EXTERNAL REFLECTORS FOR LARGE SOLAR COLLECTOR ARRAYS, SIMULATION-MODEL AND EXPERIMENTAL RESULTS

A model for the calculation of incident solar radiation from flat- and CPC-shaped external reflectors onto flat plate solar collector arrays has been developed. Assuming an infinite length of the collector/reflector rows, the basic calculations of incident radiation in the collector plane from the reflector become very simple. The direct radiation from the sun is projected into a vertical plane perpendicular to the collector and reflector plane. The incident radiation onto the collector, including corrections for shadowing and lost radiation above the collector, can then be calculated using 2-D geometry. For very short collector/reflector rows a 3-D model is given for correction for the loss of specular radiation in the east west direction. The diffuse radiation is assumed to be isotropic. The diffuse radiation in the collector plane is calculated using View factors. CPC-shaped reflectors can be treated with the same models by introducing an equivalent flat reflector. The incidence angle for the solar radiation from the reflector onto the collector is in most cases higher than the incidence angle for the radiation directly from the sun. Therefore the incidence angle characteristics of the collector glazing and absorber become more important in this application. Equations are given for the incidence angles for diffuse and beam radiation. An annual performance increase of over 30%. 100-120 kW h/M2, has been measured for aged (four operating seasons) flat reflectors in the Swedish climate. With a CPC-shaped reflector and new reflector materials, a performance increase of up to 170 kW h/M2 is not unrealistic. This means that the collector and ground area requirement can be reduced by more than 30% for a given load.