Low power ORC-ORC systems for heat pump applications

This paper presents a study of a residential (about 20kW) ORC-ORC (Organic Rankine Cycle) thermally driven heat pump. The system works with three different temperature levels similar to absorption heat pumps. The proposed concept is a hermetic system using the same working fluid in both ORC’s with a radial compressor and turbine on a single high-speed shaft. The use of gas bearings enables the system to be oil-free. This gives the system the advantage of low maintenance costs. Using the same working fluid in both cycles does not come without difficulty due to the large difference between the high and low temperature levels. Depending on the application, the temperature difference in the heat exchanger at the heat source might be substantial which results in large exergy losses. Nevertheless preliminary calculations show that COP’s higher than 1.6 could be expected which would be competitive with absorption heat pump alternatives.