Off-grid PV solar for domestic HW
PV2
| Posted in GBA Pro Help on
I am planning a small off-grid home and would like to send all surplus energy from the 4.8 kw solar array to a standard electric water heater, probably of 60 or 80 gallon capacity. In effect, it would serve as an additional “battery” once the main battery bank is fully charged. Does anyone have any suggestions or experience on how this might be done in a fairly simple and reliable fashion?
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Peeter,
It's fairly routine to have a diversion load or a dump load -- although it's more common for hydroelectric or wind systems than PV systems.
Here are some charge controllers that offer diversion control:
http://www.morningstarcorp.com/en/tristar
http://www.wholesalesolar.com/products.folder/controller-folder/xantrexC60.html
More info on load diversion:
http://windandsunpower.com/store/index.php?main_page=index&cPath=3_6
Most PV-powered off-grid homes don't bother setting up a dump load. Trust me -- you won't get much hot water with this method. If your batteries are almost charged, and it looks like a sunny day, just do a load of laundry.
Ok, thanks for the info and links. But would you not say that by adding some more pv, then the surplus could generate a significant percentage of domestic HW in a more effective, reliable and lower cost manner than with a solar thermal system? I want to keep things as simple and reliable as possible and solar thermal does not seem to be particularly simple. I am in zone 6A so long stretches of pretty cold temperatures are a reality.
Peeter,
No, you can't heat water with PV if you are off-grid. An electric water heater is a huge load. You can meet it easily if you have grid power, but you could never afford the lead-acid batteries required to store enough electricity in September to heat your hot water in October. Heck, you can't even afford the batteries to store electricity on Monday to heat water on Tuesday.
That's why an off-grid home is a good place for a solar-thermal system. It's just about the last type of house where solar thermal makes sense.
@ the OP:
Of course you can heat water with electric resistance, no matter where the electricity comes from.
There are ready to go PV systems (" of-the-shelf") available for your purpose, however most of them cover only about 80 % of the total electricity demand including water and space heating.
That's what manufacturers and serious sellers recommend.
To go for 100% coverage in an island situation ("off-grid") can be done technically. But the costs for batteries would be high, they take only x-time charging and then they 'decay', loose their power capacity.
And replacements or new aditions of batteries cost money.
At what power demand are you looking ?
Have you checked with the major PV manufacturers already for their of-the-shelf systems?
I should have clarified that the plan is to generate HW in the winter with a coil in the woodstove set up with a thermsiphon loop throough the electric water heater tank. Since a PV system sized to provide all or almost all non-heating power in the winter will end up being considerably over-sized in the summer, I wanted to set up an automatic control system that would send all surplus pv power to the electric water tank. So the question was really one of how to get the control information as to when is there suplus power (i.e. state of battery charge) and how to send it to the water heater. It looks like the charge controller I am looking at using has an auxiliary output that can be used for controlling a relay for sending power to the heater, based on state of battery bank charge. Perfect, as long as it really works. I am hopeful that for summer use, this is a less costly and more effective and reliable method than solar thermal. Any comments? Martin, you had a very interesting article on pv now being less costly than thermal, in grid-tie applications. What about a summer only off-grid application as described here? Would it not be true that with a big enough tank, the pv supply will be averaged out sufficently to be workable? There are only 2 occupants and we use very little hot water. In any event, a small propane fueled heater could fill in the gaps - we have a 10 gallon one now in our off-grid cabin and we seem to use on average 2 to 3 fillings or roughly 25 gallons per day.
If PV arrays could meet the energy supply demands of resistence heaters and heat exchangers we would have gone off oil decades ago. PV's are inefficient and expensive ways to make electricity, not heat. Converting from heat (sun) to electricty (PV's) and back to heat again (DHW) is not really a worthy model.
Direct solar heating (infrared) is the only cost-effective way to get heat (DHW) from the sun.
If the incredibly efficient evacuated tubes were reasonably priced, every house would be retrofitted.
There are amaziningly good and much more affordable, high performing direct solar HW designs to be found on the web.
These latter two solutions do not freeze; they heat water all year; but they need sunshine.
If, might, would , could .....
PV systems to provide full coverage are readily available.
Here a smaller one allowing for 5 kW tapping:
http://singapore.conergy.com/Conergy-solar-storage-solution.aspx
http://singapore.conergy.com/PortalData/1/Resources/master/pdf/conergy_complete/200/Complete_200_300_SF_ENG_110601_web.pdf
These systems are modular, can be extended by adding another battery. Or two, or three .....
And yes, not only home heating and DHW can be covered with electricity but cars and aeroplanes can be propelled with it as well.
All energy manufactured on the home's roof. And stored for usage on demand, day and night.
As said: these storage units are readily available. Ask your electrician.
Hein,
Of course it's possible to buy a huge battery bank, and the PV modules necessary to keep the batteries charged. It's even possible to buy a battery bank large enough to power an electric-resistance water heater. But you'd have to be crazy to do it.
Trust me, Hein. I've lived in an off-grid house for 37 years, and many of my friends live in off-grid homes. No one recommends that off-grid homes use electric-resistance water heaters.
Remember, off-grid homes are mostly drawing their electricity from batteries. The number of hours per day when the sun is actually shining enough to create electricity is usually only 5 out of 24 or perhaps 6 out of 24. For 18 or 19 hours a day, the electricity comes from the batteries.
And in November, you don't get sun for three weeks at a stretch.
There is no way to make the economics work for electric-resistance domestic hot water in an off-grid home.
But in the summer there is considerable surplus capacity so why not send this surplus to an 80 or 100 gallon electric water heater? A 4800 watt array could make a lot of hot water during a long and sunny summer day when few other loads are on. The incremental cost is very low (a relay and some wire) if the same tank is already there as part of a simple thermosiphon setup using a coil in a woodstove.
Peeter,
As I explained in my original answer, you can use a water tank as a dump load if you want. I have no problem with the dump load plan.
However, you later modified your question. You proposed something other than a dump load when you wrote, "By adding some more PV, then the surplus could generate a significant percentage of domestic HW in a more effective, reliable and lower cost manner than with a solar thermal system?"
I don't think this makes any sense for an off-grid home. But if you like to experiment, be my guest.
Thanks Martin,
I like the idea of perhaps oversizing the array for other reasons also, faster charging being one of them particularly on overcast days. The economics seem to be changing almost monthly - I paid $1.29 per watt for my 240 watt panels last spring and when I looked today I see 240 watt panels can be had for 68 cents per watt!
Prices for PV-panel prices aren't the deciding factor for off-grid installations anymore, they come at a rate cheaper than double glazing window panels.
The type of battery resp. its price is decisive.
US-made batteries seem to be the cheapest at the moment.
Here panels sold for 50 € cents/Watt:
http://www.alma-solarshop.com/2-solar-panels
@ Martin:
Old days experience with PV systems isn't what is up-to-date nowadays.
We run cars with PV and supply homes with it. In paralel, for family homes. All-year round.
Check the past events of the solar decathlon in Washington.
They have been send to the desert for the next event, the establishment did complain ... about the lawn.
Duke Energy (the atomic sow) is now investing in home made PV.
Hein,
My experience with off-grid PV systems isn't "old days experience," as you call it. It's this morning's experience. Every morning when I turn on the light, I'm using PV power. And every time I check the level of distilled water in my batteries, I'm experiencing off-grid PV.
Every year or two, I buy a few PV modules, so I'm familiar with the trend of PV module pricing. I'm also familiar with battery costs; I bought my latest set of new batteries 18 months ago.
I've been watching my volt meter, my amp meter, and my amp-hour meter every day for 37 years. I know how much sun I get in November, and how much I get in July.
Hein, how many years have you lived in an off-grid house?
What seems to be missing is a relevance between the small amount of energy solar panels generate compared to the MASSIVE amount of energy an electric water heater requires. While the water heater can act as a peak dump load, to design it to use PV power is terribly inefficient is basically whats being said. If money is no object, it can be done though.
The most efficient use of those occasional peaks is to - as noted - perform and energy intensive activity like do a load of laundry.
Here a device from the UK :
http://www.immersun.co.uk/technical.php
Sure that's snake oil, Martin?
Here a data resource usefull for the calculation of PV-harvests:
http://re.jrc.ec.europa.eu/pvgis/solrad/index.htm#databases
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" Hein, how many years have you lived in an off-grid house? "
: Not that long.
Hein,
Lots of grid-connected homeowners with PV arrays have electric resistance water heaters. In fact, I wrote a blog on the topic -- and recommended the method as a more sensible approach than the use of a solar thermal system.
However, off-grid homeowners aren't doing that. In fact, the website you linked to is a device intended for grid-connected homes, as is clearly revealed by this quote on their home page: "My immerSUN was installed on August 10th ... In the first three month the immerSUN has diverted over 300kWh into my immersion heater instead of feeding it in the grid."
But perhaps it does make sense in my specific application i.e. simple woodstove coil and 80 or 100 gallon tank for hot water in the winter and solar PV to the electric element in the summer, on the theory that there will be sufficent surplus PV during summer months for a 2 person household.
My supplier modeled (with Retscreen) 9.3 kwh daily production from the PV array in December and 17.9 kwh in June. The difference (surplus) is 8.6 kwh, enough to to raise the temperature of 40 gallons of water by 80 degrees F. So it appears to me that it is worth pursuing this idea.
It is so indeed, Peeter.
The immersun device does not need to be grid connected.
You can use the PV-power for whatever you like, charge a battery, a car, run the sewing machine or heat the house, whatever.
There are many people doing this already, off-grid and on-grid.
There are many countries(most!) which have no efficient legal provisions for small scale electricity producers. These small scale producers are - an many cases - simply not allowed to feed power to the grid without following complicated procedures and technical devices, punished with taxes and fees.
Still, PV can be used there as well. To heat things like domestic water, central heating radiators, UFH etc..
Hein,
The Immersun looks very interesting but unless there is a North American version (60 hz) It won't work here. I skimmed through the installation instructions and it certainly seems to be a very well executed design - thanks for posting. Are you aware of a North American version and if so distributor contact information?
No, sorry, Peeter.
Check with an electrician,the PV-specialists etc., they'll know where to purchase something like it.
Maybe the immersun people in the UK can help as well.
I've come a few years to late to this discussion, but I'll ask anyway.
I have a weekend mini-cabin in the woods in SW Wisconsin. The indoor area of the cabin is about 270 square feet. 6 inches of fiberglass insulation in the roof and floor, 3.5 inches in the walls, and heated with a wood stove in winter. In the summer a 200' loop of black poly tubing along the driveway when the sun shines gives me 110F water to wash dishes or take a shower. In winter I disconnect the water supply.
I'm just there about twice a month on weekends. The wood stove heats up the cabin in an hour. In between visits, the cabin is pretty much just outdoor temperature. Currently I have 3 deep cycle 12v batteries that give me enough power for some LED lights, a mini DVD player, CPAP machine, and a few small items. My inverter runs a few tools when needed, but I have a generator I turn on a few times each year when I want to run a bigger power tool. This is Phase 1 and it has worked okay for a few years.
Phase 2 will involve buying a bunch of solar panels, maybe 6 to 9 panels at 300 watts. Way more than I need to charge my current batteries. Phase 2 will let me perhaps add a mini DC refrigerator, use my laptop and work from the cabin for a week at a time, perhaps have a medium sized LED TV for watching DVDs. Phase 2 also will let me keep the cabin just above freezing without running the woodstove in winter while I am away. I've had a few minor problems when taking the cabin from -10F to 80F in the winter with the wood stove. If I leave cans of soup or beans behind in the cabin, they freeze up, and when I start to warm up the cabin on arrival, they get covered with frost and later drip water all over. I can't leave behind bottles of anything that will freeze up and then expand and crack and make a mess. Frost can form on the inside of kitchen cabinets and steel tools. Without getting on the grid, I want to add enough heat to the cabin while I'm away to keep it above freezing all the time in winter. No problem if it gets up to normal room temperature, but I'd like the cabin to not totally freeze even if there are overcast days and cold nights.
Lets say I set up an array of nine 300w panels, and can count on about 4 hours of partial sun in the darkest part of winter. Let's say I'm generating way more power than I'll ever need to charge up my batteries.
What I'd like to know is the best simple arrangement to use a diversion/dump load capable charge controller that will turn my surplus power into heat, to keep the cabin above freezing, without involving draining the batteries at night. I just want to pump enough heat in the cabin during the day so that it stays above freezing all night long. What method would work best? Immersion resistive heater in a 55 gallon drum of antifreeze that would radiate heat at night and warm up during the day? Infrared heater pointed at a pile of cinderblocks? RIght now if the cabin is warm from the wood stove before bed time, and the fire goes out, it is still at least 50F in the morning with just the retained heat in the cabin and the metal of the stove. I'm just fishing for ideas and hoping to eliminate any faulty plans from my imagined solutions. I'll still use the wood stove for heat in winter while I'm there, but the 4 hours of 2700w (or whatever the real world numbers would be) surplus from the panels is enough to warm up the cabin, I'd have to use a lot less wood.
Thanks
Dave,
It's fine to experiment, and I'm eager to see your data. But your cabin will still freeze.
Four hours of sunlight a day is a fantasy from Nov. 15 to Feb. 7. I remember certain Novembers when I got maybe 4 or 8 hours of sunlight during the entire month. Here in northern Vermont, it's easy to get three weeks of clouds. So that's your basic problem.
I agree that when the sun is shining, you might have 2,000 watts available. That's a hair dryer, and you can do a lot with a hair dryer. Your idea of buying a 2,000 watt electric heater and aiming it at a pile of concrete blocks is as good a method as any. Just make sure that you have a low-voltage cut-off control to protect your batteries.