Operation control strategy of direct-expansion solar-assisted heat pump system for heating

Abstract: Revolutionary changes can take place in the energy field in the context of the dual carbon strategy. In particular, the energy conversion chain can be changed from the "heat generation by fuel and power generation by heat" to the "green power production and heat generation by electricity", indicating the significant electrification trend of terminal energy consumption. Fortunately, the solar energy can be used to heat the refrigerant in the evaporator of a heat pump (called a direct-expansion solar-assisted heat pump system, DX-SAHP), where a solar collector can be employed as the evaporator. The heat pump system can be applied for the building heating to reduce the consumption of conventional fossil energy and pollution. But the intermittent and unstable characteristics of solar radiation have led to the reduction of solar energy utilization efficiency and operational reliability of the system. As a result, a control strategy can be widely expected for the safe, efficient, and stable operation of the system using the environmental parameters. In this study, a DX-SAHP operation control of the system was developed for the radiant floor heating platform with propane as the working medium in Qingdao, Shandong Province, China. The system was mainly composed of a micro-channel heat collector/evaporator with an area of 2.93 m2, a constant frequency rotary compressor with the rated power of 732 W, an electronic expansion valve, a micro-channel condenser with a single surface area of 0.83 m2 surrounding a 200 L water tank, a circulating water pump with a rated power of 46 W, and a radiant floor heating chamber with the heating area of 12.96 m2. The hot-water storage tank was directly connected with the radiant floor heating coil to transfer the heat into the heating chamber. The data of the control system was collected, including the solar radiation intensity, temperature, pressure, wind speed, flow rate, and power at the 5-second interval. The function of the control system was realized, including the start-stop control of the compressor, the start-stop control of the circulating water pump, and the opening control of the electronic expansion valve. An experiment was conducted to clarify the effects of solar irradiation intensity, ambient temperature, and supply water temperature on the thermal performance of the DX-SAHP for the radiant floor heating system. The operation control strategy was then proposed under the premise of indoor thermal comfort. The linear fitting of experimental data was utilized to obtain the lower limit of supply water temperature under various ambient temperatures. The water temperature inside the water tank increased to the set value, according to the actual demand for the heat storage of solar radiation. The superheat and discharge temperature of the compressor were controlled in a certain range for the safe operation of the system. The two-position control of indoor temperature was modified for the indoor thermal comfort condition with reduced energy consumption. The experimental results show that the DX-SAHP control strategy for the radiant floor heating system performed stable and efficiently under various working conditions, indicating a fully comfortable heating demand. The average coefficient for the performance of the system was between 2.02-3.58, the superheat of the working medium at the inlet of the compressor was stably maintained at 7-11 ℃, and the discharge temperature of the compressor was lower than 90 ℃. A comparison was given for the indoor temperature and power consumption of the system with/without the control strategy. The heating effect of the system with the control strategy performed better than that of the without one, where the power consumption was reduced by 8.5%. The new control strategy was greatly conducive to the DX-SAHP system for the radiant floor heating system.