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This reradiation loss can be considered as either a mirror or as an absorber problem, although it has always been treated as a mirror problem.
All solar-thermal space power research programs in the past have considered that the critical and missing element of a solar thermal electric power system is the lack of a quality space deployable mirror. The current mirror design program at Boeing Company (Ref. 3) makes this same assumption. The possibility of economical, lightweight, large space power stations based on solar thermal energy is sufficiently attractive for Boeing to consider an elaborate system of servocontrols for individual segments of a large space deployable mirror. The alternative approach, ie., assume that the reradiation loss problem is an absorber problem instead of a mirror problem, has not been explored. This alternative view has not been seriously considered because of the widely accepted, but invalid belief that any absorber which absorbs sunlight well must also be an efficient radiator with large reradiation losses at high temperature.
"Absorber for Solar Power", Appendix B, is a new concept in solar absorbers which appears to essentially eliminate reradiation. It thus appears possible that the space deployable mirrors tested more than a decade ago may be more than adequate for cheap, lightweight, everlasting space power. It appears that this new absorber can be both hotter and larger than the focal spot of a perfect-mirror black-body absorber system and still have far less reradiation losses. Initial and incomplete analysis indicates that reradiation may be reduced 1000-fold! Thus it appears feasible to enlarge the absorber entrance diameter several times so as to capture all of the sunlight concentrated by imperfect but practical space deployable mirrors developed years ago.
It should be noted that, although NASA lost interest in solar thermal electric power and eventually let project SUNFLOWER die, the Brayton cycle thermal energy converter system for space power is in a highly developed state. (More than 30 million dollars 3.
has been invested by the U.S.Government and private industry.) As presently envisioned, radio isotopes rather than the sun will supply heat for Brayton-cycle space power plants. However, solar heat appears to be an attractive alternative if either a quality mirror or a reradiation free absorber could be developed. Economy, weight, pe rmancence and no radiation danger are the principal advantages for solar heat. In low orbit the occulted sun is a disadvantage for the solar system but short term energy storage (either battery or thermal) may be less massive than the isotope supply's radiation safety shield.
The proposed solar absorber provides a high a/e ratio by combining established principles into a new mode of operation. High efficiency can be maintained with uncritically focused sunlight, allowing for realistically attainable optical collectors.
As illustrated in Figure 1, the proposed solar absorber basically consists of a cavity whose length is significantly greater than its diameter and which is cooled at its entrance end by a mass fluid flow through the entrance end. The fluid extracts heat from the walls of the cavity absorber during passage through the cavity. The heated fluid is removed from the absorber at the end remote from the entrance. The energy in the heated fluid can either be stored or directly utilized for power generation. Fluid flowing through the absorber is in thermal contact with the interior walls of the absorber and creates a temperature gradient in the cavity with the temperature being higher at the closed end of the absorber than at the entrance end. Therefore the entrance end of the absorber can be held at a relatively low temperature which significantly reduces reradiation losses from the absorber. But temperatures at and near the remote end of the absorber can be relatively high because most of the solar energy entering the absorber is absorbed deep in the cavity after many reflections on its shiny mirror walls.
Heating of the mass of fluid flowing through the cavity to a usefully high level occurs while reradiati on losses from the remote hot sections of the cavity are greatly reduced because the mirror walls do not permit the heat radiation energy to "mirror" its way back out and escape. That is, the cavity walls are like a second surface glass mirror in that solar radiation is reflected ever deeper into the tubular cavity but heat radiation cannot "see" through the glass-like wall to be reflected and is consequently reabsorbed within the cavity with little chance for escape. Folded optics, not illustrated in Figure 1, can also be used to prevent any direct path for the escape of heat radiation.
Inexpensive, long-lasting power systems are essential to future space missions. Low lift-off weight, immunity to radiation degradation, and uncritical modular designs should be the features of future power systems, especially for Shuttle users.
The economics of the Shuttle are that experimental and operational costs for Shuttle payloads can be reduced by not requiring payloads to be as intensively designed and configured as with conventional launch vehicles. One facet of this economic advantage is that "commercial" equipment could be used in Shuttle payloads. However these equipments would not be the efficient (and costly) "power-misers" specially designed and packaged for space use. Equipment for commercial use was designed with the assumption that unlimited electric power was available at the wall socket. Intrinsic therefore in the Shuttle program concept of economical, non-custom design instruments and equipment is the provision for reasonably large self-contained power modules with the possibility that these modules could be left in orbit to reprocess waste water and store hydrogen and oxygen for the next space mission. That is, not only could a solar based power plant
immune to radiation damage function as the electric utility for a series of Shuttle experiments with high power demands but it could also function as a fuel production depot. However, all of these concepts depend upon the success of the new absorber if the expense, weight and radiation degradation problems of solar cells are to be avoided.
Preprototype Solar Collector, 15 KW Solar Mechanical
Feasibility of Large-Scale Orbital Solar/Thermal Power