8.6: Hydroelectric Power (Hydropower)

This is the second largest source of renewable energy used, next to biomass energy. The majority of hydropower currently comes from dams built across a river to block the flow of river water. The water stored behind the dam contains potential energy (see chapter 4) and when released, the potential energy is converted to kinetic energy as the water rushes down. This energy is used to turn blades of turbines and causing a generator to generate electricity. Electricity generated in the powerhouse of a dam is transmitted to the electric grid by transmission lines while the water flows into the riverbed below the dam and continues down river. An alternative approach considered less disruptive involves diverting a portion of the river’s water through a pipe or channel and passed through a powerhouse to generate electricity and returned to the river. Another approach involves pumping water from a lower reservoir to a higher reservoir and then allowed to flow downhill through a turbine, generating electricity. This approach, however, requires energy input to pump the water.

Figure \(\PageIndex{1}\): a) Hydroelectric facility (credit: Denis Belevich, Wikimedia Commons); b) An illustration showing how water is funneled through a portion of the dam to rotate turbines and generate electricity. (Image from Wikimedia, public domain).

Figure \(\PageIndex{2}\) shows the Hoover Power Plant located on the Colorado River. In the U.S., hydroelectric plants account for about 10% of total production and account for about 35 % of the United States' renewable energy consumption. In 2003 capacity was at 96,000 MW and it was estimated that 30,000 MW capacity is undeveloped. Energy produced can be calculated and modeled as shown in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{2}\): a) A view of the Hoover dam from above (credit Adam Kliczek). b) Hydroelectric generators at the Hoover dam (credit: Jon Sullivan).

Figure \(\PageIndex{3}\): (a) Water gushes from the base of the Studen Kladenetz dam in Bulgaria. (credit: Kiril Kapustin; http://www.ImagesFromBulgaria.com) (b) In the absence of significant resistance, water flows from the reservoir with the same speed it would have if it fell the distance h without friction.

Environmental Impacts of Hydroelectric Power

Hydropower (hydro-electric) is considered a clean and renewable source of energy since it does not directly produce emissions of air pollutants and the source of power is regenerated. However, hydropower dams, reservoirs, and the operation of generators can have serious environmental impacts. A dam that is used to create a reservoir or to divert water to a run-ofriver hydropower plant can obstruct migration of fish to their upstream spawning areas in areas where salmon must travel upstream to spawn, such as along the Columbia River in Washington and Oregon. Hydro turbines kill and injure some of the fish that pass through the turbine although there are ways to reduce that effect. This problem can be partially alleviated by using ‘fish ladders’ that help the salmon get up the dams.

A reservoir and operation of the dam can affect the natural water habitat due to changes in water temperatures, chemistry, flow characteristics, and silt loads, all of which can lead to significant changes in the ecology and physical characteristics of the river upstream and downstream. Construction of reservoirs may cause natural areas, farms, and archeological sites to be covered and force populations to relocate and result in the loss of scenic rivers.

Carbon dioxide and methane may also form in reservoirs where water is more stagnant and be emitted to the atmosphere. The exact amount of greenhouse gases produced from hydropower plant reservoirs is uncertain. If the reservoirs are located in tropical and temperate regions, including the United States, those emissions may be equal to or greater than the greenhouse effect of the carbon dioxide emissions from an equivalent amount of electricity generated with fossil fuels (EIA, 2011 p. 333).

Potential of Tidal Power

Tidal power involves placing turbines in zones of the ocean with significant tides and currents, and using the power of flowing water to turn the blades of a turbine to generate electricity. Ocean power systems are still being researched and currently still experimental. For example, the Bay of Fundy, which has a 15 m tide, a dam constructed across the estuary would let water enter on the incoming tide, then release the water through turbines at low tide. The energy potential is great http://www.ialtenergy.com/tidal-power-news.html , and so is the environmental cost. Tapping tidal energy resources involves building major dams on inlets and estuaries that are prized for other purposes, so few tidal energy facilities have been developed. Harnessing waves and currents on a significant scale will involve designing turbine structures that are large, inexpensive, and can operate for long periods under the physical stresses and corrosive forces of ocean environments. Though proposed, a tidal power plant has not been constructed at Fundy. There is a 240,000 kW tidal plant at La Rance, France. Figure \(\PageIndex{4}\) shows a diagram of a tidal power plant. During high tide, water flows into the tidal basin (a). During low tide, water flows out of the water basin back to the main body of water (b). These produce power by spinning the generator in opposite directions. This is shown in the graph of Figure \(PageIndex{4}\) (c). More details can be found at Alternative Energy Tutorials.

Figure \(PageIndex{4}\): (a) Diagram of a tidal power plant at high tide. The water is able to flow into the tidal basin This generates the ebb tide power shown in (c). (b) The tidal power plant at low tide when the outgoing flows back into the primary body of water. This generates the flood tide power shown in (c). Source: https://www.alternative-energy-tutorials.com/tidal-energy/tidal-barrage.html

Wave Power

Ocean waves represent a significant, yet largely untapped, source of renewable energy. Capturing the energy of these traveling waves involves various technologies that convert the wave's motion into usable electricity. These technologies include point absorbers, attenuators, overtopping devices, and oscillating water columns, each with unique designs and working principles. See Figure \(\PageIndex{5}\) for examples of these devices.

How it works

• Point Absorbers:

  These devices, often buoy-like, move up and down with the waves, driving a generator to produce electricity according to EKT Interactive. 

• Attenuators:

  These devices are designed to move with the waves, using the resulting mechanical energy to drive a generator. 

• Overtopping Devices:

  These capture water as waves break over a structure, then channel the water through a turbine. 

• Oscillating Water Columns:

  These devices trap air pockets within a chamber. Wave action compresses and decompresses the air, driving a turbine to generate power. 

Figure \(\PageIndex{5}\): Types of wave energy extraction. (a) Point absorbers have a buoy that moves relative to a fixed weight and attenuators bend with waves generating energy from the mechanical bends. (b) Overtopping devices "catch" waves and use the gravitational potential energy to turn a turbine. (c) Oscillating water columns use trapped air columns to drive turbines.

Challenges

• High Production and Installation Costs: Developing and deploying wave energy converters can be expensive.
• Maintenance Complexity: The harsh marine environment can lead to maintenance challenges and costly repairs.
• Potential Environmental Impacts: There are concerns about the impact on marine life from both the devices themselves and the noise they generate. 

Potential

Wave energy could play a significant role in the future energy mix, particularly in densely populated coastal regions and remote locations. It offers a more consistent and predictable power source compared to wind and solar, making it a valuable addition to the renewable energy portfolio.