A review of compressors for high temperature heat pumps

The development of high temperature heat pumps for waste heat recovery benefits industrial processes by meeting demand, increasing energy efficiency, and reducing emissions. The aim of such systems is to upgrade waste heat streams, typically around 50 degrees C to 100 degrees C, to higher temperatures ranging from 100 degrees C to around 200 degrees C. A primary challenge in getting the required high temperatures is the compression system. There is a plethora of recently published research papers including reviews that address high temperature heat pumps. However, there has been no comprehensive review addressing compression systems, despite their major influence in the successful development of high temperature heat pumps; this paper provides a comprehensive review of such compressors. Firstly, an overview of heat pump systems is provided, which covers cycle arrangements and working fluid selection. This is followed by a review of the different compressor technologies used, and the development of relevant modelling and design tools. Finally, suggestions for future directions in research for high temperature heat pump compressors are provided. It was found that screw compressors have been the obvious choice for heat pumps due to the experience gained from the refrigeration industry. However, the temperatures they can handle are constrained by the maximum possible limitation to avoid oil degradation. For higher temperatures, better efficiency, and larger capacity, it seems that the alternative is turbo-compressors. Nevertheless, there is a lack of experience in this area and more research and development efforts are required to enable these machines to achieve their potential in high temperature heat pumps.