It's cliché to claim that large-scale energy storage is the holy grail or missing link of renewable energy.  

But the problem remains -- energy storage technology is too expensive. Haresh Kamath of EPRI's Technology Innovation Group has said, "Storage is a great idea -- except for the cost." Steve Berberich of the California ISO recently said of energy storage, "It's good stuff, but it's expensive, and we have to find business cases."

Today, the only economical method of storing energy at a large scale is pumped hydro (pumped hydro accounts for almost all large-scale electricity storage) or compressed air energy storage (CAES). Unfortunately, both of those technologies require easy access to an immense airtight underground cavern or a couple of large reservoirs. 

Back in 2010 we covered a U.K.-based firm, Isentropic, that claimed it could change the cost structure of energy storage. Today, the firm announced a large funding event to get it to demonstration scale.

The U.K. government-backed Energy Technologies Institute (ETI), a private-public partnership, is investing $22 million in Isentropic's Pumped Heat Electricity Storage (PHES) system in order to build a full-scale demonstrator of its technology. The goal is to deploy a 1.5-megawatt, 6-megawatt-hour storage unit on a U.K. primary substation owned by Western Power Distribution,  a distribution network operator for the Midlands, southwest England and South Wales with 7.7 million customers. 

Jonathan Howes, the Chief Technical Officer of U.K. startup Isentropic Energy has claimed large-scale storage costs that are an order of magnitude lower than lithium-ion batteries or other stored energy technologies: $55 per kilowatt-hour currently, with a path to get down to $8 per kilowatt-hour.

Isentropic's technology is compact, has no geographical constraints and claims a round-trip efficiency of 75 percent.

Pumped Heat Electricity Storage

Isentropic's Pumped Heat Electricity Storage (PHES) system is based on the First Ericsson cycle and uses a heat pump to store electricity in thermal form. The storage system uses two large containers of gravel, one hot (500 degrees C) and one cold (-150 degrees C). Electrical power is input to the machine, which compresses/expands air to 500 degrees C on the hot side and -150 degrees C on the cold side. The air is passed through the two piles of gravel, where it gives up its heat or coldness to the gravel. In order to regenerate the electricity, the cycle is reversed. The temperature difference is used to run the system as a heat engine.

The startup claims that its reversible engine/heat-pump boasts three critical features:

  • Very high round-trip efficiency
  • High reversibility -- the machine works as both an engine and a heat pump. High reversibility means that if it first turns electricity into a temperature difference, it can then regenerate most of the electricity from the temperature difference.
  • Gas cycle machine -- no use of damaging refrigerants, chemicals, or water


Isentropic's innovations include using aircraft engineering techniques to reduce piston weight and cost, designing new valving to eliminate pressure losses, and using a new sealing technology.

Hopefully, this startup can get the job done and we can finally retire that 'energy storage as holy grail' cliché.