motivation - [principle] - concentrator - stirling enginereceiver - costs & markets

Fig.3: Polartracking

Metal Membrane Concentrator

There are two different tracking modes - azimuthal and polar. For azimuthal tracking the concentrator rotates around an axis perpendicular to the local horizontal earth’s surface (azimuthal axis) and around an another axis which is horizontal (elevation axis)(Fig.5). For polar tracking the collector rotates around an axis pointed at the North Star at a constant rate of 15 degrees per hour. The other axis of rotation is perpendicular to this local polar axis and is adjusted once a day to compensate the seasonal solar declination (Fig.2 and 3).

Since the Stirling engine is an externally heated engine which can utilize various heat sources, a hybrid receiver can be integrated to the system. This allows for Stirling engine operation with combined solar/gas power sources and enables the system to operate not only during sunshine hours on pure solar energy but also during cloud passages and during night time on gas - preferably with bio-gas locally produced. Tests on our prototypes described below have demonstrated net solar-to-electric conversion efficiencies as high as 22%. This is significantly higher than any other solar technology today.

A Dish-Stirling system concentrates the solar radiation and converts it at a very high efficiency into electrical energy. It’s components are:

- the parabolic concentrator (mirror)
- the tracking system
- the solar heat exchanger (receiver)
- the Stirling engine with electric
- generator (Fig.2)

Fig.2: A 7.5m/9kw polar tracking Dish-Stirling system (For azimuthal tracking see Fig. 5)

The parabolic concentrator concentrates the direct incident solar radiation in a focal point where the solar heat exchanger of the Stirling engine is located. The solar radiation is absorbed by the receiver, thus heating the working gas (helium) of the Stirling engine to approximately 650° C. The Stirling engine converts the energy of the hot working gas into mechanical energy. An electric generator coupled directly to the crankshaft of the engine converts the mechanical energy into electricity.

The solar radiation reflected from the mirror must be focused onto the heat exchanger during the whole day. Therefore, the concentrator must continuously follow the path of the sun.