Solar image characteristics of solar concentrators
Phillips, Paul Gregory
Master of Science
Solar beams can be concentrated by various optical systems in order to obtain energy at high temperatures. Among the various concentration devices the main criteria for classification deal with the optical systems used. In this work only concentrators using reflecting systems are considered. Host of the previous work done with these concentrators dealt with mathematically well defined systems. The method of analysis of these concentrators was simple but subjected to the problem of having-nonuniform intensity distributions on the absorber. The lack of a uniform intensity profile can disrupt the performance characteristics of the collection system and even ruin the absorber. Changing the geometries of the reflectors could alleviate this problem. The geometries, though, will no longer be mathematicaly well defined. A method of analysis developed in this work will study these mathematically complicated reflectors. Finite elements are used to develop a model for solar concentrators. This model accommodates the size of the solar disk when considering the incident field, and can also study complex reflector and absorber geometries. The present work has some limitations such that it is not general enough to handle many types of concentrators. These concentrators include secondary concentrators of double reflector systems and any other system that deals with ill-defined or broad incident fields. The model has been developed so that improvements in relaxing its limitations can be made. The method of analysis makes use of simple analytic geometry for the solution of specular reflections. All elements are assumed to be planar and specular properties are interpolated over the elements. The use of these elements is studied in three different models and only one of them is used in the final computer program. Two of these models use a type of ray tracing technique. This involves subdividing the incident field into small beams. Each beam is represented by a ray, and the ray is traced through all its reflections. The first method uses only one ray per element and does not lend itself well to accounting for solar size. The second method divides the beam on a reflector element into nine rays. This method is better at accounting for solar size. The specular factor model is the final variation of use of the basic analysis. It uses specular factors which are analogous to view factors for diffuse radiation to anlayze solar concentrators. The results are very accurate for single reflections. The second of the foregoing ray tracing models is chosen for use in the computer program. These results are compared with the previous works and some original results dealing with conical reflectors are examined. Although improvements in the division of the beam into nine rays and in handling various incident fields need to be made, the developed model and its computer application work quite well.