EUROTROUGH A NEW PARABOLIC TROUGH COLLECTOR WITH ADVANCED LIGHT WEIGHT STRUCTURE

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1 for submission to: Solar Thermal 2000 International Conference, 8 10 March 2000, Sydney, Australia. EUROTROUGH A NEW PARABOLIC TROUGH COLLECTOR WITH ADVANCED LIGHT WEIGHT STRUCTURE Eckhard Lüpfert, Michael Geyer DLR, Plataforma Solar, Apartado 39, E Tabernas (Almería), Spain, Fax: (+34) , ECKHARD.LUEPFERT@PSA.ES Wolfgang Schiel Schlaich Bergermann und Partner, Hohenzollernstr. 1, D Stuttgart, Germany Fax: (+49) , W.SCHIEL@SBP.DE Eduardo Zarza CIEMAT-PSA, Apartado 22, E Tabernas (Almería), Spain, Fax: (+34) , EDUARDO.ZARZA@PSA.ES Rafael Osuna González-Aguilar INABENSA, División Taller, c/manuel Velasco Pando 7, E Sevilla, Spain, Fax: (+34) , ROSUNA@INABENSA.ABENGOA.ES Paul Nava Pilkington Solar International, Mühlengasse 7, D Köln, Germany, Fax: (+49) , NAVA@PILKSOLAR.DE Abstract A parabolic trough collector is under development for various applications in the C temperature range in solar fields up to the hundreds Megawatts range. The design of a lightweight support structure of the collector included concept studies, wind channel measurements, FEM analyses and resulted in a structure with a central framework element. This torque box design will have less deformation of the collector structure than the other designs considered. Therefore it would be possible in future to connect more collector elements on one drive which would decrease the total number of drives and interconnecting pipes, thus reducing the thermal losses. In terms of the degree of material usage further weight reduction would be possible. The presented design has a significant potential for cost reduction, the most important goal of the EuroTrough project. The prototype set-up and testing at PSA is under preparation. Only minor license charges have been concluded for license takers by the consortium to enable distribution and benefit of the technology. 1. INTRODUCTION Parabolic trough collectors have been the key element in the commercial application of concentrating solar thermal power plants in California. Although other concentrators promise higher concentration factors and higher system efficiencies the parabolic trough will continue paving the way for concentrating solar power. Considering this importance a European consortium is developing the next generation of a parabolic trough collector basing on the long experience of operation of Luz collectors LS-2 and LS-3 in California. The work targets collector development for a wide range of applications in the Ctemperature range in solar fields up to the hundreds Megawatts range. The new design shall incorporate newest features in lightweight construction, drive technology, control technology and concentrator technology with collector weights below 30kg/m², view the aspects of a mass manufacturing, transport and assembly concept, that allows the economic implementation of parabolic trough collectors for electricity generation, desalination and process heat applications achieve the largest automation of operation, enable for the minimization of O&M requirements, reduce solar collector costs below 200 Euro/m², Applications of the parabolic trough collector are in the following fields of technology:

2 Solar thermal electricity generation in co-generation plants Solar thermal process heat applications in a wide range of process steam Solar thermal sea-water desalination in MED processes 2. STATE OF THE ART The successful parabolic trough design in the 80s has been implemented in the Californian SEGS plants by LUZ, and the collectors still operate for grid-connected electricity production. The operating companies have worked thoroughly on improvement of operation and maintenance (Cohen et al., 1999). Features of the different collector types in terms of performance and durability became obvious. The main characteristics of these collectors however establish the minimum requirements for a new generation of parabolic trough collectors. Key elements of the existing technology are: support structure: steel frame-work structure with central torque tube or double V-trusses (figure 1). drive: gear drive, hydraulic drive tracking control: clock controlled, sunsensor controlled reflector panels: parabolic mirrors, from glass or metal sheets absorber tubes (heat collecting element = HCE): with evacuated glass envelope fluid: mineral oil, synthetic oil, water/steam The components depend in part on the application, especially on the temperature range. 3. EUROTROUGH CONCEPT Figure 1: LS-3 collector structure mounted on a jig at PSA The concept of the EuroTrough collector is basing on the boundary conditions mentioned above. Key characteristics are given in table 1. Significant effort was put on a sophisticated design of the support structure. Apart from survival of the structure, the stiffness of the collector is important to keep up collector performance with wind-loads during operation. This has been examined from two aspects: Bending and torsion forces have been determined in wind channel experiments. Horizontal forces and pitching moments have been evaluated for different wind speed and direction, different collector positions in the field and various elevations of the collector. Secondly the results were included in FEM calculations to determine collector deformation and estimate losses (spillage of radiation due to wind).

3 Table 1: EuroTrough characteristics layout parabolic trough collector support structure steel frame work, pre-galvanized, three variants light weight, low torsion collector length 12 m per element; 100 m (125 / 150 m) collector length drive hydraulic drive max. wind speed operation: 14 m/s, stow: 31 (40) m/s tracking control clock + sun sensor, <2 mrad parabola y = x 2 /4f with f = 1.71 m aperture width 5.8 m reflector 4 glass facets absorber tube UVAC or other, application dependent fluid oil, steam, application dependent cost < 200 Euro/m 2 Three different concepts of collector structures have been analyzed: the existing LS-3 design, a torque tube design and a torque box design, with the intention to find out about weight and possible deformation of the structure (figure 2). Different load cases have been analyzed. Existing Regular Version Torque Tube Design A Torque Box Design A Figure 2: FEM results from the bending (top) and torsion (bottom) analysis due to wind loads for three structural concepts. Deformation is up-scaled with factor 60 for visibility.

4 spillage in % LS-3 LS-3 average twist of SCE on wind ET4 average twist of SCE on wind ET6 average twist of SCE on wind spillage losses due to twisting 5 m/s wind speed (m/s) 10 ET Twist in mrad Figure 3: Comparison of optical performance due to wind with example for 5 m/s. The graph shows that the stiffness of the EuroTrough collector (ET4, 100m) results in reduced optical losses as compared to LS-3 collectors. This is still valid for the extended ET6-collector of 150m up to 6 m/s wind speed. 4. DESIGN The actual EuroTrough collector support structure design is shown in figure 4. It is composed of a rectangular torque box with mirror support arms. The rotational axis is in the center of gravity, close to the torque box. The design comprises new mirror supports that make use of the glass facets as static element, but at the same time reduce the forces onto the glass sheets by a factor of three. This promises less glass breakage with highest wind speeds. Figure 4: Sketch of the EuroTrough collector torque-box design: Central element is a steel construction which absorbs torsion and bending forces. The reflector panels are supported with attached arms, using a new type and changed location of fixing pads to reduce breakage. The drawings have been finished and the mounting of the first prototype collector at PSA will be finished in summer. The collector will be set-up in east-west direction and with cautious

5 temperature sensor installation for improved testing capabilities. The test program for the prototype mainly includes thermal performance tests with synthetic oil up to 390 C. Further tests aim at the mechanical evaluation of the collector. As wind influence is highly transient and difficult to measure with the thermal output, stress and acceleration sensors as well as angular encoders on several pylons are foreseen. absorber tube reflector panels drive pilon local controller Figure 5: The EuroTrough collector prototype to be tested at PSA will have 4 collector elements to one side of the drive only, with the extension option to 6 collector elements. Following improvements will be achieved with the new design: weight reduction drive cost reduction (including cabling and control) reduced glass breakage higher thermal performance by: - Reduced distortion of surface under dead and wind load - Reduced global torsion and bending under dead and wind load - Reduced tracking error due to stiffer SCE Reduced assembly cost Reduced variety of parts Site preparation cost reduction (tolerable inclination of ground) 5. OUTLOOK Worldwide interest in solar thermal technology as depicted in figure 6 is the driving force for the ongoing work on the parabolic trough collector design. The design is available for interested license takers, only minor fees are charged by the consortium. The main challenges for the EuroTrough technology are electricity generation, and process heat in a wide temperature range, including desalination. The prototype testing and analysis will proof the theoretical results presented here. Like in other scientific work on trough collectors, e.g. direct steam generation, the project aims at further cost reduction and market penetration for near term future. Acknowledgements The presented work is being funded by the European Commission (JOR3 CT ) and the EuroTrough consortium partners Instalaciónes Abengoa, S.A. (INABENSA), Schlaich Bergermann und Partner (SBP), Fichtner Solar GmbH, Pilkington Solar International, Deutsches Zentrum für Luft- und Raumfahrt e.v. (DLR), Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), and Centre for Renewable Energy Sources (CRES).

6 KJC BECHTEL BOEING SMA World Bank DukeSolar Gamesa ABENGOA Ghersa KfW FICHTNER ONE EEA/NREA Solel NEPCO AGO Figure 6: Projects and key players in actual solar thermal project developments. REFERENCES Cohen, G., Kearney, D., Price, H. (1999) Performance history and future costs of parabolic trough solar electric systems, 9 th Int. Symposium on Solar Thermal Concentrating Technologies, Odeillo, France, June 1998, J. Phys. IV France 9 PR3 pp De Laquil, P., Geyer, M., Kearney, D., Diver, R. (1992): Advanced Solar Thermal Electric Technology. In: Renewable Energy - Sources for Fuels and Electricity. Island Press Washington, D.C., 1992, ISBN Geyer, M., Kistner, R., Heller, P., Kolb, G., and Cordeiro, P. (1999) Identification of international solar thermal project opportunities. Reports from the IEA SolarPaces START Mission, 9 th Int. Symposium on Solar Thermal Concentrating Technologies, Odeillo, France, June 1998, J. Phys. IV France 9 Pr3, (1999), Geyer, M., Kistner, R., Kolb, G. (1998) Case Studies of Solar Thermal Power Projects from the IEA SolarPACES Start Missions. Selected Proceedings of the Initial Implementation Conference ISES Utility Initiative for Africa, Johannesburg, März 1998, p. 49 ff. Hennecke, K., Zarza, E. (1999) Direct Solar Steam Generation in Parabolic troughs (DISS) - Update on project Status and Future Planning, 9 th International Symposium on Solar Thermal Concentrating Technologies, Odeillo (France), J. Phys. IV France 9 Pr3, (1999), Pilkington Solar (1996): Status Report on Solar Thermal Power Plants - Experience, Prospects and Recommendations to Overcome Market Barriers of Parabolic Trough Collector Power Plant Technology. Cologne ISBN Schillig, F., Geyer, M., Kistner, R., Aringhoff, R., Nava, P. and Brakmann, G., (1998) The THESEUS project 50 MW solar thermal power for Crete PowerGen ( ) Milano, Italy.