Testing and evaluation of large-area heliostats for solar thermal applications

Abstract

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.).

Wind gust distribution analysis and potential effects on heliostat service life

“…An earlier test conducted at Sandia (Strachan and Houser, 1993) shows the heliostat dynamic response to winds for the 200 m 2 SPECO heliostat; the reported beam movement for that test showed a beam centroid variation of about +-4.65 milli-radians. The design requirement was that the heliostat have a pointing error of the beam less than about 1.5 milli-radians for winds less than 10 mph (4.47 m/s) and 2.0 milli-radians for winds up to 35 mph (15.6 m/s).…”

“…They showed a variation of the Strouhal number multiplied by 2 as a function of Reynolds number that ranged from about 1 to 1.5; that is, the conventional Strouhal number was about 0.16 to 0.24. There are comprehensive wind tunnel tests of model heliostats (e.g., Peterka, et al, 1989), CFD models (Wang, et al, 2009), and field tests (Strachan andHouser, 1993, Ho, et al, 2012) that address various aspects of heliostat wind induced loads together with the wind speeds and their transients. The maximum static azimuth moment is typically determined for near-vertical heliostats canted into the wind with angles of attack in the range of about 20 degrees (Heller and Peters, 1989).…”

An investigation on factors influencing dust accumulation on CSP mirrors

“…The production drops from 100% to 40% of the clean collector production level. The degradation of heliostat efficiencies caused by dust accumulation has been studied by Strachan, who has pointed out an average reflectivity decrease of 6.3 and 8.8% respectively for the two types of heliostats investigated in their study [9]. Given a thermal energy loss production of 1.2% for each 1% point of reflectivity drop, the identification of the optimal balance between more and less O&M activities (cleaning) represents an important aspect for CST plant economic feasibility [10].…”

Optical performance of the TBC-2 solar collector before and after the 1993 mirror lustering

“…This report presents the results of the dish's optical performance before and after this lustering process. The tests described in this report were performed using Sandia's Beam Characterization System (BCS) [ 1,2], as well as cold water calorimetry (CWC). The BCS facilitates the measurement of the flux in the dish's focal region; it includes a diffusively reflective flux target that is positioned in the focal region, and a digital camera that is used to capture an image of the flux reflected from the target.…”