Not all renewable energy comes from the sky (sun and wind) or the sea (waves and tides). Two other sources come from the Earth itself and from living organisms: geothermal heat and biomass.
Geothermal Energy
The Earth's interior is naturally hot — and at certain locations, this heat is close enough to the surface to be exploited. Geothermal power plants extract heat from inside the Earth (via deep wells that bring up naturally heated water or steam) to drive a chain identical to that of a steam power plant: hot steam spins a turbine, which spins a generator, producing electricity.
| Advantage | Challenge |
|---|---|
| Available continuously (not intermittent like wind and solar) | Requires suitable geological locations (volcanic/thermal activity close to the surface) |
| Very low emissions | High cost of deep drilling |
Notice the similarity with a nuclear power plant: in both cases, only the initial heat source changes (the Earth's interior instead of nuclear fission instead of fuel combustion), while the steam → turbine → generator chain remains the same — another application of the energy conversion chains principle.
Biomass Energy
Biomass consists of organic materials (agricultural residues, wood waste, animal waste, energy crops) that store solar energy absorbed during their growth through photosynthesis. It converts to electricity through two main methods:
- Direct combustion: burning biomass to heat water and generate steam that drives a turbine — very similar to the boiler of a steam power plant, but with renewable organic fuel instead of coal.
- Anaerobic digestion: decomposition of organic matter in the absence of oxygen produces biogas (mostly methane), which is used as fuel in a gas plant or generator engine.
Why Is Biomass Classified as Renewable Despite "Burning"?
The fundamental difference from fossil fuels: biomass renews on a relatively short timescale (a growing season, years of plant growth) — unlike coal and oil, which form over geological ages. Additionally, the carbon released during combustion was originally absorbed by the plants from the atmosphere during their growth — making it part of a much shorter carbon cycle, unlike releasing carbon that was locked underground for ages.
Geothermal and biomass are reminders that the list of renewable sources is broader than just "wind and solar" — any energy source that renews at a rate comparable to our consumption, whatever its form (heat from the Earth, or a plant grown this season), belongs to this family.
A common mistake is automatically linking "combustion" with "fossil fuel and non-renewable". The true criterion for renewability is the relative speed at which the source replenishes — and biomass is burned, but it replenishes within short seasons, unlike coal and oil.
Sample answer: What they have in common is that both change only the initial heat source, while the subsequent conversion chain (steam spinning a turbine, which spins a generator) remains exactly the same as a traditional steam plant — in geothermal, the source is the Earth's natural internal heat, while in nuclear it is nuclear fission. As for biomass being classified as renewable, the criterion for renewability is the relative speed at which the source replenishes compared to its consumption: biomass (agricultural residues or plants) replenishes within relatively short growing seasons, unlike coal and oil, which form over extremely long geological ages — combustion itself is not the criterion, but the speed of replenishment.
Assuming that any energy source that is "burned" is necessarily a non-renewable fossil fuel. The decisive criterion is the relative speed at which the source renews compared to its consumption, not the presence of combustion itself — and biomass is a clear example of a source that is burned and yet classified as renewable.
Want to understand power generation step by step?
Follow trainer Fahad Refai's Electrical Machines and Power Plants courses — a practical walkthrough from the principle of generation to plant operation and grid synchronization.
Browse Fahad Refai's Courses