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Solar Power Towers

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Solar Power Towers

Postby Prasanth » Fri Jun 08, 2018 6:48 am

SOLAR TOWER

• Solar power towers also known as Concentrated Solar Power (CSP) Towers generate electric power from sunlight by focusing concentrated solar radiation on tower mounted heat exchangers.

• 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 MW range.

• The limited supply of fossil hydrocarbon resources and the negative impact of CO2 emissions on the global environment dictate the increasing usage of renewable energy sources.

• Concentrated Solar Power (CSP) is the most likely candidate for providing the majority of this renewable energy, because it is amongst the most cost-effective renewable electricity technologies and because its supply is not restricted if the energy generated is transported from the world's solar belt to the population centres.

• In power tower systems, heliostats (A Heliostat is a device that tracks the movement of the sun which is used to orient a mirror of field of mirrors, throughout the day, to reflect sunlight onto a target-receiver) reflect and concentrate sunlight onto a central tower-mounted receiver where the energy is transferred to a HTF.

• This energy is then passed either to the storage or to power-conversion systems, which convert the thermal energy into electricity.

Heliostat field, the heliostat controls, the receiver, the storage system, and the heat engine, which drives the generator, are the major components of the system.
For a large heliostat field , a cylindrical receiver has advantages when used with Rankine cycle engines, particularly for radiation from heliostats at the far edges of the field.
Cavity receivers with larger tower height to heliostat field area ratios are used for higher temperatures required for the operation of turbines.

These plants are defined by the options chosen for a HTF, for the thermal storage medium and for the power-conversion cycle. HTF may be water/steam, molten nitrate salt, liquid metals or air and the thermal storage may be provided by PCM (phase change materials) or thermally insulating materials.. Power tower systems usually achieves concentration ratios of 300–1500, can operate at temperatures up to 1500o C.

To maintain constant steam parameters even at varying solar irradiation, two methods can be used:

 Integration of a fossil back-up burner; or

 Utilization of a thermal storage as a buffer

By the use of thermal storage, the heat can be stored for few hours to allow electricity production during periods of peak need, even if the solar radiation is not available.

Examples of heliostat based power plants:

• The 10 MWe Solar One and Solar Two demonstration projects in the Mojave Desert, which have now been decommissioned.

• The above 10 MW Solar Tres Power Tower in Spain builds on these projects. In Spain the 11 MW PS10 Solar Power Tower was recently completed.

• In South Africa, a solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m².


Major parts of solar tower are –

1)Heliostats

2)Central Receiver

3)Molten Salt

4)Steam Generator


Heliostats:

Heliostats are reflective surfaces or mirrors which track the suns rays and reflect it onto the central receiver. Relatively few heliostats have been manufactured to date, and their cost is high (>$250/m2). As the demand for solar power increases, heliostat mass production methods will be developed that will significantly reduce their cost.
Research is currently being conducted under the Solar Manufacturing Technology (SolMaT) Initiative to develop low-cost manufacturing techniques for early commercial low volume builds. Prices are a strong function of annual production rate.

Since the heliostat field represents the largest single capital investment in a power tower plant, advancements in technology are needed to improve the ability to manufacture, reduce costs, and increase the service life of heliostats.

In particular, a lower cost azimuth drive system is needed (i.e., to rotate the heliostat around an axis that is perpendicular to the ground).

Recent Developments in Heliostat designs:

The modern R&D efforts have focused on polymer reflectors and stretched-membrane heliostats.

A stretched-membrane heliostat consists of a metal ring, across which two thin metal membranes are stretched.

A focus control system adjusts the curvature of the front membrane, which is laminated with a silvered-polymer reflector, usually by adjusting the pressure in the plenum between the two membranes.
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