Renewable Energy: Water, Wind, Sun, Biomass, Earth Heat and Tides – Power and Energy Production in the World (Senior 6 Geography) | Ganzaa

Power and Energy Production in the World

Lessons

Senior 6 · Geography

Power and Energy Production in the World

4 lessons

Lesson 2 of 4

Renewable Energy: Water, Wind, Sun, Biomass, Earth Heat and Tides

25 min

Three countries, three different answers

Some countries already make most of their electricity without fossil fuels. Study the table, taken from IEA statistics, and suggest what physical feature of each country explains its main source.

Country (2024)	Main sources of electricity	What you might expect to find there
Ethiopia	Hydropower 97.1%, wind 2.7%	High plateaus, heavy seasonal rain, deep gorges (the Blue Nile)
Norway	Hydropower 88.9%, wind 9.2%	Mountains, glaciated valleys, high rainfall and snowmelt
Kenya	Geothermal 43.2%, hydropower 28.3%, wind 14.0%	The Rift Valley with volcanoes and hot rocks; windy northern plains

The pattern is clear: renewable energy is where nature puts it. Rwanda follows the same rule. REG reports that hydropower produced about 35% of the electricity on Rwanda's grid, from plants such as Nyabarongo I (28 MW) and the Rusumo Falls plant (80 MW), which Rwanda shares with Burundi and Tanzania (REG, 2024 Review).

Definition

Renewable energy: energy from a source that is naturally replaced as fast as it is used, such as sunlight, wind, flowing water, plant growth, the Earth's internal heat and the tides. Renewable sources are flow resources: they cannot be used up, but they can be wasted or damaged.

Hydroelectric power (HEP): electricity generated by water falling through a turbine.

Head: the vertical height through which the water falls, from the reservoir surface to the outflow.

Intermittent source: one whose output rises and falls with the weather or time of day, such as solar and wind power.

Capacity factor: actual energy produced in a period divided by what the plant would produce running at full capacity all the time: $CF = \frac{E_{\text{actual}}}{P_{\text{max}} \times t}$ .

Biomass: plant and animal material used as fuel: wood, charcoal, crop waste, dung, and biogas or biofuels made from them.

Geothermal energy: heat from inside the Earth, brought up as steam or hot water.

Tidal range: the difference in height between high and low tide.

Water: hydroelectric power

Hydropower gave 14.7% of the world's electricity in 2024, more than any other renewable source (IEA). In a dam scheme (Figure 1), water from a reservoir enters an intake, rushes down a steep pipe called a penstock, spins a turbine joined to a generator, and leaves through the tailrace back to the river.

Figure 1: A hydroelectric dam in cross-section.

The power available depends on how much water flows each second and how far it falls:

P = \rho\, g\, Q\, h\, \eta

where $\rho = 1000\,\text{kg m}^{-3}$ is the density of water, $g = 9.81\,\text{m s}^{-2}$ , $Q$ is the discharge in $\text{m}^3\,\text{s}^{-1}$ , $h$ is the head in metres and $\eta$ is the efficiency of turbine and generator (about 0.8 to 0.9). This is why the best sites combine a large, reliable river with a big drop. The giants include the Three Gorges Dam on the Yangtze in China (22 500 MW) and the Grand Ethiopian Renaissance Dam on the Blue Nile (5 150 MW). The Congo River at Inga has some of the greatest untapped potential on Earth.

Worked example 1: power and energy from a small dam

Problem. An illustrative hill river in Rwanda has a discharge of 12 m³ s⁻¹. A dam gives a head of 45 m and the equipment is 85% efficient. (a) Calculate the power output. (b) If the plant has a capacity factor of 0.60, how much energy does it produce in a year of 8 760 hours?

Solution.

\text{(a) } P = 1000 \times 9.81 \times 12 \times 45 \times 0.85 = 4.50 \times 10^{6}\,\text{W} \approx 4.5\,\text{MW}

\text{(b) } E = 4.5\,\text{MW} \times 8\,760\,\text{h} \times 0.60 \approx 23\,700\,\text{MWh} \approx 23.7\,\text{GWh}

The capacity factor is below 1 because the river runs low in the dry seasons (roughly June to September and December to February in Rwanda), and the plant must stop for maintenance.

Wind and sun

Wind power gave 8.0% of world electricity in 2024, and solar photovoltaic (PV) power 6.6%, the fastest-growing source of all (IEA). A wind turbine's blades turn a generator; the power in the wind passing through the blades is

P = \tfrac{1}{2}\, \rho_{\text{air}}\, A\, v^{3}

where $A$ is the area swept by the blades and $v$ the wind speed. Because power rises with the cube of speed, a slightly windier site is far more valuable, which is why wind farms are built on coasts, open plains, ridges and offshore. Africa's largest example is the 310 MW Lake Turkana wind farm in northern Kenya, where winds funnel between highlands.

Solar energy is captured in two ways. Photovoltaic panels turn light directly into electricity, from a single roof panel to a huge field. Concentrated solar power (CSP) uses mirrors to focus sunlight, heat a fluid and raise steam; because the heat can be stored in molten salt, CSP can keep generating after sunset. Morocco's Noor Ouarzazate complex uses both. In Rwanda, the 8.5 MW Rwamagana plant, opened in 2014, was among the first utility-scale solar fields in East Africa, and off-grid solar kits brought electricity to 22.5% of households by March 2024 (REG).

Worked example 2: why a windier site matters

Problem. Two possible sites for the same turbine have average wind speeds of 6 m s⁻¹ and 9 m s⁻¹. By what factor is the power in the wind greater at the second site?

Solution. The air density and swept area are the same, so only $v^{3}$ changes:

\frac{P_2}{P_1} = \left(\frac{9}{6}\right)^{3} = 1.5^{3} = 3.375 \approx 3.4

A wind speed 1.5 times higher gives about 3.4 times the power. Conclusion: careful measurement of wind speed for a year or more is the first step in siting a wind farm.

Biomass, the Earth's heat and the tides

Biomass. Wood, charcoal, crop residues and dung are the oldest fuels and still the main cooking fuels for most of sub-Saharan Africa. In Rwanda's 2022 census, 76% of households cooked mainly with firewood (NISR). Biomass is renewable only if trees are replanted as fast as they are cut. Modern forms include biogas from animal dung and human waste in digesters, and biofuels such as the ethanol Brazil makes from sugar cane.
Geothermal. Near plate boundaries and rift valleys, hot rock lies close to the surface. Wells tap steam that drives turbines. Kenya's plants at Olkaria, in the Rift Valley near Lake Naivasha, produced 43.2% of the country's electricity in 2024 (IEA); Iceland, Indonesia, the Philippines and New Zealand are other leaders. Western Rwanda lies on the western branch of the Rift and has hot springs; REG has explored areas such as Karisimbi, Kinigi, Gisenyi and Bugarama.
Tidal. A barrage across an estuary traps water at high tide and lets it out through turbines as the tide falls. It needs a large tidal range, roughly 5 m or more. The best-known stations are La Rance in France (240 MW, opened in 1966) and Sihwa Lake in South Korea (254 MW). Tidal power is very predictable but suits few coasts, and tide still gives almost 0% of world electricity (IEA, 2024).

Comparing renewable sources

Source	Conditions it needs	Strengths	Limits
Hydropower	Large, reliable river; steep gradient or gorge; hard rock for a dam	Cheap power once built; can be stored and released to meet peaks	Floods land and villages; silting; output falls in droughts
Wind	Strong, steady winds: coasts, plains, ridges, offshore	No fuel; quick to build; land below can still be farmed	Intermittent; noise and harm to birds; needs backup or storage
Solar	Many sunny hours; open land or roofs	Works at any scale, even one home; costs have fallen fast	No output at night; needs batteries; large fields take land
Biomass	Forests, plantations, crop and animal waste	Available locally; biogas also gives fertiliser	Deforestation if overused; indoor smoke harms health
Geothermal	Hot rock near the surface: rifts, volcanoes, plate boundaries	Steady day and night; small land area	Few suitable places; costly, risky drilling
Tidal	Estuary with a large tidal range	Perfectly predictable	Very few sites; barrages change estuary ecosystems

Warning

Common mistakes. "Renewable" does not mean "harmless": dams flood land and displace people, and wood smoke kills. Nor does a renewable plant run at its full capacity: a solar plant gives no power at night, so compare plants by the energy they produce (MWh), not only by their capacity (MW). And do not confuse geothermal energy (the Earth's internal heat) with solar heating of the ground.

Tip

Memory hook. Link each renewable to the landform that supplies it: gorge → hydro, ridge or coast → wind, desert → solar, rift → geothermal, estuary → tidal, forest or farm → biomass. In an exam, a named place for each earns more marks than a list.

TED-Ed: "Can 100% renewable energy power the world?" by Federico Rosei and Renzo Rosei. Watch for the main obstacles the video names, especially intermittency and storage, and compare them with the limits in the table above.

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Practice

Define renewable energy and explain why it is called a flow resource.
State three physical conditions needed for a hydroelectric power station.
A river has a discharge of 20 m³ s⁻¹ and a head of 30 m; the equipment is 80% efficient. Calculate the power in MW. (Take g = 9.81 m s⁻².)
Wind speed at a site falls from 8 m s⁻¹ to 4 m s⁻¹. By what fraction of its former value does the power in the wind fall?
Explain why geothermal power is important in Kenya but not yet in Rwanda.
Give two advantages of solar home kits for rural Rwanda compared with extending the national grid.
"Biomass is a renewable resource, so its use in Rwanda is sustainable." Evaluate this statement.

Show answersHide answers

Energy from sources that are naturally replaced as fast as they are used; it is a flow of energy (sunlight, wind, water) that keeps arriving rather than a fixed stock that is used up.
Any three: a large and reliable flow of water; a steep gradient, waterfall or deep gorge giving a high head; hard, impermeable rock for the dam's foundations; a site for a reservoir; a market and transmission lines nearby.
P = 1000 × 9.81 × 20 × 30 × 0.8 = 4 708 800 W ≈ 4.7 MW.
(4 ÷ 8)³ = (½)³ = ⅛, so the power falls to one eighth of its former value.
Kenya's Olkaria field lies in the eastern Rift Valley, where hot rock and steam are close to the surface and have been proven by drilling. Rwanda lies on the western Rift and has hot springs, but exploration has not yet proved a reservoir hot and permeable enough to be worth developing.
For example: they are cheap and quick to install in scattered homes on steep hills, where poles and lines are costly; and each home's power does not depend on a distant power station or long lines.
Biomass is renewable only if trees are replanted at least as fast as they are cut. With 76% of households cooking mainly with firewood (NISR, 2022), demand can exceed regrowth, causing deforestation, soil erosion and fuel shortages, and smoke harms health. It can be made sustainable with woodlots, improved stoves, biogas and cleaner fuels, so the statement is only partly true.

Summary

Renewable sources are flows that keep arriving: water, wind, sunlight, biomass, the Earth's heat and the tides.
Each needs particular physical conditions, so renewable energy is produced where nature provides it: Ethiopia's gorges, Kenya's rift, Morocco's desert, the Rance estuary.
Hydropower output depends on discharge and head ( $P = \rho g Q h \eta$ ); wind power rises with the cube of wind speed.
Wind and solar are intermittent; hydro, geothermal and biomass can give steady power.

Sources and credits

Country and world electricity shares: Energy Statistics Data Browser, International Energy Agency (IEA), 2024 data, retrieved 2 October 2026. Data cited, not copied.
Rwandan facts: Rwanda Energy Group, Rwanda Least Cost Power Development Plan, 2024 Review (2025), and the REG pages Hydro power, Solar, Geothermal and Facts and figures; cooking fuel from the Fifth Population and Housing Census, 2022, NISR. Facts cited, not copied.

Photos: Noor 1 and 2 - Ouarzazate Solar Power Station (Richard Allaway, CC BY 2.0) and Rance Tidal Power Station (Sw271149, CC BY-SA 4.0), via Wikimedia Commons.

Video: "Can 100% renewable energy power the world?", TED-Ed, embedded from YouTube.

Figure 1, the text, worked examples and exercises: written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Three countries, three different answers

2Water: hydroelectric power

3Wind and sun

4Biomass, the Earth's heat and the tides

5Comparing renewable sources

Three countries, three different answers

Water: hydroelectric power

Wind and sun

Biomass, the Earth's heat and the tides

Comparing renewable sources