Pumped Heat Electricity Storage – A Promising Green Battery

Wind and solar are considered promising alternative technologies. However they have certain demerits such as intermittent and inconsistent output, geographically constrained and so on. Such undesirable features render them unfeasible as anything other than secondary power sources. Thus there is a pressing need for the industrial and scientific communities to design and develop a utility storage solution that could effectively store the energy and help reduce the loads on the transmission network. An ideal utility scale storage solution should be cost effective, efficient, not geographically constrained, environmentally inert and safe and scalable. A team of engineers from Isentropic, a UK based developer of cost effective heat engines/heat pumps, has recently developed an in-house pumped heat electricity storage system (PHES) to address the challenges of utility scale storage.

The Isentropic PHES system utilizes a highly reversible heat engine/heat pump to pump heat between two storage vessels containing gravel. A gas is employed in this process from which work is extracted. At first, the compressed gas is made to pass through one of the stores. This heat up the gravel up to 500 degree Celsius and during heat exchange, the temperature of the gas almost drops to ambient condition. In the second step, this gas is made to expand to its original pressure and as a result the temperature drops to -160 degree Celsius. It is then passed through the other store and it exchanges heat with the gravel by direct contact. In this process, the gas is warmed back to its original temperature. The energy supply is mainly by employing a motor as it not only supplies electrical power to it but also acts as an energy storage element. The cycle is reversed to release the energy. As the energy passes fro hot to cold, it powers a generator from which electricity is produced.

The research team claims that its PHES system yields round trip (electricity-in to electricity-out) efficiency in the range of 72 to 85 percent. The research team is of the opinion that each individual process within the system should be performed with minimum losses so that high efficiency could be realized. Talking to Technical Insights, the Chief Technical Officer (CTO) Jonathan Howes says that the two storages have long energy storage capacity. From his calculations, an unused silo that is 50 m tall and has 50 m diameter would approximately lose half of its energy through its walls at the end of three years. Furthermore, CTO claims that the system’s life expectancy is close to 25 years that could be attributed to the absence of harmful refrigerants, chemicals and water.

Isentropic is currently working on this process so as to demonstrate a storage build cost of $10 per kWh for a utility-scale application and hence prove to be one of the cheapest sources of storage technology. The company has recently validated performance predictions when it experimented on two prototype machines. However, the company is undertaking a series of Research and development activities in order to improve the system and thus design for a wide range of energy harvesting.

The PHES system would be a promising method to store and to generate energy as it is relatively cheaper and poses negligible threat to the environment. However, the design could be further improved by carefully selecting suitable conducting materials. Furthermore, it could be used to convert intermittent renewable energy sources. For instance, PHES could be coupled with wind power to store the energy generated by a turbine and to supply when it is needed.


Jonathan Howes

Chief Technical Officer


10 Jesus Lane, Cambridge

Cambs CB5 8BA

United Kingdom

Phone: +44-0-845-206-2070

Fax: +44-0-845-206-2071

URL: www.isentropic.co.uk

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