Solar thermal. Vacuum tube collectors. Testing institute warns against generalised judgements

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1 Solar thermal Vacuum tube collectors Tubes hot or not? Vacuum tube collectors are a contentious subject both their technology and their importance on the market. Some see them as the future of solar thermal, while for others their importance is more likely to dwindle. Frank Kameier, professor at the Düsseldorf University of Applied Sciences in Germany, has carried out use assessments for small solar thermal systems and come to a rather surprising conclusion: Flat plate collectors should only be used at especially high yield sites in southern Germany, says a report published in December It is a remarkable result, as it contradicts previous experience. The solar heating expert and consumer consultant Oskar Wolf from the Solar Energy Information and Demonstration Centre (Solid) in Fürth (also Germany) has a different opinion, for example: This result is completely unfathomable. We can show from the over one hundred thousand solar systems in Germany that it is right to use flat plate collectors because they are a) cheaper, b) longer lasting and c) essentially provide exactly the right temperature levels for use. The statements by Frank Kameier are stoking a disagreement which is as old as the modern collector technology itself. While some say that vacuum tube collectors work more efficiently when a solar thermal system is to provide space heating in addition to hot water, others assert that this is just a fairy tale. It is certainly much more valid to say that the supposedly higher yield from vacuum tubes comes at a high price because of the higher collector costs. Testing institute warns against generalised judgements In the Paradigma vacuum tubes chemically stable water flows around in place of a glycol mix. Photos (3): Paradigma Andreas Bohren, head of solar testing at the Swiss solar institute SPF in Rapperswil, warns against heeding such generalised statements: The question of performance is a tough one because there is no easy answer. The right answer is that you can t give a clear answer and have to look at each case individually. Although some vacuum tube collectors are more expensive than flat plate collectors, other cheaper products are also on sale. Tube collectors tend to work better at higher temperatures than flat plate collectors, but that s all that can be said on the matter. That they are better for heating support and in winter is partially true because higher temperatures are more desirable for heating support than for preheating. The collector tester qualifies the statement 42

2 that tubes provide heat more efficiently in winter: In the winter a solar heating system in central Europe doesn t deliver that much anyway, regardless of whether it is a tube or a flat plate collector. After all, the days are much shorter in winter. Plus, you should take into account that compared to a flat plate collector a vacuum tube will tend to stay snowed under after it snows because of the good insulation. An advantage thus becomes a disadvantage in practise. Above all, critics are negative about the high system temperatures which vacuum tube systems can reach. If there is overheating, the glycol in the heat transfer fluid can be destroyed. If there is stagnation, for example if the pump breaks down while the collectors continue to generate heat that the storage can no longer take up, then the temperature of the system can rise to way over 250 C quite a test for some system components. Heatpipe tubes provide a certain amount of protection in such cases. Here, the solar fluid doesn t flow directly through the tubes. Instead, a carrier medium circulates in a special absorber, vaporises when the sun shines and gives up heat to the solar fluid via a heat exchanger. An integrated temperature limiter prevents overheating. This may appear to be true superficially, admits Solid consultant Wolf, but the question is: what happens in the system as a whole and how is the system joined together? After all, you ve got 20 to 30 tubes per collector connected one behind the other. It s all about the emptying process. How is the collector tank set up, for example? Is it above or below? If it is below, then the system can drain well and the systems will have few overheating problems if there is stagnation. Many suppliers still have the tank above, however, says Wolf critically. Then the system can usually only empty partially, and if there is stagnation, you have the risk that the anti-freeze will decompose. Manufacturers with inherently safe systems The vacuum tube collector systems supplied by Paradigma Deutschland GmbH, easily the most important German supplier in this sector, thus use water instead of a glycol mix. We realised that we couldn t make progress with the tubes if we didn t get away from using anti-freeze. Our systems have thus been using water for the last few years. Stagnation is not a problem for us anymore, says Rolf Meißner, head of large solar systems at Paradigma. During stagnation the system boils just as an anti-freeze system would, except that water is fully chemically stable. The water in the collectors evaporates into the expansion tank. The systems can thus withstand stagnation with ease, which Meißner says brings another positive effect with it. We dimension the system based on the winter or the transition periods, even though this means that the system can sometimes start to boil at around midday in the summer. The storage simply won t take up any more heat, and you can t use it in the summer months anyway. A large storage capable of taking up so much summer heat would result in a lot of heat loss in the winter. Additionally, water is a safe, unproblematic heat transfer medium. Meißner concludes: A craftsman who has worked with water generally won t want to deal with anti-freeze ever again. The Swiss company AMK-Solac Systems AG provides a different solution to the problem of stagnation. It runs its system in such a way that a controlled evaporation is initiated at somewhere between 120 and 150 C. For this, the solar fluid and the internal pressures in the solar heating system must be tuned to each other. The Head of AMK-Solac, Kurt Amrein, A heat transfer sheet, pressed closely against the inside of the tube, transfers the heat to a heat transfer fluid flowing inside a U-tube. Photos (2): AMK-Solac With tube collectors by the Swiss company AMK-Solac Systems AG, there is controlled vaporisation in the case of stagnation. 43

3 Solar thermal vacuum tube collectors According to Paradigma s experience, a craftsman who has worked with water generally won t want to deal with anti-freeze ever again. explains the principle as follows: When you limit the pressure in the system, this automatically limits the temperature. Through the use of flexible stagnation temperature settings, optimally dimensioned expansion tanks and specially developed solar stations with an integrated system for oxygen evacuation, his systems are able to run with a minimum of maintenance. The solar station, specially developed for the AMK-Solac solar systems, makes sure that the vaporisation doesn t occur in the expansion tank, but that the excess energy is removed within the heat exchanger in the storage tank. Improved glass to metal transition A weakness of vacuum tube collectors is often seen to be the metal to glass transition between the vacuum tube and the absorber plate. Because of the different expansion coefficients of the materials, they can work against each other and cause microporosity at the interface. There is thus a relatively high risk that the tubes will lose their vacuum after a certain amount of time, says Wolf. For previous products this was sometimes the case after three to five years. To counter this scepticism Narva Trade Solartechnik GmbH has developed a new design. With their vacuum tubes the glass is not put under any tension. The metal end of the tubes is made of a nickel-iron alloy which has the same expansion coefficient as the glass used. Crossstresses and vibration through wind loads and expansion also don t destroy this new glass to metal transition, as the design allows bending forces transferred from the hot water pipes to be taken up by the metal, leaving the glass practically unaffected, is how the German company from Brand-Erbisdorf describes its product in a brochure. The heatpipe cannot overheat because the special fluid in the evaporation tube is completely in the gaseous phase at 160 C and the thermosiphonic principle then ceases to function. Some critics of vacuum tubes are concerned about their safety. The reports of burst Schott pipes in the summer of 2007 seem to validate these concerns. But Rolf Meißner counters the critics with the stable design of the Sydney tubes. The principle for this type of tube is to have two glass tubes, one within the other, which are fused together at 44

4 one end. The air in the gap between the tubes is then evacuated. Unlike in other vacuum tubes the absorber is not flat but consists of a cylindrical, selectively coated glass tube. Closely pressed against it on the inside is a heat transfer sheet, via which the heat is transferred to a U-tube through which the heat transfer fluid flows. It is a system which is completely mechanically sound, stresses Meißner. He continues with further advantages of these vacuum tubes. As they are insulated by a vacuum, they have the lowest conceivable heat losses, and as there is no insulation material such as mineral wool present, there is nothing which can rot. There are no condensation problems, as there are with flat plate collectors. As they have a flat performance line, they deliver high temperatures all year round regardless of whether it is June or February. And then there is the minimalised construction. The whole collector consists just of glass, aluminium and stainless steel. All are materials which are super for recycling. Additionally, the weight is exceptionally low, so there are no problems with building statics. Meißner also sees advantages for vacuum tubes in terms of manufacturing costs. In the future they will become cheaper than flat plate collectors; the more sparing use of materials alone will achieve this. Today, however, things are still the other way around because the quantities are not yet that large and the high development costs and the cost of the machines are included in the current prices. This estimation may also be splendidly argued about, of course. The future will show whether vacuum tubes become dominant on the European collector market in a few years time with a share of around 90 %, as is the case today in China, or whether flat plate collectors can consolidate their leading role in Europe. Until then, the discussion on advantages and disadvantages may continue. Joachim Berner Further information: Düsseldorf University of Applied Sciences: Solid: Schweizer Solarinstitut SPF: Paradigma: AMK-Solac Systems AG: Narva Trade Solartechnik GmbH: Collector assembly: A vacuum tube collector is light as it consists only of glass, aluminium and stainless steel. 45

5 Solar thermal vacuum tube collectors Tubes hot or not? Statements! Our correspondent Bärbel Epp asked manufacturers and experts for their opinions about vacuum tube and flat plate collectors. Choose the most appropriate collector according to the project conditions Top collectors in comparison Eta At around 50 C the efficiency curve of the flat plate collector drops below the U-pipe vacuum tube collector. Therefore different application temperatures can require various collector types. Diagram: Wagner & Co T m -T a [ C] Calpak produces a full range of collectors from a flat plate collector with a black paint sandwich absorber up to a very efficient U-pipe vacuum tube collector with parabolic reflector. The reasons are obvious. After many years of experience Calpak was founded in 1976 and numerous national and international projects, we have learned that it is very important to select the appropriate collector according to the different project conditions, in order to increase efficiency and minimize the amortization period of a solar thermal investment. Most solar water applications in Greece are provided with simple flat plate collectors whereas vacuum tubes are very appropriate for space heating, especially under-floor heating. We use this technology in a lot of projects in Cyprus where the system has to supply hot water, space heating and pool heating. Summing up, we could say: whenever the application temperature is high or the demand in winter is high, we recommend vacuum tubes. The characteristic curves of the efficiency of different Evangelos Nikolopoulos, Business Development Manager of collector producer Calpak Solar Energy, Greece Photo: Calpak collector types can be seen in the diagram on the left. In the vertical axis, the instantaneous efficiency of the collectors is depicted whereas on the horizontal axis, it is the ratio of difference between the medium liquid temperature inside the collector (T m ) and the ambient temperature (T a ). Usually flat plate collectors with high instantaneous efficiency have bigger losses, that is why their curve drops down. In contrary, the curve of the vacuum tube collector starts with a lower instantaneous efficiency, but stays up higher, because of the minimized heat losses. So based on where we are and what the system needs, we choose the collector which will give us the greatest area between the difference of our highest ambient temperature and liquid temperature and the difference of our lowest ambient temperature and liquid temperature. Evangelos Nikolopoulos, Calpak Michele Speri, Export Manager of the vacuum tube collector manufacturer Kloben, Italy Photos (3): Bärbel Epp Increasing reliability also in latest applications Vacuum tubes this is what we are specialized in since That shows our strong belief in this technology. Our aim is to increase our market share in new solar thermal energy applications, like process heat and solar cooling. This is where vacuum tubes show their advantages, since with raising working temperatures their efficiency increases a lot compared to other kinds of collectors. Another advantage of this technology is the higher energy gain by low outside temperatures, since the vacuum has demonstrated to be the best insulation for thermal solar collectors. This is of high importance in process heat applications, where the energy demand is constant all the year round. We use only vacuum tubes of Sydney type, due to the higher reliability, constant efficiency in the long term and best cost/benefit ratio, if compared with other types of tubes. Kloben guarantees a high quality of the vacuum tubes used in its collectors, thanks to our own tube production in a joint venture company. This allows continuous research and development of new materials and several vacuum tube types, in relation to the requested usage. For 2010 we are planning the launch of a new generation vacuum solar collector, specially projected for process heat and solar cooling, with highest output and granting a higher working temperature range. The future of Kloben will be orientated towards specialization in the latest application fields for solar thermal energy. Michele Speri, Kloben 46

6 Jiao Qingtai, Vice President of the Chinese collector manufacturer Jiangsu Sunrain Solar Energy Co., Ltd, China China is a very good market for flat plate collectors Sunrain is among the five biggest vacuum tube collector manufacturers in China with an annual amount of square metres of more than 2 million. Even so we decided to start producing flat plate collectors last year, due to the dominance of this technology on the global market except China. Both technologies have advantages and disadvantages. One Vacuum tubes cannot be an integral part of the façade Werner Weiß, Director of the Austrian research institute AEE INTEC Vacuum tube collectors will remain a niche segment of 10 to 20 % in Europe on the long run, due to one very simple reason. They cannot be an integral part of the building envelope as simply as a flat plate collector could be. There are several good arguments, why collectors must be an integral part of the roof or the façade in the long run. If you want to reach an appreciable share of solar thermal in the heating demand in Europe, you need to install several square metres per inhabitant. In the residential sector this can only be guaranteed, if the building shell is actively used for heating. According to our latest potential study, a collector area of 3 m 2 /inhabitant in 2030 is necessary to cover 15 % of the end energy heating demand in Europe. It costs nearly twice as much to mount the collector field on a flat roof than to make it part of the façade, especially, if you have high wind loads. For space heating the collector is best positioned in the façade, because the sun is low in winter and therefore the output of the collector is maximized during the cold season. In the commercial sector, surfaces are not so crucial. Therefore, as well as due to the higher temperatures necessary, vacuum tubes could play a more important role in this sector today as well as in the future. Werner Weiß, AEE INTEC advantage of vacuum tubes is that efficiency is high in winter or in cold areas. Vacuum tubes can be used widely not only for hot water but also for space heating and solar cooling. They have a good freezing resistance and they are a hi-tech product. But there are also arguments against vacuum tubes: glass tubes are not tempered, there may be a problem of breakage by sudden attacks. Furthermore, in some cases installation is more complex than that of flat plate collectors. Flat plate collectors, however, are easy to install, easy to integrate into the building and safer because of tempered glass as the cover. On the other hand, efficiency of flat plate collectors is low in winter or a cold area and they have a low freezing resistance. In China, flat plate collectors are welcome by the government officers and the architectures and engineers. Therefore I think that China is a very good market for flat plate collectors and the market will grow in the future. Jiao Qingtai, Jiangsu Sunrain Solar Energy 47

7 Solar thermal vacuum tube collectors Marek Szymanski, Technical Department Manager of the Polish collector manufacturer Watt Ltd Photo: Watt Security measures make vacuum tube collectors more expensive Vacuum tube collectors are gaining importance on the solar thermal market. We were one of the first Polish system suppliers to integrate vacuum tubes into our portfolio, actually as early as We purchase double-glass Sydney tubes from China and combine them with a U-pipe absorber at our factory in Silesia, Poland. We encountered several problems with the vacuum tube collectors in the beginning mostly because installers did not handle them properly. In one case, the plumber left the delivered collector modules two weeks in bright sunshine. The stagnation temperature reached 800 C and the copper pipes melted and became useless. Other installers burned themselves while working on the roof, because they were not aware of the fact that collectors in stagnation can reach extremely high temperatures. We therefore had to consider security measures. So, today we de liver all our collectors under a polystyrene form that covers the entire tube area. Do not unwrap until joined to the heating system! is the important rule. We furthermore implemented some additional measures to guarantee a secure and correct operation of the vacuum tube collectors. Unfortunately, these measures drive up the price as well: To prevent steam at high temperatures from reaching the solar station, the pumps and the valves in the cellar, we recommend an expansion vessel that is six times bigger than with a flat collector. An additional check after filling ensures that the solar circuit is free of air bubbles, because U-pipe collectors are much more difficult to vent. Furthermore, air bubbles can stop the flow in the U-pipe and decrease efficiency. An automatic pump increases the pressure in the circuit to 5 bar, so that leakages will be spotted and air bubbles removed. Sydney tube collectors are much more difficult to vent due to the construction of the collector. Additionally, air bubbles can have a negative effect on the U-tube, which will not work when being left in that condition. Nothing will be able to flow through and, hence, there will be no heat exchange: the result will be an overheating of the tube itself. Another emerging difficulty besides the aforementioned can be hot spots that will occur if vacuum collectors have not been correctly vented. The temperature sensor may show error readings in some particular cases of operation. Last but not least, we use a glycol that is specifically adapted to vacuum tube collectors. If one adheres to these security measures, vacuum tube systems will perform well. Today, more than half of our collector area is sold with this kind of technology. Marek Szymanski, Watt Your manufacturer for high end VACUUM COLLECTORS AkoTec Produktionsgesellschaft mbh Grundmühlenweg Angermünde Germany info@akotec.eu Advantages of our collector: - maximum power 3014 Wp - plug in connection tube - manifold - tubes 360 rotatable - tube distance always the same - collector and tubes made in Germany - Narva vacuum tubes with double coated absorber - short energy amortisation "In an environmentally friendly way, highly competitive and certainly." 48

8 Specialist and German market leader in the production of tube collectors Industrially manufactured top quality products High quality, high performance Distribution exclusively to OEM partners Certificates and authorisations: EN 12975, RAL UZ 73, Solar Keymark / ISO Ritter Solar GmbH & Co. KG Kuchenäcker Dettenhausen Germany Tel.: +49 (0) Fax: +49 (0) info@rittersolar.de

9 Solar thermal vacuum tube collectors Volker Kallwellis, Head of Development at Wagner & Co Solartechnik GmbH, Cölbe, Germany Photo: Wagner & Co There is an unbeatable cost-effectiveness Until two years ago we also had vacuum tube collectors in our range. Because of the high performance of our flat plate collectors, the demand we had for vacuum tube collectors declined steadily. Plus, the use of vacuum tube technology requires a significantly higher technical effort and has regularly been linked to various problems in the past. From 2007 onwards we took the product out of our range completely and are now concentrating solely on flat plate collectors. We can list a number of reasons for this: Flat plate collectors have a better price-to-performance ratio for hot water systems and for heating support. The specific yield increase for vacuum tube collectors per square metre can be generated more cheaply by using a larger area of flat plate collectors. Flat plate collectors are robust, long-lasting, low in maintenance and are fairly forgiving of certain installation errors, whereas tube collectors have a series of technical disadvantages during installation and operation that can also lead to larger problems e.g. very high stagnation temperatures. As a result, the heat carrier fluid ages faster and must be checked and replaced more often. If there are often periods of stagnation and maintenance is not carried out, then in some tube types mainly those with poor drainage characteristics and small tube cross-sections the heat transfer fluid degrades and leaves tar-like blockages in the absorber tubes. The collector is then unusable. Purely water-based systems do not have this problem. Here there is the danger, however, that temperatures below freezing can destroy the whole collector field if active safety systems, such as the heating of the collectors in winter, fail through a blackout, faulty parts or incorrect use. The sun shining on the collectors before they go into operation also tends not to be a problem for flat plate collectors, even over a period of weeks, whereas tube collectors quickly suffer from considerable corrosion and damage to the inner copper tubing. Unlike flat plate collectors, tube collectors do not use pre-stressed safety glass. This makes their use in façades and on sloping roofs questionable. If glass tubes break, there is a considerable risk of injury from falling glass shards and larger pieces: in the worst case scenario even spears of glass. And it is known that glass breakage can occur even without further mechanical loading through minimal previous damage or construction faults. Hailstorms have also caused considerable damage to glass vacuum tubes again and again in the past, something which just doesn t happen to this extent with flat plate collectors. Tube collectors with CPC reflectors often lose performance over time. The cause: a dimming of the reflector through dirt and corrosion, as well as moss growth on the rear of the working tubes through condensation and droplet formation. It is undisputed that vacuum tube technology can play its advantage of having lower heat losses at temperatures of around 70 C and above. However, the main yields for service water heating and heating support are obtained at considerably lower temperatures, so that a better cost-effectiveness can generally be achieved for these uses with flat plate collectors. Serious economic comparisons should take into account all costs and yields, as well as the amount of solar coverage which can be achieved, though. Volker Kallwellis, Wagner & Co Solartechnik 50

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