Permaculturing in Portugal

One family's attempts to live in a more planet-friendly way

Solar water heating: Part 1

Besides a dining table and water from a tap, there’s another thing noticeable for its absence these last 18 months.

That’s a shower! Bucket baths are all well and good and serve their purpose, but once in a while there’s nothing to beat the feeling from a shower.

After plumbing cold running water to the yurt, the next logical step was to think about hot water too. And thinking of hot water made no sense without thinking about a shower at the same time. It’s always been my intention to create a solar water heating system for summer use on the quinta, and to have a solar shower, but we never quite managed to get around to it last year despite having most of the raw ingredients sitting around the place waiting to be put together. So yesterday I finally got around to starting work on the first part of the system.

Here is the recipe:

Solar water heating - take one old single-panel flat radiator

Take one old single-panel flat radiator (obtained a couple of years ago through Freecycle in Scotland)

Solar water heating - connect it, test it, flush it through, rub it down, and paint it matt black

Connect it, test it, flush it through, rub it down, and paint it matt black

Solar water heating - build a shallow box to contain the radiator

Build a shallow box to contain the radiator (from timber that was used for the temporary solar panel framework and for shuttering)

Solar water heating - panel box with cork insulation

Insert 2 panels of 50mm cork insulation (left over from the roof) to prevent heat loss through the back of the radiator

Solar water heating - insulated panel box on temporary framework

Cut openings for the pipework and erect on temporary test framework

Solar water heating - radiator mounted in box

Put radiator into place and test

It works like this. The cold water supply enters the radiator at the bottom. A tap regulates the flow so it can be adjusted to keep pace with the rate the water is heated. The heated water exits through the top of the radiator and, through a combination of convection and pressure (no pump is necessary), rises to fill a 200 litre tank on the terrace above. (For this to work, the solar collector must be below the storage tank.) The hot water in the tank can then be brought back down again to feed a shower and a hot water supply to the yurt.

There was enough sun this morning before the daily thunderstorms arrived to establish that this works. The radiator still needs a lot of flushing, judging by the colour of the hot water filling the tank, and its box needs a glass or perspex cover to magnify the heating effect of the sun and minimise the cooling effects of air, wind and rain. I’m going to make an insulated cover for the collector to minimise heat loss once the sun is no longer shining on it or to shut off the heater if temperatures get too high. The tank on the terrace above also needs an insulated housing to keep the water hot once it’s no longer being fed by the radiator, but so far this is looking promising.

I was wondering how the radiator would perform given that the input and output are on the same side of the panel, but increasing the flow of water and testing which parts cooled down first showed that the water does in fact circulate fairly effectively.

Main concerns at this stage are that I might need to replace the hosepipe and plastic tank with pipes and tank designed for much higher temperatures, but the radiator holds a large volume of water and by keeping it moving through at the right rate, it ought to be possible to prevent temperatures from getting too high.

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  1. Vin July 8, 2014


    Love your project. What temp water enters and what temp water leaves the unit?
    Have you double glazing panel to cover top of unit?

    Try also to paint cork black which will help radiate heat to the panel in the now air tight capsule.

    Good luck

  2. Quinta do Vale July 8, 2014

    Hi Vin. Thanks for your input. The radiator heater in this post would have been covered with a piece of clear perspex … had I been able to get the backing off the sheet I had (but that’s a whole other story … and the piece of perspex is now functioning as a whiteboard). The water temperature going in was about 8°C and coming out about 60°C, but you could control the output temperature with the tap by reducing or increasing the speed and volume of water passing through the radiator.

    This shower and heater has now been retired since we started using our cob bathroom and solar water heating system designed specifically for it. We now use a 100m roll of 40mm black pipe which acts as its own tank as it holds 80 litres of water and it’s fed continually from the new 12,000-litre tank supplying the entire quinta. It can provide many showers throughout the day, giving 2-3 good showers at a time and only taking an hour to heat up to over 60°C in full sun. This isn’t covered with anything. Sure it would keep temperatures a bit higher, but i) it’s a refinement that’s unnecessary in the Portuguese sun and ii) it’s problematic. Perspex only lasts 2-5 years before needing replacement because it deteriorates in the sun and glass is an unacceptable hazard laid flat on a terrace which the javali (wild boar) frequently visit. Not to mention that either would be prohibitively expensive!

  3. George June 19, 2023

    Hi, thanks for the detailed description.
    I recently got similar idea and did 3 projects using panel radiator, single and doubles.
    The first one I just lay single radiator on roof of my shed, the hot water is really hot!
    The second, I put one more radiator on the shed and using garden hose, heating my greenhouse floor, and it is good.
    Third project, I noticed, that my loft is getting really hot in summer, so I put three double radiators mounted just under the roof as conture to the roof pitch and angle.
    All of three projects are only for summer and otum or spring use, when I start using them, I realised how much money and energy I’m saving on daily basis.
    Than, I realised that I should have it done long time ago, this is much more economic and effective, not like solar panels, which I did use before.

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