Special Test Bench Development for Golf Ball Load Tests

© Fraunhofer CSP
Fig. 1: Experimental design, schematic diagram.
© Fraunhofer CSP
Fig. 2: Electroluminescence image of a crystalline solar module before (left) and after(right) the drop test. The blue markings (right) denote the points of impact.

Photovoltaic modules were damaged by golf balls in a module park close to a golf course. These modules were to be replaced by more resilient modules able to withstand the impacts without difficulty. For this reason, a test special test setup was designed at the Fraunhofer CSP, with which the effects of golf-ball impacts (anticipated impact speeds, typical: 100 km/h; maximum: 300 km/h) can be estimated experimentally once the PV module is hit. The object is to find out how the various module types cope with these forces.

Fig. 1 shows the experimental arrangement chosen for the drop tests. A guide tube can be driven over each position of the solar module by means of a mobile holding device. The tube serves to guide the falling object which can be dropped on the test piece from various heights. In its fall, the falling object accelerates and finally hits the test sample at the impact point with the impulse p. the acceleration distance and thus the impulse of the falling object can be varied by varying the fall height.

The falling object consists of a semicircular steel ball and a cylindrically shaped end-piece, which serves to increase the mass and enable its guidance inside the guide tube. The radius of the semi-circular steel ball equals that of a standard golf ball in order to ascertain the same Hertzian pressure at the point of impact. Steel was used for two good reasons: firstly, because of its greater mass in order to obtain the same impulse as a swiftly hit golf ball even with a much lower impact speed, secondly, because of the greater hardness compared to a golf ball. Golf balls consist of plastic materials which display a time-dependent material behavior. A high-speed golf ball hitting a target does so with a higher stiffness than a slower golf ball. For this reason, steel was used as it assures that the falling object has a comparable or even greater stiffness on impact (worst case scenario). A high-speed camera was used to determine the impact velocity of the falling object.

In addition to obvious failure scenarios such as breakage of the front glass in case of thin, hardly toughened glasses, a cell fracture might occur under the undamaged pane if the impulses are strong enough. This can be seen in electroluminescence images (Fig. 2) only, however, it might result in a significant decrease in module yields in the course of time through an exposure to temperature changes or mechanical stress.