Renewable and Non-Renewable Energy Sources

Lessons

Senior 3 · Physics

Renewable and Non-Renewable Energy Sources

4 lessons

Lesson 4 of 4

Rwanda's Energy Sources and the Move to Renewables

25 min

What is on the fire tonight?

At supper time, smoke rises from kitchens across the hills. According to the National Institute of Statistics of Rwanda, firewood was the main cooking fuel of 92.5% of rural households in 2023–24, about half of urban households cooked with charcoal, and only 5% of all households used modern stoves such as gas or electric ones (NISR, EICV7). Meanwhile, most of the country's electricity comes from water, gas from Lake Kivu, peat and imports.

So Rwanda's energy story has two parts: the fuel used for cooking, still mostly wood, and the sources of electricity. This lesson classifies both, and asks why the move towards clean, renewable energy matters.

Experiment 4: how much wood does it take to boil water?

You need: two identical cooking pots, 4.0 L of water, dry firewood, a kitchen scale, a thermometer, a stopwatch, three large stones and an improved (enclosed) clay stove. Do this outdoors with your teacher.

Weigh a bundle of firewood. Put 2.0 L of water in a pot on a fire between three stones.
Keep the fire going until the water boils. Record the time, then put out the fire, let the unburnt wood cool, and weigh it. The difference is the mass of wood burned.
Repeat with the other pot of water on the improved stove.

Figure 4.1: The improved stove surrounds the fire with clay walls, so more of the heat goes into the pot.

Stove	Wood burned (kg)	Time to boil (min)
open three-stone fire	0.40	25
improved stove	0.20	18

These are illustrative results. The improved stove boils the same water with about half the wood, and faster. Less wood burned means fewer trees cut and less smoke in the kitchen.

Important

Safety. Work outdoors, away from buildings and dry grass, with a bucket of water or sand nearby. Only your teacher lights and puts out the fires. Keep back from the smoke, use cloths to handle hot pots, and never leave a fire unattended.

Worked example 1: the efficiency of a stove

Heating 2.0 kg of water from 20 °C to boiling needs about 0.63 MJ of energy (you will learn to calculate this in the unit on heat). Dry firewood releases about 16 MJ per kilogram when it burns (a typical value). Use the illustrative results to find the efficiency of each stove.

\text{Open fire: } \frac{0.63\ \text{MJ}}{0.40\ \text{kg} \times 16\ \mathrm{MJ\,kg^{-1}}} \times 100\% = \frac{0.63}{6.4} \times 100\% \approx 10\%

\text{Improved stove: } \frac{0.63}{0.20 \times 16} \times 100\% = \frac{0.63}{3.2} \times 100\% \approx 20\%

Even the better stove wastes four fifths of the wood's energy, but it doubles the useful share.

Classifying Rwanda's sources of electricity

Figure 4.2: Installed generation capacity serving Rwanda's grid, from Rwanda Energy Group's Least Cost Power Development Plan, 2024 Review (September 2025). "Imports and shared plants" includes 100 MW imported from Uganda and Rwanda's share of plants built with neighbouring countries, such as Rusumo.

Source	Renewable?	Installed capacity (MW)	Share of electricity generated
hydropower	yes	109.8	35%
methane from Lake Kivu	no (used faster than it forms)	85.8	35%
imports and shared plants	counted as renewable by REG	144.8	20%
peat	no	85.0	8%
solar	yes	12.05	1.2%
diesel and heavy fuel oil	no	28.8	1%

Source: REG, 2024 Review, Table 3 and Figure 6. Notice that the capacity of a plant and the energy it actually produces are different things: diesel plants have 6% of the capacity but give only 1% of the electricity, because they are kept for emergencies and peak times. Access has grown fast: by March 2024, 76.7% of households had electricity, 54.2% from the grid and 22.5% from off-grid systems such as solar kits (REG, Facts and Figures).

Worked example 2: how renewable is Rwanda's electricity?

Use the shares of electricity generated in the table. REG counts hydropower, solar, and imports and shared plants as renewable. Find the renewable share, and the share from methane and peat together.

\text{Renewable: } 35\% + 1.2\% + 20\% = 56.2\%

\text{Methane + peat: } 35\% + 8\% = 43\%

REG reports the renewable share of generation as 56.21%, which matches. About two fifths of the electricity still comes from local non-renewable sources.

Why the move to renewable energy matters

Climate. Burning methane, peat, diesel and wood that is not replanted adds carbon dioxide to the air, which warms the climate. Rwanda's farming depends on regular rains, which climate change makes less reliable.
Health. Smoke from open fires and simple stoves damages the lungs and heart. The World Health Organization estimates that household air pollution caused about 2.9 million deaths worldwide in 2021, and that around 2.1 billion people still cook with polluting fuels (WHO fact sheet, 2025). Women and children, who spend most time near the fire, suffer most.
Forests and soil. REG warns that biomass "may be non-renewable unless properly managed"; cutting trees faster than they regrow leads to erosion and landslides on Rwanda's steep hills (REG, Biomass).
Cost and security. Diesel must be imported and is the most expensive way to make electricity, and peat and Lake Kivu methane will not last for ever. Sunshine and rivers are free and local.

The move is already happening: more hydropower (such as the 43.5 MW Nyabarongo II project, which REG also plans to use for water supply, irrigation and flood control), solar plants, mini-grids and solar home kits, and cleaner cooking with improved stoves, biogas and gas.

Worked example 3: the wood saved by better stoves

REG reports that an average household cooking with a traditional stove uses about 1.8 tonnes of firewood a year. If an improved stove halves this, how much wood do 1000 households save each year?

\text{saving per household} = \frac{1.8}{2} = 0.9\ \text{t}

\text{for 1000 households} = 0.9 \times 1000 = 900\ \text{t per year}

Warning

Capacity is not output. A 28.8 MW diesel plant that runs only at peak times produces far less energy than a 26 MW methane plant that runs day and night. When comparing sources, look at the energy each actually produces (MWh or the share of generation), not only the megawatts installed.

A lorry loaded high with sacks of charcoal on a dirt road in Gisovu, Rwanda. Charcoal is made by heating wood with little air; making it wastes much of the wood's energy, so each sack of charcoal uses up much more wood. Photo: JEsa, CC BY-SA 4.0, via Wikimedia Commons

Tip

Judge a source with four questions: Will it run out? What does it do to the air, land and water? Is it available when needed (night, dry season)? What does it cost to build and to run? A good energy plan mixes sources so that the weaknesses of one are covered by another.

Rwanda's Largest Solar Field Also Empowering Orphans (Voice of America). A news report on the solar plant at Rwamagana, built on land belonging to a youth village; watch for the rows of panels and what the plant means for the community.

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Practice

Classify each as renewable or non-renewable: hydropower, peat, solar, diesel, methane from Lake Kivu, biogas, firewood from a replanted woodlot.
From Figure 4.2, which single source has the largest installed capacity inside Rwanda? What percentage of the total (about 466 MW) is it?
Give two reasons why Rwanda wants more of its energy to come from renewable sources.
Explain how cooking on an open fire harms health, and give two ways to reduce the harm.
A stove burns 0.30 kg of wood (16 MJ per kg) to deliver 0.63 MJ to a pot of water. Find its efficiency.
According to NISR, what percentage of rural households did NOT use firewood as their main cooking fuel in 2023–24?
Hydropower output falls in the dry season. Which of Rwanda's sources can keep the grid supplied then, and what is the drawback of each?
A remote village with no grid must choose between a diesel generator and a solar mini-grid with batteries. Evaluate the two options and recommend one.

Show answersHide answers

Renewable: hydropower, solar, biogas, firewood from a replanted woodlot. Non-renewable: peat, diesel, methane from Lake Kivu (at present rates of use).
Hydropower: $\frac{109.8}{466} \times 100\% \approx 24\%$ . (Imports and shared plants are larger in total, but they are not all inside Rwanda.)
Any two: to cut carbon dioxide emissions and slow climate change; to protect health from smoke; to save forests; to avoid costly imported diesel; because peat and methane will run out.
Smoke from burning wood contains fine particles and harmful gases that damage the lungs and heart. Reduce it with an improved stove or a clean fuel (biogas, gas, electricity), a chimney or good ventilation, and dry wood.
$\frac{0.63}{0.30 \times 16} \times 100\% = \frac{0.63}{4.8} \times 100\% \approx 13\%$
$100\% - 92.5\% = 7.5\%$
Methane (runs day and night, but non-renewable and emits $\mathrm{CO_2}$ ); peat (local, but non-renewable, emits $\mathrm{CO_2}$ and damages wetlands); diesel (quick to start, but imported and costly); imports (depend on neighbours).
Diesel: cheap to buy and gives power at any time, but fuel is costly, must be transported, and it is noisy and polluting. Solar mini-grid: costs more to build and needs batteries for the night, but its fuel is free, it is clean and quiet, and Rwanda has good sunshine. Over many years the solar mini-grid is usually the better choice; a small diesel back-up can cover long cloudy spells.

Key points

Most Rwandan households cook with firewood or charcoal; better stoves and clean fuels save wood and protect health.
Rwanda's electricity comes from hydropower, Lake Kivu methane, peat, solar, diesel and imports; REG puts the renewable share of generation at 56%.
Installed capacity (MW) is not the same as energy produced (MWh).
Moving to renewables matters for the climate, health, forests, cost and energy security.

1What is on the fire tonight?

2Experiment 4: how much wood does it take to boil water?

3Classifying Rwanda's sources of electricity

4Why the move to renewable energy matters

What is on the fire tonight?

Experiment 4: how much wood does it take to boil water?

Classifying Rwanda's sources of electricity

Why the move to renewable energy matters