| Based on a simple geometrical description of the sky hemisphere and the magnitude of the horizontal diffuse radiation, a model for estimating diffuse radiation impinging on sloping surfaces is developed. The radiance of the sky is assumed to be constant throughout the dome, except in a disc centered on the sun's position and a band at the horizon, where radiance is enhanced. The magnitudes of radiance enhancements within these two zones are treated as functions of the solar altitude, the amount of diffuse radiation received on the horizontal, and the relative importance of direct radiation at the earth surface. These functions are established statistically by analyzing a month of one minute global solar radiation data recorded on slopes of different orientations in Albany, NY.;A method is proposed to apply the model to collectors with limited fields of view such as Compound Parabolic Collectors, (CPC). The application of the model to these collectors accounts partly for the role played by forward scattered radiation in the total energy they receive. The importance of this factor is found to be maximum for concentration ratios of the order of two and may be considered as negligible for higher concentrations.;An optimization of CPC's geometrical characteristics is performed for photovoltaic electricity generation in the area of Albany, NY. This calculation is used to assess the relative effects of meteorological conditions and economic assumptions on optimum concentration values, and provides the reader with information pertaining to the variation of the cost of electrical energy produced as a function of the cost of silicon solar cells.;Tests against data show that substantial improvement is achieved over the classical isotropic model for any collector slope or orientation. Improvement is found for instantaneous as well as accumulated data. The good result obtained for a test against a data set totally independent of the one used for establishing the model suggests that the use of the latter is not confined to the specific region where it is established. |
