Downwind: Why didn’t we think of that? Electricity stored as a temperature difference

The recent droughts across the U.S., specifically in California, have stressed the importance of power generation and storage strategies that use as little fresh water as possible. Energy production by fossil fuels and nuclear use water as a key part of power generation.

Today approximately 99% of grid-scale energy storage relies on pumped hydropower. Beyond the huge amounts of water required by this process, pumped hydropower is limited geographically to where there is a mountain and a water source.

But according to Jonathan Howes, a British aeronautical engineer, there soon may be a viable alternative. Howes, with partners James Macnaghten and Mark Wagner, co-founded Isentropic, Ltd., a company that is currently developing a new storage technology called pumped-heat electricity storage (PHES), which stores electricity as heat and cold. PHES, Isentropic claims, is cheaper than pumped hydro, is deployable anywhere in the world, and is comparable—and in some cases superior—to pumped hydro with a round-trip storage efficiency of 72 to 80%.

Isentropic silos

The square silo to the right is the cold store. The silo to the left is the hot store. The pistons in the middle adiabatically compress and expand the gas.

PHES uses two large silos of crushed gravel referred to as the “cold store” and “hot store.” A piston system is used to pump heated and cooled argon gas through the circuit.

The process begins when electricity is fed into the system, which powers pistons filled with argon gas. The pistons rapidly compress the argon gas adiabatically from 1 bar to 12 bar, heating the gas to 500°C as a result. The heated gas then travels to the top of the hot store silo filled with the crushed gravel.

The upper portion of the gravel is heated by the gas to 500°C while the bottom region remains at ambient temperature. The hot thermal front between the ambient zone and the heated zone near the top gradually moves downward into the ambient zone as more of the gravel is heated. The gas is cooled in the process as it moves through the gravel and transfers its heat, but it remains at 12 bar. When the gas exits the bottom of the silo, it moves to another set of pistons which rapidly expand the gas back to 1 bar, cooling it to -160°C in the process.

In the final stage of the charging cycle, the gas moves to the cold store and enters at the base of the silo. Similar to the hot store, this cooling effect creates a cold thermal front that moves upward toward the ambient zone as the gas cools more of the gravel. At the top of the silo, the gas is once again at ambient temperature and has remained at 1 bar. Finally, the gas exits the cold store and returns to the beginning of the cycle where the pistons compress and heat the gas. In this way, electricity is stored as the temperature difference in the gravel and not as compressed gas. Furthermore, no water is required in the energy storage process.

The entire system cycle can be reversed to begin a discharge cycle and extract the stored power by changing the timing of the pistons, causing the system to operate as a heat engine and allowing electricity to flow out of the system. This switch from charging to discharging, says Isentropic, takes less than a second.

An independent study by Parsons Brinckerhoff reports that PHES costs 30% less than pumped hydropower with a per hour storage cost of $103/kWh. Currently the technology is scaled to support up to a 2,000-home town. For this scale, the building housing the system is estimated to be 20 to 40m tall. Currently the first commercial-scale system is under construction in Fareham, England. In an interview from May 2014, Macnaghten ventured to guess that Isentropic is 5 to 6 years away from making the system commercially available.

Watch the model demonstration video of the PHES technology, visit


  1. I would agree with Bill Jackson if the PHES did not recapture some energy from pistons that drop the pressure from 12 bar back to 1 bar. It seems to me that the PHES system will work relatively efficiently other than the typical mechanical compressor inefficiencies, the modest pressure drop through the gravel beds, and the thermal heat losses of the vessels and equipment. It seems to me the cost challenge for this technology will be the cost of the high pressure heated storage vessel.

  2. Bill Jackson says:

    This seems to fail due to Carnot cycle irreversibility.
    Pumping water uphill and then getting the power out, is just like using a rope and a weight. The pump going up is close to 90% efficient and the turbine =~90%. Maybe more if large. Net is 75~80% on stored water power. The eficiency of this system can not be above 30%, I place no faith in Parsons and I doubt they said what they are supposed to have said,.

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