2. Hydroelectric energy

Hydroelectric energy

In hydro energy, the gravitational descent of a river is compressed from a long run to a single location with a dam or a flume. This creates a location where concentrated pressure and flow can be used to turn turbines or water wheels, which drive a mechanical mill or an electric generator. An electric generator, when there is excess energy available, can be run backwards as a motor to pump water back up for later use.


Pros:
Hydroelectric power stations can promptly increase to full capacity, unlike other types of power stations. This is because water can be accumulated above the dam and released to coincide with peaks in demand.
Electricity can be generated constantly, because there are no outside forces, which affect the availability of water. This is in contrast to wind, solar or tidal power, all of which are far less reliable.
Hydroelectric power produces no waste or pollution.
Hydropower is a renewable resource; oil, natural gas, and coal reserves will be depleted over time.
Hydroelectricity secures a country's access to energy supplies.



Cons:
The construction of a dam can have a serious environmental impact on the surrounding areas. The amount and the quality of water downstream can be affected, which affects plant life both aquatic, and land-based. Because a river valley is being flooded, the delicate local habitat of many species are destroyed, while people living nearby may have to relocate their homes.
Hydroelectricity can only be used in areas where there is a sufficient supply of water.
Flooding submerges large forests (if they have not been harvested). If such vegetation decayed, it could release methane, a greenhouse gas.
Dams can contain huge amounts of water. As with every energy storage system, failure of containment can lead to catastrophic results, i.e. flooding.
From wikipedia