Addressing Water Quality, Economic and Environmental Concerns with Hydropower Processes
By Art Haddaway, WaterWorld Editor
When it comes to efficiently producing and managing energy, water has always played an integral role in the process. Used to cool thermal plants, extract and transport raw fuel materials and cultivate biomass feedstock crops, for example, water contributes to many forms of power generation, especially concerning the area of hydropower.
Hydropower is the process of producing power from the energy created by moving water flowing through rivers, lakes and streams that's captured in specifically-designed dams and facilities. Serving as the fundamental element to nearly all aspects of operation, this water is utilized to rotate large turbines, which in turn direct energy to central generators that ultimately produce power for various residential, municipal and industrial purposes.
Hydropower isn't new -- in fact, it's often regarded as one of the earliest known forms of producing power. Compared to more conventional methods of power and electric generation, such as fuel, coal, natural gas, or nuclear energy, hydropower is one of the cleanest and most renewable sources of energy today. According to the U.S. Geological Survey (USGS), it produces about 7 percent of the nation's total power and represents about 19 percent of total electricity production throughout the world.
In the U.S., most hydropower is produced at large facilities built by the federal government, with the majority of the largest dams in the western region of the country, as indicated by the U.S. Energy Information Administration. It noted, for example, that the states of Washington, Oregon and California comprise over half the nation's hydroelectric capacity for electricity generation, with Washington generating approximately 29 percent of total U.S. hydropower in 2013, where the Grand Coulee Dam -- the country's largest hydroelectric facility -- is located.
A growing, stable and more environmentally friendly source of energy, hydropower offers a number of advantages, including reduced greenhouse gas emissions, low maintenance and operating costs, near immediate output, and little to no waste byproducts, among others. Further, as a domestic source of energy, hydroelectricity reduces reliance on international fuel sources, and these dams can also improve overall flood control, irrigation and water resources, according to the U.S. Department of Energy.
There are, however, a number of economic and environmental challenges associated with hydropower, especially with regard to water quality issues. While the hydropower process does not necessarily require the captured water to be of a particular quality when used to generate electricity, it has the potential to alter the state of the water discharged back into the waterway. This could cause changes to the temperature of the stream or river, increase dissolved oxygen (DO) levels and boost the presence of nutrients such as nitrogen and phosphorus, to name a few.
Other issues can include thermal stratification, occurring within the hydro project's waterbody or storage area as a result of surface water warming; total dissolved gas (TDG) supersaturation, arising when water spilled over the dam accumulates nitrogen-filled air and causes instability in the waterway; and water level/flow variations, or inundations, stemming from process infrastructure and peaking energy demands. These issues can ultimately affect the chemical makeup of the waterway, potentially posing a threat to native fish and wildlife; riverbeds, riparian zones and the surrounding environment; and overall public health.
"When water is taken from the bottom of the reservoir and used to generate electricity, it can and often does have water quality issues that are of concern," said Gerrit Jöbsis, southeast regional director for American Rivers (Columbia, S.C.). "Additionally, you can have constituents that are liberated from the sediments because of the anaerobic conditions, and they then get discharged downstream." This is of particular concern if the sediment contains toxins.
According to the Foundation for Water & Energy Education (FWEE; Spokane, Wash.), ecosystem impacts are dependent upon varying factors that involve, for example, the size and flow rate of the waterway; existing climatic and habitat conditions; size, design, and operation of the project; and upstream or downstream location in relation to other projects. Moreover, different types of hydropower projects can also affect the quality of the water and how it's used. These include:
- Impoundment facilities, which store stream or river water in a reservoir
- Run-of-river projects, which allow water to pass through a dam at about the same speed as the river naturally flows
- Pumped storage, which essentially recycles water by transferring (pumping) water to a reservoir above a dam when electricity demand is low, then releases the water to generate power when demand is high
- Diversion facilities, which channel portions of the waterway through a canal
Given the degree to which water is used during the hydropower process, it is essential to properly regulate both the influent and effluent, as it not only impacts the hydropower process but also other important resources. "The fuel for [these dams] is a public resource -- the water," said Jöbsis. "The utility has to meet the public's overall needs for that water, not just the hydroelectric power needs."
To manage water-related challenges -- such as temperature variations and increased DO -- hydropower facilities can consider certain effective solutions. For example, Jöbsis noted that the temperature of discharges can often fluctuate based on the depth at which operators draw water from a stratified reservoir. Water is colder at deeper levels and warmer closer to the surface, especially during the summer. "One way to deal with temperature is to either combine some of the warmer surface water with the colder, deeper water or have variable level intakes that would allow you to adjust the temperature as it's discharged," he said. On the contrary, the water is well-mixed during the winter, minimizing problems associated with stratification.
With regard to preventing DO, Jöbsis noted that oxygen injection is a viable option, where liquid oxygen is added to the water as it flows through the turbine. Forebay oxygenation is another strategy, whereby diffuser systems are used to aerate large volumes of water to handle fluctuating levels of power generation. Another technique, turbine venting, allows air to mix with the water as it is being passed through the turbines. "Turbine venting can add one, two or three milligrams per liter of oxygen generally, sometimes up to four," he said, "but if you have to go beyond that, then something like the oxygen injection or forebay oxygenation would be needed."
As a whole, hydropower facilities should base these solutions around three distinct strategies: protection, mitigation and enhancement, according to Andy Dunau, executive director of FWEE. Hydroelectric projects can better protect watersheds and wetlands by identifying and preserving areas within them that are ecologically important. Through mitigation, they can also buy land, restore habitats or build hatcheries, for example, to address declining fish populations. Further, enhancement projects can involve creating more fish passages, changing water flow conditions or stabilizing stream banks with vegetation.
Focusing on these three areas, "you get a mix of what can be done at the project itself, whether it's the turbine or spillway area or fish bypass system, to improve the environmental situation, and what strategies can be done offsite to mitigate, protect or enhance to address effects," said Dunau. "You're going to see more demands to move water through the system in a way that's fish- and habitat-friendly -- you have to balance out what we might be losing in power versus what we might be gaining in environmental mitigation."
Hydropower is likely to remain a profitable and efficient source of energy for certain parts of the country, particularly in the west, but Jöbsis said it's unlikely that many major new dams will be built. Rather, he noted, there will probably be a steady increase in the modification of existing dams to make them produce more power, be more energy efficient and use water more effectively.
"There's going to continue to be hydroelectric power generated from our existing dams, and that's going to be done in a way that's compatible with both generation needs and environmental needs of our communities," said Jöbsis.
By rehabilitating older dams, he said, we can "get the improvements put in place that can meet the overall public benefits a river should provide. There is an opportunity to make a really big-scale change."
