## 2 System Specification  
### System Apparatus and Operation Strategies  
There are 44 student apartments at the selected university. The schematic diagram of the original solar-assisted electric boiler water heating system is shown in Figure 1.  
It consists of a solar collector, an electric boiler, a hot water storage tank, etc. The collector installed on the roof has an area of 218 m\({}^{2}\). The hot water storage tank (volume 15 m\({}^{3}\)) and the electric boiler (120 kW) are mounted in the equipment room. The system's water supply temperature is 60 \({}^{\circ}\)C.  
The operating strategies of the system are as follows: (i) Solar collector system: the collector temperature, T\({}_{1}\), rises with the increase in solar radiation intensity, and when T\({}_{1}-\)T\({}_{2}\)\(\geq\) 7 \({}^{\circ}\)C, the circulating pump, P1, opens. When T\({}_{1}-\)T\({}_{2}\)\(\leq\) 3 \({}^{\circ}\)C, the circulating pump, P1, stops. (ii) Water supply system: the water supply pump, P2, continuously operates for 24 h unless the water level in the tank is less than 0.1 m. (iii) Auxiliary heating system: like pump P2, the auxiliary heating water pump, P3, also operates for 24 h and will close when the water level in the tank is less than 0.1 m. During normal operation  
Figure 1: Schematic diagram of the original solar-assisted electric boiler water heating system. of the system, the outlet temperature of pump P3 is monitored in real time, and when the temperature is \(<\)50 \({}^{\circ}\)C, all four of the heating tubes of the electric boiler are turned on. When the heating process occurs, the water temperature gradually increases, and when it increases to 53 \({}^{\circ}\)C, one heating tube is turned off. The second electric heating tube will be turned off when the temperature reaches 60 \({}^{\circ}\)C. When the outlet water temperature rises to 69 \({}^{\circ}\)C, all the heating tubes in the electric boiler will be turned off. (iv) Water supplement system: this system is controlled by the liquid level. The water supply valve is opened when the liquid level is lower than 1.5 m and closed when it reaches 1.8 m.  
The two student apartments have the same building area and solar collector area and similar water consumption and usage modes. The hot water system of one of the student dormitories was reformed, while the other was not modified. The schematic diagram of the rebuilt SA-ASHP water heating system is shown in Figure 2.  
The transformation mainly consisted of:  
1. A new hot water storage tank (volume 6 m\({}^{3}\)) was added as a solar water collection tank, and the original water tank with a volume of 15 m\({}^{3}\) was used as a constant temperature water tank. The two water tanks were connected to form a water supply system, and a thermostat was installed on the pipeline between them. 2. Three air source heat pumps were selected as auxiliary heat sources instead of the electric boiler. One was connected to the solar water collection tank, and the other two were set to control the temperature automatically and connected to the constant temperature water tank to form a constant temperature water supply system. The heat pump model was KFXRS-38 II with a heating capacity of 38 kW. 3. To meet the student requirements for bath water temperature, to improve the efficiency of the air source heat pump and solar collector and to reduce the heat loss of solar collectors and hot water pipes, the water supply temperature was reduced from 60 \({}^{\circ}\)C to 40-50 \({}^{\circ}\)C.  
The reformed system is equipped with a control system and a remote monitoring system, which can be monitored and measured remotely. In the control system, a programmable logic controller is used as the core control element, with the functions of timing, constant temperature, low water level water supply, over-temperature water replenishment and low-temperature water return.