Flat plate solar water heaters are very simple devices, which use solar energy quite effectively to heat water.

This article discusses, at a limited conceptual level, a  simple cost effective DIY design for a flat plate solar water heater.

How do Flat Plate Collectors Work?

Flat plate solar water heaters use the greenhouse effect to trap solar energy and thus heat up some form of metal plate, which in turn heats up water in pipes thermally attached to that plate.

In this case, the greenhouse effect is caused by the radiant properties of glass.  Incoming solar radiation energy has wavelengths that can pass through glass. This energy is absorbed by a metal plate, or sheet, on the other side of the glass (which is normally painted or coated matte black to maximise radiant energy absorption). The greenhouse effect comes about because the heated plate emits its own radiant energy, but at much higher wavelengths, which cannot pass back out through the glass as easily. So the incoming solar radiant energy is effectively trapped behind the glass. Transparent plastic does not posses this property to the same extent that glass does. This greenhouse effect can be used to markedly increase the efficiency of the unit, in terms of the proportion of solar energy that falls on the collector, which actually ends up heating water.

Basic Components of the Design

The basic components used in this design are readily available, mostly low cost materials that can be bought at most local hardware shops.

The diagram below shows a cross-section of the collector’s basic components:

The core of the design is based on lengths of 15mm copper pipes that are firmly attached (using something like fence bailing wire) to a length of corrugated steel roof sheeting. The pipes are attached to every second “valley” in the sheeting. Both the pipes and the sheet will be painted matte black to maximise radiant energy absorption.

The ends of the pipes should stick out past the top and bottom of the sheet to provide space to join them together. This can be done using 22mm pipe sections and T-piece fittings. The ends of those 22mm cross-pipes will extend out through holes drilled in the timber frame, which will then provide four points at which water can be plumbed into, or out of, the unit. (Note that you should allow for expansion/contraction as the pipes heat up and cool down each day.  So the holes drilled through the frame should be slightly larger to allow some movement).

To join the 15mm pipes to the 22mm cross-pipes, one can either use compression fittings (which are more expensive, but easier to use), or alternatively, the type that require soldering, which are much cheaper.

When the assembled unit is mounted properly, and pointed towards the sun, incoming solar radiation will heat up the matte black coated corrugated roof sheeting and attached pipe work. The heat gained will mostly be trapped inside the frame. The air gap between the glass and the corrugated sheet will minimise conductive heat loss, while the glass itself will minimise radiant, as well as convective heat loss. A sheet of polystyrene foam behind the corrugated sheet will minimise heat loss in that direction, as will the timber frame around the sides. This will help maximise the amount of energy available to heat water within the pipes inside the frame. (High temperature paint will probably be necessary for the pipes and sheeting for this reason).

Tempered glass is recommended so that it is strong enough to withstand hail.

What Size Collector Do You Need?

This depends on the amount of solar energy reaching the ground at your location on earth, which can vary quite significantly. Luckily, thanks to the US government, we can easily get that data using the NASA Atmospheric Science Data Center.  Just click on the link and then enter the coordinates of your location on earth. And then select “Insolation on horizontal surface (Average, Min, Max)” and then press the “Submit” button.

NASA’s measurements for incoming solar radiant energy for your selected location (kWh/m2/day) will then be presented to you. It’s probably best to use the average annual figure (this will mean that your corrugated sheet will be adequately sized “on average”, but might not produce quite enough heat during winter). Note that it’s better to go for a slightly smaller corrugated sheet, than too large, because if it’s too large, the water might overheat and cause potentially dangerous problems.

Once we have that figure, you can use the calculator below to estimate the size of corrugated sheet you need. (Please note that the results also depend on an estimate of how efficient the completed unit might be, which is uncertain. Some trial and error may be required):

Once you have determined the dimensions of the corrugated sheet you need, you can attach your pipes to that sheet (for example, by drilling small holes through the sheet and using bailing wire to fasten the pipe securely to the sheet). As previously stated, you need to make the pipes slightly longer than the sheet to allow enough space for the fittings that join them to the cross-pipes that run along the top and bottom of the sheet. After that, you can build your timber frame to fit around the entire assembly. So the dimensions of the frame depend on the size of the sheet and pipe assembly.

Once you know the frame dimensions, you can get a suitably sized sheet of glass and install that into the frame.

The same goes for the polystyrene sheet at the back.

The CAD drawing below gives an idea of what the final assembly might look like.

Disclaimer:  This article is meant to provide no more than a conceptual design of a simple flat plate solar water heater for discussion purposes.  This article should thus not be  relied upon as detailed instructions for any such project. We make no claims about how effective this design might be. Anything you attempt as a result of reading this article is done entirely at your own risk.

DIY Design for a Solar Water Heater

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