Build a Concentrated Solar Water Heater

Plans for building a cheap advanced solar water heating system

On the REUK.co.uk site there are already instructions on making simple solar water heaters (1) - for example a basic solar water heater (2) which is ideal for camping and can be made in a few hours with cheap components. These home made solar water heaters are nowhere nearly as efficient as commercially available evacuated tube solar water heating (3) but they are far cheaper.

The next step up in DIY solar water heaters is concentrated solar water heating. Concentrated solar water heaters are more complicated to construct and more expensive, however they can be very efficient and great fun to build.

Concentrated Solar Water Heating

The key concept in concentrated solar water heating is the use of mirrors or other reflective surfaces to concentrate the suns rays hitting a large area onto a tube or container full of a fluid which is heated. The overall idea is very similar to that employed in a solar oven (4).

Making the Parabolic Solar Collector

The key component of a concentrated solar water heater is a parabolic trough which focusses all of the light hitting it onto a pipe containing oil.

Obviously a large parabolic mirror would be extremely expensive, so some efficiency has to be sacrificed in order to bring costs down. The trough itself does not get very hot - primarily because it reflects away the suns rays rather than absorbing them - therefore it can be made simply of a thin layer of flexible material such as formica, MDF, or plywood. The inside of the trough must then be covered with a reflective material such as aluminiumised mylar (a reflective plastic sheet) , or aluminium flashing.

Reflections of light hitting a parabolic reflector

Bending the trough into an exact parabolic shape does not have to be very difficult. Basically two end supports need to be made which are parabolic and the reflective sheet can then be fitted to them. Below is an example parabola with a focal point suitable close to the base of the trough (vertex of the parabola). If you print this, or any other suitable parabola onto 2mm squared paper (5) (PDF), you then just have the simple task of scaling up the parabola to the required size.

When printed, the parabola displayed above is exactly 80mm wide. Therefore if you want your trough to be 1 metre (1000mm) wide you need to scale up by a factor of 1000/80 = 12.5. If this parabola is printed on 2mm square paper, then a grid of squares 2 x 12.5 = 25mm wide needs to be drawn onto the wood/plastic which you intend to use for the end supports. Then the parabola can be copied freehand very easily and a jigsaw used to cut out the finished shapes. A hole the size of the pipe to be used should be drilled into the end supports at the focal point of the parabola.

It is well worth making a small model of your proposed solar collector using mirrored card (available online and from art and craft shops) before making the full-sized version.
NEW See the results of testing a concentrated solar water heater prototype cardboard parabolic trough (6) here.

Building a Concentrated Solar Water Heater

Every parabola has a focal point at which all light hitting the parabola is focussed. For our three dimensional parabolic trough we have a line of focal points along which we position the pipe containing the fluid to be heated. Common motor oil is perfect for this job since it is made to be heated to very high temperatures, it is not corrosive, and it is cheap and easy to find. Antifreeze is another option which is often used in solar water heating systems.

Standard copper pipes must be used rather than the newer push-fit plastic pipes. Plastic pipes will melt at around 90 degrees, far below the the temperature which will be reached by the pipe in the solar collector. Painting it with black paint will increase heat absorption.

The Finished Solar Water Heating System

The oil is in a closed system with a small pump (7) used to move hot oil through a coil in the hot water tank. As the oil passes through the coil it transfers some of its heat to the water in the tank before returning to the solar collector to be reheated. Twin coil hot water cylinders (tanks) are available, but it is sometimes possible (but difficult) to add a second coil to an existing standard cylinder.

Schematic diagram for a concentrated solar water heating system

Web Link References

(1) http://www.reuk.co.uk/Introduction-to-Solar-Water-Heating.htm
(2) http://www.reuk.co.uk/Simple-Solar-Water-Heating.htm
(3) http://www.reuk.co.uk/Evacuated-Tube-Solar-Water-Heating.htm
(4) http://www.reuk.co.uk/Solar-Oven-Cooking.htm
(5) http://www.pdfpad.com/graphpaper/pdf/e-m-2-a4.pdf
(6) http://www.reuk.co.uk/Test-a-Concentrated-Solar-Heater-Design.htm
(7) http://www.reuk.co.uk/Pumps-for-Solar-Water-Heating.htm

Article from REUK.co.uk:
http://www.reuk.co.uk/Build-a-Concentrated-Solar-Water-Heater.htm
Published: 12th April 2007
© REUK 2009