The Helios model simulates the optical behavior of reflecting concentrators. The model follows the incident solar radiation through the system (including the atmosphere) and includes all the factors that influence the optical performance of a collector. An important output is the flux-density pattern (W/cm/sup 2/) at a grid of points on a surface such as the absorbing surface of a receiver and its integral (power in watts) over the surface. The angular distribution of sunrays for the radiation incident on a concentrator is modified by convolution, using the fast Fourier transform, to incorporate the effects of other nondeterministic factors such as sun-tracking errors, surface slope errors, and reflectance properties. The analytical methods used for the statistics, the off-axis reflecting optics, the atmospheric effects, and the various coordinate systems are described and illustrated. This model forms a basis for the simulation code HELIOS as well as for other codes under development. Some of the HELIOS routines are described, a few of its capabilities are discussed and illustrated, and comparisons of data with calculations are presented. These capabilities have been used for performance predictions, safety studies, design trade-offs, data analysis problems, the specification and analysis of concentrator quality, and for the general understanding of solar-concentrator technology.
