San Luis solar downdraft tower – Updates and Corrections

Tower by Solar Wind Energy Inc.
Tower by Solar Wind Energy Inc.

In a previous article, I described a project by Solar Wind Energy Inc. who propose to build a giant downdraft tower in San Luis, Arizona, to generate electricity. The tower is planned to be 2,250 feet tall, 1,200 feet wide at the top, and 1,500 feet wide at the bottom.

As a result of that story, ADI was contacted by Mr. Steven Sadle, COO of the company, who complained that the story contained an error. I phoned Mr. Sadle late Friday afternoon for more information. I spoke to him again and to Ronald W. Pickett, president and chief executive officer, Monday morning.

There is an error in my original story. I took a quote from Inside Tucson Business: “Ronald W. Pickett, … said he expects the plant, when completed, to generate 425 megawatts of electricity each year.” I took that quote literally to mean the total production of electricity. Based on that assumption I made an erroneous calculation of how many houses that could serve. What Mr. Pickett really meant was that the annual average production is expected to be 425 megawatts per hour.

According to the Solar Wind Energy Inc. website, the rated capacity of the tower is 1,250 megawatts per hour under ideal conditions. However, “ideal conditions” will rarely exist, so the company anticipates an average production of 425 megawatts per hour, 34 percent of rated capacity. That is not unusual for “green” energy projects. For instance, Tucson Electric Power operates a photovoltaic system near Springerville, Arizona. Their experience over the first five years of operation is that the array produced only 19 percent of rated capacity. Wind turbines typically produce less than 25 percent of rated capacity.

Below is an explanation of why actual production from the proposed tower will be less than maximum capacity.

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.

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.

In the original story I wrote that about 100 megawatts would be necessary to run the plant, mainly for pumping water. That figure came from a Forbes Story. Mr. Pickett told me it will take 11.6 percent of electricity output at the time to run the plant. 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. As I originally wrote, the project will use about 8,000 acre feet (2.6 billion gallons) of water per year. The project will also collect and recycle water that runs down the interior walls of the tower. Mr. Pickett conceded that his statement quoted by Inside Tucson Business:”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” is not strictly accurate.

I noted in the original story that San Luis is in a seismically active zone (see map in original story). 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 and act similar to a floating cap which will dampen some seismic movement. Also, the tower will be designed to allow some movement within the tower structure itself. Mr. Pickett also noted, correctly, that seismic activity in the area is stronger on the west side of the Colorado River than it is in San Luis, east of the river.

As they say, the devil is in the details. I wish Forbes and Inside Tucson Business had published more of those details. But then, you wouldn’t have the real story from ADI.