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* Corresponding Author: kerr @macgregorsolar. com
This paper describes a novel method of building large, cheap solar water heating collectors on site. It is based on using synthetic rubber pipes with enhanced thermal conductivity which are squeezed against the underside of an aluminium solar absorber sheet. The collector can easily be built on site using unskilled labour at a relatively low cost. In addition the collectors are freeze-tolerant and can handle corrosive fluids.
Several attempts have been made (Bartelsen et al, 1999) to use polymers pipes for solar heat collectors. However, these require specially formed absorber plates to encase the pipes. This method uses plain, flat absorber sheets which are cheaper and more readily available.
The starting point is a foundation board which can be of timber or chipboard. A layer of thin insulation e. g. reflective bubble polythene is then laid on the foundation board. A grid of the polymer pipes is then laid on the insulation, spaced at about 150mmcentres. The pipes can be temporarily held in place by adhesive tape. Then a sheet of highly conductive material, e. g. aluminium, is laid on top of the pipes and is fastened using screws to the underlying foundation board. The thickness of aluminium and pipe spacing are chosen so that the polymer pipes are squeezed against the underside of the aluminium, but are not closed to the passage of fluid. The aluminium sheet is then painted with a solar absorbing paint. Alternatively, aluminium which has a selective surface can be used. Finally, glazing such as twin wall polycarbonate is located over the absorber and held by glazing bars. A diagram showing a cross-section of the collector is shown in Fig 1 below.
Fig 1: A cross-section of the absorber 3. Suitable piping material
Obviously the pipe material should have good thermal conductivity and long life. EPDM has been previously used. Its thermal conductivity can be enhanced by including materials such as carbon black or graphite in its composition. That can also increase the mechanical strength of the polymer. For example, the addition of 40phr of carbon black to EPDM can increase thermal conductivity from 0.2 to 0.5 W/mK and tensile strength from 3Mpa to 17Mpa. It should also increase resistance to UV degradation, though in this application the tubing is shielded from solar radiation.