Solar power and water heating

Discussion in 'Electronics & Electrics' started by Lostnewb, Feb 11, 2021.

  1. Lostnewb

    Lostnewb Member

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    This will be a bit convoluted, but bear with me.

    Recently upgraded my home solar setup from a dinky 2.2kW single phase to an 8kW 3 phase unit. The standard nowadays is to go overboard with the solar panels so the inverter runs absolutely flat out at 110% rating when it's sunny, and down to about 40% when it's cloudy. I wonder at the longevity of the inverter, having it running so hard during summer, but they did say it was fine. /shrug.

    My power bills come mainly from water heating in winter and air conditioning in summer. The aircon has now been taken care of by running the split systems flat out after the inverter starts putting out enough power to compensate for them, and it's working well, but the water heating is more difficult.

    The solar hot water system is a good one, but during winter it needs the electric booster on for a few hours to give enough hot water. Currently a 3.6kW, but I have a 2.4kW element I can throw in there.

    What I want to do is power the single phase electric booster using solar. With at most 3-4kW solar during cloudy days, it would be best to time it to be on for a good 4 or 5 hours during the middle of the day, giving plenty of hot water at night.

    But that's 2.4kW single phase, on a system that will put out around 3.5kW 3 phase. That sucks.

    Any thoughts on using my old 2.2kW inverter to power the electric booster? The method would be using a 3 phase full bridge rectifier with appropriate smoothing cap to give a nice DC input, providing the actual DC voltage won't exceed the specs for the inverter, and then take the output from the inverter and feed the 2.4kW element. Since it's a grid tie inverter, I'm not entirely sure it would work.

    The other option would be to use a couple of 415V - 120V 1500VA ish sized single phase transformers in open delta, but I don't HAVE any of them and they'd be pretty damn expensive. I'm going for economy here.

    Won't do a tldr, but see the blue text bit if needed.

    Cheers
     
  2. heller44

    heller44 Member

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    How about keeping it simple.. Most non-crap inverters have a programmable dry contact to use for switching something external. Use that to control a slimline contactor to feed the water tank element when you are in pv excess, and have a manual overide for when even that isn't enough.
     
  3. rickbishop

    rickbishop Member

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    Ok, if things are set up properly, your HWS booster will be on a separate Controlled Load meter. This gives you cheaper rates per kWh that is used purely for heating water, and you get those cheaper rates because they only guarantee power within certain timeframes i.e. between 10pm and 7am.

    If you want to do away with this, you'll need to get the Controlled load meter removed, and the HWS put on to the normal power. Dunno what it costs in perth, but in Queensland, a metering change is $100, regardless of what they're doing.

    Then, you just put a timer on your HWS, so that it only turns on for say, 4 hours in the middle of the day.

    How does this work when the single phase electric booster is more than a single phase of the solar system? 3 phase solar systems use NET METERING. So even if one phase is sucking down some power from the grid, the export on the other two phases will compensate for it.

    The downside of this is that you will be paying full price to heat your water when it rains for a week and your solar system is producing fuck all power.



    Oh, and your solar inverter can be overdriven by 1/3 and still run fine (so panels can output 133% of the inverters rating). Don't even bother with the old system, it won't work.
     
  4. OP
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    Lostnewb

    Lostnewb Member

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    I haven't investigated the programmability of the inverter, it's a Huwawei (or however you spell it) and it needs a wifi signal to give any information at all. No LCD on it or anything, so I use an external power meter to figure out what it's putting out. I'm not going to let the thing have access to my wifi, since the installers just said that it pretty much only told you what you were using.
     
  5. OP
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    Lostnewb

    Lostnewb Member

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    I used to be on the REBS scheme, but it ended this year. But I've never had a smart meter installed, I don't have variable rates for my electricity supply. The HWS is already on a timer, which is good.

    However, the net metering thing, if it's true for Synergy (WA's supplier), makes the entire thing moot. That's all I need. Best news I've had in days, thanks mate :)
     
  6. rickbishop

    rickbishop Member

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    Controlled load isn't a function of a smart meter. My controlled load runs on the old spinny meter that has a relay connected to it. The grid operator sends a signal down the wires that switch all of those relays at the same time. However, looking at synergys website, it seems they don't have controlled load over in WA, and they call Queensland backwards...

    Having said that though, I've never heard of any 3 phase setup that's not net metered. It should be fairly easy to check just by looking at the switchboard. If you've got a single meter that displays your grid usage as a single number and your solar export as another single number, it's a net meter.
     
  7. heller44

    heller44 Member

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    Controlled load isn't a thing with Synergy. Also you are correct it is net metering here.
     
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    Lostnewb

    Lostnewb Member

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    Woah, I never even considered that. I am going home after work to have a close look at my meter. Cheers mate.
     
  9. OP
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    Lostnewb

    Lostnewb Member

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    Excellent news, I'll still have a gander at the meter, just out of curiosities sake. This is all good :)
     
  10. rickbishop

    rickbishop Member

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    Yep, no controlled load meter and you've already got a timer on your HWS, so it means you don't have to spend any money. Just set the timer to turn on between 11pm and 2pm and ignore. If it's big enough, that should be more than sufficient for all your hot water needs overnight and in the morning.
     
  11. barywhite

    barywhite Member

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    I have one of these - https://www.acsolarwarehouse.com/products/solaredge-3-6kw-immersion-heater-controller/

    Because the element in the hot water system is a resistive load it doesn't care what its given it just gets hot.

    This box matches the excess PV production and diverts only export power to the hot water system.

    It works well, however I have found what looks to be a bug/fault in its monitoring where it doesn't correctly indicate how many watts are going to the hot water system. Solaredge are not being helpful at all.
     
  12. rickbishop

    rickbishop Member

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    The main problem I've got with things like that is that they only off-load excess PV production. The problem with that is if it rains for a week, you've got no excess PV production, and therefore no hot water.
     
  13. barywhite

    barywhite Member

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    Ah, sorry. I should add it is 'smart' so that in the case where the system doesn't have enough PV production it will switch the hot water system on. You tell it what time you want the hot water ready and it does it. Ours is set to 4pm. I've seen it come on (looking at the graphs) twice in the past 6 months. Have not run out of hot water yet!
     

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