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Solar Hot Air
I’ve recently built my own 4-by-8
box space heater. Due to location,
it sits alongside my house and is
ducted through holes in the wall.
I am using a bathroom exhaust
fan (plugged into the grid) at 70
cfm to pull the heated air into the
house. I have four hours of sun
in the morning to use the heater.
This is giving me a 4°F increase
in temperature (from 65°F to 69°F),
when outside it has been in the high
20s to 30s. I’ve been looking for a
fan and a PV module to power it that
would be strong enough to circulate
the air. I’ve seen the DC Venturi
fans, but don’t know if they can do
the job. The ducts are 4 inches in
diameter. The 70 cfm is not enough.
I’m trying to heat a 24- by 20-foot
room, and believe I need about 500
cfm minimum.
I’m also planning ahead for
summer and have been looking at
the solar-powered swamp coolers.
Have there been any studies done
on these as far as efficiency, value,
and cost? I was wondering if I could
convert my standard home-model
swamp cooler into a solar-powered
setup. Maybe this subject could be in
a future magazine article. Any help or
ideas would be appreciated.
Dennis Mann • Reno, Nevada
Hi Dennis, A 4- by 8-foot collector should
have 6-inch duct openings. The limitation
of 4-inch ducts is huge. It has to do with
static pressure, and a bigger blower might
just make things worse, depending on the
curve of the blower. A 4-inch duct has
less than half the cross-sectional area of a
6-inch duct and makes the friction loss of
the moving air (static pressure) too large.
All blowers have different performance
curves and some are so poor that their
cut-off static pressure will not allow them
to be used on a good solar collector, which
should have some static pressure built
into it.
On 4-by-8 collectors, we use a 1/12 hp,
AC-powered, 380 cfm permanent splitcapacitor
blower with 6-inch ducts. We
have a few thousand of these installed, and
time has proven the match is a good one.
A 176 cfm, 12 VDC blower is available
for about US$90. It takes a 50 to 75 W PV
module to run it. It doesn’t perform quite as
well as the 380 cfm AC blower, but it does
a good job. We use it for PV power systems
and have never had a complaint. As I said
at the start though, the big problem in your
system is the inlet and outlet size—they are
too small for the size of collector.
A well-built 4-by-8 collector with
adequate airflow should have about a 30°F
to 70°F rise in temperature, inlet to outlet.
The temperature difference is dependent
on the time of day and year, the outside
temperature, and flow through the collector.
In spring, it will produce a good deal more
heat than in the middle of winter.
Southwest Solar (www.southwest-solar.com) manufactures DC swamp coolers in a
variety of sizes (650–4,000 cfm.) Both 12
and 24 VDC models are available, as well
as 120 and 240 VAC units. You could
probably also convert your AC cooler by
using a DC pump and fan. Please be sure to
let us know what you end up doing and how
it worked out.
Chuck Marken • AAA Solar
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