J u n e 2 0 - 2 3 , 2 0 0 6 S e v I l l e , S p a i n

TEST FACILITY FOR VOLUMETRIC ABSORBER

M. Ebert**, G. Dibowski*, M. Pfänder**, J.-P. Säck*, P. Schwarzbözl*, S. Ulmer**

German Aerospace Center (DLR), Institute of Technical Thermodynamics

* Linder Höhe, 51147 Cologne, Germany

** Apartado 39, 04200 Tabernas, Spain

1 3 t h I N T E R N A T I O N A L S Y M P O S I U M

on Concentrating Solar Power and Chemical Energy Technologies

1Financial support for part of this work by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety

in the Project KOSMOSOL is gratefully acknowledged.

Qualification of volumetric absorbers for open solar tower receivers

Long-time testing of volumetric absorber modules is an inevitable measure to gain the experience and reliability required for the commercialization of the open volumetric receiver technology. While solar tower test facilities are necessary for performance measurements of complete volumetric receivers, the long-term stability of individual components can be tested in less expensive test setups. For the qualification of the aging effects of operating cycles on single elements of new absorber materials and designs, a test facility was developed and constructed in the framework of the KOSMOSOL project1. In order to provide the concentrated solar radiation level, the absorber test facility is integrated into a parabolic dish system at the Plataforma Solar de Almería (PSA) in Spain (Fig. 1).

Specification of the absorber test facility

The design of the test setup is based on the operation conditions of the 3 MWth volumetric receiver tested at the CESA-1 power tower at the PSA [1]. As shown in Fig. 2 the test setup is equipped with two blowers, one blower for the main air stream through the absorber (blower V1, absorber outlet temperature T1) and another one for the simulation of the air return system (blower V2, air return temperature T2). In order to simulate different default mass flows and temperature gradients an adjustment of the blower speed is possible. The absorber test facility is equipped with a programmable logic controller that allows remote operation in different operation modes. For the evaluation of the cycle tests, the blower speed and air flow temperatures are logged.

Solar flux distribution

The solar flux distribution generated by the dish is more irregular than the distribution generated by the heliostat field of solar tower plants. In order to homogenize the flux distribution, the parts of the mirror of the dish concentrator causing distinct peaks in the flux density were identified with the color target method [2] and then masked with a special scattering foil. Fig. 3 shows the resulting improved flux density distribution, the ideal absorber position (black rectangle) and the possible shift caused by offset limitations of the dish tracking (dashed line).

Infrared measurement

With a solar blind infrared measurement system [3] the temperature distribution of the absorber surface at different mass flows, flux density distributions and outlet temperatures can be observed. Fig. 5 shows a typical surface temperature distribution of the rectangle absorber module. The resolution of the thermogram is sufficient to resolve the safety wire net that was installed to protect the concentrating mirror from fragments falling down after a possible damage of an absorber module.

Cycle tests

The flexibility of the dish and the test setup enables a fast reproduction of temperature gradients and transients typical for the operation of a solar power tower plant. For cycle tests of new absorber prototypes the temperature gradients are in the range of 100 - 150 K/min with maximum absorber outlet temperatures of 650 – 750 °C. Fig. 6 shows a typical cycle test day.

Development of new prototypes

The test facility allows the pre-testing of prototypes before manufacturing a batch of absorbers for a complete receiver.

So far four different prototypes of absorber elements were developed and tested. While the present design had a glued bond between both parts of the absorber (the cup and the volumetric monolith), the new bonds were realized by positive connection (see Fig. 4).

These prototypes were tested successfully with over 1000 cycles.

Outlook

While cycle tests of the testing facility will go on to gain more experience and measurement results, the operation of the whole volumetric receiver with the first charge of improved and pre-tested absorbers is scheduled for spring 2006.

References[1] Téllez F, Romero M, Heller P, Valverde A, Dibowski G, Ulmer S, Thermal Performance of “SolAir 3000 kWth” Ceramic Volumetric Solar Receiver. Ramos C, Huacuz J, editors. 12th International Symposium Solar Power and Chemical Energy Systems, 2004, Oaxaca, Mexico, SolarPACES International Symposium, Instituto de Investigaciones Eléctricas, ISBN 968-6114-18-1

[2] Ulmer S, Heller P, Reinalter W. Slope measurements of parabolic dish concentrators using color-codified targets. Proceedings of the 13th International Symposium Solar Power and Chemical Energy Systems 2006, Seville, Spain

[3] Pfänder M, Lüpfert E, Heller P. Pyrometric temperature measurements on solar thermal receivers. ISEC 2005, Proceedings of the International Solar Energy Conference 2005, Orlando, Florida.

Fig. 5

Fig. 2Fig. 1

Fig. 4

Incident Flux [kW/m²]

Fig. 3

Fig. 6

Fig. 1:

Fig. 2:

Fig. 3:

Fig. 4:

Fig. 5:

Fig. 6:

The test facility installed in the dish

system

Design of test facility

Flux map of adapted dish concentrator

Two designs of new prototypes

Example of the temperature distribution

at the sample

Typical cycle test