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re: heat cells- an alternative to evs.
17 feb 1998
 wrote:

>some twenty years back, i took out a patent on a device i referred to
>as a 'heat cell', which was simply a thermally insulated box
>containing a material with favourable heat storage characteristics...

sounds like a nice way to keep a house warm, using say, a woodstove inside a
well-insulated sauna containing lots of thermal mass, eg an 8' cube with 24
55 gallon drums full of water (c = 10,560 btu/f) inside 6" of fiberglass
insulation (us r20) in 384 ft^2 of exterior cube surface with a thermal
resistance r = r20/384ft^2 = 0.052 f-h/btu. heating the water to 180 f would
store (180-80)10560 = 1,056,000 btu of useful space heat with a minimum
usable temperature of 80 f. the water would lose about 24h(180-70)/0.052
= 50.8k btu in a day in a 70 f room, cooling 50.8k/10.56k = 4.8 degrees 
to 175.2 f, or more accurately, rc = 0.052f-h/btux10560btu/f = 549 hours,
so the cube would cool to 70 + (180-70)exp(-24/549) = 175.3 f in a day,
or to 80 f in -549 ln((80-70)/(180-70)) = 1,316 hours or 55 days...

>my calculations showed that such a cell could have maybe a tenfold
>advantage over the conventional lead acid battery in terms of
>(stored energy)/weight ratio.

a diehard might store 12vx100a-h = 1200 wh in 60 pounds, ie 20 wh/lb...

>the best material for storage is probably graphite. others, like
>aluminium or sio2 (rock) are less suitable.

the 1993 ashrae hof says a pound of graphite ("karbate") stores 0.259 btu/f,
vs stone at 0.2, aluminum at 0.214, sodium at 0.293, coke at 0.356, and
water at 1.

>at 1000 degrees c, one kilogram of graphite holds about as much energy
>as a 50 ah storage battery and about 1/30 that of a kilo of petrol.
>i.e., 300 kilos of hot graphite = about 10 kilos (2.5 gals) petrol.

i guess "usable energy" and efficiency increase as the hot - cold temp
difference increases. what was that formula? (th-tc)/th? so with a min temp
of 100 c (373 k), efficiency is at most (1273-373)/1273 = 0.71, ie 71%, and
the extractable energy is (1000-100)x1.8x0.259x0.71x2.2 = 655 btu or 192 wh,
ie 50 ah at 4 volts or 87 wh/lb, a factor of 4, at least... 
 
>heat cell losses and engine efficiencies might be poor but they are
>probably no worse than ices and the overall process of generating,
>storing and re-using electricity.

i guess most of the loss is in converting heat to motion, either way, and
a non-mobile house might want lots of btu/ft^3 or btu/$, vs btu/lb, and
the "waste heat" from making electrical power could heat the house via 
baseboard radiators.

>one favourable aspect of the heat cell is that it can be heated
>directly by concentrated sunlight...

sure, although collection efficiency decreases as concentration increases.

nick




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