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We recently had photoelectric panels installed. We have 8 kw. In Germany this is subsidised and we reckon to break even in about 9 years, just by selling it and by own use during the daytime.

Anyway, the point is not to make a profit, but to make our own electricity, to at least feel a bit self-sufficient. So we need a method of storing our nice power until we need it, e.g. to charge an electric car or to heat water.

There are many ways to do this; batteries seemed to me to be the obvious solution but they are extremely expensive, and some of them are fire-risks. Then the solar man suggested compressed air, although this is apparently noisy. Then there's turning water into hydrogen and back again - which sounds like a fire risk to me.

In this erudite circle there is certainly someone who can point me in the right direction; I'm fine with new technology but it has to be

  • practical
  • economical
  • maintenance-friendly

edit: Hmm, environmental impact. I gather our solar panels will now generate more power in their official 20-year lifespan than were used in their manufacture. But this is a different question. But really it's about self-sufficiency - a suspicion that if Germany gets any greener they will start rationing electricity (this has been mentioned, in passing, by the Greens). Also, turning off the nuclear power stations and iffy gas deliverers may well mean the power just goes off on dark nights with no wind.

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    Are you more concerned about self-sufficiency or the environmental impact of your choice? – longneck Nov 5 '14 at 18:54
  • Are there actually any residential-scale ways of storing energy other than batteries? I've heard of compressed air and hydrogen, but thought those were only for commercial-scale arrays. I can't find any information about home-scale installations. – Hank Nov 5 '14 at 23:04
  • It was the domestic solar man who suggested compressed air and hydrogen, but I'm not sure he meant it seriously - otherwise he'd have followed it up. – RedSonja Nov 6 '14 at 11:37
  • I have heard that some people use large batteries like the ones used in forklifts. I don't know of any other practical means to store power than batteries for homeowners. – Jason Hutchinson Nov 6 '14 at 23:11
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We're still living in complicated times when it comes to figuring what's best in terms of energy usage and storage.

If the goal is to become purely fiscally self-sufficient, I think ben's answer is really good. Sell your excess for cash. Then use said cash to exchange it for 'cheap' (relatively speaking) storable energy sources like a propane tank + generator.

Whether we like it or not, fossil fuels are still the cheapest way to store energy for extended use.

If the goal is to help the environment, then ideally you are never drawing power from the grid at all if the grid is powered by fossil fuels. On the other hand, if the grid is powered by wind or solar (or, arguably, nuclear) then it actually may be better for you to draw from the grid rather than storing it yourself.

If the grid is fossil fuel based, then you're back to finding a device to store energy with.

Options:

  • chemical energy = batteries (likely most common for household usage)
  • kinetic energy = flywheels (my favorite concept, but not really affordable or practical at a household size)
  • kinetic energy = compressed air (already mentioned)
  • kinetic energy = water pumping
  • heat energy = ideal if a large part of your energy needs are for heating

and plenty more options: http://en.wikipedia.org/wiki/Energy_storage

Based purely on your 3 bullet point needs, I'd say the propane tank + generator are your best bet.

  • This is the most practical answer to having my own energy reserves in the cellar. I hadn't thought of this one. – RedSonja Nov 6 '14 at 11:42
  • I went to that Wiki link, lots of excellent ideas there. Also the idea of storing the heat in a hole in the ground... some of my neighbours have bore holes for getting heat out of the earth, but I never thought of using it as a store. – RedSonja Nov 7 '14 at 13:28
  • The answers are all good, but I ticked this one (investing the money in propane) because it was a good engineering answer I hadn't heard before. Though I probably won't do it, the spousal unit has a thing about gas in the house. – RedSonja Nov 7 '14 at 13:34
  • If the issue is literally gas within the walls of the house, that's easily remedied--as the propane tank and generator should be outside anyways. – DA01 Nov 7 '14 at 15:15
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By selling the electricity, it can be stored as money because the fungibility of money allows it to be exchanged for electricity. Furthermore, this is likely to be competitively efficient with various physical storage equipment. In addition money can be converted into reduced demand by allocating it toward more efficient load side equipment.

The goal is really sustained access to a particular level electrical service. Generating and storing electrical power on site are just two possible strategies for maintaining such access.

  • Point of clarification, it's not stored as money. It's converted to money. – DA01 Nov 5 '14 at 21:59
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    @DA01 Technically, electricity is exchanged for money. If then later an equivalent monetary value is exchanged for electricity , the practical effect is that electricity has been stored. But I think my meaning was already clear. – ben rudgers Nov 5 '14 at 22:43
  • Indeed, your meaning was clear. Point of clarification was really for the OP's sake as it's not clear if they're looking for literal storage or not. – DA01 Nov 5 '14 at 22:51
  • @DA01 What would make compressed air or hydrogen literal storage? – ben rudgers Nov 5 '14 at 22:56
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    @RedSonja - You don't need to rely on the grid to get your electricity back when you need it. For example, you can use the money to buy fuel for a generator. – mbeckish Nov 6 '14 at 13:54
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Remember to figure in the costs of maintaining the storage system. Batteries wear out fairly rapidly, have efficiency limits, and produce a moderately nasty waste stream. There are good reasons that solar took off when it did, and efficiency is only part of the answer; net metering (the "sell it back to the electric company" mechanism) really is one of the best solutions available.

  • +1. The cost of batteries (including regularly replacing them) is a major component of the cost of an off-grid solar setup. If the OP has not accounted for the cost already, it will have a major impact on the break-even time. – Hank Nov 5 '14 at 23:01
  • Yes, that's why we didn't get the batteries straight away. The solar man suggested the price would fall and we should wait a year, which we have now done. – RedSonja Nov 6 '14 at 11:40
  • Well, hybrid cars have been helping to drive the price of large batteries down. (In fact there's a proposal that would allow the power companies to use hybrids plugged in for recharge as auxilliary storage should they have to tap a few megawatts back out to balance their network load.) But I suspect that, unless you have strong reasons to want to be off the grid, net metering is still going to be more cost-effective. – keshlam Nov 6 '14 at 14:04
  • Note too that net metering is eligible for SREC credits. I'm not sure local-storage systems are. – keshlam Nov 6 '14 at 16:35
  • Since I want to get an electric vehicle (more likely a scooter) it would actually be cheaper to get a 2nd battery for the vehicle, which would be charging during the day when I am out at work with the other one. – RedSonja Nov 7 '14 at 13:26
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Since you are thinking of storing energy for heating water, the obvious answer IMHO is to store energy in hot water! You can use the water itself directly if it is the right temp, or if the stored water is much hotter than you need, use a heat exchanger and mix with cold water. Even during times where the water never gets hot enough to use directly, you can still pre-heat water that will be further heated by other means. Every watt of energy that comes from solar thermal is a watt of energy you don't need from electricity or gas. There are many possible ways to do this and a number of them have are discussed here:

http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

That said, if you want to store energy in water for space heating or domestic hot water, you really ought to be collecting heat energy directly from the sun instead of via PV panels. That's because you get far more energy (watts/square meter) from a solar hot water heater than you do a PV panel. Of course, you have less flexibility with a solar hot water system since there is no easy way to convert the hot water to electricity. If you have enough area, consider both PV and solar hot water.

  • Yes, the heating man was there recently, and he said this too. He also said, get a bigger hot water tank, because it needs to hold enough water and heat to keep you going over a few days without sun. So when we next need a new tank we'll get a big one. We would then heat the water with a heat pump, seems to be the best way for our set-up. – RedSonja Nov 7 '14 at 13:24
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Heating water is already suggested here. Someone on 'Sustainable living' presented figures for how much energy u can store in just 100 liters. I was surprised! You could add Glauber's salt and make it store even more. It would not be practical to try to turn it back into electricity, but u could have the tank sitting somewhere in your house where it will radiate this heat to the inside of your house. This would save a lot on heating bills.

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