Chasing the sun
Each SCA is an independently tracking assembly of parabolic trough solar collectors. Every collector includes parabolic reflectors (mirrors), a metal support structure or space frame assembly, a receiver tube, and a tracking system. In contrast to the LS-2 and LS-3 designs used at the SEGS plants, each SCA has a 16.4-ft aperture (like the LS-2) but is 328 ft long (like the LS-3). The mirrors are identical to those used on the LS-2 (Figure 6)
6. Capturing the sun. An organic fluid is heated by solar energy concentrated by the Solargenix solar collector assembly. Courtesy: Arizona Public Service
Eight SCAs are pieced together to form each solar collector loop. The ends of the three loops in the solar field are attached to cold and hot HTF headers, which are routed along one side of the solar field to and from the OEC power block. The SCAs rotate around the horizontal north/south axis to track the sun as it moves across the sky over the course of a day. The axis of rotation is located at the collector's center of mass to minimize required tracking power. The drive system uses hydraulic rams to position the collector. A closed-loop tracking system relies on a sun sensor for the precise alignment required to focus the sun on the HCE with a precision of +/- 0.1 degrees.
Tracking is controlled by a local controller on each SCA. The local controller also monitors the HTF temperature and reports operational status, alarms, and diagnostics to the main solar field control computer in the control room. The SCA is designed to operate normally in winds of up to 25 mph and with somewhat reduced accuracy in winds of up to 35 mph. The SCA modules can withstand 70-mph winds in stowed position.
The SCAs start to absorb heat once the sun has risen 10 degrees above the horizon. The rate of HTF flow at the main Solargenix header is 235 gpm, at a design operating temperature of 550F and a maximum of 600F. The temperature of the HTF changes during the day with the intensity of solar insolation. Because the site's minimum ambient temperature of about 50F is higher than the minimum allowable working temperature of the HTF, there is no danger of HTF freezing in the collectors.
Lessons learned
The Saguaro Solar Power Plant was funded by APS customers using money from the environmental porttfolio standard. APS's cost to build was 7% to 15% more—on a per-kW basis—than the large, tracking, "pure" PV plants that APS has deployed in the past. Much of the additional cost was eaten up by auxiliaries associated with the trough plant—namely, the evaporation pond, cooling tower, site development, and flood control. Obviously, the per-kW cost would be much lower if the plant were made larger. Indeed, it appears that a plant of this type with a capacity of 5 MW or more would be cost-competitive with next-generation solar thermal systems now under construction (see box).
Another lesson that APS learned was that wet cooling significantly improved power-cycle efficiency during the Tucson summer. Cost was a huge driver of the cooling water system design. A dry system would have cost about $400,000 more than the wet one, including the evaporation pond. The former approach also would have necessitated much more O&M. Finally, a performance penalty was caused by a lack of cooling capacity as well as the parasitic losses incurred by the use of more fans. APS would have considered hybrid cooling system for a larger plant.
The author would like to recognize the many contributors to the success of this project. They include the management and staff of APS, Solargenix, Ormat, Sandia National Laboratories, and the National Renewable Energy Laboratory, as well as the many site construction contractors.
—B. Scott Canada is a project engineer in the Renewables Engineering Department of Arizona Public Service; Jeff Lee is Saguaro's plant manager.
Email
Comments
Print
Reuse
