[Greenbuilding] Another solar yard furnace

[nick pine]

>... we might well solar heat an existing house without architectural 
>changes or loss of floorspace by:

> 1. Measuring its thermal conductance by looking at fuel bills and indoor 
> electrical use, or by heating it with electric space heaters with 
> thermostats for a while and reading the house electric meter; and
>
> 2. Lowering that conductance with airsealing, more insulation, or seasonal 
> foamboard window inserts; and

After blower door testing and airsealing the uninsulated 2x4 walls with a 
dense pack cellulose insulation retrofit, Terri's 100-year-old house (photo 
on page 8 of http://sbse.org/newsletter/issues/newsf08.pdf ) used 208 therms 
of natural gas at 95% efficiency (19.8 million Btu) and 551 kWh of 
electricity totaling 21.6 million Btu in January of 2011, when the average 
outdoor temp was 32 F, so the house conductance is now 21.6M/(65-32)/31d/24h 
= 881 Btu/h-F, so it would need 24h(65-36.6)881 = 600KBtu for an average 
36.6 F December day near NYC.

Indoor electrical use provided about 61K Btu, leaving a need for 538K 
Btu/day of solar heat in an average year, including gas water heating 
energy.

> 3. Adding enough solar siding to heat it for 24 hours on an average 
> December day, if possible; and

PVWATTS says the southeast and southwest walls receive 685 and 701 Btu/ft^2 
of sun on an average December day, with an average 40 F daytime temp and a 
potential solar siding gain of 0.8x685-6h(70-40)1ft^2/R2 = 458 Btu/ft^2 on 
the southeast and 471 for the southwest. An 8'x40' patch of SE siding would 
contribute 147K Btu/day, with 151K for SW.

> 4. Adding a solar yard furnace with enough glazing to heat the house on an 
> average December day (if steps 2 and 3 won't accomplish that) and enough 
> heat storage to keep the house warm and heat water for showers for 5 
> cloudy days in a row.

According to PVWATTS, an R2 twinwall yard furnace roof with 80% solar 
transmission and a 60 degree tilt would receive 967 Btu/ft^2 on an average 
December day. It could collect about 0.8x967-6h(70-40)1ft^2/R2 = 684 tu/ft^2 
of heat.

With no solar siding, the yard furnace roof could be 538K/684 = 787 ft^2, eg 
40'x20'. Adding 320 ft^2 of SE solar siding above the porch roof would 
reduce this to (538K-147K)/684 = 572 ft^2, eg 32'x20'. Adding 320 ft^2 of SW 
siding above the driveway would reduce the yard furnace roof size to 
(538K-147K-151K)/684 = 351 ft^2, eg 24'x16'.

But Terri would like to enclose the whole south wall in a transparent 
lean-to sunspace. It might have a $4500 40'x36' clear vinyl panel from 
http://www.farmtek.com/farm/supplies/prod1;ft1_canopies_tents;pg105657_105659.html 
that rolls down a quarter-cylindrical frame made with 6 $5 double curved 1x3 
beams on 8' centers with a 22' radius connecting the 2nd floor eave with the 
ground 14' to the south of the porch with a pressure treated 2x4 staked to 
the ground on edge with 1/2" rebar as the foundation. My first sunspace had 
a $50 32'x24' piece of cloudy 4-year polyethylene greenhouse film in lieu of 
vinyl.

Alternatively, Terri could have a sunspace like my current version, with a 
low-slope 40'x16' corrugated polycarbonate roof extending 16' to the south 
of the 2nd floor eave 8' above the flat porch roof connecting to a 40'x16' 
vertical wall made with 20 $100 4'x8' flat 10 mil HP92 polycarbonate 
double-glazed panels gently inflated with welding argon via tire valves. The 
roof and wall would collect 0.8x40'x16'(510+685) = 612K Btu/day and lose 
6h(70-40)640ft^2/R2 = 58K, for a net gain of 554K. Twinwall endwalls would 
collect more. It would be easy to add more insulation to the outside of the 
enclosed existing house walls.

This would enclose the existing porch and add 8'x40' of enclosed deck roof 
floorspace and enclose 8'x40' on the ground, which might include a picnic 
table with an awning and a December tomato and herb and red bell pepper 
garden and a patio over a heat store, which might supply heat to a hot tub 
as well as heat water for showers with a $60 1"x300" pressurized NSF plastic 
pipe coil as a heat exchanger.

The house needs 5dx538K = 2.7 million Btu for 5 cloudy 36.6 F December days 
with about 97% solar house heating in December. The sunspace could have 
2.7M/(140-80)/62 = 720 ft^3 of water cooling from 140 to 80 F in a 
well-insulated 8'x16'x6'-tall tank with a $300 folded 20'x32' EPDM liner, 
dug 6' into the ground.

Now all we need is SRCC approval--not :-)

Nick