How to extract solar energy from a wall?

Question

I have a vertical wall which faces the sun (North in my case as I am in New Zealand) and it gets hot in Summer.I should like to take some of that heat and transfer it into a nearby swimming pool as it seems a shame to waste it.

My plan is to fix 20mm HDPE pipe to the wall on the sunny side and pump water from the pool through it, then back to the pool.Most of the internet based plans have big spirals of exposed black pipe, but this is ugly (though effective) and wastes pipe.It should be possible to apply the pipe to the wall, spaced appropriately then render over the pipe so that the pipe is protected, concealed and also able to extract the energy from the bits of the wall between the pipes. 100% efficiency is not needed but from an engineering perspective the issues are:

1. The less pipe I can use the better as this will limit costs, reduce flow resistance and of course make it easier to apply the render however the cost of the render is quite high so I need to extract as near as possible all the solar energy from the wall.

2. I will use electronics to detect when the temperature of the wall is higher than that of the pool so there is no need to plan for winter or not so sunny days - it will just run when it's going to heat the water rather than cool it.

3. There is no realistic prospect of the pipe freezing - it's a sunny place with gentle winters and the water is salty.

4. My plan would be to render 25mm deep - the depth of the pipe plus 5mm over the top. chicken wire over the pipe will hold the pipe and the render in place nicely and should deal with expansion in the pipe well enough. Fewer pipes will make this less of an issue of course.

5. If the pipes are placed too far apart, they will not collect all the energy from the sun.

So what is the question?

How far apart should I place the pipes to have most of the solar energy absorbed by the wall enter the pipe, and how should they be configured.

It would be possible to make the render deeper or the pipes narrower.

Two configurations spring to mind also:

a. A spiral. This has the advantage of simplicity and low material cost. No leaks either, but is a very long pipe so resistance is high and airlocking might be a problem. (not likely as I can just use a more powerful pump of course as there will be no joins) I favour this option.

b. A parallel array with large infeed pipe along the bottom of the wall, similar atop it and vertically placed risers between. Lots of joints in this case but much lower pump force needed and it is easy to bleed any air out.

Will this all just fall apart due to differential expansion in heat?

Answer 1

You might want to take a look at commercial designs -- for example there are solar panels with fluid loops on the underside to extract waste heat exactly as you want to do.

Now, on a technical note: ideally you'd like to maximize the heat transfer into your fluid. If you could cover the tubing (and/or the wall) with a material whose absorptance is high in the visible wavelength range but low in the infrared, then you will minimize radiative heat loss & thus improve the conductive heat transfer to the fluid.
The other thing do to is to use tubing on the wall that is rectangular or elliptical in cross-section, so the tubing maximizes solar absorption and minimizes the physical distance from the surface to all the fluid.

Answer 2

While black pipe in any configuration will collect heat from the sun, I recommend not reinventing the wheel. There are many companies around the world that manufacture low-cost black-vinyl solar collectors specifically for pool heating. If you do some price hunting you can likely get them cheaper than you could get the equivalent surface area of tubing.

Heliocol Solar Pool Panel Manufaturer
UMA Solar Pool Heating Distributor
Heliocol vs Fafco

If you insist on making your own collector I recommend looking at manufactured panels and following them as a guide for achieving low cost and best efficiency.

Also, while putting them on a wall will work, to get the most energy per solar collector you should put them at an angle that depends on your latitue. This solar angle calculator has New Zeland. Choose the angle for the time of year you need the most heat. This will probably be the spring/autum angle if you do not plan on using the pool in the winter.