16-05-2012, 05:35 PM
SOLAR POWER TOWER
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System Description
Solar power towers generate electric power from sunlight by focusing concentrated solar radiation on a tower-mounted
heat exchanger (receiver). The system uses hundreds to thousands of sun-tracking mirrors called heliostats to reflect
the incident sunlight onto the receiver. These plants are best suited for utility-scale applications in the 30 to 400 MWe
range.
In a molten-salt solar power tower, liquid salt at 290ºC (554ºF) is pumped from a ‘cold’ storage tank through the
receiver where it is heated to 565ºC (1,049ºF) and then on to a ‘hot’ tank for storage. When power is needed from the
plant, hot salt is pumped to a steam generating system that produces superheated steam for a conventional Rankinecycle
turbine/generator system. From the steam generator, the salt is returned to the cold tank where it is stored and
eventually reheated in the receiver. Figure 1 is a schematic diagram of the primary flow paths in a molten-salt solar
power plant. Determining the optimum storage size to meet power-dispatch requirements is an important part of the
system design process. Storage tanks can be designed with sufficient capacity to power a turbine at full output for up
to 13 hours.
The heliostat field that surrounds the tower is laid out to optimize the annual performance of the plant. The field and
the receiver are also sized depending on the needs of the utility. In a typical installation, solar energy collection occurs
at a rate that exceeds the maximum required to provide steam to the turbine. Consequently, the thermal storage system
can be charged at the same time that the plant is producing power at full capacity. The ratio of the thermal power
Sunlight
SOLAR POWER TOWER
provided by the collector system (the heliostat field and receiver) to the peak thermal power required by the turbine
generator is called the solar multiple. With a solar multiple of approximately 2.7, a molten-salt power tower located
in the California Mojave desert can be designed for an annual capacity factor of about 65%. (Based on simulations
at Sandia National Laboratories with the SOLERGY [1] computer code.) Consequently, a power tower could
potentially operate for 65% of the year without the need for a back-up fuel source. Without energy storage, solar
technologies are limited to annual capacity factors near 25%.
The dispatchability of electricity from a molten-salt power tower is illustrated in Figure 2, which shows the loaddispatching
capability for a typical day in Southern California. The figure shows solar intensity, energy stored in the
hot tank, and electric power output as functions of time of day. In this example, the solar plant begins collecting
thermal energy soon after sunrise and stores it in the hot tank, accumulating energy in the tank throughout the day. In
response to a peak-load demand on the grid, the turbine is brought on line at 1:00 PM and continues to generate power
until 11 PM. Because of the storage, power output from the turbine generator remains constant through fluctuations
in solar intensity and until all of the energy stored in the hot tank is depleted. Energy storage and dispatchability are
very important for the success of solar power tower technology, and molten salt is believed to be the key to cost
effective energy storage.