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re: storing solar heat
17 mar 1997
willie schmit, crl wrote:
>>seems like most people would like some sort of indoor pond to store solar
>>heat, with a waterfall and lots of plants and granite statues. i wonder how
>>to do that. if the pond were a lithium chloride solution it might release
>>heat by absorbing water vapor from plants in a ground-coupled garden or
>>a lawn in a sunspace. licl is a salt that can absorb about 12 times its
>>weight in water. a house might get 500k btu (about 150 kwh) of heat for a
>>cloudy week by allowing about 50 pounds of dryish licl solution to absorb
>>500 pounds of water vapor, ie about 55 gallons of water. it would take 20
>>insulated 55 gallon drums full of 130 f water cooling to 80 f to store that
>>much heat. can we store heat with no loss in an uninsulated pond with twenty
>>times less volume, and use the licl in some sort of passive ground-coupled
>>heat pump system? as a rule of thumb, each square foot of licl pond surface
>>can release about 40 btu/hr of heat as it absorbs water vapor.
>if you look carefully you will notice a common thread to all this talk
>about collection and storage of heat - the answer is mass.
well, the paragraph above describes evaporating and condensing water,
ie phase change, vs sensible heat storage in thermal mass.
>i would sooner have 10 yards of concrete in my greenhouse than a stack of
> expensive, leaky, clumsy 55 gallon drums (the ones that were filled with
>toxins are usually cheap)
hard to know where to begin... i get 40 free plastic drums every month from a
food processing plant, and they used to contain edible products like frying
oil or sauterne wine, or non-toxic products like soap or propylene glycol.
and drums in a greenhouse may work reasonably well for you in nm, but in
cloudy climates this cripples the solar house heating performance of a
sunspace by storing solar heat all day in the drums, which mostly leaves
through the low-thermal-resistance solar glazing on cloudy days and at night.
better to put the drums inside the house, and let warm air flow through a
hole in an insulated wall during the day, heating the drums during the day,
and let the sunspace get icy cold at night, perhaps leaking some warm house
air deliberately and controllably into the greenhouse on a cold night as
needed to keep the plants from freezing. better still to live outside the
heat battery and its temperature swings, and keep it at more than 70 f on
the average, so the house can be controllably heated by leaking warm air
from the warmstore to the house.
and concrete only stores about 1/3 as much heat as water by volume, with an
undesirably higher effective thermal resistance. rock beds have ok thermal
resistance, but they require lots of fan power compared to a room full of
containers of water.
>btw - get a keg of beer and a bunch of guys with rakes - plus 1 or 2 that
>know concrete - and pick up the phone - they deliver.
the beer sounds nice, but concrete costs more than water.
then again, concrete is more rodent-resistant.
in our home solar heating system we used water as the thermal storage
medium for an air-transfer unit, the water being contained in 1000
one-gallon polyethylene bottles stacked so that air could flow between
them. they worked satisfactorily until some desert pack rats invaded
the storage bin, making nests of the insulation and chewing holes
in the water bottles.
p 468, _applied solar energy_, by
aden b. meinel and marjorie p. meinel
addison-wesley, 1976
nick
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