Project title: Two-phase polytropic energy storage

The project aims to prove the feasibility of a grid-scale energy storage technology based around thermal energy

stores in the form of un-pressurised insulated containers of rock gravel. One store will be at high temperature and

the other at low temperature. The concept is scalable to very large size and should have storage time-constants

well beyond diurnal. Unlike pumped storage schemes, such storage systems could be built almost anywhere and

close to or within large electrical load centres such as cities. In the storage phase, electrical energy will be

transformed to mechanical energy and thence to heat and cold in the gravel stores via thermodynamic

compression and expansion processes not unlike those used in refrigerators and heat pumps. Energy recovery to

generate electricity will be by the reverse sequence. To allow the use of cost-effective un-pressurised thermal

rock stores, the working fluid of the compression and expansion processes will be a combination of a gas and a

liquid which will will transport thermal energy between the pressurised gas and the un-pressurised thermal

stores. .