Performance visualisation of a transcritical CO2 heat pump under fault conditions

Abstract

Heat pump systems have gained acceptance and appeal as an energy efficient alternative to electrical geysers for the purpose of water heating. This paper investigates a next-generation transcritical heat pump system using carbon dioxide as its working fluid. The water-to-water heat pump system used in this study simultaneously produces cooled and heated water. The heat pump system consists of four components. An evaporator is used to extract heat from a stream of water in order to evaporate and then superheat the working fluid. Chronologically after the evaporator comes the reciprocating compressor that raises the pressure and temperature of the working fluid to enable its circulation through the system. The gas cooler follows the compressor and is responsible for transferring heat from the working fluid to the second stream of interacting water. The expansion valve causes the working fluid, that leaves the gas cooler component, to undergo a large pressure drop and phase change. The working fluid that leaves the expansion valve enters the evaporator component for evaporation and superheating. The continuous conversion between different forms of energy, as enabled by the heat pump's components, make heat pumps susceptible to many different types of fault conditions. In this paper, the identification of faults that can occur during the operation of a heat pump and the degrading effects thereof on system performance were investigated. Fault detection and monitoring of the heat pump system via various visual representations are proposed. Specifically; fouling, working fluid leakage and coinciding water pump failure as system faults were investigated. The visual representations could uniquely identify and distinguish between the investigated fault conditions. The graphs were also able to monitor the severity and progression of system faults and their contribution to performance degradation