Summary Report Impact Resistance Testing(2)

The ice balls proposed are:

Table 5: Ice balls allowed to be used according to prEN ISO 9806

The productions as well as other requirements on the ice balls are identical with the IEC 61215.

The steel ball method was not changed from EN 12975.

A new reporting was proposed to give more detailed and transparent information. To test a collector on the “pass” criteria of the hail resistance test the steel ball test is

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accepted. As there is the assumption that the impact generated by a dropping steel ball is always harder to with stand than from a comparable ice ball which is deforming in the moment of impact.

In case a collector does fail the hail testing using the steel ball, one has to repeat the testing with the ice ball method to either back up or revise the fail/pass decision.

6. On-Going and future work

As there is consensus among the experts that the two methods are not directly comparable in their results, and as there is consensus that hail testing is a relevant test in most cases, future work is needed to find one common method to use. The shooting areas will be more clearly defined, this is shown in the following two pictures, which will be parts of comments to the draft.

Figure 1: Sketch specifying the shooting areas for a typical ETC

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Figure 2: Sketch specifying the shooting areas for a typical FPC

A possible development of the testing procedure could also be, to substitute the ice balls by another material like clay or plastics. Empirical rows of shooting ice balls on glass probes (very standardized equipment from microscopy) at Fraunhofer ISE in 2010 showed that the impact impulse is very repeatable. Therefore the homogeneity of the projectile is an important factor of course. To produce ice balls in such a way that they are homogenous and fulfil the normative requirements on diameter

tolerance and weight is not trivial. So to make the method cheaper a substitute at lower effort could be a solution.

Contact Info

Address: Korbinian Kramer

Tel. : +49 (0)761 4588 5139

E-mail: korbinian.krame@ise.fraunhofer.de

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7. Annex A “Working paper of QAiST”

Working paper for methods of the hail stone

resistance test

Introduction

The hail stone testing procedure of collectors by using ice balls is a reliable and repeatable method to test the impact resistance and therefore the durability of the modules against hailstones. For this reason it is demanded to change the optional hailstone test to a mandatory test method in the revision of the EN12975-1,2:2006. It is recommended to establish the second testing method (using ice balls) according to EN12975-1,2:2006.

In the following the testing requirements as they might most likely be established and required in Switzerland were taken into account. Please see references for more detail.

Discussion:

Is the objective of this test procedure the assessment of the extent to which a

collector can withstand the effects of heavy impacts caused “only” by hailstones or an assessment of the ability of collector modules to withstand impacts in general (independent from the kind of impact).?

S.Mehnert: From my understanding the objective of this test procedure should firstly assess the effects of hail impacts against ice balls. A general assessment of the impact resistance against impacts caused by different materials is a too wide area. Than we have also to take into consideration the shape of the impact material and … Methods

1 General

According to this testing procedure the hailstone resistance of solar thermal

collectors is evaluated. Thermal collectors are classified in different hailstone classes depending on its hailstone resistance.

2 Area of application

The testing method contains all solar thermal collectors coming under the norm EN12975.

3 Test items

Group I: flat plate collectors with only one single cover .

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Group II: Large scale collectors modularly constructed, having multiple identically covers. It is sufficient to test one representative single module with an accordant cover surface.

Group III: Vacuum tube collectors. An entire module (not a single tube) needs to be tested with the minimum of required tubes (see below).

4 Test setup

The test setup need to fulfil at least the standards according to EN12975-1,2:2006

4.1 Pre-storage of the sample

Generally no pre-storage of the sample is necessary. In the case of collectors

including at least one material other than glass or metal at the relevant impact spot, the collector needs to be exposed outside for at least 15 days and gaining at least 250 MJ/m2 irradiation. The irradiation may also be applied by a solar simulator according to EN12975-2:2006. A maximum of 14 MJ/m2 and day needs to be

observed and the collector needs to be exposed for at least 12 hours a day outside.

4.2 Ice balls

The ice balls are made of water and should be entirely free of air bubbles and must not have cracks that are visible to the unaided eye. The applied ball diameters are mentioned in table 1. In principle it is allowed to test with different diameters analogously. The testing procedure is being conducted with ice balls having a temperature of -4°C ± 2°C

Table 1: Mass of the ice balls and testing velocities

Annotation: The velocities given in this table are equal to the velocities given within ISO IEC 61215:2005 for the ball diameters 25 mm, 35 mm and 45 mm. The other diameters are not given within this standard.

4.3 Testing instrument

The testing instrument needs to fulfil the requirements according to EN12975.

5 Shooting area and angle

The firing area and angle are adapted to different collector types:

a. Glazed flat plate collectors:

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Collectors with a transparent cover: The impact point needs to be maximum distance of 5 cm from the edge and maximum distance of 10 cm from the corner of the collector cover.

For each diameter during the testing procedure a different corner needs to be chosen. The shooting angle is vertical to the collector surface. Each testing procedure with a certain diameter comprises 10 shots.

b. Unglazed collectors: The same definitions as for the glazed collectors are valid.

Furthermore it needs to be assured that also the tubes containing the fluid are hit. Other reasonable impact points need to be considered if it is not possible to hit the fluid containing tubes due to geometrical reasons.

Unglazed collectors need to be filled with water or with an adequate solar fluid. The collectors are tested under atmospheric pressure.

c. Vacuum tube collectors: For each diameter of ice ball four randomly chosen tubes of the collector are tested. The impact point needs to be in a distance less than 15 cm from the upper or lower end (visible aperture).

Two tubes of them are being shot at the upper end (?up“). The other two tubes are shot at the lower end (“down”). The shooting angle is vertical onto the tube axis.

One cracking of one tube is permitted and needs to be compensated with an additional testing of two more tubes at the same position. Example: cracking of a lower tube end: two additional tubes need to be tested at the lower end.