Module 4 — Technologies in the electricity system
Lesson 4 of 9
Hydroelectricity
Learning objectives
By the end of this lesson you should be able to:
- Understand how hydroelectric power stations generate electricity.
- Distinguish between conventional hydroelectric and pumped-storage hydroelectric plants.
- Explain the strengths and limitations of hydroelectric generation.
- Recognise why hydroelectricity is one of the most flexible electricity technologies.
- Appreciate the role hydroelectricity plays in supporting system reliability.
- Understand why hydroelectric resources should be valued according to the services they provide, not simply the energy they generate.
Introduction
Hydroelectricity is one of the oldest forms of renewable electricity generation.
For more than a century, rivers and reservoirs have supplied clean, dependable electricity to homes and businesses around the world.
Unlike wind and solar power, however, hydroelectric generation is often highly controllable.
Many hydro stations can increase or decrease their output within seconds, making them invaluable for balancing electricity systems.
Some hydroelectric stations also store energy, allowing electricity generated during one period to be used many hours—or even days—later.
As electricity systems become increasingly reliant on variable renewable generation, hydroelectricity has become more valuable than ever.
How hydroelectricity works
Hydroelectric power stations convert the potential energy of stored water into electricity.
The basic process is straightforward.
- Water is stored at a higher elevation.
- Water flows through large pipes known as penstocks.
- The flowing water turns a turbine.
- The turbine drives a synchronous generator.
- Electricity is supplied to the network.
The greater the height difference between the reservoir and the turbine, the more energy can be extracted from each unit of water.
Sources of hydroelectric power
Hydroelectric schemes vary considerably in size and design.
Some rely on:
- Large dams.
- Natural lakes.
- Mountain reservoirs.
- River diversions.
Others operate directly on flowing rivers without requiring large reservoirs.
These are known as run-of-river hydroelectric stations.
Conventional hydroelectric generation
Most hydroelectric stations operate by releasing water from an elevated reservoir.
The operator controls the amount of water flowing through the turbines.
Increasing water flow increases electricity generation.
Reducing flow decreases output.
Because water flow can usually be adjusted rapidly, conventional hydroelectric stations are highly flexible generators.
Run-of-river hydro
Not all hydro stations have large reservoirs.
Run-of-river schemes generate electricity using the natural flow of rivers.
They generally have:
- Lower environmental impact.
- Smaller reservoirs or none at all.
- Lower storage capability.
Because they depend upon river flow, their output is less controllable than reservoir-based hydroelectric stations.
They therefore behave somewhere between conventional hydro and other renewable technologies such as wind.
Pumped-storage hydroelectricity
One of the most important forms of hydroelectric generation is pumped-storage hydroelectricity.
Unlike conventional hydro stations, pumped-storage plants both consume and generate electricity.
They operate using two reservoirs located at different elevations.
When electricity is plentiful or inexpensive:
- Water is pumped uphill.
When electricity is scarce or expensive:
- Water is released downhill through turbines to generate electricity.
In effect, pumped-storage hydro functions as a very large rechargeable battery.
Energy storage rather than energy creation
It is important to recognise that pumped-storage stations do not create additional energy.
Some energy is always lost during pumping and generation.
Instead, they shift electricity from one time to another.
This process is known as energy arbitrage or energy shifting.
The value lies not in producing extra electricity, but in making electricity available when it is most useful.
Operational flexibility
Hydroelectric stations are among the fastest responding generators in electricity systems.
Many can:
- Start within minutes.
- Increase output rapidly.
- Reduce output equally quickly.
Some pumped-storage stations can reach full output within seconds.
This makes hydroelectricity exceptionally valuable for balancing short-term fluctuations in electricity demand and renewable generation.
System services
Hydroelectric generators provide many services beyond energy production.
Because they use synchronous generators, they naturally contribute:
- Inertia.
- Frequency response.
- Reactive power.
- Voltage support.
- Fault current.
Many hydro stations also provide:
- Operating reserves.
- Black start capability.
- Fast balancing services.
This combination of services makes hydroelectricity one of the most versatile technologies within modern electricity systems.
Strengths of hydroelectricity
Hydroelectric generation offers numerous advantages.
Renewable energy
The primary energy source is the natural water cycle.
No fossil fuels are burned during operation.
Very low operational emissions
Hydroelectric stations generate electricity with extremely low operational carbon emissions.
Excellent flexibility
Output can usually be adjusted much more rapidly than most thermal power stations.
Energy storage
Reservoirs effectively store energy in the form of elevated water.
Pumped-storage stations actively move this stored energy between periods.
Long operating life
Many hydroelectric stations remain operational for many decades with appropriate maintenance.
Grid stability
Hydroelectric generators provide important stability services alongside electricity generation.
Limitations
Hydroelectric generation also has several constraints.
Geography
Suitable locations require favourable terrain and water resources.
Not every country possesses suitable sites.
Environmental impacts
Large dams may:
- Flood land.
- Alter river ecosystems.
- Affect fish migration.
- Change local habitats.
Environmental impacts must therefore be carefully managed.
High capital costs
Constructing dams, tunnels and reservoirs requires substantial investment.
Water availability
Electricity production depends upon rainfall, river flow and reservoir levels.
Extended droughts can significantly reduce available generation.
Finite storage
Even large reservoirs eventually become depleted if inflows remain low.
Hydroelectric storage is therefore valuable but not unlimited.
Hydroelectricity in modern electricity systems
Historically, hydroelectric stations often supplied baseload electricity.
Today their role is increasingly strategic.
Many hydro stations now operate to:
- Balance wind and solar generation.
- Provide reserves.
- Respond to sudden disturbances.
- Supply electricity during periods of peak demand.
Rather than simply generating as much electricity as possible, operators often preserve stored water until it delivers the greatest system value.
Why timing matters
Imagine a reservoir contains enough water to generate:
100 MWh.
The operator faces two options.
Option A:
Generate immediately during a period of low demand.
Option B:
Wait until evening when electricity demand is high and renewable generation has fallen.
Although the total energy produced is identical, the second option often provides much greater value to the electricity system.
This demonstrates an important principle.
Stored energy has both an energy value and a timing value.
Hydroelectricity versus batteries
Pumped-storage hydro and batteries often perform similar functions.
Both:
- Store energy.
- Respond rapidly.
- Support balancing.
However, they differ in important ways.
Hydroelectric storage generally offers:
- Much larger energy capacity.
- Longer discharge durations.
- Lower energy losses over long storage periods.
Batteries often provide:
- Faster response.
- Easier deployment.
- Greater flexibility in location.
The two technologies therefore complement rather than replace one another.
Why £/MWh is not enough
Suppose two generators both produce electricity at £60/MWh.
One produces electricity continuously whenever water is released.
Another stores electricity during one period and supplies it during another.
Their energy costs may appear similar.
However, the second technology provides additional services including:
- Energy shifting.
- Fast response.
- Operating reserves.
- Frequency support.
These services create substantial value that is not reflected by the cost of energy alone.
Hydroelectricity therefore illustrates why electricity technologies should be evaluated according to the complete set of services they provide.
A key insight
Hydroelectricity is much more than a source of renewable electricity.
It combines:
- Low-carbon generation.
- High flexibility.
- Energy storage.
- Grid stability.
- Rapid response.
Few technologies provide such a broad range of system services simultaneously.
As electricity systems become increasingly dependent upon variable renewable generation, the flexibility offered by hydroelectric resources becomes increasingly valuable.
Key takeaways
- Hydroelectric power stations convert the potential energy of stored water into electricity.
- Reservoir-based hydro is highly controllable, while run-of-river hydro depends more directly on natural water flows.
- Pumped-storage hydro stores energy by pumping water uphill when electricity is abundant and releasing it when electricity is scarce.
- Hydroelectric generators provide rapid response, reserves, inertia and other important system services.
- Geography and water availability limit where hydroelectric schemes can be developed.
- Hydroelectricity plays an increasingly important role in balancing renewable electricity systems.
- The value of hydroelectric resources depends not only on how much electricity they generate, but also on when and how flexibly they provide it.
Looking ahead
Hydroelectric reservoirs have long been the world's largest form of electricity storage.
The next lesson explores electricity storage more broadly, examining batteries and other storage technologies that are becoming increasingly important as electricity systems transition towards low-carbon generation.