Applications

Two of the largest and most famous hydropower plants in the world are found at the Hoover and Grand Coulee Dams. These marvels of engineering, construction, and sheer human effort, which were built more than 60 years ago, are testaments to viability of hydropower as a cost-effective, reliable, renewable source of electric energy production, with extremely long service life.

The largest hydropower plant in the United States (though about half the capacity of the largest in the world) is the enormous Grand Coulee Dam in Washington Sate. Figure 14-7 provides an aerial view of the dam and power plant. It has an installed capacity of about 6,800 MW, with 27 Francis turbine-driven units ranging in capacity from 10 to 800 MW The plant also includes an additional 300 MW of pump-generator capacity. With nearly 12

Fig. 14-8 Cross-Sectional View of Grand Coulee Dam and Third Power House Drawn to Scale. Source: Bureau of Reclamation

million cubic yards (9.2 million m3) of concrete, the dam is said to be the largest concrete structure ever built. With a rated head of 330 ft (100 m), this barricade, which raises the water 350 ft (107 m) above the old riverbed, is 5,233 ft (1,595 m) long and 550 ft (168 m) high. The average water release from the dam is 110,000 ft3/sec (3,115 m3/sec). Figure 14-8 provides a cross-sectional view of the dam and third powerhouse, drawn to scale. Notice the massive penstock required to provide the necessary water flow to the turbine-generators.

After operating for some 60 years, with several expansions and renovations along the way, this hydroelectric facility remains highly cost-effective. Annual operations and maintenance (O&M) costs, which represent the majority of the operating costs, range between 2 and 3 cents per kWh produced and, of course, there is no real fuel-cost component. The Grand Coulee Dam project was undertaken as part of the Bureau of Reclamation's Columbia Basin Project, which, in addition to providing hydropower production, was intended to provide flood control, irrigation, recreation, stream flows, and fish and wildlife benefits. FDR Lake, behind the dam, is 151 miles (252 km) long, with over 5 million acre-ft (6.2 billion m3) of active storage. Additionally, 550,000 acres (2,225 km2) of irrigation is provided to the Columbia Basin.

The average annual power generation is about 21 billion kWh. This corresponds to a load factor of about 35%. Since Grand Coulee has such a large generation capacity relative to baseloads connected to the distribution system, Bonneville Power Authority (BPA) varies the output considerably by time-of-use, creating a peaking load profile. In addition to daily output variation, monthly variation is also considerable, ranging from a typical year output of 1.5 million MWh in October to just under 2.5 million MWh in June. The actual availability of the plant has varied from about 80 to 90% in each of the last 10 years.

While less than one-third the capacity of the Grand Coulee, The Hoover Dam, pictured in Figure 14-9, is itself a massive concrete thick-

Fig. 14-9 Aerial View of Hoover Dam and Power Plant. Source: Bureau of Reclamation

arch structure that is 726 ft (221 m) high and 1,244 ft (379 m) long at the crest, with a power generation capacity of more than 2,000 MW with 19 generating units. The Hoover power plant, located in the Black Canyon of the Colorado River, was developed with similar water management objectives as the Grand Coulee, using electricity sales as a means of making the project self-supporting and financially solvent. Floodwaters of the Colorado River are impounded by the Hoover Dam and released in response to downstream water orders. The quantity of water available for release through the power plant is, in part, based upon the water orders. Water for generation is conveyed through four penstocks from four intake structures immediately upstream and contiguous to the dam. Spillway structures use 16 ft (5 m) by 100 ft (30 m) drum gates, which provide for an additional 16 vertical ft (5 m) of storage capacity in Lake Mead, the reservoir impounded upstream of the dam. Lake Mead is the largest reservoir in the United States with a capacity of 28.5 million acre-feet (35.2 billion m3), a length of 110 miles (177 km), a shoreline of 550 miles (885 km), a maximum depth of 500 ft (152 m), and a surface area of 157,000 acres (635 km2). At 576 ft (176 m), the rated head of this plant is fairly high, almost double that of the Grand Coulee.

The ten-year average plant output is greater than 4.5 million MWh annually and has reached as high as 5.8

million MWh. O&M cost is fairly high for a large hydropower plant, ranging from 4 to 6 cents per kWh. While some plants are even higher, many medium- and large-capacity plants have demonstrated significantly lower long-term average O&M costs. Availability of the Hoover plant is very high, at an average of nearly 90% from year to year. Overall load factor on all installed capacity is fairly low at about 30%, with significant seasonal variation. Monthly electricity output generally varies from 300,000 MWh in winter to 500,000 MWh in spring.

While the famous massive hydropower plants draw the most attention, the majority of hydropower plants are much smaller. Many are similar in concept and design, but at a scale of one tenth or one hundredth the size. Projects of much smaller capacity and with rated heads of about 65 ft (20 m) or less are termed low-head plants. These can still be sizable systems of 20 MW or more, though many are under 1 MW of capacity. Low-head dams may often be located closer to where the real electric loads are, reducing the power lost in transmission. They may also be designed as run-of-the river plants, which use power in the river water as it passes through the plant without causing appreciable change in the river flow. These systems generally impound very little water and, in some cases, do not require a dam or reservoir. This reduces the likelihood of water quality changes, such as higher temperature, lower oxygen, increased phosphorus and nitrogen, and increased siltation.

Figure 14-10 shows the Boise River Diversion Dam on the Boise River, about 7 miles southeast of Boise, Idaho. It is a 68 ft (21 m) high rubble-concrete, weir type structure with a hydraulic height of 39 ft (12 m). It was

Fig. 14-10 Boise River Diversion Low-Head Dam. Source: Bureau of Reclamation

originally built to provide irrigation and to supply power for the construction of the nearby Arrowrock Dam. The power plant, shown in Figure 14-11, consists of three vertical 500 kW electric generation units. The plant began operation in 1912 and retains many unique engineering features of the era, including double Francis turbines, wooden turbine bearings, and belt-driven auxiliary systems. When operating, the plant supplies power to irrigation loads and delivers any surplus power to the BPA for marketing and distribution to regional industries and municipalities. The power plant has been placed in standby mode, but is presently targeted for rebuilding. It will be up-rated to 2.1 MW (710 kW each unit) and special care will be taken to restore its historical significance. As part of the rebuilding effort, the plant will be automated and controlled as part of the southern Idaho automation system, which will remotely control several southern Idaho power plants.

On the other end of the spectrum from the Grand Coulee are projects such as the Lewiston Dam and power plant (Figure 14-12), a 40 year old, 350 kW plant on the Trinity River in California. Like the Grand Coulee and Hoover Dams, it was designed with similar multiple purposes. However, in stark contrast, this dam stands only 91 ft (28 m) tall, with a rated head of about 60 ft (18 m), and has a water release discharge rate of 350 ft3/sec (10 m3/sec). Given its age and small capacity, annual O&M costs, which have ranged widely from as low as 3 cents to as high as 8 cents per kWh over the past decade, are considerably higher on average than what is now generally achieved in larger, more modern plants. An impressive statistic of this plant, however, is that both annual load factor and availability factor have been very high, exceed-

Fig. 14-11 Three Vertical 500 kW Turbine Generator Units Installed in 1912. Source: Bureau of Reclamation

Fig. 14-12 Lewiston Dam and Small Capacity Low-Head (350 kW) Power Plant. Source: Bureau of Reclamation

Fig. 14-11 Three Vertical 500 kW Turbine Generator Units Installed in 1912. Source: Bureau of Reclamation

Fig. 14-12 Lewiston Dam and Small Capacity Low-Head (350 kW) Power Plant. Source: Bureau of Reclamation ing 90% and even approaching 100% in many years.

Load factor variation in hydropower plants that have high availability factors may result from seasonal variations in available water flow, permit limitations, and obligated downstream releases. In addition to the need to regulate water flow in the river, storage is intentionally used for enhanced reliability and for shaping of electricity output to match system load requirements. In some cases, hydropower plants are purposely operated as peaking plants and this may be enhanced by pump storage systems. Still, some plants, including many small-capacity, low-head plants, show annual load factors in excess of 90%.

In assessing a potential hydropower application, it is necessary to evaluate the source to determine how much water can be delivered to the turbine and how high the water source is above the turbine. The length of the penstock or feed-in pipe must be considered to calculate friction losses. It is also necessary to distinguish between gross and net head. Gross head is the vertical height from the tailwater level to the intake level when the turbine is stopped. Net head is the head available to drive the turbine when it is running at normal full load, with all losses, such as pipe friction and rise in tail water level, taken into account. In the case of an impulse turbine, the height of the jet center above the tailwater level must also be deducted. With a long pipeline on a high head scheme, or with a restricted tailrace on a low head scheme, the difference between gross and net heads may be as much as 20%. Initial gross head and flow assessment can be done through direct measurement or from previously established sources and maps. For proper equipment selection and system design, it is important to know not only average stream flow, but also minimums and maximums to be expected. Normalized hourly or daily flow data is necessary to make an accurate assessment of expected annual power production and to project economic performance.

Beyond the initial assessment of approximate system capacity and power production, is the extensive environmental assessment. This aspect of project development can take several years and will greatly influence not only if a project can be implemented, but the design constraints under which the project must be developed.

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