Thermal energy storage as a way to improve transcritical CO2 heat pump performance by means of heat recovery cycles

Abstract

Electrification of the heating sector is a major target of energy transition towards a more sustainable, efficient, and less polluted future. Heat pumps are considered more suitable than electrical heaters or fossil-fueled boilers; however, common refrigerants cause ozone layer depletion, which exacerbates the greenhouse effect. Natural refrigerants, such as CO2, perform comparably and even better than hydrofluorocarbons while minimizing the negative aspects. This study presents a model of a water-heater CO2 transcritical heat pump in a configuration that increases the overall coefficient of performance (COP) by introducing thermal energy storage (TES). The thermodynamic cycle was divided into two separate phases. After heating the TES (charging), warm water was used as the heating fluid in the evaporator to increase the evaporation temperature and pressure of CO2, which reduced the work of the compressor. As the water temperature decreased progressively, the discharge cycles improved the total COP. The case study focuses on dairy processes and suggests a more straightforward and cheaper method to improve cycle efficiency than the current processes, such as ejector-expansion systems or double compression.