Winter has passed, giving us time to sit and review how our solar furnace worked. It’s a bit tricky to evaluate exactly how effective it was since we made several other changes to the house last fall (sealing up gaps around windows, turning our thermostat down to 59) but we feel it made a huge difference. Here are our thoughts and I’ll dig into the numbers down below.
- Mmmmm, warm. We enjoyed the sunny days when the solar furnace raised our indoor temperature to 72 while our gas furnace thermostat stayed set at 59.
- Warm air did circulate fairly well through our house (although it did stay warmest in our living room, which is where Will has his home office and where the dog likes to spend her time).
- The solar furnace reduced our output of carbon dioxide by 1,792 pounds between the electricity and natural gas conserved.
- Since this was a demonstration project cost-shared by SIREN, it was a low-cost way for us to enjoy solar technology.
- Solar furnaces do absolutely nothing when it’s cloudy and can not fully replace a regular furnace. (Having one made me much more conscious of when we have sunny days.)
- Our dollar savings were pretty minimal, about $52 for the season.
- Payback time would be 58 years (40 if you get the tax rebate) but “normal” people would see higher savings (see note below)
- Someone who keeps their thermostat at 70 during the day would see much, much greater savings because they would be replacing a lot more gas/electric furnace time.
- Someone who relies on electricity for heat would see much greater savings because it’s more expensive than a gas furnace (even counting both the gas and electricity needed to run a gas furnace).
- Someone who lives in a place with more expensive electricity would also see higher dollar savings – our electricity is nominally 9.7 cents per kWhr, which is really about 12.5 cents with the fees added in.
- It’s likely our solar furnace would be even more effective if we increased the thermal mass in our home to absorb that heat on sunny days and keep it longer into the night. We could also use more insulation in our attic and crawlspace.
- All these numbers should be taken with a grain of salt as we changed a lot of variables at once, which is a common challenge as people green their homes – who wants to make just one change and wait a whole year to document how effective it was?!
Now some numbers. Last winter (November 2009-April 2010) we used about 352 CCF of natural gas (one CCF = 100 cubic feet of gas) while this past winter with the solar furnace (November 2010-April 2011) we used about 236 CCF. That’s a reduction of about 33%. Will took a look at the outdoor temperatures and overall they’re pretty comparable from winter to winter.
Part of the savings we saw is due to lowering the thermostat on our gas furnace. We turned our thermostat down about six degrees during the day, from 65 to 59, and about 3 degrees at night, from 62 to 59. Past studies have shown that homeowners reduce their energy usage about 1% for every 8-hour period per degree. However, we’re guessing those studies were looking at people turning their thermostat down from 72 degrees and we’d expect energy savings to be smaller as you get lower. (This is because it takes more energy to heat when there’s a larger differential between the outside air temperature and the inside air temperature. Somewhere I saw a very pretty exponential curve showing this but I of course can’t find it right now.)
Let’s say down in our temperature range the savings is more like 0.6% for every 8-hour period per degree. So during the day (roughly 16 hours) we would save 0.6% x 6 degrees x 2 time periods = 8%. At night (roughly 8 hours) we would save 0.6% x 3 degrees x 1 time period = 2%. That gives us 10% total.
We also caulked around all our windows and insulated our electrical outlets and attic doors. Past studies say that can reduce energy usage 10-15% but our house really seemed pretty tight beforehand so let’s say it was just 8%.
The solar furnace is the rest, (so 33% – 10% – 8% =) 15% savings. Not enormous but we enjoyed a much more comfortable home since the solar furnace would routinely heat our house up to 68 or 70 while we left our gas furnace thermostat at 59. Dollarwise, our energy is cheap and therefore our savings were modest. We pay about 55 cents per therm/CCF. So our overall gas savings for the winter was a modest $64 ($0.55 x 116 CCF). Out of that, only about $27 can be attributed to the solar furnace (45% of the total savings).
However, our gas furnace also uses quite a bit of electricity to run the fans. Again, I had to back calculate to estimate how much since we can’t put a kilowatt on our furnace. Our total electricity usage for the 2009-10 winter was about 1943 kW-hrs over 6 months. For the 2010-11 winter, it was 1,156. That’s a difference of 787 kW-hrs but a big chunk of that can be attributed to our new refrigerator, which we found uses 1.9 kW-hrs less per day, or about 342 for the winter. That leaves us with an overall reduction of 445 kW-hrs (a 22% savings over last year), which at 12.5 cents each would be a savings of $55. Out of that, about $25 can be attributed to the solar furnace (about 45% of the savings) while the rest is due to the lower thermostat and weatherizing.
So our total solar furnace savings = $27 from gas + $25 from electricity = $52. That means our payback time on a $3,000 solar furnace system (installed price) would be about 58 years. Not super. However, there is a big tax rebate available that will refund 30% of the system cost, bringing the price down to $2,100 and the payback time to 40 years. Still pretty slow but we are very sure the savings would be much better for someone who normally keeps their thermostat around 68 or 70.
In terms of carbon dioxide, we saved both on natural gas and electricity. Natural gas generates about 12 pounds of CO2 per CCF of natural gas (according to the Carbon Fund). So 116 CCF x 12 pounds = 1,392 pounds of carbon dioxide saved. Electricity generated from coal generates about 2 pounds of CO2 per kW-hr (according to Kilowatt Coal). So 200 fewer kW-hrs would mean 400 fewer pounds of carbon dioxide generated. (As a side note, natural gas actually produces less carbon dioxide than coal if you convert everything to kW-hrs for a side-by-side comparison but in this case we’re sticking with CCF since that’s what appears on the gas bill.)
Total carbon dioxide savings? 1,792 pounds from the solar furnace, or about 9/10ths of a ton. Whew!
So, our bottom line conclusions are:
- Qualitatively, we really like our solar furnace. It really cranks out the heat and we enjoy waiting to see when it will magically kick on and give us hot air from the sun. On sunny days it will run from about 11:00 to 4:00 and can easily heat the house up 10+ degrees. The fan isn’t too loud and with our open floor plan, the warm air circulates fairly well. We have considered putting in a fan or two towards the far end of the house to improve circulation even more but have decided we like having our main living area be the warmest and our bedrooms be the coolest. We would like to experiment with adding a little more thermal mass to the house (to soak up the heat and radiate it back into the house at night) and of course more insulation.
- Quantitatively, we’re a little disappointed in the payback time but we think that’s mostly because of our radically low furnace thermostat setting. The solar furnace performs the best during the spring and fall “edge” seasons when there tends to be more sun and a lower differential between indoor and outdoor temperatures. If you’re determined to keep your indoor air temperature at 68 or 70, your furnace will run a lot during those periods and the solar furnace can make a huge offset. However, we were often opening our windows during those periods to let the “warm” 60 degree air into our house. And even during the coldest and darkest parts of winter, the gas furnace never really had to run very much to keep the house at 59 so there wasn’t a lot of usage to offset.
I’m going to try to get some data from two other friends who have solar furnaces who I believe keep their gas furnace thermostats at a higher setting, to see what they have observed. I would also love to figure out how the solar furnace compares to installing insulation, in terms of return on investment. What kind of savings would you see from $3,000 worth of insulation? Maybe that will be our next experiment…
**UPDATE** Our friend Woodie Bessler ran through some calculations to see how a $5,000 investment in a solar furnace from RREAL compares to buying $5,000 in photovoltaic panels at $5/watt and using them to run an electric heater. His conclusion? The solar furnace produces more heat in BTUs. This made us feel good about our system. However,
- The output is comparable if the cost of PV panels drops to about $3.85/watt.
- Excess electricity from the PV panels can be used to power other things, whereas excess heat from the solar furnace will do nothing (and may even be undesirable).
- An even better option may be to use PV panels to power a geothermal heat pump, which would produce three times the heat, meaning an equivalent amount of heat could be produced for about half the cost.
You can see a pdf of his calculations here.