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solar tank water heater - Page 2

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Posted by Gary on April 11, 2004, 2:59 am
Dave wrote:

The length of the shadow is:

LS = Height/ tan(Elev)

    Where LS = length of shadow
    Height = height of obstacle
    Elev = Elevation angle of sun

For example if obstacle Height = 15 ft, and sun Elev 50 deg, then
the length of the shadow is:

    LS = 15/ tan(50) = 15/1.19 = 13 ft

    Decreasing the Elev by 10 deg to 40 deg:

    LS = 15/ tan(40) = 15/0.84 = 18 ft

    Decreasing Elev another 10 deg to 30 deg:

    LS = 15/ tan(30) = 15/0.58 = 26 ft


I think you are wise not try to get your wife to change her shower time :-)

If you don't put in the extra storage tank, the water in your collector
tank might have an overnight temperature drop of about:

    Qnight loss = (Aglaz)(Tamb - Ttank)(12hr)/ Rglaz

            = (30ft^2)((120F+95F)/2 - 60F)(12hr) / 1 = 17000 BTU

    Ttank drop =  Q/(Wwater)(Cwater) = 17000/(750lb)(1) = 23F

    (This assumes 30 sq ft of glazing, a 90 gal tank/collector,
    afternoon tank temperature = 120F, and average night temperature
    of 60F.  Insulated back as sides on collector)

So, your tank temperature might drop from about 120F to about 97F
overnight.  Even with this drop, you are still getting quite a bit of
benefit from the solar heater (i.e. your hot water tank is getting 97F
water, rather than 50F(?) water it would get from the "street" supply).
For water draws later in the day, you would get fully heated water.
You could reduce the night time drop with double glazing, or with some
form of manually or automatically deployed insulation over the
tank/collector or the glazing.

Given this, I'm not sure its worth adding the complexity of pumps and
circulation loops etc???


Posted by Dave on April 11, 2004, 3:40 pm
Thanks Gary, Now thats the kind of info I need :) I've got the tank
pressure testing now after cleaning and removing the old mineral
deposits. This is the second tank as the first one is maller and from a
gas water heater and has the hole down the center for the flue. I was
thinking that tank may have some kind of advantage (like getting heat
through a 4" tube for winter heat) but I believe there are better ways
for that. I am going to start with one tank and no pumps and go from
there...I appreciate the feedback,
Gary wrote:

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