The ASHRAE clear sky model-an evaluation

Abstract

The performance of the ASHRAE method for estimating solar radiation under cloudless conditions is evaluated, using measured data collected during 1977 at 31 NOAA monitoring stations. The performance of a modified version of the ASHRAE model is examined, using elevation-corrected optical air mass instead of seasonal clearness numbers. Based upon the comparative results, a revised procedure for estimating clear day solar radiation is suggested. Results show that mean bias errors of less than 10% occurred in approximately 90% of the cases for both versions of the model. The general validity of the ASHRAE approach to estimating solar radiation under cloudless conditions is confirmed. The comparative superiority of an alternative version of the ASHRAE model is demonstrated, using elevation-corrected optical air mass rather than the clearness number concept. It is concluded that the revised model allows for decreased attenuation due to all relevant atmospheric constituents with increased station elevations. The ASHRAE technique is extended to sites other than those for which clearness numbers are available.