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Today's solar heated water in Leeds

Discussion in 'Home Power and Microgeneration' started by Mary Fisher, Jun 25, 2006.

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  1. Mary Fisher

    Mary Fisher Guest

    There has been at least one person who's doubted that in cloudy England we
    could get any useful hot water from a solar panel.

    Well, we've had no sun at all today and it's not really all that warm. Even
    I have put on jeans and sweat shirt instead of shorts and short sleeved
    shirt.

    Yesterday we had a Build-your-own-Pizza party for ten of us to celebrate the
    first firing of the stone built bread oven. We started in the afternoon and
    finished at about 11 pm, eating and drinking most of the time (not jus
    pizzas). It was cool and raining part of the time but we'd erected a canvas
    cover so stayed outdoors. There was a LOT of washing up which couldn't be
    done in the dishwasher and we were too tired to do it last night.

    So this morning I had a marathon washing up session and was quite surprised
    that there was plenty of hot water to do the job, especially since last
    night I'd put on the hot water supplied washing machine to launder the large
    linen tablecloth which of course had wine spilt on it.

    I've just noticed that the cylinder temperature now is 45C (113F) despite
    having even no Yorkshire sun.

    No doubt there will be those who won't believe this but tough, those are the
    facts.

    I hope gb10 reads this.

    Mary
     
  2. Mary Fisher

    Mary Fisher Guest

     
  3. R L driver

    R L driver Guest

    As a rtelative newcomer to the group , what type of collector/ system do
    you have Mary..... Flat plate or tubes, home made or bought?
    I'm down in Wales, just built a flat plate system and trying to get the
    circulation rate down / temperature up. Its beginning to seem to me that
    building a collector is one half of the job, optimising its output at
    reasonable cost seems to be the other half. I Know Leeds reasonably
    well, my Mum lives up there, it gets cold in the winter.
    Steve the grease
     
  4. Would you describe your system in detail, and the problems you're having
    in 'tuning' it? I'm planning to build a solar water heating system and
    your experience would be a great help.

    Thanks,

    brian mitchell
     
  5. Mary Fisher

    Mary Fisher Guest

    Since ours is a commercial product there are no controls, it just does its
    thing - reliably.

    We have a daughter who has a farm in Wales, she gets more snow in winter
    than we do! I know that because we have a tiny caravan on her farm and often
    spend Christmas there, I have some stunning photographs of a monochrome
    landscape :)

    Mary
     
  6. daestrom

    daestrom Guest

    Careful there. Don't assume that reducing flow and raising collector outlet
    temperature will collect more energy.

    I know it's counter-intuitive, but reducing flow and raising the collector
    outlet temperature can actually *lower* the total amount of energy you
    collect. For best results, you want more flow through the collector, not
    less. For each increase in flow, yes the outlet temperature will go down.
    But physics tells us the higher flow will offset the drop in temperature.
    And cooler collectors lose less heat to their surroundings.

    Just so long as the collector outlet temperature is *some* amount above the
    inlet temperature, you're winning.

    daestrom
     
  7. Guest

    And raise the amount you store.

    Nick
     
  8. daestrom

    daestrom Guest

    Care to back up that article of faith with some math??

    Raising collector outlet temperature by reducing flow does not change the
    amount of heat stored in a tank connected to such a loop.

    It just raises the collector temperature and ambient losses in said
    collector.

    daestrom
     
  9. This may satisfy the laws of physics but probably won't please the
    domestic energy-saver very much. Since the temperature of the stored
    water can never be greater than that of the collector output, who wants
    a tank full of tepid water?
    Not if the inevitable losses in the rest of the loop equaled that "some
    amount". Also, there must be energy use which has to be accounted for in
    circulating the water at a higher rate.

    It seems (intuitively) to me that what you say would only be *usefully*
    true if you needed to pre-heat a large quantity of water. I imagine most
    domestic energy-savers would think the cost and commotion of installing
    a solar system to be not worth their while unless it delivered
    appreciably hot water which needed little topping up, if any.


    brian mitchell
     
  10. Guest

    We might consider usable energy, eg water above a certain minimum usable
    temperature, eg 100 F. And we might discuss a large stratified tank with
    60 F water at the bottom...

    Nick
     
  11. R L driver

    R L driver Guest

    wall polycarbonate sheet on the front.
    and back up about 7 or 8 metres
    insulation. The hot feed goes to the hot
    the cyl where the cold water goes in.
    twin" website for the general idea and layout.
    temperature as you use the water directly rather
    the flow rate low enough, running the pump at
    designed for 12 volts it wont start at that voltage ,
    If I was going to sell a thousand of them
    system, which I guess is what solartwin have done .
    pump . A feed back loop from collector
    feedback loop would have a remote
    when the water reached a certain
     
  12. Define tepid? If I have a storage tank full of 110F-120F water, it's going
    to be more use to me than a storage tank where the top half is 140F and the
    bottom half is 100F. If, otoh, the whole tank is less than 100F, that's
    too cold.
    To me, it seems (intuitively) that I just need the output temperature from
    the collector to produce water in the tank (ie, after any losses between
    collector & storage tank) at approximately 120F. Anything else is
    unnecessary. However, that's based on the assumption that the collector
    _can't_ produce enough heat to heat my _whole_ tank above 120F. I think
    Nick's comment about being able to _store_ more if you move the fluid more
    slowly makes sense if you can actually get enough heat to bring the whole
    thing to 150F (don't forget to install the anti-scald mixing valve...).
     
  13. daestrom

    daestrom Guest

    Circulating the water faster will reduce collector losses and bring your 60F
    water (a 'negative' amount of usable energy) up to the 100F point faster.

    If the tank is stratified and 100F at the top and 60F at the bottom (in a
    linear profile), then you have an average of 80F water (20F *below*
    'usable'). Circlating the water slowly so that the 60F is heated to 100F at
    the collector outlet doesn't raise the average temperature of the tank any
    faster than circulating the water quickly.

    Think overall system and apply the first law. The solar energy in-flow to
    the collector is the same, ambient collector and return piping losses are
    higher with low flow system, so rate of energy storage in the system is
    smaller. Has to be. Unless you want to re-write some of that 100 year old
    physics you're so fond of....

    daestrom
     
  14. daestrom

    daestrom Guest

    As long as the return water is warmer than the water leaving the tank to go
    up to the collector in the first place, you're gaining. You won't gain more
    by throttling flow.
    As I said, as long as the return is at least warmer than the tank. If
    'inevitable losses' can cool the collector output to below the tank
    temperature with high flow, then how much more are those losses when the
    return water is 20 degrees hotter and how much more heat will it lose as it
    spends more time in the return piping??
    I'm talking about circulating between a collector and a storage tank. This
    tank can have auxilary heating, or feed a separate 'conventional' heater.

    If you're talking about a 'once-through' system where you take cold supply,
    run it once through the collector and then into the conventional heater
    where it is 'topped up' before use, then the flow is governed by the end
    use, not anything we can change here. Putting a 'splitter' in such a once
    through system so that only part of the supply goes through the collector
    (and thus exits the collector at a higher temperature) would be ludicrous.

    daestrom
     
  15. It's true no actual temperatures were specified. Your 100F sounds about
    right. In fact, body temperature. Any less feels cool. Of course, the
    water may have come from the mains feed at a temperature of 50F and the
    collector heroically raised it a further 50F, but it will still *feel*
    tepid.
    Why so? Do you always use all the water in your storage tank whenever
    you use any?
    I speak from a sublime and virginal ignorance about the whole subject
    really, but I suppose there must be times when the collector can't, as
    you say. There should also be times when it can, though, and then you'd
    want to be able to take advantage of it. Since the energy input into the
    collector is not easily controllable, perhaps what is really needed is a
    variable flow rate to adjust to the conditions obtaining.


    brian mitchell
     
  16. And placed it right among the pigeons! (Actually not).
    Thanks. How have you thermally connected the aluminium sheet to the
    copper pipe? And what kind of insulation have you used?
    Did you use rubber pipe for the same reason Solartwin do, to allow for
    freezing expansion, or is that to conserve heat? If the latter, what's
    your anti-freezing strategy?
    I'd think you'd want a low rate of flow for either direct or indirect systems.
    Have you measured the temperatures you have achieved?
    You would have this loop whatever kind of pump you end up with?

    Is there the possibility of electronic control for the pump you already
    have. There are current control devices, as opposed to voltage control,
    and reduced current may be what you want. Also possible to deliver a
    starting current higher than running current via electronics (not that I
    know precisely how to do it). Just a thought. I'd be glad to know how
    you finally resolve this.


    brian mitchell
     
  17. R L driver

    R L driver Guest

    Going backwards thru the list

    I'd love to do all this with electronics, but I'm well out in the sticks
    here ( ie B+Q or M+S = 100 mile round trip) and whilst I know a few
    people who can mend amps stereos etc I dont know anyone who can design
    electronic circuitry so Im stuck with solutions I can fix up. (Any
    boffins reading this please post!)

    From what I can see for an indirect cylinder you will always get a tank
    full of luke warm water, at least initially, with a direct system with
    stratification you should get hot at the top of the tank

    Where I live is in a coastal area where we get very little frost, maybe
    5 or 6 night a year, but the idea is to drain and cover the collecter
    when frost is forecast, I've incorporated bleed valves at the cylinder
    to help draining back etc

    I used rubber pipe because I believe that with small bore moves the heat
    where you want it more rapidly than having it sat round in a larger pipe
    moving slowly. So hopefully less thermal losses.

    If the flow rate could be reduced enough then i dont think i would need
    the feedback loop ( solar twin dont use one I dont think)

    Insulation = 50 mm foil covered urethane foam ( easy to get)

    The 50 feet of microbore is cable tied to an ally sheet roughly every 2
    inches ( lots of holes!) then painted with aluminium wood primer which
    has lots of tiny aluminium flakes in it, filled in the little gaps, good
    for heat transfer?? All painted matt black obviously, seems to work , on
    a test run from a 25 Litre drum at low flow rate the water was too hot
    to run over my partners hand ( so 50 deg ish?) but on a very hot day,
    but I would like to move away from the dull day = luke warm water syndrome.

    Steve the grease
     
  18. Google provided this very simple PWM control circuit using only one
    chip, one transistor and a few other components:

    http://www.solorb.com/elect/solarcirc/pwm1/index.html

    and Maplin

    http://www.maplin.co.uk

    will send you all the parts through the post. If you go to their
    "projects and modules" section they have a PWM controller in kit form as
    well for about £16. But getting one of your people who can mend amps to
    assemble the above circuit should cost less, even with the drink. PWM
    control gives good torque at low voltage, so should overcome the
    starting difficulty. A variable control such as this would allow you to
    find the best setting.

    What you don't say is *when* the pump has to operate. What turns it on
    and off at the moment? Whatever you currently use would probably replace
    the on/off toggle switch in the circuit.

    (If you're not used to looking at circuit diagrams, this one is a *lot*
    simpler and easier than it may seem at first)

    Hope this may be of use.

    That's the scientific spirit! Test it on your assistant :)


    brian mitchell
     
  19. Guest

    This may not be negative if we are preheating water for showers, with
    a conventional water heater to follow.
    And if we have a step profile?

    Nick
     
  20. Guest

    But you may store more.
    I'm thinking about an unpressurized stratified heat storage tank with
    a pressurized pipe coil heat exchanger that preserves stratification.

    Nick
     
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