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Hybrid PV-Thermal Roof
The most efficient energy-gathering device for a homeowner is a roof
that can both heat water and produce electricity.
By SETH MASIA
Seth Masia is deputy editor of
Solar Today. Contact him at
smasia@solartoday.org.
The arithmetic is pretty simple: In bright midday sun, roughly 1,000 watts of solar radiation falls on every square meter of the Earth’s surface. A high-quality
silicon photovoltaic (PV) module can produce about 140
watts from that energy, and a thin-film module, laminated
to a standing-seam metal roof, can produce about 65 watts
per square meter.
It’s nice to have that power, but it’s obviously only a
small fraction of the energy available. The rest is usually
lost as heat. A metal roof typically cooks up to about
120°F at noon ( 50°C), or even 160°F (70°C) in midsummer. That heat can be harvested by running hydronic
tubes behind the roof panels. Water, or an anti-freeze
solution, runs through the tubes and is heated to roughly
the temperature of the roof panels. That heat can be
used for space heating, industrial processes or domestic
hot water.
Dawn Solar Systems Inc. ( dawnsolar.com) in New
Hampshire has installed nearly 200 building-integrated
metal-roof water-heating systems, many of them faced
with UniSolar PV laminate. Bill Poleatewich, president of
Dawn Solar, notes that the heat harvested by these certi-
fied collectors can be equivalent to about 250 watts per
square meter, depending on the thermal load. At peak
sun, a hybrid industrial PV-thermal system therefore may
generate up to 4. 8 times as much total power as the thin-
film module alone. Since solar thermal energy harvests
are limited by the size of the thermal load, a residential
system may produce as little as 60 watts per square meter
of usable heat energy, still nearly double the total solar
electric harvest.
