Solar

To encourage the development of molten-salt power towers, a consortium of utilities led by Southern California Edison joined with the U.S. Department of Energy to redesign the Solar One plant to include a molten-salt heat-transfer system. The goals of the redesigned plant, called Solar Two, are to validate nitrate salt technology, to reduce the technical an d economic risk of power towers, and to stimulate the commercialization of power tower technology. Solar Two has produced 10 MW of electricity with enough thermal storage to continue to operate the turbine at full capacity for three hours after the sun has set. Long-term reliability is next to be proven.

The conversion of Solar One to Solar Two required a new molten-salt heat transfer system (including the receiver , thermal storage, piping, and a steam generator) and a new control system. The Solar One heliostat field, the tower, and the turbine/generator required only minimal modifications. Solar Two was first attached to a utility grid in early 1996 and is scheduled to complete its startup phase in late 1997.

The Solar Two receiver was designed and built by Boeing's Rocketdyne division. It comprises a series of panels (each made of 32 thin-walled, stainless steel tubes) through which the molten salt flows in a serpentine path. The panels form a cylindrical shell surrounding piping, structural supports, and control equipment. The external surfaces of the tube s are coated with a black PyromarkTNpaint that is robust, resistant to high temperatures and thermal cycling, and absorbs 95% of the incident sunlight. The receiver design has been optimized to absorb a maximum amount of solar energ y while reducing the heat losses due to convection and radiation. The design, which includes laser-welding, sophisticated tube-nozzle-header connections, a tube clip design that facilitates tube expansion and contraction, and non-contact flux measurement devices, allows the receiver to rapidly change temperature without being damaged. For example, durin g a cloud passage, the receiver can safely change from 290 to 570°C (554 to 1,058°F) in less than one minute.

The salt storage medium is a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate. It melts at 220°C (428°F) and is maintained in a molten state (290°C/554°F) in the 'cold' storage tank. Molten salt can be difficult to handle because it has a low viscosity (similar to water) and it wets metal surfaces extremely well. Consequently, it can be diffic ult to contain and transport. An important consideration in successfully implementing this technology is th e identification of pumps, valves, valve packing, and gasket materials that will work with molten salt. Accordingly, Solar Two is designed with a minimum number of gasketed flanges and most instrument transducers, valves, and fittings are welded in place.

The energy storage system for Solar Two consists of two 875,000 liter storage tanks which were fabricated on-site by Pitt-Des Moines. The tanks are externally insulated and constructed of stainless steel and carbon steel for the hot an d cold tanks, respectively. Thermal capacity of the system is 110 MWh t. A natural convection cooling system is use d in the foundation of each tank to minimize overheating and excessive dehydration of the underlying soil.

All pipes, valves, and vessels for hot salt were constructed from stainless steel because of its corrosion resistance in the molten-salt environment. The cold-salt system is made from mild carbon steel. The steam generator system (SGS) heat exchangers, which were constructed by ABB Lummus, consist of a shell-and-tube superheater, a kettle boiler, and a shell-and-tube preheater. Stainless steel cantilever pumps transport salt from the hot-tank-pump sump through the SGS to the cold tank. Salt in the cold tank is pumped with multi-stage centrifugal pumps up the tower to the receiver.

Solar Two is expected to begin routine daily power production in late 1997. Initial data collected at the plant show that the molten-salt receiver and thermal storage tanks should perform as predicted during design. For example, dat a collected on March 26, 1997, revealed that the receiver absorbed 39.8 MWt, which is 93% of the design value. Considering the fact that the heliostat field had significant alignment problems at the time of the measurement, th e receiver is expected to reach 100% of the design after realignment. This was reaffirmed by efficiency tests conducte d in October 1997 which indicated an 87% value; this is nearly identical to the design prediction. The hot tank withi n the thermal storage system has also exhibited excellent thermal characteristics. Figure 3 depicts a month-long coo l down of the hot storage tank when it was filled with molten salt. It can be seen that the tank cools very slowly (abou t 75°C/167°F over one month) and the measured thermal losses are within about 10% of the design prediction.

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