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heat storage - can licl be a solution?
thu, 13 dec 2001
i, as many of you, have an interest in storing heat. heat storage simply
makes renewable source heat convenient, comfortable and even more

we've talked about all kinds of storage methods over the years,
including bulk water, pcms (phase change material) salts and paraffin,
rock piles, etc. at one point i was ready to go lookin for a couple tons
of lead until you kind folks straightened me out.  :)   now the hot
ticket seems to be cast concrete septic tanks, of course still using
water :(

recently, one of our esteemed contributors has been mentioning a
chemical, licl (lithium chloride) and its capacity to store and release
much larger amounts of heat than any previously discussed
medium/methods. this caught my attention as i believe heat storage is
vital to the growth of several renewable technologies.

let me dig right into the discussion as this is going to be rather long
and complex.  i'll begin by barrowing some text from a current thread
"fireplace heat recovery" for the setup: and then concluding
with a question and answer piece that i exchanged with nick pine.  much
of the information is new and is posted at his suggestion. some of the
math is way over my head, so i'll be digesting and learning this along
with you.

here's where it started:

...the amount of heat storage you can get in 55gal drums filled with
water is insignificant for house heating.

 "nick pine"  wrote:
hmmm... 55x8x(180f-80f)1btu/f = 44k btu, like running a furnace
full blast for an hour. otoh, a drum full of 50% licl solution
on top of a basement woodstove might store 55x8x1300 = 572k btu,
enough for a day or so.
so, basically nick, you seem to be saying that licl (lithium chloride?)
can store about 13 times as much energey as water.

 question is, how expensive is the raw product. i seem to remember
seeing balls?  of pcm that were fairly pricey, but then, so is
insulating 13 times as big of a storage area. i see licl mentioned in a
lot of your
posts, is this because it is one of the better pcm's, or because it is a
cheaper product?

or for that matter is licl even a pcm?
 "nick pine"  wrote:
consider the difference between a drumful of water on top of a basement
woodstove and a drumful of 50% licl solution. evaporate water when the
woodstove is running, and absorb water vapor from a damp basement floor
when it is not, as a chemical heat pump,  making the floor colder and
the drum warmer. to work at a reasonable rate, this probably needs
additional heat exchange surface for the absorption part, eg a layer of
plastic film under the basement ceiling, and we only get 1300 btu/lb
with infinite dilution, so the real yield is less, given a finite tank
volume to hold the weak solution and the fact that weaker solutions
provide lower temp heat at lower rates.

>how expensive is the raw product?

cyprus foote-mineral sells dry licl in 55 gallon drums for $4/lb.
a 2:1 mix of cacl2 (an ice-melting salt at about 50 cents/lb) and
licl can work almost as well.

> or for that matter is licl even a pcm?

it's a desiccant (water absorbing) salt, vs a pcm. another advantage
is that it can store heat forever with no loss of heat over time and
no need for insulation. there's no risk of fire, it doesn't get tired
of changing phase, and it can be used in a simple heat pump.

to better understand the chemical and how it works, here are several
links i included in the "fireplace heat recovery" thread:
the classic lab demo (which nick repeats below) :

more recently this property has been put to practical application in
this form:

this is a 'how it works' pictorial which is linked at the bottom of the
above cited page:

so this caused me to write to nick with a number of questions and in
true nick pine form,  he cut my bloat and gave me formulas, with
examples  :)     here's the q & a:

(me)>...can lithium chloride be used efficiently to store heat?

(nick) i think so.

>...there are certain practical limits which in turn reduce its
> advantage.

sure. for instance the licl solution storage tank size.

>for example, if one were to use it to store heat produced by a cogen,
>about the highest temperature from the water jacket is 200f...

that may be fine. that limits the max concentration, as does humidity.
if we keep the solution at some constant high temp for a long time, it
will evaporate water until it rises to an equilibrium concentration at
which the vapor pressure near the solution surface equals the vapor
pressure of the water in the surrounding air...

>what temperatures would one use to work with this material?

i'd say hotter than domestic hot water, eg 140-240 f, vs 110 f.

>...what dilutions would give what amounts of heat.

you can figure that out from the equations in

"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, v. 50, pp 59-73, 1993.

i'll append a program that uses those equations to find concentrations.

i have only measured the approximate heat of solution from 100% (dry
licl) to 50% (half water by weight.)

the crc chem/phys handbook says licl's heat of solution is 8,850
cal/mole (with infinite dilution) and a calorie (capital c) raises 1 kg
of water 1 c (1.8 f), so a calorie is about 4 btu, and a mole of licl
weighs 42 grams,  vs 454 grams/lb, so diluting dry licl to a 0% solution
by adding water releases about 8.8x4x454/42 = 379 btu per pound of licl.
if we can arrange to add water vapor (vs liquid), the solution gains the
additional heat of condensation, about 1000 btu per pound of water

when i mixed equal weights of 70 f dry licl and water in a styrofoam cup
to make a 50% solution, the temperature initially jumped to 187 f
(measured with a raytek ir thermometer with a 500 ms sampling time), so
the heat of solution to 50% dilution is about 187-70 = 117 btu/lb (a
conservative estimate, ignoring the licl's heat capacity), so the 50%
solution would release another 379-117 = 262 btu/lb on further dilution
to 0%. if it absorbs water vapor vs liquid, it also gains 1000 btu/lb of

>would a temperature above the boiling point of water boil the water off
>or would the salt change the boiling point so that higher sensible heat
>is also stored?

adding salt increases the boiling point of water, but it might be more
helpful to think about evaporation than boiling. evaporation happens
more quickly at higher temps. sensible heat storage happens too, but
that is small compared to the latent heat and heat of solution.

>how many gallons of water would have to move to move what amount of
> heat?

ignoring the heat of solution, it seems to me that allowing a huge licl
lake to absorb a gallon of water vapor would add about 8,000 btu to the
lake. adding a pound of 50% licl solution to a huge water lake would
release 262 btu. you can calculate how much heat is released between
these extremes.

>am i thinking straight here?  you move water in and back out to release
>and restore the latent heat?

adding water releases "heat of solution." adding water vapor releases
the latent heat of the vapor as it condenses. evaporation from the
solution at a higher temp concentrates it, which restores latent heat
and the heat of solution.

>i know i would appreciate a gallon or 2 but couldn't say 20 or 30
>gallons of water released into a dwelling over the course of a day,
>in the middle of winter, cause some problems?

it might, if it weren't absorbed by something else, eg your licl lake.
and this could all happen inside a closed container.

>and would it be just 20 or 30 gallons?

could be more... allowing a licl lake to absorb 30 gallons of water
releases about 240k btu of heat, like burning about 2 gallons of oil.

here's one possiblity. you might have a damp basement floor, with
a layer of plastic film near the basement ceiling under the rafters
of the first floor of your house, to make a licl lake that slopes
towards one edge, with a slow drain and a licl storage tank below it.

the lake could be open around the perimeter, with a gap above, so
it could absorb water from basement air. when the house needs heat,
you pump some licl up into the lake with a float switch to keep it
from overflowing, and it drains back into the lake when the house
becomes warm enough. meanwhile, you sprinkle the basement floor with
water when the basement air becomes too dry, say 50%, using
a solenoid valve and a humidistat.

>could this be overcome by exchanging heat into outdoor ventilation?

sounds like you are talking about regeneration now, ie reconcentrating
the solution. if you are actually using a woodstove (vs a closed solar
still in the attic), you might condense the vapor that comes off the top
of an open licl tub on top of your basement woodstove in an air-air heat
exchanger that brings in outdoor air, but who's to say that your heating
and ventilation requirements are directly related? you might rather keep
the latent heat in the house, and condense the water vapor from the tub
on the woodstove under the ceiling lake, with water above and a gutter
below to collect condensation from the underside of the lake and a vapor
barrier built into the floor above the lake so the living space doesn't
get too humid. or condense it in a poly duct that covers the top of the
tub and runs the length of the basement ceiling.

here's a program that estimates the equilibrium licl concentration after
a long high-temperature bake, and the vapor pressure, dew point and rh
of nearby air after the solution cools to 90 f...

10 a1=12.7409'licl vapor pressure constants from hawlader paper
20 a2=-.065536
30 a3=-8.2416e-04
40 b1=-4675.4
50 b2=+29.31
60 b3=+.66911
70 c1=372690!
80 c2=-1689.8
90 c3=-187.1
190 w=.0119'humidity ratio for midland air in july
200 pv=25.4*29.921/(1+.62198/w)'vapor pressure in july (mmhg)
210 for tf=120 to 300 step 20'solution temp (f)
220 tc=(tf-32)/1.8
230 tk=273.1+tc
240 c=a1+b1/tk+c1/tk^2-log(pv)/log(10)
250 b=a2+b2/tk+c2/tk^2
260 a=a3+b3/tk+c3/tk^2
270 conc=(-b-sqr(b^2-4*a*c)/(2*a))'equilibrium soln conc (wt%)
340 tsf=90'soln temp after cooling (f)
350 ts=(tsf-32)/1.8'soln temp (c)
360 tk=ts+273.1'soln temp (k)
370 ap=a1+a2*conc+a3*conc^2
380 bp=b1+b2*conc+b3*conc^2
390 cp=c1+c2*conc+c3*conc^2
400 pvc=10^(ap+bp/tk+cp/(tk^2))/25.4'vapor pressure at tk ("hg)
410 td=9621/(17.863-log(pvc))-460'dew point (f)
420 rh=100*pvc/(exp(17.863-9621/(tsf+460)))
430 print tf,conc,pvc,td,rh
440 next

bake             vapor        dew
temp   conc      press        point      rh
(f)    (%)       ("hg)        (f)        (%)

120    35.62    .4452         55.26      30.74
140    43.17    .2296         37.61      15.86
160    50.35    .1048         18.21       7.238
180    57.28    .04254        -2.30       2.938
200    64.02    .01543       -23.36       1.066
220    70.61    .005037      -44.48        .3478
240    77.05    .001492      -65.22        .1030
260    83.32    .0004051     -85.25        .02798
280    89.41    .0001022    -104.3         .007059
300    95.30    .00002432   -122.3         .001679


wow nick!  you're an incredible head with numbers  :)

now, can this be put to practical application storing heat?

what could be some possible means and methods?

steve young

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