HIGHLIGHTS FROM SOLAR TODAY’s E-BULLETIN FOR SOLAR PROS.
Low-Tech Concentrator Can Boost Collector Efficiency
By RoB Stout
Aconcentrating reflector surface can effectively double the radiant energy falling on a water-heating collector, used for space heating, domestic hot water or both. A flat,
fixed-position reflector placed above the collector, so as to maximize midwinter gain, can
also provide partial shade in summer to reduce the risk of overheating. This is especially
useful in desert climates.
Reflector design can be simple and robust. The
patent-pending design described here is made
of flat sheet metal with a Reflec Tech (reflectech
solar.com) mirror-film surface, supported on a rack
made of L-angle braces. The roof should be reinforced to bear the
wind load, so it’s strongly recommended that the roof and reflector
be engineered at the same time.
Determining Collector Angle Can Be Tricky
The angle of the collecting surface is important in maximizing
the energy absorbed by the system. The ideal angle is perpendicular
to the sun during the heating season. Panels are sometimes laid flat
on the roof for aesthetic reasons. While that look is less intrusive, the
roof is rarely steep enough for appropriate seasonal collection. The winter heating and hot water require-
ments are three times that of summer. In order to match this seasonal energy demand, a
steeper panel angle is required to provide the additional heat in winter. This is even more
pronounced in higher latitudes, as the sun is lower in the sky. A rule of thumb when deter-
mining the collector’s angle is latitude plus 15 to 20 degrees to maximize solar collection
during the heating season. At this angle, there is extra heat in summer beyond the needs
of the domestic hot water system. This heat has to be transferred somewhere, or the pan-
els will continue to get hotter until the pressure relief valve opens and spews antifreeze all
over. The system then needs to be recharged with antifreeze. Summer overheating can
be avoided by adding a summer dump loop, but this adds complexity and cost. Another
way to deal with the extra heat is by tipping the panel toward the lower midwinter sun at
30 to 35 degrees plus the latitude, but this gives up some collection efficiency in winter. I
have used this strategy for many years to simplify the system, always looking for a better
way to balance the solar collection with the seasonal heating requirements.
Concentrating Panel Offers Solution
Concentrating the sun’s energy with a reflector is a way to increase winter collection
while providing only enough heat in summer for the domestic hot water. This is done
by tilting the panel perpendicular to the winter sun ( 15 to 20 degrees plus latitude) and
adding a mirror to the top of the panel. The effective collection area is increased in winter,
and the overhanging reflector casts a partial shadow on the panel in summer to protect
from overheating. Correct proportions enable the system to produce enough heat in
summer for domestic hot water and double the energy of the panels in winter.
the author’s concentrator panel boosts efficiency in the winter and
prevents overheating in
ob Stout (solarrs@
mail.com) has been
esigning solar homes
nce the 1970s. He has
egrees in engineering
d architecture. The
n provides 95 percent
the energy to run his
-year-old house. He
aches green architec-re at Santa Fe Com-unity College and
oes a radio show on
stainable living. His
rrent design work is at
very near zero energy.
Get more details on the author’s concentrating reflector system at solartoday.org/sw, where
you can read this article in full. While you’re there, sign up for your free subscription to Solar@ Work.
66 July/August 2010 SOLAR TODAY solartoday.org
Copyright © 2010 by the American Solar Energy Society Inc. All rights reserved.