Solar One and Solar Two

Transcription

1 Solar One and Solar Two Solar One generated electricity between 1982 and ( ) Solar One generated steam directly from water in its receiver, but its direct steam system had low efficiency in storing energy and Solar One s operation was frequently interrupted by passing clouds. The best performance for a single day was a net of 80 MWh during June Figure E (181) shows some of the Solar One monthly operating record. The heliostats (Fig. E21.6.2) reflected and focused sunlight onto the nearly 100 m tall tower (see Fig. E21.6.3). Because of cost overruns in construction, the heliostat field ended up with 15% fewer heliostats than the original design called for, which caused degradation of performance. (188) Because of Solar One s poor cost experience, industry experts mostly dismissed the central receiver option for large-scale power. (183) Fig. E Monthly energy production of Solar One ( ). (Southern California Edison)

2 Energy, Ch. 21, extension 6 Solar One and Solar Two 2 Fig. E Heliostats at Solar One in Daggett, California. The hot water either went to turbines, where it generated electricity at 35% thermodynamic efficiency, or was sent to a heat exchanger, where it heated oil that was then sent to a thermal storage tank. The tank had a volume of about 4230 m 3, and contained 4,120 tonnes of crushed granite and 2,060 tonnes of sand (181) through which the oil circulated. A temperature gradient existed in the tank (the idea was from Rocketdyne, and is proprietary) from the low temperature (244 C) to the high (entry) temperature (304 C). The heat from the tank could be drawn back through the heat exchanger to produce steam for the turbine at 274 C. (181,182) In addition, the thermal storage allowed some buffering for passing clouds and so kept the plant operating through short changes in weather conditions. The plant needed to be operating at least 2 hours per day to make it cost-effective to start up the pumps. (179) In 1989, a decision was made to redesign Solar One; after 2 years of redesign planning, it was decided to make even more changes and begin Solar Two. (188) The Department of

3 Energy, Ch. 21, extension 6 Solar One and Solar Two 3 Energy funded just under half of the $58 million cost with a match by Southern California Edison and a consortium of utilities. The rebuilding took from 1992 to Fig. E The tower with mirrors focused. (U.S. Department of Energy, National Renewable Energy Laboratory) The energy cycle was redesigned to use molten nitrate salt at 56 C as storage medium instead of oil. (14z,184) The reconstruction was completed in Most of the equipment from Solar One (the large heliostat field, tower, and turbine) was reused, but a new receiver (Figs. E21.6.3, E21.6.4, and E21.6.5), storage system (visible at the base of the tower in Fig. E21.6.5), control system, and connecting piping were built. The tower is much larger in area than in Solar One. In addition, some heliostats were added to the heliostat field.

4 Energy, Ch. 21, extension 6 Solar One and Solar Two 4 b. Fig. E Solar Two s receiver, atop a 90-meter tower could absorb 800 times the normal intensity of the sun. (U.S. Department of Energy, Ref. 189) Fig. E Solar Two. (U.S. Department of Energy, Ref. 189)

5 Energy, Ch. 21, extension 6 Solar One and Solar Two 5 Fig. E Schematic of electricity generation using molten-salt storage. Stages shown are 1. Sun heats salt in the receiver; 2. Heated salt is stored in the hot storage tank; 3. The hot salt is pumped through the steam generator; 4. The steam drives a turbine/generator to produce electricity; 5. The salt returns to the cold storage tank to be recirculated. (U.S. Department of Energy, Ref. 190) Fig. E Solar Two has become quite predictable. (U.S. Department of Energy, Ref. 189)

6 Energy, Ch. 21, extension 6 Solar One and Solar Two 6 The molten-salt approach of Solar Two overcame storage and intermittancy difficulties. The molten salt was able to keep hotter longer than the system used in Solar One. Figure E shows the schematic of the molten salt storage facility. The salt is heated to 545 C in normal operation. In addition, much was learned from Solar One about the behavior of thermal tower systems; Fig. E shows just how predictable Solar Two s operation had become after the initial shaking out period was over. Table E shows the results from Solar Two. TABLE E Solar Two Results Dispatchability: Utilizing its unique and highly efficient thermal storage system, Solar Two delivered electricity to the grid around the clock for 153 straight hours (nearly a full week). Power Output: Solar Two produced 1633 MWh over a 30-day period, exceeding its longterm performance measure of 1500 MWh of power production; the plant also produced a record turbine power output of 11.6 MW. Reliability: During one stretch in the summer of 1998, the plant operated for 32 of 39 days (4 days down because of weather, 1 day because of loss of offsite power, and only 2 days for maintenance. Parasitic Power Use: The electrical parasitic energy load (electricity required to run the plant) was reduced significantly and routinely met the design goal. Efficiency: The receiver efficiency was measured at 88%, as per design specifications. Source: Ref. 189 The greatest output from Solar Two was 11.6 MW, as Table E showed. This was a bit in excess of the design, which was nominally 10 MW, just as for Solar One. Solar Two s success demonstrated that the technology could be scaled up to commercial use. As with Solar One, the greatest output comes at a time when Southern California is using electricity the most (reaching peak output) for air conditioning. This feature will make

7 Energy, Ch. 21, extension 6 Solar One and Solar Two 7 such solar tower systems attractive in sunny climates. Of course, the disadvantage of all solar systems is the high upfront cost, and this problem is shared by Solar Two. The proposed Spanish project, Solar Tres, would use all the proven molten-salt technology of Solar Two, but scale it up by a factor of 3 in just such an environment. It would be funded completely by commercial interests. Future versions, if Solar Tres is built, may incorporate provisions to keep the heat balance over the collector area the same as it would have been without the concentrating mirrors. Australia is considering a solar tower of its own. (191) If completed, it would be the world s tallest structure at 1 km.