From the MEWS photo archive, this low temperature differential Stirling engine was displayed at a Guildford exhibition. The low-temperature differential (LTD) Stirling engine is a type of Stirling engine that can run with a small temperature difference between the hot and cold ends of the displacer cylinder and so making it able to operate with various low-temperature heat sources. In some cases that can be a cup of coffee or just the warmth of a human hand.