The complete SOLGATE receiver system consists of three receiver modules that are connected in series (Fig. 2). There, the air coming from the compressor with 290°C is heated by solar energy up to max. 1000°C. The receiver modules have a hexagonal entrance aperture and are arranged like a honeycomb in the focal spot (Fig. 4). For higher power levels the complete focal spot can be covered by a number of low, medium and high temperature modules that are interconnected in serial and parallel way.

Two different receiver technologies for air heating in gas turbine cycles were developed:

• a volumetric receiver capable for temperatures up to 1000°C

• a low temperature receiver module at significantly reduced cost for the low flux

regions of the focal spot

To allow for outlet temperatures of 1000°C the absorber, the absorber mounting and the window cooling had to be modified [3]. A scheme of a pressurized volumetric receiver is shown in Fig. 5.

For the high temperatures a highly porous SiC ceramic foam absorber with a pore size of 20ppi is used. The pressure resistant, domed quartz window is actively cooled at the atmospheric side by air jets. For the low temperature receiver the aim was to achieve an overall cost reduction at the first, low temperature stage of the receiver cluster by utilizing simple, less expensive modules. The selected concept is a multi-tube coil attached to a hexagonal secondary concentrator, with the air being convectively heated while flowing through the tubes. The bent tubes are very flexible and thus reduce mechanical stresses from thermal expansion of the tube material. The final layout consists of 16 tubes connected in parallel, each with a length of 2.3 m and a diameter of 28 mm. According to

the design calculations the absorber should have a temperature increase of about 200 K and an associated pressure drop of 100 mbar.

After pre-assembly of several components like secondary concentrators, receivers, or the gas turbine skid on the ground, the pieces were lifted onto the tower for final integration. A emergency shut down procedure was established to be able to shut down the solar power in less than two seconds with the help of a fast shutter, i. e. a white ceramic curtain in front of the receiver cluster.

The tests were divided into two parts. During Phase 1 (winter 2002/2003) the 3rd receiver stage was equipped with a metallic wire mesh absorber capable for 800°C. This phase was intended to demonstrate the general operational capability of the gas turbine together with the receiver cluster. In Phase 2 (summer 2003) the metallic absorber was replaced by a ceramic absorber for 1000°C. In addition an active external window cooling was installed, supported by a new high resolution infrared-scanner to measure the window temperature.

Test Phase 1

After commissioning the gas turbine generated for the first time electric power with a small solar fraction on December 15, 2002. In the following weeks, receiver temperature and electric output power were increased gradually. In March 2003 operation at 800°C receiver temperature and more than 230 kW electrical output was performed for several days. The system performed quite well and was able to handle all kinds of solar transients.

Test Phase 2

After installation of the new absorber, the window cooling and the IR scanner operation was resumed in June 2003. Measurements with the scanner indicated window temperatures that were about 100°C lower than predicted (eventually due to better optical quality of the quartz window). After increasing the receiver temperature above 800°C, the window cooling was activated. Temperature data from the window showed that the window cooling worked as expected.

After several weeks of successful operation, testing was interrupted by an emergency shut down, caused by a control error. Due to a too slow air evacuation through the emergency blow off pipe the second turbine shaft coupled to the generator reached overspeed. This resulted in some damage to the turbine. Anyway, the operation continued with reduced power but continuously increasing temperatures for some days. A problem with temperature peaks on the absorber could be solved by an improved heliostat aiming precision. Finally, a maximum temperature of 960°C was achieved, limited by the turbine damage.