The used absorber system is built of a standard sydney tube (see Fig. 4). This kind of absorber con­sists of a double walled glass pipe (1 and 2) with an evacuated annular gab. This reduces heat losses due to heat convection and heat conduction. On the outer surface of the inner glass wall (2), a selective layer is applied facing the evacuated gap, which allows the highly efficient absorption of the solar irradiation und avoids the heat losses by infrared radiation. A heat conducting system (3 to 5) is inserted over the whole length inside the inner glass pipe. It consists of a round bent heat conducting sheet of aluminium (3), which surrounds a U-bended copper pipe (4 and 5). The heat conducting sheet is attached to the inner side of the inner glass wall and transports the heat from the absorber layer to the copper pipe. The heat transfer medium streams through the U-bended copper pipe and transports the heat out of the absorber system. Inlet and outlet are located on the same side of the vacuum tube. The described kind of vacuum pipe is used in common evacuated tube collectors.

During the tests, it turned out that this kind of absorber is improper for the application in parabolic through collectors. The main reason is the temperature range, since the vacuum tube is supposed to work at temperatures between 100 °C and 150 °C at continuous operation and at a stagnation tem­perature around 200 °C, while the application of parabolic trough collectors starts at these tempera­ture ranges. Calculations have shown that at outlet fluid temperatures of 160 °C at continuous op­eration, the temperatures at the inner wall of the glass pipe already reach 210 °C.

The described effects increase because of the following reasons: the relatively high fabrication tol­erance leads to air gabs between the heat conducting sheet and the inner glass wall. The air acts like a heat insulation, which limits the transport of heat from the selective layer to the heat con­ducting sheet, with the result of an increase of temperature of the inner glass wall. For this reason the thermal losses increase and the efficiency of the system decreases.

During the measurements the glass pipe broke several times. Presumably the copper tube expanded due to the high temperatures in its length and U-turn, which resulted in a burst of the inner glass tube (see Fig. 5).

Planned optimization: The German manufacturer NARVA will develop a new absorber system with the required characteristics. A conceivable concept might be an evacuated one walled glass tube with a heat conduction system inside. The heat conduction system might consist of a U-tube combined with a bent metal sheet, which is applied with a selective layer on the surface. The inlet and outlet pipes are located on the same side of the absorber. This design is likewise the sydney tube but unlike the prototypes’ concept the U-tube does no longer implicate the danger of destroy­ing the absorber system in the new concept. It can expand inside the one-wall glass tube without colliding. Thus no critical tension is reached inside the glass.

Another opportunity would be to design a direct flow absorber with the inlet on the one end and the outlet on the other end of the absorber. Such kind of absorber would allow an easy piping of sev­eral collectors in one row as it is standard for large scale collector systems like the EuroTrough.