Investigation and analysis of a CO₂ heat pump chiller with novel two-stage evaporator

This study was carried out to investigate the performance of a transcritical CO₂ heat pump chiller that provides heating and cooling simultaneously. There are several applications where such CO₂ systems are implemented, i.e., hotels, commercial kitchens, fishing vessels, etc. Depending on the type of applications, these systems can be designed to achieve the desired hot and cold fluid temperature on the secondary side. The system investigated in the present study is utilized to produce hot water (up to 90°C) and chilled water (4°C). The evaporator utilized for chilled water production is a novel two-stage evaporator. The first stage of this evaporator operates on gravity-fed mode while the second stage operates on ejector-supported mode. The secondary loop is internally connected within the plate heat exchanger. This evaporator configuration gives the possibility to achieve a higher temperature gradient on the secondary fluid side. To analyze the performance of this proposed CO₂ system with the two-stage evaporator, a simulation model was developed in Modelica. Using this model, simulations were carried out to analyze the performance of the system under different operating conditions. Results show that the integration of the two-stage evaporator enhances the overall performance by over 5 % as the cooling capacity is shared equally between the two stages and the suction pressure of the compressor is elevated by utilizing the ejector.