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re: sizing a heating system
19 jun 2000
jack  wrote:

>could you tell a little more about the viability of chemical heat
>storage, i.e. what chemicals to use, what measures to take, etc.?

a few clues...

0. dessicants can store about 20x more heat by volume than sensible
heat stores, with no heat loss over time. lithium chloride (licl) can
absorb about 11 times its weight in water vapor, so a pound of licl
can store about 11k btu of heat as it absorbs 11 pounds of water, vs
11 pounds of water that stores, say, 550 btu of heat as it cools 50 f.

1. dave robison of stellar processes and tom lawand of brace research
institute both mentioned an old canadian paper suggesting that a cubic
meter of zeolites was sufficient to store house heating energy for an
entire season.

2. i've heard of a sailboat fridge that used zeolites.

3. there's a german website describing a portable refrigerator that
works by concentrating sun with a small dish onto a metal cylinder
containing a licl solution to evaporate water from the licl. then
you attach the cylinder to some sort of heat exchanger and pump out
the air to prime it for self-sustaining operation.

4. some israeli greenhouses use "brine dehumidifiers."

5. "unglazed collector/regenerator performance for a solar assisted
open cycle absorption cooling system" by m. n. a. hawlader, k. s. novak,
and b. d. wood of the center for energy system research, college of
engineering and applied sciences, arizona state university, tempe, az
85287-5806 usa, in solar energy, vol. 50, pp 59-73, 1993 describes:
"an ordinary black shingled roof... used as a collector/regenerator
for the evaporation of water to obtain a strong solution of [lithium
chloride] absorbent... experimental results [using a 36' x 36' roof] 
show a regeneration efficiency varying between 38 and 67%. cooling
capacities ranged from 31 to 72 kw (8.8 to 20 tons)", ie about 1 ton
per 100 square feet of roof area.

in the house "water [the refrigerant] is sprayed into an evaporator,
evacuated to about 5 mmhg of pressure, where it immediately flashes
into vapor... cold water, pumped from the bottom of the evaporator,
flows through a fan coil... that blows cool air into the conditioned
space. the absorber acts as a vapor compressor and condenser for the
system. water vapor from the evaporator flows over the absorber where
it is absorbed by the concentrated absorbent. the continuous absorption
of water vapor maintains a low pressure in the system and permits
flashing of water in the evaporator... the product of the absorption
process, a weak absorbent solution, collects at the bottom of the
absorber to be pumped [up over the roof] for concentration."

"the dilute licl solution was delivered to the collector surface
through a spray header spanning the top of the roof and made from
50.8 mm (2 in) diameter cpvc pipe fitted with 35 evenly spaced
brass nozzles. the concentrated solution collected at the bottom...
in a pvc rain gutter, and returned via gravity feed to a 1608 l
(425 gallon) fiberglass tank... in the event of of a rain, fluid
flowing off the collector could be manually diverted to a 946 l
(250 gallon) wash tank or to a roof drain. during the initial phase
of rain, residual salt would be washed from the roof and collected
in the wash tank to be stored for later regeneration. after sufficient
rainfall, the rainwater is diverted to the roof drain."

in a heater vs air conditioner, the concentrated licl solution might
absorb water vapor from groundwater in an evacuated heat exchanger with
groundwater flowing through some sort of vessel under some licl flowing
through another vessel. the water vapor might rise up in a tube between
them, warming the licl as it absorbs the vapor and cooling the groundwater
solution as it evaporates. evacuating the heat exchanger seems to make
this go a lot faster, for the same heat transfer area, since the water
vapor molecules don't have to battle their way through air molecules
to get to the licl solution. perhaps the ground water evaporator could
be a damp basement in a house, and the absorber could be a licl pond or
fountain upstairs.

6. cyprus foote mineral sells licl for about $4 per pound. it's fairly
non-toxic, and a residential system might require 200 pounds.

nick




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