The Bad Creek Project
STORING ENERGY: WHY AND HOW
During the clay, customer demand on a generating system ranges from a minimum, or base load, to a maximum, or peak load. On a summer day, for example, customers would use less power in the cool morning hours, and the peak load would come during the hottest part of the day.
Duke Power uses its nuclear units and its most efficient coal-fired units to meet the base load. As the load increases, less efficient coal-fired units are brought on line. And as the load reaches the peak, hydro units are started up.
Why this hierarchy? Nuclear plants have lower fuel costs than coal-fired plants. Efficient coal plants generate more kilowatts per ton of coal than less efficient plants. Hydro plants can be started and stopped much more quickly than nuclear or coal-fired plants, making hydro well suited for meeting peak demand.
The limited water supply in Duke's service area works against greater use of hydro power. One way to circumvent this barrier is to reuse the water that is available. That's done through pumped storage.
Pumped storage requires two bodies of water, one higher than the other, with a powerhouse in between. During peak hours, water is released from the upper reservoir. As in conventional hydro plants, the water flows through the powerhouse, turns the turbines and empties into the lower reservoir.
During the early morning hours, when Duke's large units are producing more power than the system needs,
power is fed into the generators. The generators then function as electric motors, turning the turbines.
Operating in reverse, the turbines function as pumps and return water to the upper reservoir. Water is then “stored” in the upper reservoir until another period of peak demand.
A pumped-storage plant typically consumes more energy than it produces. The power consumed in the pumping mode is greater than the power produced in the generating mode. The key is the time of day—and meeting the peak demands placed on the system.
THE TECHNICAL DETAILS
The Bad Creek Hydroelectric Station is being carved out of the Blue Ridge Mountains of northwestern South Carolina. A paved access road snakes across the 1,650-acre site, stretching 4.7 miles from the site entrance at S. C. Highway 130 to the shore of Lake Jocassee. Elevation at the site entrance is 2,6oo feet above sea level while at Lake Jocassee it is 1,120feet.
The road, with its steep grade, hairpin curves and occasional rock slides, has ramps for runaway trucks. Trucks and construction equipment are equipped with special brakes that rely on compression from their own engines to slow their descent.
South Carolina's mountains are cooler than the state's lowlands, but are still moderate. Bad Creek's southern exposure further moderates the winter weather, Even though heavy snows are infrequent, progress was slowed in 1988 when 20 inches of snow blanketed the project.
Figure 1. The typical daily demand cycle showing how excess power in the demand troughs can be used to pump water into the upper reservoir of a hydro storage facility.
Figure 2. A simplified diagram of a pumped-storage hydro facility. Water is pumped into the upper reservoir at night and released to generate peaking power during the day.
Heavy precipitation of any sort— rain, snow or sleet—slows progress. Mud roads become treacherous, the soil too soggy to meet compaction requirements for the dams. Over the last 20 years, the site has averaged about 84 inches of precipitation annually. The driest year since construction began was 1987, when 56 inches of moisture fell. The wettest year on record was 1989, with 118 inches of precipitation—including 30 inches between mid-June and mid-July.
Looking north in the powerhouse cavern on February 22, 1988. Hydraulic truck cranes of 50 and 250 ton capacity were being used at this stage. The cavern is nearly as large as one-half of a modern flexible roof stadium such as the Pontiac Silverdome.
Bad Creek's upper reservoir required damming two small streams and building a dike. The main dam, crossing Bad Creek, stretches more than a half mile across and is 360 feet high. The west dam crosses West Bad Creek, and the east dike fills a low area between the hills on one side of the lake.
The three embankments required about 13.8 million cubic yards of fill (earth core, rock shell). The main dam alone required more than 12.3 million cubic yards.
Most of the water for the upper reservoir will come not from the small streams, but from the plant's lower reservoir, Lake Jocassee. Lake Jocassee, created in the 1970s to serve as the upper reservoir for another pumped-storage station, covers 7,565 acres at full pond and has a maximum drawdown of 30 feet.
The upper reservoir will cover 313 acres at full pond and have a maximum drawdown of 160 feet. This will result in a high head pressure for generating a lot of watts but for only a short cycle time. At full pond, the reservoir can supply enough water for about 24 hours of operation. It will take about 32 hours to refill the reservoir.
The powerhouse, which contains the four generating units, is completely underground. It required excavating a cavern 164 feet high, 75 feet wide and 433 feet long. More than 2.75 miles of tunnels and shafts have been built both to carry water when the plant becomes operational and to provide access during construction.
In the generating mode, water will drain from the upper reservoir through an intake shaft. The intake shaft, 55 feet in diameter tapering to 29.5 feet, drops 856 feet to the power tunnel. The power tunnel, 29.5 feet in diameter, carries the water at a 7 percent grade almost three-quarters of a mile to a manifold tunnel. The diameter of the 470-foot-long manifold tunnel tapers from 29.5 feet to 13.8 feet.
Four penstocks, each 13.8 feet in diameter tapering to 8.4 feet, carry water from the manifold to the plant's four pump/turbines. After passing through the turbines, water flows out four draft tubes, each 16.4 feet in diameter and averaging 316 feet long. The draft tubes converge into two tailrace tunnels, which carry the water out to Lake Jocassee. Each tailrace tunnel is 875 feet long and 26.25 feet in diameter.
Water flow follows the same path in the pumping mode, except in reverse.
Access to the powerhouse is through a main access tunnel or a vertical access shaft. The main access tunnel is 1,198 feet long, 29.5 feet wide and 26.2 feet high—just large enough for truck traffic. The vertical access shaft is 544.5 feet deep and 29.5 feet in diameter. The shaft extends upward from the powerhouse to the equipment building at surface level.
The equipment building, located in the transformer yard, houses control systems for monitoring and operating the plant. It receives power from the powerhouse, regulates the voltage and connects the plant to the Duke system.
The mountainous terrain holds few sites suitable for a construction yard. The construction yard contains temporary offices, shops, warehouses, material storage yards and parking lots.
Looking north in the powerhouse cavern on February 6, 1989. In the rear can be seen the superstructure over Unit #1. Forward from it can be seen the completed spiral case of Unit #2. In the foreground, the spiral case is being placed over the discharge ring of Unit #3. Above is the 225-ton bridge crane and, above it, the temporary 10-ton dome crane. To the left between and above Units #2 and #3, is the cantilever platform used for unloading concrete trucks which have received ready mixed concrete through the vertical access shaft.
The site chosen for the construction yard is about halfway down the mountain. Duke employees park at the construction yard. Then they take a company bus 2.5 miles to the main access tunnel, about a 10-minute ride. Then they take a 5-minute walk down the quarter-mile-long tunnel to the powerhouse. Coming out is a bit tougher, because the tunnel slopes up from the powerhouse at about an 8 percent grade.
Bad Creek is an hour's drive from the Greenville, S. C., metropolitan area. The lack of a large urban center closer to the project has hampered recruiting.
GEOLOGY AT BAD CREEK
When Duke Power Co. decided to build the Bad Creek pumped-storage station, engineers began an extensive subsurface investigation program at the site. They tested hundreds of core samples, developed a test quarry and -excavated a pilot tunnel a quarter of a mile into the mountain. They found a few surprises.
The Bad Creek Project, named after a tiny stream the Cherokees said was unfit to drink, lies in the Blue Ridge geologic province. Duke geologist Malcolm Schaffer says, “The cover rocks in this province are between 600 and 700 million years old. The basement rocks are anywhere from 1 billion to about 5 billion years old.
“The Bad Creek site itself is located in what is known as the Toxaway Dome. The basement rock here is called Toxaway gneiss and its age is 1.2 billion years old. Most of the underground structures are built in this Toxaway gneiss.” Toxaway gneiss is a banded rock, deeply weathered on the surface but generally sound underneath.
Though the testing was extensive, it wasn't possible to test every square inch of a 1,650-acre site. As the earthwork and tunneling got underway, no one could predict exactly what would be found down below.
Duke's only other pumped-storage facility, the Jocassee Hydroelectric Station, was completed in the early 1970s. It is just a few miles from the Bad Creek site. Lake Jocassee, which serves as the upper reservoir for the Jocassee station, serves as the lower reservoir for Bad Creek.
At the Jocassee project, rock was blasted in a quarry and moved directly to the dam. The rock required very little processing. It was hoped that a similar situation would exist at Bad Creek. It didn't work out that way. Because of natural fracturing, rock in the Bad Creek quarry tended to shatter when blasted.
“It's good quality rock,” says Bill Lindsay, Bad Creeks geotechnical engineer. “But we weren't able to blast it without producing fines.” Schaffer adds, “It is such a hard, brittle rock. Either you bust it up real good or you don't bust it up at all.”
“Fines” are pieces of rock too small to meet Bad Creeks gradation requirements for the shell. At Jocassee, the dam was not designed for free drainage. But because Bad Creek's upper reservoir will draw down so quickly, the rock shell of the Bad Creek dam was designed for free drainage-without fines. The result was that more rock had to be processed at Bad Creek than at Jocassee. To avoid delays at Bad Creek, Duke added a larger rock crusher and expanded its processing operation.
The main dam held other geologic surprises. An earth slide interfered with the work on the west abutment of the dam. Workers also uncovered a large formation of fractured rock on the dam's west abutment. Called “blocky rock” because of its appearance, this fractured rock was the remnant of an ancient landslide.
To halt the earth slide, Duke placed a rock buttress against the west abutment. The blocky rock was bolted to the dam foundation. Voids in the rock were filled with concrete to prevent water paths.
The project's most serious geological problem lay below the surface, in the powerhouse cavern. Excavating the powerhouse released enormous stresses that had built up inside the mountain over millions of years. This caused the powerhouse crown to crack. Shotcrete, used to line the rock surface, began to peel off, causing a hazard to the workers below.
To repair the crown, Duke had to scale off loose material, install more than 1,000 rock bolts and add a steel safety net.
PROTECTING THE ENVIRONMENT
Unlike many construction companies, Duke does not leave after the project when it is completed. The company has operated in the Carolinas since the early 1900s. Its employees live in the service area. They use Carolina parks, forests, rivers and lakes for recreation. Most are active in their communities.
A cross-section through the powerhouse and one of the generating units showing elevations in meters.
In the 1920s, long before federal regulations to protect the environment, Duke Power set up a biological sciences group to watch over its waterways. Today, at the Bad Creek project, Duke continues its strong commitment to protecting the environment.
Minutes from the site entrance, just over the North Carolina line, visitors can hear the roar of Whitewater Falls, the highest cascade in eastern North America. Laurel, hemlock and other flora common to the southern Appalachians abound in the area.
Rare plant species can be found here, too, such as the beautiful Oconee Bells. For their protection, some of these plants were transplanted early in the project.
Bear, deer, turkeys and other wild animals roam the hillsides. But hunters must watch their step, to avoid the area's copperhead and rattle snakes.
Lake Jocassee, like other lakes created by Duke Power, attracts boating and fishing enthusiasts from miles around. It's especially noted for its record size trout.
Trout thrive in bands of cold water. To protect the trout, Duke built an underwater weir in the lake, a few hundred yards out from the Bad Creek discharge. Made of rock blasted from the tunnels, this weir forces the warm discharge water to the surface, where it loses its heat, leaving the trout undisturbed.
Duke also has protected the area's lakes and streams by erecting more than 27 miles of silt fences to block erosion during construction.
To protect one stream, Duke built a pumping station to divert the stream around the main dam. Duke environmental engineer David Meachum says, “Stream augmentation is strictly environmental. It was put thereto compensate Howard Creek for flow that we will be impounding in the dams. “Howard Creek is one of the few trout streams that originate in South Carolina. Most of the trout streams in South Carolina originate in North Carolina.”
Bad Creek's upper reservoir will drain too quickly to be safe for recreation, so it won't be open to the public. But Duke has offset this by building a public hiking trail through the mountains. Duke's 43-mile-long Foothills Trail links trails maintained by the S.C. Park Service and U.S. Forest Service. Together they form an 80-mile trail between the Table Rock and Oconee state parks in South Carolina.
Duke's onsite environmental group monitors compliance with government regulations and the company's own standards. This group has emphasized the control of hazardous substances and conservation of the area's natural environment. Duke tries to disturb as little area as possible and asks employees to leave their work sites in better condition than they were found.
Duke has spent over $25 million to protect the environment at Bad Creek and has been recognized by environmental groups. In 1984, Duke became the first corporation to win the National Wildlife Federation's Conservation Achievement Award. In 1986, South Carolina's Land Resources Commission commended Duke for its erosion and sedimentation control measures at Bad Creek, And in 1987, Duke received the Merit Award from the Soil Conservation Society of America for erosion control at Bad Creek.
BAD CREEK PROJECT ORGANIZATION
Duke's Bad Creek Project is headed by an “overall manager,” with the authority and accountability for managing all aspects of the project. The overall manager reports directly to an executive vice president rather than to a department head, Each of the departments involved with the project has assigned a representative to work with the overall manager. The overall manager coordinates their work and verifies their solutions to technical problems. These are the major departments involved with the project:
- Design Engineering designs the facility,
- Construction and Maintenance builds the facility,
- Operating reviews the plant's design and construction to make sure it will be reliable and easy to maintain and will operate the plant upon completion,
- Transmission designs and installs the systems needed to tie the plant in with the company's distribution system,
- Quality Assurance provides testing and inspection services as required by design specification or as requested by the construction organization,
- Purchasing procures materials,equipment and services that meet quality, quantity and schedule requirements at the lowest possible cost, and
- Project Control makes sure department schedules are integrated and reports project status for executive review.
As at its other divisions, CMD is organized along functional lines at Bad
Creek. At the heart is the craft organization, which does the physical work. The other groups supporting the crafts include:
- Engineering interprets design requirements and provides other technical support to the crafts.
- Human Resources is responsible for personnel services, such as employment, employee relations, training and safety.
- Materials and Equipment makes sure the crafts have the materials, tools and equipment they need—at the right time and place.
- Planning and Cost Control works with the other groups to plan, schedule and budget for the division's work,
CMD also has two staff positions at Bad Creek that it doesn't have at its other divisions. One is a geotechnical engineer who provides technical support to the tunneling and earthwork contractors. The other is a contract administrator who represents Duke's interests in dealing with the contractors on site.