EGEE 101
Energy and the Environment

Hydroelectric Power

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Hydroelectric is one of the leading electricity sources for renewable energy, providing 7% of the US total electricity supply.

Renewable water power has been used for years to grind corn and grains into flour or to pump water. With hydropower, we convert stored potential energy into kinetic energy to do work or to generate electricity.

Picture of a historic water mill.
Wheels like this were used for centuries taking the flow of water from rivers and streams to provide power.

Potential Energy PE = mgh

M is mass
G is the acceleration due to gravity (~10 m/s2)
H is the height above a reference point

Imagine carrying a sack of potatoes up the stairs of a warehouse. Two things influence how much work you will have to do; the mass of the sack of potatoes, and the number of floors (the height) that you will have to carry the potatoes.

Our potato example would look like this in a formula;

To move a 60 kg sack 5 meters above the ground level becomes:

60 (kg) x 10 (meters per second per second) x 5 (meters)
= 3,000 kg m2s2
= 3,000 Joules (J) or
= 3 kJ

If you recall the energy laws, energy cannot be created or destroyed, and so, if we drop the sack of potatoes, we will obtain the same amount of energy back (but in various forms). Thus, potential energy is stored energy. Hydroelectric energy works in the same way. We use the stored energy in water to produce electricity by flowing the water through a turbine (to spin a generator as usual). The water flows because of the influence of gravity, and that it is above the reference height.

Hydroelectric energy is renewable because the water that flows through the turbine to create electricity is replaced naturally by the water cycle. The sun (solar energy) evaporates water from the seas and lakes; some of the water will form clouds high in the atmosphere that will drift over land, and the water will fall back to earth in some form of precipitation (hail, fog, snow, or rain). The water flows into rivers and streams and the kinetic energy can be used for useful work. In the case of the large hydroelectric operations, the rivers have been dammed to generate large lakes. The dam holds the water back and raises the level of the water above the reference level (the bottom of the dam). In the U.S. we have already used many of the primary sites for large-scale hydroelectric production and those that are left are unlikely to be dammed due to the damage it inflicts on the ecosystem. Hydroelectric power (large scale) was one of the cheapest of the renewable options and a source for a significant quantity of electricity in those regions where it could be employed at scale. For a long time, hydroelectricity was the leading renewable electricity source but has recently been surpassed by wind power in the U.S.

Annual electricity generation from wind and hydro2009 - 2019
Credit: International Energy Agency

China is the leading nation for electricity production and has been increasing its generation capacity with very large-scale additions such as the Three Gorges Dam (almost twice the size of other large-scale sites). The U.S. hydroelectric production has however remained steady for decades.

Hydro electric power in various countries
Hydropower generation in terawatt-hours (TW) by country
Click for a text description.
  • China, 1,302 TW
  • Canada, 398 TW
  • Brazil, 386.95 TW
  • United States, 274 TW
  • Russia, 190.29 TW
  • India, 162.1 TW
  • Norway, 125.77 TW
  • Turkey, 87.09 TW
  • Japan, 86.67 TW
  • Venezuela, 72 TW
Credit: Statista

The energy we are trying to capture is kinetic energy:

Kinetic Energy = 1/2 mv2

Unfortunately, to change all the kinetic energy into electricity would require the water to have no velocity whilst exiting the turbine. This is impractical, as we do not transform all the kinetic energy into electrical energy. We can obtain more energy by increasing the flow rate of the water (increasing the mass of water) and by increasing the velocity of the water (increasing the height of the dam increases the water pressure and thus increases the velocity and the mass). PLEASE SEE THE HTML SOURCE BELOW

Additional types of hydroelectricity production

See the visualizations provided by the "Energy.gov" website, and their section on hydroelectric power. Give it a look with the goal of understanding how hydroelectric power produces electricity. 

Advantages

  • It is able to easily follow the demand curve (change electricity production rapidly) that was covered in lesson 01
  • It is also a cheap large-scale electric source but quality locations are limited
  • One of the major advantages is that some sites can be used to store energy through pumped storage (particularly useful for the storage of renewable energy): The issue of energy storage is a major one. Listen to this audio file for an explanation (transcript).
  • Hydroelectric is a clean energy source producing no air pollution (once setup), and it is a renewable, domestic energy supply. Changes in the turbines might increase the capacity, and small scale additions may be added but with little impact on the overall contribution

Disadvantages

  • One of the environmental issues with hydroelectric power is the alteration of the species in the river. The fish that like the fast-moving cold waters of rivers do not do well in the warmer, deeper lakes. Thus, there is a reduction in the resident species when dams are built. Fish ladders and elevators can help move the fish around the physical barrier, but the impact on both the surrounding area and the ecology is devastating. There is a move now to dismantle some of the smaller hydroelectric facilities and return the rivers to their natural state. Run-of-river hydroelectric schemes might be used but they won't generate large quantities of electricity in a single site
  • Flooding of valleys and the destruction of that ecosystem
  • There is not much growth potential in the US as the likely spots are already used
  • Dams use a great deal of concrete that produces lots of CO2 (a greenhouse gas) for construction
  • With climate change and annual variations, there can be reduced snowfall in locations such as the Pacific Northwest. This has caused reduced hydroelectric electricity supply to California at times. This is one of the main variables related to the production of hydroelectric power — that it is very dependent on precipitation to replenish the water supply
  • Earthquakes or terrorist attacks can also destroy the dams, producing flooding; and a deadly, fast-moving flow of water. During the Iran-Iraq war, Iraq deliberately opened floodgates to flood (impact) a major oil-producing region in Iran. 

Three Gorges Dam

Photograph of the Hoover Dam showing water gushing from the outlets under high pressure
Hoover Dam
Credit: United States Bureau of Reclamation

The worlds' largest dam is the Three Gorges Dam. Producing 20,000 MW (for scale the Hoover Dam, pictured here, generates about 1,400 MW). The river navigation and flood control will also improve because of the dam (flooding has killed 300,000 last century). Hydroelectricity will provide clean electricity for the rapidly growing industries of eastern and central China. This is desirable because most of China's electricity comes from coal power plants close to the cities—lots of polluted air.  Of course, such a large project has rather large costs as well; one million-plus people will have to be relocated, fish migration will be stopped, and there may be increased threats to the survival of the Yangtze River dolphin, along with many other animal species. As is common with most issues of energy and the environment, however, there is plenty of disagreement over whether the dam is really the right thing for this country.

Future Directions for the Hydroelectric Industry

Some consider the hydroelectric industry as "mature" and that the technical and operational aspects of the industry have not changed in the past 60 years. Research is currently underway that concentrates on new concepts for the industry, and one project is testing new turbine designs. This project will hopefully recommend a final blade configuration that will allow safe passage of more than 98% of the fish that are directed through the turbine (I wouldn't want to be part of the 2%.) The US Department of Energy has identified more than 30 million kilowatts of untapped hydroelectric capacity that could be constructed with minimal environmental effects at EXISTING dams that do not have generating facilities at the present time, and also at EXISTING dams that are underutilized, and at a number of sites where dams do not presently exist. This research and planning activity suggests that hydroelectric power could continue to be an important part of the US energy picture for some time to come. In-river hydro is also an option (kinetic energy from flowing water WITHOUT the dam). (See Ocean Energy Technologies of D.O.E.) 

Here is a good overview video (3:50 min) by the U.S. Department of Energy. It is a few years old so it still (now incorrectly) claims that hydroelectricity is the leading renewable energy source for U.S. electricity.

Click here for a transcript.

PRESENTER: People have been capturing the energy and moving water for thousands of years. And today, it's still a powerful resource that can generate clean, renewable, and affordable electricity. You see, we harness energy from flowing water and convert it to electricity. That's what we call hydroelectric power, or hydropower. Water flows from a higher elevation to a lower elevation. And a hydropower facility uses turbines and generators to convert this motion into electricity.

America has been using hydropower to generate electricity for more than 100 years now. And today, about 7% of all our electricity is generated from hydropower, making it the largest source of renewable power.

[MUSIC PLAYING]

So what makes hydropower renewable? It's simple, water. Water evaporates into clouds and recycles back to Earth as precipitation. The water cycle is constantly recharging, and can be used to produce electricity along the way. How does it work? Basically, there are several ways hydropower technologies can generate electricity. You may recognize dams, like this one. This technology is called an impoundment. The impoundment stores water in a reservoir. When the water is released, it flows through and spins a turbine, turning a generator that produces electricity.

Here's another technology. This is a diversion. It channels a portion of a river through a canal or pipe into a turbine and generator system. What's cool about this method is that it uses the natural flow of the river, and usually doesn't require a large dam.

And have a look at this. This is called pumped storage hydropower. Basically, it works like a huge battery. To charge the battery, water is pumped back up into a reservoir during periods of low energy use, often during the night when people are using fewer appliances. Then when people need more power during the day, the water can be released to produce electricity.

[MUSIC PLAYING]

As long as we've been capturing energy from water, you may think there's nothing new in hydropower technology. Think again. The Department of Energy is helping to upgrade older facilities by increasing the efficiency of the turbines and generators. Operators of neighboring hydropower facilities are also working together to optimize energy production across whole river systems, instead of each dam working alone. And we can add generators or retrofit dams that were built without power, like dams used to water crops or prevent floods.

Today, there are about 80,000 dams in the US. But less than 3% of these dams produce power. That means there's a big opportunity to generate more clean renewable power at dams we've already built.

New technology is also making hydropower even more environmentally-friendly. For example, researchers are reducing adverse impacts on fish with fish-friendly turbines. And fish ladders like these let them swim around dams.

Hydropower is an essential, reliable, and renewable source of clean energy with a rich history, and it's meeting substantial energy demands today. With new technologies, it will be even more efficient and have greater production capacity, powering US homes and businesses for centuries to come.

Credit: The Department of Energy