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re: solar water heater designs
2 mar 2003
yahugh  wrote:

>can anyone point me to some well proven designs for home-built solar hot
>water heaters?

well-proven panels tend to be expensive and inefficient. you might build
a preheating system onto a south wall and get some house heating as well.

>my site is in sunny but chilly southeast idaho at 6200 feet...

nrel says 810 btu/ft^2 of sun falls on a south wall on an average 24.8 f
december day in sunny pocatello, with an average daily max of 33.7.
averaging the averages makes the average daytime temp 29.3.
the average annual (groundwater) temp is 46.4. 

a greywater heat exchanger seems in order. if 100 f water drains from a
shower, 300' of 1" plastic pipe coiled in a 55 gallon drum might preheat 
cold water to 46.4+0.98(100-46.4) = 98.9 f. pt industries (800) 44 endot
pbj10041010001 1"x300'100psi nsf-certified pipe is tested to 500 psi...
buy it for $59.99 from any true value hardware store. lowes sells 90 degree
elbow pvc combo insert/fipt fittings (with a 1" barb and a 3/4" female pipe
thread) for $0.74 each (2 required) and pipe-to-hose-thread adapters for
$1.83 each. you might pump the greywater out of the drum and into the septic
system using boatus item 168226, a $33.99 attwood v1250 bilge pump near
the bottom, with boatus item 168228, a $20.99 attwood float switch and
10' of 1.125" hose, item 218818 at $1.19/ft.

the v1250 can move 400 gpm 6.7' with 12v at 2.6a (31w), so pumping 75
gallons of greywater per day takes about 0.1875 h or 5.85 wh/day,
something like a 3 watt pv panel charging a 12v diehard.

heating 98.9 f water to 110 f for 4 10 min showers per day and 20 minutes
of dishwashing at 1.25 gpm would take 8x75g(110-98.9) = 6.7k btu per day.
heating 46.4 f water would take 8x75g(110-46.4) = 38.2k btu per day.

argo industries #1560 fin-tube has about 5 btu/h-f-ft of water to air
conductance. an 8' piece with 2" fins on 3/4" copper pipe costs $15.67.
with an average 105 f water temp, two pieces in 120 f air might collect 
6h(120-105)80 = 7.2k btu/day. collecting 38.2k btu/day in average 78 f 
water might take 38.2k/(6h(120-78)) = 151 btu/h-f-ft, eg 4 8' pieces.

if 2 1 ft^2 layers of r1 south polycarbonate glazing with 90% solar
transmission gain 0.9x0.9x810 = 656 btu/day (vs 12% more with snow or
whitewashed stone on the ground) and lose 6h(120-29.3)1ft^2/r2 = 272,
the net gain is 384 btu/ft^2, so we need 6.7k/384 = 17 ft^2 of glazing
with a greywater heat exchanger, or 38.2k/384 = 100 ft^2 without one. 
a larger wall-warmer might also provide house heat with a motorized
damper and thermostat.

the fin-tube could preheat an additional tank-type water heater with
a small circulating pump and a differential thermostat. the thermostat
could turn on the pump for freeze protection. it would also be nice to
choose a pump that allows enough thermosyphoning flow to prevent freezing
the fin-tube when the power is off. 

nick




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