It seems a bit odd to be writing about solar water heating while it’s raining, but rain (and teens still sleeping off a night’s clubbing) is a great excuse to catch up the blog and leave the vegetable garden to water itself for once.
When we completed the plumbing and electrics for the cob bathroom back last November, I mentioned we’d allowed for a yet-to-be-built solar water heater to be used instead of the wood-burning water heater or bailarina in Summer. With such a wet and late Spring, it was April before we got the solar heater construction under way, but we’ve been enjoying hot showers from it since.
It’s a very simple concept. A roll of black water supply pipe laid out in a coil flat on the ground with some insulation underneath. The 100m coil of 40mm black pipe sits on a bed of Leca (lightweight expanded clay aggregate) which is around 15cm/6″ deep. The roll holds 90 litres, so acts as both storage tank and water heater. It’s situated on the terrace above the bathroom, midway between the cold water storage tank and the bathroom, and laid flat so it gets the sun from all angles. This means it’s exposed from around 10.30am to 7.00pm at summer solstice, reducing to 11.30am to 5.30pm 2 months either side of solstice. It remains to be seen how long into the Autumn we can use it, but by winter solstice it will get no sun at all.
Amount in litres contained in 100m coils across the range of pipe diameters
The idea of covering it with glass, perspex or acrylic for extra insulation and to reduce wind cooling was carefully considered and eventually rejected. Any kind of covering is superfluous in summer – the water gets well hot enough without it and since the sun leaves the coil around the time we’re wanting showers at the end of the day, there’s not a lot of value in having a covering. Certainly not enough to justify the expense. (If I was to use anything at all, it would be something like polystyrene to keep the heat in once the sun’s off the coil.) Not only that, but plastic/perspex/acrylic lasts just 2 years in the Portuguese sun before it goes opaque, brittle and fractures, so regular and expensive replacement would be required. Glass is a hazard laid on the ground – we have wild boar who regularly visit the quinta – and would need regular cleaning. And there’s little chance of pre-heating water for the bailarina in winter since no sun falls on this north-facing slope, so the system is designed as an either/or – either bailarina or solar – and is switched over by closing off one supply and opening the other. Once the heater ceases to get any sun, we will drain the solar system down to prevent water freezing in the pipes and cover it over for winter.
So we kept it very simple. And cheap. Total materials cost for this solar heater was about €60. The pipe will eventually degrade in the sun, but it will be inexpensive and easy to replace.
We dug a circular pit, with extra depth where the coil of the pipe would sit
The pit is edged with broken pieces of roof slate to hold back the edges of the soil
The raw materials – a roll of black plastic pipe and some Leca
Lining the pit with horticultural fabric to suppress any plant growth
The pipe is wired to a cross of rebar to keep the coil in place
The cob bathroom. The solar water heater is just to the right of the stand of wild plum trees (light green leaves) on the terrace above, about where the loquat tree and bamboo in the foreground obscures the view of the terrace.
The distance between the solar heater and the bathroom does mean we have to run the water in the bathroom a while before it runs hot, but since the grey water from the bathroom goes to irrigate the vegetable garden, the water isn’t wasted.
The water heats very quickly and is even warm enough to shower with on a cloudy day. Having the solar heater in a direct line from the cold water storage tank above it means we have great water pressure and plenty of hot water for laundry and showers.