[Below are excerpts from this 45 page document. Dams not only provide power but also water for agriculture, drinking water, cooling water for thermal power plants, ecosystem health, fisheries, and so on. All dams have a finite lifespan of 50 to 200 years due to siltation and the limited lifespan of concrete. Within the next 20 years, 85% of U.S. dams that cost taxpayers $2 trillion dollars will have outlived their average 50-year lifespan.]
Figure 1: Top 10 Hydropower-Generating States and Their Reliance on Hydro Sources for Electricity, 2009, total hydroelectric power generation 273 MWh. These states together produce more than 80% of the Nation‘s total hydroelectric power.
ID 80% WA 71% OR 59% MT 35% NY 21% CA 14%
TN 11% AL 8% AZ 6% NC 4%
The U.S. Department of Energy (DOE) and the U.S. Department of Homeland Security (DHS) collaborated to examine the interdependencies between two critical infrastructure sectors – Dams and Energy. The study highlights the importance of hydroelectric power generation, with a particular emphasis on the variability of weather patterns and competing demands for water which determine the water available for hydropower production. In recent years, various regions of the Nation suffered drought, impacting stakeholders in both the Dams and Energy Sectors. Droughts have the potential to affect the operation of dams and reduce hydropower production, which can result in higher electricity costs to utilities and customers. Conversely, too much water can further complicate the operation of dams in ways that can be detrimental to hydropower production and to the infrastructure of the dams.
The requirements for providing sufficient water for irrigation, environmental protection, transportation, as well as community and industrial uses are already in conflict in many places. Low water conditions (e.g., drought) and high water conditions (e.g., flood) resulting from extreme weather variability can strain the operation of dams.
Although hydroelectric facilities are a type of asset that falls under the auspices of the Dams Sector, they are also an important element to the Energy Sector because the electric power they generate is critical to maintaining the reliability of the Nation‘s electricity supply.
The National Infrastructure Protection Plan (NIPP) provides an overarching framework for the protection and resilience efforts for the Nation‘s 18 critical infrastructure sectors.
DOE and DHS support and coordinate the protection and resilience activities for the Dams and Energy Sectors‘ critical infrastructure as defined below: Dams Sector assets include dam projects, hydropower generation facilities, navigation locks, levees, dikes, hurricane barriers, mine tailings and other industrial waste impoundments, and other similar water retention and water control facilities. Energy Sector, as delineated by Homeland Security Presidential Directive 7 (HSPD-7), includes the production, refining, storage, and distribution of oil, gas, and electric power, except for hydroelectric and commercial nuclear power facilities.
Chief among these concerns is the fact that hydroelectric power generation is affected by extreme fluctuations of water flow, as well as long-term issues surrounding the management and uses of water supply to generate hydroelectricity. In recent years, various regions of the Nation suffered droughts affecting stakeholders in both the Dams and Energy Sectors.6 Although recent drought conditions have not caused a serious problem in terms of electricity supply and reliability, they have the potential to affect the operation of dams by decreasing hydropower production,
The report investigates how different variables might affect the operation of hydroelectric facilities and the supply of hydroelectric power, especially in times of drought and other extreme weather events. Such variables include: The relationship between hydroelectric power generation and the variability of hydrology and weather patterns; Operation of major reservoirs and streamflow regulations at these reservoirs; and Management for flood control, fish habitat protection, and power generation.
Importance of Hydroelectric Dams for Power Generation
Historically, hydroelectric sources have been a vital source of electric power generation that accounted for as much as 40% of the Nation‘s electricity supply in the early 1900s. Although the share of hydropower generation has declined to 7% of the U.S. total electric power generation as production as other types of power plants grew at a faster rate, hydroelectric dams remain an important power source. Hydropower is critical to the national economy and the overall energy reliability.
- Hydroelectric sources produce 7% of the U.S. total annual electric generation.
- Hydroelectric generating capacity constitutes 8% of the U.S. total existing generation capacity.
- The top ten hydropower-generating States produce more than 80% of the U.S. total hydroelectric generation.
- The 20 largest hydroelectric dams produce almost half of the U.S. total hydroelectric generation.
- Hydroelectric power generation has declined in most parts of the country during the 2007-2009 period compared to the historical average.
Hydropower is important because it’s:
- The least expensive source of electricity, as it does not require fossil fuels for generation;
- An emission-free renewable source, accounting for over 65% of the U.S. total annual net renewable generation;
- Able to shift loads to provide peaking power (it does not require ramp-up time like combustion technologies); and
- Often designated as a black start source that can be used to restore network interconnections in the event of a blackout.
Hydropower serves an essential purpose of enhancing electric grid reliability, and can rapidly adjust output to meet changing real time electricity demands and provide black-start capability to help restore power during a blackout event. Black start capability is defined as the ability to start generation without an outside source of power. Because hydropower plants are the only major generators that can dispatch power to the grid immediately when all other energy sources are inaccessible, they provide essential back-up power during major electricity disruptions such as the 2003 blackout. With black start capability, hydropower facilities can resume operations in isolation without drawing on an outside power source and help restore power to the grid.
Hydroelectric Power Capacity vs. Generation. As seen in figures 2 and 3, hydropower generation capacity has remained steady in the last 20 years, whereas production from hydro sources has fluctuated dramatically year-to-year. According to EIA, hydropower capacity grew at an annual rate of 0.3 percent or a total of 4,600 megawatts (MW) in the past 20 years (1990: 73,925 MW vs. 2009: 78,525 MW).
The interannual variability of hydropower generation in the United States is very high—a drop of 59 million megawatt hours (MWh) (or 21% of the U.S. total hydropower generation) was seen from 2000 to 2001. Sensitivity of hydroelectric power generation to changes in precipitation and river discharge is high; in the range of 1.0+ (a sensitivity level of 1.0 means that one percent change in precipitation results in one percent change in generation). Although it is evident that precipitation is a determining factor in available hydropower generation for a given period of time, the variability of weather patterns impose uncertainty in the operation of hydroelectric facilities. Hydropower operations are also affected indirectly by the changes in air temperatures, humidity, and wind patterns which change water quality and reservoir dynamics. For example, reservoirs with large surface areas (such as Lake Mead in the lower Colorado River) are more likely to experience greater evaporation, which affects the availability of water for all uses including hydropower. In addition, altering snowfall patterns and associated runoff from snowpack melt are a matter of concern, particularly in the Pacific Northwest, where snows are melting earlier and the proportion of precipitation in the form of snow is decreasing.
A 20-year period from 1990 to 2009 was examined to see the changes in hydropower production at the State level. The results indicate that the national annual average of hydroelectric power generation between 2007 and 2009 was 11 percent less than that of the historical average between 1990 and 2006 in the top 10 hydropower generating states, which all experienced a decline, with certain States losing up to 28% of their normal annual hydropower generation.
Largest Hydro Dams. According to the 2010 Dams Sector-Specific Plan, the total number of dams in the United States is estimated to be around 100,000. However, most dams were constructed solely to provide irrigation and flood control, and only about 2% (or 2,000) of the Nation‘s dams produce electricity.
Table 1 provides a list of the 20 largest hydroelectric dams in the United States ranked by summer capacity as of December 2009. These 20 hydroelectric facilities account for 40% of the Nation‘s hydroelectric power capacity; they provided 44% of the hydropower generated in the United States during the 20-year period from 1990 to 2009. The majority of the 20 largest hydroelectric power plants are located in the Columbia River basin in the Pacific Northwest, all of which experienced decreased production in the 2007 to 2009 time span compared to the historical average between 1990 and 2006.
EIA reports that the largest hydroelectric facility in the United States is the Grand Coulee Dam with a summer capacity of 6,765 MW, located in the Columbia River basin. It is also the largest hydropower producer. To compare the magnitude of the Grand Coulee, the next two largest dams, Chief Joseph and Robert Moses Niagara, each have only about a third of Grand Coulee‘s capacity. Note, however, that the capacity factor at hydro plants varies significantly, generally in the range of 30 to 80%, with an average capacity factor of about 40 to 45%. To illustrate this varied capacity factor of hydroelectric plants, the capacity factor of the Grand Coulee Dam is about 36%, whereas the Robert Moses Niagara Dam has a relatively high capacity factor of 71%.
Table 1. 20 Largest Hydroelectric Dams in the United States Plant Name Owner State
Drought can play a significant role in hydropower production—it can decrease upstream flow and require the diversion or retention of water that would otherwise go to produce electricity or to other water purposes during times of scarcity.
The Columbia River basin is the predominant river system in the Pacific Northwest, encompassing 250 reservoirs and about 150 hydroelectric projects. The system spans seven western States: Washington, Oregon, Idaho, Montana, Wyoming, Nevada, and Utah, as well as British Columbia, Canada.
Today, the Columbia River system operations serve multiple purposes — flood control and mitigation, power production, navigation, recreation, and environmental needs—that are guided by a complex and interrelated set of laws, treaties, agreements, and guidelines. These include the Endangered Species Act, a Federal law that protects threatened or endangered species— protection that can result in setting restrictions on the time and amount of allowed flow and spill—as well as numerous treaties and agreements with Canada dealing with flood control and division of power benefits and obligations.35 Streamflow in the Columbia River system does not follow the region‘s electricity demand pattern in which the peak occurs during winter when the region‘s homes and businesses need heating. Although most of the annual precipitation occurs in the winter from snowfall, most of the natural streamflows occur in the spring and early summer when the snowpack melts. About 60 percent of the natural runoff occurs during May, June, and July (see figure 7). Thus, the objective of reservoir operation is to store snowmelt runoff in the spring and early summer for release in the fall and winter when streamflows are lower and electricity demand is higher.
Hydropower supplies approximately 60 to 70% of the electricity in the Pacific Northwest Region. In the Columbia River system, power generation operations are generally compatible with flood control requirements. However, under the current operating strategy, conflicts between power generation and fish protection are generally resolved in favor of fish protection.
The current strategy requires increased water storage in the fall and winter and increased flows and spill during the spring and summer to benefit migrating juvenile salmon. This approach does not provide an optimal operating strategy for power generation as it results in more water for fish protection, but reduced hydropower generation during the peak demand periods. As a result, BPA is often likely to purchase power frequently during high load periods in the winter and sell surplus power in the spring and summer.
The Pacific Northwest has been affected by widespread temperature-related reductions in snow pack, as well as a changing annual runoff pattern. Recent studies indicate 1) a transition to more rain and less snow and 2) a shifting pattern of snow melt runoff in western North America— contemporary snow melt runoff has been observed 10 to 30 days early in comparison to the period from 1951 to1980. To adapt to these changes, the ability to modify operational rules and water allocations is critical to ensuring the reliability of water and energy supplies, as well as to protecting the environment and critical infrastructure. However, the current set of laws, regulations, and agreements is intricate and creates institutional and legal barriers to such changes in both the short and long term. In 2010, the Pacific Northwest experienced the third driest year in the last 50 years and the fifth lowest water level on record since 1929, causing low runoff in the lower Columbia River. According to BPA‘s 2010 Annual Report, BPA‘s gross purchased 37%, from 2009, mainly due to below normal basin-wide precipitation and stream flows, resulting in insufficient power generation to fulfill load obligations.
Not only droughts, but too much water can also bring challenges to hydropower operation. After a dry winter, spring 2010 river flows were expected to stay fairly low. However, in June 2010, a strong Pacific storm system brought heavy precipitation that almost doubled the stream flows in the Columbia River.45 During the month of June, dam operators faced the challenges of managing flooding and an oversupply of hydropower and, at the same time, complying with Federal regulations for fish protection that restricted the amount of spill allowed. Since water that goes through power turbines does not increase dissolved gas levels, thus maintaining safe conditions for fish, dam operators were forced to produce power for which they could not find a market.46 As a result, BPA disposed of more than 50,000 MWh of electricity for free or for less than the cost of transmission and incurred a total of 745,000 MWh of spill for lack of market in June 2010.47 Figure 10 shows that BPA balancing authority generation significantly exceeded load in early June.
High flows in the Columbia River system are common, resulting from above average snowpack and/or early warming periods that result in rapid snowmelt. However, operating the Columbia River system through those events has become much more complex in recent years due to the following new factors: 1) multiple flow and storage requirements to protect threatened and endangered salmon and steelhead under the Endangered Species Act; 2) changing uses of the transmission system in a deregulated electric power market; and 3) the significant addition of variable, non-dispatchable wind power capacity (3,400 MW as of February 2011) with financial incentives for operation—production tax credits of $21 per MWh and renewable energy credits of $20 per MWh.48
The Colorado River System is considered one of the most legally complex river systems in the world, governed by multiple interstate and international compacts, legal decrees, and prior appropriation allocations, as well as federally-reserved water rights for Native Americans.52 The river basin extends over seven U.S. States— Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming and parts of northwestern Mexico (see figure 11), serving about 25 million people in the Southwest. Its water yield is only 8% of the annual flow of the Columbia River.
In the early 21st century, water use issues intensified as the Colorado River region experienced some of the Nation‘s highest population growth, as well as the start of a long period of drought considered to be the worst drought in the 100-year recorded history (hereinafter referred to as the ?early 21stcentury drought?). The Colorado River region is of particular concern because of the continuing trend of rising temperatures seen across the region that contributes to increased evaporative losses from snowpack, surface reservoirs, irrigated land, and vegetated surfaces.
Lakes Mead and Powell comprise approximately 80% of the basin‘s entire storage capacity.
In October 2010, Lake Mead stood at 39% capacity or 1,084 feet in elevation, curtailing power generation at the Hoover Dam, the region‘s largest hydro facility. For every foot of elevation lost in Lake Mead, Hoover Dam produces 5.7 MW less power. That is because at lower water levels air bubbles flow through with the water causing the turbines to lose efficiency. As a result, electricity available from Hoover Dam declined 29% since 1980, which meant that local utilities had to buy power on the open market where rates were up to four times higher.
The Tennessee River System territory includes most of Tennessee and parts of Alabama, Georgia, Kentucky, Mississippi, North Carolina, and Virginia, serving more than 8.7 million people. TVA manages the Tennessee River and its reservoirs as a whole, regulating the flow of water through the river system for flood control, navigation, power generation, water quality, and recreation. TVA is also the Nation‘s largest public power provider, wholly owned by the U.S. Government; it maintains 29 conventional hydroelectric dams.
On average, the Tennessee Valley gets 51 inches of rain a year, which is more than double the average rainfall in the southwestern United States. Nonetheless, the Tennessee Valley has experienced water shortages during the 2007-2008 droughts that forced communities around the watershed to restrict water withdrawals and take conservation measures. In December 2010, Gary Springston, TVA program manager for water supply, stated that the present situation was still tenuous and ?even systems connected to the Tennessee River system could face conflicts between instream flow needs to support water quality and aquatic life and withdrawals for offstream uses such as public-water supply, industry, thermoelectric power generation, and irrigation. Water supply concerns continue to increase due to population growth and interbasin transfers, especially since the Tennessee River is surrounded by areas that may require more water to accommodate growing needs.
The 2007-2008 droughts in the TVA region were among the worst on record, during which low reservoir water levels caused TVA to lose almost half of its total hydroelectric generation. At the same time, coal prices more than doubled, forcing TVA to rely on additional natural gas purchases to meet electric generation needs while keeping prices as low as possible. Even with the increased reliance on natural gas as opposed to coal, TVA raised rates by 20% in October 2008 to absorb more than $2 billion of increased costs for coal, natural gas, and purchased power costs associated with infrastructure modernization can become an issue. Financial resources to design and implement facility upgrades generally come through public funds and/or power sales for publicly held hydropower infrastructure, and from rate increases approved by public utility commissions for privately held facilities. Although payback periods could be as short as 3-5 years for technology upgrades, securing the initial investment can be challenging. Some owners have received offers from investors and other utility companies to enter into a variety of energy savings performance contracts that would provide the initial investment for modernization in return for a share of the subsequent increased energy production. None of the participants indicated that they were presently involved in such contracts and several raised concerns as to whether they could legally enter into such arrangements.
The potential for technology upgrades at some hydropower infrastructure may also be limited or made more expensive due to the age or physical condition of the facility.
Although operators want to retain as much water as possible in the reservoir for hydropower production, storing it in the reservoir during high water conditions may be hard to manage, as it might impact residences surrounding the reservoir.
Many dams have multiple missions; for some, the requirement for flood control takes precedence over hydropower production. Adherence to this primary mission may require passing high volumes of water through the dam turbines even though there may be low power demand. These increased flows may also require downstream dams to pass through water and not be able to sell the resulting power at a reasonable price. Even if flood control is not a facility mission, owners do their best to avoid or minimize downstream harm when they manage high water conditions. Debris buildup associated with flooding can be dangerous to the facility infrastructure and affect operations. Trees, lumber, sheds, animals, and other debris can be swept into rivers from floods and can build up against dams. The cost and personnel resources required to remove this debris can be significant.
Hydroelectric facilities serve multiple purposes that can include flood control, recreation, industrial and community water supply, irrigation, and transportation. The demands for water for these uses can come into conflict with hydropower production in terms of how much water can be used for nonpower generation and the condition of the water associated with power generation. For multifunction facilities, the combination of existing water rights, treaties, contracts, laws, or court cases determine who gets how much water and when they receive it. Modifying these controlling forces to consider reduced water availability can be difficult because they may involve multiple States and parties, and sometimes, international partners. In addition to these legally binding obligations on water delivery, softer forces, such as providing or storing water to protect recreational uses or the value of residences around the reservoir, can also limit the availability of water for hydropower generation. The condition of the water used in producing hydropower may also be heavily controlled through Federal and State laws and regulations, operating permits and licenses, and court cases related to the protection of natural resources and the environment. These controlling forces may stipulate water conditions such as tail water temperature, streamflow, and dissolved oxygen levels. Operating stipulations are primarily designed to protect species designated as threatened or endangered under Federal or State laws. They may also serve to protect downstream banks, channels, and river branches.
Southern Co. 85 2007 “Georgia Power’s hydroelectric power generation was down 51% in 2007, forcing the company to spend $33.3 million for purchasing coal and oil to replace lost hydropower generation although hydropower sources account for less than two percent of Georgia Power’s generation portfolio.” – Nov. 2007, Atlanta Business
Chronicles Manitoba Hydro86 2003 “A net loss of $436 million was reported in Manitoba Hydro’s 53rd annual report for the fiscal year ending March 31, 2004. The loss was primarily due to the prolonged drought conditions that affected normal electricity production at the utility’s 14 hydroelectric generating stations.” – 2004, Manitoba Hydro
Water is used as the primary coolant in the condensers in both steam and natural gas-fired, combined cycle plants; the amount of water used for cooling in these plants can be significant, depending on the type of cooling system used. Plants that use “once-through” or “open-loop” cooling systems withdraw large amounts of water from nearby surface water sources. This water passes through a condenser as a coolant and, in doing so, transfers heat energy from the hot steam to the coolant water, raising the temperature of the water. After moving through the condenser, the water is released to the original lake, pond, or river source. The increased temperature of the discharge water also increases the rate of evaporation for the body of water. The quantity of water lost from the hydrological system by evaporation caused by elevated temperatures is said to be “consumed.” Closed-loop cooling
Coal Transport by Barge. Transportation on the inland waterways and Great Lakes is an important element of the domestic coal distribution system, carrying approximately 20% of the Nations‘ coal, enough to produce 10% of U.S. electricity annually. Barge transport is often used to transfer coal from the initial source to a railroad, from a railroad to the coal-fired power plant, or the entire distance from the mine to the plant. Barge traffic is particularly important in the Midwestern and Eastern States, with 80% of shipments originating in States along the Ohio River. The amount of waterborne transported coal has remained relatively constant over the last two decades. Barge transport and the amount transported on a single barge are dependent upon the depth of the river on which the barge travels. Reducing the barge load is costly. Losing one foot of draft typically means losing 17 tons of cargo on a single barge and 255 tons on a typical 15-barge tow. In addition, idle tow-boats cost shipping companies $5,000 – $10,000 per day. Droughts have the potential to reduce the rate at which all goods, including coal, can be transported by barge. Some river systems, like the Missouri River, have a system of reservoirs that are used to control river depths. When river levels are low, water is released from the reservoirs to increase river depths and permit barge travel. To mitigate the potential for low water levels to significantly disrupt electric power generation, most coal-burning plants with barge access can also receive coal shipments by rail. However, because barge is the cheapest mode of transportation, utilities pay a higher rate for transportation.
By affecting the availability of cooling water, drought has had an impact on the production of electricity from thermoelectric power plants. The problem for power plants becomes acute when river, lake, or reservoir water levels fall near or below the level of the water intakes used for drawing water for cooling. A related problem occurs when the temperature of the surface water increases to the point where the water can no longer be used for cooling. The Southeast experienced particularly acute drought conditions in August 2007, which forced the shutdown of some nuclear power plants and curtailed operations at others in order to avoid exceeding environmental limits for water temperature. A similar situation occurred in August 2006 along the Mississippi River, as well as at some plants in Illinois and Minnesota.
Thermoelectric freshwater withdrawals accounted for 41% of all freshwater withdrawals in 2005; however, it is important to note that only 3% of the withdrawn water is consumed and the rest is returned to natural flow.
Limitations of the Study. To maintain the focus of the study, this report is limited to issues that specifically relate to electric power generation at hydroelectric dams. Specifically, this study examines issues pertinent to overall management of reservoirs and stream flows at dams that are affected by the variability of weather patterns. In-depth analysis of certain topics considered outside of the scope of the study is omitted from the report. These include: climate change, new hydropower technologies, renewable energy credits, the value of hydropower‘s avoided greenhouse gas emissions, and the effects of reduced hydropower generation on the overall power market. There are three types of hydroelectric power plants: conventional, pumped storage, and diversion facilities. The focus of this report is on the conventional hydroelectric facilities, which are the most common type of hydroelectric power plant. The U.S. Energy Information Administration (EIA) defines a conventional hydroelectric power plant as a plant in which all of the power is produced from natural streamflow as regulated by available storage. Most pumped storage units have closed-loop systems in which water can be stored and reused; therefore, electricity production at pumped storage is more resistant to drought or changing weather patterns. For this reason, the discussion of and data on hydroelectric power generation provided in this report excludes generation from pumped storage, unless noted otherwise.
