Concentrating collector

The concentrating collector composes of concentrating, absorption and tracking system. According to the theory of solar concentrating, it can be divided into two types, reflected type and refracted type in which can be divided into several kinds according to the variable concentrating device.

In order to satisfy the demand of utilization of solar energy, simplify the tracking system, increase the reliability and reduce the cost, diverse concentrating collectors have been developed in this century but its promotion quantity was far less than the flat plate collector.

Among the reflected type concentrating collector, rotating parabolic concentrating collector (point focus) and groove shape parabolic concentrating collector (liner focus) are mostly applied. The former can obtain higher temperature but is required to implement 2-dimension tracking; the latter can obtain medium temperature and is only required to implement 1-dimension tracking. Both types of the collector were applied in the early of this century, and have been improved, for example to increase the process accuracy of the reflected side, to research high reflection material and develop high reliable tracking system. Now both types of the collector can satisfy variable requirement on medium and high temperature in utilization of solar energy. Due to its high cost, its mass applications are limited.

In 1970s, a kind of combined parabolic collector (CPC) was launched in the world, which composed of 2-groove shape parabolic reflection mirror and was not required to follow the sun¡¯s path and was only required to make a little adjustment along with the changing of season so as to focus the light and obtain higher temperature. The concentrating ratio was normally under 10. If it was under 3, CPC can be fixed and was not needed to adjust. At that time, CPC was awarded high praises, even was considered to be an important breakthrough in the utilization technologies of solar energy and was predicted to common applications. But several decades passing, CPC was still applied to few demonstration projects other than the mass production of flat plate collector and evacuated tubular collector. CPC has once researched in China in 1970s to 1080s, but was less applied and even stopped.

Other reflected concentrators include taper reflecting mirror, ball reflecting mirror, bar shape reflecting mirror, dipper model groove shape reflecting mirror, plane-parabolic mirror concentrators. Moreover, the heliostat, another tracking device applied to the tower type solar power station, composed of many plane reflecting mirrors or curved surface reflecting mirrors which will reflect the sunlight into an identical absorbers resulting in a higher temperature and huge energy obtained.

The refracting collector is utilized according to the light refraction theory. A performance had been showed in Paris, France in history by using 2 lenses to focus the sunlight to melt the metal. Some utilized a group of lenses and plane mirror to assemble the high temperature solar stove. Obviously, the glass lens was heavy in weight, complex in process and high in cost, which may not be mass-produced. Therefore, the refracting collector had not been well developed in a long time.

In 1970s, large scale Fresnel lenses was researched in the world intending to use for manufacturing solar concentrating collector. The Fresnel lenses were plane concentrating with light weight and lower cost, which also can also be divided to point focus and liner focus. It was made by organic glass or other transparent plastics or glass mainly used for the concentrating solar cells generation system

Research on Fresnel lenses in solar energy device was carried out in China from 1970s to 1990s. Stem pressing method was employed to manufacture large area of flexible transparent plastic Fresnel lenses. Or assembled cutting tool was employed to manufacture point focusing Fresnel lenses with the diameter of 1.5m. The results were not satisfactory. Recently, stem-pressing method was employed to manufacture liner glass Fresnel lenses which was no good at accuracy and needed to improve.

There are another two types of innovative solar concentrator by use of complete reflection theory, which are not applied in practice, but have instructiveness. One is the light-guide fiber concentrator, which is composed of light-guide fiber lenses and connected light guide fiber. The sunlight will be focused through light-guide fiber lenses and transited to end-use through light-guide fiber.

Another is fluorescence concentrator, which actually is a kind of transparent plate (organic glass) added with fluorescence pigment. It can absorb the sunlight of which wavelength is consistent with the absorption band of fluorescence, and then emit the fluorescence by the wavelength of irradiation band which is longer than that of absorption band. Due to the discrepancy between the plate and the surround medium, the emitted fluorescence is to lead to the edge of flat plate in a way of complete reflection within the plate. Its concentrating ratio depends on the proportion of flat plate area and the edge area, which easily reaches 10 to 100. Such flat plate can absorb not only the incident light from variable direction, but also scattered light without tracking the sun.