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re: looking for a solar heating system
3 oct 1997
ricardo agudelo wrote:
>i live in a very sunny area, and are very interested in building a solar
>heater in my home roof. where can i find information about design and
>parts needed to built one?
>ricardo agudelo
>maracaibo, venezuela
let's see: if 4 people wash dishes and take showers for 10 minutes every day
with 20 liters per minute of 40 c water, and the incoming water is 20 c, we
need to heat 4x10x20 = 800 liters of water, ie 800 kg, by (40-20) = 20 c,
which means collecting 800x20 = 16,000 (capital c) calories or 4.19x16,000
= 67,000 kilojoules (about 65 k btu, since a btu is about 1 kj) or 50 mj/3.6
= 19 kwh per day of solar heat, worth $1.90 us per day at 10 cents/kwh.
i wonder how much your electrical energy costs, when it's available.
duffie and beckman's solar engineering book says the coolest month in
"maracibo, venizuela" (is that the same as maracaibo, at latitude 10.6 s?)
is december, with an average outdoor temperature of 27 c (80.6 f) and an
average daily 14.4 megajoules per square meter (4 kwh/m^2 or 1270 btu/ft^2)
of sun that falls on a horizontal surface. if there were no thermal losses,
it looks like 5 square meters of solar collector would be enough. (the book
says caracas is cooler, with 20 c and 13.5 mj/day in december.)
>> i wonder what the roof is made of. wood?
>no we don't have wood constructions here, the housing here is rather masonry.
so it's a masonry roof...
>> does a part of the roof face south?
>
>no, is in east-west direction, but as we are 12 over the equator, i think it
>doesn't affect too much
sure...
>> how much is the slope of the roof?
>
>about 10 degrees, but i'm thinking in a flat section of 32 square meters
sounds like a nice flat section... i suppose its reasonably strong. no snow.
won't rot underneath. making a water heater there might be easy...
>> does the temperature where you live ever drop below freezing?
>
>we would love that!, we are about the hundreds all year around. locals have a
>season near christmas they call "los hielitos" (the little ices) when it drops
>about the nineties.
the water heater can be simpler if it never freezes. how about a shallow pond
on the roof, with a black plastic pond liner underneath and a transparent
plastic film cover? keep it filled with a float valve, or rainwater that
falls on the cover, to make a gravity-feed hot water system?
or make it deeper, like a hot tub?
what combination of area and depth can supply most of the hot water needed
for a day or two with only a small change in temperature? suppose we try
to make the water 130 f on an average day, and ask it to stay at least 110 f
for 48 hours with no sun. if the box has good insulation underneath and on
the sides, and a us r1 cover (one or two layers of greenhouse polyethylene
film), after 48 hours,
110 f = 80.6+(130-80.6)exp(-48/(rc)),
so the time constant needs to be at least rc = -48ln((110-80.6)/(130-80.6))
= 24.9 hours, ie we need a water depth of at least 24.9/64x12 = 4.7". the
daily heat loss from the box would be about 24h(130-80.6)/r1 = 1,186 btu/ft^2,
and the daily solar gain is about 1,270 btu/ft^2. looks like we need more
layers of plastic film, or solar concentration or nightime insulation.
the night insulation might be a 3" thick rigid foam cover that automatically
folds up during the day and folds back down over the box at night. with a
used automobile windshield wiper motor and a photocell and a couple of diodes
and limit switches to control it. a small pv-panel might charge the battery,
and also act as the photosensor. sounds like a nice product for jade mountain.
this system might be inside a small greenhouse, if you have lots of wind. the
foamboard would lower the losses to something like 700 btu/ft^2/day, so the
box needs a top surface of about 50k/(1200-700) = 100 ft^2, about 9 m^2.
if the water were 12" deep, the box would weigh about 6,400 pounds, and it
might have 300 ft^2 of us r20 surface, ie a thermal conductance of about
15 btu/h-f. if the water inside had an initial temperature of 130 f, and
the box were closed up for 48 hours, it would lose about 48h(130-80.6)15
= 36k btu of heat, cooling the water to about 130 - 36k/8k = 124 f. adding
1600 liters, ie 3500 pounds of 80 f water to the box would lower the
temperature to about (124(6400-3500)+80(3500))/6,400 = 100 f.
nick
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