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Capacity calculation questions...? - Page 2

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Posted by SJC on November 28, 2005, 4:34 am
You can get a general idea of efficiency of flat plates collectors by looking at
this link:
Evacuated tubes (the Dallas link I posted) are much higher efficiency at low air
and high water temperatures than flat plates, because they use a vacuum for

Hey Guys:

Thanks for the information and links, that should keep me busy for a few
days... especially since tomorrow its "back to the day job..."

BTW, the panels are "hand me downs."  I have no idea who the
manufacturer is or was and doubt I could ever find out.

The 65 deg F mentioned in the example was the estimated entering water
temperature at start up.  I didn't know where to start so I just used
the temperature of the tap water I would be filling the system with.

I will follow the links and do some more reading,  maybe I can ask
better questions next time...


SJC wrote:

Posted by nicksanspam on November 28, 2005, 11:51 am


The air temp doesn't matter much with evacuated tubes. Their efficiency
isn't 100%, but their vacuum insulation is so good that they act like
solar current sources.

NREL data indicate January is the worst-case month for solar heating in
Ft. Worth, when 910 Btu/ft^2 of sun falls on the ground and 1260 falls on
a south wall on an average 43.4 F day with a 54.1 daily max. That's
sqrt(910^2+1260^2) = 1554 Btu/ft^2 at an atan(1260/910) = 54 degree tilt.

At 100% solar collection efficiency :-)

A square foot of flat plate collector with an ordinary surface with 2 layers
of R1 glazing with 90% solar transmission and a 100 F temp diff might collect
1554x0.9x0.9 = 1259 Btu and lose 6hx100x1ft^2/R2 = 300, for a net gain of 959
Btu/ft^2 per day.

If you started every day with cold tank water, or had a temperature-
stratified tank with cold water in the bottom and hot water on top.

You might find the tank temp on an average day, given average weather and
an average hot water demand, and store heat for a few cloudy days.

Ideally, you can turn it off with no damage in summertime. July is the
warmest month in Ft. Worth, when 2220 Btu/ft^2 falls on the ground and
720 falls on a south wall on an average 85.3 day with a 96.5 max. Harry
Thomason suggested raising the differential thermostat's threshold in
summertime so the pump runs less often and uses less electricity.


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