Collector materials glazings

Polymeric materials are used In all solar technologies. In addition to such conventional applications as adhesives, coatings, moisture barriers, electrical and thermal Insulation, and structural members, polymers are used as optical components In solar systems. Mirrors on parabolic troughs are made up of metallized fluoropolymers and acrylics. Commercial flat-plate collectors are glazed with fluoropolymers and ultraviolet-stabilized polyester/ glass fiber composites. Photovoltaic (PV) cell arrays are encap-... 

The ideal glazing would transmit all the solar energy into the collector and, at the same time, reflect the long-wave reradiation from the collector. Research with glazing materials to improve the transmittance and reduce reradiation losses could improve the economics of a solar heat collector. It is, therefore, recommended that these two avenues of investigation be considered by researchers in this field. 

Materials cost reduction has been achieved through the use of thln-fllm polymeric materials In both the absorber and glazing portions of the collector. The films, attached to a lightweight bent-metal frame, form a set of stressed membranes that contribute to the overall strength of the panel. 

Elastomeric materials are used in thermal solar collectors as gasket and caulking compounds. In addition to these sealant applications, polymeric materials are also widely employed as thermal insulation and occasionally as glazing and frame components. This paper provides a supplement and a continuation of a previously reported study( 1) of the endurances of several commercially available sealants to the harsh environment of the collector. 

Glazing - Transparent or translucent material (glass or plastic) used to admit iight and/or to reduce heat loss used for building windows, skyiights, or greenhouses, or for covering the aperture of a solar collector. 

Soiar Distiiiation - The process of distilling (purifying) water using solar energy. Water can be placed in an air tight solar collector with a sloped glazing material, and as it heats and evaporates, distilled water condenses on the collector glazing, and runs down where it can be collected in a tray. 

The glazing (cover plate) material for thermal solar collectors has to be able to resist wind, snow, and gravitational loads over the range of temperatures achieved by the collector. This can be in the range of —40 to 120°C in normal operation and as high as 200°C when the fluid is not flowing in the absorber. 

Besides durability, premium sealants are judged by special properties as shown in Table 4. The ability to take on greater elongation and compression is measured by movement capability in terms of joint width. The stability to UV exposure is important for those glazing and insulation compounds used in modern high-rise structures. Thermal stability is in demand for solar collectors, or for other structural materials. On the basis of these evaluations, we can foresee future trends of sealants as shown in Table 4. Silicones appear to out-perform others. In the meantime, technical advances will provide low-modulus polysulfides, and better movement ability for both polysulfides and polyurethanes. Their cure time will be decreased and the UV stability will be improved to match or compete with silicones. All three will be developed for better adhesion under the un-primed conditions. 

Applications for FEF include chemical process pipe linings, wire and cable, and solar collector glazing. A material similar to FEF, Hostaflaon TFB (Hoechst) is a terpolymer of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride. 

The cost of solar collectors can be reduced by using polymeric glazing and absorber materials, but the durability of their optical and mechanical properties must be demonstrated. Polycarbonate glazings with ultraviolet screening layers have been shown to survive accelerated light intensity for an equivalent 20 years outdoor exposure in Miami, FL. The mechanical properties of two candidate absorber materials— metallocene-based multi-density polyethylene and polypropylene— have been measured as a function of wet and dry thermal exposure. 

To assess the thermal stabihty of candidate polymeric absorber materials experiencing elevated temperatures associated with glazed collectors, mechanical propsties (tensile modulus and strength, and strain at break) arc measured as a function of time of thermal exposure to dry air and to heated deionized water. Samples are prqjared and tested as po- ASTM D638. Based on the cMculated sovice tempaature distribution, samples were tested for wet exposure at 60X 75X and 90 C. Measurements were made after 1, 3, and 6 months of exposure. 

We surveyed and screened a number of candidate glazing matmals (5,6), which include a varirty of polyesters, fluoropolymers, and polycarbonates (PC), as well as acryhc, polyetherinride, poly ylene (PE), polystyrene, and polyvinyl chloride. Bas on accelerated screaiing tests, we find that fluoropolymers and acrylic arc UV weatherable, and all other materials tested lose transmittance and yellow. However, fluoropolymers are relatively expensive and realistically would be limited to use as thin-film glazings. Acryhc tends to be too brittle and exhibits thomal sag as a glazing candidate for solar collector apphcations. 

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