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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorjms452
    • CommentTimeSep 24th 2013
     
    When reading previous' PV + battery' threads the aim has been to hold enough power to last a few days in winter meaning you need >10s of kwH and it all gets rather expensive.

    I was musing from a different perspective.

    We are on grid.
    Even during overcast winter days we are still exporting >100W through most of the day.
    During this time it is the short bursts of power hungry appliances that result in us buying in electricity.

    This makes me think that a relatively small capacity battery system could dramatically improve our self usage.

    So does a long cycle life battery system with ~1kwH capacity and max output of ~3kW exist (if not which part of this requirement is the limiting bit).

    If it exists how much would it cost and many cycles would the batteries be expected to last?

    cheers

    John
    • CommentAuthorEd Davies
    • CommentTimeSep 24th 2013
     
    Pretty much any sensible battery technology can be scaled to those sorts of capacities and discharge powers. Which is best from the cost per cycle is a matter for debate.

    I'd have thought the harder part is that it would take a rather specialized inverter to follow the load appropriately to prevent import. A bit like these immersion controllers which prevent export but the other way round. Grid-tie inverters generally produce as much output as they can from the source available and don't work well with batteries as they just suck them dry and kill them. Maybe a Sunny Island or the like can be programmed to work in this sort of mode.
    • CommentAuthorSeret
    • CommentTimeSep 25th 2013
     
    Posted By: jms452
    This makes me think that a relatively small capacity battery system could dramatically improve our self usage.


    What's your aim of doing this? Cost or being green? Batteries in a grid situation tend to fail on both counts. The grid is a greener battery than you think, it's also likely to be lower or similar costs, and it's definitely less hassle and safer.
    • CommentAuthorjms452
    • CommentTimeSep 25th 2013
     
    I am thinking cost.
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 25th 2013
     
    You have to balance the storage capacity of the batteries with their power delivery. The kWh of the battery tells you very little about it.
    • CommentAuthorPeterW
    • CommentTimeSep 25th 2013
     
    You could look at repurposing a UPS unit from someone like APC and seeing how that smoother things out - it has the charge, battery and socket setup that you need.

    They come in various sizes from a few KWh to very large

    Cheers

    Peter
    • CommentAuthorEd Davies
    • CommentTimeSep 25th 2013
     
    So how do you get the UPS to provide power from the battery when there's nice mains it can provide it from instead?

    Like jms452, I'm thinking cost. Even when the energy in a battery is replaced for free the cost of the cycle in terms of depreciation of the life of the battery is likely to approach, if not exceed, that of mains electricity.
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 26th 2013
     
    Have you made a profile for your energy usage?
    Mine is like this.

    It may be possible to get a micro inverter or 2 connected to a battery bank to supply your extra power.
    • CommentAuthorjms452
    • CommentTimeSep 26th 2013
     
    <blockquote><cite>Posted By: SteamyTea</cite>Have you made a profile for your energy usage</blockquote>

    I have the data (although not on me) but the salient points are.

    We buy on average 3kwH/day although approximately 30% during daylight hours, 30% evening and 40% economy 7
    Peak load is ~3kW
    1kwH total available storage would greatly eat into the first two 30%s
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 26th 2013
     
    Posted By: jms4521kwH total available storage would greatly eat into the first two 30%s
    Only at 100% cycling efficiency. If you took a very hopeful 80% cycle efficiency the numbers are not so good.
    But no matter as this is an interesting problem.
    Just knowing how much energy is needed is only a small part of it. The when is important as you need time to recharge the batteries. No good depleting them at sundown.
    Then there is the volt drop on the batteries, say they work between 10 and 12 volt, then a 1 kWh, 12V battery can only store 170Wh, so you would need 6 batteries to store your kWh (all these numbers need to be checked against real battery data).
    • CommentAuthorjms452
    • CommentTimeSep 26th 2013
     
    I should have also said:
    average generation is just over 10kwH/day.
    Average export ~ 10kwH/day
    Daytime/Evening electricity ~20p/kwH

    <blockquote><cite>Posted By: SteamyTea</cite>Only at 100% cycling efficiency. If you took a very hopeful 80% cycle efficiency the numbers are not so good.</blockquote>

    I'll have a 1.25kwH system then please.

    <blockquote><cite>Posted By: SteamyTea</cite>Just knowing how much energy is needed is only a small part of it. The when is important as you need time to recharge the batteries. No good depleting them at sundown</blockquote>

    I'm happy with estimating how much cash it could save the part I am struggling with is how much it will cost!

    Probably over simplistically I could buy a 1.2kwH deep discharge lead acid battery for £60 which would payback in ~300 cycles or as little as six months.

    It is the non battery kit I am struggling with.
    • CommentAuthorjms452
    • CommentTimeSep 26th 2013
     
    If this sort of thing was financally viable from a self use perspective and a large number of households with PV fitted then in the short term 'eco' perspective I understand that this would be slightly negative.

    However, as we move towards Germany's fraction of PV a few hundred thousand 1-5kwH battery systems starting to eat in to the evening peak would be rather useful.

    I was wondering if this could be a way of driving such a distributed storage without the need for additional 'incentives'...

    Once the batteries are there maybe they could be interfaced to smart meters to charge whenever the price was very low (windy at night etc.)?
    • CommentAuthorEd Davies
    • CommentTimeSep 26th 2013
     
    Posted By: jms452: “Probably over simplistically I could buy a 1.2kwH deep discharge lead acid battery for £60 which would payback in ~300 cycles or as little as six months.”

    Rule no 1: so-called deep-cycle batteries (leisure batteries) aren't. If you cycle one down to 100% DOD (depth of discharge) regularly it'll die - probably well before you get to your 300 cycles. Generally you want to keep them to 10 to 20% DOD to have any reasonable life at all.

    Traction (fork-lift) type batteries are a bit more tolerant of deep discharge. Still, they like to be recharged fairly promptly. I suspect leaving them full discharged overnight regularly would be less than ideal.

    In other words, with lead-acid batteries you really need a lot more nominal capacity than you actually use.

    There are a few people who do this sort of thing: on-grid but with PV-charged batteries. Two blogs on the subject:

    http://solarbodge.blogspot.co.uk/
    http://green-power-gen.livejournal.com/165925.html

    Both do it for ideological and tinkering reasons rather than because it makes immediate financial sense though I think it probably saves enough that it's a cheap hobby. In both cases they run specific appliances or circuits off the off-grid system and switch things back and forth depending on the time of year, etc.

    You might like to talk to the owner of the first of those blogs (“Outtasight”) who you can track down via the last comment on his last blog post. I've had an email conversation with him about other stuff (http://edavies.me.uk/2013/07/lifepo4-charge/) and know he's been doing some commercial work in this area.

    I agree, there is some significant potential for distributed power storage to shave the peaks off the grid and it would be good if people could have it at home. I just don't think the battery and other technology or the grid pricing is right for it yet - particularly not for somebody who's not very familiar with the problems.
    • CommentAuthorjms452
    • CommentTimeSep 26th 2013 edited
     
    Cheers Ed,

    I'll have a poke around those sites later.

    Is there such a thing as a graph of discharge fraction used to expected lifetime for lead acid or other technologies?

    edit - found one
    http://www.resilience.org/stories/2012-09-19/blow-blow-pv-system-efficiency-case-study-storage

    But is seems to suggest that even using only 5% of the capacity rather than 100% only gets 50% more energy stored over the whole battery life
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 26th 2013 edited
     
    If you were a company that used compressed air or low vacuum (and you could get rid of the leaks), then this could be used for storage.
    Now there is a thought.:bigsmile:
    • CommentAuthorjms452
    • CommentTimeSep 26th 2013 edited
     
    I think its been thought!

    if you loose the grade low grade heat released during compression its only about 30% efficient:sad:

    Keeping said heat energy is possible but the complexity increases and/or maximum power output decreases.
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 26th 2013
     
    Posted By: jms452I think its been thought!
    Yes it has, as a dedicated storage system. But if you need compressed air anyway then it becomes more viable.
    But as Ed says, the domestic FITs is not set up to encourage this.
    • CommentAuthorjms452
    • CommentTimeSep 26th 2013
     
    I see what you mean now I reread your previous post
    • CommentAuthorEd Davies
    • CommentTimeSep 26th 2013
     
    • CommentAuthorEd Davies
    • CommentTimeSep 26th 2013
     
    Posted By: jms452: “edit - found one
    http://www.resilience.org/stories/2012-09-19/blow-blow-pv-system-efficiency-case-study-storage

    Huh, I'd read about Tom Murphy's system (I'm subscribed to his blog: http://physics.ucsd.edu/do-the-math/ not that he's been very active for a while) but had forgotten it otherwise I'd have included it above.

    “But is seems to suggest that even using only 5% of the capacity rather than 100% only gets 50% more energy stored over the whole battery life”

    Probably true for forklift or off-grid batteries (like those Trojans than Tom Murphy uses or the similar Rolls http://lifeattheendoftheroad.wordpress.com/2013/02/24/just-like-a-holiday/ ones) but practical experience by a few people on the Navitron forum shows that the leisure-type deep-cycle batteries (which look like car batteries and are sold for caravans and boats, etc) don't stand it so well.
    • CommentAuthorMartinH
    • CommentTimeSep 26th 2013
     
    There is a Dutch firm called NEDAP who make a battery/inverter system which can kick in to "top up" your PV supply when a bigger load is switched on - hence minimising import from the grid. It also works as a UPS in power cuts. I think there are other manufacturers too - Sunny Island?
    • CommentAuthorjms452
    • CommentTimeSep 27th 2013
     
    cheers

    I have some more reading to do now!
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 27th 2013 edited
     
    Posted By: jms452It is the non battery kit I am struggling with.
    Missed this line

    What you need is some sort of controller that can divert excess PV to a storage system, then when you need more power than your PV can supply a controller to deliver from the storage
    What you have there is a standard off grid system

    So how about some kind of controller that can connect or disconnect the grid coming into your place when there is either not enough storage (connect to grid) or export excess when the storage is full (connect to grid). The normal state would be disconnected from grid.

    I think you could make your own from a large contactor to switch the mains coming into the place. The controller would just have to sense the voltage on the storage (cuts the grid supply at over say 10.5V and reconnects it at less than 10.5V, connects to grid when over 12.5V or whatever is needed).
    That can't be hard to design.
    • CommentAuthorEd Davies
    • CommentTimeSep 28th 2013
     
    So all you need for that is a grid-tie inverter which magically also works off-grid (i.e., one which can certifiably tell the difference between a power cut and your contactor opening).
    • CommentAuthorsnyggapa
    • CommentTimeSep 29th 2013
     
    and also reconnect the grid if the load exceeds a certain, probably not very high amount

    Wouldn't like to see what happens if you turn a simple kettle on - taking 3Kw load from a 12v battery is erm 250 amps, assuming 100% efficiency - not sure how quick that would melt any reasonably sized low voltage wiring

    I think as stated above the only practical way to do this is run dedicated devices permanently from the UPS/battery (things with constant low load) and manually/auto switch the mains in if the battery fails

    all you would then needs is a "sunny whatever" immersion controller to steal the excess energy generated to charge the UPS , and a mallet to smash the UPS "beep" alarm into a million pieces when it thinks the mains power has been lost!
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