Evacuated tube collector
The vacuum tube collector is a construction of solar collectors and part of a thermal solar system . It is used to heat water and / or water- antifreeze mixtures. The heat medium is isolated from the outside environment by a vacuum.
Vacuum tube collectors consist of evacuated glass tubes in which there is a tube with a layer that absorbs solar radiation and through which a heat carrier flows. The following variants are known, for example:
- U-tube or coaxial tube is inserted into the vacuum tube from one side.
- a heat pipe is inserted into the tube from one side and heats the heat transfer fluid in a busbar.
Both variants can be implemented with a simple glass tube or with a double-walled glass tube: the latter, so-called “Sydney” tubes, are two concentric glass tubes that are fused together like a vacuum jug ; the space between the tubes is evacuated .
The Sydney tubes have the advantage that the vacuum is only closed off by a glass wall and therefore technologically difficult glass-metal transitions in the vacuum vessel are not required. The strong and increasing distribution is not least due to the politics in China, where companies and suitable technologies are promoted and thus - without funding of private individual projects - a cost-efficient mass production was created. So is z. B. in Beijing, hot water heating with solar collectors is mandatory. For this purpose, vacuum collectors are advantageous because of the higher temperature level, the smaller area and the less critical mounting angle.
Another variant are systems that consist of two glass tubes that are sealed by another material. Due to the different expansion coefficients of the glass and the sealing compound, these tubes have a tendency to stress cracks and thus air to enter.
The absorber is used to absorb ( Latin absorptio , suction) the heat radiation of sunlight. In some models, the inner glass tube contains a coated sheet metal strip, in other systems a coating is applied to the surface of the inner of the two glass tubes. Particularly in the case of vacuum collectors with an elongated focusing mirror, a central copper pipe guides the heat transfer fluid and also carries the absorber layer. In the past, a selective absorber was electroplated onto copper as such, black chrome . The wavelength-selective coatings consist of materials that have the highest possible degree of absorption in the spectral range of sunlight and the lowest possible at the spectral radiation maximum of their own temperature. For example, three metal films are applied by means of magnetron sputtering in approximately twelve individual evaporations.
Since radiation is the only remaining path for heat loss with a vacuum collector, selective absorption is of particular importance and is therefore the subject of research and development.
With vacuum tube collectors, the insulating effect is achieved by a vacuum in one glass tube or in the space between two concentrically arranged glass tubes, which greatly reduces the heat dissipation to the environment due to the prevented convection and the lack of heat conduction. Particularly in winter, vacuum collectors, due to their better insulation compared to flat-plate collectors, produce significantly higher yields than these, but they thaw more poorly when covered with snow or ice.
Resistance to temperatures below 0 ° C must be achieved by adding antifreeze to the circuit water, by using alternative heat transfer media, by draining ( drain back ) or heating. Heat pipes are also at risk of frost because water is apparently used as a carrier medium.
The efficiency of evacuated tube collectors is around 20% higher than that of conventional flat-plate collectors and the achievable temperature level is higher at up to 150 ° C.
Evacuated tube collectors achieve significantly higher operating temperatures than air-filled flat-plate collectors of the same size and are therefore also suitable for generating industrial process heat . The absorber temperature and thus also the liquid temperature can bring the heating medium to a boil or evaporate, especially in summer. If the heat is not dissipated in time (e.g. in a buffer storage tank ), the liquid evaporates in the collectors. To prevent overpressure or bursting, a system z. B. designed so that the resulting steam presses the solar fluid out of the collectors ( Steam Back System ). A solar expansion tank takes up the liquid volume of the collectors that are almost empty (filled with steam). This prevents the overpressure from bursting or releasing the overpressure valve.
A pump standstill has a similar effect in pressure systems, e.g. B. by a power failure. If the liquid is no longer circulated and the collector is no longer cooled, the solar liquid can evaporate. After the collector system has cooled down, the solar fluid is pressed from the solar expansion tank back into the collectors and the system is ready for operation again.
Summer yield surpluses due to excessively large collector surfaces or unsuitable system constellations may stress the heat transfer fluid (a water- glycol mixture; limited heat stability) in such a way that, in the worst case, it becomes unusable due to thermal cracking . However, if the evaporation temperature is below the temperature that triggers the cracking (typically in the region of 170 ° C), this effect can be limited because only the volume of steam in the collector is extremely heated. The evaporation temperature is determined by the system pressure.
For operation with pure water, the water is only pumped from the storage tank into the collectors when heat is to be obtained (drain-back systems). For this purpose, the pipe system must be designed in such a way that there is no residual water anywhere. In Asia, pressureless systems are primarily used, in Central Europe steam baking systems.
- Klaus Oberzig (Ed.): Solar heat - from collector to house system 2010 . Fraunhofer IRB Verlag. ISBN 978-3-8167-8317-6
- Heinz Ladener, Frank Späte, Elmar Bollin: Solar systems - manual of thermal solar energy use . 11., update Edition 2011. Ökobuch Verlag u. Shipping. ISBN 978-3-936896-40-4
- https://www.soltop.ch/de/solarwaerme/roehrenkolleorien.html Company Soltop on vacuum tube collectors, accessed on November 19, 2019
- https://blog.paradigma.de/solarthermie-in-china-was-europa-lernen-kann/ Sven Tetzlaff: Solar thermal in China - What Europe can learn , contribution for Ritter Energie- und Umwelttechnik GmbH & Co. KG, August 18, 2014. Retrieved November 19, 2019
- https://blog.paradigma.de/solarthermie-in-china/ Solarthermie in China , online article by Ritter Energie- und Umwelttechnik GmbH & Co. KG, August 18, 2014, accessed on November 19, 2019
- https://www.solarserver.de/2012/03/26/solarthermie-in-china-peking-macht-solare-warmwasserendung-fuer-neubauten-verpflichtend/ Solarthermie in China: Peking makes solar water heating mandatory for new buildings. , Article by EEM Energy & Environment Media GmbH from May 26, 2012, accessed on Nov. 19, 2019
- https://www.spektrum.de/lexikon/physik/selektiven-absorberschichten/13191 Spektrum.de Lexicon of Physics: selective absorber layers , accessed on November 2, 2019
-  (PDF file; 523 kB) English text that also explains the sputtering (coating) of tubes
- Test report on the frost resistance of heat pipes (PDF file; 646 kB)
- https://www.carmen-ev.de/sonne-wind-co/solarthermie/technik/222-solarkolleorien Publication of the Central Agricultural Raw Material Marketing and Energy Network eV on solar collectors, accessed on November 2, 2019
- http://www.solarplumbingdesign.com/ess1.html Company Solar Plumbing Design ; Jessica Baldwin: Solar Water Heating Fundamentals , accessed Nov. 2, 2019
- Optimized solar systems with perfected drain-back
- Lecture by the Fraunhofer Institute for Solar Energy Systems ISE ( Memento of the original from July 10, 2012 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF file; 894 kB)
- Solar domestic hot water use - Skrit TU Graz