THE DEVELOPMENT AND APPLICATION OF A SIMPLIFIED THERMAL PERFORMANCE EQUATION FOR A SHEET-AND-TUBE PHOTOVOLTAIC THERMAL COMBI-PANEL
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PV/Thermal technology is the combination of solar thermal and photovoltaics - two mature and widely understood technologies. Combining the two technologies complicates existing standardized rating procedures and performance modeling methods. Currently a standardized performance test method does not exist for PV/Thermal (PV/T) panels. Existing and developing PV/T panels are commonly tested using separate standardized solar thermal and photovoltaic test procedures. Solar thermal performance is rated in terms of temperature difference whereas photovoltaic performance is dependent on absolute temperature level. The thermal and electrical performance of a PV/T panel is, however, coupled so performance equations derived using traditional test methods may not accurately reflect the performance of a combined PV/T panel over a wide range of conditions. The purpose of this work was to develop an efficiency equation for a PV/T panel which can be derived from a minimal amount of empirical test data and still accurately predict its thermal and electrical performance over a wide range of conditions. To accomplish this, a quasi- 3-dimensional steady-state model of a sheet-and-tube PV/T collector was developed and used to generate a broad data set from which a simplified PV/T performance equation was developed. Using this numerically generated data set, and introducing additional coefficients into the traditional solar thermal performance characteristic, a modified PV/T efficiency equation was derived which expressed the electrical and thermal efficiency in terms of ambient temperature, incident solar irradiation and the temperature difference between the inlet fluid and the ambient. It was also shown, for the case studied, that the efficiency equation can be produced from as few as 6 data points and still accurately predicts the performance at a wide range of operating conditions. A TRNSYS  model was developed to demonstrate how the performance equation can be used to simulate the annual performance of a PV/T collector in a domestic hot water system. It was shown that a performance equation, derived from 6 data points, performed as well as a performance map which used over 1000 data points. The annual thermal and electrical production predicted by both models was within 1.5% of each other. The PV/T efficiency equations were also shown to perform well for a range of electrical parameters, thermal properties and substrate thermal conductivity values. Future work is recommended to validate the PV/T performance equation using real empirically derived data for a range of collector designs.