## 1 Introduction  
In the past decade years, more and more attention was paid to the utilization of solar energy in China. China's photovoltaic (PV) power capacity reached 136 million kilowatts by the end of June 2019, ranking first in the world [1]. For crystalline silicon solar cells, more than 80% of the solar irradiations cannot be converted into electricity, and most of them are converted into thermal energy. This leads to a temperature rise of solar cells and a drop of electrical efficiency. As the PV cell temperature increases above 25 \({}^{\circ}\)C, the electrical efficiency decreases by 0.04-0.08% per degree centigrade rise [2]. Photovoltaic/thermal (PV/T) collectors using air or water as working fluid can produce electrical and thermal energy simultaneously, which makes the PV cell temperature decrease and electrical efficiency increase to a certain extent.  
Building-integrated photovoltaic (BIPV) systems consist of the integration of PV modules into building envelope [3]. In BIPV systems, PV modules simultaneously serve as building envelope material and power generator, and have been widely used for exterior shading [4; 5]. There is a likelihood of performance degradation in a BIPV system due to various factors, including the increase of PV module temperature [6]. Building-integrated photovoltaic/thermal (BIPV/T) systems can recover the heat from PV modules incorporated with the building envelope, and often utilize air as working fluid [7; 8; 9].  
Some studies about PV/T water collectors have been conducted, which are reviewed by [PERSON] and [PERSON] [10]. Moreover, the performance of PV/T water collectors integrated into a building envelope (roof or facade) should be evaluated according to practical operation conditions. [PERSON] et al. performed an experimental and numerical investigation on thermal and electrical performance of a water-based BIPV/T collector system on the roof of a house in Boulder, Colorado [11]. The analysis results indicated that the PV cell efficiency can be raised by 5.3% and that the outlet water temperature is suitable for domestic hot water use. [PERSON] et al. investigated the thermal and electrical performances of a multifunctional PV/T hybrid solar window in Avkarleo, Sweden, which can simultaneously produce electricity and hot water and can serve as a sunshade [12]. The solar window mainly consists of reflector screens located on the inside of the window. The reflector screens are integrated with PV cells and water pipes. [PERSON] et al. investigated the performance of a building-integrated PV/T roof collector using polyethylene pipes as the thermal absorber in Nottingham, UK [13]. The previous studies about water-based BIPV/T systems rarely dealt with their shading performance.  
Most BIPV and BIPV/T systems are connected to power grids. Stand-alone photovoltaic systems have been used for outdoor lighting, electric vehicles, and outdoor water pumping, etc. [14; 15]. In this paper, a stand-alone BIPV/T solar water heating system for exterior horizontal shading of residence windows is presented. The system can not only produce domestic hot water but also help to increase the electrical efficiency of PV cells. In the presented system, the collectors of decentralized solar water heaters are integrated with building envelope. This study aims to evaluate and optimize the performance of such system applied in a typical city of south China. The thermal, electrical, and shading performances of such system are analyzed through simulations.