Most photovoltaic/thermal (PV/T) systems have an air layer between the glass cover and the cells to minimize heat loss and to improve overall efficiency. This feature is not found in typical PV systems. The frame border that supports the glass cover casts a shadow over the cells near the frame when sunlight is slanted. This design is common in PV/T systems. The objective of this study is to determine the effects of a frame shadow on PV/T systems. A mathematical model of a PV module in a PV/T system is established. An experimental research on a partly shaded single cell is then conducted. Based on the module model and the results of the experiment, a single cell model that simulates the performance of partly shaded cells is developed and integrated into an existing system model. The new model can be applied to simulate frame shadow in a PV/T system. Results show that the performance of a partly shaded single cell can be calculated as the cell operates below the average irradiation intensity of the shaded and unshaded parts. In the worst-case scenario, the frame shadow can cause a 39.3% decrease in photoelectric efficiency. Although the parallel circuit performs better than the series circuit in minimizing loss caused by the frame shadow, the latter has a higher total output. (C) 2014 Elsevier Ltd. All rights reserved.