Turning the Tide: The Story of Hydropower’s Role in the Renewable Energy
By Allen Huang
Hydropower harnesses the dynamic movement of water to generate electricity and is one of the oldest and most widely used renewable energy sources in the world. It is one of the world’s oldest renewable energy sources, dating back 2,000 years to a time when both the ancient Greeks and Romans used water wheels to grind grain and other tasks.
Today, hydropower is the largest renewable electricity source, generating around 16% of the world’s total electricity. China, Brazil, the United States, Canada, Russia, India, Norway, Venezuela, Sweden, and Japan have all been successful in using hydroelectric power to feed their electricity grid but hydro power comes with numerous points of view, debates and controversies. Let’s take a closer look at hydropower’s development journey and its outlook for the future.
How Does Hydroelectric Power Work?
Hydroelectric power is generated by harnessing the potential energy of water as it flows from higher elevations to lower ones, reminiscent of the dramatic rush of waterfalls. This movement is captured using turbines that are set in motion by the flowing water. As the turbines spin, they drive generators that convert this mechanical energy into electricity.
When electricity is needed, water is released from the reservoir and flows through a penstock (a large group of pipes) to a turbine at a lower elevation.
The moving water spins the blades of the turbine, which is connected to a generator. The generator converts the mechanical energy of the spinning turbine into electrical energy, using an electromagnetic process.
The electricity is then sent through transformers to increase its voltage for efficient transmission over power lines to homes and businesses. Such a process relies heavily on the hydrological cycle, where the sunlight evaporates water downstream, leading to precipitation that flows through rivers, providing a consistent and renewable source of energy.
You can calculate the efficiency of hydroelectric power generation depending on factors such as the flow of water (Q), measured in cubic meters per second; the height or head (H) through which the water falls, measured in meters; and the overall efficiency (η, also known as Eta) of the system, which is usually around 90 percent.
When you multiply these factors together with the gravitational constant (the acceleration that objects experience when falling freely near the Earth’s surface, which is 9.81 meters per second squared), you get a power output measured in kilowatt hours (kWh).
Let’s take an example of a hypothetical dam built on the Hudson River based on this formula. The average flow rate (Q) of the Hudson river is around 600 cubic meters per second. Assuming a dam being built at a 50 meter head (H) and the 90% efficiency (η), this dam will lead to approximately 265,000 kWh, which means this one dam produces enough electricity to power 25 American households for a year in just an hour.
Hydroelectric plants can range in size from small micro-hydro systems generating a few kilowatts to massive installations like China’s Three Gorges Dam, which has a maximum installed capacity of 22.5 gigawatts, equivalent to what 25 households use in a year within an hour.
Regardless of scale, the underlying principle remains the same: converting the gravitational energy of falling water into usable electricity. However, the site-specific nature of hydroelectric power means that each project is designed to fit the local geography, hydrology, and energy needs.
Water has flowed across the Earth for millennia, shaping landscapes and powering human innovation. The energy generated by this movement has long been harnessed to drive various industries, even before the Industrial Revolution, in the form of watermills and aqueducts. While the use of water wheels to grind grain and power machinery dates back thousands of years, the birth of modern hydroelectricity occurred in the late 19th century, during the Second Industrial Revolution.
In 1882, the world’s first hydroelectric power plant began operation on the Fox River in Appleton, Wisconsin, lighting two paper mills and a residence. Early installations, similar to that in Appleton, emerged in regions with access to significant water resources, such as the Alps and the Great Lakes basin in North America.
A key reason for the advancement in hydroelectric power was the evolution of turbines. The transition from water wheels to turbines allowed for greater energy output from higher falls. The development of impulse turbines like the Pelton wheel further enabled hydroelectric projects in high-head environments that waterwheels couldn’t do.
Just a few decades later, hydropower plants were being built at a rapid pace around the world, ushering in a new era of renewable energy. By the 1940s, hydropower accounted for around 40% of electricity generation in the United States.
The pièce de résistance of all this industrial hydroelectric boom was the Hoover Dam on the Colorado River, situated between Nevada and Arizona. Built during the Great Depression as a means to control the waterflow, manage irrigation and produce electricity, its construction took six years and nearly a billion dollars in today’s money. Meanwhile, the Grand Coulee Dam on the Columbia River in Washington State, which was built around the same time, has an installed capacity of 6809 mWh, is the largest dam in North America, and among the largest hydroelectric projects in the world. However, due to the social and environmental costs of building these projects, large impoundment constructions have stopped in the United States for decades.
The post-World War II period saw a significant expansion of hydroelectric power projects, particularly in developing countries. Governments and international organizations recognized the potential of hydroelectric power to provide clean, renewable energy and stimulate economic growth, but it would gradually give way to fossil fuel and nuclear generation despite its continued usage and development to satisfy the need for energy use in sectors such as infrastructure, military and transportation.
Not All Hydroelectric Power Plants Are The Same
There are several types of hydroelectric plants, each suited to different environments. The most common type is the impoundment facility, which is the most common type of hydroelectric dams in the world and the one that many would most easily recognize in photos.
They feature a large dam wall that contains and stores river water in a reservoir. When electricity is needed, water is released from the reservoir, and flows through turbines, to generate power. The reservoir provides a stable, controllable flow of water, allowing the plant to ramp electricity output up or down relatively quickly to meet changing electricity demand, with the Hoover Dam being perhaps the most famous example.
Other notable examples of this infrastructure include Canada’s Site C Dam and Churchill Falls Generating Station, both integral to the country’s power grid. In Brazil, the Belo Monte dam and Itaipu dam (which crosses between Brazil and Paraguay), serve as cornerstone facilities, supplying a substantial portion of the country’s electricity. Norway’s Sima Power Plant is another example, utilizing impoundment hydropower to support national energy needs.
What makes hydropower one of the most controversial in renewable energy is the well documented environmental and ecological issues created as a result of these constructions, particularly impoundment facilities.
Dams can harm aquatic ecosystems in several ways. They disrupt the flow of sediments, which are important for wildlife and farming. When plants and trees are submerged underwater due to dam construction or flooding, they can decay and release harmful greenhouse gasses like methane and carbon dioxide, contributing to climate change. This loss of vegetation also disrupts local ecosystems and wildlife habitats.
As rivers naturally flow, water transports sediments downstream to replenish soils and maintain the ecosystem’s circulation and overall health. When dams are constructed, the sediment will start to build up in reservoirs, resulting in a lack of supply and the eventual erosion of the habitats downstream and build up behind the walls of the reservoir causing damage to the dam or require frequent removal of sediment. The degradation of habitats will impact both the wildlife and local agriculture.
Dams block the migration route of fish, especially species like salmon, that rely on free flowing rivers to spawn. While there have been efforts to mitigate the effects through fish ladders and bypass systems, the success is at best varied (actually mostly unsuccessful). At times, the failure of the bypass measures have resulted in the costly removal of dams at times in emergency efforts to revive fish populations:
Elwha River, Washington: The Elwha and Glines Canyon Dams were removed in 2011 and 2012, respectively. This restoration project aimed to revive the salmon populations that had significantly declined due to the dams blocking their migration routes.
Breach of the Klamath River Dams, California/Oregon: In 2021, four dams on the Klamath River were removed to restore salmon and steelhead populations. The dams had disrupted fish migration and contributed to ecological issues in the river.
Maine’s Penobscot River: A series of dam removals began in 2004 with the removal of the Great Works Dam and continued with other dams. This project aimed to restore fish passage and improve habitat for species like Atlantic salmon, which had been severely impacted by the dams.
The impact of dams to oxygen levels is perhaps one of the lesser-known, but most dire consequences of these structures. When vegetation and soil are submerged by reservoirs created by dams, they decompose without oxygen, a process that produces carbon dioxide and methane. The gasses that build up are then released into the atmosphere, either directly from the water surface or through dam turbines.
In the Himalayan country of Bhutan, which relies heavily on hydroelectric power, methane emissions from submerged vegetation have been linked to the melt on glaciers, causing concerns of disastrous flooding, and led to the power plants run on a much lower capacity than its full capability. Bhutan’s situation is far from an anomaly: a global study on reservoirs across the world, many of which were created as a result of dam constructions, are responsible for between 6-8% of anthropogenic methane emissions
The most serious impact, and the most significant environmental challenge to the large hydroelectric power plants in the form of dams, are the risks of Glacial Lake Outburst Floods (GLOFs) and dam failures. GLOFs occur when the natural dam containing a glacial lake fails, releasing large volumes of water downstream, often impacting hydropower infrastructure. With climate change accelerating the melting of glaciers as a result of greenhouse gas emissions, the concern has become even graver.
There are a lot more dams than we think, many of which are no longer serving as functional hydroelectric power plants and are way past their expected lifespan. There are more than 91,000 dams in the United States alone, most of which were built more than 50 years ago, and over 2,300 of them are under “high-hazard-potential,” according to the American Society of Civil Engineers.
Another variation on the impoundment system is pumped storage hydropower (PSH), which provides a more flexible source of energy to meet varying demands. They consist of two reservoirs at different elevations, connected by pipes or tunnels. During times of low electricity demand (often nights or weekends), excess electricity from the grid is used to pump water from the lower to the upper reservoir, effectively storing energy.
One of the advantages of pumped storage hydropower (PSH) is its ability to respond quickly to changes in electricity demand. Unlike some other renewable energy sources that can take time to ramp up production, PSH can typically begin generating electricity within minutes of a demand spike.
When electricity demand increases, the water stored in the upper reservoir is released through turbines. This process is quick because the system relies on gravity to allow the water to flow back down to the lower reservoir, which can happen almost instantaneously.
The turbines that generate electricity are connected directly to the water flow, allowing them to spin and produce power as soon as water is released. Since PSH systems are already set up to generate electricity at a moment’s notice, they can adjust their output rapidly in response to real-time changes in demand. This rapid response capability makes it invaluable for grid stability, especially as more intermittent energy sources like wind and solar are integrated into the energy mix.
One of the most famous pumped storage hydropower (PSH) facilities is the Bath County Pumped Storage Station in Virginia, USA. It’s one of the largest PSH plants in the world, with a total capacity of around 3,000 megawatts. The facility features two large reservoirs and plays a critical role in balancing energy supply and demand for the region, storing excess energy during low demand periods and generating electricity when demand is high. Bath County is often cited as a key example of the effectiveness of pumped storage in enhancing grid reliability and supporting renewable energy integration
In this way, PSH acts as a giant battery, helping to balance electricity supply and demand. PSH is the largest form of grid energy storage available today, accounting for over 90% of global storage capacity. Additionally, many PSH facilities can operate efficiently for decades, with some even exceeding 50 years of service, demonstrating their long-term viability as a key component of sustainable energy systems.
A Less Environmentally Damaging Hydropower System
As well as impoundment style hydroelectric plants with dams there are run-of-river systems. These are favored due to their lower environmental impact, compared to the more traditional impoundment style dams, as they maintain a more natural water flow.
They may have a small dam or weir to create a head and divert water to the turbines, but they do not significantly impede the river’s flow. As a result, their power output is more dependent on seasonal conditions and may fluctuate more than impoundment facilities. They are often built in series along a river, forming a cascade of projects.
Because they don’t require large reservoirs, run-of-river systems are better for preserving local ecosystems and fish migration patterns as they do not need areas of land to be flooded. For example, in British Columbia, Canada, run-of-river projects can generate electricity while allowing salmon to navigate upstream, ensuring that vital wildlife populations are supported alongside renewable energy production.
The People Problem
The other issue with dam based systems is that if a dam structure fails due to architectural or engineering weaknesses, lack of maintenance, or natural disasters, it can lead to massive flooding, destruction of ecosystems, loss of human life and property and long-term, likely irreversible environmental degradation.
The other issue is that as a result of building large-scale dams and creating reservoirs hundreds of thousands people across the world are displaced. It was estimated that nearly 80 million people have been displaced, or involuntarily settled, as a result of these types of dam construction projects.
For example, during the construction of the Three Gorges Dam in China, 1.2 million people across the Yangtze River were moved away from their homeland, as hundreds of towns and villages were submerged under water to create new reservoirs. These displacements not only uproot people physically but also lead to the loss of livelihoods and cultural identity.
Emerging New Hydroelectric Technologies
In recent years, as technologies have developed, other forms of hydropower constructions have emerged to challenge the conventional techniques and systems.
One of them is underground hydropower plants, which are built through excavation deep below the earth’s surface, making use of underground water flows or reservoirs. The main components, such as the turbines, generators, transformers, and control systems, are housed in underground caverns, which are connected by tunnels to the water source and outlet. These plants channel water from a high-altitude reservoir through tunnels that lead to turbines located deep beneath the surface, generating electricity efficiently.
Notable examples include the Obervermuntwerk II in Austria, situated in the Montafon Valley. This facility achieves a capacity of about 110 megawatts as part of a larger hydroelectric system in the region. Similarly, the Huangdeng Pumped Storage Power Station in China utilizes underground caverns for its turbines and generators, enabling efficient energy storage and generation, crucial for balancing grid demands.
In the United States, the Glen Canyon Dam and Hells Canyon Dam incorporate underground elements that enhance their operational efficiency. The Glen Canyon facility features tunnels that direct water to its turbines, while Hells Canyon leverages underground infrastructure as part of its extensive hydroelectric project.
Additionally, Bhutan’s Tala Hydroelectric Plant utilizes underground tunnels to transport water to its turbines deep within the mountains, boasting a significant installed capacity of about 1,020 megawatts. These plants exemplify how underground hydropower can effectively harness energy while minimizing surface disruption and maximizing efficiency.
Another emerging new hydropower technology are gravitational vortex energy power plants which represent an eco-friendly way of generating electricity from low-head water flows.
This technology works by creating a swirling vortex in a circular basin where water flows in, rotating a turbine located at the center of the vortex. The continuous motion of the vortex keeps the turbine spinning, generating energy even in smaller streams or rivers.
Examples of gravitational vortex energy applications include the Vortex Bladeless turbine being tested in Spain, which harnesses energy from both wind and water. Although primarily wind-focused, similar principles can be applied to vortex technology in water environments. Another project is the HydroVortex in Canada, which aims to demonstrate the efficiency of vortex turbines in various water conditions. As this technology matures, more projects could emerge, showcasing its potential for sustainable energy generation.
It’s All About the Turbines
The turbine is the most crucial component of a hydropower plant because it is the central mechanism that converts the energy of flowing or falling water into mechanical energy, which is then transformed into electricity. Without the turbine, the immense potential energy stored in water due to its flow or elevation would remain untapped.
There are two main types of hydropower turbines: impulse and reaction turbines. They differ in design and how they capture the energy from flowing water to generate electricity. The choice between these turbines depends on several factors, including the head (the height from which water falls), flow rate, and specific site conditions. A higher head usually means more potential energy, which can be converted into electricity more efficiently.
Impulse turbines work best in situations where water falls from a great height but has a relatively small volume. They convert the energy of a fast-moving jet of water, which hits the turbine’s buckets or blades, causing the turbine to spin.
In contrast, reaction turbines are better suited for low-head, high-flow conditions because they can efficiently convert the energy from large volumes of water with less vertical drop. These turbines operate by utilizing both the pressure and velocity of the water to spin their blades. Water enters the turbine through a spiral casing, which guides the flow onto the turbine runner, allowing for effective energy conversion in situations where the water falls from a shorter height
As water flows through the runner, which is the rotating part of the turbine that converts water energy into mechanical energy, it causes the blades to rotate, generating electricity. Reaction turbines are often used in large hydroelectric projects with low head and high flow rates. The Francis turbine is versatile, suitable for various head and flow conditions. Kaplan turbines, with adjustable blades, are ideal for low-head, high-flow situations and are commonly found in run-of-the-river projects, where the natural river flow is harnessed without large dams or reservoirs.
The Benefits and Continued Success of Hydropower
One of hydropower’s most significant advantages is its renewable nature. Hydroelectric power is classified as a form of renewable energy because it relies on the natural water cycle, which is continuously replenished by solar energy. When the sun shines, it drives evaporation from the Earth’s surface, forming clouds that eventually release precipitation. This precipitation flows back into rivers and streams, creating a continuous supply of water. However, it’s the movement of this water—whether from flowing rivers or falling waterfalls—that is harnessed to generate electricity.
As long as this cycle continues and the water is in motion, hydropower remains a viable and renewable source of energy.
Unlike fossil fuels, which are finite and release greenhouse gasses when burned, hydroelectric power generates energy without diminishing water resources or producing emissions. In hydroelectric systems, water is not consumed or altered, making it a sustainable and clean energy source for the long term.
Reservoirs created by hydroelectric power plants store water during periods of low demand and release it when energy needs peak, ensuring a steady supply of electricity. Moreover, hydropower plants are highly efficient, with some facilities converting up to 90% of the available energy into electricity, a much higher rate than most other energy sources (such as the 17-20% in solar and the highly varied conversion rate among wind turbines).
As a result of the high rate of conversion, hydroelectric power provides critical support to the stability of electricity grids. Hydropower plants can adjust output rapidly, allowing them to respond to fluctuations in demand. This ability to ramp up or scale back water flow within minutes makes hydropower an ideal source for balancing electricity grids, especially as more intermittent renewable sources like wind and solar are integrated.
Additionally, pumped storage hydropower plants, which store energy by pumping water to a higher elevation and releasing it during peak demand, offer a highly effective way to store excess energy, helping to ensure a reliable electricity supply even when renewable sources are less productive.
Many hydroelectric facilities are multipurpose, providing benefits such as flood control, irrigation, and recreation. Reservoirs created by dams can prevent floods by regulating water flow and storing excess water, which can also be used for agricultural purposes during dry periods. These reservoirs often become recreational areas (for example, Lake Mead was formed as a result of the Hoover Dam), supporting activities like boating and fishing, which can further boost local economies.
What does Hydropower’s Future Look Like?
The future of hydropower is shaped by both its immense potential and the challenges it faces in the context of climate change and sustainable development. Its status as the world’s largest source of renewable energy is unlikely to change in the near future and continues to exist as an enduring and stable option of energy supply, which means countries striving to meet decarbonization targets will still vye for hydropower to do so.
However, achieving these goals will require significant investments in new hydropower capacity, especially in developing regions where untapped resources remain abundant. According to the International Renewable Energy Agency (IRENA), hydropower’s installed capacity may need to more than double by 2050 to meet the global climate target of 1.5 degrees celsius. This will demand not only massive financial investments, but also technological innovations and policy reforms to ensure the sustainability and resilience of future projects.
Another significant trend shaping the future of hydropower is the shift toward smaller, more sustainable projects. Large-scale dams are known for their environmental and social impacts, including the displacement of communities and the damage of local ecosystems. In response to these concerns, there is growing interest in low-impact hydropower technologies, such as hydrokinetic turbines and micro-hydropower plants, which generate energy without the need for large reservoirs or significant alterations to river systems.
In recent years, solar panels have been combined with the reservoirs on impoundment facilities to provide spaces to generate electricity through more diverse means. The photovoltaic panels, which float on the top of the reservoir overlooking the dam, can balance each other to meet the demand when there isn’t enough waterflow to generate the adequate amount of electricity through the dams alone. One research from the Renewable Energy journal estimates that if this pairing is implemented across the world, they will be able to drastically reduce the cost of solar electricity and meet 40% of the global electricity demand.
Threats to Hydropower
Hydropower generation relies on predictable water flows, which are ironically, and increasingly, threatened by changing precipitation patterns, glacial melt, and extreme weather events. In many regions, such as the Himalayas and the Amazon Basin, climate change is causing both increased flood risks and prolonged droughts, which can severely disrupt hydropower production.
The variability of water availability caused by climate change could make hydropower less reliable in certain areas, particularly in regions where glacial meltwater or seasonal rainfall is a critical component of river flow. Researchers suggest the incorporation of climate resilience models into their design and operation, including adaptive reservoir management and the use of hybrid systems that integrate solar or wind energy.
Hydropower’s future depends on effectively addressing the significant environmental and social challenges associated with its construction. As a crucial player in the transition to renewable energy, hydropower must adapt to these emerging issues while continuing to play an essential role in our energy systems. The success of the global shift toward renewable energy will rely on how well hydropower navigates these challenges.
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