Stoves and furnaces – Solar heat collector – With concentrating reflector
Reexamination Certificate
2000-06-19
2001-03-27
Yeung, James C. (Department: 3743)
Stoves and furnaces
Solar heat collector
With concentrating reflector
C126S694000, C359S869000
Reexamination Certificate
active
06205998
ABSTRACT:
The present invention is directed generally to a solar concentrator having a gap between a primary reflector and a solar receiver. In particular the invention is directed to a solar concentrator having an optimized positioning of a receiver (collector) relative to a primary reflector and a nearby groove or cavity in the profile of the primary reflector.
The need to design a new generation of nonimaging solar concentrators has seen renewed interest in the so-called “gap problem”—that is, how to design near ideal concentrators in which the reflector must accommodate a space or gap around the absorber for thermal isolation or mechanical reasons. This has always been of very great practical significance. The subject dates back almost 20 years, and some of the early developments have substantial disadvantages.
Prior art nonimaging solar concentrator designs have typically required the reflector and absorber to touch. The reason for this is compelling and worth recounting. These designs forced the input and output etendue to match by the judicious use of reflectors. The argument is as follows: The etendue can be expressed in terms of optical path lengths along maximum angle rays (the “edge rays”) both at the input to and at the output of the concentrator. The output rays are, in turn, incident on the absorber. This relationship is very general and makes use of the Hilbert Integral, a concept borrowed from the calculus of variations and adapted to optics. The etendue matching requirement is satisfied by making sure the optical paths lengths of the extreme edge rays, which are at the ends of the input and output, are the same. This is accomplished by mirror segments at both input and output which bisect the angles between the extreme edge rays. One extreme ray is reflected into the other, causing it to retrace the same optical path. In this way the equality of optical paths is enforced by making the paths identical. This methodology requires mirror segments contiguous with input and output and in particular touching the absorber. However, practical considerations dictated a gap between absorber and reflector. The outlook as a result of the Hilbert Integral theorem was that interposing a gap would result in loss of optical throughput, loss of concentration or both.
For solar thermal concentrators a physical space or “gap” was needed between the hot absorber and the reflector to insulate the absorber, with a vacuum space or with air, or simply to prevent damage to the reflector. This practical need for a gap led to various attempts at modifying the design with the aim of reducing or eliminating potential loss of radiation through the gap while maintaining close to maximum concentration. An important step was taken in developing a loss-less W-shaped cavity with gap (g) up to (g+r)≈2 r, where r is the radius of the cylindrical absorber. This provides proof of the existence of a loss-less solution, but of course, one gives up light concentration. Then, a microstructure was found that redirects all radiation outside the angular substance of the absorber into the absorber. This progressed to the case of g=r where the exterior etendue just matches the etendue subtended by the receiver. This limiting case is straightforward: for g=r the angle subtended by the absorber at the cavity wall is 60°. The etendue subtended by the absorber is (up to constant factors) 2 sin 30° while the exterior etendue is (up to constant factors) 2 (1−sin 30°). These are equal. Of course, one still gives up some concentration because the requirements that all radiation outside the angular substance of the absorber is certainly sufficient, but is more than necessary. Some portions of the exterior etendue are empty because light rays from the sky will not reach it. Nevertheless, this solution is practical because all the elements of the microstructure are identical. The structures are V-shaped and are designed by the conventional method of images. In the large-gap limit g=r, and the micro-grooves have 120° opening angle. Practical implementations usually needed are only a single V which works perfectly up to g≈0.27 r and quite well for even larger gaps. However, the requirement of zero optical loss is unduly restrictive since it results in significant loss of concentration. Various design approaches optionally permit maintaining maximal concentration while accepting optical losses, eliminating optical losses at the expense of concentration or some compromise between these two extremes. It is even possible to design to a virtual absorber which is larger than the physical absorber, thereby producing even more than ideal concentration. Of course this is at the expense of loss of optical throughput as radiation escapes through the gap. This became known as the “ice cream cone” design from it's suggestive shape. What is needed is an approach to optimize the trade-off between optical loss and concentration. This trade-off should also consider economic and manufacturability issues.
It is therefore an object of the invention to provide an improved solar concentrator design.
It is another object of the invention to provide a novel solar concentrator gap design.
It is another object of the invention to provide an improved solar concentrator having a particular spatial relationship between a primary reflector, coupled V-notch portion and a solar receiver.
It is a further object of the invention to provide a novel solar concentrator having a receiver's real space position defined by forming a mirror image based on a tangent to a virtual receiver's surface, the tangent line defined by connecting two points (in a two dimensional construct) to establish the end of the primary reflector and the beginning of the V-notch.
It is an additional object of the invention to provide an improved solar concentrator having a receiver's position defined by a mirror image of a virtual receiver across one edge (or plane in three dimensions) of a V-notch and a tangent to the vertical receiver.
Other objects and advantages of the invention will become apparent from the description herein and drawings described.
REFERENCES:
patent: 4230095 (1980-10-01), Winston
patent: 4359265 (1982-11-01), Winston
patent: 4387961 (1983-06-01), Winston
patent: 4419984 (1983-12-01), McIntire
Foley & Lardner
Rechtin Michael D.
Solar Enterprises International, LLC
Yeung James C.
LandOfFree
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