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3. Water Powered Electricity

Turbines driven by the power of flowing water are used to generate electricity for hydroelectric, wave, and tidal power. Hydroelectric energy accounts for over 6% of U.S. electricity generation but wave and tidal power are new technologies that are mainly in demonstration or pilot plant production around the world and not yet used as major power generators.

3.1 Hydroelectric Energy

Hydroelectric power, or simply hydropower, uses the energy of water flowing in a river to generate electricity. This is most often accomplished by building a dam that blocks the flow of water allowing it to collect into a reservoir upstream of the dam. Water from the reservoir is then directed through a pipe (called the penstock) to drive turbines located within the dam structure and generate electricity (Figure 3C.3.1)

Image of how a hydropower plant works. The water flows from behind the dam through penstocks, turns the turbines, and causes the generators to generate electricity. The electricity is carried to users by a transmission line. Other water flows from behind the dam over spillways and into the river below.
Figure 3C.3.1  Diagram illustrating how a hydroelectric power plant works. Source: EIA (n.d.). Public Domain. Found here.

The volume of water flowing through the dam and the change in elevation from the reservoir to downstream side of the dam control how much energy is produced. This means larger dams with larger reservoirs can produce more energy over a specific time frame than smaller dams with smaller reservoirs.

Hydropower can produce large amounts of energy and can be used as a baseload energy source, one that can provide continuous, not intermittent, energy. The amount of water let through the penstock can be increased or decreased to generate more or less energy, respectively, as needed to meet energy demands.

While hydropower is renewable and unquestionably a cleaner energy source than fossil fuels, the reliance on dams creates greenhouse gas emissions and other environmental impacts as discussed in Chapter 2B.5.

A different type of hydropower that attempts to negate the impacts associated with large dams is run-of-river (ROR) hydropower. This can be defined as hydropower where inflow to the dam equals outflow from the dam over a relatively short period of time. The exact time frame over which these flows must be equal varies by country and the regulations that have been set there but can be almost continuous to over a period of a week. The simplest use of a system like this would be water wheels used to grind wheat at a mill. For power generation, however, this has a very different look that is often not too dissimilar from a traditional dam, albeit with a smaller reservoir.

With ROR, a portion of water is diverted from the river into the penstock to generate electricity while the rest of the water can flow freely downstream. The method used to divert the water can range from a weir, where water is slowed down behind a small wall, pooling slightly upstream before flowing over the wall, to a full dam that allows water to run through it more consistently and freely than at a traditional hydropower plant (Figure 3C.3.2). The amount of water that is diverted through the penstock can range from minimal to about 95% of the river.

left: diagram of ROR with increased water flow upstream of the weir. Water flows through the intake into the penstock then through the turbine and back into the river through the outfall. The outfall is located some distance downstream of the weir so between the outfall and the weir lies a section of river where water flow is greatly reduced. Right: photograph of an ROR project in the UK showing the stark water reduction in the zone between the weir and the outfall location.
Figure 3C.3.2 Run-of-the-river hydropower. Left: Diagram illustrating the process of ROR projects highlighting the reduced water flow in the river between the weir and the outfall from the penstock. Right: Image of a ROR project in New Mills, Derbyshire, UK. Source: Left: Lindsay Iredale (2024). CC BY-4.0. Right: Anderson et al., (2014). CC BY-4.0. Found here.

Because the water runs more freely in ROR, the size of the reservoir created is smaller than in traditional hydropower so less land is flooded behind the dam or weir. A smaller reservoir means the power plant’s output is impacted by water levels in the river, such that during times of drought or low water the power plant may not be able to produce sufficient energy to meet needs. The amount of land flooded can still be quite large though depending on the size of the ROR facility. The Dalles Dam in Oregon is a run-of-the-river hydropower plant that is among the largest 10 hydropower plants in the U.S. (Figure 3C.3.3). Emplacement of this dam created a 24 mile (39 km) long lake upstream of the dam, flooding the waterfalls that used to be present and inundating the former Native American community of Celilo.

Photograph of the Dalles Dam with a dam spanning the majority of the river to divert water. River levels upstream of the dam are noticeable higher than downstream of the dam.
Figure 3C.3.3 Photograph of The Dalles Dam in Oregon. Source: USGS (2004). Public Domain. Found here.

Because water is more consistently flowing in ROR projects, the impacts associated with stagnant water in a reservoir, such as methane production and water temperature increases are reduced. However, because water is slowed behind the weir or dam, sediment still becomes trapped altering sediment supply to downstream locations. In addition, the presence of a weir or dam still impacts fish migration along the river.

3.2 Tidal Energy

Tidal energy converts the energy from flowing tidal currents into electricity. Tides are very long waves that move through the oceans generated by the gravitational pull from the Moon and to a lesser extent the sun. The combination of the gravitational pull from the moon and the centrifugal force created by the Earth-Moon rotation creates two large bulges of water, one directed towards the moon and the other exactly opposite the Moon. As the Earth rotates under these bulges of water, coastal areas experience high and low tides (Figure 3C.3.4). Most locations have two high and two low tides every day at very predictable times.

Gif of tidal bulge creation. Earth is in the middle of an oval shaped circle representing ocean water. A circle representing the moon is to the right. The oval of ocean water is oriented so the long axis points towards the moon and the oval is thicker in the direction of the moon and on the opposite side of Earth directly away from the moon. The thicker portion of the oval represents high tide and the thin portion represents low tide. As the Earth spin under this oval of water coastal areas experience high and low tides depending on whether the area is under the thick bulge of water or the thin part of water.
Figure 3C.3.4 Two tidal bulges are created by the gravitational pull of the Moon (tidal bulge on the right) and the centrifugal force from the spinning Earth-Moon system (tidal bulge on the left). Earth’s landmasses rotate through the tidal bulges. A person standing on the Earth sees the tides rise when passing through the bulges, and fall when passing through the low points, or out of the bulges. Of course, in reality the Earth is not a smooth ball, so tides are also affected by the presence of continents, the shape of the Earth, the depth of the ocean in different locations, and more. Source: NASA/Vi Nguyen (n.d.). Public Domain. Found here.
Series of 3 illustrations showing how a tidal barrage works. Top: sluice gates (gates that allow water to pass through a dam) in the tidal barrage (dam) open during incoming tide to allow water to fill the tidal basin. Water is up to high tide levels in the tidal basin and in the ocean. Middle: sluice gates close following high tide, keeping water levels in the tidal basin high while the tide goes out. Water is at high tide level in the tidal basin but is dropping on the ocean side of the barrage. Bottom: At low tide, the sluice gates open allowing water to flow through the turbine in the dam, generating electricity. Water level is low on the ocean side and is dropping on the tidal basin side.
Figure 3C.3.5 Tidal energy system using a tidal barrage. Top: water fills the tidal basin during high tide. Middle: Water is trapped in tidal basin as tide goes out. Bottom: Tidal basin water flows out through the turbine in the dam to generate electricity during low tides. Source: Lindsay Iredale (2024). CC BY-4.0.

The first use of tidal power for electricity was similar to hydroelectric energy with the use of a tidal barrage or dam placed across an estuary near the mouth of a river. The dam traps incoming high tide water creating a filled reservoir, or tidal basin, behind the dam (Figure 3C.3.5, top). Sluice gates in the dam close once the tidal basin is filled, keeping water in the basin while the tide goes out (Figure 3C.3.5, middle). Once the tide has gone out, the water level on the ocean side of the dam is much lower than the water level in the tidal basin and the sluice gates open allowing water to flow out from the tidal basin through the turbine and generate electricity, just like in hydroelectric power (Figure 3C.3.5, bottom). Some tidal barrages are designed to generate electricity as water is flowing in and filling the tidal basin in addition to when it is flowing out.

As this style of tidal energy relies on a dam, many of the same environmental impacts associated with dams are also issues with this energy source. It is also not possible to use this style of tidal energy everywhere, as many locations do not have a large enough height difference between high and low tide, or tidal range, to generate sufficient energy or be economical to operate (building and maintaining dams is expensive!). A tidal range of at least 7m (22 ft) is needed for tidal barrage energy. Only two major tidal barrage plants are currently in operation. The largest starting producing energy in in South Korea in 2011 and has a power generating capacity of 254 megawatts (MW). The other has been operating in France since 1966 and has a generating capacity of 240MW of power. For comparison, a large hydroelectric dam is considered as anything over 30MW and very large dams, like the Hoover Dam, can produce over 2000MW. Coal and nuclear power plants are often in the 500-1000MW range or larger. Other smaller pilot plants are in Canada in the Bay of Fundy, which has the largest tidal range in the world, and in Russia.

An alternative, and newer, form of tidal energy is under development that would alleviate the need for dams. This form is called tidal stream and uses underwater turbines, essentially underwater windmills, which are turned as tidal currents come in and out of an area (Figure 3C.3.6). While this still needs a location where tidal currents are predictably strong to be both economically viable and supply sufficient power, there are more sites favorable to this style of tidal energy than to tidal barrage. The first tidal stream plant, the MeyGen Project, opened in Scotland in 2018 with a 6MW facility comprised of four 1.5MW underwater turbines. Future phases of this project will expand this to a 398MW facility.

Diagrams of tidal stream technologies. Left: underwater turbine looks like a wind turbine but is attached to the ocean floor and the turbine blades are below sea level. Right: floating turbine where the turbine is underwater but extended down from a floating apparatus on the surface.
Figure 3C.3.6 Different styles of tidal stream energy generators. Left: tidal stream turbines are attached to the ocean floor. Right: tidal stream turbines extend down into the water from a floating apparatus. Source: Left: Feldoncommon (2007). CC BY-3.0. Found here. Right: Ocean Flow Energy, Ltd. (n.d.). Copyright, use allowed with attribution. Found here.

Tidal energy is inexhaustible and low in greenhouse gas emissions; the emissions associated with this energy come mainly from the construction of the dams and turbines. Unlike some other forms of renewable energy like wind and solar, tidal energy is also highly predictable. Unfortunately, the timing of when tides are flowing, and therefore when tidal energy can be produced, does not always align with peak energy usage times. The intermittent energy from tidal sources means it cannot be relied on as the only source of energy and would need to be used in conjunction with other sources that can supply more constant energy.

3.3 Wave Energy

Waves are generated by the wind as it blows over the surface of bodies of water. Ocean waves are typically generated by large storms far offshore in ocean basins and as wind blows across the ocean it transfers some of its energy to the surface water creating waves that approach the shore as wave trains (the series of waves that hit the beach when you are on your sunny beach vacation). The energy in these waves can be captured and turned into electrical energy through wave energy converters (WECs).
The technology to collect this energy for electricity production is new, with the first wave energy test facility being established in northern Scotland in 2003. This area of the Atlantic and North Sea is very rough with constant wave action and therefore an ideal place to test wave energy technologies. The WEC developed and placed here was an attenuator style WEC called the Pelamis (Figure 3C.3.7).
Photograph of the Pelamis: set of 5 linked metal cylinders floating snake-like on the surface of a wavy ocean with storm clouds. Metal cylinders are painted red and very large: 4 meters in diameter and total length of 180 meters.
Figure 3C.3.7 Photograph of the Pelamis Wave Energy Converter on site at the European Marine Energy Test Centre (EMEC) in Scotland. Source: P123 (2008). Public Domain. Found here.
This style of WEC is a segmented snake-like converter that utilizes the bobbing motion of the waves to flex segmented cylindrical sections back and forth driving hydraulic pistons inside the segments to generate electricity (Figure 3C.3.8, left). There are many other designs for wave energy including a clam-shell shaped design, called an oscillating wave surge converter, where the base of the “shell” is attached to the ocean floor and the wave motion moves the lid of the shell back and forth, driving a hydraulic piston (Figure 3C.3.8, middle), and a floating buoy design called a point absorber that bobs up and down as waves pass also driving a hydraulic piston (Figure 3C.3.8, right). All of these are anchored to the ocean floor and have power cables that bring the generated electricity to shore.
Diagram illustrating the how three different styles of wave energy converters work as described in the text.
Figure 3C.3.8 Three styles of wave energy converter: attenuator, oscillating wave surge converter, and point absorbers. All three of these styles rely on the bobbing motion of waves to drive hydraulic pistons to generate electricity. Source: Lindsay Iredale (2024). CC BY-4.0
Wave action cannot generate energy at a constant output; larger storms generate larger waves, which means more energy. However, it is rare for waves to stop altogether on large bodies of water. So, while the exact amount of energy produced will vary depending on wave size, there will always be some amount of energy being produced, even if it is minimal. The same cannot be said of tidal, solar, or wind.
Because water is denser than air and therefore has a higher potential energy than moving air, tidal stream and wave energy could potentially generate far more energy than the wind. However, as they are both new technologies, the cost is still very high relative to the amount of energy produced. As technology advances it is expected that costs will fall and the amount of energy that can be harnessed per tidal turbine or WEC will increase to make this a more economically viable energy in the future.
Another potential issue with both tidal and wave energies is the environmental impact from these. The newness of these technologies means long term studies on environmental impacts are limited. There is the possibility that shipping routes and sea animal migrations could be affected, so careful planning and placement of energy devices is required.

Check your understanding: Water powered electricity

References

Anderson, D. H., Moggridge, H. L., Warren, P. H., & Shucksmith, J. (2014). The impacts of ‘run-of-river’ hydropower on the physical and ecological condition of rivers. Water and Environment Journal, 29(2), 268–276. https://doi.org/10.1111/wej.12101

EMEC: European Marine Energy Centre. (n.d.).Wave devices. https://www.emec.org.uk/marine-energy/wave-devices/

Ocean Energy Council (2014, March 25). Tidal Energy https://www.oceanenergycouncil.com/ocean-energy/tidal-energy/

SIMEC Atlantis Energy. (2024, February 13). MEYGEN – SAE Renewables. SAE Renewables. https://saerenewables.com/tidal-stream/meygen/

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