A Maryland company, Solar Wind Energy Inc., proposes to build a giant first-of-its-kind wind tower near San Luis, Arizona, a seismically active area that could cause the structure to collapse from a medium-sized earthquake, while producing enough humidity to negatively affect a town up to 62 miles away. San Luis is in the southwestern part of the state south of Yuma.
The proposed tower, essentially a hollow hyperbolic cylinder, is planned to be 2,250 feet tall, 1,200 feet wide at the top, and 1,500 feet wide at the bottom. It would be the second tallest structure on the planet. The project will cost $1.5 billion, apparently from private investors, and generate an average of 425 megawatts.
This project has been described previously in Forbes (see here and here) and more recently by an August 8, 2014 story in Inside Tucson Business. Both sources contained some confusing ambiguities. Earlier this week (August18, 2014) I spoke by phone to Steven Sadle, COO, and to Ronald W. Pickett, president and chief executive officer of Solar Wind Energy Tower Inc. to get better information.
The technology is based on work done in Israel in the mid-1970s (see Wikipedia article here). The tower is supposed to work as follows: Water is sprayed into the tower near its top. The water evaporates and makes the air in the tower heavier and cooler than the ambient air. This difference causes air in the tower to drop down the structure at up to 50 mph. It exits the tower by passing through turbines which produce electricity. You can see a more detailed explanation and a short video at the website of Solar Wind Energy here. As far as I can find out, the technology has been tested only with a 4-foot model. The Arizona project will be the first practical test.
The design capacity for the tower is 1250 megawatts. That is under ideal conditions of very hot, dry ambient air. Increases in humidity or cooler temperatures in ambient air both reduce the amount of electricity produced by this method. That means electricity production in winter will be much less than in summer, for instance. Even though the proposed tower can operate 24/7, it is still subject to the vagaries of local weather. Mr. Pickett anticipates an actual production averaging 425 megawatts. Mr. Pickett told me it will take 11.6 percent of electricity output at the time to run the plant. Most of that is for pumping water. The amount of water pumped is variable and depends upon local conditions. The company will closely monitor humidity and temperature so they know how much water they need to pump at any given time. According to Mr. Pickett, the project will actually use less water on hot, dry days because the evaporation rate is more efficient compared to cooler, more humid days.
This project will use 8,000 acre-feet (2.6 billion gallons) of water per year, the equivalent use of about 1,500 residences. The water will come by pumping groundwater from the aquifer in the San Luis area. In the Inside Tucson Business story Mr Pickett was quoted as saying,”We will be taking water out of the ground and putting it back through evaporation, so we will not be a net user of water.” In my conversation with Mr. Pickett, he backed off from this claim and conceded that the water would not replenish the local aquifer. The company will, however, collect and recycle water that runs down the interior of the tower.
There may be problems with local humidity. According to the 1970s Israeli research a plant of the proposed size will humidify the air enough to make “a community even 100 kilometers away… unpleasantly affected.”
Another possible problem is that the area is near a region prone to earthquakes. Most earthquakes occur in California just west of the Colorado River. San Luis is east of the river. Using the Arizona Geological Survey’s hazard viewer, I produced the map below which shows the incidence of earthquakes of magnitude 5 and greater. San Luis and the proposed tower lie in the southwest corner of Arizona close to a cluster of past earthquakes.
The vertical cross-section of the tower is a hyperbolic curve, wide at the base, narrowing with height. This technology, invented in 1918, is widely used in cooling towers at power plants because it is more stable than a vertical-walled cylinder and has better physics for air flow. However, there may still be problems with building these towers in seismically active areas. There has been much research with some cautionary conclusions. (Note: I have access to only the abstracts of the papers cited below.)
For instance, the paper “The Damaging Effects of Earthquake Excitation on Concrete Cooling Towers” which studied cooling towers in earthquake-prone Iran, “concluded that for the (typical) cooling tower configuration analyzed, the columns that are instrumental in providing a load path are influenced greatly by earthquake loading, and for the earthquake data used in this study the representative cooling tower would be rendered unstable and would collapse under the earthquake forces considered.”
Another paper “Dynamics of axisymmetric hyperbolic shell structures” notes previous failures of these cooling towers and notes that structural integrity is very sensitive to the curvature of the hyperbolic curve. Both too little and too much make the structure unstable.
A third paper from China “Study on Seismic Performance of Large Hyperbolic Cooling Tower in Different Fields” notes that the base of the structure is very sensitive and concludes that these towers should not be sited in seismically active areas.
Given these data, the tower will have to be carefully designed to take into account the extra stress that may be caused by earthquakes in the San Luis region. I brought up this subject with Mr. Pickett. He told me that the tower will be built on piles which will act similar to a floating cap that will dampen some seismic movement. Also, the tower will be designed to allow some movement within the tower structure itself.
Solar Wind Energy Inc. is in the process of securing necessary permits.
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