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Heating and cooling: Solar PV or Hot Water

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  1.  
    • CommentAuthordovecote
    • CommentTimeApr 22nd 2012 edited
     
    If you put aside initial installation costs would using solar PV to contribute to the running of an immersion heater be just as effective as solar hot water?

    I realise there are hybrid systems that do both am interested in the basic comparison.
  2.  
    Dovecote, you can edit the title to prevent silly people like me making silly comments!

    Nick
    • CommentAuthorJoiner
    • CommentTimeApr 22nd 2012
     
    :cool:
    • CommentAuthorEd Davies
    • CommentTimeApr 22nd 2012 edited
     
    Even taking initial installation costs into account PV can sometimes win: when the insolation is low and the output temperature required is high (e.g., DHW in winter). Previous discussion:

    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=8396

    My own thoughts:

    http://edavies.me.uk/2012/01/pv-et-flat/

    and also:

    http://edavies.me.uk/2011/12/pv-heating/
    http://edavies.me.uk/2012/01/solar-per-area/

    It seems to me that some sort of hybrid system would be best using solar thermal for initial warming of water and space heating then using PV to run a heat pump to boost the last 20 °C or so needed for domestic hot water. I have, on and off, been trying to figure out a neat way of exploring the trade-offs in such systems without coming to a simple conclusion.

    The flexibility to use the PV output for various purposes and to use electricity from another source (e.g., a wind turbine) is also attractive.
    •  
      CommentAuthorJSHarris
    • CommentTimeApr 22nd 2012
     
    <blockquote><cite>Posted By: dovecote</cite>If you put aside initial installation costs would using solar PV to contribute to the running of an immersion heater be just as effective as solar hot water?

    I realise there are hybrid systems that do both am interested in the basic comparison.</blockquote>

    From a position of heat output per unit panel area, then the answer is a resounding no, I'm afraid. PV panels are at very best around 20% efficient (in terms of energy per unit area) whereas solar thermal panels are typically around 60 to 70% efficient. This means that you'll get around three times the volume of hot water from solar thermal that you would for PV of the same area.

    From an economic standpoint thing also seem to favour solar thermal, as the cost of a PV array to provide the same energy as a smaller solar thermal array would be more expensive.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 22nd 2012
     
    Posted By: JSHarrisThis means that you'll get around three times the volume of hot water from solar thermal that you would for PV of the same area.

    Is that the right way to gauge it though? After all you need water that is at a minimum temperature, rather than just water that is hotter than it was (say mains temperature. I think that was what Ed was getting at a while back.

    Then it gets more complicated because of the 'deemed export' business.

    I am not sure how hot a 3 kW, 230V, 13A resistance water heater would get when it is only receiving 200W, or less than an Amps worth of juice, and how much difference that would make to water that is already at say 40°C. I am sure it is not that hard to find out.

    This is a classic case a kWh not being the best unit to use when comparing two different methods of heating.
    • CommentAuthorjamesingram
    • CommentTimeApr 22nd 2012 edited
     
    I've been considering this conumdrum for my place. (currently gas boiler for HW)
    Got room for 2-3 more PV or 2 thermal panels , I've both to hand
    (also have a good nick free indirect second cylinder so would go for the 2 cylinder setup if I went with ST-HW)
    already got grid tide PV. so was considering a seperate direct DC trickle feed to immersion from 2-3 PV panel, as mentioned on another thread, or I could just rig up a system to dump excess generation into the immersion

    I'll probably go for ST-HW but I cant help thinking I'm adding complexity and its various negatives.
    Just sticking to PV aided HW seems a far neater, simplistic solution
    •  
      CommentAuthorJSHarris
    • CommentTimeApr 22nd 2012
     
    The advantage of direct PV to immersion heater is simplicity and lack of maintenance. Under pretty much any condition a PV to immersion heater try to transfer as much energy as it can to the hot water, provided there's some sort of MPPT to ensure that the PV is best matched to the load. The element will always run at a delta T relative to the water, even with only a few watts of input.

    It won't be as efficient, in terms of energy per unit area, as a solar thermal system, but maybe the relative simplicity and ability to make some use of relatively low solar input compensates for this to some extent.
    • CommentAuthortony
    • CommentTimeApr 22nd 2012
     
    but without FIT would be a far more costly route to travel.

    The big problem with this discussion is the huge influence FIT has, in other countries it would not even be a serious option.

    Here I think a lower voltage than 240 would work OK and nicely with say a switching circuit and and an old builders transformer joined to a normal immersion hearer.

    Dumping to the grid when HW was satisfied.
    • CommentAuthorMegacycles
    • CommentTimeApr 22nd 2012
     
    If you are prepared to take the non-MCS/FIT route then the price of solar PV per watt is getting close to thermal.

    A 30 x 47mm ET panel from navitron is around £700 although from eco-nomical an equivalent 20 x 58mm panel is £475. Test data suggest an output from these around 1kW at 1000w/m2, but depends on temp differential .

    Looking around ebay PV panels are appearing from around 70p per watt so its the same price as a navitron thermal watt.

    The other considerable cost would be the inverter but the big advantage is you don't need to replace any tanks (although you could use a drop in solar coil).
    If you have the roof space its worth investigating.
    • CommentAuthorjamesingram
    • CommentTimeApr 22nd 2012 edited
     
    Posted By: tonybut without FIT would be a far more costly route to travel

    I've most of the materials about cheap/free so slightly different for me but,
    Even outside FITs, PV panels now available <80p/W , small inverter 600W approx £250, could be easy to rig a small system, possibly not that different from DIY solar thermal in price??

    Jeremy , Yes it's that simplicity of PV that's making me question going for the ST-HW solution
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 22nd 2012
     
    Posted By: jamesingramYes it's that simplicity of PV that's making me question going for the ST-HW solution

    What I like about it too, much easier to run a cable than a pipe. If you keep it outside of the FITs it is easy to take it with you if you move.
    Probably get a 4 panel, 1 kW system up and running for £1500, if your in a sunny location that could give you a MWh of hot water every year.
    On E7 that would cost about 60 quid. £1500 in an ISA would give you about £50 a year.
    • CommentAuthordovecote
    • CommentTimeApr 23rd 2012 edited
     
    Thanks all for the comments so far. Apologies for the initial title mistake (another satellite in the Pacific Ocean).

    Perhaps I can turn my conundrum around though - if I had a normal FIT PV set up, could I justify heating [some] of my hot water via immersion rather than my oil boiler?

    I understand the costs & watts / m2 points, but I can't help thinking that a PV setup on my roof is more flexible than a hot water one given that I can do many things with electricity but few with hot water.
  14.  
    posted by dovecote: >I can't help thinking that a PV setup on my roof is more flexible than a hot water one given that I can many things with electricity but few with hot water.

    How about using the PV electricity to (partly) power lights, kettle, cooker, plasma TV, etc.
    These devices will turn your PV electricity into heat. Use this to (partly) heat your house.
    Then collect the 'waste' heat using exhaust-air ASHP (ecocent etc) to heat your hot water.

    That way you can have your cake and eat it 3 times...

    If oil prices in your area are inflating as fast as they are near me, it is already cheaper to heat HW with immersion than with oil, FITs or no FITs.

    Not saying an ISA wouldnt be a better investment though.
  15.  
    Posted By: WillInAberdeenHow about using the PV electricity to (partly) power lights, kettle, cooker, plasma TV, etc.
    These devices will turn your PV electricity into heat. Use this to (partly) heat your house.
    Then collect the 'waste' heat using exhaust-air ASHP (ecocent etc) to heat your hot water.

    That way you can have your cake and eat it 3 times...
    If you want heat to be added to the house then the ASHP needs to be outside or use outside air. Supplying an ASHP from the air in the house means that cold outside air will be drawn in elsewhere & an additional source of heat will be required.

    Combined with PV & FIT this may make sense, but it wouldn't make sense to put energy into your house at peak rate electricity prices & then use more peak rate electricity to move this heat from your house to your hot water tank.

    David
  16.  
    OK, I was being facetious (sp?), but here goes!

    Posted By: davidfreeboroughIf you want heat to be added to the house then the ASHP needs to be outside or use outside air. Supplying an ASHP from the air in the house means that cold outside air will be drawn in elsewhere & an additional source of heat will be required.


    That's the bit about running additional lights, TVs etc to turn PV into additional heat. Plus you'd need to ventilate anyway, half the GBF seem to be installing MHRVs.

    Combined with PV & FIT this may make sense, but it wouldn't make sense to put energy into your house at peak rate electricity prices & then use more peak rate electricity to move this heat from your house to your hot water tank.

    David


    It does if youre not on gas...

    Turn on some lights, boil the kettle, and thus put 2kWh of heat into your house. Use an additional 1kWh to turn this into 3kWh of DHW, using CoP3 ASHP.
    Net gain is 3kWh of DHW for £0.13 per kWh, and the house got lit, heated and ventilated and tea was made for free along the way. Much cheaper if on E7 (but use storage heaters instead of the lights/kettle unless you like your tea at 2am).

    Compare with oil heating for DHW which costs (me) 12p per kWh, and rising, even before I switch on the kettle.

    I'm joking here... only a bit!
  17.  
    Posted By: WillInAberdeenThat's the bit about running additional lights, TVs etc to turn PV into additional heat. Plus you'd need to ventilate anyway, half the GBF seem to be installing MHRVs.
    MVHR isn't relevant because it requires balanced flow. Any air supplied by the MVHR needs to be matched by an equal volume of air extracted by the MVHR.

    For the ASHP to extract heat from the house, either its input & output ducts both connect to the internal air making the MVHR irrelevant. Or it's configured as an exhaust heat pump, unbalancing the MVHR flows & increasing ventilation heat loss.

    Posted By: WillInAberdeenTurn on some lights, boil the kettle, and thus put 2kWh of heat into your house. Use an additional 1kWh to turn this into 3kWh of DHW, using CoP3 ASHP.
    Net gain is 3kWh of DHW for £0.13 per kWh, and the house got lit, heated and ventilated and tea was made for free along the way.
    The net gain to the DHW is 3kWh. The net loss to the house is 1kWh. So the house got lit, it got cooled by 1kWh &, if the ASHP just connects to inside air, it wasn't ventilated any more or less than before. This can make sense in a climate that requires active cooling, but not in one that mainly requires heating.

    You could configure the ASHP as an exhaust heat pump to increase ventilation, but then the 1kWh of cooling would be provided by trickle vents or MVHR unbalancing.

    However, why wouldn't you just use the 1kWh of electricity consumed by the ASHP to pump heat from outside the house to inside the house/hot water tank?

    David
    • CommentAuthorborpin
    • CommentTimeApr 23rd 2012 edited
     
    <idiotic comment removed> Doh!
    • CommentAuthorborpin
    • CommentTimeApr 23rd 2012
     
    Posted By: WillInAberdeenIf oil prices in your area are inflating as fast as they are near me, it is already cheaper to heat HW with immersion than with oil, FITs or no FITs.
    Can't you just drill for the stuff? :bigsmile::bigsmile:
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 24th 2012
     
    How do you drill for FITs :wink:
    •  
      CommentAuthorDamonHD
    • CommentTimeApr 24th 2012
     
    I'd suggest that resting the drill bit against the wisdom tooth of the energy minister and saying "this won't hurt much" might do it, but I still want him to come along and open our school fair! B^> (Other than never answering my emails, which is a heinous crime obviously, Davey seems to be a nice chap.)

    Rgds

    Damon
  22.  
    Why would you ever consider inefficient PV when you can have both PV and hot water on a tracking system.

    I do think the Zenith Z20 is the future with multiple PV layers increasing efficiency upto a claimed 38% and the PV kept cool by a usable water cooling system.
    •  
      CommentAuthorDamonHD
    • CommentTimeApr 24th 2012
     
    Not really on for mounting on roof tops in urban areas, though, is it? No space, no GPDOs, and at the moment almost no direct sun...

    Rgds

    Damon
  24.  
    Posted By: DamonHDNot really on for mounting on roof tops in urban areas, though, is it? No space, no GPDOs, and at the moment almost no direct sun...

    Rgds

    Damon


    Damon

    If you wanted a flat panel then there is no reason why you cannot still use the PV water cooler of the Z20 installed in your loft but use a fibre optic flat panel and cables to divert the sunlight to the PV module.
    •  
      CommentAuthorDamonHD
    • CommentTimeApr 24th 2012
     
    So, do you have a link for that arrangement? Sounds interesting, but what comes up for "Zenith Z20" is very unlikely to suit many in the UK, which is why I query it...

    Rgds

    Damon
    • CommentAuthorborpin
    • CommentTimeApr 24th 2012
     
    Posted By: SteamyTeaHow do you drill for FITs
    Oil - Aberdeen (never so good when you have to explain it...)
  27.  
    Posted By: DamonHDSo, do you have a link for that arrangement? Sounds interesting, but what comes up for "Zenith Z20" is very unlikely to suit many in the UK, which is why I query it...

    Rgds

    Damon


    Damon

    One of many companies who do natural solar lighting to fibre optics this company does upto a maximum 20 mtr fibre optic cable as standard.

    http://www.parans.com/eng/
  28.  
    Damon

    This is the high efficiency PV unit used in the Z20 which can be used with the fibre optics

    http://azurspace.de/index.php?mm=152
    •  
      CommentAuthorDamonHD
    • CommentTimeApr 25th 2012
     
    Solar lighting vs piping light to PV may be entirely different animals, eg in terms of wavelengths, power transmitted, etc.

    Is there an off-the-shelf product you can point me to?

    Rgds

    Damon
  30.  
    Posted By: DamonHDSolar lighting vs piping light to PV may be entirely different animals, eg in terms of wavelengths, power transmitted, etc.

    Is there an off-the-shelf product you can point me to?

    Rgds

    Damon


    Damon

    If you look at the Parans collector it is only a lens focussing the full range of wavelengths into a fibre optic cable. The cable transmits as much light as possible of all wavelengths then the Z20 is made up of 3 PV panels each capturing a different band of wavelength with the infra red heat dissipated into the water cooling system.
    I dont know how much more off the shelf you want as Parans do a standard large skylight flat panel collector already attached to standard 5 mtrs of cable. Azurspace do there standard either 100mm x100mm or 55mmx55mm 1000 sun concentrated PV/CHP system. I dont know of anybody selling them as a combined kit.

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