Renewable and Non-Renewable Energy Sources
Senior 3 · Physics
Renewable and Non-Renewable Energy Sources
Lesson 3 of 4
25 min
Deep in Lake Kivu lies an unusual store of energy: methane gas dissolved in the water. According to Rwanda Energy Group (REG), the lake holds 60 to 70 cubic kilometres of methane, of which about 44.7 km³ can be extracted, and natural processes add about 0.14 km³ more each year. Since 2015 the KivuWatt plant at Karongi has burned this gas to make electricity, and since 2023 a second large plant in Rubavu has done the same.
The methane is slowly being made again, so is it a renewable source? To answer, we need to compare how fast a source is used with how fast it is replaced. That comparison is the key to this lesson.
Definition
A non-renewable energy source is one that is formed far more slowly than it is used, usually over thousands or millions of years, so the stock we can reach will one day run out. Examples: coal, crude oil and the fuels made from it, natural gas, peat and uranium.
A fossil fuel is a fuel formed from the remains of plants and animals that lived millions of years ago, buried and changed by heat and pressure: coal, crude oil and natural gas.
You need: a candle (candle wax is made from crude oil), a clean, dry, cold drinking glass, a gas jar with a lid, limewater, matches, safety glasses and a tile to stand the candle on.
Figure 3.1: The products of burning a fuel made from crude oil.
Observations: droplets of water mist the glass, and black soot collects on it. The limewater shaken with the gas from the jar turns milky; the limewater shaken with ordinary air stays almost clear.
Explanation: fossil fuels are made mainly of compounds of carbon and hydrogen (hydrocarbons). When they burn in oxygen, the carbon becomes carbon dioxide and the hydrogen becomes water. The soot is unburnt carbon, from burning with too little air. Methane, the gas from Lake Kivu, burns in the same way:
Every fuel that contains carbon (coal, oil products, natural gas, peat, wood and charcoal) releases carbon dioxide when it burns.
Important
Safety. Wear safety glasses, tie back long hair and keep sleeves and paper away from the flame. The glass and jar get hot: hold them with a cloth. Limewater irritates the eyes; wipe up spills and wash your hands afterwards.
Millions of years ago, dead plants in swamps were covered by water and mud before they could rot completely. Layer upon layer built up. The first stage is peat: brown, spongy, partly decayed plant material that is still forming in wet valleys today. Buried deeper, heat and pressure squeezed out water and gases, turning peat slowly into lignite and then coal. Crude oil and natural gas formed in a similar way from tiny sea plants and animals buried under the sea bed. All of this takes far longer than a human lifetime, which is why these fuels are non-renewable.
| Stage | What it is | Carbon content and energy |
|---|---|---|
| living plants | trees, reeds and papyrus in a swamp | lowest |
| peat | partly decayed plants, wet and spongy | low; must be dried before burning |
| lignite (brown coal) | peat compressed for millions of years | medium |
| coal | hard, black rock formed deeper and hotter | highest |
Figure 3.2: Extracting methane from Lake Kivu. At depth the high pressure keeps the gas dissolved, as in a closed bottle of soda. When the water is pumped up, the pressure falls and the gas bubbles out, just as a soda fizzes when it is opened.
| Source | Advantages | Disadvantages and effects on the environment |
|---|---|---|
| coal, oil products, natural gas | a lot of energy in a small mass; easy to store and transport; power on demand, day and night | will run out; release carbon dioxide, which causes climate change; smoke, soot and sulfur dioxide pollute the air; oil spills; mining damages land |
| peat | found inside Rwanda, so no import costs; power on demand | will run out; releases carbon dioxide; draining and digging bogs destroys wetlands, which store water, prevent floods and shelter wildlife |
| Lake Kivu methane | found inside Rwanda; steady, day-and-night supply; burns more cleanly than coal or oil | used much faster than it is replaced; burning it releases carbon dioxide; extraction must be done carefully so that the lake's layers are not disturbed |
| nuclear fuel | huge energy from a small mass of fuel; no carbon dioxide while running | radioactive waste; very costly to build; serious accidents are rare but very harmful |
| Electricity from | Greenhouse gases over the whole life of the plant (g of -equivalent per kWh, median) |
|---|---|
| coal | 820 |
| natural gas (modern plant) | 490 |
| solar PV (large plant) | 48 |
| hydropower | 24 |
| wind (on land) | 11 |
Figures from the Intergovernmental Panel on Climate Change (IPCC), Fifth Assessment Report, Working Group III, Annex III, Table A.III.2 (2014). They include the building of the plant and the mining of its fuel, which is why solar and wind are not zero.
Worked example 1: is Lake Kivu methane renewable?
REG gives 44.7 km³ of extractable methane, with 0.14 km³ added naturally each year. (a) How long would nature take to replace the whole extractable stock? (b) If gas were taken out at 0.50 km³ a year (an illustrative rate), how long would the stock last?
Whenever the gas is taken out faster than it forms, as in (b), the stock falls: it behaves as a non-renewable source, and REG groups methane with the non-renewable thermal sources. It would only be renewable if we took out no more than about 0.14 km³ a year.
Worked example 2: the carbon footprint of a household's electricity
A household uses 50 kWh of electricity a month. Using the IPCC medians above, how much carbon dioxide-equivalent is released if it comes from (a) coal; (b) hydropower?
The same electricity from coal causes about 34 times more greenhouse gas than from hydro.
Warning
"It is found in Rwanda" does not make a source renewable. Peat and Lake Kivu methane are local, which saves on imports, but both are used much faster than they form. Renewable means replaced as fast as it is used, not "local" or "cheap".
Tip
Rate test: compare the rate of use with the rate of replacement. Faster use than replacement means non-renewable, whatever the fuel is called. And "contains carbon" means "releases when burned".
The 'Killer Lake' Powering Rwanda (BBC Click). Watch how methane is separated from the deep water of Lake Kivu and sent to a power station.
Practice
Key points
Sources and credits
Written by Ganzaa: the lesson text, the candle experiment, the diagrams, tables, worked examples and practice exercises.
