On-griders (and battery storage)

michaels

In another thread on another board it was suggested that using old fashioned lead acid batteries off-griders could achieve a storage cost of 3.5p/kwh

This got me thinking so I looked up economy 7 tariffs and found that you could get 7p/kwh. Add on 3.5kwh for storage and it is 10.5p/kwh, marginally cheaper than the cheapest standard tariff. Not a huge saving but if it meant you could also keep all your PV output rather than exporting possibly 50% of it (for effectively nothing with the deemed export) that would be worth a fair bit too (for me 1750 units at 7p/unit = £120pa)

I guess the difficulty is the kit needed in terms of divertors, invertors etc and what losses you might suffer but it seems like the economics may not be that far off?

Martyn1981

Hiya Michaels (and all), for clarification here's what I posted, with information from an off-gridder chatting on the Navitron forum:

£3000 buys you today a 1300 AH 48 Volt @C24 forklift battery ..... so about 62 kWh total storage capacity , my 40 kWh forklift battery is now 12 years old laugh ( probably 25 -30 kWh capacity left - but i never drain her so much )

So, 62kWh at a DoD of 40% (to protect lead acid batts (80% DoD for Li-ion)) is 25kWh. Over 10 years that's around 90,000kWh, or 3.33p/kWh (£33.33/MWh).

So, where to start. Firstly, there is the issue of economies of scale. Smaller systems will cost more. Next, as you say, there is the additional equipment, charge controller and inverter to change the DC storage back to AC for us.

There's also the consideration that off-gridders are happy(ish) to maintain a battery store, but for us, we'd have to consider where to place batts of this type, with consideration to ventilation for any hydrogen gas that may be given off.

But, most of those issues are solved if you look at the integrated PV inverter/battery chargers, so an all in one system. Then couple these with Li-Ion, rather than lead acid, but again this will push the cost up.

So, currently, for smaller scale systems, using more expensive Li-Ion the numbers aren't so good, but will hopefully improve quickly.

If you find this stuff interesting (I think it's fascinating) then a search of Navitron for on-grid discussions will hopefully find you posts by Nowty who operates both an on-grid FiT PV system, and an off-grid PV system with batts, that contributes to household demand via an Islanding inverter which can be used to coordinate multiple inputs. Here's his autosig to get an idea what I'm going on about:

4.0kw PV (grid tie) + Immersun diverter to hot water / storage heaters.
5.4kw PV (off grid) + 1000Ah 24v Forklift battery bank + Solamiser diverter to hot water / convector heater.
200 litre hot water pre heat tank.
500 litre underground water harvesting tank.

In describing his system he's been very honest and said it doesn't make financial sense (yet) but it works well and fulfills his personal desire to try.

I think this subject is enormous and complicated, but hope what little I know may keep you interested.

Mart.

EricMears

michaels wrote: »

In another thread on another board it was suggested that using old fashioned lead acid batteries off-griders could achieve a storage cost of 3.5p/kwh

This got me thinking so I looked up economy 7 tariffs and found that you could get 7p/kwh. Add on 3.5kwh for storage and it is 10.5p/kwh, marginally cheaper than the cheapest standard tariff. Not a huge saving but if it meant you could also keep all your PV output rather than exporting possibly 50% of it (for effectively nothing with the deemed export) that would be worth a fair bit too (for me 1750 units at 7p/unit = £120pa)

I guess the difficulty is the kit needed in terms of divertors, invertors etc and what losses you might suffer but it seems like the economics may not be that far off?

Suspect you'd be very lucky to get more than 50% of the power used to charge a battery when discharge it and feed through an inverter. That of course effectively doubles the E7 price !


And storage is VERY expensive. I've just bought an electric car and enquired what the replacement cost of a battery pack (rated at 18kWh) would be. Would you believe £11k + vat and + fitting charge (which gets very close to total I paid for car :eek: ). It shouldn't actually bother me as battery is under warranty for 7 years anyway and I really think that with two or 3 deep cycles a week Li batteries ought to last a great deal longer than that - by which time the proposed Tesla factory ought to be going and prices might have dropped.

michaels

Hmm - our electric car was about £16k new with a 24kwh battery - perhaps I should have immediately broken it for parts?!

michaels

Martyn1981 wrote: »

Hiya Michaels (and all), for clarification here's what I posted, with information from an off-gridder chatting on the Navitron forum:

Thanks Mart, I shall take a look.

In the back of my mind I have a plan to have an indoor pool paired with chp (we have mains gas) but cheap pv storage options may also make sense at some point especially if they could be paid for by storing economy 7 leccy....

Martyn1981

EricMears wrote: »

Suspect you'd be very lucky to get more than 50% of the power used to charge a battery when discharge it and feed through an inverter. That of course effectively doubles the E7 price !

Hiya Eric, well, here's a surprise for you, I thought lead acids were about 80% efficient from energy in to energy out, but I remember Nowty doing a review of his system and it came in at around 90%. His system can monitor kWh that go to storage, and kWh that come out.

I think Li-Ion may be a bit better, and maintain efficiency longer (as it can take a bit more abuse over discharge levels). Also You can buy half the capacity in Li-ion as an 80% DoD (depth of discharge) is ok, v's 40% for LA if you won't to maintain efficiency.

Mart.

Martyn1981

michaels wrote: »

Hmm - our electric car was about £16k new with a 24kwh battery - perhaps I should have immediately broken it for parts?!

Now it's getting interesting! If you want to see what someone does with a Nissan Leaf battery (£1,200 from a wrecked car), then check out the build (and dare I say battery !!!!!!!) in this thread from reply#20 onwards.

5875Wp installation in Norway

And I am working on setting up an Sunny Island for increased self consumption.
The battery pack will consist of an complete battery from an Nissan Leaf.
Lithium with about 20+KWH of storage capacity.

Mart.

EricMears

michaels wrote: »

Hmm - our electric car was about £16k new with a 24kwh battery - perhaps I should have immediately broken it for parts?!

No, I don't for a moment suppose you'd get anything like that for a s/h battery. Indeed, I doubt a new one costs VW anything like that amount - they'll just be trying to build up a savings pot for the expected diesel costs

Martyn's post at 11:47 quoting £1200 sounds more like the true value of s/h.

And to answer his point about battery efficiencies, it's not just the battery efficiency that you have to consider. There are losses in changing a 230V supply to DC, more losses if power not recovered immediately, more losses when DC is converted back to AC. And I haven't even taken ageing effects of battery into account yet let alone loss of interest on purchase price etc. .

Martyn1981

EricMears wrote: »

And to answer his point about battery efficiencies, it's not just the battery efficiency that you have to consider. There are losses in changing a 230V supply to DC, more losses if power not recovered immediately, more losses when DC is converted back to AC. And I haven't even taken ageing effects of battery into account yet let alone loss of interest on purchase price etc. .

Hi Eric, I suspect the other losses are quite small (5 to 10%), especially if charging batts directly from DC PV - though I appreciate that michaels did mention E7, so AC.

If you're interested in the performance figures then you might find his posts regarding his system and its performance interesting, here's the thread, and the performance figures are at post #10:

My Off Grid System Is Now Complete

Regarding battery ageing, yes performance will drop off. However, all battery manufacturers will give cycle lives at varying depths of discharge. That's why I suggested a 40% DoD to get into the 3,000 (10 year(ish)) cycle figures, to prevent earlier damage to batts and their performance.

Regarding lost interest, that's a fair point, but probably outweighed (punchline before the joke) by the scrappage value of the batts, currently around £400 per tonne for LA batts, as lead scrap is around £900.

Just to reiterate though, I'm just trying to give some figures, I doubt this is a serious consideration for most on-gridders, though a simplified plug n play version with Li-Ion might be very interesting, assuming prices fall substantially.

Mart.

Martyn1981

michaels wrote: »

This got me thinking so I looked up economy 7 tariffs and found that you could get 7p/kwh.

Hiya. I saw this article and thought of you.

It's a just a test to see how the economics stack up, and for balancing out generation and demand peaks.

Technically, domestic storage like this doesn't require PV, but combining them improves efficiency and could mean cost sharing of batts between homeowners, leccy suppliers and DNO's. This could prove economical collectively, when not individually.

North Star Solar to test solar-plus-storage systems in London boroughs

North Star Solar is set to deliver a new solar battery storage pilot scheme in four London boroughs as it tests four battery combinations across 40 council homes.

Peter Sermol, co-founder of North Star Solar, said: “The household uses the PV generated electricity during the day which also charges the battery for evening and night time usage. During the winter months the household will use morning time electricity from the battery, so they are paying 7p/kWh instead of 15p.

Mart.