When I set out to design and build my own flat plate solar collector I decided to take a different route from the conventional single glazed copper pipe grid in the hope of improving the performance of the collector itself.

The configuration I finally settled on was to allow hot air to circulate inside the collector itself by using two panels of polycarbonate glazing situated above each other and gaps between the insulation for air flow.

A single, thin, tall copper tank containing 10.8 litres (2.85 gallons) of water with the bottom half encased in an aluminium sheet absorber completes the design and provides a number of efficiency advantages over the normal flat plate solar collector...

Copper Tank Design

1. The tube is made from 0.5 mm copper sheet metal and is 90 mm in diameter and 1.7 metres long with caps at both ends.
2. There are three 19 mm soft copper inlet pipes near the bottom of the tube to allow the colder water to enter and three pipes right at the top where the hot water circulates into the stainless steel tank using the thermosyphon process.
3. The absorber sheet is 0.5 mm aluminium painted matte black with three coats of Coastcote Etch Primer which is temperature resistant up to 200 °C. I decided to bend the aluminium around the back of the copper tank where the sun doesn't reach to improve the water heating properties there.
4. Ten evenly spaced 95 mm pipe clamps do the job of anchoring the aluminium absorber to the copper tank. To curb galvanic corrosion, I coated the sides of the galvanised clamps in contact with the other metals using a thick layer of black gasket silicon, which is non-corrosive and temperature resistant up to 300 °C.
5. The final assembly 'floats' on top of a 50 mm polyisocyanurate insulation board touching only where the circulation pipes exit the collector to reach the storage tank.

Advantages and Disadvantages

As I said, there are some definite advantages to using this tank in the collector rather than the thinner copper pipes found in a conventional flat plate solar collector...

- The total surface area of the collector is smaller which means it will take up less space on a roof or wherever you decide to mount it and be cheaper to manufacture. The dimensions of this system are 700 mm wide x 2440 mm long for a total of 1.7 sq metres to heat 150 litres of water.

- The circulation of the hot water from the copper tank into the insulated stainless steel tank by means of thermosyphon can take place faster than a normal flat plate solar collector because there are three circulation pipes rather than only one.

- The circulation will take place even when a cloud covers the sun for awhile because the thermal mass of the water and the double glazing keep it hot for longer. The larger mass of water also means it will be less prone to boiling and stagnation is less of a problem.

- There is a large surface area of copper in contact with the aluminium sheet which makes for an efficient heat transfer from the absorber into the water.

It can't all be good unfortunately so here are some of the disadvantages of this setup...

- There is no freeze protection so in areas where frost occurs and temperatures go below 0 degrees centigrade the copper tank will freeze and there is a good chance it will burst. Fortunately I live in an area of South Africa that is frost free so it won't be a problem here.

Internal Air Circulation Design

Getting the air to circulate within the flat plate solar collector does add more complexity, weight and cost to the design, so why bother?

Better efficiency and performance is why.

Without double glazing and air circulation the losses to the surrounding atmosphere by convection and conduction are large, especially at higher temperatures. When they are added, heat loss from the lower layer of glazing is decreased and more energy goes directly into the water contained in the copper tank, making it hotter than it would normally get.

This is especially helpful because of the larger mass of water to be heated, 10.8 litres compared to about 3 litres in a conventional, single glazed collector using smaller copper pipes. It also speeds up the thermosyphon circulation which means you get a tank full of hot water sooner than otherwise.

All this improved efficiency makes the extra effort worthwhile and even though there are several different ways of achieving this, here's what worked the best for me within this flat plate solar collector design...

The pictures above complement what I'm trying to describe so if you follow along with the numbers it will hopefully make things clearer.

1. The 50 mm rigid polyisocyanurate insulation panels I use are ideal for forming air circulation passages. After lining the base of the collector box with one panel I cut two 1,7 metres long, 50 x 40 mm wide pieces to act as the bottom supports for the next layer of insulation board.
2. The gap is 40 mm high which allows the hot air to circulate freely from the top of the collector to the bottom.
3. This is what it looks like after the first layer of polycarbonate glazing is added. The copper tank and the aluminium absorber plate gets sealed inside this glazed enclosure at a later stage.
4. The 40 mm gap at the lower end of the collector that allows the hot air to rise and then circulate back to the top end again, insulating the glazed enclosure with heated air in the process.
5. This is after the top sheet of polycarbonate glazing is added leaving a 30 mm gap between the two layers. The hot air flows between the two corrugated sheets of plastic and then dives down into the 40 mm gap at the top to circulate to the bottom end of the collector and then up again. This encapsulating flow of hot air insulates the bottom layer of glazing and lowers the convective and conductive losses from the copper tank enclosed there, increasing the overall performance and efficiency of this flat plate solar collector system.
6. The completed flat plate solar collector with air circulation passages showing the copper pipe enclosed in the lower layer before the black aluminium sheet is added.