# Making electricity: thermal, nuclear, hydro, wind and solar: Hydro and wind: electricity from moving water and air Source page: https://www.materialinsights.org/learn/energy/making-electricity/p4/ [Understanding Energy](https://www.materialinsights.org/learn/energy/) / [Module 7: Making electricity: thermal, nuclear, hydro, wind and solar](https://www.materialinsights.org/learn/energy/making-electricity/) Part 4 of 5 ## Hydro and wind: electricity from moving water and air Hydro and wind plants skip heat altogether. They take the kinetic or potential energy of flowing water and moving air and turn it straight into shaft rotation, so there is no second-law penalty and no fuel. What limits them instead is geography and weather: how much water falls how far, and how fast and how often the wind blows. ### 4.1 Hydro output is flow times height, and the best sites are finite The power available from falling water is easy to estimate. Multiply the flow in cubic metres per second by the height of the fall in metres, the density of water and gravity, and you get watts. In practice a handy rule is that one cubic metre per second falling 100 metres gives about one megawatt, and modern water turbines capture around 90% of it. High-head plants in mountains use small flows dropping hundreds of metres; low-head plants on big rivers use enormous flows dropping only tens of metres. Hydro is the oldest and still the largest source of renewable electricity. Ember's review puts it at about 14% of world generation in 2025, more than wind or solar alone. The biggest dams are among the largest machines ever built: China's Three Gorges has 22.5 GW of turbines and Itaipu on the Brazil-Paraguay border about 14 GW. A reservoir is also a store. Operators can hold water back and release it when demand peaks, making hydro the most flexible large renewable and, through 'Pumped hydro storage', the main way the world stores electricity. Smil stresses the costs. Large reservoirs flood farmland, forests and villages, and have displaced tens of millions of people worldwide. Dams trap the sediment that rivers would carry to their deltas, block fish migration and change water temperature downstream. In the tropics, vegetation drowned in shallow reservoirs rots without oxygen and releases methane, which can make some reservoirs surprisingly large sources of greenhouse gas. Output also depends on rainfall: in drought years hydro generation can drop by a fifth or more. In rich countries most good sites are already used, so remaining potential lies mainly in Africa, Asia and South America. India's main potential is in the Himalaya, where many projects are run-of-river schemes with small storage. These avoid huge reservoirs but face landslides, earthquakes, glacial floods and local opposition, and several have suffered severe damage or long delays. Process flowchart From rain to electricity at a dam Inputs **Water cycle**rain and snowmelt collected in a reservoir Process steps 1 **Reservoir** water stored at height; energy = weight x height 2 **Penstock** falling water gains speed and pressure; a few% lost to friction 3 **Water turbine** about 90% of the water's energy → shaft rotation 4 **Generator and transformer** about 98% → high-voltage electricity Outputs **Electricity**about 85-90% of the water's potential energy; 1 m3/s falling 100 m gives about 0.9 MW No heat engine is involved, so there is no thermodynamic ceiling of the kind that limits steam plants. Source: Smil, Energy at the Crossroads: Global Perspectives and Uncertainties (2003), via Smil Mastery Course, M7 'Hydro and wind electricity' - Smil, Energy at the Crossroads: Global Perspectives and Uncertainties (2003) · via Smil Mastery Course, M7 'Hydro and wind electricity' - Smil, Energy Transitions: History, Requirements, Prospects (2010; 2nd ed. 2017) · via Smil Mastery Course, M7 'Hydro and wind electricity' - Ember, Global Electricity Review 2026 (April 2026) · hydro share 2025, https://ember-energy.org/countries-and-regions/world/ ### 4.2 Wind power rises with the cube of wind speed The power carried by wind through a turbine's rotor depends on the swept area and, crucially, on the cube of wind speed. Double the speed and the power rises eightfold; a site with average winds of 8 metres per second offers about twice the energy of one at 6.3 metres per second. No turbine can capture all of it, because the air must keep flowing out behind the blades. The theoretical ceiling, the Betz limit, is about 59%, and good modern machines capture around 45 to 50% at their best wind speeds. That cube law drives two things. First, siting is everything: a few hundred metres of ridge or a coastline can change a project's output substantially. Second, it pushes turbines to grow taller and wider, because winds are stronger and steadier higher up and because swept area grows with the square of blade length. Smil traces the growth from machines of about 50 kW with rotors of 15 metres in the early 1980s to offshore turbines of 10 to 15 MW with rotors over 200 metres across, and Chinese manufacturers have since tested prototypes above 20 MW. Onshore, typical new turbines are now 4 to 7 MW. Wind is variable at every timescale: gusts in seconds, weather systems over days, monsoons over seasons. Capacity factors, the average output as a share of the maximum, are typically about 25 to 35% for good onshore sites and 40 to 50% offshore, where winds are stronger and steadier. Smil warns that calm spells can cover large regions for several days, so grids that rely heavily on wind need storage, other flexible plants or long transmission links. India's best onshore wind lies in a few states, chiefly Gujarat, Tamil Nadu, Karnataka and Rajasthan, and is strongly seasonal: the south-west monsoon months bring a large share of the year's output. That pattern complements solar, which dips in the cloudy monsoon. Ember reports that India's wind generation rose 28% in 2025. Worked example #### Estimating a turbine's output A rotor 150 m across sweeps about 17,700 m2. Air at sea level weighs about 1.2 kg per m3. At 10 m/s the wind carries half of 1.2 x 17,700 x 10 cubed, about 10.6 MW, through that disc. Capturing 45% gives about 4.8 MW. At 5 m/s the wind carries one eighth as much, about 1.3 MW, and the turbine delivers only some 0.6 MW. That is why average wind speed decides a project's economics. - Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020) · via Smil Mastery Course, M7 'Hydro and wind electricity' - Smil, Power Density: A Key to Understanding Energy Sources and Uses (2015) · via Smil Mastery Course, M7 'Hydro and wind electricity' - Ember, Global Electricity Review 2026 (April 2026) · India wind growth 2025, https://ember-energy.org/latest-insights/global-electricity-review-2026/2025-in-review/ ### 4.3 Onshore wind is now the cheapest new electricity, but it is built with fossil fuels IRENA's Renewable Power Generation Costs in 2025 report gives onshore wind a global weighted-average levelised cost of electricity of about USD 33 per MWh for projects commissioned in 2025, the lowest of any technology it tracks. In China the figure was about USD 27. Installed costs fell 8% in a year to under USD 1,000 per kW. Offshore wind is far more expensive, about USD 78 per MWh, because foundations at sea, installation vessels, subsea cables and maintenance in rough water all cost more, although its higher capacity factor offsets part of the gap. The levelised cost of electricity, or LCOE, spreads all of a plant's lifetime costs, including construction, finance, operation and fuel, over all the megawatt-hours it is expected to produce. It is a useful yardstick for comparing generators, but it is a cost per unit of energy, not a value. A megawatt-hour of wind that arrives at three in the morning in a windy week, when prices are low, is worth less to the grid than one available on demand on a still summer evening. Integration costs for backup, storage and transmission are not in the LCOE. Smil adds a material point that cost figures hide. Put bluntly, a big wind turbine is built out of fossil energy. Its tower and nacelle contain hundreds of tonnes of steel, smelted with coke from coal. Its foundation needs hundreds of cubic metres of concrete, made with cement from kilns fired by coal or petroleum coke. Its blades are glass or carbon fibre bound with resins made from oil and gas, and they are hard to recycle. None of this makes wind dirty, because over its life a turbine returns its embodied energy many times over, typically within months. It does mean that building wind and solar at scale depends for now on the steel, cement and plastics industries. In India, grid connection and land acquisition often delay wind projects more than turbine supply does. Common misreading #### Cheapest per MWh is not the same as cheapest system If one source has the lowest LCOE, it does not follow that a grid built only from it would be cheapest. Wind and solar output varies, and as their share rises, more of their output arrives when it is least needed, while storage, backup and new lines must be paid for. Studies of high-renewable grids generally find the cheapest system mixes sources whose output peaks at different times, plus storage and transmission. - [IRENA, Renewable Power Generation Costs in 2025 (2026)](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2026/Jul/IRENA_TEC_RPGC_in_2025_2026.pdf (executive summary)) · web - Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M7 'Hydro and wind electricity' - Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020) · via Smil Mastery Course, M7 'Hydro and wind electricity' ### See it in the table [H₂O Water](https://www.materialinsights.org/commodities/water/)[St Steel](https://www.materialinsights.org/commodities/steel/)[Cem Cement](https://www.materialinsights.org/commodities/cement/)[O2 Oxygen](https://www.materialinsights.org/chemicals/oxygen/)[WOn Onshore wind](https://www.materialinsights.org/energy/e_onshore_wind/)[WOf Offshore wind](https://www.materialinsights.org/energy/e_offshore_wind/)[PHS Pumped hydro storage](https://www.materialinsights.org/energy/e_pumped_hydro/) ### Check your understanding Average wind speed at site A is 10% higher than at site B. Roughly how much more energy does site A's wind carry? Power scales with the cube of speed: 1.1 x 1.1 x 1.1 is about 1.33, so about a third more. Which statement about large hydro is correct? Drowned biomass decays without oxygen and emits methane. Output varies with rainfall, and remaining potential lies mostly in Africa, Asia and South America. What does the levelised cost of electricity leave out? LCOE spreads lifetime costs over expected output but treats every MWh as equal, ignoring when it arrives and what integration costs it imposes. [← Part 3: Nuclear electricity](https://www.materialinsights.org/learn/energy/making-electricity/p3/)[Mark read · Part 5: Solar electricity and the 2025 mix →](https://www.materialinsights.org/learn/energy/making-electricity/p5/) ## Other sections - [Module overview](https://www.materialinsights.org/learn/energy/making-electricity/index.md) - [Steam: the workhorse of electricity](https://www.materialinsights.org/learn/energy/making-electricity/p1/index.md) - [Gas turbines and diesel engines](https://www.materialinsights.org/learn/energy/making-electricity/p2/index.md) - [Nuclear electricity](https://www.materialinsights.org/learn/energy/making-electricity/p3/index.md) - [Solar electricity and the 2025 mix](https://www.materialinsights.org/learn/energy/making-electricity/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/energy/making-electricity/review/index.md)