Solar efficiency

Discussion in 'Eco Talk' started by DIY womble, Mar 1, 2019.

  1. DIY womble

    DIY womble Well-Known Member

    Hi diy simple interface for solar panels ....
    12v panel into 12v heating element in your immersion tank , use gas / mains elec as back up , any issues , would need a cut out to prevent overheating water I expect, cheers
     
  2. DIY womble

    DIY womble Well-Known Member

    Is it socially acceptable to bump your own post
     
  3. chippie244

    chippie244 Well-Known Member

    It's more socially acceptable than bumping your own leg.
     
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  4. masterdiy

    masterdiy Active Member

  5. DIY womble

    DIY womble Well-Known Member

  6. chippie244

    chippie244 Well-Known Member

    I thought for heating water you were better to basically stick black radiators on your roof rather than covert light to electricity to heat?
     
  7. DIY womble

    DIY womble Well-Known Member

    We don't get enough sun , solar panels work out much more efficient w/m2
    The market seems directed towards having half your roof decorated and quite expensive inverter and control system installed , I wondered if a half way budget version would yield better returns
    One or two 150w panels just directly plugged into hot water tank and a relay to prevent overheating
     
  8. chippie244

    chippie244 Well-Known Member

    If you don't have enough sun to efficiently heat water directly how can you have enough sun to inefficiently convert light to electricity to heat?
     
  9. DIY womble

    DIY womble Well-Known Member

    Solar water heaters only heat when the sun shines , solar panels can still give you power on a cloudy day
     
  10. chippie244

    chippie244 Well-Known Member

    Ok, I known very little about this and was just interested, how much power do you get off a 150W pv on a cloudy day?
     
  11. Dam0n

    Dam0n Active Member

    I looked into this recently for myself and after doing the maths it just didn't add up .

    300w working at 100% efficiency would heat a 150 litre cylinder up by 40°c in 23.33 hours.

    In reality the panels won't produce 300w and in anything other than full sunshine they would produce much much less.

    You'd need a much larger system to make it viable.
     
  12. DIY womble

    DIY womble Well-Known Member

    That's my point , it seems you need lots of panels to justify the set up but if I go 12v direct to hot water cylinder surely payback would be quicker
     
  13. Dam0n

    Dam0n Active Member

    What I just mentioned there was for 300w panels directly heating the water via a 12v immersion.
     
  14. chippie244

    chippie244 Well-Known Member

    In a year you might make a cup of luke warm gravy.
     
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  15. DIY womble

    DIY womble Well-Known Member

    Not wanting to go off grid just room for 2 panels that are solar tracking to do what they can and elected to top up
     
  16. MGW

    MGW Well-Known Member

    That is the big question, clearly electric solar panels do give some output, unlike water solar panels in Mid UK. I know even a basic solar panel works in Turkey, and I also know in North Wales even the computer controlled water solar panels don't work. So there must be a cut off point some where between the equator and poles where water solar panels become useless.

    There most also be a point where electric solar panels fail, but I would guess there are less losses in a wire carrying electric than there is in a pipe carrying hot water?

    Second to use water in same way as electric would require a heat pump, as the electric solar panel feeds a controller, which turns the DC to AC and transformers it to the voltage required, so the panel is running at the most economical voltage which changes through the day to extract the maximum from the panel.

    So in theory you could use same solar panel for 230 volt as 12 volt, as the controller will do all the cleaver stuff getting the voltage correct.

    So as to immersion heater, I suppose you can build a controller which will alter incoming supply to match outgoing, however it could also select more or less elements in the immersion heater, I seem to remember a salt is used, can't remember which salt, but the compound turns into liquid at around 70 degs C and so is able to store energy as latent heat, there were some experiments in the 70's idea was summer energy could be stored to see you through a good part of the winter, however the system seemed to have been forgotten, clearly something wrong with it, may be corrosion.

    However the idea of using an electric solar panel purely to heat water is not so daft, main gain over direct water is less losses, and the ability to control the output likely by increasing or decreasing the resistance of the immersion heater array, all it needs is for some one to design and market a controller.
     
  17. DIY womble

    DIY womble Well-Known Member

    I thought the voltage stayed the same just less current, would the heater element still work?
     
  18. JustPhil

    JustPhil Active Member

    The current is proportional to the sun light, the voltage will be defined by the load. In other words it’s a light controlled current source rather than a voltage source. Problem is a heater is just a resistor, so the voltage will vary considerably over sunlight range. That is not itself an issue so long as maximum voltage ratings aren’t exceeded, the issue comes when you consider efficiency. Panels only deliver their rated power into a load chosen to give the correct voltage. You could choose a resistive heater that will give good efficiency in full sun. First problem is that will not be a commonly available immersion heater. Worse though, as soon as you’ve got lower light levels the efficiency will collapse, and you’re back to the problem you’re trying to avoid.
    What you have to use is a MPPT solar controller that will present a varying load impedance to the panel, optimising power output in all weather conditions, and supplying appropriate output to a common immersion heater coil.
    I’ve been looking into this today cos I’m curious about trying this too; I think this controller on eBay might be the ticket.
    https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/263190867548
    (Not a recommendation, just something I’ve discovered)
    Obviously this makes the install more expensive. I suspect in my case, firing up an oil boiler during the summer is costing me a lot, so investing in this and about 300w of panel might not be unreasonably cost efficient, but will also save me burning a lot of oil, which I don’t mind paying a bit over the odds for.
    I also found this forum post interesting, it’s not all correct but most is (I’m an electronics engineer so can make sense of the power calcs, but not a heating engineer so haven’t tackled the overall energy consumption/cost effectiveness calcs... yet)
    https://www.solarpaneltalk.com/foru...-panel-directly-connected-to-immersion-heater
     
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  19. JustPhil

    JustPhil Active Member

    Just to elaborate on that, a ‘12v panel’ will actually output around 22v in full sun and no load.
     
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  20. DIY womble

    DIY womble Well-Known Member

    Cheers Phil , been plastering since 7 , only just finished , will read later
     
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