Selective solar absorbers

Randich, E., and Allred, D., CVD ZrB2 as a Selective Solar Absorber, Thin Solid Films, 83 394-398 (1981)... 

The formation of periodical structures in the nanoscale is a busy field in the physics of materials. Submicrometer stractured materials have, and are expected to have, various apphcations [1 ], like optical filters and gratings, an-tireflective surface coatings, high density data storage, selective solar absorbers, microelectronics, optical switches, waveguides with low lost, chemical and biochemical sensors and resonant cavities for small lasers. 

Westin, G., and Wackelgard, E. (2005) Low cost selective solar absorber coatings characteristics of carbon-in-silica synthesized with sol-gel technique. /, Sol-Gel Sci Technol, 36, 33-43. 

The conversion efficiency is probably the best single indicator of the potential usefulness of a selective solar absorber. The conversion efficiency is dependent on both the solar absorptance and the thermal emittance. For the ideal surface mentioned before, operating at 600 K, CE = 0.98. Only a few absorbers have been developed that give a CE(600 K) greater than 0.3 without resorting to solar concentration. Many of the known selective surfaces are unable to give a CE(600 K) greater than 0. 

Only about 10 elements, ie, Cr, Ni, Zn, Sn, In, Ag, Cd, Au, Pb, and Rh, are commercially deposited from aqueous solutions, though alloy deposition such as Cu—Zn (brass), Cu—Sn (bronze), Pb—Sn (solder), Au—Co, Sn—Ni, and Ni—Fe (permalloy) raise this number somewhat. In addition, 10—15 other elements are electrodeposited ia small-scale specialty appHcations. Typically, electrodeposited materials are crystalline, but amorphous metal alloys may also be deposited. One such amorphous alloy is Ni—Cr—P. In some cases, chemical compounds can be electrodeposited at the cathode. For example, black chrome and black molybdenum electrodeposits, both metal oxide particles ia a metallic matrix, are used for decorative purposes and as selective solar thermal absorbers (19). 

Improved selectivity of potash recovery may be obtained by high ammonia concentrations in water, but this has not been commercially adopted. Solution mining of potash has matured from the pioneering work of Kalium Chemicals at Belle Plaine, Saskatchewan [39], to encourage adoption by at least two operations formerly based on conventional mining [40]. These operations are sited to use solar evaporation for potash recovery. One uses 162 hectares (1.62 km ) of PVC-lined evaporation ponds to produce 236,000 tonnes of potassium chloride per year [29]. Experiments with solar absorbing dyes may lead to further increases in production. 

Selective Absorber - A solar absorber surface that has high absorbtance at wavelengths corresponding to that of the solar spectrum and low emittance in the infrared range. 

Selective Surface Coating - A material with high absorbtance and low emittance properties applied to or on solar absorber surfaces. 

Abstract. Usual plate solar collectors, based on a metal absorber (Cu, Al) selectively coated are technologically very sophisticated, expensive to produce and they are great consumer of fossil raw material. Polymeric materials are considered as a promising alternative for many interesting properties easy moldability, corrosion resistance, they also offer a significant cost-reduction for solar thermal collectors, and a mass production may thus benefit to a broader utilization of solar energy. Most drawbacks of polymers are their low thermal properties essentially thermal conductivity coefficient may strongly affect the solar absorber efficiency and deteriorate the collector performance. 

In the literature, several polymers have been used as absorber in flat-plate collectors. P. T. Tsilingiriss reported upon the use of the groups of polyolefin and EDPM, to overcome the undesirable effects of the poor thermal conductivity, he had exploited the design of the solar collector [4]. Polycarbonate selectively coated has been used as solar absorber plate double walled, by A. I. Kudish et al [5]. In another case, K. Sopian had developed a solar collector system using black fibreglass reinforced polyester (GFRP) [6]. 

Spectrally selective solar surfaces are commonly used for conversion of solar radiation into thermal energy. The optical properties of these surfaces are such that they possess high solar absorption (on the 400—2500 nm range) to maximize the fraction of solar energy transformed to heat, and low thermal emittance to limit infrared radiation losses. The energetic balance on a thermal solar absorber was detailed in fig 1. 

The conversion efficiency, CE, is a good criterion for evaluating a solar surface for both selectivity and absorbing effectiveness. It indicates the amount of heat that can be withdrawn at the operating temperature ... 

The spectral selectivity of a solar absorber can be judged by comparing its solar absorptance and thermal emittance with the ideal values. Because of the overlap of the solar and thermal spectra, as seen in Fig. 1, the solar absorptance of the ideal surface is limited to a maximum of about 0.99 and the total emittance (600 K) is limited to a minimum of about 0.01. 

The spectral reflectance curve of a surface can readily be compared with the ideal curve to give a visual indication of spectral selectivity. Figure 4 shows the the reflectance of the ideal solar photothermal absorber with the transition at 2 pm. The reflectance curve for silver is also included. Silver reaches a maximum reflectance of about 0.985 in the long-wavelength region, thus placing a practical limit on longwave reflectance of a solar absorber. 

There is some evidence that composites based on ZrB including ZrB -SiC and ZrB -MoSi, which have a superior oxidation resistance up to 1600°C, also possess a pronounced degree of intrinsic spectral selectivity (Bogaerts Lampert, 1983 Kennedy, 2002) which could be useful for solar absorbers. However, studies of spectral emissivity characteristics of UHTCs refer to films and coatings and not to bulk materials (either porous or dense). 

Selective absorber coatings for solar energy collectors. 

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