Photovoltaic thermal module and solar thermal collector connected in series to produce electricity and high-grade heat simultaneously

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The photovoltaic thermal (PVT) module can produce electricity and heat simultaneously, while its outlet temperature is usually not high and might not be able to meet the requirement for hot water and spacing heating in buildings. In this study, an innovative integrated system is proposed that a PVT module is series-connected with a following solar thermal (ST) collector, renamed as PVT-ST, to overcome the drawback of low output temperature of PVT and no electrical generation of ST. The feasibility of the proposed system is discussed first, and a steady two-dimensional model is developed to investigate the thermal, electrical and overall performance of the PVT-ST system. The mathematical model is also validated based on the experimental data, revealing that the numerical results agree well with the collected data, with root mean square deviation lower than 1.39%. A comparative study is then conducted, and indicates that the PVT-ST system has slightly lower electrical efficiency but can output relatively high-grade thermal energy and performs much better in primary energy saving efficiency, the duel effects could make the system promising in residential buildings which need both electricity and heat. The PVT-ST system, occupying 1.65 m2 for both the PVT and ST collector, can output in average 0.82 kWh/day electricity and 5.75 kWh/day heat in Shanghai, as a result 83.48% of primary energy saving efficiency is achieved. Finally, a parametric analysis explicates that solar radiation and mass flow rate shows positive effects on overall performance, whereas a negative effect is observed from the inlet water temperature.