Angle-Resolved Direct Emissivity Measurements on Unencapsulated Solar Cells for Passive Thermal Control

The emissivity of two unencapsulated photovoltaic cells has been measured by a direct radiometric method, between 30 and 50 °C, and indirectly at room temperature. The advantages of the angle-resolved direct method for the determination of the emissivity of the bare cell, and the challenge of using direct measurements around room temperature, are discussed. This method allows calculating the total hemispherical emissivity by incorporating directional data, which are usually lacking. These data also reveal important differences between the infrared responses of both cells, despite the similarities in the more commonly reported normal spectra. These differences stem from the structure of the cells. A comparison to the integrating-sphere indirect method illustrates the improved accuracy of the emissivity values and their potential for evaluating the thermal performance of the cells and potential radiative cooling gains in select applications, where the emissivity of the bare cell is not concealed by encapsulation. An estimation of the magnitude of this effect, obtained using a simplified heat-transfer model, is significantly less than 1% in absolute efficiency for realistic parameter values. More work is needed to quantify gains from encapsulated cells, which are expected to be even lower because of the higher emissivity of silica glass.