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re: looking for solar heater for open stock tank
13 aug 1999
kristen schulz wrote re:
>...a small, outdoor solar powered water heater for use in an
>open stock tank for watering horses.
if the tank were refilled with a water supply and a float valve,
a small hole in the bottom might help...
>my horses are far enough from the main barn that i'd rather not have to
>dig trenches and run wire/outlet to the turnout areas to plug in a
>regular portable water heater... i know that similar outdoor devices
>exist; we currently run our 2000 feet of electrical fencing off of
>a 6v parmak solar powered fence charger.
the fence charger's unlikely to be able to provide enough power to run a
heater to keep the water from freezing, but you might possibly run 12 vdc
through the fence wires with an inductor in series with a dc heater in
the tank (the inductor avoids shorting out the fence charger signal.)
or you might cover the tank with some large sealed translucent baggies with
a little water inside to keep them from blowing away and a few drops of oil
to reduce condensation. these "coverites" (us patent no. 4,467,786, issued
8/28/84) would allow the horses to drink (and give them something to play
with :-) and allow rainwater to fill the tank. if a few broke or leaked and
sank, the remainder would close up ranks so the surface stayed insulated.
the worst-case month for solar heating in worcester, ma is december, with
an average outdoor temp of 27.4 f and an average annual (deep soil) temp
of 46.7 f. an average of 480 btu/ft^2 per day of sun falls on a horizontal
surface, and 860 falls on a south wall. the average windspeed is 10.3 mph.
an 8' diameter x 2' deep tank with r10 (2" styrofoam) sidewall insulation
and an r1 cover has a thermal conductance of about 55 btu/h-f. with 90% solar
transmission and water temp t(f), 0.9x480x50ft^2 = 24h(t-27.4)55btu/h-f, so
t = 43.8 f. a 4' high half-cylindrical reflector above the north wall would
gather more sun (about 20k btu/day, warming water ~15 f more) and reduce the
heat loss by wind.
at night, the tank might lose about 18h(44-27)55 = 16.8k btu. with 200
gallons of water, ie a thermal capacitance c = 1600 btu/f, the tank cools
about 16.8k/1600 = 10 f by morning. after 5 cloudy days in a row it loses
about 5x24h(32-27)55 = 33k btu, freezing 230 pounds of water to ice (33
pounds, on andy mckegney's planet), in a layer less than an inch thick,
mostly inside the slippery coverites.
nick
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