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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

  • 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.

  • 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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