Solar thermal power is considered an important step towards developing large scale sources of clean electricity, but within this sector there are some very distinct applications of the technology.  Bright Source Energy, with offices in Oakland, California, and Tel Aviv, Israel, is building next generation “power tower” solar thermal power plants.

The power tower. (Photo: Bright Source Energy)

The stated advantages of power tower technology seem to make a lot of sense.  The solar field of mirrors require no plumbing going to each mirror, containing a thermal transfer fluid, because the two-axis tracking mirrors point to a central boiler.  This saves considerable expense to install and maintain plumbing throughout the solar field. 

Also, because each mirror sits atop a single independently placed post, the ground underneath the solar field can be left relatively irregular and uneven.  With parabolic trough technology, for example, the ground beneath the troughs must be almost perfectly smoothed, meaning far more site preparation is required.

Less obvious but also significant are the costs saved by utilizing super heated steam coming from one central boiler atop a tower, because this design allows the water to be air cooled instead of water cooled.  In order for solar thermal power to require minimal input of water, the water needs to be continuously recirculated - it heats up in the boiler, drives the turbine, then must be cooled and condensed before returning to the boiler for heating.  If this isn’t done, in a closed loop the back pressure of the steam after passing through the turbine would largely counteract the pressure of the incoming steam, ruining the efficiency of the device. 

Because a power tower concentrates the entire energy of the solar field into one boiler, the steam is superheated to 550 degrees centigrade.  In the parabolic trough designs, where the heat transfer fluid flows into dozens of distributed heat exchanging tubes above the focal point of dozens (or hundreds) of separate mirrors, the energy of the solar field is less concentrated, achieving a significantly lower top temperature of 300-350 degrees centigrade.

Because the differential between the super hot 550 C steam is so much greater than the ambient air temperature, even in the desert, air cooling is viable with a power tower design, but is not viable with trough designs.  Air cooling systems are less expensive than water cooling systems, and they use less water.  Bright Source estimates their process loses about 1/2 an acre foot for every megawatt-year of electricity they generate, compared to about 20x that amount for designs that require water cooling - even though all of these designs recirculate.

Not only is Bright Source Energy using what could emerge as the most cost effective solar thermal design, but they are well on their way to implementing their technology.  Their pilot plant in Israel, with a 60 meter tower and 1,600 mirrors, is in testing currently and will go active in mid-June.  The plant will generate 5.0 megawatts of thermal energy, which with a boiler efficiency of 74% and a turbine efficiency of 45% will output 1.5 megawatts of electricity.  That is just the beginning.

The solar field and power tower. (Photo: Bright Source Energy)

With a management team that includes several of the executives who built the original solar thermal plants at Kramer Junction in California in the early 1990’s - still operating profitably with an output of over 350 megawatts - Bright Source Energy is likely to be the first company to build new large scale solar thermal plants in California for 20 years.  Their application, filed with the California Energy Commission in Sept. 2007, was the first one filed since 1989, and proposes a 400 megawatt solar complex to be built in Ivanpah, California, in the Mojave desert near the Nevada border.

The power tower - looking across a reflecting mirror. (Photo: Bright Source Energy)