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Heating and cooling: Basic solar PV water heating.

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1 to 27 of 27

  1.  
    • CommentAuthorwertert
    • CommentTimeSep 17th 2013
     
    Hi All.

    I have been thinking about installing a solar PV system which is used solely to heat water in 3 hot water tanks.(3 x 110L ). No Connection to the grid/FIT etc. All fitted by me ;-)

    Roof space is limited and west facing so I see no pointing in spending alot. The idea is just to knock something off the gas bill each month i.e preheat. I probably have room for only 6 standard panels.

    Each tank has an immersion heater which I would drive with the output from the panels.

    What would be the minimum amount of equipment I would have to buy to achieve this ? One thing I can't find an answer to is what happens if you aren't connected to the grid and the tanks are all at temperature ? What happens to the electricity produced by the panels !? Don't want a meltdown :shocked:

    Fitted my own solar thermal system a few year back. it's still working ! ( different tank )

    Anyroad just an idea at the moment and any advice would be great, even if it's ... get a man in...

    J
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 17th 2013 edited
     
    Posted By: wertertOne thing I can't find an answer to is what happens if you aren't connected to the grid and the tanks are all at temperature ? What happens to the electricity produced by the panels !? Don't want a meltdown
    Nothing will happen as no current is being drawn, I am assuming you have a thermostat fitted. There will be a voltage there, so don't touch any wires.

    Right if you have room for 6 modules, get the highest wattage ones you can, they are getting close to 350W now. So that would be 2.1kWp.
    Trouble is it is a rare day that you will get that, most of the time you will getting 30% of that (this is down to time of year, time of day and weather related), so now you are down to 630W ( or a couple of fish tank heaters). So not much, but usable in a small tank.

    Control is your hardest thing. DC voltage, especially at high amperage does not like being switched off, it sparks and arcs a lot.
    So may be worth considering going AC, you may find a suitable inverter that does not have to be grid connected, but they are not cheap.
    For safety reasons I would not just wire the modules to the heater, DC voltage and bodies do not mix well. DC is also a good way to make hydrogen and oxygen from water, not the most explosive mixture, but enough to hurt, if even from just a pressure build-up.

    Why not fit more ST, probably suit you better.

    Give you some idea of what can be produced, I have looked at my weather data and charted this:
    • CommentAuthorwertert
    • CommentTimeSep 17th 2013
     
    Hi SteamyTea

    Thanks for that. I would be looking at an inverter. ( off grid type ) as don't fancy the DC side of things.

    The existing ST system is connect to a 250L tank in the basement ( 20m round trip ! ). I could expand this but ideally I would like to connect the new system to the attic tanks as they currently have nothing. Don't fancy changing the tanks in the attic for solar twin coils.

    On reading this forum this morning I also think my ST system may not be working properly. I never see the temps reporting by some people !

    Thanks again
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 17th 2013
     
    If you get 10m^2 of modules (about 6) then you can multiply my numbers by 10.
    So that 6 weeks I plotted would have yielded about 220 kWh (though there is some missing data), say there are 10% system losses, then about 200 kWh. Assuming that the heating element is always hotter than the water then you can a constant 1.4 kW, that should heat a fair bit. I use about 3 kWh/day for water heating. But it was August.
    Have a look at the Stecca Inverter as they kick in at a low 80 V, that will help with the low light levels, not sure if they do an off grid one.
    • CommentAuthorEd Davies
    • CommentTimeSep 17th 2013
     
    My thoughts on the general subject:

    http://edavies.me.uk/2012/11/pv-immersion-gotchas/

    Note the update at the bottom about MPPT devices aimed directly at this sort of application.

    Yes, you have to be a bit careful with DC but, as Steamy says, the only real problem is that switching it with normal contacts is a bit tricky. Switching it electronically (with MOSFETs, etc) is no big deal.

    Not sure about going for high performance panels - you tend to spend more pounds per watt doing that which might not make sense in a case like yours. You can sensibly pay less than £0.65/W incl VAT for the panels themselves.
    • CommentAuthorskyewright
    • CommentTimeSep 17th 2013 edited
     
    Posted By: wertertDon't fancy changing the tanks in the attic for solar twin coils.

    If that's what's putting you off a solar thermal approach, then a side arm Plate Heat Exchanger (PHE) is an alternative means of heating a tank & can be retro-fitted to an existing tank.

    I've not used one myself, but here's a project that used one: http://wookware.org/pics/solar/index.html

    This is the PHE: http://wookware.org/pics/solar/035-IMG_2548.JPG.html
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 17th 2013
     
    Think I forgot that there is 7 days in a week. So a constant 200 W.
    • CommentAuthorwertert
    • CommentTimeSep 17th 2013
     
    @Ed Davis

    Very interested in the www.techluck.com Solar Hot Water Controller. Seems so simple. I could use 6 panels, 2 panels per heater. Ed you mentioned 2 MPPT trackers of which techluck is one. Do you have details of the other ?
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 17th 2013
     
    If you have a fixed resistance drain, will MPPT help at all?
    • CommentAuthorEd Davies
    • CommentTimeSep 17th 2013
     
    Posted By: wertertEd you mentioned 2 MPPT trackers of which techluck is one. Do you have details of the other ?
    Sorry, no. If I had I'd have linked to them. I saw one and thought I should link to it but forgot then somebody wrote to me asking my opinion on that second one. I know they're different because the other one had a less 1990s looking web site. Also, I think it could deal with more than just the 500 W or whatever it is that the TechLuck one does.
    • CommentAuthorEd Davies
    • CommentTimeSep 17th 2013
     
    Posted By: SteamyTeaIf you have a fixed resistance drain, will MPPT help at all?
    Absolutely. Without concocting silly examples it's the case where MPPT will help most - more than a fixed voltage load like a battery being charged. If you think otherwise then have a read of my page linked above.
    • CommentAuthorwertert
    • CommentTimeSep 17th 2013
     
    website says ... ' Direct connect MPPT solar power is by far, the most efficient method of using PV panels to heat water.':bigsmile:
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 17th 2013
     
    Would have to knock up a spreadsheet with some real values:
    http://farside.ph.utexas.edu/teaching/302l/lectures/node62.html
    • CommentAuthorEd Davies
    • CommentTimeSep 17th 2013
     
    Posted By: SteamyTeaWould have to knock up a spreadsheet with some real values:
    Why? I've already posted some real values.
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 17th 2013 edited
     
    So if I understand it right, the load is fixed and the source has a variable internal resistance that is dependant on light levels.
    To match impedance to the fixed resistance load, an MPPT will have to change the resistance of the source. Won't that always mean there is less overall power because a resistance has been introduced. Not the same as varying the resistance of the load for a fixed source resistance. as the losses are on the load side (just become a temperature rise).
    Maybe I have misunderstood it all, but it has always bothered me as I think the wrong things have been compared i.e. the two resistances rather than the power in (from the sun) to the power out (from the heater). So yes it may give maximum, but limited, power but not maximum efficiency.
    • CommentAuthorEd Davies
    • CommentTimeSep 17th 2013
     
    Posted By: SteamyTeaWon't that always mean there is less overall power because a resistance has been introduced.
    No, that's not how MPPTs work. Apart from the incidental resistances in FETs, inductors and capacitors they don't have resistors in the main power path. They're just DC-DC converters; think of them as working like transformers (Vout × Iout = Vin × Iin (less any losses)) only with DC. The MPPT bit is that they tune their output voltage and current to match the available power.
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 18th 2013 edited
     
    Just sat down with a mate of mine who designs electronic stuff and he has tried to explain how the MPPT works to me, it soon changed to how a Current Controller works with not only the MPPT but also the Buck Boost and the H Bridge Inverter. He lost me completely. But when I pointed out that the internal resistance of the modules changes, hence the need for MPPT he said he would ask someone else as he was unsure.

    Not trying to be difficult, but I really don't see how an MPPT is going to help in this sort of design (I can see how if helps when grid tied).

    We then moved on to the best way to switch DC on and off, solid state relays may be the simplest, though not always the cheapest.

    Then we went back to discussing the MPPT and did a draft (or is it daft) design using a fixed reference voltage and an MPPT that can be bought from ebay
    http://www.ebay.co.uk/sch/i.html?_odkw=buck+boost&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR8.TRC0.A0.Xmppt+solar&_nkw=mppt+solar&_sacat=0

    Just a matter of getting the right resistance heater for the module voltage (which varies).

    Must admit I got a bit lost, but when I was doing my renewables degree we had to design a charge controller. Mine was the only one that used a fixed reference voltage and I got top marks for it (was asked to build it and demonstrate it but that never happened). So it seems to me that when dealing with relatively low power inputs and outputs, keeping it simple is not as easy as it could be.
    • CommentAuthorEd Davies
    • CommentTimeSep 18th 2013 edited
     
    Posted By: SteamyTeaBut when I pointed out that the internal resistance of the modules changes, hence the need for MPPT he said he would ask someone else as he was unsure.
    That's, at best, a very confusing way of looking at it. A PV panel is more like constant-current source: the current at any time depending on light level.

    (Actually, as you reduce the load resistance the current does increase a bit but not much. Also, it's also voltage limited - as the resistance is increased the overall voltage increases until the forward biased diodes intrinsic to each cell start to conduct dumping any further voltage increase.)

    Not trying to be difficult, but I really don't see how an MPPT is going to help in this sort of design (I can see how if helps when grid tied).
    So how do you think it should be done?
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 18th 2013
     
    Posted By: Ed DaviesSo how do you think it should be done?
    Just wire it in directly with a suitable simple inverter and switch/thermostat.

    Do we know the internal resistance range of a working PV module? Could work out the resistance needed for the heater then.
    • CommentAuthorEd Davies
    • CommentTimeSep 18th 2013
     
    OK, suppose you've got the panels suggested in my blog post (4 × 250 W panels in series with 25 V Vmp and 10 A Imp). What inverter and immersion would you use with those?
  21.  
    How should fuses be provided for these installations.
    If theres a short ct or earth fault can the PV deliver significant overcurrent or is it self limiting.

    10A DC is a meaty amount could make something hot other than the water.

    What about earthing.
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 18th 2013
     
    I think you don't earth the DC side as this can introduce a false reference voltage point, making the other wire live.
    Fusing the DC side is normal on off grid systems but not on grid tied, I have no idea why.
    Earth the AC side and fuse as normal.
    But it does depend on the inverter and just to make it more confusing the 17th Edition is a bit vague about it.
    • CommentAuthorEd Davies
    • CommentTimeSep 18th 2013 edited
     
    Posted By: WillInAberdeen10A DC is a meaty amount could make something hot other than the water.
    Yes, you have to make sure the connectors, etc, are properly made up. Serious systems will have a lot more current than that.

    Posted By: WillInAberdeenIf theres a short ct or earth fault can the PV deliver significant overcurrent or is it self limiting.
    PV is self-limiting, the short-circuit current is only 10% or so greater than the maximum power point current. It's one of the problems as there's no easy fault detection.

    Posted By: SteamyTeaFusing the DC side is normal on off grid systems but not on grid tied, I have no idea why.
    Fusing is required when there's a source which can supply a lot of current. Batteries can source a lot of current.

    Also, off-grid PV arrays are usually run at lower voltages (say a bit less than 150 vs 600) so typically have more strings so need some protection from faults in one string being fed by the current from two or more other strings.
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 18th 2013
     
    Posted By: Ed DaviesWhat inverter and immersion would you use with those?
    Thought I had replied to this from my mobile while out, but I had not.

    Look at the Stecca range, they seem to know what they are doing on the small stuff.

    Not sure about the actual heater element. Could possible look at the most likely range of solar input and try and work out the best voltage/resistance/power for the immersion from that. Would require knowing knowing more about the modules though.
    • CommentAuthorEd Davies
    • CommentTimeSep 18th 2013
     
    OK, since you don't want to think this through let's consider two likely possibilities:

    1) A 1 kW element. Whenever the sunlight is much less than fully bright the inverter will try to draw more current from the PV panels then they can give, the voltage will collapse and the inverter will switch itself off/fail to work so you'll get no output. Then the voltage will rise - rinse and repeat or the inverter will sit their with its low-voltage-disconnect alarm going or what?

    2) A 500 W element. That should work across a substantial chunk of the operating range but in bright sunlight half the power will be wasted.

    Apart from the ease of breaking the connection with the thermostat you're worse off in the first case and no better off in the second than if you directly connected the panels to the heater.
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 19th 2013
     
    Posted By: Ed Daviesyou're worse off in the first case and no better off in the second than if you directly connected the panels to the heater.
    Yes, I agree, because the two resistances are closer together.
    •  
      CommentAuthorSteamyTea
    • CommentTimeSep 19th 2013
     
    I am going to re-read this and see if it can answer my doubts about the merits of MPPT with a fixed resistance load.
    http://en.wikipedia.org/wiki/Maximum_power_point_tracker

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