Hydropower is a renewable energy that is generated from water moving in the hydrological cycle. Hydropower uses the energy of water moving from higher to lower elevations on its way back to the ocean, driven by the force of gravity. The primary source of energy is the sun since the water movement is produced by solar radiation.


Solar radiation reaches the land or the sea surface and then is absorbed by them, causing an increase in the medium temperature of the Earth and creating evaporation in those places where water is available. Close to 50% of all the solar radiation reaching the Earth’s surface is used to evaporate water and drive the hydrological cycle. For this reason, the potential energy of this cycle is huge but not all the energy can be technically used. The resulting evaporated water moves ups and increases the water vapour content in the atmosphere.

Wind moves the air and also the vapour content over the surface of the Earth, up to thousands of kilometres from the origin of evaporation. Eventually, the vapour condenses and falls as precipitation: about 78% on oceans and 22% on land. This creates a flow of water from the oceans to the land surface of the Earth, and an large flow of water back to the oceans through rivers and groundwater runoff. The flow of water in rivers can be used to generate hydropower.


The potential of hydropower can be estimated by total available water flow multiplied by head and a conversion factor. Most precipitation falls in high elevations. For this reason, the highest potential for hydropower development is in mountainous regions, or in rivers coming from such areas.The total annual runoff has been estimated as 47,000 km3, out of which 28,000 km3 is surface runoff.

The theoretical potential of hydropower has been calculated in 41,784 TWh/yr (for 2004) and it has been also estimated in 39,894 TWh/yr (for 2010). The efficiency that is achieved depends on the amount of water, the water loss due to leakage, differences in head (between upstream and downstream), energy losses due to friction and velocity change and the design of the electromechanical equipment.


Hydropower facilities


This kind of facilities stores the water in a reservoir for later consumption. The generating stations are located downstream. The reservoir is connected to the generating station through pipelines. Gravity causes the water going downstream acquiring kinetic energy (the energy that is caused by the speed of water flow). The kinetic energy of the water flow is converted into mechanical energy by a water turbine and then, the mechanical energy is converted into electricity thanks to an electrical generator.



Run-of-river systems are ideal for streams or rivers with a minimum dry weather flow or those systems regulated by larger hydropower facilities upstream. Projects with poundage can store water for peak load demand or continuously, especially in wet seasons. The dams in ROR systems are smaller than in a conventional storage hydropower. This means that ROR don not require a large impoundment of water. Small and well-located ROR facilities can be carried out with minimal environmental impacts (larger hydropower facilities cause more environmental impacts). For this reasons, ROR systems are often referred to as environmentally friendly.


Micro hydro is considered as a ROR system meaning that water diverted from the stream or river is redirected back into the same watercourse. Micro hydro are small plants that normally produce up to 100KW of electrical energy using the natural flow of the water. These facilities are suitable for providing power to a small house or community and if it is permitted, they are connected to the electrical grid.


Pico hydro can be considered also a run-of-river system since dams are not used.The difference between Micro hydro and Pico hydro is the amount of electricity generated.The generation of hydroelectricity in pico hydro is under 5kW while in micro hydro is up to 100kW. For this reason, pico hydro must be used in smaller facilities than micro hydro. Pico hydro is used in small communities that have a low demand of electricity.



Water is pumped from a lower reservoir into an upper reservoir. The pumped storage plant can provide a large energy storage system benefits.



Conduit hydropower is based on the use of existing pipelines, tunnels, canals, aqueducts and other structures that drive water. These structures are combined with electric generation devices. Conduit hydropower is also known as small hydro and it is able to extract power from water turning it into electricity without the need of large facilities as a dam.

Conduit hydropower facilities are efficient and environmentally friendly since:

  • They can generate electrical energy from existing water flows. This means that changes in water cycle are not produced in this kind of projects.
  • They are affordable: only minor new civil engineering works are needed.
  • It is a continuously source of renewable electrical energy.
  • These facilities are non-pollutant (not gasses or heat are released).

Benefits to the environment

Coolmyplanet has choosen hydropower as a solution to climate change because:

  • In contrast to energy produced in fossil fuel power stations, the energy production process in hydropower does not release CO2 to the atmosphere.
  • Hydropower is a renewable energy because it relies on the hydrological cycle driven by the sun.

However hydropower also has disadvantages: big dams have negative environmental impacts:

  • CO2 and CH4 emissions.
  • Habitat perturbation.
  •  Water cycle modification.
  • Croplands sumerged.
  • Lost of biodiversity

For these reasons, research in hydropower should be concentrated on small-scale hydropower applications.


It has been shown that large hydroelectric power plants are not environmentally friendly. That is why there is a trend in the growth of small-scale hydropower facilities as Micro hydro or Pico hydro. The hydropower potential depends mainly in the annual change of runoff of the place. The changes in runoff are understood in terms of water availability. In the following map we can observe the large-scale changes in water availability for the period 2090 to 2099:


IPCC has estimated changes in the hydropower generation capacity by 2050:


We can notice an increase in power generation in Asia and a decrease in power generation capacity in Europe.

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