Hydroelectric power

Also called: Hydel power, Hydroelectricity · Topic: Energy Resources of India: Conventional and Non-conventional · NCERT: Class 10, Ch 5 "Minerals and Energy Resources"; Class 12, Ch 5 "Mineral and Energy Resources"

Meaning

Hydroelectric power (also called hydel power or hydroelectricity) is electricity made when fast-flowing or falling water spins a turbine (a wheel with blades). The turbine turns a generator, which makes electricity. The water is not used up. It flows back into the river.

It matters because it is a renewable source of power that gives almost no smoke while it runs. It can also start within minutes, which helps the grid at the busiest hours. Most of India's untapped hydro potential lies in the Himalayan rivers of the north and north-east, and Arunachal Pradesh has the largest share of any state.

Explanation

How it works: from water to electricity

• Step 1, storing water at a height: a dam holds water in a reservoir, high above the turbines.
• The difference in height between the water and the turbine is called the head.

• A bigger head means more force and more power.

• Step 2, water falls: water rushes down a large pipe called a penstock.

• Step 3, power is made: the falling water spins the turbine, and the generator makes electricity.
• Step 4, water returns: the water goes back into the river downstream, so the water itself is not used up.
• Same product as thermal power: once electricity is made, it is exactly the same. A bulb cannot "tell" whether its power came from a dam or a coal plant (NCERT Class 10).

Types of hydel projects

• Storage (dam-and-reservoir) projects: a big dam stores a large amount of water and releases it when power is needed.
• Many are multipurpose projects: one dam scheme serves several uses, such as irrigation, power, flood control, drinking water and fisheries.

• Examples: Bhakra Nangal and Damodar Valley Corporation (DVC).

• Run-of-river projects: they use the river's natural flow and keep only a small amount of water stored.

• Example: the run-of-river-type Subansiri Lower project, 2,000 MW (8 units × 250 MW), built by NHPC Limited [2].

• Pumped storage projects (PSPs): two reservoirs, one upper and one lower, that work like a giant water battery.

• When power is cheap or plentiful (sunny afternoons, windy nights), surplus power pumps water up to the upper reservoir.
• At peak demand (evenings), the water flows back down through the turbines and makes electricity.

• Result: solar and wind power, which comes and goes, is stored for later. This keeps the power grid steady.

• Small hydro and large hydro: projects up to 25 MW are "small hydro". Projects above 25 MW are "large hydro". Since March 2019, large hydro also counts as renewable.

Conditions it needs

• Water that flows all year: rivers fed by snow and glaciers do not dry up in summer.
• Steep slopes: a steep fall gives a large head.
• Good dam sites: narrow valleys where a dam can hold a big reservoir.
• Safe ground: low earthquake risk and few landslides. Many Himalayan sites are weak on this point.

Benefits and costs

• Benefits:
• Renewable: the water cycle keeps refilling rivers.
• Clean to run: almost no smoke or CO₂.

• Flexible: it can start and stop within minutes, so it is useful at times of peak demand (the hours when the most electricity is used).

• Costs:

• Displacement: people living in the reservoir area lose their homes and land. Many of them are tribal people.
• Sediment trapping: the dam holds back silt.
  • Fields downstream lose fertile soil.
  • The reservoir slowly fills with silt (siltation), so it can store less water.
• Ecological damage: forests are flooded, fish can no longer swim upstream, and the river's natural flow changes.

Where it occurs

• India's potential:
• The CEA (Central Electricity Authority, India's top technical body for power) reassessment study (2017–2023) puts India's exploitable large hydro potential at 1,33,410 MW [4].

• CEA has also identified 1,76,280 MW of pumped storage potential [4].

• Himalayan rivers (most untapped potential): the Indus, Ganga and Brahmaputra systems in the north and north-east.

• Arunachal Pradesh has the largest share of any state.

• Why: the rivers are fed by snow and glaciers, so they flow all year, and they fall steeply, which gives a large head.

• Peninsular rivers (less potential): these rivers are rain-fed, so their flow drops in the dry season. The plateau has gentler slopes. Most good sites (Western Ghats, Krishna, Kaveri) are already in use.

• Major hydel and multipurpose projects:

Project	River	State(s)
Bhakra Nangal	Sutlej	Himachal Pradesh (Bhakra dam) and Punjab (Nangal)
Damodar Valley Corporation (DVC)	Damodar (tributary of the Hooghly)	Jharkhand–West Bengal; India's first multipurpose river valley authority (1948)
Kopili	Kopili (tributary of the Brahmaputra)	Assam (Dima Hasao district)
Subansiri Lower	Subansiri	Arunachal Pradesh–Assam border, at Gerukamukh [2]

• Subansiri Lower, India's largest hydro project:
• River: the Subansiri rises in Tibet (China) and flows south-east through Arunachal Pradesh into the Assam plains. It joins the Brahmaputra from the north (right bank), near Majuli. It is the Brahmaputra's largest tributary.
• Status: units began running from 2024. 2 of the 8 units have been commissioned, three more are due shortly, and the rest are due in phases in 2026-27 [2].
• Output when complete: about 7,422 million units (MU) a year [2].

• Protests downstream in Assam (Lakhimpur and Dhemaji districts) stopped work from 2011 to 2019.

• Pumped storage sites: many are planned in the Western Ghats (Maharashtra, Karnataka), in Andhra Pradesh and elsewhere. Steep scarps (cliff-like slopes) give a large height difference between two reservoirs a short distance apart.

• PSP status [3]:
• 10 PSPs (6.2 GW) commissioned
• 8 PSPs (8.5 GW) under construction
• 5 PSPs (5.8 GW) approved by CEA
• 46 PSPs (64.8 GW) under survey and investigation
• Target: 39 PSPs of 47 GW by 2029-30 [3][5].

Don't confuse with

• Thermal power: this is made by burning coal, oil or natural gas to boil water into steam, which then spins the turbine. It is non-renewable and polluting. Hydel power uses falling water, so no fuel is burnt.
• Small hydro vs large hydro: small hydro (up to 25 MW) was already renewable under MNRE. Large hydro (above 25 MW) was declared renewable only in March 2019, and a separate Hydro Purchase Obligation was created. (A purchase obligation means power-buying companies must buy a fixed share of their power from this source.)
• Pumped storage vs conventional hydro: a conventional plant makes power from a river's flow. A PSP mainly stores surplus power by pumping water uphill and releases it at peak demand. It works more like a battery than a primary source of power.
• Run-of-river vs storage dam: a run-of-river project keeps only a small amount of water stored and relies on the river's natural flow (for example, Subansiri Lower). A storage dam holds a large reservoir (for example, Bhakra).

Prelims Hooks

• Project–river pairs: Bhakra Nangal–Sutlej; DVC–Damodar (a tributary of the Hooghly); Kopili–Kopili (Assam, Dima Hasao district).
• The state with the largest untapped hydro potential is Arunachal Pradesh, not Himachal Pradesh or Uttarakhand.
• Subansiri Lower: 2,000 MW (8 × 250 MW), built by NHPC, on the Arunachal Pradesh–Assam border, on a north-bank (right-bank) tributary of the Brahmaputra, about 7,422 MU a year [2].
• Trap: India's identified pumped storage potential (1,76,280 MW) is larger than its large hydro potential (1,33,410 MW) [4].
• Large hydro (above 25 MW) was declared renewable in 2019. Small hydro (up to 25 MW) was already renewable.
• The 2023 Ministry of Power guidelines on PSPs allow exhausted (worked-out) coal mines to be used as reservoirs. They also exempt PSPs from free-power and Local Area Development Fund obligations [5].

Mains Points

• Spatial mismatch (GS-I):
• Hydel potential is concentrated in the north-east Himalaya, far from where most electricity is used.
• This makes long transmission lines necessary.

• It also creates conflict between upstream and downstream areas. In the Subansiri case, Assam feared for dam safety in a high-earthquake zone, sharp day-to-day changes in river flow (sudden floods and dry spells), and harm to floodplain fisheries and farming.

• Constraints on Himalayan hydro (GS-III, disasters and environment):

• Seismicity (earthquake risk: the Himalaya lies in Seismic Zones IV–V), landslides and GLOFs (glacial lake outburst floods: a lake held back by loose glacial rock bursts and floods the valley).
• Displacement of tribal communities, and siltation of reservoirs.
• Shared rivers: the Brahmaputra and Subansiri rise in China, and the Indus Waters Treaty applies with Pakistan.

• Solutions: basin-wide cumulative impact assessment (studying the combined effect of all dams on a river), dam-safety monitoring and fair resettlement.

• Hydro and pumped storage as the "battery" for the 500 GW non-fossil target by 2030 (GS-III, energy security) [6]:

• Solar and wind output varies by hour and season. PSPs store surplus power and release it at the evening peak.
• Policy support:
  • Large hydro counted as renewable since 2019.
  • PSP guidelines (April 2023) [5].
  • CEA's time to approve a DPR (Detailed Project Report) cut from 125 to 90 days [3][5].
  • Inter-State Transmission System (ISTS) charges waived for PSPs whose construction is awarded by 30 June 2028 [3].
  • A 47 GW PSP target for 2029-30 [3][5].

Related concepts

• Thermal power
• Subansiri Lower Hydroelectric Project
• Pumped storage hydropower

Read more

• Electricity: thermal and hydel power