Refrigerant Selection and Compressor Performance Analysis for High Temperature Heat Pumps

Abstract

It is prevalent that a push towards a more sustainable energy future is needed and is in action. Heat pumps (HPs) are a known technology that are able to offer an energyefficient alternative to other heat sources, such as furnaces and electric heaters. However, conventional HPs are only able to deliver heat up to 80°C. High temperature heat pumps (HTHPs) are an effective technology capable of providing heat above 80°C and could reduce the energy consumption of many residential and industrial applications. HTHPs however, have several barriers before they are widely used in industry, which includes but are not limited to the lack of available working fluids and suitable equipment. This project looked into finding a promising working fluid for a HTHP and the complications that it entailed. The chosen HTHP that was investigated was to provide a heat delivery of 100°C from common ambient conditions; hence the condensing and evaporating temperatures were 120 and 5°C respectively. A broad study was first completed to test the suitability of working fluids that may or may not have been used in a refrigeration or heat pump system before. These fluids were then screened upon their thermophysical properties and their performance. It was found that ethanol had the highest coefficient of performance (COP) of 2.73. Ethanol also had a few intrinsic issues, from being flammable, having a high compression ratio, in addition to the system having to operate under vacuum pressures. A multi-stage compressor consisting of the cross-vane mechanism was analysed to address these issues. The mechanism allows the introduction of intercooling between the two compression stages as well as the isolation of vacuum pressures to the atmosphere. A mathematical model was formed to analyse the geometry, kinematics, thermodynamics, mass flow and power of the chambers. The model showed a mechanical efficiency of 49% with the current geometry and operating conditions. It was found that the mechanical efficiency was significantly influenced by the geometry and the operating conditions, where the highest mechanical efficiency was found to be 70% with the change in the end face clearances.