Big valve offers big opportunities in heat storage

A 300mm diameter valve has been developed that opens and closes off air or gas flows at up to 12bar pressure, in around 2 to 2.5ms. It takes the form of a pneumatically operated slide valve that uncovers and covers a very large number of individual ports. It forms a vital part of a novel heat engine which is being developed by Jonathan Howes, technical director of Isentropic; a technology company based just outside Cambridge. A reservoir filled with gravel is heated up to 500^°C while another gravel reservoir is cooled down to -150°C. Reversing the engine allows the temperature difference to be converted in to mechanical energy. Howes says: "I built my first Stirling engine when I was 15, but it's almost impossible to get close to its optimal theoretical efficiency because of the need to exchange a large amount of heat very rapidly." The engine has one cylinder, with a thermal barrier piston in the middle and special valves at the top and bottom. One end of the cylinder is used for compression and the other for expansion. When energy is to be extracted from the hot and cold gravel beds, argon is passed through the cold bed where it undergoes isobaric cooling which reduces the volume of the gas. It is compressed to 12bar and then rapidly injected in to the hot gravel bed. Here it undergoes sudden heating enabling it to perform work as it expands on the other side of the cylinder. A crucial aspect of the machine is the valves which have to open and close very quickly, produce very little pressure drop and minimise turbulence. There has been considerable commercial interest in using the valves for other purposes, and it is likely that they will find themselves in industrial service before the heat engine. The other interesting innovation on the heat engine is its non contact piston seal. This is a new design and is also proving to be of commercial interest. The main barrier for Howes and his team has been money. Although there has been a grant from the Carbon Trust and investment by Credit Suisse Securities Europe, a full scale demonstrator is expected to cost some £2million and a manufacturing facility would be around £25million. But the biggest opportunity is likely to be the use of the valves to shut off air or gas flows very quickly. This has a host of applications such as an emergency shut off valves in chemical, gas, heating and ventilations environments.