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re: solar central heating
9 oct 2000
anthony stamp wrote:
>...we are now planning a major kitchen extension and so have the
>opportunity of installing an insulated under-floor cistern (of 4-6
>cu metres).
about 10k pounds (5 m^3) of water, enough to store (149f-77f)10.4k = 748k
btu or 219 kwh of heat as it cools from 65 c (149 f) to say, 25 c (77 f),
probably not enough to heat the whole house in december.
>would it be feasible to use solar panels to heat this up to say 65 degc
>by day and then pump this water (or water heated from it through a heat
>exchanger) through radiators when required?
sounds feasible, but expensive for the solar panels. this might be simpler
and more efficient without the heat exchanger or radiators. how about an
insulated wine cellar with a large plastic tank in the middle surrounded by
rocks or plastic water jugs?
>how can i calculate whether the size cistern we could realistically
>install would be sufficient?
it seems sufficient to heat the kitchen or make hot water for the house.
an airtight lxlx2m tall kitchen with 0.08l^2 m^2 of metric r1 windows and
8l-0.08l^2 m^2 of metric r4 (us r22.7) walls (including the studs, if any)
and an l^2 m^2 r4 ceiling has a heat conductance of l^2/4 for the ceiling
plus 2l-0.02l^2 for the walls plus 0.02l^2 for the windows, for a total of
l^2/4+2l w/c, so 219 kwh can keep it 20 c when it's 0 c outdoors for five
cloudy days if 219 kwh = 24hx5d(20c-0c)(l^2/4+2l) = 600l^2 + 4800l, or
l <= 15.5m (51 feet :-)
you might heat the water with a reflective parabolic solar trough built
into a kitchen attic with an ew ridgeline and a transparent south roof...
if 1 kwh/m^2 of sun falls on a horizontal surface and 3 falls on a south
wall on an average december day, and half is beam sun, and a 10x10m kitchen
requires 24(20-0)(10^2/4+20) = 21.6 kwh of heat, and the windows provide
6.6 kwh on an average day, leaving 15 kwh, the south roof needs at least
15kwh/(0.9x0.9x0.9x1.5kwh) = 13.7 m^2 (148 ft^2) of surface, for instance
one layer of corrugated polycarbonate greenhouse roofing with 90% solar
transmission, and a layer of 90% reflective metalized mylar under the
north roof (made approximately parabolic by some saw cuts in the rafters),
and a 30" round uv greenhouse polyethylene air duct with 90% transmission,
laid flat on the attic floor near the base of the north roof, and partially
filled with water during the day.
a 10m x 10m attic with a 2m tall ridge in the middle with a 54 m^2 south
roof might collect 1.5x0.9x0.9x0.9x54 = 58.6 kwh/day. at dawn, the focus
line would be y^2/4x = (2m)^2/(4x5m) = 0.2 m (8") from the north wall.
the relatively small target trough and diffuse solar warmth in the attic
would minimize the heat loss during solar collection.
nick
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