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re: solar panels/turbines/electric radiant heaters
6 apr 1999
wrote:
>preferred is calcium chloride hexahydrate (1 pt cacl, 6 pts h2o). melts when
>sun shines (absorbs energy) and freezes when it cools down (releases energy).
if these are like "glauber's salt" (na2so4-10h2o), they have some problems.
after a few hundred cycles they stop changing phase, and need elaborate
mechanical decoherers to keep working. and they crystallize inside their
containers from the walls in, and the crystals act as insulators, which
slows their heat transfer... nobody's found a good reliable phase-change
material yet, to the best of my knowledge.
>the number but typically stated in kilo-calories/gram. definitely 'much'. in
>english, the melting/freezing of 1 gram of cacl.6h2o will absorb/release
>'much' energy without it's temperature changing from it's freezing point (30
>c).
a cubic foot of glauber's salt stores 10,700 btu between 80 and 100 f, vs
1250 for water or 720 for rock, according to bruce anderson's solar home
book. with that much thermal capacity, they also need lots of airflow
(ie fan power) or relatively expensive packaging with a high surface to
volume ratio, compared to water containers, in an air-water system.
nick
then again, these salts may be less appetizing...
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
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