Experimental and numerical study of a CO2 water-to-water heat pump for hot water generation

• Published in: International journal of refrigeration 2021-12, Vol.132, p.30-44
• Main Authors: Illán-Gómez, F., Sena-Cuevas, V.F., García-Cascales, J.R., Velasco, F.J.S.
• Format: Article
• Language: English
• Subjects: Adaptive control; Adaptive systems; Carbon dioxide; Chauffage de locaux; CO2 heat hump; Evaporation; Heat conductivity; Heat exchangers; Heat pumps; Heat transfer; Heating; High temperature; Hot water; Hot water generation; Internal heat exchanger; Mathematical models; Pompe à chaleur au CO2; Production d’eau chaude; Space heating; Water; Water temperature; Échangeur de chaleur interne
• ISSN: 0140-7007; 1879-2081
• DOI: 10.1016/j.ijrefrig.2021.09.020

•Constant gas cooler pressure control is adequate to low temperature heating systems.•An adaptive control is recommended for high temperature heating systems.•Performance is similar for flooded or dry evaporator if superheating is below 4 K.•The use of an internal heat exchanger is always positive for hot water generation.•An increase in the surface always increases COP when using plate heat exchangers. This paper reports the results of an experimental and numerical study of different configurations of a CO2 water-to-water heat pump for hot water generation. The experimental installation developed to test the heat pump unit is briefly described and then the experimental results are presented. Numerical results of a model previously developed are compared with experimental results, allowing the validation of the model. Once the model is validated, it is used to predict untested conditions. The results show, for several hot water generation scenarios, the influence that different parameters have on the system performance, including the gas cooler pressure, the internal heat exchanger (IHX) effectiveness, and the heat exchangers area on the efficiency of the system. For low and intermediate heating temperatures, the optimal gas cooler pressure is nearly independent of the range of evaporation water temperature (5 to 30 °C), therefore a constant pressure control able to maintain the pressure slightly above the optimal value, can be the best solution. On the other hand, for medium, and high temperatures, an adaptive control system is strongly recommended. Although the influence of a variation in the heat exchangers area depends on the operating conditions, in general terms, the area of the gas cooler has the higher influence on the efficiency of the system and the area of the IHX has the smaller influence. As the gas cooler temperature increases or the evaporation temperature decreases, the improvement obtained by increasing the HXs area decreases.