8 Aug 2011

Unintended consequences No 12: PV for water heating

One of the more bizarre aspects of the Feed-in-Tariff is that electricity producing photovoltaics are being used to power immersion heaters for hot water tanks. The "correct" technology for this task is the solar thermal panel, as these are much more efficient in converting sunlight into hot water. They are cheaper to install and they do it directly - or more directly than PV will ever do.

• 1m2 of solar thermal panel will produce around 500kWh of hot water per annum. Installation cost around £1500.

• 1m2 of solar PV will produce around 100kWh of electricity per annum. Installation cost around £700.

• Put another way, hot water powered by solar thermal is around half the price of hot water powered by PV.

Yet such is the distorting effect of the feed-in-tarrifs that people are now thinking of using PV for domestic hot water heating because they can't think of anything else to do with the surplus electricity produced on sunny days. Around 90% of the tariff is available just for producing electricity, and its easy to end up with a system that is oversized for purpose when the weather is right. On hot sunny days, you just don't need much in the way of electricity, so heating a hot water tank makes sense. And if you install more than 15m2 of PV on your roof, then you should be producing enough power to heat your domestic hot water, at least on a par with 3m2 of solar thermal.

So perversely, the way the feed-in-tariff is set up, one of the losers is set to be the solar thermal industry.

It makes senseBut only in terms of the strange Alice-in-Wonderland economics of feed-in-tariffs


  1. Heating water using PV makes a sort of sense considering the UK's small houses. There isn't a lot of roof space, there isn't a lot of space for a more complicated hot watertank with two coils in it.
    Solar hot water needs extra plumbing and isn't quite fit and forget - you need to worry about whether there is too much heat in the summer and whether the system is going to freeze in the winter.

  2. I've made similar calculations for an off-grid system, not wanting to have anything to do with the FITs, RHI or anything. My numbers came out with PV being three times more expensive than solar thermal. If you use a heat pump with a COP anywhere approaching 3 the flexibility of PV begins to make a lot of sense.

    The points Rainbow Bob makes about stagnation and freezing are minor but worth consideration.

    Also, PV tends to generate at the times when even an ASHP might be a bit less than completely inefficient - unlike the spare generation from the grid which is available at night.

  3. Next thing you know someone will propose a zero carbon school with electric water and space heating offset by PV . . .

  4. Mark,
    Before we get carried away with PV and its onslaught on solar thermal we need to see daytime generation in the context of lifestyle.
    Two folk,showered,hair dried,light breakfast, tea (with chilled milk from the fridge) and toast, ironed shirt, quick doze of breakfast news,PC on standby then off to work.Consumption in daylight hours 5.9kwh.
    Same couple in all day with tell& nintendo on,washing machine and dryer,a bit more ironing and two hot meals consumption 14.6kwh.
    In winter at 1.7kwh/day, lucky if you will have anything left for water heating. Summer cooling will readily absorb the 7kwh/day seasonal peak.If you are lucky enough to have anything going to water,think of it like a battery storing the stuff which you would otherwise export to an overheated grid.

  5. It's nothing to do with the FITS scheme, but simply, as you say, what to do with the surplus of electricity produced e.g. on sunny summer days, regardless of FITS or not. What do you suggest? more public spirited to feed it back to the grid perhaps but a) the grid then has to store it and b) the grid pays peanuts for the feed back. Localisation says it's better to find a way to usefully store the (electrical) energy for deferred use on site. Batteries have drawbacks; big water tanks in many ways simpler and useful (if you need that much hot water). An electric Aga is another one. Anyway, wet solar panels may be cheaper per kWh (in that calc efficiency of conversion of incident radiation doesn't figure) but the cost of connecting them to a HW cyl is high - pipes, pumps, valves, thermostats. Wiring a PV setup to an immersion heater is as cheap and simple as anything.

  6. Water heater Uk small house i like them

  7. In carbon terms feeding PV generation into the grid is good, it will displace fossil power sources. So if you are grid connected then using your PV on site is the same in carbon terms as using electricity from the mains.
    So the question is, how does solar thermal compare with on-peak electricity? Not very well usually, but in some cases (like offices) an instantaneous electric water heater could have lower annual CO2 emissions than a solar+gas hot water storage system. Of course just using an immersion heater in the same old cylinder is worst of both worlds, and with enough direct heat use of your PV electricity in place of low carbon sources you could completely negate the carbon benefits and actually increase emissions over all.

  8. Interesting; So how would I go about feeding all my excess generation into my hot water tank. (I'm about the replace the immersion heater)

    I guess a cheap & chearful method might be to add 2nd, an extra low power heater element on a timer.

  9. A small hot water tank with a DC heater run of a PV, preheating water that is heated by a coombe boiler can be very cost effect as it is cheap to install. If the preheating tank is needed to get enough flow rates for more than one bathroom, the add-on cost of the DC heater and PV panel cannot be beaten.

    SuperFlow Solar-PV is an example of such a system.

  10. The advantages of Solar PVs certainly override that of solar thermal panels. Firstly the output yielded by them is convertible and can be exported to the grid for reuse. Secondly it is cheaper in terms of cost and simple to install and maintain. Thirdly it occupies less space as compared to a thermal system. You can get experience further advantages by having a surplus power manager installed with your solar panel. It diverts the surplus power to an immersion tank for heating water instead of exporting it to the grid.

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