| Work Detail |
Chinese scientists have designed a novel control method for water-to-water heat pumps using carbon dioxide. For the tests, they relied on a virtual heat pump. The coefficient of performance (COE) improved by up to 14.6%. Scientists from Dongguan University of Technology (China) have proposed a novel control method for water-to-water heat pumps using carbon dioxide (CO2) as a refrigerant. The model-based optimal control (MC) was tested against a typical control mechanism (TC) on a virtual heat pump, improving the coefficient of performance (COE) by up to 14.6%. “An efficient optimization strategy for optimal control of a water-water CO2 heat pump that allows reducing the computational burden has not yet been developed. Therefore, this study aims to develop a MC to improve the COP of the water-water CO2 heat pump by identifying optimal variables,” the group said. “The system models were derived using extensive data from a virtual CO2 heat pump whose reliability was validated before further simulations were performed.” The water-to-water CO2 heat pump contained an evaporator (ET), a liquid receiver (LR), an internal heat exchanger (IHX), a compressor (CM), a gas chiller (GC), an expansion valve (EX) and two circulation pumps (PM). The CT uses fixed setpoints and simple control loops to regulate specific parameters without considering the optimization of the entire system. To develop the more refined MC, the team has built a virtual CO2 heat pump in MATLAB and REFPROP. 3,969 cases were run to identify the system models, describing how the heat pump responds to changes in operating conditions. An experimental setup was designed to validate the system, which showed a mean error of 4.4% for the gas chiller outlet temperature and 7.4% for the compressor power. The scientists analyzed the thermodynamic behavior of the system components and developed an optimization algorithm to maximize the COE by identifying the optimal values ??of discharge pressure and outlet water temperature of the gas cooler. They tested the system in three case studies, for both the MC and TC models. In the first case study, researchers set the evaporator mix inlet temperature at 18 ºC and randomly set the gas chiller inlet water temperature between 29 ºC and 35 ºC. The target outlet temperature at the gas chiller varied between 40 ºC and 48 ºC. The MC achieved a 9.9% higher COP than the TC, with an average COP of 2.49 versus 2.265 for the TC. In the second case study, the gas chiller inlet water temperature was set at 32°C, while the evaporator mix inlet temperature ranged from 17°C to 19°C. The target outlet temperature at the gas chiller varied randomly between 40°C and 48°C. The MC outperformed the TC by 8%, with an average COP of 2.482 versus 2.3 for the TC. In the final case study, the evaporator mix inlet temperature ranged from 17 to 19 ºC, the gas chiller inlet water temperature ranged from 29 to 35 ºC, and the gas chiller outlet water temperature ranged from 40 to 48 ºC. The MC achieved 14.6% higher efficiency, with an average COP of 2.458 versus 2.145 for the TC. “The computational time of the system with multiplexed optimization strategy was reduced by 42.2% to 47.1% compared with the traditional optimization strategy,” the team noted. “These results indicated that the formulated MC could effectively improve the energy efficiency of the water-to-water CO2 heat pump.” The developed multiplexed optimization strategy effectively reduced the computational load and slightly decreased the average COPs. Thus, this study provided a guideline for the implementation of MC in water-to-water CO2 heat pumps, contributing to the improvement of COPs.” They presented their results in “ A study of optimal control approaches of water-to-water CO2 heat pump for domestic hot water use,” recently published in Case Studies in Thermal Engineering . Scientists from Dongguan University of Technology (China), Guangdong University of Technology and the Norwegian University of Science and Technology jointly carried out the research. |
