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re: put solar collector panels inside the envelope ?
7 may 2001
anthony matonak   wrote:

>terry wilson wrote:

>> ...why not just place the entire assembly inside the house out of
>> the cold. you could set it up in a sunspace or under a skylight...

good idea. many zomeworks big fin installations have bare collectors 
clipped to copper pipes under sloped sunspace glazing, thermosyphoning
plain pressurized water into an insulated tank above. the collectors
provide some shade and allow nice dappled sunlight to pass among them.
steve baer says "manage the sunspace" so it doesn't freeze, eg use an
automatic foundation vent or a freezestat and fan to circulate a little
warm house air through the sunspace on a cold night.

>many solar hot water systems are sold as retrofits to existing houses
>and cutting open the roof to add the skylights or building a sunspace 
>is usually not an option.

add-on sunspaces are common. i added a $500 32' long x 12' deep x 16' tall
lean-to plastic film greenhouse across the front of my 1820 farmhouse, and
replaced the steep south 640 ft^2 tin/asphalt/cedar roof with one layer of
clear corrugated polycarbonate ("dynaglas," $1.25/ft^2 in 4'x12' sheets.)
>why not cover your hot tub with clear glass or plexy, paint the bottom 
>of the tub black, use a big heliostat (mirrors) to focus a lot of 
>sunlight onto it?

it would lose lots of heat through a transparent vs foamboard cover, and
most people want a precise and constant tub temp (104 is too cold and 106
is too hot), which makes it hard to store much solar heat in the mass of
water. better to make and store hotter water for showers and trickle some
into the tub as needed for heat, while the overflow heats pressurized cold
water on its way to the heater via a small water-water heat exchanger.

a 6'x6'x4' tall tub with 6" of foamboard insulation has a heat conductance
of 168ft^2/r30 = 5.6 btu/h-f, so it needs (104-70)5.6 = 190 btu/h at 104 f
in a 50 f room, which might come from 190/(130-104) = 7.3 pounds per hour
(0.015 gpm) of 130 f water cooling to 104. recovering 80% of the outgoing
heat means ntu = 0.8/(1-0.8) = 4 = a30/7.3, ie a = 1 ft^2 of heat transfer
surface, eg 8' of 1/2" copper pipe or 25' of reinforced garden hose.

ozone and a copper sponge in the filter and more frequent water changing
might eliminate the need for all the tub chemicals... 


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