## 1 Introduction
The solar collector in the i-SAHP system is completely separated from the evaporator of the heat pump, and this type of heat pump is more stable. Worldwide, considerable research has also been conducted. [PERSON] et al. [19] performed a comprehensive simulation study of three i-SAHP systems in parallel, including series and dual-source systems. [PERSON] et al. [20] presented a dynamic model for the solar water heating mode of i-SAHP systems, and the results of the numerical simulation corresponded well with the experimental data. The conclusion was that the COP increased from 2.35 to 2.57 as the solar irradiation increased from 0 W/m\({}^{2}\) to 800 W/m\({}^{2}\). Under the weather conditions in London, UK, [PERSON] et al. [21] conducted theoretical and experimental research on a newly designed i-SAHP system. Simultaneously, a model for a solar thermal loop was established and verified with experimental data. To meet the need for domestic hot water in a high-performance house, [PERSON] et al. [22] designed an i-SAHP system and used the TRNSYS simulation software to model and optimize the control of the system. Moreover, a series of comparative experiments on water heating systems was carried out. [PERSON] et al. [23] established a mathematical model to compare the performance of a heat-pump-assisted solar water heater with that of an electric-heating-equipment-assisted solar water heater. The results showed that the efficiency of the water heater was higher when a heat pump was used as an auxiliary heat source. [PERSON] et al. [24] carried out a long-term test on an integral solar heat pump water heater. The average energy consumption measured was much lower than that of the traditional solar water heater. [PERSON] et al. [25] compared the electricity consumption, costs and greenhouse gas emissions of a traditional solar hot water system, a traditional air source heat pump hot water system, and a solar heat pump water heating system in Australia. Subsequently, [PERSON] et al. [26] made a similar comparative study based on the experimental system he designed in Kunming,China. [PERSON] et al. [27] conducted a comparative study on the economics of four different water heating systems, including electric water heaters, town gas water heaters, SAHP systems and conventional SWH systems in Hong Kong, China. It was found that the solar-assisted air source heat pump (SA-ASHP) system saved more energy and had the least global warming impact out of several SAHP systems. [PERSON] et al. [11] designed an i-SAHP model using TRNSYS software and compared it to a conventional solar domestic hot water system and an electric domestic hot water system. The results showed that the electrical consumption and operating costs of the i-SAHP system were the lowest. However, there are few comparative studies regarding different types of solar water heating systems under the climatic conditions of Tianjin, China. As discussed in this paper, a parallel SA-ASHP system was designed to supply domestic hot water for student apartments at a university in Tianjin, the original auxiliary heat source of which was an electric boiler. A comparative experiment was carried out on the two systems to compare their solar fraction, water supply quality, energy consumption and economy. The main objective of this paper was to propose a transformation plan for the water heating system of a university apartment to achieve a constant temperature water supply and save energy and to verify its feasibility and effectiveness through experiments. In addition, the findings can provide a reference for the design of a hot water system for a new apartment at the university. The remaining sections are organized as follows: In Section 2, the principles of the two water heating systems before and after the retrofit are introduced, and then, the test schemes are proposed. In Section 3, the test results are discussed and analyzed. In Section 4, the simulation model of the SA-ASHP hot water system is established in TRNSYS software to optimize the operation strategy.