Performance characteristics of a CO2 water heater at various gas-cooler designs and operating conditions.

This work presents a numerical model of a CO2 water heater. By using the numerical model, the design of a water -CO2 double tube gas-cooler and operating conditions by evaporation pressure were suggested. The double tube gas-cooler in the numerical model was developed using a finite volume method and the compressor efficiency and discharge temperature were calculated by an artificial neural network based on the experimental data. The numerical model was validated by experimental data and the maximum difference of the water temperature at the gas-cooler outlet was 2.62°C. To apply various gas-cooler designs to the numerical model, three gas-coolers having different inner-tube numbers (2, 4, 8) were used in the experiments. For the comparison of the performance according to the inner-tube number, a dimensionless quantity ‘GCAR’ was suggested and was defined as the ratio between the inner-tube cross-sectional area and the outer tube cross-sectional area of the gas-cooler. With the decrease of GCAR at same heat transfer area water-CO2, the heating capacity and gas-cooler thermal effectiveness increased by approximately 5% and 9%, respectively. The optimized pressure ratio of the CO2 water heater cycle to obtain the maximum heating capacity was shown at 2.65.