|
|
re: solar water heating component selection advice needed
28 jan 2001
george estep wrote:
>hi all,
hi george,
>...my goal is to provide 100% of my home's hot water needs using
>a solar water heating system... my current water heater uses
>approximately 10 kwh of electricity per day.
and in december, you get 950 btu/ft^2-day of sun on a south wall and 580
on a horizontal surface (or 1113 on a 58 degree tilt, or more, with more
tilt and reflective ground) and 697 btu of beam sun over about 3 hours on
a 1-axis ew concentrator in december, when the 24-hour average temp is
35.4 f and the average daily max is 45.0 and the deep ground temp is 57.7.
it takes 8(130-57.7) = 578 btu to warm a gallon of 57.7 f water to 130.
you might heat 10kwhx3.412/578 = 59 gallons to 130 f on an average day,
and store enough hot water for about 5 cloudy days in a row, ie about
295 gallons, if the tank were perfectly insulated. you might preheat
the cold water with a greywater heat exchanger...
aaa solar sells used 4'x10' panels for about $400 each, delivered to va,
and these are about 40% efficient at 130 f on a 30 f day in 250 btu/h-ft^2
(am2) full sun. you might use 10kwhx3.412/(0.4x697) = 122 ft^2 of them, ie
3, or maybe 2 with an overhead sunscoop reflector.
>i am looking closely at the 210 gallon tank from six rivers solar...
ah, solar solutions for every problem. have you considered building
a plywood box lined with a folded piece of epdm rubber?
>...being rectangular, it fits nicely in the space which i would like
>to use while providing a large amount of storage...
maybe not large enough. you might also store soft rainwater from the roof.
>i would also like to know the details of the potable water coil in the tank.
why use a coil, vs an rv pump with a built-in pressure switch? say you take
a shower or wash some dishes, and the drain goes into the top of a 55 gallon
plastic drum with an overflow pipe to the greywater drainfield. the drum also
contains 75' of 5/8" reinforced garden hose with cold water in and warm water
out. the garden hose goes to a 100gal/24h/60min = 0.07 gpm throttling valve
in series with a float valve that keeps the hot water storage tank full.
the rv pump empties the tank.
the hose has about 12.3 ft^2 of surface. with u = 10 btu/h-f-ft^2 and cmin
= 8x100/24 = 33 btu/h-f and ntu = 12.3x10/33 = 3.7 and e = 3.7/4.7 = 0.79,
this heat exchanger might be 79% efficient. if the greywater leaves the tub
or sink at 100 f, it might could preheat cold water to 57.7+0.79(100-57.7)
= 91 f, so you only need add 8x59(130-91) = 18.4k btu (5.4 kwh) of energy.
>i am looking at thermomax for collectors:
i have heard they are expensive, vs an unpressurized horizontal epdm
rubber bladder at 60 cents/ft^2.
>...i plan on mounting the unit verically in a south-facing orientation
>to give near peak performance in winter while balancing the winter and
>summer performance...
maybe built into a garage wall, or as a part of a garage ceiling, near the
north edge, under a parabolic north roof that's reflective underneath, with
a steep transparent south roof.
>...how do i determine what flow rate is optimum for collecting the most
>heat from the collectors?
an infinite flow rate is optimum. lower flow rates lower pumping power and
efficiency, since the downstream parts of the collector run hotter and lose
more heat to the outdoors. parallel collector plumbing is better than series.
you might end up with 0.05 gpm/ft^2, on the low side.
nick
|
|
