Wind generators are great for producing electricity < unless there isnt any wind.
But lack of wind isnt an insurmountable problem, according to a group of UA Engineering students. Theyve been experimenting with a design that doesnt depend on the vagaries of natural wind. Instead, their design produces its own airflow by trapping heat from the sun and then allowing the heated air to escape through a chimney-like tower to an area of lower pressure and cooler air.
The students built a scale model to test their theories and to develop a set of scaling laws to accurately predict the power output of a "wind tower," depending on its diameter, collector area, height and many other factors.
"Our idea was to optimize the geometry to see how the tower height and the tower diameter affect the airflow," said Mechanical Engineering senior Andy Lovelace. "We found that as the tower gets bigger, the power generated goes up exponentially. So if you double the size, you get four times the power."
Knowing how the designs variables change with size allowed the students to develop equations from which they can accurately predict the power output of wind towers of any size.
"The other part of our project was to design a scale model so we could take data to verify that our equations accurately predict wind tower performance," he said.
A REPLACEMENT FOR GAS- AND COAL-FIRED PLANTS
"Wind towers are not like solar cells, where you power a house," he added. "Were talking about competing with a gas- or coal-fired power plant."
In 1982, engineers built a small-scale wind tower in Spain that ran for eight years. It had a 640-foot-tall tower, a collection area of about 500 square feet, and a maximum output of about 50 kilowatts.
"My friend, Rudi Bergermann, developed the plant in Manzanares, Spain and brought this concept to my attention," said Professor Hermann Fasel, who sponsored the UA wind tower project. "He got me excited about doing serious research on this concept." Fasel is a professor in the Aerospace and Mechanical Engineering Department.
In addition to funding the project, Fasel was the teams faculty advisor and spent many hours mentoring the group. "This is one of the best teams Ive advised in a long time, as well as the photovoltaic power unit team that won the PDAT Best Mechanical Design Award at Engineering Design Day."
In addition to the Manzanaras plant, a wind tower with a height of 1,640 feet is proposed for construction in Australia.
The students tower is a much more modest effort, at just 12 feet tall. But its an accurate scale model from which data can be taken and then scaled up to predict the performance of commercial-sized wind towers.
The students tower has a circular collector constructed from a surplus trampoline frame covered with transparent Mylar. The chimney is a length of ABS pipe and their generator is a tiny cell phone motor modified to run in reverse. The motor originally powered a vibration alert mechanism in the phone.
A cone at the base of he tower helps to direct the airflow so it doesnt meet a 90-degree bend at the junction of the horizontal collector surface and vertical tower. "We tried to keep the flow as efficient as possible," Lovelace said.
COLLECTOR AIR HEATED TO 200 DEGREES
On a 90-degree day, the air under the collector was heated to 200 degrees Fahrenheit and created a wind speed of about 2.25 mph as it escaped through the tower. This produces a power output of about a half watt. The team had anticipated a wind speed of about 6 mph. The lower speed is caused by the prototypes short tower and its scale-model design, said team member Dave Klawon.
If the tower had been optimized for small size, it would have produced significantly greater wind speeds, but it wouldnt have provided the performance data the team needed to verify their equations for mega-watt-sized towers.
Analyzing the towers thermodynamics and applying that to developing equations and designs was the most difficult part of the process, Klawon said. "The thermodynamics was a lot more complicated than anything weve seen in class, and it was a great learning process."
Fasel intends to sponsor another wind tower team next semester for further development of the concept. This will include building a tower about 40 feet tall that has a collection area of 14 square feet. This should provide enough airflow to power a small turbine, Lovelace said.
INITIAL COSTS ARE HIGHER AND MORE LAND IS NEEDED
Although wind towers have zero emissions and many other benefits, they do have two problems, Lovelace noted: They cost more to build than conventional power plants and they require huge, greenhouse-like collection areas. However, over the long term, theyre cheaper than conventional power plants because they require little maintenance, have no fuel costs and < unlike nuclear power plants < no hazardous waste to dispose of.
In places where large amounts of open land exist, such as the American West and Australia, the large collection area isnt as big a problem, he added.
"There are so many different ways you can go into optimizing the performance of wind towers," Lovelace said. "We got a good optimization of the tower geometry, but now its up to future teams to look at the other variables."
In addition to Lovelace and Klawon, the wind tower team included Mechanical Engineering seniors Oscar Rueda and Gabriel Secrest.
Ed Stiles | University of Arizona Engineerin
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