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On-grid domestic battery storage

edited 30 November -1 at 1:00AM in Green & Ethical MoneySaving
1.8K replies 198.3K views
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  • Martyn1981Martyn1981 Forumite
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    Alan_Brown wrote: »
    With your mains power optimized to 230v, your inverter won't trip. Your inverter will always match your optimized mains power and so your export will not be above the upper voltage tolerance.

    Hiya, I can't follow that. If the grid is at 253V, how can the house export, unless it 'pumps' the leccy out at 253V+?

    Isn't this like having a pressure reducing valve on the mains, say down from 40ft head to 25ft. If you wanted to use your own water (illegal to mix, but stick with me) you'd have to pump it in at 25ft+, but to export it you'd have to pump it out at 40ft+.
    Mart. Cardiff. 5.58 kWp PV systems (3.58 ESE & 2.0 WNW)

    For general PV advice please see the PV FAQ thread on the Green & Ethical Board.
  • Martyn1981 wrote: »
    Hiya, I can't follow that. If the grid is at 253V, how can the house export, unless it 'pumps' the leccy out at 253V+?

    Isn't this like having a pressure reducing valve on the mains, say down from 40ft head to 25ft. If you wanted to use your own water (illegal to mix, but stick with me) you'd have to pump it in at 25ft+, but to export it you'd have to pump it out at 40ft+.



    You're thinking about DC rather than AC power. The grid is AC, which is a sinewave that may peak at 253 volts, but it also goes down to zero further along the sinewave. When an inverter feeds into the grid it matches the grid's voltage and frequency, but it has to lead the phase angle slightly. By doing this, at any given point the inverters voltage will be slightly higher than the grids on the rising part of the wave, and lower on the falling part.
  • NigeWickNigeWick Forumite
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    Martyn1981 wrote: »
    And you aren't wrong.
    I recently read that some independents are getting on the bandwagon too.
    The mind of the bigot is like the pupil of the eye; the more light you pour upon it, the more it will contract.
    Oliver Wendell Holmes
  • Martyn1981Martyn1981 Forumite
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    Alan_Brown wrote: »
    You're thinking about DC rather than AC power. The grid is AC, which is a sinewave that may peak at 253 volts, but it also goes down to zero further along the sinewave. When an inverter feeds into the grid it matches the grid's voltage and frequency, but it has to lead the phase angle slightly. By doing this, at any given point the inverters voltage will be slightly higher than the grids on the rising part of the wave, and lower on the falling part.

    Hands up (well one hand, the other is still bust), I'm lost. So are you saying that this could export leccy without pushing the grid voltage even higher?
    Mart. Cardiff. 5.58 kWp PV systems (3.58 ESE & 2.0 WNW)

    For general PV advice please see the PV FAQ thread on the Green & Ethical Board.
  • edited 21 September 2017 at 4:10PM
    Alan_BrownAlan_Brown Forumite
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    edited 21 September 2017 at 4:10PM
    Martyn1981 wrote: »
    Hands up (well one hand, the other is still bust), I'm lost. So are you saying that this could export leccy without pushing the grid voltage even higher?


    Yes.

    Sorry you're lost, I'll try and explain it better. With DC current, 253v is a constant 253v and your assertion that you wouldn't be able to export if your inverter output was 230v would stand.

    With AC, the voltage alternates between the peak of 253v and a trough of -253v and every voltage in between, in a waveform. You could export 10V to the grid if your inverter waveform was out of phase with the grid waveform enough so that your peak of 10v intersected the point where the grid's voltage was 9v or less.

    I'm not sure if that's any clearer, but just think that the grid's voltage hits 253v on the peak of its sinewave, then goes down to 252v, 251v, 250v .... 200v... 100v... 10v... 0v.... -10v... -100v etc. So you can export a lower voltage to the grid, you just have to hit the grid at the correct phase of its sinewave when the voltage is lower.


    PhaseShifting02.gif


    Here is a diagram of two voltages that are 90 degrees out of phase. They both have the same peak voltage, but at the point in time when V2 hits its peak voltage (say 200v), V1 voltage is half that value (100v).
  • Martyn1981Martyn1981 Forumite
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    Alan_Brown wrote: »
    Yes.

    Cheers. I think I understand that, but .... I thought voltage rises when you add more, or when demand falls, so how do you 'add' more leccy to the system without pushing up the voltage, is this 'added' leccy out of phase and damaging to the grid?

    If it's fine, why are there concerns over too much PV generation, and batts being installed in areas where PV deployment is 'too' high, under some DNO test deployments?
    Mart. Cardiff. 5.58 kWp PV systems (3.58 ESE & 2.0 WNW)

    For general PV advice please see the PV FAQ thread on the Green & Ethical Board.
  • Cardew wrote: »
    Given the weight of the Tesla battery at 1,200lbs it is certainly not a DIY job for the layman.

    Well, laymen wouldn't be swapping these, as you would need to have the new, charged, battery and how would that get to you and then the tooling and ugh, no.

    I mean its a trivial task to swap these out for a service centre, much like changing a tyre.
  • NigeWick wrote: »
    A number of places in London have already started putting charge points in lamp posts. I am sure it won't be long before petrol stations start installing chargers for when fossil burners are too expensive for people to want.

    Interestingly, some Tesla supercharging stations are getting so large that they are adding ancilliary services like a shop, coffee lounge and hey presto, you basically have a service station.
  • Martyn1981 wrote: »
    Cheers. I think I understand that, but .... I thought voltage rises when you add more, or when demand falls, so how do you 'add' more leccy to the system without pushing up the voltage, is this 'added' leccy out of phase and damaging to the grid?

    If it's fine, why are there concerns over too much PV generation, and batts being installed in areas where PV deployment is 'too' high, under some DNO test deployments?

    That's an interesting point, but it'd be a worry for the DNO not for the owner of the solar array. Perhaps it'll make the DNO more eager to keep voltages within set limits?

    A solution would be to use all of your generation either with batteries, hot water cylinder or both. Then you'd benefit from the voltage optimizer because your inverter would never switch off due to high grid voltages, and you'd not overload the grid because you're not actually exporting anything.
  • zeupaterzeupater Forumite
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    Alan_Brown wrote: »
    That's an interesting point, but it'd be a worry for the DNO not for the owner of the solar array. Perhaps it'll make the DNO more eager to keep voltages within set limits?

    A solution would be to use all of your generation either with batteries, hot water cylinder or both. Then you'd benefit from the voltage optimizer because your inverter would never switch off due to high grid voltages, and you'd not overload the grid because you're not actually exporting anything.
    Hi

    Which completes the circle ... if the voltage presented to the premises is currently that high that it regularly trips the inverter it's a DNO issue ...

    Regarding the batteries/DHW cylinder .... "you'd not overload the grid because you're not actually exporting anything" ... of course this makes three assumptions which when taken together seriously question the assertion .... (1) the net capacity of the energy storage is greater than you could ever fill .. (2) the household demand including DHW or Battery would always be greater than the maximum peak output of the inverter in prevailing conditions ... (3) the reaction time of whatever diversion control equipment is in place is faster than the inverter ramp-up or load change within the property, so when switching off say a 1kW kettle ~1cycle ...

    HTH
    Z
    "We are what we repeatedly do, excellence then is not an act, but a habit. " ...... Aristotle
    B)
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