توسعه و اعتبار مدل شبیه سازی استاتیک برای پمپ گرما CO2

A simulation model for the CO2 heat pump water heater was developed and validated in this study. Component models of the gas cooler, evaporator, compressor, and expansion valve were constructed with careful consideration for the heat transfer performances. To validate the simulation model, experiments were carried out using an actual CO2 heat pump water heater (water heating capacity: 22.3 kW; hot-water temperature: 90 °C). In simulations and experiments, the effects of the inlet water temperature and outside air temperature on the system characteristics were discussed. As a result, the average difference in COP between the simulation results and experimental results is 1.5%.

The effectiveness of the CO2 heat pump water heater in controlling global warming is being recognized in Japan. The use of this water heater in place of the ordinary water heater, which is mainly driven by gas, can lead to a significant reduction in the primary energy consumption. Therefore, the CO2 heat pump is gaining popularity in Japan. In order to improve the system performance of the CO2 heat pump, it is necessary to develop an optimum design and a control method for the CO2 heat pump water heater. For this purpose, high-precision and general-purpose simulation model is required. In this study, we have developed a high-precision and general-purpose system simulation model for the CO2 heat pump water heater and investigated the validity of this model with detailed experiments. Some previous studies have discussed system simulations for the CO2 heat pump water heater. White et al. [1] developed a simulation model for the CO2 heat pump water heater. In their system model, model of each component was derived based on the experimental data. For instance, they experimentally clarified that the pressure head (m) and the isentropic efficiency of the compressor were expressed as linear functions of swept volume, and the heat transfer rate (W/K) of the gas cooler was a linear function of only refrigerant flow rate. These were only adopted to their prototype heat pump. The average difference between the predicted COP and the measured COP was 4.1%. Cecchinato et al. [2] compared the performance of the CO2 heat pump water heater with that of the R134A heat pump by carrying out simulation studies. However, they did not investigate the validity of their simulation model. Yokoyama et al. [3] developed a simulation model for the CO2 heat pump water heater with using a domestic hot water storage tank and analyzed the effect of the outside air temperature and tap-water temperature on the performance of the heat pump. The validity of the model was investigated by comparing the simulation results with experimental results. In this study, the overall heat transfer coefficients of the gas cooler and the evaporator were constant, i.e., they used the experimental values. Sarkar et al. [4] formulated certain guidelines for the design and optimization of the water-to-water CO2 heat pump on the basis of simulations. Their model employed the supercritical heat transfer correlation proposed by Pitla Srinivas et al. [5] and the Watelet-Carlo correlation for two-phase flows. However, they did not investigate the validity of the model. Agrawal and Bhattacharyya [6] compared the performance of a water-to-water CO2 heat pump that used a capillary tube with that of a water-to-water CO2 heat pump that used an expansion valve. The simulation model used in their study was the same as that used by Sarkar et al. [4]. They compared the results of their simulation with a small amount of experimental data in order to confirm the validity of the model.

In this study, we developed a static simulation model for a CO2 heat pump water heater by taking into account the heat transfer performance and the pressure drop. Additionally, to validate this simulation model, experiment was carried out using an actual CO2 water heater. In experiment, the effects of the inlet water temperature and the outside air temperature on the COP, heat exchanger rate, refrigerant flow rate, and pressure were investigated. The range of the inlet water temperature was from 10 to 40 °C and the range of the outside air temperature was from 13 to 28 °C. As a result of comparison between the simulation and experimental data, when the inlet water temperature changed the maximum and average difference between the predicted and measured COP were 5.4% and 0.9%, respectively, and when the outside air temperature changed, the maximum and average difference between the predicted and measured COP were 5.1% and 1.5%, respectively. From these results, we believed that we developed a high–precision and general-purpose simulation model for a CO2 heat pump water heater.