Solar hot water, hydronic heating

I’ve been interested for a while in getting underfloor heating, probably of the hydronic variety.  Underfloor heating offers some benefits in terms of less air movement and noise, and of giving a more liveable ambient heat – there is some evidence that the warmth of your feet is a key determinant of how comfortable your house is in winter.

The concept has been to use a large water storage tank as thermal mass, and to use evacuated tube water heating to heat that tank.  This gives most of the benefit of having large thermal mass in your house, whilst giving control over when you take the heat out of that thermal mass.

This blog post explains why I think that’s probably not a great idea anymore, and what I’m thinking instead.

The intuitive attraction to solar hot water is that we’re taking heat from the sun and using it directly as heat.  As opposed to taking heat (well, radiation) from the sun, turning it into electricity using PV, putting that electricity into a battery, taking it out again and using it to make heat, to heat your house.  It seems more efficient to store the heat as heat.

However, the bit that I missed in my maths is the existence of heat pump water heaters.  When you create heat using resistive heating you turn most of the energy into heat.  So, if I took a unit of “heat” from the sun, I’d get:

  • About 20% efficiency converting it to electricity
  • About 90% efficiency storing it in a battery (or, alternatively, putting it into the grid and taking it back out again later with some sort of attractive feed in tariff)
  • About 95% efficiency turning it back into heat
  • In aggregate, about 17% of heat ends up heating my home

But, if I use a heat pump to move heat, instead of resistive heating to create heat, I get much better efficiency.  A top rated geothermal heat pump can return about 4 units of heat for every unit of electricity.  Better still, unlike my current air exchange heat pump, it makes no difference when during the day you heat, because it’s pulling heat out of the ground, not out of the air.  So unlike today where I set my heating to run in the middle of the day (more efficient), I could heat first thing in the morning with no efficiency penalty.

If you get 4 units of heat for every unit of electricity, then the calculation becomes:

  • 20% efficiency converting to electricity in a PV panel
  • 90% efficiency storing it
  • 400% efficiency turning back into heat
  • in aggregate, about 90% of the heat ends up heating the home

Better still, you don’t need the water storage tank, and when the sun isn’t shining you can use grid power instead.  You end up with some other bits such as inverters etc, but you also get surplus power in summer that you can sell back into the grid.  And you can use the heat pump water heater for domestic water as well, avoiding a separate water heater.

All in all I think I’m leaning now towards this solution rather than a water storage based solution.

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