My assignment is to build an off grid house in upstate NY. My initial thought was to use SIPs, geothermal and PV panels with a backup propane powered electrical generator. However, I am worried about snow collecting on the panels in the winter and cutting off the electric. What do people do about snow? Also, if anyone has any better ideas about the systems and envelope please feel free to reply. It will have a passive solar design and be super insulated.


Thanks!

Hello What I have found with solar panels, and we had them in Wisconsin, seeing they are black, they attract sunlight and the snow melts off very quickly. If we had a big snow storm and left 6 inches of snow on them and the sun could not get to them I would sweep them off with a broom, and this only happened twice. For your off grid house, and I have a on grid net metering house, it is pretty close to net zero house, I built out of sips, 6 inch walls, 12 inch roof, 8 inch walls in the basement, and I have a hybrid wind and solar system that net meters and has battery backup. This is a very neat house. If you have any question please contact me I can help you with this. david@bridlewoodconstruction.

Forget about the snow on your PV panels, the startup current & total power requirements of the geothermal is going to be expensive and not very cost-effective for in an off-grid system. Methinks you'll get better cost-effectiveness out of PV-powered active solar thermal with tank storage & propane backup on the heating end. Lots of systems integrate on-demand hot water heaters as backup to solar thermal storage. You can buy quite a bit of solar-thermal for the cost of a geothermal system. Burning propane at 20% efficiency in a generator to run a geothermal unit will NEVER be as efficient (nor the equipment as cheap) as burning the same propane in even an 82% EF on-demand HW heater (96% EF condensing versions exist too, for more money). Similarly, solar PV collected at 15%-20% efficiency to run the geothermal will at-best match solar thermal collectors, with much higher system complexity and cost.

On solar-thermal panel snow buildup is an issue too- particularly for evacuated tubes (flat panels heat up and shed the snow better). But mounting them vertical or at a very steep angle isn't a huge performance hit for the low-sun months when you need it the most, and in a well designed system you can often avoid the need for summertime heat-dumping as well. Designing in enough off-grid PV to run the solar pumps is easy, but pumps for the on-demand HW heater present a bigger load (although it's still an order of magnitude less load than a geothermal pump.)

Sizing the active solar may be more an issue of the heat STORAGE more than heat collection in a superinsulated passive solar house. Storing enough heat to coast through a couple of days of cool & cloudy is clearly easier than a week of mid-winter storms. But big tanks are a lot cheaper than another 10 peak kw of PV and a big rack of batteries- there will be tradeoffs no matter what approach you take.

Thanks for the replies. I guess it would be easy enough to brush off the panels if they are at ground level (though is theft an issue?).

Since the house is in upstate NY cooling is not as big an issue as heating. I do not know much about active solar thermal except that my father tried it in the 70's on our house and it did not work very well. I think his rock bed was poorly constructed. I suppose systems today are much more codified. Do many people use active solar thermal? Would geothermal heap pumps be preferred only in regions where cooling is an issue?

eothermal pumps are only preferred in regions where there are POWERLINES. Most good off-grid designs optimize use of passive & active solar for heating.

Water is a very good & flexible storage medium for solar thermal (far better than a bed o' rocks, which is commonly used when the transfer fluid was air, which is similarly crummy. Air/rock takes a lot more volume & fan power compared to pumps & water, especially in cold climate where a lot of storage may be required.) In combination with low-temp radiators (like radiant-flooring) water heat storage can be quite effective. Building large atmospheric-pressure water storage tanks for the heat isn't rocket-science- very low tech & reliable. Gary Creysa's approach seems adaptable to off-grid situations without many engineering changes:

http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm

http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/Tank/Tank.htm

Geothermal & off-grid are a bad match 'cuz it take a LOT of startup current to get heat pumps going, which means you start out by oversizing the inverter & battery arrays, and you need a LOT of photovoltaic to store enough energy to keep 'em going. When your end use is relatively low-temperature HEAT, both capture & storage of the available solar energy is much more efficient (and FAR cheaper.) To keep your up-front costs under control you have to think long & hard about every electrical load, and whether it's "worth it", or if that function can be provided more cheaply by some other means. A geothermal pump is a GIa-NORMOUS load compared to most off-grid living. Really- don't go there!

Do the math (both the energy capture efficiency, and the financials). Read up on Net Zero Energy Houses, etc, and you'll find that the best money investment is in a super-insulated well sealed shell. If building with SIPs, make 'em at least 10" thick EPS, and foam-seal every corner/edge etc. You need a "clear wall" R-value of 38-40 or more. Be very picky about window design size & location too- if you build right they will be your biggest heat loss. Even grid-tied most "net-zero" houses opt for solar-thermal over geothermal for very good financial reasons. But for off-grid it's really a no-brainer.

This Net Zero project in Edmonton used a tiny heat pump to boost the efficiency of the solar thermal by keeping it's required operating temp down but the primary heat capture is solar thermal: http://www.riverdalenetzero.ca/Riverdale_NetZero_house_--_project_profile.pdf http://www.riverdalenetzero.ca/Home.html In upstate NY your heat load is far less severe, so you can probably get sufficient efficiency out of the solar thermal without the heat pump by right-sizing the storage tank. (Edmonton has far colder mid-winter temps than almost anywhere in NY state- considerably colder than Ottawa & Montreal. If it can work there, it can work in NY.) Somewhere in cyberspace there's a very detailed PDF of that design, and desribing some of the analysis behind the design decisions. Try to dig it up (or other net-zero designs) Their design issues are YOUR design issues, only you don't have the luxury of grid-backup.

Try to design your solar thermal delivery system with thermosiphons instead of pumps where you can get away with it- every watt-hour you need to run the heating system is a watt-hour you'll have to collect and store with expensive PV & batteries. Good off-grid homes are not a trivial design problems- be sure to study up on it, and check your math twice.

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