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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.
Absorber for Solar Power
W. Richard Powell
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.
nested porous greenhouses with air flowing inward Solar energy can be converted into power directly through them to keep the outermost greenhouse cool as in a photocell or indirectly with the absorber's and to remove heat from the innermost greenhouse. thermal output converted into power in a thermal engine. This work is concerned only with the absorber II. Tubular Absorber of an indirect converter; but to give adequate emphasis on high temperature output, a Carnot engine effi- A. Description ciency factor will be included. Likewise, little will be
Consider a long, thin walled glass tube open at one said of the concentrator optics, but for clarity a paraboloid mirror will be assumed.
end and sealed with a black cap at the other as shown
in Fig. 1. The outer surface of this tube is coated In order for the absorber to be efficient, it must be black to sunlight; yet if it is very hot for good Carnot
with a highly reflecting silver film. A second slightly efficiency, much of the absorbed radiation may be
longer cylinder surrounds this mirror tube, leaving an reradiated and lost. This dilemma has led others, 1,2
annulus between, which is sealed at both ends except to utilize selective surfaces which for sunlight absorb
for an entrance pipe at the open end of the inner tube
and an exit pipe at the closed end to form a cooling well, a % 1, and for long wavelength radiation emit poorly, e « 1, where a and e are absorption coeffi
jacket for the inner tube. Surrounding this second
cylinder is a layer of high temperature insulationcients. At present, surfaces with ale z 10 for service below 500°C can be fabricated. In contrast, the vir
i.e., an evacuated annulus containing many separated tual surface or entrance hole of the absorber de.
coaxial layers of reflecting metal foils, the innermost scribed in the next section exhibits an effective ale >
being the shortest and covering only the end of the
inner tube most remote from the entrance and pro500 while producing exhaust temperatures in excess of 1000°C. This is made possible by combining old
gressing in length to the outermost, which extends
the full length of the absorber. Except for this detail ideas into a new principle of operation-the parts of the absorber that can radiate the long wavelength ra
made necessary by an axial temperature gradient as diation well are kept cool by transfering the heat they
well as a radial one, this radiation shield is of convenabsorb to a coolant flow, which is heated further to
tional design and construction. form the high temperature exhaust. The thermal ra
Now imagine that the closed end of this tubular asdiation from the hot parts of the absorber is prevent
sembly is pointed at the sun, while the open entrance
of the central mirror tube encompasses the image of ed from reaching the entrance and escaping by economical selective surfaces and favorable geometric
the sun formed by a large paraboloid mirror, and that factors. In concept, the device is much like a set of
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 The author is with Johns Hopkins University, Applied Physics
inner tube is much less intense than the flux across Laboratory, Silver Spring, Maryland 20910.
the entrance. If the tubular assembly is long enough Received 26 February 1974.
for most of the solar photons to be absorbed before
APPLIED OPTICS / Vol. 13, No. 10 / October 1974