Aluminium has the best ratio of thermal conductivity to cost and is the best material for this application. A thickness of approx 1mm gives reasonable collection efficiency (with 150mm pipe spacing) and also sufficient flexibility to avoid closing up the pipe completely.

4. Suitable surface finish for absorber plate

The two options are either non-selective paint or a selective surface. Comparative tests have shown an advantage in collection efficiency of 10-15% for selective over non-selective surfaces. For this low cost application, a non-selective absorber is probably optimal.

5. Suitable back insulation material

Since the collector is built on top of a material e. g. wood which already has quite a low thermal conductivity, then the additional insulation requirements are modest. For this application, a reflective bubble polythene sheet was used. It is easily available

6. Suitable glazing

Twin wall polycarbonate is cheaper and lighter than glass. It is also tough and virtually unbreakable. Its optical transmission is reasonable and its thermal resistance is better than single glass. Modern polycarbonate with UV resistant coating should last for at least 15 years before replacement. Therefore twin — wall polycarbonate (10mm thick) was chosen for this application. Standard glazing bars for the polycarbonate were used to locate the sheet.