Solar power towers 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 used steam as the heat transfer fluid. Current power towers, based on Solar Two, use molten nitrate salt 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 in the Mojave Desert. The Solar One thermal storage system worked by storing heat in the form of steam generated using solar energy in a tank 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 start-up.
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 transience and lack of effective thermal storage.
The expansion of Solar One to Solar Two required a new molten-salt heat transfer system and a new control system which allowed the heliostats to track the sun. This included 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.
The tower was 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, was used to coat the external surfaces of the tubes. The receiver design was optimized to absorb a maximum amount of solar energy while reducing the heat losses due to convection and radiation.
The design included laser-welding, sophisticated tube-nozzle-header connections, a tube clip design that facilitated tube expansion and contraction, and non-contact flux measurement devices, which allowed the receiver to rapidly change temperature without being damaged.
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 chosen: it is liquid at atmospheric pressure, provides an efficient, low-cost medium in which to store thermal energy, its operating temperatures are compatible with today’s 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.
The Salt Mixture
A mixture of 60 percent sodium nitrate and 40 percent potassium nitrate was employed as the salt storage medium and is still used in modern solar power towers. This salt melts at 220ºC and is maintained in a molten state of 290 ºC in the ‘cold’ storage tank. It then travels through the receiver where it is heated to 565 ºC 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.
Despite the central tower being torn down in 2009, the success of Solar One and Two in the US prompted many other countries to develop their own installations, including Spain and Israel.
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.
Solar power towers also reduce a country’s reliance on fossil fuels; in Israel for example, Ashalim will prevent 110,000 tons per year of CO2 emissions, being released by burning fossil fuels.
There is evidence, however, that such large installations that concentrate solar rays could be detrimental to bird populations, which can experience scorched feathers, or even death when close by. In order to avoid such tragedies, a maximum of four mirrors are focused on any one spot during standyby.
Sources and Further Reading
This article was updated on 23rd May, 2019.