Solar thermal Medium temperature collectors Concentrated power Parabolic troughs or Fresnel? The market for commercial medium temperature collectors for solar process heat is slowly gearing up. Two-thirds of industrial final energy demand is heat; a third of that does not have to exceed 200 C, says Christian Zahler, Managing Director of Mirroxx GmbH in Freiburg, Germany. Mirroxx is one of a handful of companies focusing on the sunny side of the process heat market. The International Energy Agency estimates that solar process heat potential in Europe alone amounts to more than 100 GW. Zahler estimates the European solar process heat market is worth, at least 100 billion Euros. Applications range from direct heat use in the food industry, which can even use temperatures below 100 C, to heating for absorption refrigeration systems (see table 1 on page 53). While double glazed flat plate collectors can be used for temperatures of up to 120 C and specially designed vacuum tube collectors are suitable for temperatures of up to 150 C, the realm of parabolic troughs begins beyond the 150 C range. The long, shiny silver troughs with their parabolic cross-section have become a fixture in power plants. Andasol 1, the world s largest solar power plant, was commissioned on July 1 in southern Spain using precisely this technology to generate electricity. Despite a degree of technological maturity in parabolic troughs, manufacturing processes for the technology have continued to develop. NEP Solar Pty Ltd of Australia won an innovation prize for solar thermal Mirroxx Fresnel collector with a 352 m² aperture area located in Seville, Spain. The process heat is used to air condition a building via an absorption refrigeration system. Photos(2): Mirroxx GmbH 52
technology at Intersolar 2009 for its Polytrough 1200, which at first glance looks like a traditional parabolic trough collector. What impressed the judges was the system s manufacturing process. The aluminium for the primary reflector is applied to fibreglass in a single step. The Australians are proud of their lean construction method, even though the process is not a unique selling point. The disassembled collectors can be transported in less space. Another way to concentrate solar irradiation on an absorber tube is based on a principle discovered 200 years ago by French physicist Augustin Jean Fresnel. Instead of using parabolic reflectors, numerous flat, rotating mirrored strips reflect sunlight onto the absorber tube either directly or via a curved secondary mirror. In 1964, Giovanni Francia and Marcel Perrot generated 450 C steam with just such an arrangement. But it took several decades of cheap oil and gas before researchers in Freiburg, Germany would once again take up intensive research on this process. Mirroxx GmbH is a subsidiary of Fraunhofer ISE spin-off PSE AG, which has been operating demonstration plants in Europe and North Africa since 2005. The largest such system, with an aperture surface area of 352 m² is located in the Spanish city of Seville; the newest such plant (2008) is located in the northern Tunisian town of Grombalia. Mirroxx has generated simulation data for its Fresnel collector for three locations (Fig. 1). Not surprisingly, direct solar irradiation in Hurguada, Egypt (usually spelled Hurghada ) is nearly three times as high as in Freiburg, Germany. However, the fact that heat yield is four times as high can only be explained by the much higher annual efficiencies in Egypt (40 % in Egypt, compared to less than 30 % in Freiburg). The higher efficiencies in Egypt are due to a lower proportion of low-angle incident light. Mirroxx provided measurement data taken on a July day in 2008 for a Fresnel collector located in Zingonia, Bergamo, in northern Italy (Fig. 3). The system, with an aperture surface area of 132 m² supplies 180 C heat to an absorption refrigeration unit in an office building. The system s reflectors were designed to provide a constant collector outlet temperature if necessary, the mirrors are actually turned away from the sun. Thus, on the day in which the measurements were taken, the highest output (nearly 40 kw) had already been achieved in the plant s warm-up phase before 10 a.m., even though the plant has a theoretical peak of up to 66 kw. This enabled the system to DNI Gross heat yield Annual efficiency Fig. 1: Direct normal irradiation (DNI), annual efficiency, and gross heat yield in simulations with the Mirroxx Fresnel collector at three locations: Freiburg im Breisgau/Germany, Seville/Spain, and Hurguada/Egypt. Graphic: Mirroxx Fig. 2: Efficiency curves for the NEP PolyTrough 1200 parabolic trough collector based on direct irradiation and temperature difference. Graphic: NEP Table 1: Markets and uses for medium temperature process heat Market Use Temperature foodstuffs washing, cooking, pasturization, sterilization 50 to 250 C textiles dyeing 50 to 170 C chemicals distillation, pressing, cooking, flocculation 60 to 300 C wood drying, pressing 50 to 180 C metal Galvanization 50 to 110 C hotels air conditioning (absorption refrigeration) 160 to 180 C supermarkets cooling (absorption refrigeration) 160 to 180 C buildings air conditioning (absorption refrigeration) 160 to 180 C Source: Mirroxx Your manufacturer for high end VACUUM COLLECTORS AkoTec Produktionsgesellschaft mbh Grundmühlenweg 3 16278 Angermünde Germany www.akotec.eu 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." 53
Solar thermal Medium Temperature Collectors capacity temperature Fig. 3: Measurement data from a Fresnel collector in Zingonia, Italy, taken on July 17 th, 2008. Between 11 a.m. and 5 p.m. the collector reached its target temperature of 180 C. Red: collector outlet temperature; blue: collector inlet temperature; orange: direct irradiation; black: ambient temperature; green: collector output. Graphic: Mirroxx maintain a constant temperature of 180 C between 10:30 a.m. and 5 p.m. These days, Mirroxx is busy in Freiburg commissioning a further demonstration system designed to generate steam at a pressure of 40 bar in a demineralized water filled receiver tube with a 132 m² Fresnel collector. The operators will use the system for extensive testing and are focusing on a future client base of paper, textile, and food industries in areas with high direct solar irradiation, such as southern Europe and North Africa. It looks as if the Fresnel collector is poised to become a serious competitor to parabolic troughs. Table 2 lists key basic data for the Mirroxx Fresnel collector, comparing it to two parabolic trough collectors (from NEP and Solitem). In a comparison of peak output adjusted for surface area, Solitem s parabolic trough collector has a healthy lead over both competitors, with roughly a third better performance. Solitem s stated annual efficiency figure of at least 60 % is also impressively high. However, the company s competitive advantage is moderated somewhat when one considers the system s installed space requirement. Parabolic trough collectors have to be set up with a very large space in between to prevent them from shading one another. Fresnel collectors, on the other hand, can be installed right next to one another. Besides an annual efficiency figure of up to 45 %, NEP gives expected efficiency curves based on the level of direct irradiation and the temperature gradient (Fig. 2). When comparing annual efficiencies, however, we have to bear in mind that a common standard calculation does not exist. In terms of operating temperature variance, however, the Fresnel collector has a leg up on the competition. Of course, the 400 C temperature can only be achieved with the thermal oil variant, not with the normal water version of the system. Obviously a price comparison between the various systems would also be interesting, but Solitem is holding its cards close when it comes to cost. Mirroxx cites a figure of approximately 500 /m² of aperture area ex works, adding that systems in excess of 1 MW efficiency are less expensive. That puts the cost of a four-meter module at some 11,000. NEP cites a figure of less than 400 /m² of aperture area. NEP provides a cost comparison based on the price of a barrel of oil. According to the comparison, Table 2: Key data for commercially available medium temperature process heat collectors Manufacturer Mirroxx GmbH NEP Solar Pty Ltd Solitem GmbH Product name Mirroxx Fresnel process heat collector PolyTrough 1200 PTC 1800 parabolic trough collector Principle of operation Fresnel collector parabolic trough collector parabolic trough collector Length modular in 4 m segments 24 m modular in 5.09 m segments Width 7.50 m 1.21 m 1.80 m Height 4.00 m 1.58 m 1.50 m Aperture width (= total width of the primary reflectors) 5.50 m 1.20 m 1.80 m Net aperture area 22 m² per module 28.8 m² 9 m² per module Peak output at 1,000 W/m² of direct irradiation approx. 500 W/m² of aperture area approx. 550 W/m² of aperture area Primary reflector thermally tempered, silver-backed clear glass mirrors aluminium/fibreglass aluminium Secondary mirrors highly reflective aluminium mirrors n/a n/a Receiver Schott PTR 70 vacuum absorber tube proprietary design, no vacuum n/a Tracking single-axis, individually tracking rows of mirrors; a 7 W electric motor provides tracking for a maximum of 8 primary mirrors with a length single-axis single-axis of 32 m Optical efficiency based on direct radiation 62 % > 68 % 68 % Typical annual efficiency up to 40 % depending on the location up to 45 % depending on the location 60 % Operating temperature up to 400 C up to 220 C up to 250 C Special technical features remote control and monitoring via LAN or internet remote control via internet n/a 680 W/m² of aperture area Source: manufacturers information, DLR 54
Stainless steel receivers The heart of every parabolic trough collector is its receiver, which collects concentrated solar irradiation and converts it into heat. Switzerland s Energie Solaire S.A. has long years of experience in coating stainless steel tubes with a selective black chrome layer. We started doing this more than 15 years ago, reports Head of Sales Bernard Thissen. Now the company is putting this experience to use to produce receivers. After all, in recent months demand for these products has risen sharply, Thissen says. The company is able to coat tubes in various lengths with diameters ranging from 25 mm to more than 100 mm, tailored to customer requirements. Energie Solaire also uses the selective coating for its cushion absorbers. This coating has long since proven itself in extreme conditions in our ES Solar Roof system and is very well suited to coating stainless steel tubes, Thissen says. The manufacturer says that its coating is suitable for sustained temperature loads of up to 300 C. Energie Solaire receivers are already being used in NEP Solar s Polytrough. Further information: Energie Solaire S.A.: www.energie-solaire.com The NEP PolyTrough 1200 collector solar process heat from an NEP parabolic trough collector is equivalent to an oil price of US$ 40 to 120 per barrel, depending on the location. American company HelioDynamics Ltd also offers Fresnel collectors. Spain s Abengoa Solar, S.A. and Sopogy, Inc. of the US are among the companies offering commercial parabolic trough collectors for process heat. None of these companies provided Sun & Wind Energy with technical data for this overview. Photo: NEP In sum, the great variance in construction, materials, and data even among the parabolic trough systems confirms that the market is still very immature, both in terms of age and technology. It is a market that is sure to develop in innovation, refinements, and more streamlined products. Alexander Morhart Further information: Mirroxx GmbH: www.mirroxx.com NEP Solar Pty Ltd: www.nep-solar.com Solitem GmbH: www.solitem.de FLABEG SOLAR MIRRORS IMPOSSIBLE TO SURPASS FLABEG redefines mirror bending precision achieving FDx <= 8mm, measured by DLR www.flabeg.com FLABEG Holding GmbH Waldaustr. 13 D-90441 Nürnberg Tel. +49 (0) 911 96 456-0 info@flabeg.com
Solar thermal Medium Temperature Collectors What we lack is concrete experience with subsidized projects. Sun & Wind Energy spoke with Matthias Rommel about using parabolic troughs and Fresnel collectors to generate process heat. Rommel, a physicist and former head of the Thermal Collectors and Applications Group at Germany s Fraunhofer ISE in Freiburg, began his tenure as Director of the Institute of Solar Technology SPF at Switzerland s Rapperswil HSR Technical University in July. Under the leadership of Professor Matthias Rommel, the Fraunhofer ISE developed a testing facility for process heat collectors. Photo: Fraunhofer ISE S&WE: What are the strengths and weaknesses of parabolic troughs and Fresnel collectors? Matthias Rommel: Both technologies are still in very early stages when it comes to industrial process heat applications; and whether solar process heat is usable does not necessarily depend on whether a parabolic trough or a Fresnel collector is used. The differences are also not so great that only one or the other technology will come out on top. One of the strengths of parabolic troughs is that they ve already been thoroughly researched and used; we can draw on the principle results of their use in power generation and transfer that knowledge to other applications. However, technological development has shown that other materials and collector designs are indeed needed for the process heat in the 100 to 250 C range. The average aperture width for medium-temperature parabolic troughs is between 1 and 2 m; in power plant collectors it is 6 m. Power plant collectors require a completely different kind of mechanical stability mounting bases for process heat collectors are much more costeffective. For instance, you can use aluminium reflectors for process heat applications, but power plant collectors always use coated glass mirrors which are heavier and demand a more complicated substructure. S&WE: And what about the other way round using process heat collectors for power plants? Rommel: Absolutely, in Australia there are already projects in which Fresnel collectors are used for power plant applications. I see a possible advantage of Fresnel collectors in the fact that process heat collectors are designed to be set up on roof tops. Because of the wind loads that these systems are subjected to, I can imagine Fresnel collectors being better suited to roof installations because the reflector elements are parallel to the surface of the roof and thus are not subject to wind loads. Parabolic troughs are exposed to significantly greater wind forces, which then have to be absorbed by the roof. And another thing if you compare the efficiencies based on installed surface area, Fresnel collectors come out a bit ahead of parabolic troughs, since you don t have to worry about the troughs shading one another. S&WE: To what extent can the traditional collector equation be applied to these collectors? Rommel: It can be applied, however, a few other factors come into play in testing and characterization. One factor is that when you characterize concentrating collectors, the efficiency has to be based on direct irradiation. Another factor is that certain effects have to be calculated in the trough-end losses, for instance. After all, the collector has a finite length. The shorter the trough, the more apparent it is that sun reflected back toward the axis of the trough does not hit the receiver because the collector ends there. On top of that, the small collectors tested in the laboratory are kept clean. S&WE: Aren t large collectors cleaned? Rommel: If you ve got a whole field of collectors you can t always keep them clean, so you have to factor in the effect of dirt on the collectors. Another issue in testing collectors is that the measurements are taken at a constant wind speed of 3 m/s. To get the constant wind speed, you use fans to generate artificial wind when you re testing small collectors; but this would not be practical on the scale of a collector field. We still don t have enough experience to know the extent of the influence of wind on collector efficiency. A further issue is that for Fresnel collectors the incidence angle modifier (IAM) is dependent on the installation location (longitude and latitude) and the orientation of the collectors. It s very important when performing measurements on large collector fields to take into account run-time effects of the fluids as they pass through the collector field. To be able to account for those effects, we have to further develop dynamic testing processes. The basic approach to determining efficiency, however, is similar to the approach that collector standard takes. S&WE: ISE has a special test bed for process heat collectors. Why is ISE s measurement method special? Rommel: Yes, we can perform measurements on small collectors with some 6 m² of area at temperatures as high as 200 C. All of the other research organizations can only take measurements up to 120 C. At higher temperatures, they can only extrapolate efficiencies. Knowing those efficiencies is especially important for collector development. 56
S&WE: Are there other approaches, or is research being done on other types of collectors? Rommel: Yes, fixed mirror solar collectors (FMSC), where the reflector is fixed and the receiver is on a tracker. The advantage of that system is that the reflector surface can be designed as a fixed roof surface. There is an FMSC research collector on Mallorca. S&WE: Which projects for medium-temperature process heat are currently underway? Rommel: The EU issued a call for tenders in March and had a hard time getting bids for large, hightemperature plants. The systems built so far use industrial process heat for washing and other processes that require temperatures of up to 80 C. Solar cooling is what drives many projects. The advantage of solar cooling is that with high operating temperatures you can even use a two-stage cooling system with a high Carnot efficiency to generate sub-zero temperatures for industrial processes. S&WE: Which applications are the most promising? Rommel: Applications in which conventional heated steam in existing processes, generated by gas or oil firing, can be directly replaced in breweries for instance. Typically gas or oil-fired systems generate 170 degree saturated steam. To easily replace this energy, you need a collector that can deliver steam at that temperature. You can actually achieve these temperatures using concentrating collectors in highinsolation countries that have a large proportion of direct irradiation. The other way to do this would be to change the systems in the brewery so that they could make do with lower temperatures. But when you talk to the breweries they say, We don t want to change our systems; we just want to use new energy technology. Generally, when dealing with industrial process heat, it is important to learn as much as possible about the processes so that you can supply solar heat as simply as possible. S&WE: So the solar industry has to employ brewers? Rommel: No. Solar technology has to work handin-hand with process technology planners. And what we lack is concrete experience with subsidized projects. The research facilities have the development instruments and the expertise. But the only way to convert this intellectual capital into something useful is through subsidized projects in which plants are built and operational experience is evaluated. Testing the durability of the individual components is very important for instance, looking at the effects of hail on reflector construction and materials. We also need to do a lot of development on fluids finding out how to prevent fluid break-down at high temperatures (glycols/ mineral oils) for instance, or how to prevent water from freezing. The interview was conducted by Alexander Morhart.