What is a Solar Power Tower and How Do They Work?

Introduction
Solar One - The First Generation of Power Tower Plant
Solar Two - The Next Generation Solar Power Tower
Advantages of Using Molten Salt
Design and Construction of the Solar Two Power Tower
The Salt Mixture
Metal Corrosion in the Molten-Salt Environment
What are the Benefits of Solar Power Towers?
Environmental Impact of Solar Power Towers

Introduction

Solar power tower convert sunshine into clean electricity. The technology uses many large, sun-tracking mirrors commonly referred to as heliostats to focus sunlight on a receiver at the top of a tower.

A heat transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional turbine-generator to produce electricity. Early power towers such as the Solar One plant uses steam as the heat transfer fluid. Current power towers such as Solar Two use molten nitrate salt. Nitrate salt is used because of its superior heat transfer and energy storage capabilities.

Solar One - The First Generation of Power Tower Plant

Solar One was the world’s largest power tower plant, which operated from 1982 to 1988. The Solar One thermal storage system works by storing heat in the form of steam generated using solar energy in a tank which is filled with rocks and sand and using oil as the heat-transfer fluid.

The Solar One thermal storage system extended the power generation capability of the plant into the night and provided heat for generating low-grade steam for keeping parts of the plant warm during off-hours and for morning startup.

Unfortunately, the Solar One thermal storage system was complex and thermodynamically inefficient. Solar One also showed the disadvantages of a water/steam system, such as the intermittent operation of the turbine due to cloud transcience and lack of effective thermal storage.

Solar Two - The Next Generation Solar Power Tower

The conversion of Solar One to Solar Two required a new molten-salt heat transfer system and a new control system. This includes the receiver, thermal storage, piping, and a steam generator. The Solar One heliostat field, the tower, and the turbine or generator required only minimal modifications.

Advantages of Using Molten Salt

A variety of fluids were tested to transport the sun's heat, including water, air, oil, and sodium, before molten salt was selected as best. Molten salt is used in solar power tower systems because it is liquid at atmosphere pressure, it provides an efficient, low-cost medium in which to store thermal energy, its operating temperatures are compatible with todays high-pressure and high-temperature steam turbines, and it is non-flammable and nontoxic. In addition, molten salt is used in the chemical and metals industries as a heat-transport fluid, so experience with molten-salt systems exists for non-solar applications.

Design and Construction of the Solar Two Power Tower

The Solar Two power tower is composed of 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.

A black Pyromark™ paint which is robust, resistant to high temperatures and thermal cycling and absorbs 95% of the incident sunlight is used to coat the external surfaces of the tubes. The receiver design has been optimized to absorb a maximum amount of solar energy while reducing the heat losses due to convection and radiation.

The design including 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.

The Salt Mixture

A mixture of 60 percent sodium nitrate and 40 percent potassium nitrate is employed as the salt storage medium. This salt melts at 220ºC and is maintained in a molten state of 290ºC in the ‘cold’ storage tank. It is then traveled through the receiver where it is heated to 565ºC and then on to a ‘hot’ tank for storage.

Hot salt is pumped to a steam generating system when power is needed from the plant. These hot salts produce superheated steam for a conventional Rankine-cycle 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.

Metal Corrosion in the Molten-Salt Environment

All pipes, valves, and vessels for hot salt were constructed from stainless steel because of its corrosion resistance in the molten-salt environment, while the cold-salt system is made from mild carbon steel.

What are the Benefits of Solar Power Towers?

Like all solar technologies, solar power towers are fueled by sunshine and do not release greenhouse gases. Solar power towers are unique among solar electric technologies in their ability to efficiently store solar energy and dispatch electricity to the grid when needed, even at night or during cloudy weather.

Environmental Impact of Solar Power Towers

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.

Source: AZoCleantech
Last Update 6th January 2008

Date Added: Jan 7, 2008
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