It is of major importance to determine the influence of the fluid medium and the fluid dynamics inside the collector on the heat transfer. The heat transfer through a surface depends not only on the thickness and the heat conductivity of the material but also — and in some cases even predominantly — on the heat transfer coefficient between the fluid and the wall which is determined by the fluid dynamics in the vicinity of the surface. Thus one of the main objectives of this work was to study in detail the behaviour and the characteristics of the fluid flow by means of numerical simulations. By using computational fluid dynamics methods (CFD) it was possible to calculate the fluid flow in the absorber, the heat distribution in the solid materials and the radiation interaction between the internal and external surfaces.

In order to develop a systematic way to compare the mechanisms and the performance of different collector types, calculations for a simple flat plate collector were first made and the influence of various parameters was examined.

For symmetry reasons, the numerical model was reduced to a stripe of 1cm width and symmetric boundary conditions were set for both sides. The model consists of approx. 100’000 finite volumes (Fig. 1). CFD simulations were performed in order to quantify how the heat transfer depends on

• the length of the collector.

• the position of the absorber (above, in between or below the carrier fluid).

• the height of the fluid channel.

• the mass flow rate.

Through series of systematic calculations the following dependencies were discovered:

The temperature difference between inlet and outlet is proportional to the length (Fig. 2).

The pressure drop in the fluid channel is proportional to the length.

The temperature difference increases if the height d of the channel decreases.

The pressure drop increases reciprocally with the channel height d.

AT is constant if the mass flow rate Фщ is held constant for different collector geometries.

The pressure drop is proportional to the mass flow rate.

Furthermore, the simulations showed that the position of the absorber does not influence the outlet temperature of the carrier fluid. However the temperature of the surrounding solid materials depends on the position of the absorber. Thus this parameter is of relevance for the design of the absorber for material or stability reasons. This topic is discussed in Section 4.