The extra-thick double-glass vacuum tubes by German company Soltark have survived a test with 40 mm hailstones at the Swiss test institute SPF Solartechnik in Rapperswil. A systematic investigation of vacuum tubes at SPF Rapperswil two years ago showed that most of the 26 tested vacuum tube collectors were damaged after several hits with 35mm hailstones. In large parts of Switzerland, however, hail with a thickness of 30 mm must be expected at least once in 50 years (red-coloured areas in the above map). More and more Swiss building insurers are adopting rules for the hail resistance of solar collectors, which the owners hope to incorporate into already existing insurance contracts.
“We want to improve the image of vacuum tubes, and dispel the prejudice that vacuum tubes are not capable of withstanding hail showers,” Daniel Lochmaier, CEO of Soltark, explains the motivation behind the three-year development in cooperation with an OEM production partner in China. The reason for the high stability of the new Soltark double-glass tubes is two-fold: The thickness of the outer tube with an diameter of 58 mm is 2.2 mm – 1.6 or 1.8 mm is very common in the industry. The heating and cooling processes in the coating chamber and the vacuum oven have additionally been adjusted to minimise stresses in the glass tubes. The disadvantage of the process: It takes a bit longer, because the tubes are slowly brought to temperature, and then also slowly cooled down again.
Soltark´s vacuum tube collectors with thicker outer tubes have already been available as standard products in three different sizes: 1.78, 3.18 and 4.54 m2. “Swiss solar thermal system supplier Helvetic Energy - former Conergy - has already integrated our new collectors into its portfolio,” says Lochmaier. In Switzerland, the solar thermal sector is more sensitised towards the topic of hail protection, because building insurers take the issue very serious. “In some Swiss cantons, a building insurance only covers damages to a solar thermal system if the collector achieves the required hail resistance class for the respective location,” Andreas Bohren, head of the testing facility of SPF, explains. “It is important to know that this strategy not only concerns solar thermal products: These requirements apply to all products used on the exposed side of buildings. The idea behind them is not to disadvantage renewable energies, but simply to prevent excessive damage.” The hail classes compose several levels, from HW1 (10 mm hailstones) to HW5 (50 mm hailstones). Only classes HW1 to HW3 are considered relevant to solar thermal collectors, since it is assumed that the lifetime of a collector is no more than 50 years.
Flat plate collectors protected by a cover made of toughened safety glass with a minimum thickness of 3 mm can be classified as HW3 without further testing. In the case of vacuum tube collectors, the situation is not as easy because of the aforementioned comprehensive SPF tests of 26 vacuum tube collectors. 44 % of the tested products had already been damaged by a bombardment of 25 mm grains. Most of the remaining collectors had broken at the 35 mm hailstone mark, therefore not reaching the HW3 level required in large parts of Switzerland (see the map above). In contrast, the 15 tested flat plate collector models all resisted the bombardment of 35 mm balls.
So far, the hail test has been a voluntarily part of the European solar thermal standards. But: “The hail test is likely to be included in the standards' upcoming revisions,” says Bohren. As the climate changes and the risk of hailstorms increases, other countries may soon find themselves in the same discussion about the risks to solar technology.