Solar Thermal Electricity (STE)
Page 2: Solar Energy Generating System (SEGS)
Kramer Junction, California
The Solar Energy Generating System (SEGS) solar trough plants in Kramer Junction, California have been operating for decades, and will last at least that much longer without requiring replacement costs as other electricity generating technologies require. The operating and maintenance (O&M) costs of SEGS are only 2 cents per kilowatt-hour (kWh), which includes cleaning the collectors and periodic refurbishing of the steam turbines. Cost reduction work on one of the plants has lowered O&M costs to 1.22 cents per kWh, and future plants will have O&M costs below 1 cent per kWh (< US $ 0.01 / kWh).
A solar trough power plant with 30 MW turbine was built in 1985 in Dagget, California, followed with solar trough power plants built in Kramer Junction during the late 1980s that power five more 30 MW turbines. The same design was then used in 1989 and 1990 to build solar trough arrays that power two 80 MW turbines at Harper Lake. All of these plants continue to operate successfully, requiring only maintenance.
An earlier 14 MW plant built in 1984 in Dagget used a different design. That first plant used oil tanks for heat storage which were destroyed by fire. That plant now operates without heat storage. The subsequent 8 plants did not use heat storage (and still don't). Solar trough plants being built now use molten salt heat storage (instead of oil storage) to generate electricity at night. Molten salt stores heat safely and efficiently.
The plants at Kramer Junction are numbered III through VII:
Figure 2.1 [ Final Report ]
Figure 2.2 One of the five 30 MW turbines at Kramer Junction.
Solar trough plants can use turbines that generate 250 MW to 500 MW of electricity per turbine, including the General Electric D Series steam turbine (up to 375 MW) and the Siemens SST-5000 (up to 500 MW). Use of even larger turbines is also possible.
Figure 2.3 SEGS solar trough power plants. The 30 and 80 Megawatt (MW) plants were originally designed to use larger turbines (160 MW each for plants VIII and IX), but were scaled back.
Figure 2.4 Actual 1997 operating and maintenance (O&M) cost of plant VI (US$ 107 per kilowatt per year, which is 1.22 cents per kilowatt-hour), and Sandia Labs 1997 estimates for planned solar trough plants (all less than 1 cent per kWh). The subsequent plants were not built.
Collectors extend in the north-south direction, along a constant longitude (a longitudinal meridian).
Figure 2.5 Parabolic reflectors. Reflection of sunlight on the reflector pipe does not need to be uniform (astronomical optical precision is not required).
Figure 2.6 Rows of collectors. The white rectangle along one of the rows of reflectors is the image of the underside of the receiver pipe being illuminated by the reflectors.
The most effective method of cleaning the collectors is to point them upward when rain or snow is predicted. Snow is reported to do an especially good job. However, the site is sunny 98 percent of the time, so other cleaning methods are required.
Washing is done once per week during peak summer season, using demineralized water. The water is demineralized on site. Two washing methods are used: the classic deluge wash, and the twister method which is more effective, but takes longer and costs more, so it is not done every cleaning. Both methods use less than one-fourth gallon of water per square meter of collector area for each washing.
The twister method is almost as good as natural snow cleaning. It uses a tractor with twirling nozzles, towing a water tank:
Figure 2.7 Twister method of washing the collectors. Notice that water collects a little bit at the bottom of the collector. Every 2 or 3 years, manual cleaning of the bottom edge of the collectors is performed, using a hard brush, to remove gradual buildup.
For the deluge method, the collectors face each other so that two rows of collectors are washed at once. A large tanker truck with fixed sprayers drives between the collectors:
Figure 2.8 Deluge method of washing the collectors, using a water truck.
Figure 2.9 Newer deluge washing truck, with improved spray nozzle positions.
The cost of deluge washing is US $ 0.14 per square meter of collector area per year.
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