SEGS Plant Operating Experience

The nine operating SEGS plants have demonstrated the commercial nature of the Luz parabolic trough collecto r technology and have validated many of the SEGS plant design concepts. Additionally, many important lessons hav e been learned related to the design, manufacture, construction, operation, and maintenance of large-scale paraboli c trough plants [7,8,9].

Solar Field Components: A simple problem with a single component, such as an HCE, can affect many thousands o f components in a large solar field. Thus it is essential that each of the SCA components is designed for the 30-year life of the plant and that a sufficient QA/QC program is in place to ensure that manufacture and installation adhere t o design specifications. Luz used three generations of collector during the development of the nine SEGS plants. Eac h time a new generation of collector was used, some form of component failure was experienced. However, one of th e major achievements of Luz was the speed with which they were able to respond to new problems as they wer e identified. Problems with components were due to design or installation flaws. An important lesson from the plant s has been the recognition that O&M requirements need to be fully integrated into the design. Three components i n particular are worthy of discussion because they have represented the largest problems experienced: HCEs, mirrors , and flexhoses.

Heat Collection Elements (HCEs): A number of HCE failure mechanisms have been identified at the SEGS plants, with all of these issues resolved through the development of improved installation practices and operation procedures, o r through a design modification. Loss of vacuum, breakage of the glass envelope, deterioration of the selective surface, and bowing of the stainless steel tube (which eventually can lead to glass breakage) have been the primary HC E failures, all of which affect thermal efficiency. Several of the existing SEGS plants have experienced unacceptabl y high HCE glass envelope breakage rates. The subsequent exposure to air accelerates degradation of the selectiv e surface. Design improvements have been identified to improve durability and performance, and these have bee n introduced into replacement parts manufactured for the existing plants. In addition, better installation and operational procedures have significantly reduced HCE failures. Future HCE designs should: (1) use new tube materials t o minimize bowing problems; (2) allow broken glass to be replaced in-situ in the field; and (3) continue to improve th e selective coating absorptance, emittance, and long-term stability in air.

Mirrors: The current low iron glass mirrors are one of the most reliable components in the Luz collectors. Separatio n of the mirror mounting pads from the mirrors was an early problem caused by differential thermal expansion betwee n the mirror and the pad. This problem was resolved by using ceramic pads, a more pliable adhesive, and therma l shielding. In addition, methods have been developed that allow the O&M crew to retrofit the older mirror pad design and strengthen them to greatly reduce failures. Mirror breakage due to high winds has been observed near the edge s of the solar field where wind forces can be high. Strengthened glass mirrors or thin plastic silvered film reflectors have been designed to circumvent this problem. In general, there has been no long-term degradation in the reflective quality of the mirrors; ten year old mirrors can be cleaned and brought back to like-new reflectivity. However, the glas s mirrors are expensive and for the cost of the collector to be reduced, alternative mirrors are necessary. Any new mirror must be able to be washed without damaging the optical quality of the mirror. Front surface mirrors hold potential t o have higher reflectivity, if the long-term performance and washability can be demonstrated.

Flexhoses: The flexhoses that connect the SCAs to the headers and SCAs to each other have experienced high failur e rates at the early SEGS plants. Later plants used an improved design with a substantially increased life tha t significant ly reduced failures. In addition, a new design that replaces the flexhoses with a hard piped assembly wit h ball joints is being used at the SEGS III-VII plants located at Kramer Junction. The new ball joint assembly has a number of advantages over flexhoses including lower cost, a significant reduction in pressure drop, and reduced hea t losses. If ball joint assemblies can be proven to have a life comparable to the new longer-life flexhoses, then they will be included in all future trough designs.

Mirror Washing & Reflectivity Monitoring: Development of an efficient and cost-effective program for monitorin g mirror reflectivity and washing mirrors is critical. Differing seasonal soiling rates require flexible procedures. Fo r example, high soiling rates of 0.5%/day have been experienced during summer periods. After considerable experience, O&M procedures have settled on several methods, including deluge washing, and direct and pulsating high-pressur e sprays. All methods use demineralized water for good effectiveness. The periodic monitoring of mirror reflectivit y can provide a valuable quality control tool for mirror washing and help optimize wash labor. As a general rule, th e reflectivity of glass mirrors can be returned to design levels with good washing.

Maintenance Tracking: In recent years, computerized maintenance management software (CMMS) has found wid e acceptance for use in conventional fossil power plant facilities. CMMS systems can greatly enhance the planning and efficiency with which maintenance activities are carried out, reduce maintenance costs, and often result in improve d availabili ty of the power plant. CMMS programs have been implemented at trough power plants as well, but th e software is not ideally suited for the solar field portion of the plant. CMMS systems excel in applications that hav e a thousand unique pieces of equipment, but are not really suited to handle systems with a thousand of the same kin d of equipment, like SCAs in a solar field. For this reason, custom database programs have been developed to trac k problems and schedule maintenance in the solar plant. These programs have proven to be an essential tool for tracking and planning solar field maintenance activities and should be considered to be essential for any new project.

Collector Alignment: Operational experience has shown that it is important to be able to periodically check collecto r alignment and to be able to correct alignment problems when necessary. Collector designs should allow field alignment checks and easy alignment corrections.

Project Start-up Support: Operation of a solar power plant differs from conventional fossil-fuel power plant operation in several ways, primarily due to the solar field equipment and operations requirements, integration of the solar fiel d with the power block, and the effects of cyclic operation. Much knowledge has been gained from the existing SEG S plants that is applicable to the development of procedures, training of personnel, and the establishment of an effective O&M organization.

Thermal Cycling and Daily Startup: Typically, parabolic trough plants are operated whenever sufficient solar radiation exists, and the backup fossil is only used to fill in during the highest value non-solar periods. As a result, the plant s are typically shut do wn during the night and restarted each morning. The plants must be designed to not only be started on a daily basis, but also to start up as quickly as possible. Since the current SEGS plant design does not includ e thermal storage, the solar field and power block are directly coupled. The use of thermal storage can significantl y mitigate these problems. In general, equipment/system design specifications and operating procedures must b e developed with these requirements in mind. Both normal engineering considerations and the experience from th e SEGS plants provide important inputs into these needs. Mundane design features such as valves, gaskets, and seal s and bolt selection can be an expensive problem unless properly specified.

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