Possible Breakthrough

I went to a Solarsavers seminar yesterday in High Wycombe to discuss energy options with a number of suppliers. I had expected to be most interested in the solar water heating options.

There were some good nuggets in the presentations:

  • “the 19th was the century of coal, the 20th of oil and the 21st will be the century of solar”
  • PV costs around £3,000 per kW. For Hedgerley a 4kW system would cover 40 sq metres, cost around £10,000 net of grant and produce around 4,000 kWh a year, or a third of our current electricity needs.
  • Oxford Solar Initiative is another local advice centre, which has some additional grants on offer.
  • The Solar Trade Associated is said to be a good kitemark against cowboys.
  • UK has 60% of the solar radiation of the equator.
  • Normal required temperature for domestic hot water is around 50 degrees.
  • pipe insulation should equal the thickness of the pipe.
  • most solar water heating uses a dual coil boiler.
  • 20% of the savings from solar water heating can be lost in electricity to power the pump.

But the real disappointment was that shade is a killer even for vacuum tube systems – contrary to what I had read. I am arranging a survey anyway with Gary Fowles from Rayotec, but it doesn’t look hopeful. One speaker said that more than 2 hours of shading a day made systems uneconomic.

But then the final presentation was on a technology which I had somehow managed to ignore up to now – ground source energy pumps. The speaker was a very enthusiastic engineer from ICE Energy in Eynsham, David Greenwood. It’s not a new technology, having been used for decades in Scandinavia. It takes heat from the ground through long pipes of circulating anti-freeze linked to a heat pump, working as a refrigerator in reverse. It does require an input of electricity to drive the pumps, but offers 4 kW of heat for every 1 kW of electricity used. And the result is round-the-year hot water and central heating, though there are issues I need to understand about the lower temperatures achieved without boosting.

As applied to Hedgerley, this would mean a 200 metre looping trench, one metre deep, down into the edge of the woods, which should be no problem, although if we hit the chalk we apparently have to add sand. Then the combined pump and cylinder would replace the current cylinder and boiler in the same space. David couldn’t give a total cost, because they only supply the heat pump side, but it sounds like around £8,000 all in. The next step is to arrange a survey and go and look at the pumps themselves.

Finally, he said that a good approach is to undersize the heat pump slightly and use a wood-burning stove on particularly cold days. So that could the the combination we end up with.

Overall this seems like a very exciting option, providing steady heat and hot water with no carbon implications and lower running costs. It might even allow us to use the swimming pool again in the summer when the ground heat isn’t needed for the house.

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