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Utility-scale, energy storage technology could ease shortfall in electricity generation capacity

28 October 2009

Isentropic, a UK company, has designed and built the first ever truly reversible heat engine based on an Ericsson thermodynamic cycle. In other words, it can be used interchangeably as a highly efficient engine or as a heat pump. Isentropic’s success in optimising the engine’s internal processes is not only revolutionary, but also provides the key enabling technology for low cost, widespread storage of electrical energy, without the environmental impact of pumped hydro storage. Low cost and straightforward to manufacture, it is also suitable for heating and refrigeration. 

“Britain faces a looming shortfall in electrical generating capacity,” explains Jonathan Howes, Isentropic’s Technical Director. “Even now, the UK is not making efficient use of its power stations and yet we’re all being urged to use energy more efficiently. Building electricity storage would allow existing power stations to be used much more efficiently, reducing the need for new ones.

“Although we have sufficient electricity in total, a shortfall in generating capacity means we won’t have enough electricity at peak times. We already have more capacity than we need at night. Being able to store that excess night-time capacity and feed it into the grid during the day would allow the same number of power stations we have now to cope with greatly increased peak demand, rather than switching on inefficient and dirty generating plants to cover peak times as happens at present.”

How Isentropic’s reversible heat engine works as an energy storage device: To store energy, the heat is pumped from one container of thermal storage material (normally gravel) to another similar container with the same material. It cools the first container to around -160°C while heating the other container to around 500°C. In principle, this is similar to but much bigger than the heat pump driven by electricity in a domestic fridge to chill the inside of the fridge and warm the back.  Unlike a domestic refrigerator, which generally utilises a range of chemical working fluids, the Isentropic machine can use air or any other gas with significant implications for the reduction of harmful atmospheric chemicals.

The storage containers hold large volumes, so this heat and cold can be stored for a considerable time. When discharging the process is reversed and it acts as a heat engine with the heat from the hot container transferring back to the cold container via the machine and releasing most of the energy originally used in charging. This is used to drive a generator to release the stored energy as electricity.

With Isentropic’s technology, around 75% of the energy supplied to the system as electricity can be returned as electricity. This is comparable to pumped hydro electrical storage. However, the added advantage is that the engine and stores use 300 times less land and are not dependent on geography, so can be installed anywhere.

Told it would not be able to achieve the low pressure losses and internal system efficiencies it was seeking, Isentropic has proved the sceptics wrong by a considerable margin and now has its second prototype running. The next step is to build a demonstration storage plant for which it is currently looking for investor funding of £6.5 million. 

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