[Greenbuilding] distance limitation for delivering active, solar
gary at BuildItSolar.com
Sun Mar 29 21:28:10 CDT 2009
Re: distance limitation for delivering active solar
I have a setup somewhat similar to what you want to do:
In my case the collector bank is about 120 ft from the house, and the
420 gallon storage tank is just under the collectors.
Some things to think about:
-- 300 ft is a long ways -- I think it is pushing it, but it may be
workable with very very good insulation. This is the insulation scheme
I think this is a pretty good scheme for getting high R values at a
reasonable cost, and its pretty easy to install. You would want to have
much more thickness than I used. You also want to have some insulation
between the supply and return pipes.
-- I would use PEX instead of the CPVC I used -- too many joints in the
CPVC. Be sure to pressure test before you backfill :)
-- You need to get the trench below the frost level in your area -- if
you are in a cold climate, this will make the trench a pretty time
consuming digging process. Tree roots etc make it even worse.
-- You need to put a lot of thought and some calculation into the size
of pipe and the size of pump you use to make the heat transfer from the
tank to the house. Among the things to consider: loss from the
underground pipe depends on how much of the time you run hot fluid
through it, high flow rates minimize transfer times losses, but the
house may not be able to use that rate of heat delivery. High flow
rates mean either big diameter pipes, or high pumping energy. Big
diameter pipes leave a lot of water in the pipes after pump shuts off,
and the heat in the water left in the pipes is quickly lost to the earth
even with good insulation (600 ft of 1 inch pipe holds about 24 gallons,
so this is no insignificant).
I think that its very unlikely that thermosyphon flow rates are going to
-- I did some logging of temperature loss over my 120ft pipe. One
Tout from tank = 101.9F, Tin to house = 101.7F
Tout from house = 92.8F, Tin to tank = 92.4F
So, there was a 0.2F drop in temp from the tank to the house,
and, a 0.4F drop from the house back to the tank (not sure why the
So, a total loss of 0.2F + 0.4F = 0.6F on the round trip.
This may not seem like much, but typically the floor loops in the house
only drop the temperature about 10F, so the 0.6F loss represents 6% of
the heat delivered to the house. If I had it to do over, I would
increase the insulation levels to cut this 6% down somewhat. I also
have a higher flow rate pump to put in, but have not done so yet.
Note that these measurements are pushing the capabilities of my loggers,
but I did make sure that the 4 thermistor sensors I used gave nearly
identical readings when dipped in the same bath of 100F water.
-- I would do a careful solar survey at the location you want to put the
collectors. You don't want to go through all this work to find out the
collectors are shaded half the day through Dec and Jan.
This 300 ft away collector seems like a stretch to me -- is there not
some simpler way to get this done? Maybe this is a case where you
concentrate extra hard on the efficiency of the thermal envelope, and do
without the active solar?
Or, maybe you just let the collectors heat the water in the storage tank
during the day, and then truck the whole tank up to the house at night
to harvest the heat :)
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