10-04-2017, 07:59 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.
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
Solar One
Solar One, which operated from 1982 to 1988, was the world s largest power tower plant. It proved that large-scale power production with power towers was feasible. In that plant, water was converted to steam in the receiver and used directly to power a conventional Rankine-cycle steam turbine. The heliostat field consisted of 1818 heliostats of 39.3
m2 reflective area each. The project met most of its technical objectives by demonstrating (1) the feasibility of
generating power with a power tower, (2) the ability to generate 10 MWe for eight hours a day at summer solstice and four hours a day near winter solstice. During its final year of operation, Solar One s availability during hours of sunshine was 96% and its annual efficiency was about 7%. (Annual efficiency was relatively low because of the plant s small size and the inclusion of non-optimized subsystems.)
Solar Two
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 and 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.
System Application, Benefits, and Impacts
Overview
To date, the largest power towers ever built are the 10 MW Solar One and Solar Two plants. Assuming success of the Solar Two project, the next plants could be scaled-up to between 30 and 100 MW in size for utility grid connected applications in the Southwestern United States and/or international power markets. New peaking and intermediate power sources are needed today in many areas of the developing world. India, Egypt, and South Africa are locations that appear to be ideally suited for power tower development. As the technology matures, plants with up to a 400 MW rating appear feasible. As non-polluting energy sources become more favored, molten-salt power towers will have a high value because the thermal energy storage allows the plant to be dispatchable. Consequently, the value of power is worth more because a power tower plant can deliver energy during peak load times when it is more valuable. Energy storage also allows power tower plants to be designed and built with a range of annual capacity factors (20 to 65%). Combining high capacity factors and the fact that energy storage will allow power to be brought onto the grid in a controlled manner (i.e., by reducing electrical transients thus increasing the stability of the overall utility grid), total market penetration should be much higher than an intermittent solar technology without storage.
Environmental Impacts
No hazardous gaseous or liquid emissions are released during operation of the solar power tower plant. If a salt spill occurs, the salt will freeze before significant contamination of the soil occurs. Salt is picked up with a shovel and can be recycled if necessary. If the power tower is hybridized with a conventional fossil plant, emissions will be released from the non-solar portion of the plant.
Technology Assumptions and Issues
Assuming success at Solar Two, power tower technology will be on the verge of technology readiness for commercial applications. However, progress related to scale-up and R&D for specific subsystems is still needed to reduce costs and to increase reliability to the point where the technology becomes an attractive financial investment. Promising work is ongoing in the following areas:
First Commercial System
Ideally, to be economically competitive with conventional fossil technology, a power tower should be at least 10 times larger than Solar Two [4]. It may be possible to construct this plant directly following Solar Two, but the risk perceived by the technical and financial communities may require that a plant of intermediate size (30-50 MW) be constructed first. The World Bank will consider requests for funding power tower projects following a successful two-year operation of Solar Two. However, countries interested in the technology have indicated they may need to see a utility- scale plant operating in the U.S. before they will include power towers in their energy portfolio.