Technology
Production set for solar energy collector Seven firms will make first industrial versions of compound parabolic concentrator developed at Argonne National Lab Argonne National Laboratory has awarded contracts worth $230,000 to seven firms for the production of prototype solar energy collectors. They will be the first industrial versions of the compound parabolic concentrator that has been under development at Argonne for the past year (C&EN, Aug. 5, 1974, page 18). Production of the prototypes is in line with the Energy Research & Development Administration's policy of fostering greater industrial participation in solar energy technology, and will provide the developers with commercially produced collectors that can be evaluated under industrial constraints. The compound parabolic concentrator was first proposed by Dr. Roland Winston of the Enrico Fermi Institute and the physics department at the University of Chicago. His original interest was in devising a means to detect the faint Cherenkov radiation produced in some nuclear reactions. A subsequent private conversation with Argonne's director, Dr. Robert G. Sachs, led to the adaption of the collector for solar energy and a development project funded by ERDA. Unlike most solar energy collectors Argonne researcher adjusts prototype
that must track the sun to keep the sun's image focused on the absorbers, the concentrator may be stationary. This results in very simple designs that do not require complex tracking machinery. The device is made in long channels with each wall of the channels being a portion of a parabolic surface. The versions developed at Argonne are made of a stable epoxy material on which aluminum is vapor-deposited to provide the mirror surfaces. Because of the large collection angle and the peculiarities of surface geometry, the solar energy collector can operate effectively with only diffuse sunlight. It can concentrate the sunlight by a factor depending on the relative input and output areas of the channels. The two versions exhibited by Argonne have concentration multipliers of 3 and 10. For a given solar flux, the higher the concentration multiplier, the smaller the required collector area. A bank of the collectors, about 1 sq m in area, will provide an average daily output of about 500 watts in the Chicago area. Both versions at Argonne are covered with a glass sheet to keep the collectors clean and to aid in trapping heat. In discussing the potential for the collectors, Dr. John H. Martin, senior physicist at Argonne, tells C&EN that the collection multiplier could be increased significantly over the present value of 10. Values of 100 or more theoretically are possible with the basic design. However, very high values may be limited by the nature of the collector surfaces. As the multipliers get higher, the surfaces must be extremely
smooth. With conventional reflecting surfaces, there appears to be a practical limit of about 50. Martin also notes that the collector is not limited to producing heat. It also may be used to concentrate sunlight on a photovoltaic surface with the corresponding enhancement of the electrical output and, possibly, an improvement in electrical generating efficiency. Much depends on the photovoltaic surface used. At present, silicon is the most commonly used material, with a maximum efficiency of about 14% in existing commercial units. This might be improved with the Argonne collector and/or with a better material. The collector test units at Argonne are equipped with heat absorbers in the bottom of each channel. Underlying the absorbers is a network of channels through which water is pumped to provide the heat sink. The test units easily can heat water to the boiling point in a single pass. With larger units, or with units having higher multipliers, there is no doubt that the collectors could produce steam easily. Coincident with the exhibition of the compound parabolic concentrator at Argonne, the ERDA-Honeywell mobile solar energy laboratory demonstrated the use of a flat-plate collector to heat and cool two mobile vans. One of the problems with the flat-plate collector is the low temperature of the water produced. It is high enough to operate a small absorption refrigeration unit but not high enough to run a small Rankine engine. If the Argonne collector were used in place of the flat-plate collector, a Honeywell representative speculates, sufficiently high temperatures would be available to run the model of compound parabolic concentrator Rankine unit. This would be a quantum jump in the application of solar collectors to space heating and cooling. In the near future, scientists at Argonne believe that A-frame buildings could use the compound parabolic concentrators for space heating and cooling at an investment only marginally greater than that for a conventional system. The biggest impediment in solar systems at present is not technology, but educating architects, bankers, and the public to the value of solar heating and cooling. Technologists have estimated that about half the heating and cooling in the Chicago area could be taken care of with solarpowered units. The investment per dwelling unit is about $5000. If the solar units can be mass produced successfully, the price will drop. D
