I have an off grid house with enough solar for my current needs. The system was installed in 2007. But I have a propane wall heater that I need to either move to a different wall because it gets blown out by ferocious wind, or just junk entirely and get a new furnace. I've noticed that things solar have dropped since I put in my system.A lot! So I've been wondering if (I know it sounds nuts but please hear me out) perhaps I could put in an electric system of some sort. Maybe Z mesh in floor heat, or an electric stove. The thing is this is a SIPs house with walls about 10 inches thick, so it doesn't take all that much to heat it. I can spend $1200 to $3200 on another propane set up, or perhaps I could put that money into more solar. My current setup is maxed out, so I would have to ( I think!?) get another inverter, controller, batteries - the whole shebang I guess. Since I would not have to pay for propane to heat the house in the winter I was wondering if this might work out in the long run. I'm not sure how much of my propane bill is for heat, but I'd guess around $400 a year. Is this a totally nuts-o idea, or could it maybe work out? I wouldn't even mind being a little financially on the losing end if it meant I was entirely independent energy wise. I'd appreciate any input from those more knowledgable than I am. Thank you. Kavade

If you are considering heating solely with electric and you have a limited supply, look at a high efficiency electric heat pump, either conventional air source, mini-split, or geothermal. If your heating area is relatively small and open, one of the newer high efficiency mini-split heat pumps might serve your needs. With a heat pump, you will generally get 3 times as much heat output for a given amount of electricity as compared to resistance heating elements.

What about a wood stove? No electricity required, or only a small amount for a blower if so equipped.

Thank you. I'll check into heat pumps. I'm not wild about chopping wood, or wood smoke, so I think the heat pump is the thing I should look into.

$400/year won't pay for much solar. But if you do go solar, I'd make sure everything is tight (ie, do a blower door test) and then use an efficient (highest possible COP) air source or ground source heat pump. Plus keep some propane capability in case the sun doesn't shine while you are on vacation.

You might also consider hydronic solar panels + a water tank (old school active solar).

With a ground source or air to water heat pump, I'd look at always running the heat pump while the sun shines and store the extra heat in a water tank. Tanks don't wear out like batteries do.

Kavade-

A potential disadvantage of using solar PV energy for heating is that the energy source and energy need are usually out of phase. In most climates, the sun shines more during the summer, and heatiing is needed in the winter. During winter, when the sun is shining, passive solar is heating the house at the same time that solar electric is available for heating. Conversely, you need to the most heat at night or often on cloudy days when the solar PV is less available.

However, sounds like you already live off-grid and know what you are doing. The cheapest electric heating system is a relatively simple resistance type, but the electrical usage is greater than for heat pump types. However, resistance heating is using high-grade energy (electricity) in a sledge hammer way. Your relatively low heating costs of $400/annually seem too low to justify something like ground-source heat pumps, but you could trade off the lower initial costs of a resistance type heater versus something like an air-source heat pump like a mini-split, which should be more expensive initially but 2 or 3 times cheaper to operate.

As mentioned by jonr, if the objective is space heating and you want to do it with solar, the more straightforward approach is to use solar hot water. The collection efficiency for the panels is much greater for solar hot water than for solar PV since the solar thermal collects a wider spectrum of the solar spectrum than a solar PV panel. Also tanks for storing hot water should be MUCH more reliable and lower maintenance than batteries for storing electricity that would be used for heating.

Thank you very much for your replies. I will investigate all of these suggestions. I also ran across solar air heaters, and I've been wondering how well they work. When the sun is out , very well I gather. I suppose they could lessen one's dependence on other fuels, but that's all. I wonder if they are actually something a reasonably handy person could make, and make properly? Has anyone here made one and/or used one?

Posted By kavade on 08 Dec 2011 05:58 PM
Thank you very much for your replies. I will investigate all of these suggestions. I also ran across solar air heaters, and I've been wondering how well they work. When the sun is out , very well I gather. I suppose they could lessen one's dependence on other fuels, but that's all. I wonder if they are actually something a reasonably handy person could make, and make properly? Has anyone here made one and/or used one?

Absolutely...they work great.  You just have to remember to size the unit based on the fact you have no storage other than the mass of the structure, as over heating can be an issue on sunny days.  A very loose rule of thumb is 10% of the square footage of the area you want to heat in collector area (ie, 100 sq ft of collector for 1000sq ft of heating).  In your case, a hydronic system might make more sense, as you can set up storage.  It could also help with your water heating and is a way more efficient way to use the solar energy than pv is for heat.  Another idea is to build a low mass sun space on the south side of your place and circulate the warm air through windows on sunny days.  Again, no storage, but if you live in a sunny area and your house is well sealed and insulated, maybe a good solution in itself.

Check out the solar space heating ideas here on Build it Solar:

http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm

There are so many good, affordable, easy to build ideas there.  There are also, some pages there that have calculators for determining heat loss of your house...a good place to start so you can determine how much supplemental heat you need.

My solar air heater project is the larger of the 2 downspout heaters here:

http://www.builditsolar.com/Projects/SpaceHeating/DownSpout/DownSpout.htm

It's a heating animal when there's sun (not much in my area in the winter).  Currently, it's in storage as we just put in a new geo system, and I want to see how it works on it's own this winter.  If I had the project to do over, I'd go with screen as the absorber material rather than downspouts...probably a little more efficient, and a much cheaper way to build the collector.  I'd also have paid more attention to making the ducting larger.

There are also 2 very active solar groups in yahoo groups, "Simply Solar" and "Solar Heat".  Both groups have lots of ideas and help for solar thermal applications.

Have fun.  DIY solar can be addictive.

Scott

Sorry, couldn't get my links to work...you'll have to cut and paste them.

Thank you Scott. I will indeed check into this.

How much propane (in gallons liters,pounds, anything but dollars) do/did you use per year?

It takes quite a bit of PV, inverter and battery to run even the tiniest mini-split heat pump, but that would be 2.5-3x more heat per unit of power than any type of resistance electric heater.

It takes relatively little PV/battery to run the blower on an active thermal-air-panel, and your effective coefficient of performance would be many times that of a mini-split. (I've built a handful of them over the past 3 decades, from ~25 square feet up to ~100 square feet.) Definitely not difficult, but also definitely not a primary heat source for anything but a super-insulated house.

I'm guessing I use about 450 gallons a year of propane. The house is well insulated - 8 1/4 inch SIP walls are about R-30, and the ceiling at 10 1/4 inches is about R-39- though in thinking it over I could do a little work to make it even better insulated than it is. I have a ton of windows double paned, though I wish I'd gotten triple paned.But I can improve the window situation. I think I would like to make, or have made, an air collector, but I have also wondered if there are some super simple things I could do to enhance my heat gain. I'm curious to know what effect it would have to have a dark colored south wall. I remember that when I lived in the desert there was a noticeable difference in heat between my dirt drive area and my neighbor's asphalt area. Surely her asphalt - right next to her house- made her place hotter than mine in summer? I wonder what the temperature difference would be if two identical houses, one with dirt and one with asphalt, were measured? Of course, her asphalt was a couple of inches thick, and was dense, whereas a wall painted black....?Well, walls are dense too. Hmm. Any comments on simple things like a dark south wall?

If you have an unobstructed southern exposure every square foot of vertically oriented glazing on a low temp thermal air panel will offset about 1.5-2 gallons off the propane bill. So if you have some place where you can put up a 100 square feet of thermal air panel and do it right you could conceivably cut propane consumption roughly in half, assuming something like 100-150gallons is for hot water/bathing. There are diminishing returns with more glazing, since above some point you need to turn off the blower to keep from overheating.

A line-voltage thermostat (available at box store home centers) mounted in conditioned space, wired in series with a snap-disc mounted to the heat exchanger in the air panel that comes on at 110F, breaks at 80-90F (eg. WHITE-RODGERS Model # 3F01-111, available at Grainger and elsewhere) makes a fairly cheap, simple and reliable control system for an active-blower thermal air panel. In winter set the T-stat for the maximum amount of overheating you'd want to tolerate, then dial it back in spring as the solar gains through your regular windows are enough.

Simply painting a south facing exterior wall black buys you nearly nothing- your wall is R30, and low mass. The gains are slow, but the drop in surface temp at night on the exterior is fast, and highly emissive unobstructed south facing wall on a clear cold night can drop as much as 10F below the ambient air temp offsetting some of the dired gains. The vast majority of the solar gain will be through the windows (or glazed thermal air panel), and re-radiated heat off of dark emissive mateirals like asphalt is largely rejected by windows. Glazing over the south wall and circulating air from the gap into the conditioned space whenever the temp in the gap is above 110F makes it a building-integrated thermal air panel.

If you live where there's a consistent winter snow cover you get a significant boost in solar gain from snow-reflected heat, which might add another 1/2 gallon's worth of reduced propane use per square foot of south facing glazing.

Okay Dana. Thank you for taking the time to explain. A thermal air panel it will be. Yes, I have a south wall with enough room for the panel you suggest. I'll let you know how it turns out.

Snoop around the build-it-solar site for design & construction tips. There are many decent examples of mid-to-large sized active thermal air panels there, eg:

http://www.builditsolar.com/Projects/SpaceHeating/SolAirColWithDHW/AlAirCol.htm

http://www.builditsolar.com/Experimental/AirColTesting/Index.htm

http://www.n3fjp.com/solar/solarhotair.htm

(google is your friend- there lots of decent passive & active thermal air panel designs out there.)

you could conceivably cut propane consumption roughly in half

That math doesn't add up for me, even if there is great thermal storage (unlikely).


kavade,

You have mentioned $400/year for heat and 450 gallons propane/year total. That suggests that hot water should be your primary focus and an air system won't help you there. Use a hydronic solar system that will. You might even add drain water heat recovery at that level.

I think the biggest use of propane, outside of heat, is a generator for pumping my well.I have an on demand water heater. I would guess the well pump accounts for about a third of my propane use.

At current PV & propane prices it's probably cost effective to run the well system off an inverter & batteries, and buy more PV. You'd have to be running it quite a bit (for way more than mere well pumping) to be burning anything like 0.4 gallons a day for pump power, unless you're washing your car & watering the lawn daily.

Unless yours is a 1- person household that is extremely stingy on the bathing front you're using more than 100 gallons of propane for hot water. For 2-3 people showering daily (who aren't hot water hogs) 150-180 gallons would be about right. With drainwater heat recovery that can be cut by about 1/4, maybe more, depending on how hot water is being used other than in showers. (DWHR with baths would require storage of greywater but with showers it's a fairly straightforward heat exchanger on the drain pipe and incoming water.) A batch solar HW heater plumbed in series with the incoming water to the tankless can cut even more deeply into the HW fraction, but in deep-freeze climates it can be tricky and more expensive to implement. (Where are you?)

In "typical" stick-built New England homes & climate an unobstructed 30-100 square foot TAPs will offset ~3 gallons of space-heating propane (~2 gallons of oil) per square foot per year, and that's without a particularly high-mass interior. For a moderately high-R and tight building the load is lower so discounting that to 1.5-2 gallons is prudent, but it depends on just how much overheating you're willing to tolerate to achieve that. Retrofitting more interior thermal mass can moderate the temperature swings, but adding a lot of mass can sometimes be awkward or bulky. It's worth assessing- how much stone, brick, concrete & wallboard is there inside the insulation boundary of the house (basically everything inside the interior face of the exterior walls and ceiling)? With a lot of site, construction, and climate-specific information it's possible to model it to get a better handle on the performance to expect, but it's possible to over-analyze it, and time spent building it is probably more worthwhile.

Going with thermal-air has the advantage over hydronic systems as being quite cheap to implement as a DIY project, and uses very little power (and only while the sun is shining on the PV), with no standby losses for the controls. But in a very low thermal mass building it becomes self-limiting. Active (pumped) hydronic heating systems are almost never the right solution for off-grid applications, but sometimes thermosiphoning passive hydronic can work.

Kavade said:
"Any comments on simple things like a dark south wall?"

I'll tell you something really cheap and simple that will help some. If we assume window screens cut down solar energy to 74% of what would be present without them, then going the other way, removing screens, will increase solar energy by 1/0.74 = 1.35, or a 35% increase in solar energy at no cost! I take off my screens and clean the windows at the same time in the fall to tune up my "passive solar collectors" for the heating season. I have not measured this increase directly, but will in the future.

Okay! It seems reasonable to me that I would gain a wee bit more solar if I take off the screens. Thanks for pointing that out. So I have decided to make a DIY solar air heater, and I would guess it will make a noticeable difference, perhaps not astonishing but noticeable certainly, in my propane usage. Now I just have to decide on a design and figure out if I can do it or if I should have the handyman up the road do it. For me to do it the design would need to be dead simple. Time to Google. Thanks everyone!

Depends on where those screens are.  Outside, sure...inside makes no difference.

Edit...maybe I spoke a little quick.  If the screen area inside the window was quite a bit warmer than the room temp, then conductive losses back out the glass would be more than without the screen.  If the screen were light colored, it might reflect some light back out.  But a dark screen decreasing the light coming into the room is not an issue in itself, cause lost light is turned into heat at the screen inside the house.

That being said, I pull some of mine off too.  Not sure why, though :)

Scott-

It is good to point out the difference for interior versus exterior screens. However, when you state "(for) inside makes no difference (in the loss of solar energy)," you are making the assumption that all the screen looses are due to absorption of light by the screen, and some solar energy is absorbed, but some, perhaps the majority, is reflected back through the window outside.

kavade-

You say, "...I would gain a wee bit more solar if I take off the screens" You would get about a 35% increase in passive solar heating through the windows. You said that you had a lot of windows. Therefore, if you are getting only 2.9 wee bits of solar energy through all your windows, then you would only get a wee bit of increase by taking off the screens. If you make lots of wee bits of improvements, and add an energy source, you can get to net zero energy! Not so much if you don't pay attention to detail.