It is well known, that the primary deficiency of polymeric materials with regard to their usage in solar collectors is their poor heat conduction. However, the heat conductivity A is not the only quantity that determines the heat transfer Q from the polymeric absorber surface to the fluid medium. From the formula of the heat transfer

1

1 d T

+ +

a1 A a2

it becomes obvious that the heat transfer induced by convection (the a’s) is at least as important. This heat transfer is determined by the exact fluid behavior at the contact surface and in turn the fluid behavior depends on the geometrical shape of the surface. Since polymeric materials can be given almost any form the question arises if it is possible to optimize the shape of a polymeric absorber surface so that the heat transfer by convection overcomes the lack of heat conductivity.

In this paper a first attempt towards optimizing polymeric collector surfaces is presented. Several CFD simulations were carried out for different geometries of the absorber. To illustrate the approach two of the candidate forms are considered (Fig. 3).

Computations of the fluid dynamics within these two models were made where the carrier fluid was water and the mass flow rate ranged from 10 to 40 kg/m2h. They resulted in identical final outlet temperatures and final power for the two shapes. Fig. 4 illustrates the velocity distributions in the chosen geometries.

This outcome is not really astonishing since the Reynolds numbers for all geometries are extremely low (Re<1) when the flow rates of water remain in the range mentioned above. This means that

changing the geometry of the absorber alone will not lead to significant enhancement of the power of water driven collectors. However this goal could be achieved if the Reynolds number will be increased by operating at much higher inflow velocities and / or by using other carrier fluids than water. Such systems are currently under investigation.

Q = Фm Cp AT.

Values for all these quantities can be obtained from the results of the CFD simulations. The numerical studies showed a nonlinear behavior of both the outlet temperature and the power. In Fig.

5 AT and Q are plotted over the mass flow rate. The resulting curves can be interpreted as characteristics that allow comparing different collectors.

The graph shows clearly that there is a trade-off between the two parameters that matter with regard to quality of the collector. These plots can help to choose the collector with the maximum power given a desired working point (e. g.

given outlet temperature). On the other side the diagram shows how the operating conditions for high flow and low flow collector systems differ.