NREL says 800 Btu/ft^2 of sun (300 diffuse) falls on a south wall on
an average 31.8 F December day with a 24.4 and 39.2 daily max and min
in Allentown, near the PA Renewable Energy Festival, 9/22-23/07,
http://www.paenergyfest.com , where I'll be talking about the system
below at 2:30 on Saturday and Nathan Hurst will talk about his Mazda
radiator solar heating experiments in Australia at 3:30 on Sunday.
Rich Komp (author of Practical Photovoltaics) will discuss energy-efficient
food storage at 3:30 on Saturday and new PV developments at 4:30 on Sunday.
We will all be exhibiting ourselves and a $5 1995 Mitsubishi 2.0 Eclipse
radiator at an "Ask the Engineer" table near Booth 24 in the exhibit area.
If a house is 65 F on average indoors (eg 70 F for 12 hours per day and
60 for the other 12) and a frugal 300 kWh/mo of indoor electrical use
provides 34K Btu of heat on an average day and a 4'x8'x3'-tall EPDM-lined
plywood heat storage tank on the ground containing 4x8x3x62.33 = 5984 pounds
of 140 F water warms the house using an 800 Btu/h-F radiator for 5 cloudy
days until it cools to Tmin and the house thermal conductance is G Btu/h-F
and we keep it 70 F on a 24.4 F morning, (Tmin-70)800 = (70-24.4)G makes
Tmin = 70+0.057G.
On an average day, we need 24(65-31.8)G-34K = 796.8G-34K Btu of heat energy.
If (140-Tmin)5984 = (140-(70+0.057G))5984 Btu = 5d(796.8-34K), G = 136 max,
and Tmin = 78 F, and the house needs 74.4K Btu/day of non-electrical heat.
A 1024 ft^2 house with a 640 ft^2 loft might look like this,
viewed in a fixed font:
. . 12'
. 24' .
. . . . . . .
8'. . . 8'
. . .
. . . . . . . . .
. . .
. . .
. . .
. . .
. . . 32'
. . .
. . .
. . .
. . .
. . . . . . . . .
If made entirely of Structural Insulated Panels (18 SIPs?) with R-value Rv
and 1024 ft^2 of ceiling and 2304 ft^2 of walls and no air leaks, G = 136
= 3328ft^2/Rv makes Rv = 24 ft^2-F-h/Btu min; 8" R32 SIPs make G = 104. With
good airsealing and 32 cfm of air leaks, we might have G = 136 Btu/h-F.
With 136 ft^2 of R30 walls and a 136/30 = 4.5 Btu/h-F conductance, the tank
would supply 24h(140-65)4.5 = 8K Btu of house heat on an average day, leaving
a need for 74.4K-8K = 66.4K Btu/day of solar air heat (line 150 in the calc
below.) Sunspace air keeps the house 70 F during collection time and stores
heat in the house mass (line 340) to keep it warm overnight as it cools from
70 F at dusk to 60 at dawn.
If 500 Btu/ft^2 of 250 Btu/ft^2 full sun arrives in 500/250 = 2 hours on
a 6-hour solar collection day and 300/(6h-2h) = 75 Btu/h-ft^2 arrives in
the other 4 hours, we can model AS ft^2 of $/ft^2 Thermaglas Plus U0.58
twinwall polycarbonate "solar siding" with 80% solar transmission over
a 1 foot air gap over a dark south wall like this, viewed in a fixed font:
0.8x250AS = 200A Btu/h 1/(0.58A)
--- -------www---------- TSF
|---|-->|---------- TSF |
--- | - | 35+200A/(0.58A) = 380 F
| - ---
1/(0.58A) | - -
35 F----www----- |
TSF is a Thevenin equivalent (no load, stagnation) sunspace air temp in
full sun. With A = 192 ft^2 (line 170) and a 140 F auto radiator and
its 2 30 watt 1000 cfm 12 V fans to heat tank water:
RS TAF Q Btu/h
1/111 1/1000 | ---
-------www-------www-----*------|-->|---- 65 F
| --> | ---
| 380 F I | |
--- 4K Btu/h | | 1/800 140 F |
- v --www---------| |--|
We can collect 8K Btu/h of tank heat in 2 hours of full sun if the sunspace
air temp TAF = 140 + 4K/800 = 145 F. At the same time, we can collect Q Btu/h
of warm sunspace air. With RSER = RS + 1/1000 = 1/100 and I = (380-145)/RSER
= 23.5K Btu/h (6.9 kW at $5K, for PV fans :-), Q = I-4K = 19.5K Btu/h.
In diffuse sun, we have:
0.8x75x192 = 11.5K Btu/h 1/111
--- -------www-------- TSD
|---|-->|---------- TSD |
--- | - | 35+60/0.58 = 138 F
| - ---
1/0.58 | - -
35 F----www----- |
1/111 | 1/1000
-------www---------www----- 65 F
| 138 F I
I = (138-65)/RSER = 7300 Btu/h. We collected 2Q = 39K Btu of the 66.4K/day
air heat in full sun. We can collect the rest in (66.4K-39K)/I = HDIFF < 4
hours, so 4 4'x12' sheets of twinwall suffices. We could verify this with
a simple simulation using NREL's Allentown TMY2 weather file with measured
hourly weather data for a Typical Meteorological Year.
20 TAVG1.8'24-hour Dec temp in Allentown (F)
30 TMAX9.2'average daily max (F)
40 TDAY=(TMAX+TAVG)/2'average daytime temp (F)
50 GSUN0'south wall global sun (Btu/ft^2-day)
60 DSUN00'south wall diffuse sun (")
70 FSUN=GSUN-DSUN'south wall full sun (")
80 HSUN=FSUN/250'full sun hours
90 HDAY=6'daytime hours
100 GHOUSE6'house conductance (Btu/h-F)
110 HHOUSE$*(65-TAVG)*GHOUSE'average day house heat (Btu)
120 UELEC00'indoor electrical use (kWh/mo)
130 HELEC412*UELEC/30'electrical heat gain (Btu/day)
140 HTANK00'tank heat (Btu/day)
150 ESSA=HHOUSE-HELEC-HTANK'sunspace air energy (Btu/day)
160 PRINT HHOUSE,HELEC,HTANK,ESSA
170 A=4*4*12'sunspace glazing area (ft^2)
180 RS=1/(.58*A)'glazing resistance (F-h/Btu)
190 ISF=.8*250*A'full sunspace heatflow (Btu/h)
200 TSF=TDAY+ISF*RS'full sunspace equivalent temp (F)
210 CFM00'fan cfm
220 RSER=RS+1/CFM'sunspace series resistance
230 GRAD0'radiator conductance (Btu/h-F)
240 TAF0+HTANK/GRAD/HSUN'full sunspace air temp (F)
250 PRINT TSF,TAF,HSUN
260 FSSA=HSUN*(TSF-TAF)/RSER-HTANK'full sunspace air heating (Btu)
270 ISD=.8*A*DSUN/(HDAY-HSUN)'diff sunspace heatflow (Btu/h)
280 TSD=TDAY+ISD*RS'diffuse sunspace equivalent temp (F)
290 ICAP=(TSD-65)/RSER'house cap heatflow (Btu/h)
300 TAD=TSD-ICAP*RS'diff sunspace air temp (F)
310 HDIFF=(ESSA-FSSA)/ICAP'house heating hours)
320 PRINT TSD,TAD,HDIFF
330 HCOLL=HSUN+HDIFF'solar collection hours
340 ESTOR=ESSA-HCOLL*((70-TDAY)*GHOUSE-HTANK/24)'overnight heat (Btu)
350 HCAP=ESTOR/(70-60)'house heat capacity needed (Btu/F)
360 PRINT A,HCAP,HCOLL,HDAY
GHOUSE HELEC HTANK ESSA
Avg day electrical tank heat Warm air
heat (Btu) heat (Btu) (Btu) heat (Btu)
108364.8 34120 8000 66244.81
TSF TAF HSUN
Full sun Sunspace Full sun
eq temp (F) temp (F) hours
380.3276 145 2
TSD TAD HDIFF
Diff sun Sunspace House heat
eq temp (F) air temp (F) hours
138.9483 72.40973 3.65525
A HCAP HCOLL HDAY
Glazing House mass Collection daytime
area (ft^2) (Btu/F) hours hours
192 4159.546 5.655251 6
The radiator and its fan could be at the top of a vertical duct that returns
sunspace air to the lower sunspace without mixing with room air. On cloudy
days, pump water up through the radiator to warm the house. A pressurized
plastic pipe coil heat exchanger in the tank could heat water for showers,
with the help of a greywater heat exchanger.
Air might flow as below, conceptually, with a single fan and an upper
motorized sdamper hinged at the bottom (use Honeywell's 6161B1000 $0 2W
damper actuator or a $5 DC gearmotor from Grainger or a windshield wiper
motor with limit switches or a 12V damper from an auto heater) that opens
inwards up to 90 degrees (moving counterclockwise below) to block room
airflow when it is in the horizontal position:
top 2' top
a. r motor s. | | |
d. fa <--> d. | | adamper |
a. d a. | | |
m. <== ai m. 2' | | sdamper | 2'
p. a p. | | |
e. nt e. | | |
r. o r. s | | |
| r-.........-| u | |-----------| west
| 4' | n | | |
| | s | | |
| ^ | p | 20' | |
| | | a | | |
| room | c | s | |
| air | e | o | |
| | | u | |
.a .s | t | |
.d room sunspace .d ^ | h | |
.a air air .a | | | adamper |
.m ==> ==> .m | | |
.p .p | | sdamper |
.e .e | | |
.r .r | | |
---------------------------- Drawing not to scale.
a| r s|s |
d| fa d|d |
a| d a|a |
m| <== ai top <== m|m 4' | 12' south
p| a p|p |
e| nt e|e |
r| o r|r |
1. To heat the tank, pull sunspace air through the radiator with its fan
and return it to the sunspace below, with the motorized sdamper horizontal.
A (redundant?) lower one-way lightweight plastic film convection sdamper
opens (to the right) over a vertical hardware cloth grate when the fan runs
and prevents reverse sunspace thermosyphoning at night.
2. To heat the house, pull room air through the radiator and out to the room
via the upper adamper. The upper and lower adampers should be heavy enough to
prevent room air thermosyphoning up through the vertical duct when room heat
is not required.
3. To do both, open the damper halfway, or give house heating priority.
1. For more exact room temp control and less mass, add mass to the upper
24'x32' of the ceiling, with a ceiling fan and a room thermostat to keep
the house exactly 70 F when occupied and 60 when it's unoccupied. An open
wintertime door in place of the upper adamper can let sunspace air flow
into the room to heat the ceiling and return to the sunspace without mixing
with room air... 120 F ceiling mass can store 7 times more heat than 70 F
mass, with a shiny surface beneath to avoid room overheating by radiation.
2. Blowing room air through the lower adamper with a window fan could raise
efficiency and prolong the life of the radiator fans. Bearing guru Dave Pine
says auto radiator fan bearings last 3000-4000 hours (some last 7000 hours)
when he tests them at 225 F, and life doubles with every 10 C decrease, so
they might last 4000x2^((225-145)/1.8) = 87K hours at 145 :-)
IIRC, he was using driving them with 60 watts total in a PWM circuit
vs 120 W full speed. Or maybe 16 watts... Each? This is a bit vague.
Sure. Over some range, power is proportional to cfm^3.
That means moving about 200 cfm, with perfect heat exchangers.
If a fan uses 80 watts at 1000 cfm, it might use 80(200/1000)^3
= 0.64 W at 200. Probably more. Maybe not much more.
"Large format," vs lots of boxes.
One therm per day?
Collecting warm air and hot water at the same time can be more efficient
than collecting them separately. Give the water priority in full sun,
and take as much air as possible at the same time.
I suppose that's what was in the local auto salvage yard.
I abandoned a dead '97 Ford Taurus after my mechanic said it would take me
3 days to remove its radiator. Remove the front bumper, then some large
structural metal pieces above and below the radiator, and so on. Toyotas
and other late model Japanese cars have 4 plastic posts... 2 drop into
holes below and 2 fit into plates above with 2 screws. Easy.
Leave longish pieces attached to help connect them to standard plumbing
and pick cars with manual transmissions and no AC, other things being equal.
We don't export lots of valuable goods or services nowadays. Movies,
intellectual property, high-tech medical devices. It helps to reduce
waste and use what we have, eg dead cars and the sun. People say our
only growing resource is trash.