Thursday, July 18, 2013

Recycle a freezer into a heat storage tank

This 24.9 ft^3 chest freezer could hold 24.9x7.48 = 186 gallons of water in a 10'x12' folded EPDM liner...

It uses 568 kWh/year, eg 3x568x3412Btu/kWh/365d/24h = 664 Btu/h with a COP of 3 at 0 F in a 70 F room, with a 664Btu/h/70F = 9.5 Btu/h-F thermal conductance and about 80 ft^2 of surface with an R-value of 80ft^2/9.5Btu/h-F = R8.4 ft^2-F-h/Btu, which could be raised with fiberglass or foamboard. R10 foamboard over the top and sides would reduce the conductance to 65ft^2/R18.4+15ft^2/R8.4 = 5.3 Btu/h-F.

It could live above a 4'x16' bare fin 2.56 gpm DIY collector made from $67 worth of 0.018" brown- or black-painted aluminum flashing as described at

The collector could have 128 ft of 1/2" PEX-Al-PEX tubing on 6" centers, with 2 Ts to make 2 64' 1.28 gpm flow paths. says 64' of 1/2" PEX tubing with a 0.475" ID would have a 1.28 gpm flow with a 3.7' pressure drop, and an $85 Grundfos 15-58C pump can deliver 2.56 gpm with a 8' pressure drop on speed 1, using 60 watts.

A $100 differential thermostat controller or an Arduino controller could run the pump when the collector is less than 40 F to avoid freezing or when the tank is warmer than 160 F to avoid overheating the EPDM liner. The tank could could contain a 1"x300' pressurized PEX coil as a heat exchanger, or it could be a gravity-pressurized hot water supply with a float valve. (Rich Komp has a gravity hot water supply with a 3' head (a 55 gallon drum) for a shower with a rainwater supply in his off-grid house in Maine. The flow is adequate, but it won't knock you back against the wall like a shower in a hotel.)

Where I live near Philadelphia (not an easy climate for solar heating), 1000 Btu/ft^2 of sun falls on a south wall on a 30 F average January day with a 34 F daytime temp. If 0.8x8'x16'x1000 = 102.4K Btu enters 8'x16' of R2 twinwall glazing with 80% solar transmission over 6 hours and 51.2K falls on a 4'x16' 160 F bare collector which also gains 6h(Ta-160)4'x16'x3 Btu from both sides from Ta (F) air inside the glazing and

102.4K = 51.2K + 6h(Ta-160)4'x16'x3 + 6h(Ta-34)8'x16'/R2,

then Ta = 161.8 F, with 51.2K + 6h(Ta-160)4'x16'x3 = 53.3K Btu of water heating on an average January day, ie 30% more than the 41K Btu/day SRCC 0G-300 spec, in colder, cloudier weather, eg 53.3K/(110-60)/8.33 = 128 gallons of 110 F water heated from 60 F.

On the 1st cloudy day after a long string of average days, the tank can supply 53.3K Btu and cool to 160-53.3K/(186x8.33) = 125.6 F, with no backup heat requirement.

On the 2nd cloudy day, as it cools from 125.6 to 110 F, the tank can supply (125.6-110)186x8.33 = 24.2K Btu, enough to heat 24.2K/(110-60)/8.33 = 58 gallons of water from 60 to 110 F. Adding 128-58 = 70 gallons at 60 F will reduce the tank temp to (60x70+110x186)/(70+186) = 96.3 F, with an average (110+96.3)/2 = 103.2 exit temp, and heating that 70 gallons to 110 requires (110-103.2)70x8.33 = 3986 Btu, making the backup fraction 3986/53.3K = 0.075 after the 2nd cloudy day.

On the 3rd cloudy day, adding 128 gallons of 60 F water lowers the tank temp to (60x128+96.3x186)/(128+186) = 81.5, with an average (96.3+81.5)/2 = 88.9 exit temp, and heating that 128 gallons to 110 requires (110-88.9)128x8.33 = 22.5K Btu, making the backup fraction 22.5K/53.3K = 0.422 after the 3rd day.

On the 4th cloudy day, adding 128 gallons of 60 F water lowers the tank temp to (60x128+81.5x186)/(128+186) = 72.7, with an average (81.5+72.7)/2 = 77.1 exit temp, and heating that 128 gallons to 110 requires (110-77.1)128x8.33 = 35.1K Btu, making the backup fraction 35.1K/53.3K = 0.634 after the 4th day.

On the 5th cloudy day, adding 128 gallons of 60 F water lowers the tank temp to (60x128+72.7x186)/(128+186) = 67.5, with an average (72.7+67.5)/2 = 70.1 exit temp, and heating that 128 gallons to 110 requires (110-70.1)128x8.33 = 42.5K Btu, making the backup fraction 42.5K/53.3K = 0.769 after the 5th cloudy day.

(If I did that right. I should learn how to use spreadsheets :-)

Assuming cloudy days are like coin flips, the expected backup heating fraction is approximately

0x2^-1+0x2^-2+0.075x2^3+0.422x2^-4+0.634x2^-5+0.769x2^-6 = 
0         +0        +9.4E-3      +26.4E-3     +19.8E-3    +12.0E-3       = 0.0676,

so, ignoring the tank heat loss, the expected solar heating fraction is 1-0.0676 = 93% in January.  


Now that all of us are plagued with the pollution resulting from the overabundance of devices we have purchased, perhaps government or church groups should sponsor a series of "you don't need it" commercials. Instead of the bright uniformed "service personnel" of the Ace Air Conditioning Company briskly delivering and installing the latest gadgets, the commercials would show the expensive equipment misused: a bored housewife growing geraniums in her new dishwashing machine; a small child casually dismantling a TV-stereo combo with a claw hammer...

from the book "Sunspots," by Steve Baer, 1979.

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