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Solar Power Tower

A solar power tower, sometimes called a heliostat, uses many solar reflectors to concentrate sunlight on a single focal point located in a tower several hundred feet off the ground. In many ways, the solar power tower is just a derivation of a parabolic dish system where the dish and the heat engine are separated.


The reflectors for a solar power tower are usually made of flat glass panels with reflective silver coatings. They track the sun using a dual axis system and are capable of concentrating solar energy several hundred fold.

The receiver is located high from the ground in order to minimize shading and thus increase efficiency of the system. By building mirrors on tiers rather than on a horizontal plane, the amount of surface area that solar power towers occupy can be drastically reduced.

Original power towers simply heated steam with the energy from the mirrors. It was found that molten salt, because it retains its heat energy from longer periods, was a better alternative so many systems how use molten salt, which generates steam that is used in a limited circuit to power turbines. Because the tower is several hundred feet tall, the power generation apparatus is far from the location where the solar energy is collected, making efficient transport essential. The overall efficiency of a tower system is usually on the order of 17%.

These systems are quite large and often take up several acres of land. As such, they are only viable for on large-scale grid power supply. The first operational plant, called Solar One, was built by the United States government in the Mojave Desert and produced 10 MW of power. There are 15 MW, 11 MW, and 20 MW plants operating in Spain and a 5 MW tower in California. These are the only four operational solar power tower systems in the world at this time.

The cost to produce electricity using solar power towers is about 31 cents per kWh. When compared to coal at 9 cents per kWh and nuclear at 11 cents per kWh, solar power towers are quite expensive. However, the U.S. government estimates that mass production and investment in such technology could reduce the cost of solar power tower electricity to 5.47 centers per kWh by 2020.

Advantages

The single largest advantage of solar power towers is cost. The large scale implementation of such systems means that thy benefit from economy of size when compared to single unit installations. At an efficiency of 17%, solar towers are also the second most efficient form of concentrated solar power generation, which helps to reduce cost as well.

Because solar power towers generate a great deal of heat in a single location, storage becomes somewhat simplified. Most of these systems use molten salt storage tanks to store unused heat energy. Due to the fact that these systems often supply electricity to large populations, they have been designed to run at near capacity and thus only store about 1 hour of reserve energy.

As a result of their “near peak” utilization, all solar power towers currently in operation are hybrid systems. Of all CSP systems, solar power towers are the simplest to hybridize. This stems from the fact that the solar collection system and steam turbines are separated in solar power towers. This makes hybrid implementation much simpler.

One interesting advantage to power towers that has nothing to do with energy generation is materials testing. The high temperatures achieved by the tower can be used to test the thermal performance of materials such as those used in the flight or in space travel. Light from power towers has even been used for satellite calibration and for simulating nuclear thermal flash. There is a good deal of research potential for solar power towers that goes beyond their use in electricity generation.

Disadvantage

The major disadvantage of solar power towers is that they cannot operate at night and do not store enough energy to continue producing power without burning fossil fuels. Of course, such problems could be offset by adding additional units to generate excess heat energy for storage, but that also has problems. Storage of thermal energy requires careful attention, vast automation centers, and resistant materials. Nevertheless, this hurdle is can be overcome, making solar power towers a very viable possibility for energy generation if the grid is upgraded.

 Interestingly, land use is not a major problem. The 5 MW solar test facility in the United States uses only 9 acres of land, less than most traditional power generation facilities. The way the land is used is also not of tremendous impact either as the reflectors cause no appreciable environmental damage and do not interfere with animal movement on the ground if mounted high enough. The only potential source of animal interference is for birds that fly through the intensely concentrated light near the tower. At 750 degrees Celsius, the temperature is high enough to cause death in a matter of seconds.

Solar power towers are high maintenance facilities due to the complexities of the dual axis tracking mechanisms and the sheer number of mirrors involved. However, maintenance is not beyond that of traditional power facilities and may even be lower overall.

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