What is hydropower and how does it work?

Perhaps one of the most visible renewables, the workings of hydropower and its role in energy storage are as interesting as they are unknown

Contemporary to the great Greek philosophers such as Plato and Aristotle and a fundamental instrument for the growth of the Roman Empire, hydraulic energy has been of great use to mankind for millennia. However, it was not until the rise of the industrial revolution and electric power that hydropower took a major leap forward to become the key to the development of 19th and 20th century societies. Today we will take a look into the past, but also into the future of this type of renewable energy; and we will also take a look at how it works. Will you join us on this journey through the power of water to discover how hydroelectric energy works?

Controlling the forces of nature and harnessing them to our advantage has been one of mankind's greatest endeavours. Thus, generation after generation, people have been modifying our environment to our advantage in order to guarantee our survival in the first place and to improve our quality of life as a second goal. The great civilisations of antiquity carved out their world to use natural resources to their advantage and to grow thanks to them. The power of water, one of the basic elements of our planet, is no stranger to this; rather, it is the main protagonist. The Greek waterwheel, the Roman mill or the Egyptian water lifting sāqiyah are but a few examples of how hydraulic energy was fundamental for tasks such as pumping water or grinding cereals. Although its role was important at the time, it became crucial with the technological and scientific explosion experienced during the industrial revolution of the 19th century, when electricity became the backbone for the development of cities and, therefore, of the economies of industrialised countries. It was then that the hydropower we know today appeared.

Boosted by the development of the electric generator and the sophistication of turbines, hydroelectric power stations began to multiply in the most developed countries. Terms such as dam and reservoir began to spread in the public's vocabulary, and plans for the construction of large civil hydraulic works to feed the growing need for electricity emerged in Europe and America at breakneck speed, changing the landscape of our natural environment. By then, a very large proportion of all electrical energy came from harnessing the power of water.

Coexisting with other types of renewable generation such as wind and solar, hydroelectric power has remained fundamental to the present day. It is not in vain that nearly a quarter of the world's electricity is produced thanks to its operation, this share being tremendously higher in countries such as Norway (99%), the Democratic Republic of Congo (97%), Brazil (96%) or Canada (60%). An example of the great importance of this technology is that some of the great civil works of our times are hydroelectric power plants such as the Three Gorges Dam (China 22,500 MW) or the Itaipu Dam (Brazil and Paraguay 14,000 MW). But do we know how hydroelectric power works?

How does hydropower work?

Let us focus on a more or less standard hydroelectric power plant. Every power plant of this type consists of several main elements for its operation. Let's look at them one by one:

• Dam: This is the civil construction, mainly made of concrete, that allows the volume of water retained for use by the power plant operator to be maintained. Some of the most impressive ones raise the water fall by hundreds of metres.
• Reservoir: This is the water storage facility that houses the installation and collects the inflow stream.
• Sluice gates: These are the movable elements that open and close to allow water to pass through them.
• Penstock: A pipeline through which water is conveyed on its way to the power generation.
• Turbines: These are the engines that the water feeds as it passes through, allowing mechanical energy to be generated.
• Electric generators: These are the elements responsible for converting the mechanical energy of the turbines into electrical energy.
• Power line: It transports the newly generated electricity to the different consumption points.
• Draft tube: This is the outlet for all hydraulic flow used during the electricity generation process.

In short, hydroelectric power plants use the force of water falling between two points located at different heights to generate electricity. In other words, the operation of the entire installation is based on harnessing the force of gravity and the mass of water to the benefit of a series of mechanical elements that will absorb this energy potential for its subsequent conversion into electrical energy.

In this way, the water retained in the reservoir circulates inside the hydraulic circuits of the dam, bridging the difference in level between two points. This artificial waterfall, which emulates those found in natural waterfalls, allows the liquid to acquire, thanks to the effect of gravity, a speed that is transferred in the form of kinetic energy to the turbines located at the lower point of the difference in level of the infrastructure. Thus, the water circulates through the turbine, which accelerates its rotation, generating mechanical energy that is transferred to the electrical generator for conversion into electrical energy.

All in all, the water leaves the installation and is drained into the downstream part of the river without causing any harm to the environment. This whole process also achieves an outstanding efficiency in which around 90% of the potential energy of the water is used for electricity generation, generating only efficiency losses in the loading of the hydraulic circuit and in the friction process of the hydroelectric group (turbines).

In this way, we can obtain 100% renewable electricity, which is one of the most positive energies, given its high level of sustainability and reuse, and which helps to reduce the emission of greenhouse gases into the atmosphere. In addition, the water used in the process can be used for household consumption or irrigation. This type of technology, which has a long service life, is also of particular interest for regulating the course of rivers and preventing floods; a fundamental reason for the construction of the Three Gorges Dam in China, whose energy purpose is added to that of preventing a flood like the one that occurred in 1931, which caused millions of people to be affected and killed.

What types of hydroelectric power plant are there?

Due to their functionalities and locations, we can today distinguish three main types of hydroelectric power plants:

• Diversion or “run-of-river” hydropower facility: This is the model that is most dependent on weather conditions or the flow of the river, as it does not have water storage capacity, but rather uses the circulating water to generate energy. This means that these plants, most of which are smaller in size and capacity than the rest, cannot adapt to the needs of electricity demand. In their favour being that they do not interrupt the course of the river, minimising the environmental impact on the surroundings.
• Impoundment hydropower facility: This is the type of hydroelectric power plant that we would all think of if we were asked. They are based on the storage of different quantities of water with which to regulate their operation based on the electricity demand at any given time. They are large semi-natural energy batteries with which we are able to meet society's energy needs, as long as water resources are available.
• Pumped storage hydropower (PSH) facility: These are impoundment facilities built at different superimposed elevations that pump water from one reservoir to the other to generate a continuous circuit of water resources to generate energy.

What is the role of hydropower in energy storage?

One of the great characteristics of hydroelectric power is its ability to supply electricity at times of peak demand thanks to the possibility of converting reservoirs into large energy batteries. This, which was of great benefit until a few decades ago, has become increasingly important with the emergence of other renewable technologies whose production flow is decreasing due to changing climatic conditions.

Thus, the role of hydroelectric power has been reinforced with the arrival of pumping facilities that use the surplus energy from photovoltaic or wind installations to convey, upstream, the water used during the hydroelectric generation process. In this way, the wind or photovoltaic energy that is not fed into the grid during generation is used in these hydraulic installations to feed the upper reservoirs of the infrastructure, allowing this water resource to be used in a renewable manner at a later date without wasting the wind or photovoltaic energy generated. Although the wind does not always blow in our favour and sunlight does not always provide us with the necessary energy, hydroelectric power would serve as a refuge for green energy generation. In this way, a reservoir becomes a gigantic battery.

A clear example of the success of this type of technology and its great potential for growth can be found in the Spanish Gorona del Viento hydroelectric power plant, located on the island of El Hierro (Canary Islands). This power plant, commissioned in 2014, supplies 70% of the total electricity demand of this small Canary Island with a 100% renewable method; even achieving entire days in which 100% of the energy is covered by this method.