Pumped Storage Hydroelectricity (Energy Engineering)

Abstract

Pumped storage hydro is a form of hydroelectric power generation for electric utilities that incorporates an energy storage feature. The fuel, water, moves between two reservoirs—an upper and a lower—with a significant vertical distance between them. Water is stored in the upper reservoir until such time as the utility determines that it is economic to use the water to produce electricity for the system, usually to keep coal-fired and nuclear power plants operating at economic levels during low load periods, such as at night and on weekends. Pumped storage is the most widespread energy storage system in use on power networks and is used for energy management, frequency control, and provision of reserves. Efficiency of any specific pumped storage facility, which is primarily dependent on the height between the upper and lower reservoirs, ranges from 70 to 85%.

INTRODUCTION

Pumped storage hydro is a form of hydroelectric power generation for electric utilities that incorporates an energy storage feature.[1] The fuel, water, moves between two reservoirs—an upper and a lower—with a significant vertical separation (see Fig. 1). Water is stored in the upper reservoir until such time as the utility determines that it is economic to use the water to produce electricity for the system. The water in the upper reservoir is stored gravitational energy.[2] When the water is released, the force of that water spins the blades of a turbine that connect to a generator, which produces electricity.[3]


Water is generally released from the upper reservoir to produce electricity during the daylight or on-peak hours. After passing through the turbines, the water is discharged into the lower reservoir. At night and on weekends, or during light-load or off-peak hours, water is pumped from the lower reservoir into the upper reservoir by the turbines, which have been reversed to work as electric motor-driven pumps.

In the upper reservoir, the water is essentially stored energy. Water can be stored for a long or short time in the upper reservoir, depending on the needs of the utility. A vertical separation of at least 100 m (328 ft) is necessary to make a pumped hydro facility economic.[4] The height difference between the upper and lower reservoirs is called the head. The amount of potential energy in the water is directly proportional to the head, so the greater the height, the more energy that can be stored.[1'5]

Pumped storage hydro was first used in Italy and Switzerland in the 1890s. In 1929, the first major pumped storage hydroelectric plant in the United States, Rocky River, was built in New Milford, Connecticut. By 1933, reversible pump-turbines with motor generators had become available. The turbines could operate as both turbine-generators and in reverse, as electric motor-driven [6,7] pumps.

About 3% of total global generation capacity—more than 90 GW—is pumped storage capacity. In 2000, the United States had 19,500 MW of pumped storage facilities in operation.

The largest pumped storage facility in the United States is in Bath County, Virginia, which has a capacity of 2,100 MW. Pumped storage plants are characterized by long construction times and high capital expenditures.1-6’8’9-1

Pumped storage is the most widespread energy storage system in use on power networks and is used for energy management, frequency control, and provision of reserves. Efficiency of any specific pumped storage facility, which is primarily dependent on the height between the upper and lower reservoirs, ranges from 70 to 85%.

Environmental issues associated with the construction of pumped storage plants often parallel those for conventional hydroelectric plants and range from water-resource and biological effects to potential damage to archaeological, cultural, and historical sites.[6]

HYDROELECTRIC GENERATION

In conventional hydroelectric generation, hydraulic turbines rotate due to the force of moving water (its kinetic energy) as it flows from a higher to a lower elevation. This water can be flowing naturally in streams or rivers, or it can be contained in manmade facilities such as canals, reservoirs, and pipelines. Dams raise the water level of a stream or river to a height sufficient to create an adequate head (height differential) for electricity generation.[10]

Pumped storage hydro configuration.

Fig. 1 Pumped storage hydro configuration.

If the dam stops the flow of the river, water pools behind the dam to form a reservoir or artificial lake. As hydroelectric generation is needed by the electric utility, the water is released to flow through the dam and powerhouse. In other cases, the dam is simply built across the river, and the water moves through the power plant or powerhouse inside the dam on its way downstream.[11]

In either case, as the water actually moves through the dam, the water pushes against the blades of a turbine, causing the blades to turn (see Fig. 2). The turbine converts the energy in the form of falling water to rotating shaft power to turn a generator, which produces electricity.[11'12] The selection of the best turbine for any specific hydroelectric site is primarily dependent on the head (the vertical distance through which the water falls) and the water flow (measured as volume per unit of time) available. Generally, a high-head plant needs less water flow than a low-head plant to produce the same amount of electricity.[12,13]

The power available in a stream of water is

P = hPghV

where: P, power (J/s or watts); 7], turbine efficiency; p, density of water (kg/m3); g, acceleration of gravity

Hydroelectric turbine-generator.

Fig. 2 Hydroelectric turbine-generator.

(9.81 m/s2); h, head (m; the difference in height between the inlet and outlet water surfaces); V, flow rate (m3/s).[14]

For SI units, this equation can be roughly approximated as

POWER (kW) = 5.9 X FLOW X HEAD

where FLOW is measured in cubic meters per second and HEAD is measured in meters.[15] In general terms, for IP units, 1 gal/s falling 100 ft can generate 1 kW of electrical power.[16]

Hydroelectric power is generally found in mountainous areas where there are lakes and reservoirs and along rivers. Hydroelectric power currently provides about 10% of all the electricity produced in the United States. Hydroelectricity provides about one-fifth of the world’s electricity. Worldwide capacity is 650,000 MW.[11,16]

Hydroelectric power is characterized as a renewable resource. The fuel, water, is replenished by rain and snowfall. With an existing plant, it is the cheapest way today to generate electricity. Producing electricity from hydropower is so economical because when the dam is built and the equipment has been installed, the flowing water has no cost. In addition, the dams are very robust structures, and the equipment is relatively simple mechanically. Hydro plants are dependable and long lived, and their maintenance costs are low compared with those of most other forms of electricity generation, including fossil-fired and nuclear generation. Many hydroelectric plants do not even require staff, further contributing to their low ongoing maintenance costs.[13]

Pumped storage hydro generation is a specific kind of hydroelectric generation requiring an upper and lower reservoir, and special equipment that can both generate power as water flows downhill and then reverse and serve as a pump to move the water back uphill.

FACILITY DESCRIPTION

A pumped storage hydro power plant typically has three major components: the upper reservoir, the lower reservoir, and the pumping/generating facilities. The pumped storage facility represented in Fig. 3 is Duke Power Company’s Jocassee plant in South Carolina.

The water from the upper reservoir used for electricity generation is allowed into the intake shaft through the opening of the headgates. Water moves through the high-pressure shaft and steel-lined power tunnel until it reaches the turbines in the powerhouse.[1] The water turns the turbines, which then drive the generators to produce electricity. Then the water moves through the tailrace tunnel until it is discharged into the lower reservoir, which in Fig. 3 is Lake Jocassee. Water discharge capacity in the upper reservoir can require several hours to several days.[6]

Pumped storage facilities can be categorized as “pure” or “combined.” Pure pumped storage plants, also referred to as modular pumped storage or MPS, continually shift water between an upper and a lower reservoir. Combined pumped storage plants also generate their own electricity like conventional hydroelectric plants through natural stream flow. Modular pumped storage systems tend to be much smaller than conventional hydroelectric power plants. They use closed water systems that are artificially created instead of natural waterways or watersheds. The water for MPS usually is put into the system only when it begins operation, either from groundwater or possibly from municipal wastewater.[8,10]

When water is to be pumped back into the upper reservoir of a pumped storage plant, it flows from the lower reservoir into the powerhouse, where reversible pump-turbines (usually, a Francis turbine design) pump it back into the upper reservoir. The hydraulic units are designed to operate as pumps when rotating in one direction and as turbines when rotating in the opposite direction. Similarly, motors that drive the pumps can be reversed to act as generators. Fig. 4 contains a schematic diagram showing the directions of water flows and rotation of the pump-turbine for each mode of operation.[8,9]

Many turbines used at pumped storage plants are Francis turbines, named for American engineer James B. Francis (1815-1892), who worked to enhance turbine design. Francis turbines are capable of applications of 2-800 m (approximately 6.5-2,600 ft). The turbines are individually designed for each site to operate as efficiently as possible and to match each site’s flow conditions. These turbines may be designed for a wide range of heads and water flows. Francis turbines are very expensive to design, manufacture, and install, but they operate for decades.[14,17-19]

The efficiency of pumped storage plants generally ranges from 70 to 85%. This means that 70%-85% of the electrical energy used to pump the water into the upper reservoir is actually generated when water flows back down through the turbines to the lower reservoir. The losses of energy occur due to evaporation from the exposed water surface (in both the upper and lower reservoirs) and to the mechanical efficiency losses during conversion from flowing water to electricity. In hot climates or windy areas, the evaporation losses will be higher. Similarly, mechanical losses are higher with older equipment.[8]

HOW PUMPED STORAGE IS USED BY UTILITIES

Pumped storage hydro serves an important function in the portfolio of resources available to utilities. They provide the most significant and most cost-effective means of storage of energy on a scale useful for a utility. It is not usually feasible and/or economic for utilities to turn down or reduce load on large nuclear and coal-fired (so-called thermal) units during low load periods—generally, at night. Using electricity from these thermal units to provide the power needed to pump water into the upper reservoir allows the utility to have the water to use during higher load periods the next day to generate electricity through the turbines of the pumped storage power plant and to keep their coal and nuclear units operating at more efficient levels.[5]

Pumped storage hydro is economical because it flattens the variations in load on the power grid, permitting thermal plants (coal-fired and nuclear) that provide electricity to continue operating at their most efficient capacity while reducing the need to build special power plants that will run only at peak demand, using more costly generation methods.1[8]

Fig. 5 shows a typical pattern for pumped storage usage by a utility. Here, power is generated by the pumped storage facility during the higher load hours of 7:00 a.m. to 10:00 p.m., and pumping (which is usage of electricity from the grid) occurs in the off-peak hours.

In addition to energy management, pumped storage systems are important components in controlling electrical network frequency and in providing reserve generation. Thermal plants are much less able to respond to sudden changes in electrical demand, which cause frequency and voltage instability. Pumped storage plants can respond to those changes in seconds. This is particularly desirable in the case of a unit’s becoming unavailable or forced out of service or on utility systems with high amounts of intermittent resources, such as solar and wind. The Dinorwig pumped storage facility in north Wales, United Kingdom, for example, can go from 0 MW to full capacity of 1,320 MW in 12 s and usually can stay at this level until other generating units on the utility’s system can be brought online.[1'4'8]

Pumped storage power plant.

Fig. 3 Pumped storage power plant.

Schematic of pumped storage hydro operation.

Fig. 4 Schematic of pumped storage hydro operation.

COST AND ECONOMICS

The cost effectiveness of a pumped storage hydroelectric facility depends on the topography of an area and on the types and sizes of power plants in the electric utility’s system. The capital cost of the facility will be dependent on the height of the head and the geography in the area. Adequate resources need to be available consistently to provide the pumping energy required economically.

Higher head ranges, varying between 300 and 600 m (approximately 1,000-2,000 ft), and relatively steep topography are generally desirable for a cost-effective facility to be designed, built, and operated. Two main parameters affect the costs of pumped storage facilities: the ratio of waterway length to head (l/h) and the overall head of the project. A low l/h ratio will result in shorter water passages and will reduce the need for surge tanks to control transient flow conditions. Higher head projects require smaller volumes of water to provide the same level of energy storage and smaller waterway passages for the same level of power generation. Most desirable pumped storage sites have heads in excess of 300 m (approximately 1,000 ft) with l/h ratio of 6 or less.[20]

New pumped storage projects have not been built in the United States for quite a number of years. In 2006, the Lake Elsinore Advanced Pumped Storage Project is being proposed in California and is under review by the Federal Energy Regulatory Commission. Two separate alternatives have been proposed, including pumped storage and pumped storage plus a conventional hydroelectric facility, at costs ranging from $720 million to $1.3 billion, including the associated required transmission for 5001,275 MW.[21]

The cost effectiveness of the pumped storage power plant during daily operation for an electric utility depends on the cost of the power used to pump the water back uphill, the efficiency of the unit, and the cost of the power at the margin during the utility’s on-peak period. Consider the example of the following hypothetical utility (Utility A), whose level of so-called production costs—costs including only fuel and operating and maintenance (O&M) costs—are as shown.

Power spectrum of a pumped storage power plant.

Fig. 5 Power spectrum of a pumped storage power plant.

Utility A has a peak load of 10,000 MW. Utility A has three 750 MW nuclear power units that generate electricity at a production cost of 1.7 cents/kWh. Additional generation resources include a variety of coal-fired generating units totaling 7,000 MW that have an average production cost of 2.5 cents/kWh. Utility A also has 1,250 MW of natural gas-fired combustion turbines that produce power at an average price of 5.5 cents/kWh. There is also 1,000 MW of conventional hydroelectric capacity on Utility A’s system.[22]

The following table summarizes the resources and the costs for Utility A. Utility A also has a 750 MW pumped storage facility that is 70% efficient (Table 1).

Because the cost of using the pumping energy is significantly lower than the production cost of using natural gas, Utility A can cost-effectively use the nuclear and coal resources at night to pump water back uphill and then use generation from the pumped hydro facility during the day instead of running its natural gas-fired combustion turbine units.

ENVIRONMENTAL ISSUES

Issues related to permitting of conventional hydroelectric and on-stream pumped storage hydroelectric facilities include

• Water-resource impacts: stream flows, reservoir surface area, groundwater recharge, water temperature, turbidity, and oxygen content

• Biological impacts: displacement of terrestrial habitat, alteration of fish migration patterns, and other impacts due to changes in water quality and quantity

• Potential damage to archaeological, cultural, or historic sites

• Visual-quality changes

Table 1 Resources and costs for Utility A
Generation type MW Production cost (cents/kWh) Cost of energy from pumped hydro using this capacity to pump
Conventional hydroelectric 1,000 0 Already used to serve load, not available to use for pumping
Nuclear 2,250 1.7 2.21
Coal 7,000 2.5 3.25
Natural gas 1,250 5.5 Too expensive to use this fuel to pump

Too expensive to use this fuel to pump

• Loss of scenic or wilderness resources

• Increased risks of landslides and erosion

• Gain in recreational resources

These concerns are much less significant for MPS plants, because MPS operate in a closed loop and are not associated with natural waterways and watersheds. Usually, MPS are specifically not located near existing rivers, lakes, streams, and other sensitive environmental areas to avoid the regulatory lag time and complexity associated with combined pumped storage hydroelectric facilities.[23]

CONCLUSION

Pumped storage hydroelectric facilities offer many benefits to utilities, including energy management, frequency control, and the provision of reserves. Utilizing water as a fuel, these facilities now provide about 3% of the world’s generating capacity. These plants are the most economical in large utility systems, where the pumping energy can come from large coal-fired and nuclear units, and can keep those units from having to reduce load at night and on weekends.

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