Fresnel collectors

The aim of the solar thermal collector development is to improve and optimize them for the temperature level from 80 °C to 250 °C. There are different approaches like double-glazed flat plate collectors with anti-reflection coated glazing, stationary compound parabolic concentrator (CPC) collectors, evacuated tube collectors, vacuum flat plate collectors, small parabolic trough collectors, linear concentrating Fresnel collectors and a concentrating collector with a stationary reflector. 

Linear concentrating Fresnel collectors use an array of uniaxially-tracked mirror strips to reflect the direct sunlight onto a stationary thermal receiver. The features of linear concentrating Fresnel collectors include their relatively simple construction, low wind loads, a stationary receiver and a high ground usage. Some applications allow for the use of the shaded area underneath the collector (e.g. as parking lots). 

A historical introduction into the uses of solar energy was attempted followed by a description of the various types of collectors including flat-plate, compound parabolic, evacuated tube, parabolic trough, Fresnel lens, parabolic dish and heliostat field collectors (Kalogirou, 2004a, 2004b). Solar thermal electricity systems utilize... 

Focusing collectors are usually cast acrylic Fresnel lenses, or mirrors of aluminized polyester film in frames of aluminum. These reflectors are either enclosed in a bubble of poly(vinyl fluoride) film, or under polycarbonate glazing, which may be covered with a fluorocarbon film to reduce the reflectivity. The absorbers for active systems are copper or aluminum since the temperatures are too high (325—370°C) for plastics. The frames, however, can be molded ABS, high density polyethylene or polyurethane, either solid or structural foam. Polybutylene or chlorinated PVC can be used for piping hot water, and tanks can be made of either reinforced polyester or blow- or rotational-molded, high density polyethylene (12—15). 

The Fresnel lens concentrator, shown in Figure 1.36 top right, uses refraction rather than reflection to collect the solar energy. These units are molded out of inexpensive plastic and provide higher efficiency than the standard PV collectors. Point-focusing Fresnel lenses are also available. In Figure 1.36 bottom left, parabolic dish collectors are shown. Both the Fresnel lenses and these collectors can be rotated around two axes to continuously track the Sun. 

Fresnel lens-type PV concentrators have operated at 26% efficiency (Amonix, Inc.). The efficiency of concentrating PV designs can reach 25-30%, and DSG thermal systems can also reach 30%. Similarly, the efficiency of dish concentrators using Stirling heat engines is also about 30%. Table 1.42 provides a summary of solar collector costs, efficiencies, and suppliers. 

Polymers have many potential applications In solar technologies that can help achieve total system cost-effectiveness. For this potential to be realized, three major parameters must be optimized cost, performance, and durability. Optimization must be achieved despite operational stresses, some of which are unique to solar technologies. This paper Identifies performance of optical elements as critical to solar system performance and summarizes the status of several optical elements flat-plate collector glazings, mirror glazings, dome enclosures, photovoltaic encapsulation, luminescent solar concentrators, and Fresnel lenses. Research and development efforts are needed to realize the full potential of polymers to reduce life-cycle solar energy conversion costs. 

---