Experimental Investigation of a Hydrocarbon Piston Compressor for High Temperature Heat Pumps

The global trend towards carbon neutral industries require sustainable and climate friendly heat supply. Heat pumps can meet this requirement, but the technical capability today often limit the heat supply temperature to below 90°C. The major challenge in the development of a heat pump that can deliver heat at high temperatures (115 oC) to the heat sink is the operability and performance of the compressor. The compressor electric motor cooling, lubrication stability and material durability are important physical properties to evaluate for a compressor working at high temperatures. In this study, compressors operating with natural fluids such as hydrocarbons are focused. Hydrocarbons, like butane (with high critical temperature, 152 oC), have become a viable alternative to synthetic working fluids for high temperature heat pumps. Some of the synthetic fluids will be phased down by the Kigali amendment to the Montreal protocol. Butane can operate with similar compressor technology that is familiar to propane as well as synthetic working fluids, due to the close thermodynamic properties. This study experimentally investigates the performance of a prototype butane compressor adapted for high temperature heat pumps. The compressor has been fitted with modifications specifically for high performance and high temperature heat delivery. The compressor is installed in a 20 kW cascade heat pump with propane in the low temperature cycle and butane in the high temperature cycle. The prototype compressor development can be applied in a heat pump for drying, pasteurization, sterilization, low-pressure steam production, pressurized hot water production and others that are conventionally supplied with heat by the combustion of fossil fuel in boilers or direct electric heating. It may also recover waste heat from industrial processes, thereby increasing energy efficiency. Experimental results show that the prototype compressor has an average total compressor efficiency of 74 % while delivering heat at 115 oC