Numerical assessment of the use of a dedicated mechanical subcooling system during hot water generation in a water to water transcritical CO2 heat pump

Abstract

This paper presents a numerical study of the use of a dedicated mechanical subcooling (DMS) system using R1234yf, for hot water generation in a water-to-water CO2 heat pump. Compressor mass flow rates and power consumptions were modeled using the manufacturer’s correlations, expansion valves were modeled as isenthalpic, and heat exchangers were modeled by deriving correlations for the evaporation/condensation pressure and heat transfer rate. In the condenser, IMST-ART was used to obtain condensation pressure and heat transfer rate. A cell-by- cell discretization model was used for the evaporator, which was a transcritical CO2, subcritical R1234yf heat exchanger. Three different systems were compared for the transcritical CO2 cycle: with internal heat exchanger (IHX), with DMS, and with IHX+DMS. Results showed that, for the conditions studied (hot water generation up to 60 ºC and evaporator water inlet temperature from 5- 25 ºC), the use of a DMS does not improve the performance of the system.