It is clear that there is no long-term solution to energy supply problems other than planned and continuous conservation. The use of low-grade energy as an unlimited resource will play an important part in maintaining the balance between the future of the world for both developed and developing countries alike. A literature survey reviewing the most promising approaches to the construction of low-grade energy engines has been conducted. The validity of each feasible system has been examined and the principles of operation described. A feasibility study concerning the adoption_of the organic Rankine cycle utilizing common suitable refrigerants (e.g. halo-carbons) as working fluids has been conducted. In particular, the use of a multiple vane expander as the prime mover has been fully investigated. A suite of computer programs has been developed to: • describe working fluid properties; • optimise the geometrical and dynamical parameters of the Rankine cycle to achieve the most efficient operation in both steady and transient states; • optimise the mechanical design of the expander depending upon the mode of operation and the source and sink characteristics. A fully flexible experimental test facility was constructed so as to be capable of testing a wide variety of prime movers. This has been operated for a real time test period in excess of 300 hours. The test results are encouraging. Efficiencies were measured in accordance with the mathematical predictions and a portfolio of suggested modifications towards a high efficiency, low-cost, robust and reliable expander capable of utilising low grade thermal energy has been produced. Fruitful links with relevant British industries have been forged. Demonstrations of the system are planned within the UK and overseas.