Imágenes de páginas
PDF
EPUB
[blocks in formation]
[merged small][merged small][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][merged small][merged small][subsumed][subsumed][merged small][merged small][merged small][merged small][graphic]

Calculated Radial Flux Density Distribution

As A Function Of Axial Plan Position Goodyear Mirror

[merged small][merged small][ocr errors][merged small]
[graphic]
[merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

To receive 100 percent of the reflected energy, it is necessary to design a flux trap to the maximum error; however, this energy may not be realized because of increased reradiation of a larger flux trap mouth. If the trap is designed to the effective mirror rating, some energy loss is accepted.

A-14

Absorber for Solar Power

W. Richard Powell

APPENDIX B

A simple, economical absorber utilizing a new principle of operation to achieve very low reradiation losses while generating temperatures limited by material properties of quartz is described. Its performance is analyzed and indicates approximately 90% thermal efficiency and 73% conversion efficiency for an earth based unit with moderately concentrated (~tenfold) sunlight incident. It is consequently compatible with the most economic of concentrator mirrors (stamped) or mirrors deployable in space. Space applications are particularly attractive, as temperatures significantly below 300 K are possible and permit even higher conversion efficiency.

[ocr errors][merged small]

Solar energy can be converted into power directly as in a photocell or indirectly with the absorber's thermal output converted into power in a thermal engine. This work is concerned only with the absorber of an indirect converter; but to give adequate emphasis on high temperature output, a Carnot engine efficiency factor will be included. Likewise, little will be said of the concentrator optics, but for clarity a paraboloid mirror will be assumed.

In order for the absorber to be efficient, it must be black to sunlight; yet if it is very hot for good Carnot efficiency, much of the absorbed radiation may be reradiated and lost. This dilemma has led others,1,2 to utilize selective surfaces which for sunlight absorb well, a 1, and for long wavelength radiation emit poorly, e<< 1, where a and e are absorption coefficients. At present, surfaces with a/e≈ 10 for service below 500°C can be fabricated. In contrast, the virtual surface or entrance hole of the absorber described in the next section exhibits an effective a/e> 500 while producing exhaust temperatures in excess of 1000°C. This is made possible by combining old ideas into a new principle of operation-the parts of the absorber that can radiate the long wavelength radiation well are kept cool by transfering the heat they absorb to a coolant flow, which is heated further to form the high temperature exhaust. The thermal radiation from the hot parts of the absorber is prevented from reaching the entrance and escaping by economical selective surfaces and favorable geometric factors. In concept, the device is much like a set of

[blocks in formation]
[merged small][ocr errors][merged small][merged small]

Consider long, thin walled glass tube open at one end and sealed with a black cap at the other as shown in Fig. 1. The outer surface of this tube is coated with a highly reflecting silver film. A second slightly longer cylinder surrounds this mirror tube, leaving an annulus between, which is sealed at both ends except for an entrance pipe at the open end of the inner tube and an exit pipe at the closed end to form a cooling jacket for the inner tube. Surrounding this second cylinder is a layer of high temperature insulationi.e., an evacuated annulus containing many separated coaxial layers of reflecting metal foils, the innermost being the shortest and covering only the end of the inner tube most remote from the entrance and progressing in length to the outermost, which extends the full length of the absorber. Except for this detail made necessary by an axial temperature gradient as well as a radial one, this radiation shield is of conventional design and construction.

Now imagine that the closed end of this tubular assembly is pointed at the sun, while the open entrance of the central mirror tube encompasses the image of the sun formed by a large paraboloid mirror, and that a cooling gas flows in the cooling jacket to prevent the mirror tube from melting. Since, as shown in Fig. 1, the average solar photon reflects on the mirror tube walls many times before being converted into heat, the thermal flux incident on the walls of the inner tube is much less intense than the flux across the entrance. If the tubular assembly is long enough for most of the solar photons to be absorbed before

« AnteriorContinuar »