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Solar PV for heating

  • 28-03-2022 10:08am
    Registered Users Posts: 12,673 ✭✭✭✭

    Hi all, I’ve recently moved house and going to upgrade the heating. My house is south facing with lots of roof space. I was hoping to use PV panels to heat my very large 2,000L cylinder. I’m a plumber by trade so would be installing it all myself.

    I can retro fit four 240v 3kw AC immersions. I want to do this very very simply.

    I will have a log boiler heating the buffer on colder duller days and oil as a final backup direct to the house.

    is it possible to run 20 or so panels directly into the immersions? I’ve no intrest in using this power for any other appliances only the heating system. I’ve been told I can join the DC panels together and wire directly into AC immersions without batteries or inverters etc. can this be done?


  • Moderators, Home & Garden Moderators Posts: 5,467 Mod ✭✭✭✭graememk

    Immersions are just resistive elements, and can work on DC or AC, But you may need to take actual voltages into account,

    Each panel can output 30-40v so you might need to parallel them to run the immersions.

    But you might not get the most out of the panels.

    The inverters (or charge controllers) are now mostly MPPT (maximum Power point tracking) and they can balance the voltage and current for max power from the panels. if you connect the panels directly it could pull the voltage down out of the optimum range for solar generation.

    Why dont you want to power the house from them?

    And if your going for heat only, what about a thermal system? PV is about 20% efficient whereas the solar tubes are 60-80% efficient.

    Also 2 immersions would be ample for a 20 panel system.

  • Registered Users Posts: 12,673 ✭✭✭✭Dtp1979

    The reason is there’s no coils in the buffer for standard solar. Plus, I thought that tubes were a thing of the past no? Everyone seems to be pushing PV so I thought that was the way to go

  • Registered Users Posts: 12,673 ✭✭✭✭Dtp1979

    the reason I don’t want them powering anything else is because I don’t want to go down the route of batteries and expensive gadgets etc. plus I’d need a lot more panels no?

  • Registered Users Posts: 1,556 ✭✭✭THE ALM

    You don't need batteries or expensive gadget. The solar is producing electricity which is fed into your consumer unit to power anything electrical as normal, be mad not to be using it for anything else.

  • Moderators, Home & Garden Moderators Posts: 5,467 Mod ✭✭✭✭graememk

    For a standard house, yes PV is much easier, panels on the roof and connect inverter and away you go, but for heating water, tubes are still king, you'll get the same power for half the area.

    With PV you add the likes of an eddi for hot water divert.

    Ah forgot the buffers dont have coils, A heat exchanger could be used, but thats adding complication again.

    You wont have to worry about batteries though, your buffer tank is your battery. 2000L cycled between say 60c to 30c is nearly 70kwh of energy. I have 8.6 kwp of panels on a 6kw inverter Split NE/SW on a low slope roof, these fantastic days in march Im generating 25-30kwh a day. In june it tops out at about 50 on a perfect day.... in winter.. im lucky to break 10... or even 5 in december..

    Running some example numbers, (eg the longi 375w panel)

    Each panel max current is about 11 amps, and a running voltage of about 35V (max open circuit is 40v with no load, but there always will be a load if directly connected)

    7 panels in series will be about 245V DC, 11 amps. Max of about 2.6kw, there would be a lot of unused capacity if paralleled.

    An Inverter is generally the easier option (even if you go off grid with it and the only load is the immersions)

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  • Registered Users Posts: 12,673 ✭✭✭✭Dtp1979

    Ok that makes a little sense to me. Are inverters expensive?

  • Registered Users Posts: 12,673 ✭✭✭✭Dtp1979

    But wouldn’t trying to heat a 2,000L buffer tank pretty much use up all the energy leaving nothing for the house? It’s a big 220sqm house, 26 rads and a lot of windows

  • Registered Users Posts: 178 ✭✭eirmail

    Have you thought about PV and thermal combined

    I know nothing about them myself, but if you have either a plumbing or electrical background it could be an exciting DIY project.

  • Moderators, Education Moderators, Home & Garden Moderators Posts: 8,027 Mod ✭✭✭✭Jonathan

    Would agree with @graememk regarding Solar Thermal, but can understand your logic if you already have a 2000L tank. If you are dead set on using a DC element, this Youtube video may be of assistance:

    Regarding your query about the number of immersions, I suggest you play around with this calculator to get a feel for how much energy and how long it would take to heat 2000L of water to 60°C.

  • Registered Users Posts: 2,331 ✭✭✭bullit_dodger

    Wow - that's a pretty unique and interesting problem DTP. I'd echo what Jonathan and graememk have mentioned, but I'd also come at it with a slight different viewpoint for you to think about. It sounds naff, but ultimately you need to decide what the goals/priorities are. Is it in heating the water, saving energy or cutting down on your fuel bill - or a mixture of all three :-)

    If the "only" thing that you care about is heating the water then you can't beat those evacuated tubes. They capture as mentioned about 50% of the suns energy, meaning that per sq meter on your roof they'll do a better job than the panels (~20%). You being a plumber you can sort most of that work yourself.

    The reason why you don't see them installed much anymore is that over the past 10 years, the prices of panels has gone through the floor where a 5Kwp system would have cost €20K a decade now cost 1/3 of that. So people then started to think, why heat just water when you can slap a load of these lower efficient (but reasonably cheap now) PV panels on your roof and power your xBox and Sky TV box .... in addition to heating the immersion.

    Failing that.....with your problem I'd be thinking some form of heat pump is the way to go. You get 2-3x times the heating power from a heat pump from the energy put in, meaning if you need 100Kwhr to heat the tank, then with a HP you could cut that down to 40-50Kwhr. Ok, now that's still not a trivial problem to power, but you've made some serious inroads into the problem

    Some other thoughts. Most inverters max out at 6Kw, so if you had a few immersion coils, you'd only be able to run 2 x 3kw which would max out a inverter. You could also go 3 phase bringing you to 11Kw, but that's a whole different ball game.

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  • Registered Users Posts: 12,673 ✭✭✭✭Dtp1979

    Re above, do PV panels not perform better in Irish type weather though? Dull cloudy etc?

    the reason I wasn’t putting PV towards the house electricity is because I felt the 10-15 panels I was looking at would barely have the legs to heat 2,000l of water

  • Registered Users Posts: 2,331 ✭✭✭bullit_dodger

    I don't know about them (the evacuated tubes) as I don't have them installed, so I can't say definitively how they perform in dull cloudy days, but if you get a good sunny day, you could heat that tank up pretty well. Then it will be a question of the insulation that you have on the tank. If it's top notch, you'll be able to hold that heat/energy for 24 hrs with only minimal losses (maybe 20-30%)?

    The comment about 10-15 panels barely making a dent. 10 panels would be about 3.7Kwp and 15 would be about 5.5Kwp. I guess it goes back to the "why heat 2000L of water"? What's its use? If it's solely for heating, then it's unlikely you need it in summer - where solar PV energy is best, and that brings us back to the original thoughts I have myself about the goals here. Ultimately whatever you capture on the roof via PV should be seen in the context of the whole house, including the 2000L tank. If the PV electricity is going into the tank or going into the house, it's energy that your not having to pay for which goes towards paying back the solar installation.

    Still reckon your heating solution of the 2000L is best served by a solar PV installation possibly driving a Heat pump. It's the "best" way to cut down on the energy required. You may still have to use the boiler/stove, but it would have substantially less work to do. Maybe heating from 40C to it's final temp rather than 20C to the final temp.

  • Registered Users Posts: 12,673 ✭✭✭✭Dtp1979

    The main reason for the PV was that I’ve read they work better in cloudy weather than tubes. I may do more research on that end of things.

    why heat 2,000L? For weather like this where there’s moderate sunshine but tomorrow it could be cold. I want a decent reserve of heat ready to go. There’s never anyone at home from 9-5 do no heating demand do the buffer can rise during the day.

    is it not adding big cost to the installation to have PV joined to the house? Heatpumps are expensive as hell and only have a 15yr life expectancy, and they require maintenance and eventually replacing. Ideally I want enough panels to have zero heating bills with no maintenance costs either.

  • Registered Users Posts: 2,331 ✭✭✭bullit_dodger

    Unfortunately, it's a question that comes up (fairly regularly) on the forums and it's just not really possible for solar PV to supply enough energy to heat your house in winter, unless you spent north of €20-30K I reckon on panels and have a mini-solar farm out your back yard. To give you some context of why this is true.... here is my actual generation of 14 panels (5.3Kwp) for November, December and January just gone.

    You can see in December I generated 58 units. Or approx 2 units a day on average. To heat your 2000L tank, you would be taking somewhere in the region of 50-80Kwhr a day. (depending on the starting/ending temps). So you can see the shortfall. You might be able to get the temperature up a bit from it's 15C-20C starting point, but you'll never have enough power captured from the sun in the dark months to fully heat it.

    In March and to take this specific march which is outstanding, then yes, you could make a serious dent into that heating requirement with Solar, but then we are getting into the time of the year where heating is less likely to be needed, so your actual benifit is minimalized.

    I know that's not the answer that you/most people (including myself) want to hear. I was also thinking similar that solar PV could get me some heating in the winter, but heating lots of water takes a huge amount of energy.

    That said, solar PV is still a great way in reducing your energy needs, I'd encourage you to have a think about it even for non-heating reasons.

  • Registered Users Posts: 12,673 ✭✭✭✭Dtp1979

    Thanks for that. Interesting to read. Any idea how solar tubes would compare in winter to PV?

    im also going to have a log boiler attached and have access to relatively cheap/free firewood, but we all know solid fuel is a pain in the hole so I want to light it as little as possible.