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  1.  
    Just read a very good Dissertation looking into this. One of the drawbacks is battery efficiency which is apparently around 40% For this reason (mostly) such schemes appear non viable.

    Anyone know where battery efficiency is heading? Of if there are batteries out there with higher efficiency?
    • CommentAuthorEd Davies
    • CommentTimeJul 9th 2013
     
    Startling. I think battery efficiencies are much higher than that, above 80% at least.

    I can only imagine getting efficiency numbers that low if you include energy which needs to be diverted while lead-acid batteries are in the absorption phase at the end of the charge cycle. This is particularly a problem when charging from generators. Still, you'd have to be pretty stupid not to be making reasonably good use of the energy diverted.
  2.  
    Thanks Ed, It surprised me as well but its not really my area...to be fair the student had a limited word count and the Literature Review was necessarily focused on other aspects of the study. The 40% efficiency is a single source but does come form a working system.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 9th 2013
     
    So, at 80%, efficiency is not the thing that holds back battery storage? In a nutshell, what is the drawback - just capital cost?
    • CommentAuthorEd Davies
    • CommentTimeJul 9th 2013
     
    Yes, capital cost and short life time making for relatively high cost per cycle.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 9th 2013
     
    I looked into this battery efficiency a while back, the tread will be up here somewhere, and I got the impression that the charge/discharge cycle was not brilliant. Others claim much better efficiency though.
    If it is just storage then why not hot water or use a flywheel, flywheels are already used on the grid.
    • CommentAuthorSprocket
    • CommentTimeJul 9th 2013
     
    I'm with Ed, efficiency can certainly be very good.

    Efficiency will depend mostly on the type and quality of the charger circuitry. A good modern (and expensive) charge controller will use a switched mode converter (much like a grid-tie inverter) that can be up to 90% efficient.
    As for the battery charge/discharge itself... that can be very efficient if you stay within the optimal window for charge and discharge rates.

    It is easy to get high losses though if everything is not running optimally. High currents and continuing to charge when the battery is full will just produce heat or cause gassing.This problem will only become worse as the battery ages.

    Cheaper/older chargers that are intended to be used with PV voltage fairly closely matched to battery voltage may just limit current and dump excess as heat and that can be a lot of lost energy so these are pretty inefficient.

    Because Lead-Acid cells are pretty forgiving they tend to be treated fairly roughly but this does not make for good efficiency. Lithium batteries, aside from having much higher capacity, are comparatively fragile and very expensive so they tend to get treated much better too which helps them appear to be much more efficient but it needn't be so,
    • CommentAuthorCWatters
    • CommentTimeJul 9th 2013
     
    Some notes on lead acid efficiency here...

    http://www.solar-facts.com/batteries/battery-charging.php

    This was interesting..

    "The state of charge of the battery will also affect charge efficiency. With the battery at half charge or less, the charge efficiency may be over 90%, dropping to nearer 60% when the battery is above 80% charged."
  3.  
    Hmm, not straightforward then. Presumably as the battery gets older, the efficiency will drop even lower than 60%?
  4.  
    Posted By: SteamyTeaI looked into this battery efficiency a while back, the tread will be up here somewhere, and I got the impression that the charge/discharge cycle was not brilliant. Others claim much better efficiency though.
    If it is just storage then why not hot water or use a flywheel, flywheels are already used on the grid.


    He did briefly look into hot water storage though that was outside the scope of the Dissertation.If you can remember which thread......
    • CommentAuthorjms452
    • CommentTimeJul 9th 2013
     
    When its arguable if we can even see the drop in demand caused by PV then the excess electricity can be used with no extra capital cost and close to 100% efficiency by your neighbors.

    That's a environmental argument rather than a financial one though.

    IMHO the PV subsidy is at the point where it should be switched round with a small generation payment and larger metered export payment. A financial incentive driving people to fit batteries, immersuns and failing to encourage daytime efficiency is not holistically rational or efficient.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 9th 2013 edited
     
    Posted By: jms452A financial incentive driving people to fit batteries, immersuns and failing to encourage daytime efficiency is not holistically rational or efficient.
    I agree, can add morally to that.

    Mike
    This is where we were discussing it,not much there though except great claims for batteries.
    I think Wookey mentioned something on another thread that started is all as well.

    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=7276
    • CommentAuthorjms452
    • CommentTimeJul 9th 2013
     
    I think individual morality is a red herring.

    Most sensible individuals will do what is best for them financially (often for moral reasons - caring for family etc.). It is up to government to make the individual choices align with the national strategy.

    2 or 3 years ago when subsidies were 40p this was more of an academic point. However now we are at sub 15p (grid parity from the owners perspective) and only large arrays are really cost effective. With the current scheme the rational PV owner will do anything other than export their excess electricity.

    I think/hope that the newish higher export rate and smart meters are intended to address this but wouldn't be surprised if it took 10 years to get smart meters and even then its only 3.2 or 4.6p export.

    I'd suggest that rather than further reducing the subsidy the export and generation elements were reversed in magnitude. Unfortunately I have a hunch the electricity companies 'like' it that the government subsidies PV generators allowing them to buy premium electricity for bargain prices (4.6p at present).
    • CommentAuthorMike George
    • CommentTimeJul 9th 2013 edited
     
    Thanks Gents. Just to give you a bit more background - The Aim of the Dissertation was 'to investigate the viability, advantages and drawbacks for the employment of electrical energy storage for domestic photovoltaic schemes in the UK, as opposed to the current preference of exporting to the National Grid.

    The efficiency of batteries is obviously a critical factor and at 40% such schemes are a non- starter. What we are interested in currently is as at what level of efficiency will such schemes become viable?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 10th 2013 edited
     
    If say a kWh of Grid Supplied is 12p, you get 15p for your own generation and an extra 3p for exporting.
    Assuming you can store all your generation and export nothing then your overall efficiency has to be a minimum of 0.8 [12/15].
    Now assume that you store half your generation and export half, then it has to be a minimum of 0.72 [12/(15+1.5)]
    If you store a third then a minimum of 0.7 [12/(15+1)].
    There will be a lower limit where you store nothing and export all, 0.66 [12/(15+3)].
    So what your dealing with, to my way of thinking, is two fixed prices and one variable. This does start to get messy as soon as you have deemed exports, variable tariffs, fixed meter rental and the big ones of variable generation and demand.
    If I have done my sums right (was a late night last night) then it is best described with the second order polynomial curve y = 0.0242x^2 - 0.1573x + 0.7998 (a polynomial is acceptable here as there are fixed bounds)
    Where y is storage efficiency and x is decimal fraction exported.
    • CommentAuthorskyewright
    • CommentTimeJul 10th 2013
     
    Posted By: jms452IMHO the PV subsidy is at the point where it should be switched round with a small generation payment and larger metered export payment.

    The missed opportunity for making export meters part of every FIT install right from the start is one of the stranger aspects of the whole set up to me. If the DNOs didn't want to fund that then fair enough but would a, say, £250 FIT registration fee (I think the meters themselves have a unit cost of around 10% of that, maybe less?) have really had much of an impact on the number of installs?

    Posted By: jms452Most sensible individuals will do what is best for them financially (often for moral reasons - caring for family etc.). It is up to government to make the individual choices align with the national strategy.

    And without an export meter people loose track of what they are really using as soon as they install microgeneration. Without an export meter

    usage = import + generation - export

    has 2 unknowns.
    •  
      CommentAuthorted
    • CommentTimeJul 10th 2013
     
    The reason export meters were not mandated for FiT is because it was thought that the roll-out of SmartMeters would follow on not far behind and therefore there was no need to incur what would have been a 'double cost' and that the 50% deemed export would be 'good enough'.

    Of course deployment of SMs has slipped and slipped.
    • CommentAuthorskyewright
    • CommentTimeJul 10th 2013 edited
     
    Posted By: tedThe reason export meters were not mandated for FiT is because it was thought that the roll-out of SmartMeters

    Thanks Ted. I should perhaps have mention the justification for the decision in my post. I just don't think it was sufficient justification. I mentioned the idea of a registration fee as a counter to the "double cost" (to the 'industry').
    •  
      CommentAuthordjh
    • CommentTimeJul 10th 2013
     
    Posted By: skyewrightI just don't think it was sufficient justification. I mentioned the idea of a registration fee as a counter to the "double cost" (to the 'industry').

    At the end of the day it is the customers, who are also taxpayers, who would pay all costs and would have been mightily annoyed at such obvious waste.
    • CommentAuthorskyewright
    • CommentTimeJul 10th 2013
     
    Posted By: djhAt the end of the day it is the customers, who are also taxpayers, who would pay all costs and would have been mightily annoyed at such obvious waste.

    A registration charge to cover the cost of export meter installation would have been a cost direct to the FIT recipient. Readings from the meters are just submitted along with those from the TGM.

    SSE were happy to install export meters at no charge. They've stopped now, presumably because they realised they were the only people doing so?

    I'll not try to put a 'value' on the usefulness of the information that an export meter supplies to the micro-generator or DNO/supplier/whatever.

    By the time SmartMeters have fully rolled out there could well be FIT installations nearing 10 years old?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 10th 2013
     
    As DNOs have to change meters every twenty years, does that mean that the generation meter will also have to be changed?
  5.  
    Posted By: SteamyTeaIf say a kWh of Grid Supplied is 12p, you get 15p for your own generation and an extra 3p for exporting.
    Assuming you can store all your generation and export nothing then your overall efficiency has to be a minimum of 0.8 [12/15].
    Now assume that you store half your generation and export half, then it has to be a minimum of 0.72 [12/(15+1.5)]
    If you store a third then a minimum of 0.7 [12/(15+1)].
    There will be a lower limit where you store nothing and export all, 0.66 [12/(15+3)].
    So what your dealing with, to my way of thinking, is two fixed prices and one variable. This does start to get messy as soon as you have deemed exports, variable tariffs, fixed meter rental and the big ones of variable generation and demand.
    If I have done my sums right (was a late night last night) then it is best described with the second order polynomial curve y = 0.0242x^2 - 0.1573x + 0.7998 (a polynomial is acceptable here as there are fixed bounds)
    Where y is storage efficiency and x is decimal fraction exported.


    I follow the first bit of that.. :cry: then my head starts to hurt :cry: Is there a % efficiency which comes out of that. I appreciate its a stab at it given the variables :bigsmile:
    • CommentAuthorSeret
    • CommentTimeJul 10th 2013 edited
     
    Posted By: Mike George
    The efficiency of batteries is obviously a critical factor and at 40% such schemes are a non- starter. What we are interested in currently is as at what level of efficiency will such schemes become viable?


    As mentioned above, I don't think 40% is really a realistic efficiency for a battery system. Actual efficiencies are well above that, but the limiting factor isn't really the efficiency, since even at 100% efficiency the high cost, limited lifespan and ongoing maintenance requirements of battery systems would make the whole thing pretty marginal.
    • CommentAuthorMike George
    • CommentTimeJul 11th 2013 edited
     
    Posted By: Seret
    Posted By: Mike George
    The efficiency of batteries is obviously a critical factor and at 40% such schemes are a non- starter. What we are interested in currently is as at what level of efficiency will such schemes become viable?


    As mentioned above, I don't think 40% is really a realistic efficiency for a battery system. Actual efficiencies are well above that, but the limiting factor isn't really the efficiency, since even at 100% efficiency the high cost, limited lifespan and ongoing maintenance requirements of battery systems would make the whole thing pretty marginal.


    Maybe, though the Dissertation does consider cost, lifespan and maintenance' with the conclusion that better efficiency may tip the balance and make some schemes viable.... That's what I am trying to test - Though it seems everyone here is cynical (maybe rightly so - I don't know)

    Battery storage is utilised in New Zealand I think - though not sure quite how much more sun they get than us
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 11th 2013
     
    Mike

    Multiply the 0.8, 0.72 and 0.66 by 100 to convert to percentages.

    The 0.66 (66%) is the very very lowest efficiency needed as you are not actually storing any power then, just a theoretical lower bound.

    Changing the the prices paid and earned for electricity will vary the efficiency needed.

    Posted By: SeretActual efficiencies are well above that
    Probably are when tested in ideal circumstances in a lab, but when put into practice with differing charge and discharge cycles, part charging, high load discharge, some low level charging, some high level charging some very low discharge and then add in the losses of the charger/controller/inverter/cabling losses and battery management unit, I think that a 50% round journey is realistic. So maybe I should have been more specific about my assumption of 50%, and it is an assumption as I have not seen any data from a working system.

    The economics is a tricky one. If you take a pure economic viewpoint the price of the equipment is written off at the very beginning and you just look at the value you get out of the electricity. So a hospital/army can afford to have batteries while a corner shop/house may not. Large server farms have backup as they are probably contractually obliged to do so, with high penalties for lose of service.
    Anything else is accountancy which plays by different rules.

    Be good if someone could monitor battery efficiency in 'the real world'. I think some of the BMUs have logging facilities.
    • CommentAuthorSeret
    • CommentTimeJul 11th 2013
     
    Posted By: Mike George
    Battery storage is utilised in New Zealand I think - though not sure quite how much more sun they get than us


    Only off-grid, unless something has changed in the 15 years or so since I've lived there. They get a little more sun, but not signicantly more so. Their far south is about the same latitude as the UK's south, so you'd be looking at similar insolation to France.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 11th 2013
     
    Not checked but I think New Zealand's south island has similar coastal cloud cover to us as well.
    The 2.5 degrees latitude difference will give them a bit more but not a great deal.
    Not knowing New Zealand at all (worked in Australia briefly, not the same), why do people use batteries rather than generators, or is it just the odd few that have batteries.
    • CommentAuthorSeret
    • CommentTimeJul 11th 2013
     
    Don't know, I've never lived out in the sticks. Generators are bog standard farm equipment though, I'd imagine that's what most folks use.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 11th 2013 edited
     
    Nearly all dairy farms will have a back up generator even with our very reliable grid and electrical service. 3 days without electricity and it has paid for itself. Not to mention the welfare issues.
    • CommentAuthorskyewright
    • CommentTimeJul 11th 2013 edited
     
    Posted By: SteamyTeaNearly all dairy farms will have a back up generator even with our very reliable grid and electrical service. 3 days without electricity and it has paid for itself. Not to mention the welfare issues.

    For a backup system against grid loss I think a genny probably wins hands down against battery storage on many counts (though storage of significant quantities of petrol in or around a domestic property might be an issue? If it's 15 miles to the nearest petrol station, popping out for the odd gallon would become a pain!).

    However storage & later reuse of 'excess' micro-generation "feels " a lot better. :bigsmile:

    This sort of subject comes up quite often on Navitron, e.g.
    http://www.navitron.org.uk/forum/index.php/topic,20030.0.html
    a topic currently running to 16 pages...
   
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