Assessment of the normative requirements
It is obvious that the diameter of hailstones is a good indicator for the damage potential of solar energy systems caused by hailstones. As shown in table 2, the standards for solar thermal collectors only define one value for the diameter. On the other hand, the standards for the quality assurance of PV- modules define in each case certain variations of the diameter. The major difficulty for the question on which ice ball diameter should be used to perform impact resistance tests is, that the kinetic energy of a hailstorm and the therewith associated damage potential, does not depend on this value alone. During a 16 years study in France (cf. 3), published in the Atmospheric Research, 3611 hailstorm events have been observed. The main topic of this study was to develop an understandable six class scale for public use to classify hailstorm events. But this is not of importance in this context. More interesting is that this study considered also the effective kinetic energy of severe thunderstorms with the result that the other most representative parameters, which directly influence the kinetic energy of hail events, are the total mass of hailstones per unit area, the total number of hailstones per unit area as well as the percentage of ground covered by hailstones [3]. Expectedly the diameter of hailstones is strongly correlated with the kinetic energy of hailstorms. But also the total mass of hailstones per unit area, which depends again on the total number of hailstones, is strongly correlated with the kinetic energy of a hailstorm. In consideration of all these parameter the resultant kinetic energy of a single hailstone against the grit size in cm is given in Fig. 7.
This shows that the values given in the standards are twice as high as those given in Fig. 7. From this point of view the normative requirement should be strong enough to ensure the impact resistance of solar energy systems. If we additionally take into account the frequency of hailstorms against the diameter of their hailstones, (cf. 3) it seems, that the usage of 25 mm grit size for testing procedures is
good to ensure the impact resistance of solar energy systems to severe hailstorms. This means, even though, that damages caused by larger hailstones, which are really seldom, have to be accepted.
The compliance of the requirements shown in Table 1 concerning the diameter as well as the mass of the ice balls are, verifiable in a easy way with the aid of the appropriate measurement equipment,. However, other requirements defining the quality of the ice balls such as “no cracks visible to the unaided eye” is subjected to the subjectiveness of the observer. Generally such a definition leads not to the same result concerning the quality of the ice balls. But the ice ball quality, independent from the listed parameters in Table 1, does have an essential influence of the result of the impact test, because a part of the energy of the impact of the ice ball is used for the destruction of the ice ball. The process of the production of ice balls therefore has to be defined in a way that the ice balls always show the same fracture mechanics. The production of the ice balls is therefore the most difficult and sensible procedure in respect to the performance of impact resistance tests. This fact results in the basic necessity to perform some representative studies to quantify the impact resistance of solar thermal collectors and photovoltaic-modules. For example, it is conceivable to define a material classification, with the objective to use other materials than ice, e. g. well defined polymer materials, which will cause the same result.
More attention should be given to the testing of solar collectors and PV-modules with respect to resistance against hailstorms. We have set up an apparatus for impact resistance tests at Fraunhofer ISE using ice balls as defined in the relevant European Standards. The present normative requirements are sufficient to ensure the impact resistance of solar thermal collectors and PV-modules to severe hailstorms. It is suggested to investigate if the test procedures can be simplified by using other projectiles than ice balls. Experimental studies have to be carried out with the objective to quantify the impact resistance of solar thermal systems.
[1] http://www. essl. org/ESWD/
[2] Dr. Matthias Fawer, Solarenergie 2007 - Der Hohenflug der Solarenergie halt an. Bank Sarasin & Cie AG, 2007
[3] J. Dessens, C. Berthet, J. L. Sanchez; A point hailfall classification based on hailpad measurements: The ANELFA scale, Atmospheric Research, Volume 83, Issues 2-4, February 2007, Pages 132-139, European Conference on Severe Storms 2004 - ECSS 2004, European Conference on Severe 2004
[4] CEI/IEC 61215: 2005-4, Crystalline silicon terrestrial photovoltaic (PV) modules - Desing qualification and approval, IEC 2005
[5] AS/NZS 2712:2007, Solar and heat pump water heaters - Design and construction, Jointly published by
Standards Australia, GPO Box 476, Sydney, NSW 2001 and Standards New Zealand, Private Bag 2439, Wellington 6020'
[6] EN12975-1,2:2006, Solar thermal systems and components - Part 2: Test methods; German Version, CEN 2006
[7] E 1038 - 5, Standard Test Method for Determining Resistance of Photovoltaic Modules to Hail by Impact with propelled Ice Balls, ASTM International
