Thousands of years ago, long before the invention of the Periodic Table, the Ancient Greeks believed that there were four elements: earth, air, fire, and water. Everything was made from the four elements.
Modern science has moved on, but we can still liken those elements to renewable energy resources.
The earth element corresponds to geothermal energy.
The air element corresponds to wind energy.
The fire element corresponds to solar energy.
So, the water element must correspond to water energy! How can we use Earth's water to generate power?
The Definition of Water Power
Hydropower, sometimes referred to as water power, is one of the oldest renewable energy sources. It uses the natural flow of water to produce power.
Water power is energy derived from the weight or motion of water.
Humans have been using hydropower for thousands of years – the Ancient Greeks used watermills to grind grain into flour.
Water has some unique properties that allow it to be utilised to create energy. The properties of water are as follows:
Water (H2O) is a liquid at room temperature. It freezes at 0ºC and boils at 100ºC.
Its molecules are cohesive (they stick together).
Water has a high specific heat capacity – i.e., it requires a lot of energy to be heated up. As a result, bodies of water tend to maintain a fairly constant temperature. This property enables organisms to thrive in water.
This unusual liquid has excellent potential as an energy resource. Let's explore why?
71% of the Earth's surface is covered by water. It can also be found underground, in the atmosphere, in rivers and lakes, in glaciers, and even in your body!
Water is a highly abundant resource found all over the planet.
Water is 800 times denser than air. As a result, it has a high energy density.
Energy density is the amount of energy stored in a given unit of volume.
Water is constantly moving. Think about rainfall, flowing rivers, and tides. Constant movement results in a large amount of kinetic energy, which can be harnessed and used to generate electricity.
Fig. 1 – Oceans constantly moving: tides, waves, precipitation, and evaporation, unsplash.com.
Examples of Water Power
There are three main methods of generating electricity using power from moving water: hydroelectric power, tidal power, and wave energy.
Hydroelectric Power
When an object is raised, it gains gravitational potential energy. Water in upland areas has gravitational potential energy, converting it to kinetic energy when it flows downhill.
Hydroelectric power is a form of electricity produced by converting the gravitational potential energy of water.
Hydroelectric dams are a popular method of generating electricity. In fact, 71% of the world's renewable energy is generated using hydroelectric technologies.
Tidal Power
The positions of the Sun and the Moon control the Earth's tides. High tides occur when the Earth is parallel to the Moon, and low tides occur when the Earth is parallel to the Sun. It takes approximately 6 hours and 12 minutes for the tides to change from high to low or vice versa.
Tidal power is a renewable source of energy produced by surging ocean waters during the rise and fall of tides.
Harnessing tidal power is a relatively new form of energy technology
Wave Energy
Winds blow across oceans, transferring their energy to the water. This causes the water particles to move in a circular pattern, creating waves. The energy of a wave is dependent on its height.
Wave energy is a form of energy harnessed from the movement of waves.
Like tidal power, harnessing energy from waves is a relatively new energy technology.
Water Power Plants
A power plant sometimes referred to as a power station, is generally connected to an electrical grid. Mechanical power is converted into electricity. In this case, the kinetic energy comes from the motion of water and is, therefore, a low-carbon, renewable power source.
A power plant is an industrial facility used for the generation of electricity.
Let's go into further detail about how hydroelectric, tidal and wave energy are harnessed so that they can generate power.
Hydroelectric Power Plants
Most hydroelectric power plants are impoundment facilities. These plants use a dam to control the flow of river water downhill. When energy is needed, water is released from the dam. The water flows downwards due to the force of gravity, where it travels through turbine blades. The movement of the turbine blades drives an electrical generator, converting the kinetic energy into electricity.
A dam is a barrier construction that holds water and raises its level, creating a reservoir, which can then be used for water storage or recreation activities.
Hydroelectric power can be harnessed to meet fluctuations in energy demands. Water stored above the dam in an upper reservoir holds a large amount of potential energy, which can be rapidly converted to electricity when required by releasing the stored water. The hydroelectric power plant then operates in the same way, generating electricity during peak times such as the weekend or evenings.
Dam operators choosing when to release the water helps deal with surges in energy demands.
Fig. 2 – The Srisailam Dam, India. The water of the Krishna River is trapped behind the dam and forced through turbines to generate electricity, unsplash.com.
Two less common types of hydroelectric power plants are diversion facilities and pumped storage facilities.
- The former uses a series of canals to channel water towards turbines, where electricity is generated.
- The latter drives water uphill into a reservoir. These are particularly useful for responding to surges in energy demands. When the demand for electricity is high, the water in the reservoir is released, travelling through turbines and generating electricity.
Tidal Power Plants
Tidal power plants harness the energy produced by the surge of ocean waters. There are three kinds of tidal power plants. All three types force water through turbines. The movement of the turbines powers generators that convert their kinetic energy into electrical energy.
The first kind of power plant is a tidal barrage. These are large, dam-like structures built across bays and estuaries. Barrage gates trap water at high tide, forcing it through turbines to return to the sea.
Another kind is tidal lagoons. These are bodies of water partly enclosed by a barrier, which can be natural or artificial. They function like a tidal barrage, trapping water and controlling its release through turbines.
Tidal barrages and tidal lagoons control the flow of water through turbines. This enables the power plant to respond to surges in energy demand by releasing water.
Wave Energy Plants
There are five main types of technology used to generate power from waves.
- Absorbers are floating structures that extract energy from movements on the surface. The kinetic energy is converted to electricity using linear or rotary generators.
- Oscillating water columns are partially submerged structures. When waves travel in and out, it forces air through turbines at the top of the structure. Movement of the turbines powers a generator, converting their kinetic energy into electricity.
- Attenuators have hinged 'arms', extracting energy from their relative movements. They are connected to hydraulic pumps, which convert the kinetic energy into electricity.
- Overtopping devices function like tidal barrages, trapping water and releasing it through turbines. Movement of the turbines powers a generator, converting their kinetic energy into electricity.
- Oscillating wave surge converters have a hinged paddle, extracting energy from its relative movement. The paddle's motion drives hydraulic pumps, which power generators to convert kinetic energy into electricity.
Advantages of Water Power
Water power can be generated in different ways, but all three water power technologies have similar advantages.
Advantage | Description |
Reliable | Water power is not influenced by weather conditions or the time of year. |
Renewable | Water is constantly moving and cycling around the Earth. It will never run out – so energy from water can always be harnessed. |
Energy efficient | Water is dense. The larger the mass of an object, the more kinetic and gravitational energy it has. Thus, more energy can be harnessed and converted to electricity. So, water-based power is a highly efficient form of energy generation. One cubic metre (1m3) of water weighs 1000 kilograms (equivalent to a tonne). |
Clean energy | Water power generation releases significantly fewer greenhouse gases and pollutants than fossil fuels. |
Cost-effective | Once in operation, it's cheap to produce electricity using water power plants. |
Longevity | Water power plants, especially hydroelectric dams, can operate for decades. |
Promotes development | Water power plants can be established in remote, off-grid locations. The electricity they generate can transform communities and encourage social development. |
Economic Advantages | Using renewable energy resources such as water power reduces the reliance on imported fossil fuels and creates job opportunities. |
Disadvantages of Water Power
No form of energy generation is perfect. Water power is no exception, below are the drawbacks to this renewable source of energy.
| Disadvantage | Description |
| Ecological Problems | Constructing water power plants alters the nearby environment. Habitat damage and flooding are common side effects. Many species die or migrate elsewhere, leading to biodiversity loss. |
| High initial cost | Constructing water power plants is expensive. However, it's expected that construction costs will fall as technology improves. |
| High maintenance | The equipment used is subjected to constant water exposure and requires regular maintenance to prevent corrosion and ensure its use. |
| Visually unappealing | Large, unsightly power plants disrupt the natural landscape. This may impact tourism. |
| Greenhouse gas emissions | Constructing hydroelectric dams can release some greenhouse gases, primarily methane, as soils are flooded. |
| Lack of accessibility | Some parts of the world have limited access to coastlines or upland rivers. |
Water Power - Key takeaways
- Water power is energy derived from the weight or motion of water. On Earth, water is abundant, dense, and always moving. These traits give it excellent potential as an energy resource.
- Hydroelectric power is a form of electricity produced by converting the gravitational potential energy of water. Dams are constructed in upland areas, trapping water and forcing it through a turbine which powers a generator.
- Tidal power is a renewable source of energy produced by surging ocean waters during the rise and fall of tides. Tidal power plants include barrages, lagoons, and in-stream turbines.
- Wave energy is a form of energy harnessed from the movement of waves. Energy is harnessed using various technologies: absorbers, oscillating water columns, attenuators, overtopping devices, and oscillating wave surge converters.
- Water power is a reliable, renewable, and efficient source of energy. However, it can cause ecological problems, have a high initial cost, and require regular maintenance.
1. Emilio F. Moran, Sustainable hydropower in the 21st century, Proceedings of the National Academy of Sciences, 2018
2. Lynn Laymon, Water World, Dive Training, 1998
3. Water Science School, How Much Water is There on Earth?, USGS, 2019
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