Two heliostats representing the state-of-the-art in glass-metal designs Ibr central receiver (and photovoltaic tracking) applications were tested and evaluated at the National Solar Thermal Test Facility in Albuquerque, New Mexico from 1986 to 1992. These heliostats have collection areas of 148 and 2(X) m2 and represent lowcost designs for heliostats that employ glass-metal mirrors.The evaluation encompassed the pcrfom_ance and operational characteristics of the heliostats, and examined heliostal beam quality, lhc effect of elevated wincks on beam quality, helioslat drives and controls, mirror mtxlule reflectance and durability, and the overall operational and maintenance characteristics of lhc two heliostats.A comprehensive presentalion of lhc results of these and other tests is presented. The resulls arc prefaced by a review of the development (in the United States) of helioslat leclmology.
MTEP,_iSTFitBLITION OF THiS DOOUMENT 16 UNLIMITED ACKNOWLEDGMENTS The authors are indebted to .lake VanDerGeest tbr his enthusiastic and tireless assistance in the testing and evalualion of the prototype heliostats. We are grateful lo Daniel Alpert who guided our efforts and provided invaluable assistance in the interpretation of our test results, including the performance of HELIOS code simulations of collector performance. Our thanks also go to Bob Edgar, Sam Dunk.in, "JJ" Kelton, Larry Yellowhorse, and Winn Erdman for their assistance in numerous technical tasks. The manufacturers of the large-area heliostats provided valuable technical supporl and many useful suggestions throughout the evaluation period; we thank David Gorman (Advanced Thermal Systems, Inc.) and Daryl Bielenberg (Solar Power Engineering Company, Inc.).
The Video Scanning hart ma^ Optical Tester (VSHOT) is a slope-measuring tool for large, imprecise reflectors. To study the accuracy of VSHOT as well as its sensitivity to changes in test setup variables, a series of experiments were performed with a very precise, astronomical-grade mirror. The slope errors of the reference mirror were much smaller than the resolution of the VSHOT, so that any measured slope errors were caused by the instrument itself rather than the mirror. The VSHOT exceeded its accuracy goals by achieving about io.5% (68% confidence) error in the determination of focal length and B.1 mrad (68% confidence) error in the determination of RMS slope error.
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