One of the achievements of LUZ International was the development of three reliable, durable PTC designs, called the LS1, LS2 and LS3, which were successfully implemented and operated in the SEGS plants. Although the LS1 installed by LUZ at the SEGS-I plant in 1984 had a unit length of
50.2 m and a parabola width of 2.5 m, which was similar in size to other designs developed during the early 1980s, it soon became evident that bigger PTCs would have to be developed for larger CSP plants. This was the reason why LUZ developed the LS2 and LS3 designs, with aperture areas of 235 and 545 m2 per collector, respectively. After the demise of LUZ, one of the barriers to the installation of large solar fields with parabolic troughs was lack of a suitable PTC. In view of this, a European consortium composed of industry, engineering firms and R&D centers was formed in 1998 to develop a new PTC suitable for large CSP plants. The result was the Eurotrough-100 (ET-100) and EuroTrough-150 (ET-150) designs, which were then improved, leading to their successor, the SKAL-ET, the collector installed at the ANDASOL-I plant in Spain in 2007. Figure 7.5 shows the
7.5 Steel structure of the Eurotrough-100 collector design.
Table 7.3 Parameters of the Eurotrough-150 collector design
steel structure of the ET-100 design, and Table 7.3 gives the parameters of the ET-150 collector design.
Highly precise assembly of the steel structural profiles is required to achieve perfect parabolic shape of the concentrator and overall structural rigidity, while at the same time keeping assembly cost low. Thus, although not all PTC designs require assembly jigs, most of the modern designs do require them to meet the design tolerances, which are usually around ±1 mm. Figure 7.6 shows the final check of an assembly jig for the EuroTrough collector design.
The collector design shown in Fig. 7.5 has a central space frame, called torque box, which provides good rigidity and prevents torsion, which is very important to ensure good PTC optical and geometrical performance under moderate wind speeds (below 35 km/h). Figure 7.6 also shows how a Eurotrough parabolic-trough module looks when all its components have been mounted in the assembly jig. In large parabolic-trough solar fields, two or three assembly lines are used in parallel to shorten the construction time. Another PTC design approach is the replacement of the torque box by a central steel tube (called the torque tube). However, assembly of the steel mirror support frames on this central tube must also be highly accurate. Figure 7.7 (bottom) shows a detail of the collector developed by the Spanish company, Albiasa Solar (www. albiasasolar. com), using the torque tube concept. The collectors developed by the Spanish companies, SENER (www. sener. es) and URSSA (www. urssa. es), in 2006-2010 also have a torque tube instead of a torque box.
The main advantage of PTC torque box designs is their usually better performance and rigidity under wind loads, while their main disadvantage is their higher assembly cost due to the number of steel profiles which require high-precision assembly. Torque tube designs, on the other hand, are usually somewhat cheaper, but are subject to more deformation from
7.6 Check out of the assembly jig for EuroTrough collectors (top) and a parabolic-trough module assembled in the jig (bottom).
gravity (bending) and wind loads (torsion) than those using a torque box. In any case, rigidity in wind loads of both PTC designs is good enough to keep deformation within reasonable limits.
There are also commercial PTC designs which provide a good stiff structure without a torque box or a torque tube, which are replaced by a metallic space frame, such as the one with a 430.8 m2 collector aperture area used by Solargenix and Acciona in the Nevada Solar One Plant, or the American Skyfuel company’s SkyTrough collector, which has a collector aperture area of 690 m2 (www. skyfuel. com). The design used by Solargenix and Acciona is also notable in that it is one of the few PTC designs that does not use an accurate jig for assembly, but instead relies on the accuracy of the preformed parts and fasteners to generate an accurate shape. Figure 7.7 (top) shows the space frame developed by the company Gossamer Space Frames for Solargenix and successfully used in the CSP plant Nevada Solar One, and which provides good torsional rigidity and performance in wind loads.
7.7 PTC designs using a space frame (top) and torque tube (bottom) to provide good torsional rigidity.
Other means of providing a stiff structure in order to reduce costs and simultaneously maintain good rigidity and assembly accuracy are under study. However, these innovative designs were only in the prototype stage at the end of 2010.
Since the size and number of components in the structure of large PTC collectors used in solar thermal power plants make correction of errors almost impossible after the structures have been installed in the solar field, an effective quality control procedure during the whole assembly process is extremely important to ensure that design tolerances and performance are met. In addition to a highly accurate assembly of the steel structures, the alignment of the parallel rows of collectors in the solar field must also be accurate to avoid solar tracking errors, especially when open-loop tracking systems based on mathematical calculation of sun coordinates are used.
Any misalignment would introduce a tracking error and a corresponding reduction in the concentrated solar radiation finally reaching the receiver pipes.
Although the main type of reflector used in commercial PTC designs is the curved back-silvered low-iron thick-glass mirror, it can also be made of polished sheet metal, or silver or aluminum-coated films that can be laminated onto a rigid parabola-shaped substrate. There are several suppliers of this type of polymer film reflector. Vikuiti™ marketed by 3M (http://www. solutions.3m. com) and ReflecTech® marketed by Reflectech Inc. (http:// www. reflectechsolar. com) are two examples of polymer film reflectors for solar applications. However, experimental data about their outdoor performance and durability are still limited and not as plentiful as for glass mirrors.