re: "radiant wall" to deliver heat
1 may 2002
>i'm planning on having a combined solar-thermal and wood-heated hot
>water supply for home heating.
wood is ongoing work and mess, with fire and air pollution hazards,
and getting its heat into water isn't easy.
>now, my wife loves the feeling of radiant heat (and i want to keep
>her happy)! but for a delivery system, i don't want to do a radiant
>floor, because i live in a warm climate (georgia), and i want to
>couple the slab floor to the earth (with perimeter insulation),
>so i can use it to keep the house cool longer into the summer.
it's harder to get coolth (vs heat) out of the ground. the ground warms up,
and heat can flow better upwards than downwards in soil, with evaporation
in lower layers and condensation above. the average yearly air (deep earth)
temp in atlanta is 61.3 f. how many cubic feet of soil can you heat to 75 f
by fall and cool to 65 by spring, at 30 btu/f-ft^3? how big is your house?
how much insulation? will it have ac or dehumidification?
>sooo, i'm toying with the idea of doing a "radiant wall." i would
>build a wall of 4" concrete block, with the cores laid sideways rather
>than the usual vertical orientation. i'd surface bond both sides. and
>i'd snake pipe (prob. pex) through the cores, back and forth row by
>row, and fill the cores with mortar. then i'd build an insulated wall
>behind it (2x4 with fiberglass), to keep the heat in the desired room.
sounds messy, with lots of bulges to "surface bond" and lots of bends
and hydraulic resistance (and a few leaks?) in the pipe and a long time
constant, so the house will overheat on a warm day and stay frozen for
a long time after you return from the bahamas.
>i will probably also use baseboard radiators in the same room, for
>quick air-temp pickup when the zone first "comes on."
>in use, then, i would run the hot water through the baseboards first
>(at ~ 140f), then through the wall to get the long slow heat, plus to
>get a little more use out of the energy in the water (and thus improve
>system efficiency somewhat).
sounds like too many heating systems.
more nrel data for atlanta (lat. 33.65 n, elev. 1034', at 14.2 psi):
24h avg temp 41.0 78.8 f
avg daily min/max 31.5/50.4 69.5/88.0 f
humidity ratio w 0.0040 0.0153 #w/#da
windspeed 9.9 7.6 mph
s wall sun 1030 710 btu/ft^2-day
horiz sun 820 1950 "
diffuse component 370 950 "
atn(s/h) 51.5 20.0 degrees
max sun elevation 32.9 80.0 "
e/w wall sun 525 1030 btu/ft^2-day
n wall sun 240 600 "
you might use r48 sips and heat and cool and dehumidify the house with
a linear parabolic concentrator on the roof, which might also make hot
water for showers and contain some pv panels under several suns in
the shallow water trough under the reflector.
a 48'x48' house made with 20 8'x24'x12" r48 sips (about $20k) with 4% of
the floorspace as r4 windows has a conductance of 96ft^2/r4 = 24 btu/h-f
for windows plus 1440/48 = 30 for walls plus 48 for ceiling. add 30 cfm
of air infiltration, and the total is 132 btu/h-f, so it needs (65-41)132
= 3168 btu/h to stay 65 f inside on a 41 f day, ie 76k btu/day. if a 300
kwh/mo indoor electric usage contributes 34.1k btu, 41.9k btu is left,
or 210k btu for 5 cloudy 41 f days.
on an average january day, 40 ft^2 of south windows and 20/20/16 ewn
with 50% solar transmission would supply 0.5(40x1030+40x525+16x240)
= 33k btu of heat, leaving 8.9k from other sources, eg a plastic 55
gallon drum used as a simple graywater-air heat exchanger.
there seem to be at least 3 ways to store heat for 5 cloudy days, in order
of mystery and complexity: a) lots of hot water, b) a licl solution, adding
liquid water on cloudy days, and c), b), adding water vapor on cloudy days...
a) requires more storage volume than b) which requires more than c), which
can also make air conditioning and dehumidification.
in a), you might have an indoor tank for heat storage and distribution,
with water cooling from 180 to 80 f over 5 days, 210k/(180-80) = 2096
pounds or 262 gallons or 33 ft^3 of water, eg a 1'x4'x8' tall tank or 5
8'x12" vertical pvc pipes in a closet.
in b), you might have a 180 f water tank and another tank that's filled
with a 180 f 50% licl/cacl2 solution on an average day, and mix them over
5 days to make a 33% solution. diluting a 50% licl solution from 50% to 0%
releases 262 btu/lb of heat... 50 to 33% might release (50-33)/50x262
= 87 btu/lb, so starting with p pounds of licl solution and p pounds of
water, 210k = 2p(180-80)+87p makes p = 732 pounds, approximately, ie 2
91 gallon tanks, or 4 8'x12" pvc pipes in a closet.
in c), you might have a pipe full of licl solution and a pipe full of
rocks to act as a "packed column" to absorb water vapor in licl, and
a pond full of rocks under the slab or outdoors to supply the vapor.
each pound of vapor absorbed releases about 1000 btu. diluting p pounds
of 180 f 50% licl to 33%, 210k = p(180-80)+87p+1000p, approximately, so
p = 177 pounds. we need a licl container that will hold 354 pounds or 44
gallons or 5.5 ft^3 of solution when full, eg one 8'x12" pipe.
the house needs 2k btu/h on cloudy days. with a thermal resistance r to
"deep earth," vapor comes out of the pond at 61.3-dt (f), where r=dt/2k.
if the (damp) soil conductance is say, 16 btu-in/h-ft^2-f, each 1" layer
under the pond has 2304 ft^2 of surface with a resistance of 1/(2304x16)
= r2.7x10^-5. if the house needs 24hx2kbtu/hx90d = 4.3 million btu over 3
months, and we cool d' of soil dt, then 4.3m = 2304x30ddt and ddt = 62.2,
but r = d/2x12x2.7x10^-5, approximately, so d = 6173r = 3.09dt, and dt
= sqrt(62.2/3.09) = 4.5 f, and the vapor emerges at 56.8 f.
how many square feet of surface do we need in the packed column to make
2k btu/h? how much room air needs to flow through the column, with what
this house would never freeze in wintertime, with condensation under
the slab. with only the slab, it might be comfortable with a sweater.
in july, we need (78.8-70)132+34.1k/24 = 2580 btu/h of cooling. drying
30 cfm of fresh air from w = 0.0153 to 0.00787 (70 f at 50% rh) requires
30x60x0.075x(0.0153-0.00787)x1000 = 1000 btu/h, for a total cooling load
of 3580 btu/h, or 64.4k btu over 18 hours.
we might turn on a fountain in the house when cooling is needed and
trickle some licl solution into and circulate house air through the
packed column, keeping the house at 50% rh. if the house becomes more
humid, we run the column without the fountain.
house air exits the column warmer than when it entered, but with less
moisture, so the net effect on the house is cooling, because it takes
more heat to raise the exit air back to 50% rh than to cool it to 70 f.
for example, if a pound of air enters the column at 70 f and 50% rh and
emerges at 80 f and 10% rh, cooling it back to 70 f requires about 2.5
btu, but its exit vapor pressure is pa = 0.1exp(17.863-9621/(460+80))
= 0.105 "hg, so it contains 0.62198/(29.921/0.105-1) = 0.00219 pounds
of water, vs 0.00787 for house air, and increasing the rh back to 50%
requires about 1000(0.00787-0.00219) = 5.7 btu, so 3580 btu/h requires
moving 3580/(5.7-2.5) = 1119 pounds of air per hour, or 1119/0.075/60
= 249 cfm.
meanwhile, the licl solution is getting warmer and more dilute. it might
warm from say, 100 to 140 f during the day, as it dilutes from 50 to 25%.
the licl tank could be plumbed through an open pond above the flat polycarb
cover of a 4' wide x l' long x 8' high linear parabolic water heater on the
roof, with 8' vertical polycarbonate south glazing to collect and recycle
condensate. during the day, warm dilute licl would thermosyphon up through
the open pond. it would cool at night and cooler more concentrated licl
would fall back into the reservoir.
there are lots of numerical pitfalls here. these papers have clues:
"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 and
"effectiveness of heat and mass transfer processes in a packed bed
liquid desiccant dehumidifier/regenerator" by viktoria martin and
d. yogi goswami in hvac&r research, vol. 6, no. 1, pp 21-39,
january, 2000 and
"a review of liquid desiccant cooling" by viktoria oeberg and d. yogi
goswami, chapter 10 in advances in solar energy, vol. 12, pp 431-470,
1998, american solar energy society publishers.