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re: solar power home heating.
6 dec 2002
niels lyck wrote re:
>... an ordinary dhw-system, where the solar collectors heat an oversize
>tank/water heater. anytime the sun shines, you will have hot water for
>free... the dimensioning rules for denmark... says 1-1,5 m2 solar
>collector per person and a tank volume of 40-70 litres per m2 collector.
>this goes for modern collectors with an absorber with selective coating
>and ensures hot water for 2-3 days after a sunny day... this type of tank/
>boiler normally has a built-in heat exchanger at the top, to connect your
>oil/gas burner/central heating as backup. the backup only heats the top
>of the tank...
the first cost might not change much with slightly larger collectors and
a very large unpressurized well-insulated water tank with a $145 bare 42
gallon 16" diameter x 50" pressurized galvanized tank inside as a heat
exchanger, and a tankless electric heater, or no backup system at all.
>...you should know that you will "never" be able to cover your space
>heating needs to 100, or even to 50%... why? because saving heat from
>summer to winter, or even from one month to another requires quite
>enormous amounts of heating capacity...
but most places have some predictable sun in every month of the year.
the op (sam) lives in new england. nrel data say january is the worst-
case month for solar house heating in boston, with 1040 btu/ft^2 of sun
(including 330 diffuse) on a south wall and 590 (on a level surface on
an average 28.6 f day. sam enquired about a 50x100' house with "pretty
standard insulation." the first step might be to make the house a lot
more airtight, say 0.1 ach, then beef up the insulation, say us r30 walls
and r50 ceiling. then stuff r10 foamboard in most of the windows for the
winter, or replace them all with r4 windows with 50% solar transmission.
we might end up with a thermal conductance of 50'x100'/r50 = 100 btu/h-f
for the ceiling plus 400/4 = 100 for the windows plus 1600/30 = 53 for the
walls plus 0.1x50x100x8/60 = 67 for 67 cfm of air leaks, a total of 320
btu/h-f, so the house would need 24h(65-28.6)320 = 280k btu to stay 65 f
indoors on an average jan day... 600 kwh/mo of internal electrical use
would reduce that to 211k. nrel says east and west walls get 410 btu/ft^2
per day in january, and north walls get 180, so 160 ft^2 of south windows
and 160 of ew and 80 n would transmit 0.5x80(2x1040+2x410+180) = 123k btu
on an average day, leaving 211-123 = 88k from "other sources."
>how to do: you need a larger collector area. in denmark, sweden and germany
>we use eg. 10, 12 or 18 m2 of collector insted of the 4-6 m2 of the
>dwh-system.
a square foot of collector with an r1 cover with 90% solar transmission
could gain 0.9x1196 = 1076 btu/day. with 100 f water inside, it might lose
6h(100-32)1ft^2/r1 = 408 btu, for a 668 btu gain, so the house above might
have 88k/668 = 131 ft^2 (12.2 m^2) of "collectors." some salt could help
a trickle collector work better... perhaps you have less sun, and possibly
smaller and better-insulated and more airtight houses.
>from the other side of the heat exchanger you now take the heat and
>introduce it into your central heating system. the simplest way is
>to use a floor heating system with a large thermal mass, eg 10 cm
>of concrete, where you can deposit some of the heat...
well, we need a floor anyhow, and overnight thermal mass for an average
day, but buried pipes can go bad, and the concrete may keep the house
warmer than needed on a warm winter day, tempting us to open windows and
waste heat. baseboard radiators with a warm water storage tank instead of
concrete would allow more control of the house temp without dumping heat
to the outdoors.
the house needs about 18/24x88k = 66k of overnight heat on an average day,
ie 6.6k of thermal capacitance, with a 10 f day/night temp swing. this might
come from 6600 pounds or 825 gallons or 103 ft^3 or a 4x8x4' boxful of water
cooling from 100 to 90 f, or a smaller box, since the house will have some
thermal mass of its own. we might put a 42 gallon tank in this average-day
box, a preheater in series with another 42 gallon tank in the cloudy-day box.
>if you want more coverage, you need more thermal capacity, eg. via a thick
>layer of sand (30-40 cm)underneath your concrete slab, and insulated form
>the latter. here, you can deposit your heat in the sand, and thanks to the
>insulation the heat will be slowly released into the room above. relative
>collector size: 12-15m2
we might keep a well-insulated cloudy day heat store hot with a small
collector instead. sand has lots of thermal resistance, not to mention
the insulation above, so it has to be hot to heat the house on a cold day,
which makes the collectors less efficient. and there's little control
over heatflow, and water can store about 3x more heat by volume than sand,
in a compact and easy-to-insulate tank vs a floor with a large s/v ratio.
>using the thermal capacity of the building materials normally comes cheaper,
nice for overnight heat storage, with a large s/v ratio and small temp
swing and low upper temp limit...
>if you want even more solar coverage, you can build a huge water tank
>into your central heating system...
maybe not so huge, if it's well-insulated and seldom-used, ie it provides
no deliberate heat for the house on an average day. if it's inside the house,
its lost heat can help warm the house on an average day.
the large house above needs 5x211 = 1055k btu for 5 cloudy 28.6 f days in
a row. that might come from 1055k/(130-80) = 21k pounds or 2638 gallons
or 330 ft^3 of water cooling from 130 to 80 f in a couple of $400 1500
gallon poly ag tanks or a couple of 4x8x8' epdm-rubber-lined plywood boxes.
if the boxes are side by side, a dramatic part of the living space with
a porthole and light and galileo thermometer (or 130 f fish?) and 384 ft^2
of r30 insulation, they would lose 24h(130-65)384/30 = 20k btu/day (which
heats the house.) a 130 f collector with 2 covers might gain 968 btu/day
and lose 6h(130-32)1ft^2/r2 = 588, for a net gain of 380, so we might keep
the boxes warm with another 20k/380 = 53 ft^2 (4.9 m^2) of collectors,
or less, with a selective surface...
nick
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