When Allison Gray’s away from her desk, it’s a good bet she is positioning mirrors, leveling lasers and entering data at an unadorned
warehouse in an industrial district of Arvada, Colo. The warehouse
is an outbuilding of the National Renewable Energy Laboratory
(NREL), and Gray, an NREL engineer, is doing the legwork that
helps ensure the nation’s concentrating solar power (CSP) plants are
operating at peak performance.
In the day-to-day, Gray quantifies the inaccuracies of the surfaces of mirrors, or facets, used in parabolic trough, power tower, compact linear fresnel reflector (CLFR) and dish-engine CSP technologies.
In some CSP applications, the reflective devices — mirror and all — are known as heliostats. Using
checkerboard-like grids, lasers and cameras, Gray measures sample facets for surface accuracy (not
reflectivity), to make sure light is being redirected to the right places. If a surface has too many inaccuracies, the light striking the facet will fail to reach its designed destination. The destination is usually
a receiver, which absorbs the sunlight, transferring its energy to a heat-transfer fluid that eventually
turns a turbine. In a real-world CSP facility, errant light could result in lost electricity.
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“It’s like a geometry problem,” Gray said of her tests. “We know the output angle of the laser, where
the laser is in space and we think we know the intersection point on the facet. So, we know where it
ideally should strike the target. Then, we measure where it actually strikes the target.” The mirror’s
surface accuracy is the difference between the ideal striking location and the actual, she said. Today,
using the laboratory’s Visual Scanning Hartmann Optical Tester (VSHO T), Gray is walking through
a test of a Flabeg RP 2 facet, a 20-square-foot (2-square-meter) mirror panel used in parabolic troughs.
At about 0.15 inches ( 4 millimeters) thick, it’s surprisingly flimsy and feels only a little more rigid than
a typical laminated restaurant menu. Other than a few special protective coatings (for weatherability)
and its slightly bent shape, there’s little to distinguish it from a household mirror. But the RP 2 is a
proven reflector — most parabolic trough CSP plants in operation today use glass mirror panels from
Germany-based Flabeg. Gray’s test facet is essentially identical to those used at the 354-megawatt
(MW) Solar Energy Generating Systems plants in California and the 75-M W Nevada Solar One plant
A Day in the
Solar Work Life
Honing in on
Heliostats
Allison Gray,
CSP Engineer I
by MIKE KOSHMRL
Mike Koshmrl is an associate editor of SOLAR TODAY.
Contact him at mkoshmrl@solartoday.org. Allison Gray
is a member of the ASES board of directors. Contact
her at allisongray2@ases.org.
Concentrating solar power engineer Allison Gray uses the Visual Scanning Hartmann Optical Tester
(VSHOT) software at National Renewable Energy Laboratory’s (NREL) optical lab. In the background is
NREL’s VSHOT camera calibration grid.
