Experimental results of a tri-partite gas cooler (D4.3)

An experimental investigation on the performance of a tri-partite brazed plate gas cooler in a transcritical carbon dioxide (CO2) heat pump water heater system is conducted. The gas cooler under investigation is partitioned to separately serve the functions of domestic water preheating, space heating, and domestic water reheating. The gas cooler is studied at a steady state for only space heating (SH) operation, only domestic hot water (DHW) operation, and simultaneous space heating and domestic hot water (DHW+SH) operation. Measurements are performed for counterow conguration with two different internal arrangements: (i) three individual single-pass heat exchangers with a U conguration and (ii) a two-pass arrangement for preheating and reheating domestic hot water. The heat pump unit is designed with a heat duty of 10 kW, 8 kW, and 10 kW under DHW, SH, and DHW+SH modes to provide the tap water heating and space heating water up to 70 C and 35 C, respectively. The effects of inlet pressure, CO2 and water mass ow rates, and water inlet temperature on the total heat transfer coefcient, heat duty, pressure drop, supply temperature, and temperature approach are analyzed in detail. To present a reference for brazed plate gas cooler optimization design in transcritical CO2 systems, the heat duty and supply temperature correlations for both space heating and domestic hot water cycles are also developed and validated with experimental data. As shown by the experimental results, the gas cooler shows promising performance and is more compact than previous designs. Under the design conditions, the heat duty meets the requirements to provide the supply temperatures of 35 °C and 70 °C for the space heating and tap water heating applications. The pressure drops in the tri-partite gas cooler are low under different operating modes and the temperature approaches for the reheating gas cooler vary between 6.52 K and 8.24 K under design conditions in DHW+SH and DHW modes, depending on the internal heat exchanger arrangement. This value is 0.65 K for the SH operating condition. This shows that CO2 liquid phase enters the throttling device, indicating good performance of the evaluated gas cooler. It is found that the operating pressure has a considerable impact on the heat transfer coefcients in the near-critical region. The maximum of heat transfer appears at the region in the vicinity of the pseudo-critical point and the peak point of the heat transfer coefcient shifts to the high-temperature state when the operating gas cooler pressure increases. Furthermore, no signicant and clear inuence of the water inlet temperature is demonstrated on the heat transfer coefcients when CO2 is in a high-temperature state. The values of heat duty increases with the increases of water and CO2 mass ow rates. The lower the water inlet temperature is, the higher the average heat duty is. Increase of CO2 mean temperature owing through the gas cooler causes a decrease in the density and viscosity of the refrigerant to maintain the ow continuity, resulting in a larger Reynolds number and then pressure drop. The increase in the hot side uid pressure can cause an increase in the density and thus a decrease in the velocity of CO2, so the CO2 side pressure drop decreases. A larger mass ow rate of refrigerant and a lower water mass ow rate increases the water supply temperature. The higher water mass ow rate is recommended to decrease the temperature approach values under DHW, SH, and DHW+SH operations. Ultimately, the values of coefcient of determination for the reheating heat duty, space heating heat duty, preheating heat duty, DHW supply temperature, and SH supply temperature are 0.88, 0.86, 0.94, 0.99, and 0.94, respectively. The results show that the correlations perform very well in the prediction of heat duty and supply temperature. The basic data for the characteristics of CO2 in the present experimental study are of signicance in designing and optimizing the transcritical CO2 systems