May 15, 2007

Focus on O&M (May 2007)

HYDRO POWER

Harnessing the Yangtze

Hydro power generators from Alstom Power Hydro (www.hydro.power.alstom.com) are converting the flow of a great river, the Yangtze, into electricity with the largest plant of any kind ever built, at Three Gorges in China (Figure 1).

1. New Great Wall. The Three Gorges project's greatest challenge was its sheer size—the dam is almost 1.5 miles wide. The entire plant is expected to be completed in 2009, 17 years after groundbreaking. Courtesy: Getty Images and Trelleborg AB

Alstom Hydro Power is based at the foot of the Alps, in Grenoble, France, where hydro power was born. Casimir Brenier began developing hydraulic turbines in 1854. Today, the company he founded is a subsidiary of Alstom Power and serves as the global technology center for Alstom's hydro power business.

"The U.S. still has the largest hydro power capacity," says Jacques Brémond, supervisor of mechanical engineering at the Alstom Turbine Technology Center. "But Asia, led by China, is the fastest growing market. In Europe and North America, power generation exceeds the demands of the population, while in China and India more capacity is desperately needed."

China's other great wall

Alstom is supplying almost half of the turbines and generators for the Three Gorges project, whose dam and plant were built for three reasons:

• To regulate the flow of the Yangtze. Its notorious floods have claimed an estimated one million lives over the past hundred years.
• To make the river navigable into the center of China.
• To create as much electricity as 18 nuclear plants and serve an estimated one-ninth of China's current electricity demand. The project, when completed, will avoid having to burn 40 million to 50 million tons of raw coal each year.

The project's greatest challenge was its sheer size. Compare the width of the Three Gorges (almost 1.5 miles) with that of a typical hydro plant dam (around 300 feet) and you begin to appreciate the scale of the undertaking.

A stunner of a runner

The main part of a hydro turbine is its runner (Figure 2), which converts the flow of water to mechanical energy. "The external diameter of the runner for the Three Gorges was 23% larger than any other we had produced before," says Brémond. "The runner is 35 feet across and 17 feet high."

2. Superheavyweight class. It wouldn't have made sense to build the runners in Alstom's workshop in Grenoble. Weighing in at 425 tons, each runner would have been too heavy for the only bridge out of town. Courtesy: Getty Images and Trelleborg AB

The weight of a single runner—425 tons—precluded making any of them in Alstom's workshop in Grenoble. After completion, they would have had to be transported over the town's only bridge, which has a weight limit of 300 tons. Consequently, the runners were manufactured in a specially constructed workshop in La Ciotat in the south of France. They were then transported by sea to Shanghai and then transferred from oceangoing vessels to riverboats. It takes six of these riverboats just to carry the runner's draft tube elbow.

In a gravity dam, the water flows down from a reservoir into a hydro turbine. It then enters the runner from one side via a spiral case, which distributes the water around the turbine. A distributor with adjustable wicket gates inside the turbine (Figure 3) controls the flow of water circulating within it. The energy of the water (head and flow) is then converted to mechanical energy (torque and rotational speed). Finally, the mechanical energy is transformed into electricity by the generator, which is on the same shaft as the turbine. Once the water has been through the runner, it is sent down a draft tube back into the river.

3. Wheel of fortune. A distributor with adjustable wicket gates inside the hydro turbine controls the flow of water circulating within it. Courtesy: Getty Images and Trelleborg AB

Given the size and weight of the runners at Three Gorges, run-of-the-mill bearings for rotating parts were out of the question. During the project's early stages, Alstom considered specifying Orkot bearings from Trelleborg AB because it had used them successfully on smaller hydro projects for nearly 10 years. Yet, "we were unable to specify Orkot bearings on the left bank of the Three Gorges," says Brémond. "The customer requested that we use a bearing tailored to the application."

To satisfy Alstom's customer, Trelleborg provided details of two independent tests on Orkot performed specifically for the application. "Based on the reports, we were increasingly confident about the performance of Orkot and decided to test it in the wicket gate lower brushes we built to refurbish a Francis turbine at the base of the Liu Jia Xia dam in China," says Brémond. "After a few more field tests, we pronounced the bearing fit enough to be used at full scale on turbines we supplied to the Alqueva Hydro Power Plant project commissioned in Portugal in 2004."