Solar Engineering of Thermal Processes, 4th Edition

Chapter 7

Concentrating Collectors

For many applications it is desirable to deliver energy at temperatures higher than those possible with flat-plate collectors. Energy delivery temperatures can be increased by decreasing the area from which heat losses occur. This is done by interposing an optical device between the source of radiation and the energy-absorbing surface. The small absorber will have smaller heat losses compared to a flat-plate collector at the same absorber temperature. In this chapter we discuss two related approaches: the use of nonimaging concentrators and the use of imaging concentrators.

Many designs have been set forth for concentrating collectors. Concentrators can be reflectors or refractors, can be cylindrical or surfaces of revolution, and can be continuous or segmented. Receivers can be convex, flat, or concave and can be covered or uncovered. Many modes of tracking are possible. Concentration ratios (the ratios of collector aperture area to absorber area, which are approximately the factors by which radiation flux on the energy-absorbing surface is increased) can vary over several orders of magnitude. With this wide range of designs, it is difficult to develop general analyses applicable to all concentrators. Thus concentrators are treated in two groups: nonimaging collectors with low concentration ratio and linear imaging collectors with intermediate concentration ratios. We also note some basic considerations of three-dimensional concentrators that ...