Payback period for $200 solar space heater?

rollandelliott
rollandelliott Solar Expert Posts: 834 ✭✭
in General Solar Power Topics #1
http://www.builditsolar.com/Projects/SpaceHeating/KenSoffitCollector/Main.htm
they would be vertical against front of home facing due south. I could angle them but I'm not sure the added piping and insulation would be worth it?

I'm thinking of making the above $200 system. My zip code is 28273, charlotte, nc.
I'd only use it from Nov1 to April1 every year because after that it's warm enough to not need any solar heat.

Is there anyway to estimate how much money I would save on heating?
I have a heat pump system. From Mid December to end of January I'm guessing it is pretty much radiant heating since the air is so cold around 32F at night or lower. currently use around.

Used around 3000kwh/month in dec/jan last year I'm guessing 2000kwh/month went towards heating big big home.

Heck I'll even take a guestimate!
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  • TnAndy
    TnAndy Solar Expert Posts: 249 ✭✭
    #2
    Re: Payback period for $200 solar space heater?

    My guesstimate is a couple/three heating seasons to pay one off. It's cheap enough that you can likely afford to simply experiment and see. Biggest limitation is gonna be the fact you'll only likely heat one room due to the venting.

    If you could build a lean to type deal at about a 50 degree angle ( like a lean to greenhouse, either using it as a greenhouse, or a smaller version ), you can gain a LOT of heat on sunny days...LOT of heat.

    We have such a greenhouse, not attached to the house, but built back in a bank with south facing slope on the polycarbonate panels, and it will be 100 degrees, easy, in there in mid winter with outside temps in the 20's.....so much heat that we have to vent some to the outside to keep things from cooking inside.

    If I didn't already heat with wood, I'd be adding a version of the greenhouse to the lower, south side of the house and running some ductwork into it for heating.
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  • rollandelliott
    rollandelliott Solar Expert Posts: 834 ✭✭
    #3
    Re: Payback period for $200 solar space heater?

    Well my windows are around 4 feet off the ground, so I could build a lean to. but wouldn't that necessitate insulating the ground and the two side walls of the lean to? 2" rigid insulation is expensive. I was also thinking of doing a leaning system like the drawing below, I dont' see how a lean to that has just as much surface area would help at all?

    However I'd probably want to take it down in the summer. Perhaps with some engineering I could make the system easy to set up and remove.

    http://www.builditsolar.com/Projects/SpaceHeating/mssungrabber.htm
    Window%20Col1.png
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: Payback period for $200 solar space heater?

    in theory this would work and i would recommend it to be up off of the ground due to snows that would accumulate and put lower bracing in place to support it. the trouble with the design is that it will radiate heat out of the house at night unless blocking the opening into the house with insulation. also the window being ajar could create an air leak between the top of the lower pane of glass and the upper pane. all sides not exposed to the sun through the glass need to be well insulated and if you can't afford the insulation then don't do this.
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  • benthere
    benthere Solar Expert Posts: 113 ✭✭✭
    #5
    Re: Payback period for $200 solar space heater?

    There are calculators out there to size solar hot water collectors. I would assume a similar collection rate to the cheaper solar water collectors (same idea, passive solar collection but not selective surface etc.) and convert to KWH from there.
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  • GaryGary
    GaryGary Registered Users Posts: 9 ✭
    #6
    Re: Payback period for $200 solar space heater?
    rollandelliott wrote: »
    http://www.builditsolar.com/Projects/SpaceHeating/KenSoffitCollector/Main.htm
    they would be vertical against front of home facing due south. I could angle them but I'm not sure the added piping and insulation would be worth it?

    I'm thinking of making the above $200 system. My zip code is 28273, charlotte, nc.
    I'd only use it from Nov1 to April1 every year because after that it's warm enough to not need any solar heat.

    Is there anyway to estimate how much money I would save on heating?
    I have a heat pump system. From Mid December to end of January I'm guessing it is pretty much radiant heating since the air is so cold around 32F at night or lower. currently use around.

    Used around 3000kwh/month in dec/jan last year I'm guessing 2000kwh/month went towards heating big big home.

    Heck I'll even take a guestimate!


    Well, if you will take a guesstimate -- here is one :)

    1 sf of of collector will see about 1600 BTU per day on a sunny winter day.
    If the collector is about 40% efficient (which is typical), then each square ft provides (0.4)(1600) = 640 BTU per sunny day.
    I think Ken's collector is about 32 sf, so (32 sf)(640 BTU/day-sf) = 20800 BTU/day.
    That's equivalent to about a quarter gallon of propane burned in a typical furnace.
    Or, to 6 KWH of electricity.

    Not all days are sunny. On really overcast days the collector will collect essentially nothing. On part cloudy or thin cloud days you get some fraction of the sunny day 21K BTU. You have to make a guess at this knowing your climate.

    The 20800 BTU is 6 KWH.
    So, if you had resistance heat, you would save 6 KWH on a sunny day. With the heat pump you might save half that depending on how good the heat pump and its install are.


    I like Ken's collector construction, but I'd consider two changes:

    - Think about using the screen absorber. Its easier to build, and it actually tested a little better than the soffit absorber collector.
    http://www.builditsolar.com/Experimental/AirColTesting/Index.htm

    - Think about making a bigger collector. If you have more space, it does not cost that much to just make it larger. Size is important in solar heaters.


    If you want a somewhat more exact method of estimating the savings, have a look here: http://www.builditsolar.com/References/HowMuch/HowMuch.htm
    There is no entry in Andy's list of collectors for an air collector, but if you use the Heliodyne Gobi collector, you won't be too far off. The Gobi is a good collector
    and your air collector probably won't be as efficient, so you might want to knock a few percent off the Gobi numbers.

    Some solar air heaters have a less than one season payback:
    http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm


    Gary
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  • rollandelliott
    rollandelliott Solar Expert Posts: 834 ✭✭
    #7
    Re: Payback period for $200 solar space heater?

    So 6kwh x 12 cents per kwh is about 70 cents a nice sunny day.

    Half our winter days might be sunny so maybe only 60 days x 70 cents = $42/winter season or 5 years to pay off.

    thanks for the insight
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  • SCharles
    SCharles Solar Expert Posts: 123 ✭✭
    #8
    Re: Payback period for $200 solar space heater?
    rollandelliott wrote: »
    So 6kwh x 12 cents per kwh is about 70 cents a nice sunny day.

    Half our winter days might be sunny so maybe only 60 days x 70 cents = $42/winter season or 5 years to pay off.

    thanks for the insight

    However, couple other things to consider. If you can do this for the $200, you'd have a small but perhaps helpful backup system if you ever needed it. Like the wood stoves some people have and never, ever use except once a decade due to power outage, etc.

    Some people, and I am one, would enjoy doing it for that little money just to do it. Well, plus you would gain some heat. If you do it carefully, do a good job. You will get heat. How much you save and/or make on a payback on the investment in materials is somewhat of a bonus if you learn from doing and are happy you did. You can stand in front of the window inlet and feel that free heat coming in and smile. And if it works well, you may then decide to expand it or build a better or larger one to heat more of the house.

    I say go for it.
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  • GaryGary
    GaryGary Registered Users Posts: 9 ✭
    #9
    Re: Payback period for $200 solar space heater?
    rollandelliott wrote: »
    So 6kwh x 12 cents per kwh is about 70 cents a nice sunny day.

    Half our winter days might be sunny so maybe only 60 days x 70 cents = $42/winter season or 5 years to pay off.

    thanks for the insight

    As you make it bigger the cost does not go up in proportion with the size as you still only need one fan and one controller. The Suntuf glazing is about $1.20 a sqft and I think that two layers of screen is about $0.6 a sqft, plus a little more for a larger frame. So, maybe $2 a sqft to make it bigger?

    If you put it vertically against the wall, which works pretty well for solar heating collectors and also looks nicer, you don't need back insulation, so the glazing plus screen and a little frame lumber is about the only cost of making it bigger.

    In Montana (where I am), this kind of collector would be free in that it qualifies for the $500 state energy project tax credit. You might check on your state rebates:
    http://www.dsireusa.org/index.cfm

    Gary
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  • rollandelliott
    rollandelliott Solar Expert Posts: 834 ✭✭
    #10
    Re: Payback period for $200 solar space heater?

    my state requires any solar heating device be certified to get a credit, good idea though.
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