14 Hydropower, Waves, and Tides

“Water flowing downhill was humanity’s first source of mechanical power beyond muscle. It remains our largest source of renewable electricity—and one of the most contested.”

Hydropower is the original renewable electricity source, generating power since the 1880s. It now provides about 15% of global electricity—more than all other renewables combined until solar and wind began their recent surge. But hydropower’s future is constrained by geography, ecology, and social impacts in ways that newer technologies are not.

14.1 Hydropower Fundamentals

14.1.1 The Physics of Falling Water

Water stores gravitational potential energy based on height: \[E = mgh\]

where $m$ is mass, $g$ is gravitational acceleration (9.8 m/s²), and $h$ is height.

Power from a flowing stream of water: \[P = \rho Q g h \times \eta\]

where:

$\rho$ = water density (1,000 kg/m³)
$Q$ = volumetric flow rate (m³/s)
$h$ = head (vertical drop)
$\eta$ = efficiency (typically 85-95% for modern turbines)

The simplicity is elegant: $P = \rho Q g h \eta$. No heat engines, no Carnot limits, no combustion. Water falls; turbines spin; generators produce electricity. This is fundamentally different from every thermal energy source we have studied. A hydroelectric plant converts gravitational potential energy directly to mechanical rotation to electricity, bypassing the thermodynamic losses that plague fossil, nuclear, and even solar thermal systems. This is why hydro turbines achieve 85-95% efficiency while the best combined-cycle gas plants top out at 64%.

Back-of-Envelope: Hydropower Potential

A river with flow Q = 1,000 m³/s and head h = 100 m: \[P = 1,000 \times 1,000 \times 9.8 \times 100 \times 0.90 = 882 \text{ MW}\]

This is substantial—comparable to a large coal or nuclear plant—from a single dam on a major river.

Now consider a small stream: Q = 1 m³/s, h = 20 m: \[P = 1,000 \times 1 \times 9.8 \times 20 \times 0.85 = 167 \text{ kW}\]

The physics works at any scale, but economics favor large installations.

14.1.2 Turbine Technologies

Different head and flow combinations require different turbine designs:

High head (>100 m), low flow: Pelton turbines—impulse turbines where water jets strike buckets on a wheel. Used in mountainous terrain where steep drops are available. Efficiencies up to 92%.

Medium head (30-100 m), medium flow: Francis turbines—reaction turbines where water flows through a spiral casing and across runner blades. The most common type globally. Efficiencies up to 95%.

Low head (<30 m), high flow: Kaplan turbines—propeller-type reaction turbines with adjustable blade pitch. Used on large, flat rivers. Efficiencies up to 93%.

All three designs have been refined for over a century; efficiency improvements are incremental.

14.1.3 Dam Types and Storage

Hydropower facilities vary enormously:

Storage dams: Large reservoirs store water for release when needed, providing dispatchable power. The Three Gorges Dam (China) has 22.5 GW capacity and 39 billion m³ storage.

Run-of-river: Minimal storage; generation follows natural flow. Lower environmental impact but less flexibility. Many small hydro installations are run-of-river.

Pumped storage: Two reservoirs at different elevations; pumping water uphill stores energy, releasing it generates power. Not a net energy source but a storage technology (see Module 4).

Diversion: Water is channeled through penstocks to turbines, then returned downstream. No reservoir but requires sufficient natural head.

14.2 Global Hydropower Status

14.2.1 Installed Capacity and Generation

Hydropower is mature and large:

Metric	Value (2024)
Global capacity	~1,400 GW
Annual generation	~4,300 TWh
Share of electricity	~15%
Capacity factor	~35% (average)

Top countries by capacity: 1. China: 420 GW 2. Brazil: 110 GW 3. United States: 103 GW 4. Canada: 82 GW 5. Russia: 55 GW

14.2.2 Geographic Constraints

Hydropower requires specific geography: rivers with sufficient flow and drop, and locations where dam construction is feasible. The best sites have largely been developed in wealthy countries:

U.S. and Europe: 70-80% of technical potential developed
Africa: ~10% of technical potential developed
Asia: ~25% of technical potential developed

Remaining potential is concentrated in:

Sub-Saharan Africa (Congo River basin)
Southeast Asia (Mekong, Irrawaddy)
South America (Amazon tributaries)
Central Asia (Himalayan rivers)

But “technical potential” doesn’t mean “should be developed”—environmental and social constraints matter.

14.3 The Dam Dilemma

14.3.1 Environmental Impacts

Hydropower’s environmental footprint is complex:

River ecosystem disruption: Dams block fish migration, alter flow regimes, change water temperature, and trap sediment. The Columbia River’s salmon runs, once the largest in the world, have declined by 90% since dam construction.

Reservoir emissions: Flooded vegetation decays, releasing methane (a potent greenhouse gas). Tropical reservoirs can have significant emissions—in some cases comparable to fossil fuel plants per kWh.

Downstream impacts: Altered flow patterns affect downstream ecosystems, agriculture, and fisheries. The Nile Delta is eroding because Aswan High Dam traps sediment that once replenished it.

Land inundation: Large reservoirs flood valleys, forests, and agricultural land. Three Gorges flooded 1,000 km² and displaced 1.3 million people.

14.3.3 The China Boom

China has built more hydropower capacity than the rest of the world combined in the 21st century:

Project	Capacity	Completion
Three Gorges	22.5 GW	2006
Xiluodu	13.9 GW	2014
Baihetan	16 GW	2022
Wudongde	10.2 GW	2021

These mega-projects displaced millions and fundamentally altered river ecosystems. They also provide enormous amounts of dispatchable clean electricity.

China’s approach reflects different trilemma priorities: security and rapid development over local equity and ecosystem preservation.

14.4 Small Hydropower

14.4.1 Definition and Scale

“Small hydro” typically means <10 MW capacity (definitions vary by country). It includes:

Micro hydro: <100 kW
Mini hydro: 100 kW - 1 MW
Small hydro: 1-10 MW

Global small hydro capacity: ~78 GW across tens of thousands of installations.

14.4.2 Advantages

Small hydro offers:

Lower environmental impact (often run-of-river)
Faster permitting than large dams
Local ownership possibilities
Distributed generation near demand

14.4.3 Limitations

But small hydro faces challenges:

Higher cost per kWh than large hydro
Limited sites with sufficient head and flow
Still affects aquatic ecosystems (fish passage, flow alteration)
Vulnerable to drought and climate change

The “low-impact hydro” certification attempts to distinguish environmentally responsible projects, but inherent impacts remain.

14.5 Ocean Energy

14.5.1 The Resource

The ocean stores energy in multiple forms:

Waves: Wind energy transferred to the ocean surface. Global wave power resource: ~2-3 TW technically available.

Tides: Gravitational interaction with moon and sun creates regular, predictable tidal cycles. Global tidal resource: ~1 TW technically available.

Ocean thermal: Temperature difference between warm surface and cold deep water. Global OTEC resource: theoretically enormous but practically limited.

Salinity gradient: Energy released when fresh and salt water mix (river mouths). Small but novel.

14.5.2 Wave Energy

Wave energy technology remains pre-commercial despite decades of research.

Physics: A wave carries energy proportional to wave height squared and wave period: \[P \approx \frac{\rho g^2}{64\pi} H^2 T\]

For typical Atlantic swells (H = 3 m, T = 8 s): \[P \approx 40 \text{ kW per meter of wave front}\]

Technologies: Multiple approaches have been tested:

Oscillating water columns (OWC): Waves drive air through turbines
Point absorbers: Floating buoys that move with waves
Attenuators: Long floating devices that flex with waves
Overtopping devices: Waves fill a reservoir; water drains through turbines

Status: Wave energy has been “five years away from commercialization” for over 40 years. No technology has achieved commercial viability despite sustained R&D investment across multiple countries. The Pelamis wave energy converter, once considered the most promising approach, was abandoned after its developer (Pelamis Wave Power) went bankrupt in 2014. Wave energy remains expensive (>$200/MWh), and the harsh marine environment (salt corrosion, storm loading, biofouling) destroys equipment faster than it pays for itself. At current costs, solar PV at $30/MWh is 6-7× cheaper.

Back-of-Envelope: Wave Energy Scale

UK wave resource: ~40 kW/m along western coast UK western coastline: ~2,000 km Theoretical resource: 40,000 MW = 40 GW

But practical considerations reduce this:

Only deep-water waves are useful
Spacing required between devices
Efficiency losses (~30-40%)
Realistic capacity: ~5-10 GW

UK electricity demand: ~35 GW average Wave energy could theoretically provide 15-30%—significant but not dominant.

14.5.3 Tidal Energy

Tidal energy is more mature than wave energy, with two main approaches:

Tidal barrage: A dam across an estuary captures tidal flow. Water fills the basin at high tide; it’s released through turbines at low tide (or both directions).

The La Rance tidal barrage (France), operational since 1966, generates 240 MW with 24 turbines. It remains the largest tidal barrage in the world. Few have been built since—environmental impacts and high costs have limited deployment.

Tidal stream: Underwater turbines in tidal currents—essentially underwater wind turbines. Emerging technology with several commercial-scale deployments:

MeyGen (Scotland): 6 MW operational, planning 400 MW expansion
Multiple demonstration projects worldwide

Advantages of tidal stream:

Predictable (tides are astronomical, not weather-driven)
Less environmental impact than barrages
Higher power density than wind (water is 800× denser than air)

Challenges:

Limited high-current sites
Harsh marine environment
High maintenance costs
Grid connection from remote locations

14.5.4 OTEC: Ocean Thermal Energy Conversion

OTEC exploits the temperature difference between warm surface water (~25°C in tropics) and cold deep water (~5°C at 1,000 m depth):

\[\eta_{Carnot} = 1 - \frac{T_{cold}}{T_{hot}} = 1 - \frac{278}{298} = 6.7\%\]

Such low efficiency requires massive water flows—tens of cubic meters per second for meaningful power output. The infrastructure is enormous; practical OTEC remains theoretical despite promising physics.

A few small OTEC plants have operated (notably in Hawaii), but none have achieved commercial viability.

14.6 Pumped Hydro Storage

14.6.1 The Concept

Pumped hydro is not an energy source but a storage technology: use cheap electricity (typically overnight or during renewable surpluses) to pump water uphill, then release it through turbines when electricity is valuable.

Round-trip efficiency: 70-85% Global capacity: ~160 GW / ~9,000 GWh

Pumped hydro provides:

95% of global electricity storage capacity
Grid stabilization (frequency response, reserve capacity)
Time-shifting of renewable generation

14.6.2 Technology

A pumped hydro facility requires:

Upper reservoir at elevation
Lower reservoir (can be natural lake, river, ocean, or underground)
Penstock (pipe) connecting them
Reversible pump-turbines

Energy stored scales with reservoir volume and head: \[E = \rho V g h\]

Back-of-Envelope: Pumped Hydro Scale

Store 10 GWh (enough for 1 hour of 10 GW demand):

With 500 m head: $V = \frac{10 \times 10^{12} \text{ J}}{1000 \times 9.8 \times 500} = 2 \times 10^6 \text{ m}^3$

That’s 2 million cubic meters—a reservoir about 400 m × 500 m × 10 m deep.

For 10 hours of storage (100 GWh), you need 20 million cubic meters—significant but feasible in mountainous terrain.

14.6.3 Geographic Requirements

Pumped hydro needs:

Elevation difference (ideally >300 m)
Suitable geology for reservoirs
Water availability
Grid connection
Environmental/social acceptability

Good sites are limited. Europe and Japan have developed most of theirs. New approaches seek to expand the resource:

Closed-loop: Artificial reservoirs (not on rivers) minimize environmental impact. More expensive but more flexibility in siting.

Seawater: Using ocean as lower reservoir eliminates one dam but introduces corrosion and environmental challenges.

Underground: Converting abandoned mines to reservoirs. Pilot projects in Germany and South Africa.

14.6.4 Role in Renewable Integration

As solar and wind grow, pumped hydro’s value increases. It can:

Store midday solar for evening peaks
Absorb overnight wind when demand is low
Provide grid stability services

But pumped hydro competes with batteries (Module 4). Batteries are falling in cost rapidly; pumped hydro capital costs are relatively stable. The competition will shape storage economics.

It is worth emphasizing that pumped hydro is not an energy source but a storage technology. It consumes electricity to pump water uphill and returns 70-85% of it when the water flows back down. Today it provides 95% of all grid-scale storage globally, but its geographic requirements (elevation difference, water supply, suitable geology) constrain expansion. Whether battery storage or new pumped hydro sites fill the enormous storage gap identified by Shaner et al. (Chapter 12) is one of the defining questions of Module 4.

14.7 The Future of Hydro

14.7.1 Climate Change Impacts

Hydropower faces climate risks:

Changing precipitation patterns alter river flows
Glacier retreat affects rivers dependent on snowmelt
Extreme drought reduces generation (California, Brazil have experienced this)
Sea level rise threatens coastal infrastructure

Some regions may see increased hydropower potential (more precipitation), but many existing installations face reduced output.

14.7.2 Limited Growth Potential

Unlike solar and wind, hydropower cannot scale indefinitely:

Best sites are developed
Remaining sites face stronger environmental opposition
Climate change increases uncertainty
Social impacts limit new large dams

Global hydropower capacity grows ~2% annually—modest compared to 25% for solar. Hydro’s share of electricity will decline even as absolute generation grows slowly.

14.7.3 Value of Existing Assets

Existing hydropower is tremendously valuable:

Long-lived (50-100+ year asset life)
Low operating cost
Dispatchable (unlike solar/wind)
Provides grid services (stability, reserves)

Maintaining and upgrading existing facilities is often better than building new dams. Turbine upgrades can increase capacity 5-10% without new environmental impact.

14.8 Key Concepts

Hydropower physics: $P = \rho Q g h \eta$—power from flow and head
Turbine types: Pelton (high head), Francis (medium), Kaplan (low)
Environmental impacts: Fish migration, sediment, reservoir emissions, displacement
Small hydro: Lower impact but limited scale
Wave energy: Pre-commercial despite decades of development
Tidal energy: Barrage (mature but limited) vs. stream (emerging)
Pumped hydro: 95% of grid storage; geography-constrained

14.9 Exercises

Hydropower calculation: A dam has 80 m head and 500 m³/s flow. At 90% efficiency, what is the power output? What annual generation (GWh) at 45% capacity factor?
Reservoir sizing: You need 1 TWh of pumped hydro storage with 400 m head. What reservoir volume is required (km³)? How does this compare to a typical lake?
Wave resource: A 50 km stretch of coastline has 35 kW/m wave resource. If devices capture 30% at 50% capacity factor, how much annual generation (TWh) is possible? Compare to UK total electricity demand (~300 TWh).
Tidal predictability: Tides occur approximately every 12.4 hours. If a tidal stream turbine generates power for 5 hours around each tidal peak, what is its capacity factor? How does this compare to wind?
Reservoir emissions: A tropical reservoir floods 500 km² of forest. If it emits 1,000 kg CO₂-eq/ha/year from decaying biomass and generates 5 TWh/year, what is the emission intensity (kg CO₂/MWh)? Compare to coal (~900 kg/MWh).
Dam retirement: An aging dam produces 200 GWh/year but blocks salmon migration. Environmental groups want removal; the utility wants to continue operation. At $40/MWh, what is the annual generation value? What factors should inform the decision beyond economics?

Framework Application

Hydropower demonstrates how mature technologies navigate the framework:

Principle: Gravitational potential energy—simple, efficient, well-understood.

Technology: Turbine designs optimized over a century; little room for improvement.

Product: Dams are site-specific, custom-engineered, not mass-produced.

Policy: Hydro predates modern renewable policy; new projects face stricter environmental review.

Outcome: Large, reliable contribution to electricity supply, but constrained growth potential.

Unlike solar and wind (where learning curves drive rapid change), hydro illustrates a mature industry where the Principle-Technology-Product chain is stable. Progress comes through Policy reform (environmental mitigation, dam removal decisions) and integrating hydro’s dispatchability with variable renewables.

This completes Module 2’s survey of “Harvesting the Sun”—fossil fuels (ancient solar), solar (direct), wind (solar-driven), biofuels (photosynthesis), and hydro (solar-evaporated water). Module 3 turns to non-solar sources: geothermal and nuclear.