Before 1984, our solar collectors either directly generated
low pressure steam for heating, distilling alcohol, cleaning aircraft parts,
and curing concrete blocks or remotely boiled water using heat-transfer oil. When
converting thermal energy into power, higher temperature is better within the physical
limits of materials. Also, tiny hot bodies lose less energy than bigger ones
at the same temperature, so smaller is better (and often less expensive).
High performance solar collectors typically direct sunlight
hitting a large area of mirrors into cavity receivers, insulated vessels open at
one end. One optical goal for developing solar concentrators: direct as much
sunlight as possible into the smallest hole. Since the sun has a finite
diameter, sunlight reflecting off flat mirrors continues to spread and images
get larger farther away. Flat mirrors that are much smaller than a target can
reflect sunlight inside the target area but making and aligning each one
takes time. Heat or other forming larger parabolic mirrors that would reflect more than
90% of the sunlight hitting it into an opening is difficult, and probably
expensive. We found it straightforward and easy to warp flat glass mirror
facets so each delivers, some distance away, an image of the sun much smaller
than itself.
This Small Receiver Intercepts Sunlight Reflected Off a Large Mirror
Area. Note: The Receiver Allows Only Brightness from Around the Sun in the
Image. The Diameter of the Actual Receiver Opening Is Half the Diameter of the
Insulated Cylinder.
These Mirror Assemblies Directed Concentrated Sunlight Into a Receiver
That Generated Power with a Turbine
In 1983 we built our first solar collector where we bent
flat mirrors into the shape that approximated a parabolic surface. Because we wanted
our mirrors to last more than 30 years, we developed a technique to encapsulate
the silver reflective surface inside glass (to isolate the silver from the environment). We modified approaches used to manufacture insulated glass windows
that also have to perform well for a long time. These glass-mirror sandwiches (360
@16.5 x 36.5 inches in the first) used spacer strips along the two long sides between the
mirror and glass. A thinner strip of adhesive was then applied midway between
the spacer strips. This warped both the double-strength (0.12 inch thick)
backing glass and the single-strength
(0.09 inch thick) mirror so each formed shallow troughs. We used
insulated glass techniques to absorb moisture and seal the volume between the
sheets of glass. The solar image on a target at the focal length of a mirror
facet at this stage was a narrow line, brighter than the sun but longer than
the facet. An aluminum strip to attach a “pull-back” was bonded on the back of the
mirror facet down the long centerline. When mounted in a mirror assembly, the
corners of each mirror facet were fixed so facet images were superimposed on a
target at the concentrator focal length. A turnbuckle connected to the center
of each facet was then adjusted until its long solar reflection became round with
an area less than one tenth the size of a mirror facet. “Pull-backs” warped the
trough-shaped facets into parabolic shapes.
Front View of the Above Solar Collector Showing Warped Glass Mirrors
Mounted on 24 Mirror Assemblies, 12 with Extensions
In 1986 we began building a larger concentrator (3,200
square feet) that used larger glass-mirror facets (392 @ 24.5 x 48 inches). We
used the same techniques as above but integrated an aluminum frame on each
facet to make them much more rugged, able to be stacked for staging and
shipping, and easier to mount on mirror assemblies.
View of Mirror Facets
Mounted on Mirror Assemblies of the Solar Collector Shown at the Top of This
Post
Close-up Showing a Reflection of the Receiver in One of the Mirror
Facets. The Actual Receiver Opening Is Too Bright to See But the Aperture Skirt
Tubing That Protects the Receiver Structure Is Visible. Curvature of Straight Elements
in Reflections Illustrates Mirror Facet Are Slightly Curved
In 1990, in our final experiment with laminated glass
facets, we adhesively bonded a large sheet (4 x 7 feet) of double-strength
glass to a parabolic structure formed out of grid of aluminum extrusions. We
bonded a single-strength mirror to this and used insulated glass techniques to
seal the volume between the sheets of glass. This large assembly intensified
sunlight over 30 times but proved cumbersome. Safely handling such large sheets
of glass requires special fixtures and would be difficult to do without
breaking some. Losing even a small corner of either a mirror or backing
would probably require recycling it. This test also uncovered another limit:
bending glass breaks if the tension at any surface exceeds the tensile strength
of the material. Any flaw, such as a scratch or chip, concentrates local forces
and greatly reduces how much stress can be applied without breaking. Thin glass
bends easily and can safely establish a spherical (or parabolic) surface with a
much tighter radius (focal length) than thicker glass. The large mirror facet
above was aimed at concentrators larger than 6,000 square feet (able to direct
more than 500 kilowatts of sunlight into a receiver).
Worldwide, serious research into high performance solar thermal technologies
dwindled in the late 1980s. We had to find work in other arenas to put our kids
through college. In my spare time I continued to make tabletop models over the
next two decades to tease out solutions to many problems and awkward approaches we encountered in earlier work. Some of our mirror assemblies had
been outdoors since the early 1970s and most of their facets were still
in good shape more than thirty years later. With the right edge treatment and
organic coatings, thin glass mirrors can weather outdoors in upstate New York
for many decades. Since we have more than a thousand new and used glass mirrors
from earlier work, I developed a technique for mounting them without a backing
glass in an array to form a mirror assembly. The next piece will cover Mirror Assemblies That Warp Simple
Glass Mirrors.
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