Collecting rays
The solar collection system has more than 19,000 PTR 70 steel and glass, 12-foot-long receiver tubes, provided by Germany’s Shott AG (www.schott.com), located on the focal line of the 219,000 parabolically shaped mirrors (Figure 2). These mirrors focus 30 to 60 times the sun’s energy on the receiver tubes. The collectors, first manufactured for the 1-MW Saguaro plant, are aligned on an east-west axis. Each mirror panel is rotated by 760 individual sun trackers to maximize the energy input to the receiver tubes. The trackers can pick up the sun when it rises just 10 degrees above the horizon in the morning. Flabeg (www.flabeg.com), another German company, provided the mirror panels (Figure 3).
2. Closed-loop system. The heated heat-transfer fluid is circulated to a steam generator to produce superheated steam in a conventional steam turbine cycle. Courtesy: Acciona Solar Power
3. Focused energy. Schott AG supplied the 19,300 solar receiver tubes, each over 12 feet long, that capture the focused energy of the sun to heat a heat-transfer fluid that circulates through the receivers. Courtesy: Acciona Solar Power
Special synthetic oil serves as the heat-transfer fluid and circulates in a closed loop through the receiver tubes, which can handle up to 750F (Figure 4). A steam generator produces superheated steam to power a conventional 75-MW SST-700 two-stage industrial steam turbine with reheat (provided by Siemens Power Generation, www.powergeneration.siemens.com), which generates the electrical power. A conventional cooling tower rejects heat from the condenser. The power generation side of the plant would be very familiar to any experienced power plant engineer. It’s how the “fuel” enters and “burns” in the steam generator that makes the plant unique (Figure 5).
4. Follow the sun. The parabolic troughs concentrate the sun’s energy on a receiver tube that is positioned at the focal point of the mirrors. A tracking system synchronizes the position of the mirrors with the sun as it crosses the sky to optimize the energy collected. Source: Acciona Solar Power
5. Familiar territory. The 74-MW (gross) steam turbine bottoming cycle will be familiar to any experienced steam plant engineer. Courtesy: Acciona Solar Power
“This is a technology the utilities are comfortable with, it has proven reliability, it lends itself to economies of scale, there clearly is still some room for price reduction, and also it’s a way to get large amounts of renewable energy deployed rapidly,” said Chuck Kutscher, principal engineer and group manager of the Thermal Systems Group at the National Renewable Energy Laboratory. “If we want to get serious about reducing carbon dioxide emissions and lower our use of fossil fuels, this is a way to quickly address that. I’m very optimistic about this technology.”
The project’s green credentials extend to its “green” 10,000-square-foot administration and control building, which uses orientation, insulation, and natural daylight monitors to reduce the need for lighting and air conditioning.
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