Giuseppe Panno

THE RELATIONSHIP BETWEEN DIFFUSE AND TOTAL SOLAR RADIATION IN COMPUTER SIMULATION OF SOLAR ENERGY SYSTEMS

ABSTRACT The diffuse to total radiation correlation developed by Liu and Jordan [1] was tested for three Italian locations. An acceptable agreement was found between the regression lines in [1] and the ones found for two of the three locations. Using measured hourly values of total solar radiation on a horizontal surface, the corresponding hourly values of total solar radiation on an inclined surface at 45° facing south were computed with both the Liu and Jordan and the Loudon [6] method for estimating the diffuse radiation. Results show that approximately the same degree of accuracy in the prediction is achievable with both methods and that the difference between the measured total monthly amount of energy incident on the inclined surface and the computed one does not exceed 6.5%. A computer model of a hot water production solar energy system was used for evaluating the error in the percent solar load estimations, when those methods are used. For this purpose comparative computer simulations were carried out making use both of the computed and the measured hourly values of diffuse solar radiation. Results show that the performance of the system is little affected by the use of the computed hourly diffuse radiation instead of the measured one.