Power’s grant is a broader federal ecosystem that steers early-stage energy hardware from lab concepts toward commercialization. Companies enter that pipeline through official portals, including the external SBIR login used by firms and researchers to submit proposals and manage awards. Agency staff and reviewers access parallel tools on the internal government side, which handles evaluation, compliance checks, and grant administration.
Those systems sit under federal data-governance rules. The U.S. Small Business Administration maintains Privacy Act notices that describe how proposal information, contact details, and performance reports are stored and used. For startups working on sensitive energy infrastructure, that framework is part of the trade-off: accessing public funding and technical support in exchange for structured reporting and long-term transparency about how taxpayer dollars are spent.
What Comes Next for Thermal Batteries
For Fourth Power and similar ventures, the next few years will hinge less on incremental efficiency gains and more on proving durability and bankability. Financiers will want to see thermal systems operate through thousands of charge-discharge cycles without catastrophic failures in pumps, seals, or insulation. Regulators will scrutinize safety under worst-case scenarios, from leaks of liquid metal to unexpected thermal runaway in carbon blocks.
If those hurdles can be cleared, thermal storage could slot into a complementary role alongside lithium-ion, pumped hydro, and emerging options like hydrogen. It would not eliminate the need for other technologies, but it could give grid planners a new tool for bridging multi-hour gaps between renewable generation and demand. The SBIR award backing Fourth Power’s high-temperature tests is an early bet that the combination of carbon blocks and TPV cells can move from physics experiment to infrastructure asset, and that solving the unglamorous plumbing problems is the key to getting there.
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*This article was researched with the help of AI, with human editors creating the final content.
