SOLAR-CELLS EFFICIENCY VARIATIONS WITH VARYING ATMOSPHERIC CONDITIONS

The results of model simulations and analyses of operating solar cell efficiencies are reported. Defined as the ratio (%) between the electrical output power of the device and the total incident radiant power, this efficiency is somewhat ambiguous as it ignores the effects of temperature, total irradiance, and the spectral distribution of the light source. Since the total intensity and spectral distribution of sunlight varies with atmospheric conditions such as cloudiness, total column ozone, turbidity, and precipitable water, the efficiency of a cell operating in a field installation may also be a function of the above factors. This paper examines variation of modeled and observed cell efficiencies with atmospheric variations. Variations in turbidity and water vapor content drive the SPCTRAL2 solar spectral radiation model of Bird and Riordan (1986). Model output, coupled with prescribed spectral response functions representing monocrystalline and amorphous solar cells, is used to model the efficiency of these cell types. The modeled efficiency rises with increasing humidity, especially for the simulated amorphous cell. Simulated efficiency decreases with increasing turbidity for both modeled cell types. The amount of increase or decrease dependents on the spectral response specified. The measured performance of different solar cells operating at the PVUSA site in Davis, California is also analyzed. The major factors found to cause variations in operational efficiency are ambient temperature and total irradiance intensity. The authors also observed increases of the apparent efficiency of amorphous cells with decreasing energy in the red portion of the solar spectrum which are consistent with model predictions.