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Specifications for switch?

whjohnson

I don't generally touch anything to do with solar kit but have been asked to come up with a method whereby an immersion heater can be switched from mains to solar or solar to mains.

A straight forword solution is simply a change over switch with 2 supplies in and one supply out to the heater, but because one of the input supplies is coming from solar - post-inverter so mains voltage - I would like to know if I can use a conventional change over switch or are there some special requiements for switching the mains output derived from a solar supply?

Comments welcome

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  • I'm a bit confused so far. If it's a conventional grid-tied PV system there won't be separate solar and mains electricity system to switch between - they're both connected together already. It might therefore be just a matter of turning on the immersion at an appropriate time - by the normal laws of physics it'll then draw what it needs, the solar system will provide what it can and if insufficient the rest will come from the grid (or the excess exported to the grid if the PV is generating more than the whole installation including the immersion needs). Otherwise you're into all sort of problems matching demand with variable PV output. Battery PV systems tend to operate on similar principle just that some of the PV output can be time delayed to try and better match the installation's demand.

    If there's no choice about when the HP need backup from the immersion, I might be as simple as letting the HP turn it on as it wishes - if there's spare solar capacity that'll be used otherwise the grid will make up the difference as normal. 

      - Andy.

  • in reply to AJJewsbury
    AJJewsbury said:
    If it's a conventional grid-tied PV system there won't be separate solar and mains electricity system to switch between

    Precisely. So, whjohnson, which is it please?

  • in reply to Chris Pearson

    This is one of the questions I have to find the answer to. At first when the heat pump went in,  and there were no panels, a cable was run to a FCU next to the tank from the aux consumer unit which also feeds some sockets and lights in the plant room. This was connected just to a 16A mcb and left switched off

    The solar guys came along, pulled out the immersion heater cable from the cu and ran it into an adaptbale box. Into this box they ran another twin and earth from upstairs and I think it comers from the solar switchgear next to the house CU which is on a different phase.

    It may well be that.the solar feeds into the house cu to the grid.

    The heat pump has its own independent consumer unit. Both this and the aux CU are fed from a different phase to that of the house. Another questions which phase is the solar feeding back to?

    Hopefully find the answers this afternoon.

    _________________________________________________

    UPDATE: Solar feedsinto main house cu and a solar-to-hot water device has already been fitted but it just cannot keep up. Short version is, no point in fitting a change over switch after all. We're keeping it simple. I have returned the installtion to it's original config, with a direct connection to the mains supply. It seems the customer gets paid more by selling juice back to the grid rather than to attempt a saving by diverting a proprtion of  it to run the immersion.

    This is the mad UK energy market at its best!

  • in reply to whjohnson

    It seems the customer gets paid more by selling juice back to the grid rather than to attempt a saving by diverting a proprtion of  it to run the immersion.

    That seems likely if they're on a smart export guarantee (SEG) tariff - most suppliers pay anything up to about 15p/kWh for export - whereas gas or grid-powered HP supplied hot water water would likely undercut that.

    Using the PV electricity to feed the heat pump (so the heat output is multiplied) is a better bet (some heat pumps have controls that'll start up the heat pump at higher target temperatures if they're told there's an excess of locally generated PV available) - if the timings are set up sensibly (i.e. HP runs when PV is available), you could get that almost automatically.

    The solar immersion diverters only really made sense with the original "deemed export" part of the feed-in-tariff - as you could use electricity that would have otherwise been exported, for free. It was a bit of an anomaly really - big-picture wise, using PV and an immersion to heat water is pretty inefficient - if that's your aim you'd be far better off with a simple solar thermal system (probably 5x as efficient in terms of roof space to tap).

       - Andy.

  • in reply to AJJewsbury
    AJJewsbury said:
    The solar immersion diverters only really made sense with the original "deemed export" part of the feed-in-tariff - as you could use electricity that would have otherwise been exported, for free. It was a bit of an anomaly really - big-picture wise, using PV and an immersion to heat water is pretty inefficient - if that's your aim you'd be far better off with a simple solar thermal system (probably 5x as efficient in terms of roof space to tap).

    I didn't appreciate that `FIT` aspect.

    One of those 'never let a good solar day go to waste' aphorism effects.

  • in reply to AJJewsbury
    AJJewsbury said:
    most suppliers pay anything up to about 15p/kWh for export
    AJJewsbury said:
    The solar immersion diverters only really made sense with the original "deemed export" part of the feed-in-tariff - as you could use electricity that would have otherwise been exported, for free.

    Thank you Andy. I must admit that I thought that the suppliers paid a lot less than that. So if you export leccy at 15 p/kWh, and import gas at 6 p/kWh, even with the relatively poor efficiency of a conventional wet CH system, you probably do better than break-even.

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  • in reply to AJJewsbury
    AJJewsbury said:
    most suppliers pay anything up to about 15p/kWh for export
    AJJewsbury said:
    The solar immersion diverters only really made sense with the original "deemed export" part of the feed-in-tariff - as you could use electricity that would have otherwise been exported, for free.

    Thank you Andy. I must admit that I thought that the suppliers paid a lot less than that. So if you export leccy at 15 p/kWh, and import gas at 6 p/kWh, even with the relatively poor efficiency of a conventional wet CH system, you probably do better than break-even.

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  • in reply to Chris Pearson

    Thank you Andy. I must admit that I thought that the suppliers paid a lot less than that. So if you export leccy at 15 p/kWh, and import gas at 6 p/kWh, even with the relatively poor efficiency of a conventional wet CH system, you probably do better than break-even.

    Indeed - but stick it through a heat pump with a COP of 2 (or better) and the figures start to lean the other way.

    I've just done a quick search, and it seems the SEG tariff is rather variable - many are in the 12-16p range, some are around 4p or 5p and there's even one only offering just 1p/kWh. I suspect many tie their SEG tariffs to the import ones, so there's a tradeoff with the import price. Still it pays to shop around.

       - Andy.

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