Palo Alto, California · Climate & Energy Storage
Terrajoule Energy
The company that decided the cheapest battery isn't a battery at all - it's a steel tank full of hot water.
The company that decided the cheapest battery isn't a battery at all - it's a steel tank full of hot water.
Here is a fact about energy that quietly runs the whole business: making electricity is easy, and holding onto it is hard. Terrajoule built a company around that second half of the sentence.
There is a certain kind of company that looks at a problem everyone is throwing money at - in this case, storing renewable energy - and decides to solve it with the least fashionable materials available. The energy world spent the 2010s and 2020s pouring hundreds of billions of dollars into lithium-ion, into gigafactories, into cobalt supply chains and cathode chemistry. Terrajoule Energy, a small outfit in Palo Alto, looked at all of that and reached instead for a steel tank, some water, and a steam engine. The steam engine, to be clear, is a technology older than the incandescent light bulb.
The logic is less eccentric than it sounds. Solar panels make power when the sun is up, which is precisely when everyone already has plenty of it, and go dark exactly when demand peaks in the evening. The valuable thing - the thing utilities will actually pay a premium for - is dispatchable power, electricity you can summon at 9pm or 3am on command. To make solar dispatchable you need storage, and storage is where the money either gets made or lost. Terrajoule's entire pitch is that it can make that storage dramatically cheaper by not using a battery at all.
Instead, its patented Dispatchable Solar Module - the DSM, in company shorthand - concentrates sunlight to boil water into steam, uses some of that steam to run an engine, and stores the rest as pressurized hot water in insulated steel pressure vessels. When the grid wants power after dark, the pressure drops, the hot water flashes back into steam, and the steam drives a reciprocating piston engine that spins a generator. The company says the round trip through this steam-water-steam phase change loses less than 2% of the energy. That is a genuinely good number, and it is the load-bearing wall of the whole enterprise.
The materials matter because materials are where cost lives. Water, it turns out, stores roughly three times as much heat per kilogram as molten salt, five times as much as concrete, and eight times as much as iron. Steel pressure vessels are one of the most thoroughly solved manufacturing problems on the planet - the natural-gas pipeline industry has spent a century learning to weld them cheaply. There are no rare earths in a Terrajoule module, no toxic chemistry, no gigafactory required, and, notably, no degradation curve. A lithium battery is a little worse every year; a steel tank of water is not.
"Storage capacity at a fraction of the cost of lithium-ion batteries from day one - no gigafactory, no rare materials, no cycle limits."
- Terrajoule Energy, on the Dispatchable Solar ModuleYou can be skeptical of the "from day one" part - hardware companies say a lot of confident things about cost before they have built at scale - and Terrajoule never grew into a large company. Public records list it as a lean, roughly two-person operation, which is either a red flag or a sign of extreme capital discipline depending on your mood. But the underlying physics is real, and it was real enough that in 2013 a serious venture syndicate wrote a serious check.
Strip away the acronyms and Terrajoule is four moves in a row. Sunlight in one end, firm electricity out the other, with a tank of hot water doing the patient work in between.
Prefabricated Linear Fresnel mirrors focus sunlight to boil water into high-pressure steam.
Excess steam condenses into a mass of pressurized hot water held in insulated steel pressure vessels.
On demand, pressure drops and the hot water flashes back to steam - losing under 2% along the way.
A reciprocating steam piston engine turns that steam into electricity, day or night.
Terrajoule's case rests on a simple physical comparison: how much heat a given mass of material can hold. Water wins, and it happens to be the cheapest and most available option too. Relative heat capacity, water indexed to 100:
The patented solar-plus-storage building block. Made from steel and glass, designed to deliver 24/7 power at a fraction of lithium-ion cost, with global manufacturing capacity already in place.
Condenses steam into pressurized hot water inside insulated steel vessels, then flashes it back on demand. No rare or toxic materials, under 2% loss per cycle.
A factory-built Linear Fresnel concentrator that ships in standard containers. Claimed 3:1 installed cost reduction and 10:1 less installation labor versus conventional designs.
Modular 2MW stations combining concentrators, storage and steam piston engines to dispatch clean power up to 24 hours a day - scalable from distributed to utility deployments.
Terrajoule was founded in 2009 by Stephen J. Bisset, an electrical engineer with a track record of starting companies that manufacture and sell capital equipment. That background is the tell. Terrajoule is not a software company that discovered energy; it is a hardware company from the beginning, and its whole personality - prefabrication, welding techniques borrowed from pipelines, cost-per-kilowatt-hour as the north star - reflects a founder who thinks in bills of materials.
In November 2013 the company raised an $11.5 million Series A. The syndicate is worth reading closely: it was led by New Enterprise Associates, one of the larger names in venture, and included ALIAD, the corporate venture arm of the French industrial-gas giant Air Liquide. A strategic investor like Air Liquide does not write a check into a pressure-vessel-and-steam company for the returns alone - it does it because it understands that particular kind of industrial hardware intimately. That is a meaningful vote of confidence in the physics.
The same year, Terrajoule stood up a 100kW field prototype near Modesto, California that produced renewable power around the clock. Then the company went quiet, as deep-tech hardware companies often do during the long grind between a working prototype and a bankable product. In December 2025, Terrajoule was acquired by Environmental Clean Technologies Limited, which folded the patented DSM technology into its own portfolio to scale it. Whether the steel-and-water thesis finally gets its moment now belongs to someone else's balance sheet.
"Terrajoule's decisively lower cost of dispatchable electricity accelerates the fossil-to-solar tipping point."
- Terrajoule EnergyElectrical engineer Stephen J. Bisset starts the company to build a new class of solar power-generation and storage equipment.
NEA leads the round with Air Liquide's ALIAD, backing distributed solar power and energy storage.
A field prototype near Modesto delivers renewable power around the clock using steam-accumulator storage.
The company advances its Dispatchable Solar Module and Prefabricated Solar Concentrator toward pilot-plant engineering.
Environmental Clean Technologies Limited acquires Terrajoule to scale the DSM solar-plus-storage technology.