Experimental and numerical investigation of a photovoltaic water flow window: Enhancing solar energy utilization and building energy efficiency in hot summer and warm winter climates

Enhancing the performance of transparent building envelopes is a key strategy for improving energy efficiency in buildings. This study introduces a photovoltaic water flow window (PV-WFW) as an innovative building-integrated photovoltaic/thermal (BIPV/T) technology. Compared to a traditional single-layer cadmium telluride (CdTe) photovoltaic window (SPV), the PV-WFW not only lowers glass temperature under intense sunlight but also enables full-spectrum solar energy utilization, reducing both air-conditioning and hot water system energy consumption. The study first presents the conceptual design and working principle of the PV-WFW, followed by a comparative experimental evaluation of its overall performance against the SPV. Numerical model validation and simulations were conducted to assess its energy-saving potential in a hot summer and warm winter climate, using Hong Kong as a case study. Experimental results show that the PV-WFW effectively functions as a solar collector, with flowing feedwater reducing the CdTe solar cell temperature by approximately 10 °C at noon and lowering the innermost glass temperature by around 14 °C. The PV-WFW achieved an overall PV/T efficiency of up to 68.8 % but did not significantly enhance photovoltaic power generation compared to the SPV. Energy performance analysis revealed a 3.6-fold reduction in annual indoor cooling loads with the PV-WFW, while its yearly-averaged thermal efficiency reached 86.9 %. Additionally, the PV-WFW resulted in annual net electricity savings of 783.7 kWh/m2, whereas the SPV incurred a net electricity loss of 65.7 kWh/m2.