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re: storing solar heat
13 mar 1997
david buchner  wrote:
>"greg burgin"  wrote:
 
>> i have an idea! build a modified ice house. superinsulate it and fill it
>> with plastic 55 gal drums full of water and maybe some antifreeze...

seasonal thermal storage requires a very large bathtub, perhaps like this:

  the lyckebo [sweden] system is a cavern of 100,000 m^3 capacity, cut out of
  bedrock using standard mining methods, of cylindrical shape, with a central
  column of rock left to support the overhead rock. the cavern is about 30 m
  high and its top is about 30 m below ground level. it is water filled, and
  inlet and outlet pipes can be moved up and down to inject and remove water
  from controlled levels. the water is highly stratified with top to bottom
  temperatures of about 80 to 30 c. figure 8.7.2 shows temperature profiles
  in the store at various dates in the second year of operation... no thermal
  insulation is used, and there is a degree of coupling with surrounding rock
  which adds some effective capacity to the system. losses occur to a semi-
  infinite solid and can be estimated by standard methods. observed losses
  from this system are higher than those calculated; this is attributed to
  small but significant thermal circulation of water through the tunnel used
  in cavern construction and back through fissures in the rock. it takes
  several years of cycling through the annual weather variations for a storage
  system of this size to reach a "steady periodic" operation. in the second
  year of its operation, while it was still in a "warm-up" stage, 74% of the
  energy added to the store was recovered. 

         from p 404 of section 8.7, "seasonal storage," of _solar engineering
	 of thermal processes_, by john a duffie and william a beckman, 2nd
	 edition, 1991, wiley-interscience isbn 0-471-51056-4

>i like the 55-gal drum idea: then i *do* get to keep my water. maybe also
>less mold and stuff. but why antifreeze? i have a sort of vague idea that
>a fairly large amount of the "cold storage" would be in the form of all
>the energy water loses to become ice.

sure. about 144 btu/lb to freeze water at 32 f and vs 1000 btu/lb to evaporate
it, unless you are trying to heat swimming pools in certain parts of canada.

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 with no moving parts? as a rule of thumb, each square foot
of pond surface can release about 40 btu/hr of heat as it absorbs water vapor.
plants can release 1 lb of water vapor per square foot per day, ie 42
btu/hr-ft^2. we might make 100k btu/day ie 4200 btu/hr with 100 ft^2 of
licl pond and 100 ft^2 of plants, or a small packed column, or 2 smaller
ponds and fountains. 

perhaps we need 2 or 3 ponds in a sunspace, licl for heat storage and summer 
dehumidification, a large ground-coupled rainwater fed pond which also serves
as a cool thermal mass with night venting and evaporative and radiative cooling
in summer, as well as a receptor for water distilled with the sun from the
licl pond, and a greywater/sewage treatment/fish pond. and some rabbits and
pvs, of course, all inside a large greenhouse with a reflective north wall,
with water vapor condensing between two thin inflated layers of polyethylene
or polycarbonate plastic during the day, and tiny cold soap bubbles filling
that space at night? 

>i see that somebody got a bunch of poly drums free. no fair! everyplace i
>can get 'em around here is *selling* them (about $15), dirty and used,
>because so many people want them to make recycling bins and swimming rafts.

$15 isn't a bad price, but if you can find someone like my local food packing
plant who is paying to have 40 per month hauled away, so much the better. you 
might try calling the drum recycling clearinghouse at 800 drum cycle, or look
around for a source of 5 gallon plastic paint pails with lids. easier to ship,
since they nest, and a desirably higher surface to volume ratio.

>perhaps you could uuencode half a dozen of them and email them to me?

right. take your laptop outside and use drumzip3.4 to decode this: *

nick



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