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Sourcing energy storage capacitors in the UK

Discussion in 'Electronic Design' started by John Beardmore, Jun 7, 2004.

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  1. We are looking to put together a low voltage energy store (probably 24
    or 48 volt) to store energy to run a 750 watt pump for a few minutes at
    a time.

    Can anybody recommend any good sources of appropriate capacitors,
    ideally in the UK please, and relate any good or bad experiences with
    such projects ?


    Cheers, J/.
     
  2. Try talking to Norfolk capacitors. They were able to help me in the past
    with a rather smaller energy demand system.
    There are a number of big questions that you are going to have to answer. If
    the voltage needs to be stable, than there is going to need to be regulation
    between the capacitor and the motor. You will need some fairly complex
    energy management electronics if the load is to be kept small when the
    supply is restored. There is obviously a balancing act between this, and a
    battery based UPS system.

    Best Wishes
     
  3. Not particularly. The control electronics needn't run at capacitor
    voltage. We can buck down to something sensible.

    Not needed for the primary load.

    Sorry, don't see what this is getting at ? Am I being thick ?

    We aim to store power in the caps, then when we have a worthwhile
    amount, dump it into the primary load until the capacitor voltage falls
    to near the load stall voltage, at which point we cut off the load, and
    let it all charge up as and when.

    Might also need an input power regulator if the load isn't needed or
    fails, but none of this seems too problematic.

    Main reason we are going for capacitors is that we hope they'll last
    longer than batteries even though they will store less energy.


    Cheers, J/.
     
  4. Tim Wescott

    Tim Wescott Guest

    Really? The salient behavior of a capacitor is that the voltage is
    proportional to charge, and to the square-root of energy stored -- if
    you just connect the cap to your motor you'll see the voltage on the
    caps fall exponentially, and the motor will see wildly varying power levels.
    If you have a constant-voltage source and just apply it straight to the
    caps you'll have a _big_ current surge. You'll need to control the
    current going into the capacitors one way or another. This is probably
    the "input power regulator" that you mention below.
    I don't know about the reliability of big energy-storage caps, but I
    _do_ know that a properly managed set of Nickel-Cadmium batteries can
    last a long time with a 10-minute discharge cycle, and 750W is quite
    reachable. Check out what the model airplane folks are doing with their
    NiCads, it's pretty amazing.
     
  5. Thanks ! Found the website. Now drafting them an email.


    Cheers, J/.
     
  6. Yes, that's fine. It's just there to pump water into a small wetland.

    If we end up having to use an inverter and mains pump rather than a DC
    unit we might need to do more, but then again, some inverters will cut
    off when the input voltage is too low and restart a few volts higher.
    That might pretty much do the job for us.

    The source will probably be solar panels. Shorting them isn't a great
    idea, but the inrush current will only be risky on initial connection,
    and that can always be done in the dark.

    PV charging applications are usually 'limited' by a forward biased diode
    and panel output.

    The interesting question is perhaps, should we use an 'optimal power
    point tracking regulator' to make transfer of power more efficient ?

    Also, should we make the solar array track the sun ?

    OK. I guess lead acid could do the job too, and with discharge rated to
    less than 10% of rated capacity, I guess sulphation could be kept to a
    reasonable minimum.


    Cheers, J/.
     
  7. Ian Stirling

    Ian Stirling Guest

    You probably want to define the problem more closely.
    And consider the pump carefully.
    Some common pumps are 10% efficiant.

    For example, I have a
    "600 watt 13000l/h pump."
    It's a foul water centrifugal pump.
    Measuring it, it draws 360W running.
    Looking at the output graph, it will pump 6000l/h to 6m.

    6000l/h is 1.6l/s.
    1.6Kg raised by 6m takes 96J, so the efficiancy is around 25%.

    Lifting a ton of water a day ten meters takes around a watt minimum.
    What head/flow do you need?

    You almost certainly do not want a big pump, but a tiny pump running for
    hours.

    You may want to consider a better design of pump, if there is room.
    Something like a big wheel with buckets could closely approach 100%.
     
  8. I hope not - it's been done to death internally already !!

    Aiming for 2000 cubic meters per year, mostly in summer.

    Agreed, but there is the matter of what is available off the shelf.

    This must be vandal proof and ideally fairly compact.

    While we'd like an efficient pump, we have to recognise that that time
    to do innovative R&D is significant.

    It will very probably be cheaper to buy more PV than make an innovative
    pump that is also very reliable and cheap to maintain.

    This said, all recommendations for efficient pumps that can lift say 4
    meters, DC or AC would be welcome, especially in the next day or
    two !

    Keep in mind that as most of the water is needed in summer the use of
    PVs is quite appropriate as energy supply maps well on to demand, but on
    the other hand, the bulk of the work has to happen during the relatively
    short periods of good sunshine, perhaps totalling 500 to 600 hours per
    year with a non tracking collector system.


    Cheers, J/.
     
  9. Ian Stirling

    Ian Stirling Guest

    Well, at least here :)
    What sort of head?

    So, 2000 tons, in 100 days, or 20 tons a day.

    How often do you envisage servicing this?

    20 tons a day, let's say it's running for 5 hours, so that's
    4 tons an hour, or about a liter a second. (this implies fairly good
    insolation and a fixed panel, or a slightly less good panel with a tracker.

    Ok...
    You later say the head is 4m, so that's 40W of 100% efficiant pump.
    160W would probably be a reasonable worst-case guesstimate for the 'right'
    AC centrifugal pump.
    There are many shelves on the internet.
    Very possibly.
    This is almost certainly the wrong group.
    Hmm.

    I'd look at the various alternative energy, and solar groups.
    Look for newsgroups with photovoltaic in the name, and you should be
    good.
    Alternatively, have a browse round the fishpond websites.
    You want to find a site with flow curves for pumps.
    Find one that does a bit over your desired flow (I suspect you'r looking
    for one with 8m maximum head, and 8000l/h flow (the efficiancy generally
    peaks at around half the max head)), and of course the minimum wattage.

    Unfortunately, the pump makers often put a bigger wattage sticker on the
    side of the pump than it really needs, which is a problem for you.
    You have looked at the various solar FAQs, and got the information for
    your location as to expected solar gain?

    However, 250W of solar panel sounds like it would probably work, with
    a 160W pump.

    If servicing every 2 years is OK, I would go with lead-acid.
    I'd want to keep the battery over 75% charge, so you want to size the
    battery so that a really bright day can charge it up to 100%, and then
    it'll keep going a bit during the night until the voltage drops below
    a threshold (lead-acid will have best cycle life if kept partially charged,
    75% may be conservative.) (read the sci.chem.battery.electrochem FAQ)
    Say the solar panel can provide 90W over the demands of the pump for
    8 hours, so that's 720Wh, which would lead to a 240Ah 12V battery.
    On a good day, the pump would be running for around 4.5 extra hours.

    I think you'r at about where you'd have to look whether a solar tracker
    is really worth it.
    Yes it might allow you to shave a few hundred dollars off the solar panel
    price, but it's not going to be free.

    You probably want a peak-power tracker with battery charger, something
    to cut out the pump when the battery voltage gets too low and if
    you can't find DC pumps, an inverter.
     
  10. As little as possible.

    Yes. Provisionally I'm assuming perhaps 320W of PV, a 357W single phase
    250V pump and a 500W sine wave inverter.

    Didn't know there were any FAQs on this in general. Then again, it's
    been a long time since I looked, and usually I'm writing them not
    reading them.

    Sounds as if we are basically taking the same line but we've built in
    more safety margin and assumed less efficient off the shelf kit.

    This is OK up to a point, but our assumption is that batteries will
    start to sulphate when discharged by as little as 10%.

    We thus aim to charge up to 100%, then discharge to 90%.

    100% charge can be determined by terminal voltage under charge, and 90%
    discharge assumed after running a known load for a know time, though
    perhaps with a low voltage cut off just in case.

    I'm assuming we can always use more energy that we generate.

    This way, the battery will only have to hold its charge for a few
    minutes, and will never be left over night at less than 90% charge.

    As I understand it, % discharge without sulphation is a bit of marketing
    issue. 10% seems to be accepted in the academic community, do for
    example you can safely take 10AH out of a 100AH battery.

    Sometimes however, a say 200AH battery might get marketed as 100AH, but
    able to accept 20% discharge without sulphation.

    Thanks ! Didn't know there was one !

    See what you mean. Run at night as well as day.

    Could do, but that takes you into longer term energy storage.

    I must confess I had thought to do the job entirely during daylight
    hours.

    Yes, though the far that we'll be up a 15 meter mast with an existing
    yaw bearing makes it more appealing than it might be !

    No, though it could be as simple as

    some studding used as a lead screw,

    a nut on the studding welded to a hinge on the panel frame,

    a couple of limit switches.


    Could run the tracker closed loop, or could just take the more naive
    position of starting facing East and turning to face West over the
    course of eight hours.

    Yes. (Assuming you mean an 'optimal power point tracking (OPT)
    regulator' ?)

    Yes, easy.

    Possibly, and many inverters will shut down if the input voltage is too
    low.

    Also add a way to isolate the PVs if the battery voltage is too high.


    Cheers, J/.
     
  11. Having chased them up, they claim to start at 20kV.

    They have no problem with our wanting 60kJ, but the 48V is a problem.

    What sort of system were you working on ?


    Cheers, J/.
     
  12. Ian Stirling

    Ian Stirling Guest

    alt.solar.photovoltaic.
    There are sites where you can look up the location, and it'll give
    you the optimal tilt, and average power output for the correctly tilted
    panel, and average power for a tracker, taking into account weather
    conditions.
    The idea is to be able to use all the energy the solar panel provides,
    and one way of ensuring this is to (probably better to discharge just
    before the morning) discharge enought that you can store the excess
    that may build up over an especially good day.
    Batteries do have a temperature coefficiant, which you probably need
    to look at if you haven't already, it alters the set-points a bit.
    Hmm, I see where you'r going.
    This would need a significantly smaller battery, but has the problem that
    a significant proportion of your energy is going to go into and out of
    the battery, incurring losses (some 30-40% IIRC) along the way.
    You'r also going to be charging and discharging the battery quite hard,
    something which is not good for life.
    Probably an idea to ask about charge philosophies there too.
    Is this mast partially underwater, or is the pump at the base?
    <snip>
    It could be, but can you ensure that it can take several thousand cycles
    when exposed to wind/rain/...
    You certainly don't want to take the battery down to the non-changable
    cut-off points of many inverters. (10.6V is common)
     
  13. "John Beardmore" <> schreef in bericht

    [snip]
    What about a mirror, and a stirling motor directly driving
    a mechanical pump?
     
  14. Harder to armour against vandals than a PV, and hard to get good
    Stirling engines off the shelf or at a sensible price ?

    Keen to know if I'm wrong about that though, though if I am, please cc
    to email as my news service is iffy at the moment.


    Cheers, J/.
     
  15. Rich Grise

    Rich Grise Guest

    Would it make sense to focus it on an actual boiler of some kind?

    The working fluid wouldn't have to be water, although it's got
    about the highest heat capacity of anything I know of - freon
    is probably not too hard to get, and would work at a much lower
    temp. Then, a little steam engine.

    I'd like to try that one myself, actually. ;-)

    Cheers!
    Rich
     
  16. Thanks for the pointers. I've quit this n.g. now due to time
    pressure...

    Thanks ! Will have a look !

    I think our plan is to use the power to do useful pumping ASAP rather
    than cycling batteries more deeply than we need to.

    Good point as this will be running at out door ambient temperature plus
    a bit. In the UK I guess that's potentially a -15 to +30 range.

    Yes, if the pump routinely uses more than the panels generate.

    OK, though it could be made relatively large, charged less 'hard' and
    cycled less deeply..

    Pump at base, base above canal.

    With all the snipping I'm not sure what had to take it. Battery I guess
    ?

    Indeed.

    Capacitors by the way proved to be a non starter for this. 120F @ say
    60V wildly expensive ! No magic wand there !


    Cheers, J/.
     
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