How Does Hydropower Energy Work?
Hydropower or hydroelectric power is the process of generating electricity by using the energy from flowing water. It is one of the oldest and most widely used renewable energy sources in the world. The first hydropower plants date back to the late 19th century in the United States and Europe.
Today, hydropower supplies 16% of the world’s electricity and over 75% of renewable electricity generation according to the Global Hydropower Tracker. Countries like Canada, Brazil, and China lead global hydropower capacity, while countries in Europe like Norway and France get a significant share of their power from hydroelectric dams.
In the United States, hydropower accounts for about 7% of total electricity generation and over 40% of renewable electricity. Major hydropower facilities are located around the Columbia River Basin, the Colorado River, and Niagara Falls.
How Hydropower Plants Work
Hydropower plants use the energy from flowing water to generate electricity. Here are the main parts of a hydroelectric plant:
Dam – A dam is built to create a reservoir, controlling the water flow. The dam stores potential energy by holding back water in the reservoir.
Reservoir – The reservoir is the body of water held behind the dam. It stores the potential energy of the water, which is converted to kinetic energy as it falls through the dam.
Intake – The intake is the entrance to the turbine system. Gravity causes water from the reservoir to flow down through the intake.
Turbine – The moving water spins the turbine blades, converting the kinetic energy of the moving water into mechanical energy.
Generator – The turbine rotates a shaft connected to a generator to produce electricity. The generator converts the mechanical energy into electrical energy.
By harnessing the natural downward flow of water, hydropower plants can generate clean, renewable electricity. The key is using the potential energy of the reservoir behind the dam to drive the turbine and generator.
Diagram: https://www.alamy.com/stock-photo/hydropower-plant-diagram.html
Potential Energy and Flowing Water
The power generation process of hydropower plants relies on harnessing the potential energy of elevated water. According to the law of conservation of energy, energy cannot be created or destroyed, only converted from one form to another. In hydropower, the potential energy of water stored at an elevation is converted to kinetic energy as the water flows downstream.[1]
The potential energy (EP) of water stored at an elevation can be calculated using the following equation:
EP = ρ x g x h x V
Where ρ is the density of water, g is acceleration due to gravity, h is the height of the water, and V is the volume of water. As water flows from the high elevation reservoir through turbines and generators, this potential energy gets converted into kinetic energy and eventually electricity.[2] The greater the volume and height of the water, the more potential energy that can be converted. This makes elevated reservoirs ideally suited for harnessing energy from flowing water.
Overall, the potential energy stored in water at a height represents a huge source of power. Hydropower plants are designed to efficiently convert this potential energy into electricity as water flows downhill through turbines.
Hydropower Turbines
The fast-moving water created by dams is directed through turbines to generate electricity. There are two main types of hydro turbines: impulse and reaction. Impulse turbines, like Pelton wheels, use the velocity of water to move the turbine blades. Reaction turbines, like Francis and Kaplan turbines, use water pressure and flow to propel the blades. The two main types of impulse turbine are Pelton and cross-flow turbines.
The three common hydroelectric turbine designs are the Kaplan, Pelton, and Francis designs. There are reaction (Kaplan and Francis) and impulse (Pelton) turbines. Kaplan turbines are propeller-like with adjustable blades. They are used for sites with lower head and higher flow. Francis turbines have fixed blades and are used for a variety of heads and flows. Pelton turbines have buckets attached to a wheel and are used for high heads.
Power Generation Process
The power generation process begins as water from the reservoir flows through a tunnel and spins the blades of a hydraulic turbine. As the turbine spins, it causes an electromagnetic field on a coil inside the generator to rotate past a magnetic field, producing electricity through electromagnetic induction.
More specifically, the turbine is connected to a rotating shaft inside the generator. As the shaft spins, it rotates coils of copper wire inside the generator around an iron core. Surrounding these coils are magnets with a fixed north and south pole. As the coils spin past the magnets, the changing magnetic field induces a flow of electrons or current within the copper wires. This flow of electrons is the electric power that gets produced and transmitted through power lines to homes and businesses.
The speed at which the turbine spins matches the generator’s rotational speed required to produce alternating current electricity at 60 cycles per second or hertz. The amount of mechanical power produced by the flowing water that gets converted into electrical energy determines the output wattage of the hydroelectric generator.
(Sources: Hydropower Technology Assessment, Assignment 3b: The Scandinavian electricity system)
Transmission of Power
After the flowing water spins the turbines to generate electricity at the hydropower plant, the power must then be transmitted to homes, businesses, and other end users. This requires an extensive electricity transmission system of power lines, transformers, and substations.
The electricity generated from the hydropower turbine is stepped up to high voltages like 115 kV or 500 kV by transformers at the power plant. This allows the electricity to travel long distances over transmission lines with minimal power losses. The transmission lines are large overhead wires suspended on tall transmission towers. Raising the voltage helps reduce power losses and allows the electricity to efficiently travel hundreds of miles.
As the transmission lines approach populated areas, substations use transformers to step down the voltage. From the substations, lower voltage distribution lines spread out to deliver the electricity to homes and businesses. Typical distribution voltages range from 4 kV to 33 kV.
Transmission systems have extensive monitoring, control, and protection systems to maintain reliability and safety. Breakers, switches, and capacitors help regulate voltage, redirect power, and prevent overloads. The interconnected network and multiple power sources provide redundancy in case a line goes down.[1]
Efficient transmission systems are crucial for delivering the power generated at hydropower plants to customers across large regions. With high voltage transmission and complex control systems, hydropower can be an important energy source even in areas far from hydro resources.
[1] https://ieeexplore.ieee.org/document/8058418
Pumped Storage
Pumped storage hydropower (PSH) facilities use two reservoirs at different elevations to store and generate electricity. When electricity demand is low, typically during nighttime hours, excess electric power generation capacity is used to pump water from the lower reservoir uphill to the upper reservoir. When electricity demand is high, the water from the upper reservoir is released back down into the lower reservoir through hydro turbines to generate electricity. This process essentially stores the energy generated during periods of low demand for later use during peak demand periods.
PSH allows energy from intermittent renewable sources, like wind and solar, to be stored and dispatched when needed. PSH facilities can start generating electricity within minutes, providing grid reliability and stability. They can operate flexibly, generating power during peak demand times and pumping water uphill during low demand times. This makes them exceptionally valuable for integrating renewables and balancing electricity supply and demand.
The upper reservoir of a PSH facility is similar to the reservoir at a conventional hydropower dam, while the lower reservoir is more analogous to the tailrace. Water is recycled between the two reservoirs as it is pumped uphill and released downhill through the turbines. Closed-loop PSH systems use reservoirs that are not connected to a natural body of water like a river or lake.
PSH is currently the largest-capacity and lowest-cost form of grid energy storage available. However, potential sites are geographically constrained as they require particular topographies with space for two reservoirs at different elevations. Only about 22 gigawatts of pumped storage capacity exists in the United States today.
Benefits of Hydropower
Hydropower provides several key benefits that make it an attractive renewable energy source. First and foremost, hydropower is completely renewable as it relies on the water cycle and rainwater to keep rivers flowing (QueryPlex). Unlike fossil fuels which are finite, hydropower can provide energy indefinitely without being depleted. This makes it one of the most sustainable energy options available today.
Hydropower systems are also extremely reliable once built, capable of providing a consistent supply of electricity 24/7. The energy output remains steady and predictable, unlike wind or solar power which fluctuate based on weather conditions (ArcGIS). This reliability reduces the need for backup generation, benefiting the overall electricity grid.
Lastly, hydropower enjoys relatively low operating and maintenance costs compared to other energy sources. The majority of expenses come from building the initial dam and infrastructure. But once operational, the availability of free flowing water minimizes fuel costs and upkeep is inexpensive. This makes the levelized cost of hydroelectricity very affordable in the long run.
Drawbacks of Hydropower
While hydropower has many benefits, there are some drawbacks to consider as well. Three major drawbacks include high upfront costs, disruptions to waterways, and drought sensitivity.
Constructing hydropower plants requires very high upfront costs. Large dams, reservoirs, turbines and other infrastructure can cost billions of dollars to build (Source). This makes financing difficult, especially in developing countries.
Hydropower facilities also cause major disruptions to rivers and surrounding ecosystems. Dams change the natural flow of rivers, affect sediment transport, and can impede fish migration routes. Reservoirs flood habitats and disrupt terrestrial ecosystems (Source).
Finally, hydropower output depends on water levels and is sensitive to droughts. During dry years, electricity generation can decrease substantially. This variability makes hydropower an unreliable source during drought conditions.
Future of Hydropower
The future looks bright for continued growth in hydropower capacity globally. According to the International Energy Agency (IEA), global hydropower capacity is expected to increase by 17%, or 230 GW, between 2021 and 2030 (source). The IEA forecasts that the majority of growth will come from pumped storage hydropower, which represents 30% of net hydropower additions through 2030 (source).
This continued growth in hydropower capacity will lead to job creation and greenhouse gas emission reductions. A report from the U.S. Department of Energy estimates that hydropower could support over 195,000 jobs across the nation and reduce cumulative greenhouse gas emissions by 5.6 gigatonnes CO2-equivalent by 2050 (source). Technological innovations, such as advanced turbines, power electronics, and pumped storage will help drive growth and improve performance.
