Fourth Power thermal battery to undercut gas peakers

Fourth Power is gambling that extreme heat can provide inexpensive, reliable power where batteries and gas turbines fall short. The Cambridge startup’s thermal battery stores electricity as white-hot heat and releases it in reverse, hitting a cost target that could, at scale, outcompete both lithium‑ion storage and new natural gas peaker plants.

How the tin-and-light thermal battery system works

The system is recharged by passing electricity through carbon blocks within argon-filled, insulated chambers. To discharge, it passes molten tin heated to around 2,400°C through graphite tubes — one of the few materials that won’t get starry-eyed at such temperatures — and then beams that heat onto thermophotovoltaic (TPV) cells, which convert high-intensity infrared light into electricity.

Think of it as a huge rechargeable thermal flashlight: store energy in the form of heat, then shine that light onto high-efficiency cells when the grid needs power. The tanks are hermetically sealed, the insulation is low-cost petroleum coke, and thermal losses approximate 1% per day, meaning that they can store for several days without exotic materials or cryogenics, according to the company.

Eight‑hour or longer daily charge–discharge cycles are the starting design point, double that of most current grid-scale lithium‑ion projects. Early testing with subscale devices has emphasized power output, cycling durability, and the ability to keep temperature distributions uniform under repetitive thermal loading.

The price to beat: gas peakers and mega-batteries

New natural gas peaker plants are constructed to operate for only a handful of hours each year, and that makes their energy expensive. Lazard’s 2024 analysis puts the levelized cost of energy for new-build peakers in the high hundreds of dollars per megawatt-hour in many cases, particularly at low capacity factors. Volatile fuel prices and capacity-market premiums mean “cheap” gas often isn’t.

Lithium‑ion continues to be the workhorse for 1–4 hour balancing; however, stacking batteries toward eight hours or more requires excess capital investment as well as risks of capacity degradation. Analysts say some of the U.S. utilities’ filings reflect installed costs for long-duration Li‑ion that are even higher than short-duration projects, while the economics degrade when batteries have to store energy for evening ramps instead of going on and off charge quickly throughout the day.

Why Fourth Power believes it can reach $25/kWh

The company’s audacious cost goal — $25 per kilowatt-hour for the energy storage half of the system, at scale — relies on commodity inputs and a short supply chain: carbon, graphite, tin, argon, and off‑the‑shelf insulation. There are no exotic cathodes, rare metals, or high-pressure vessels, and the power block (pumps plus TPV panels) is modular.

Crucially, $25/kWh is for energy capacity only rather than for the entire plant. The complete levelized cost structure of storage is based on the power hardware, roundtrip efficiency, cycling life, and utilization. Time-of-day regulation reference data from the National Renewable Energy Laboratory, and oversimplified system costs by Lux Research Center, still suggest that if one element is that cheap (energy), you can add eight to ten hours of capacity without having the cost curve shoot off into vertical space — something which electrochemical stacks cannot achieve.

Efficiency, physics and the practical limits of operation

Round‑trip efficiency won’t match lithium‑ion. Although it has been possible in a peer‑reviewed paper to show TPV cell performance above 40% at high emitter temperatures, the overall efficiency of a device will also account for pumping and thermal losses. This puts this class of storage in the 40–60% range, a solid performance depending on design — only if capital cost is well controlled and duration trumps efficiency for that particular use case.

Operating at 2,000°C-plus is unforgiving. Graphite can tolerate the heat, but the problem is erosion; sealing and thermal cycling have to be developed very well anyway. The firm’s argon environment handles oxidation, and petroleum coke insulation is both cheap and durable, although multi-year maintenance intervals will be an important proof point. That will matter to utilities and insurers if independent validation from organizations like NREL or EPRI confirms its claims.

Where this technology might fit onto future grids

The value of longer-duration storage is expected to grow as renewables continue to rise. Last year, California’s grid operator reported about 2.4 terawatt-hours of renewable curtailment — in other words, wasted energy that could be something that the heat battery soaks up and releases at night. Early customers could include data centers looking for 24/7 clean power, industrial campuses, and wind-solar hybrids.

DOE’s “Pathways to Commercial Liftoff” long-duration storage report mentions that low-cost, multi-hour technologies can cut system costs by deferring gas buildouts and transmission upgrades while firming variable generation. If Fourth Power’s economics play out, it could compete not just with peakers but also with repowering existing gas plants for reliability.

A crowded field with different bets on long storage

Thermal storage is a hot area. Antora Energy also uses heated carbon bricks with TPV, Rondo Energy stores heat in refractory brick for industrial processes, and Malta Inc. employs molten salt combined with a turbomachinery cycle. Beyond heat, iron‑air systems such as Form Energy do so by sacrificing efficiency in favor of ultra‑low cost over 100 hours. The market is big enough that more than one chemistry — and temperature — will probably persist.

What to watch next as Fourth Power scales up

The immediate milestones are a first full-scale build, continuous cycling data over the span of thousands of hours, and third‑party performance audits that convert lab metrics into bankable models. Interconnection, siting, and safety certification for ultra‑high‑temperature equipment will be just as big, if not bigger, challenges than clever physics.

If the company can demonstrate durability and reach its $25/kWh energy-capacity goal while offering eight-hour service, its sci‑fi heat battery will not only be novel — it could be a rare climate technology that truly makes “firm, cheap, clean” possible and sends new gas peakers to the back of the line.