Solar Furnace Verdict: We Like It A Lot But Our Savings Are Minimal Because We Live in the Cold

solar_furnace_2Winter 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.

The Good

  • 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.

The Bad

  • 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)

The Ugly

  • 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,

  1. The output is comparable if the cost of PV panels drops to about $3.85/watt.
  2. 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).
  3. 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.

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Insulate Yourself!

Since heating is usually a household’s largest expenditure of energy and the cost is low, insulating your house is one of the most cost-effective conservation techniques around. An even cheaper option is to insulate yourself, but how much good can that possibly do? We don’t often think about our clothing as insulation, but as long ago as 1946, scientists have examined the insulative value (similar to the R-value of house insulation) of clothes, which allows us to figure out what impact it would have.

These researchers created a scale that allows you to figure out your comfort level based on air temperature, activity level, and clothing. Their baseline, 1 ‘clo’, is the amount of clothing a normal person would need to wear to be comfortable sitting around at 70 degrees. 1 clo is the equivalent of wearing a business suit with normal underwear. To figure out the clo value of an outfit, you just add the clo values of each individual item.

Some common clo values are 0.1 for a short-sleeved T-shirt, 0.2-0.4 for a sweater, 0.25-0.35 clo for pants, and 0.22-0.77 clo for a long skirt.

For every 2 degrees Fahrenheit, you need 0.18 more clo to be comfortable (or you need to be more active). A pair of extremely insulating long underwear can be purchased for $25 and provide about 0.36 clo, which would allow you to drop your thermostat 4 degrees! Depending on where you are and what you’re heating with, that could save 15% on your heating bill.

I had a hard time believing those numbers, but I recently got a good pair of long underwear and they’ve made a huge difference in comfort.  That makes it pretty clear that insulating yourself can have a huge benefit, especially if you’re currently wearing just a T-shirt and slacks!

How can something as simple as long underwear be so efficient? Air is a good insulator, so keep still air near your body helps insulate you tremendously. Your body is already producing heat too, so all you have to do is trap it rather than using a lossy process like a heat exchange to produce heat. Best of all, you only have to hear the small area right around your body rather than a whole room or even a house.

Last year, my biggest problem was keeping my hands and feet warm. Since I primarily work from home, I had to have warm enough hands that I could type comfortably. Luckily, I’ve found that keeping the rest of your body warm keeps your hands warm too, even without gloves! When your head and torso are cold, your body adjusts by drawing heat away from your extremities. Warming your head and torso mean that your body leaves them alone and your hands and feet stay comfortable even when it’s well below 60F.

If you’d like to learn more about clos, I highly recommend the article in Low Tech Magazine that introduced me to the concept and the clo numbers I listed above (and more)!

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Freeze Yer Buns Challenge

maggie-will-hatsFellow blogger Crunchy Chicken recently launched the Winter 2010-2011 Freeze Yer Buns Challenge, encouraging everyone to turn their thermostat down this winter.  She’s open to any temperature settings that works for folks but encourages people to push their limits a bit, especially at night.  I think her target is 62 during the day and 55 at night.

I feel like we’re already participating although in a rather unique way.  We have our thermostat set at 52 degrees and are relying on our solar furnace to heat us up to a more comfortable temperature during the day.  Our main motivation is keeping our electrical bill down so that we have a shot of winning the SIREN Energy Showdown grand prize – a one kilowatt photovoltaic system!  Will also likes the idea of just acclimating to a lower indoor temperature to lower our carbon footprint all winter, since we did fairly well adjusting to life without air conditioning this summer.

For the past couple of weeks it has been unseasonably warm (several days we opened all the windows to let the warm air IN) so with the solar furnace it has been in the high sixties during the day, dropping to about 55 overnight.  Yesterday it was cold but sunny so the solar furnace ran all day and brought the temperature up to about 65.  Then last night it got cold and our furnace actually kicked on for about ten minutes this morning to heat us from 51 to 52.  It has been between 52 and 54 all day and I’ve been here working.

And you know what?  It’s cold but it’s not unbearable.  I do wish we had some sort of supplemental heat for the bathroom – I hate stepping out of a hot shower into a cold room – but otherwise it has been okay.  I wear about three layers of long sleeves plus a hat, drink a lot of hot tea, and get up to exercise every hour or two to heat my body up again.  Perhaps life with minimal heat will improve my cardiovascular health!

Or perhaps in another month or two I can coax Will into turning up the thermostat to a balmy 58.  I’ll keep you posted…

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Solar Furnace is Here!

solar_furnace_1Okay, our solar furnace is actually installed!  Now if only it were cool enough to want to use it…  Still, that will come in time and for now we’re excited to show it off.  Here are some basic pictures of the final product; we’ll post pictures of the actual construction further down the road when we have caught up with ourselves (ha!).

It’s a lot flatter than I expected, about a handsbreadth deep.  I think it looks pretty good on the side of our house and Michael B. did an excellent job with the flashing around the edges.  So what you can’t see is that there are two holes in the back of the panel that continue through the wall of the house and into the living room.  One hole has a fan attached (that unfortunately sticks further into the room than we had envisioned) for pulling air from our house through the panel.  The fan is controlled by two thermostats.

solar_furnace_4One thermostat is mounted on the wall and lets us decide how hot we want our house to be (it cranks up to 90!).  The other is located inside the panel and makes sure the panel is hot enough to heat up air as it passes through.  We had it on for an afternoon right after it was installed and it gets amazingly hot.  We could totally set up a sauna in our living room if we wanted.

So here is my very crude graphic rendition of air flow through the panel heating our house.  We were a little surprised that the panel has no channels or grooves on the inside to direct air flow; it’s just one big black box.  The advantage is that you can cut the openings wherever you need them so if your house happens to have a phone jack or an electrical outlet right where you were going to install the fan, you can move it over a few inches and just put the hole in a different place in the panel.

We’ll keep you posted on results as the weather cools down.

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Coming Soon: A Solar Furnace!

Solar Air Heater on Boggess Home (slightly different model)Guess what?!?  We were chosen to have a solar air heater installed on our house as a demonstration project sponsored by the Southern Indiana Renewable Energy Network (SIREN)!  Solar air heaters (also known as solar furnaces) are essentially big black boxes that heat up in the sun.  They are filled with air channels so that air flows through the panel, heats up from the sunlight, and then the hot air goes into the house (generally with the help of a small solar fan).  Solar air heating is a form of solar energy that’s actually more efficient than solar electricity (photovoltaic panels) but doesn’t get as much publicity.  SIREN is excited to promote the technology and also hoping to launch a pilot project installing these systems on low-income houses.

Our solar furnace will come from the Rural Renewable Energy Alliance (RREAL), a company in Minnesota that makes them and also does a lot of work making renewable energy more accessible through training, education, installations, and a unique assistance program.  They partner with agencies that serve the low-income community to install these systems for minimal cost as a way to make low-income families more self-reliant.  Pretty cool stuff!  SIREN hopes to bring a similar program down to our neck of the woods and is looking for an agency to partner with, hopefully the South Central Community Action Program (SCCAP), which is a local group working on weatherization and energy assistance for low-income families (among many other great assistance programs).  Will and I are going through the application process so we can work together to try out the system, show it off to anyone interested, and further weatherize our home.

Want to see the system?  We’ll be posting lots of pictures on our blog when the installation happens and we’ll also be opening our home up for a few tours.  One will be as part of the Simply Living Fair coming up September 25-26.  We’re giving a talk on home energy reduction as part of the Saturday workshops and then our house will be featured as part of the Solar Energy Tour on Sunday.  It should be lots of fun!  (Disclaimer: I am chairing the organizing committee for the fair so OF COURSE I think it will be lots of fun.)

Hopefully we’ll get accepted into the SCCAP program and they will help us do some insulating and caulking later this fall to further increase the energy efficiency of our home.  I’m also still contemplating putting a greenhouse on our front porch to help improve heat gain through our south-facing bay windows but I’m  not ready to make any promises.  Regardless, we are looking forward to lowering our carbon footprint this winter while enjoying free hot air from the sun!

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Beat the heat treats

Electric ice cream makerMaggie and I have been looking for a good source of organic ice cream since February. For some reason, getting an ice cream maker hadn’t really occurred to us until I got one for my brother and his wife for their wedding. After hearing of their success with some great iced concotions, Maggie and I were even more interested. We took the final leap when we found an old shopping network ice cream maker at a yard sale for $10.

As a kid, our ice cream maker was a monstrosity, with a huge bright pink bucket (or perhaps I was just smaller then). You had to dump in ice and salt and churn it continuously. It was pretty grueling work for half an hour, but spread out among the five of us it wasn’t bad. And there was ice cream at the end!

The new ones work a bit differently. Instead of hand-churning, most of them use an electric motor (which makes it a bit faster too, I think). They also replace the salted ice with a smaller metal bucket with a liquid core. You leave that in the freezer until you’re ready to make some ice cream and then toss all the ingredients in and turn on the motor.

Our first experiment was honey-flavored frozen yogurt. The recipe called for corn syrup, but we thought the honey would work better. Unfortunately, we didn’t eat it right away and it solidified in the freezer. It was rock-hard, which made it hard to sample.

This past weekend, we got the ingredients necessary for actual ice cream (primarily heavy cream). The recipe made much more than actually fit in the container, especially once we put in some peach slices from the peaches my parents brought from the NC farmers’ market. The first batch turned out pretty well, although I had to clear peach blockages out every once in a while. The second batch just never froze completely and ended up as more of a milkshake. Unlike the old ice cream makers, you can’t just add more ice, so we either had to leave it at the milkshake level or dump it into the freezer and stir manually.

We took the ice cream to Maggie’s family’s Sunday gathering and it was a hit, especially alongside the peach crumb dessert (also from the NC farmers’ market). It was more crystalline than I expected, but that wasn’t bad. The peaches were also frozen solid, which shouldn’t have surprised me. I think next time I’ll try putting the fruit bits in towards the end of the process so they’ll be chewier.

Overall, we’re very pleased with it. Now we can make ice cream as organic as we want it to be. The size is just about perfect too. A quart of ice cream is enough to take to a gathering, but little enough that we can eat it all ourselves if need be (twist my arm!).

Our other beat-the-heat treat has been fruit juice frozen into popsicles. Generally, I think I prefer those (they’re great in the hot middle of the day), while Maggie prefers the ice cream as an after-dinner dessert.

In both cases, it helps make the heat more bearable when we’ve got something cool to eat, so they’re definitely worth the (minimal) effort!

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Beating the heat

Earth exploding from heatIt’s starting to feel a lot like summer. We had a couple days of hot weather about three weeks ago, but since then it’s been in the high 70s and low 80s. In the past several days, it’s been up to the high 80s and lower 90s and should stay that way for at least a week. Since it’s still June, I expect it’ll only get worse.

We’ve only turned the AC on once, briefly, when it was hot and stuffy before bed and I’d like to keep it that way as long as possible. Working from home makes it harder, since I can’t mooch off of business AC during the day when it’s hottest. Ceiling fans aren’t an option (yet), so here’s what I’ve been doing to beat the heat.

Keep the windows close and the blinds down during the day. This makes it darker, but also cooler, since our place retains some evening coolness for quite a while. Unfortunately, this also makes it kind of stuffy, which leads to…

Use a small fan. I have a little fan that I’ve used to good effect since college. It works best in the evening when I can prop it up against an open window, but it’s also useful for some targeted cooling. When I’m working on the computer, I set it up so that it’s blowing across my face which helps me avoid the sensation of stuffy air.

Stay up late, sleep late. This doesn’t work for Maggie because she has to be up on other people’s schedule, but since I work whenever I like, I tend to stay up later and only go to sleep once it cools down around 2am. I’ve thought about also taking a siesta in the hottest part of the afternoon, but I don’t want to get my sleep schedule too out of wack. I do sometimes have to be up relatively early.

Open the windows in the evening Around here, things start to get reasonably cool around 8pm. I open the front windows and the back door (it has a screen) and let the breeze come through. It’s probably not much of a temperature difference, but it sure feels good and it only gets better as it gets darker.

It’s all pretty low-tech and common sense, but it works pretty well. I do wonder if I’m missing anything, though. I haven’t really had to deal with extreme heat since my freshman year when my dorm didn’t have AC.

What do you do to keep cool and reduce your AC usage?

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