Hydroelectric Power

As mentioned earlier, the energy from fast-flowing water has been utilized for centuries. Over 2000 years ago, residents of Ancient Greece used flowing water to turn mill wheels to grind wheat into flour. Water mills are large wheels on the banks of rivers used to create energy for a variety of functions, such as cutting or grinding, and they were very common up to the industrial revolution.

Hydroelectric power is the most common source of renewable electricity worldwide - in fact, 71% of renewable energy generated on the planet is from hydropower.

Fig. 1 – A large hydroelectric dam in the US. Source: unsplash.com

Hydroelectric Power Plants

There are three types of hydroelectric power plants.

Impoundment Facility

By far, the most common type of plant is an impoundment facility, where a dam is used to control the flow of water downhill. The aim is to raise the level of water behind the dam, creating as large a head as possible (this will be explained in more depth later!).

When more energy is required, water is released from the dam. The water flows downwards due to the force of gravity, travelling through turbine blades, which power a generator.

Diversion Facility

This type of hydroelectric plant does not use a dam. Instead, a series of canals is used to channel water towards turbines where electricity is generated.

Pumped Storage Facility

This type of hydroelectric plant collects energy from other forms of renewable energy and stores it by pumping water uphill into a reservoir. When electricity demand is high, water in the higher reservoir is released and flows down into the lower reservoir, travelling through a turbine.

Hydroelectric Power Plant Diagram

Hydroelectric Power Generation

No matter the type of power plant, hydroelectric power is generated through water travelling downwards through pipes known as penstocks. Falling water causes turbines to rotate, which drives generators that convert the mechanical energy of the turbines into electricity.

Transformers convert the alternating voltage suitable for generators into a higher voltage suitable for long-distance transmission.

Heads

The difference between the high and low elevations is known as the head, measured in metres.

The power that a volume of water can generate is proportional to its working head. The larger the head, the higher the water pressure across the turbine, thus the more power the turbine can generate.

Higher heads are preferable in hydroelectric plants because they generate more power. Additionally, the higher pressure means a higher flow rate can be forced through small turbines, which are cheaper.

New Turbine Developments

Rotating wheels and falling water have been used for a very long time to generate power. New types of wheels and their systems can improve their ability to harvest water's kinetic (meaning moving) energy.

Helical Turbines

These turbines are installed horizontally, which allows their use in shallow sites. The turbine always rotates in the same direction, independent of water flow. Thus, helical turbines are valuable in reversible tidal flows.

The three peripheral hydrofoil blades provide lots of free space, enabling safe fish passage through the turbine.

Low Head Turbines

Low-head turbines can be used as part of small-scale hydropower systems. Small heads provide minimal environmental impact.

Advantages of Hydroelectric Power

Generally, hydroelectric power holds a lot of advantages over fossil fuels.

• Renewable: hydroelectric power uses the energy of running water to produce electricity, without affecting its volume.
• Reliability: unlike some other forms of renewable energy, such as wind and solar, the availability of hydroelectricity does not fluctuate.
  • Hydroelectric energy can enter the electricity system faster than any other type of energy, so can quickly re-establish supply after a blackout.
• Environmentally Friendly: production of hydroelectric power does not emit greenhouse gases or pollution.
• Cost-effective: once built, power plants have few maintenance costs. Their lifetime can be up to a century.
• Promotes Development: energy from hydroelectric power promotes infrastructure, develops the economy and increases access to health and education.

Disadvantages of Hydroelectric Power

Ecological

• Creating dams floods river banks, destroying wetland habitats. Plants and aquatic birds are commonly affected.

• Dams often obstruct fish trying to swim upstream.

• Furthermore, operating the plant may increase the temperature of the reservoir water. Nearby plants and animals either have to adjust or migrate.

Construction Costs

• Hydroelectric power has the highest construction cost of all types of renewable energy - at $5312 per kilowatt of electricity generated. Building power plants can take between two and four years to complete.

• Building the dams themselves causes flooding, which displaces locals and destroys farmland.

Siltation

Normally, silt is carried by the flowing river, but its path is obstructed by the dam. As silt builds up on the bottom of the reservoir, the amount of impounded water slowly reduces. A decrease in water causes a decrease in energy flowing through the turbines. Some power plants only provide electricity for 20 or 30 years due to silt build-up.

Greenhouse Gas Emissions

Despite the belief that hydroelectricity is a carbon-free energy source, research has shown that reservoirs emit greenhouse gases. There are a few ways that this can happen.

Flooding

When new land is flooded after building a dam, carbon from the flooded soil can be transformed into greenhouse gases.

Water Accumulation

As water accumulates in the reservoir, aquatic bacteria can transform carbon into greenhouse gases.

Degassing

If water stays in a reservoir for a long time, the surface is warmed, but the bottom remains cold. This temperature gradient creates a thermocline, which acts as a physical barrier for small molecules such as carbon dioxide and methane.

Below the thermocline, the environment could be anoxic. Carbon is transformed into methane, instead of carbon dioxide. If this happens, methane could be released downstream through the turbines - known as degassing.

I hope that this clarified hydroelectric power for you. Remember that hydroelectric power is generated by the conversion of potential energy from fast-flowing water. Despite its drawbacks, hydroelectricity is the most common renewable source of electricity worldwide.

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