Since 2011, a test solar thermal power plant has been installed at the experimental solar power plant facility in Jülich, located near Cologne, Germany. The facility is managed by the Institute for Solar Research of the German Aerospace Center (DLR). It serves to test and further improve the solar thermal tower power plant technology and its components. Since 2017, “Synlight”, the world’s largest artificial sun, is also located on site. Among other things, it serves to optimize production processes for solar-produced fuels. sbp sonne GmbH (sbp) is undertaking long term testing for its current heliostat technology. The DLR and sbp are already conducting research for almost 30 years at the Plataforma Solar de Almería (PSA) in Spain, the largest concentrating solar technology research, development and test center for CSP in Europe. A Stellio prototype has already been tested and measured there.

Current Testing in Jülich

sbp sonne is currently testing four Stellio heliostats which were assembled as a pre-series in Jülich. The technology was jointly developed by sbp, Ingemetal solar and Masermic. The structure, drives and the control system are currently main focus of the testing. All tests are based on SolarPACES Guidelines in order to ensure comparability of the technology with industry set standards. The intention of these standards is to make the market more transparent in order to further reduce electricity generation costs.

Stellio pre-series of HelikonturPlus

Since June 2017, sbp is performing tests on optics, structure, control and calibration on four Stellio heliostats: More than 30 different tests are carried out throughout the year.

As the Stellio is supposed to work self-sufficient, a wireless Stellio version is also being considered (4): This Heliostat can be controlled directly via WiFi; in a next step, the power supply will be provided by a PV module with electrical buffer storage.

At the same time, the solar field tests offer the possibility to test the software which is used for optical calculations (sbp ray) and structural mechanics (FEM model) and to compare the accuracy of the results and models – also in long-term tests – with regard to measurements and simulations.

After assembly of the pre-series, in which the current joining processes were tested, the measuring technology was put into operation. The test series also help to check the structural calculations (FEM) on the effects of wind loads on the structure. Therefore, a measurement setup for dynamic recording of the wind profile was installed in front of the heliostat mirror surface (1). Additionally, the structural deformations are calculated with dynamic photogrammetry. These measurements are supposed to be compared with wind tunnel tests which were already carried out.

Test results

The tests for commissioning of the technology were evaluated on site with regards to their practical feasibility for large power plants. The desired high tracking accuracy on the test target of the Jülich tower (see picture below) was already achieved at the beginning.

The symmetrical image of the sun on the target, with low astigmatism even in extreme sun positions is characteristic for the Stellio. The first images on the target showed this yet again.

Due to the long test period of one year, it isalso possible to carry out various long-term tests: Thus, measurement results during the different solar positions during the seasons can provide further detailed information to simplify commissioning in commercial power plants.

An important area of the tests includes the photogrammetric measurement of the heliostats to achieve reliable values for the deformations of the mirrors, the pylon and the supporting structure due to dead loads and wind. For this purpose, more than 2000 dots were placed in a tight grid on the back of the mirror surface and on the pylon. The maximum 2D surface error (slope) of the heliostats guaranteed by sbp sonne is 1.6 mrad.

Even if the deformations are measured on a very small scale, they can have enormous effects. This is especially the case when the distance between heliostat and tower is very long, because then a part of the reflected sunlight no longer hits the receiver.

First test results are currently being evaluated and will then be considered for future development of the technology. The knowledge gained will be used to optimise all components in order to coordinate them even better and enable their commercial use in sun-rich countries, such as Spain, China and South Africa. The pre-series tests can avoid delays in commercial projects and at the same time maximise the solar yields of the subsequent systems. After completion of the test program Stellio will be the best-measured and characterized heliostat on the market worldwide.