Non-renewable and renewable energy
A non-renewable source is used up much faster than nature can make it: coal, oil, natural gas (fossil fuels) and uranium. Fossil fuels give most of the world's energy, but:
- they will run out;
- burning them gives carbon dioxide (CO₂), the main cause of global warming;
- they cause smoke, acid rain and oil spills.
A renewable source is refilled by nature as we use it: sunlight, wind, flowing water, tides, heat inside the Earth and plants. Most of them get their energy from the Sun. Using them, and using less energy (efficiency, insulation), helps solve the energy problem.
Solar energy: solar cells and solar heating
A solar (photovoltaic) cell is made of silicon. Light knocks electrons loose and makes a direct current (DC). An inverter changes it to AC for homes. Many cells make a panel.
Power out = intensity of light × area × efficiency. Good panels convert about 15–22% of the light.
Solar thermal collectors (black, often under glass) heat water. Solar cookers and concentrated solar plants use mirrors.
Plus: free fuel, no smoke, works on rooftops and in villages. Minus: no power at night, less on cloudy days, needs space and storage.
Wind, hydro, tidal and geothermal energy
Wind
Moving air turns blades connected to a generator. The power is proportional to v³, so a site with twice the wind speed gives 8 times the power. Wind farms can be noisy and changeable.
Hydroelectric
Water stored behind a dam has gravitational potential energy. It falls through a turbine: P = η × ρ × g × h × Q (η efficiency, ρ = 1000 kg/m³, h height, Q volume per second). Reliable and can be switched on fast, but dams flood land and affect rivers and people.
Tidal and wave
The Moon's pull raises and lowers the sea twice a day; the moving water turns turbines. Very predictable.
Geothermal
Hot rocks underground heat water to steam, which turns turbines or heats homes. Only in volcanic areas.
Biomass, hydrogen, fuel cells, hybrids and storage
Biomass: wood, crop waste and dung. Burned or digested into biogas (mostly methane). New plants take back CO₂, so it can be nearly carbon-neutral if replanted.
A hydrogen fuel cell joins hydrogen and oxygen without burning: 2H₂ + O₂ → 2H₂O + electricity. Only water comes out. The hydrogen is "green" only if it is made with renewable electricity.
A hybrid car has an engine and an electric motor with a battery; it saves fuel by recovering braking energy.
Storage fills the gaps: batteries, pumped hydro (pump water up when power is spare, let it fall when needed), and a smart grid that shares power. Nuclear power is low-carbon but not renewable and leaves radioactive waste.
Key formulas and definitions
- Solar: P_out = I × A × η (I in W/m², A in m², η efficiency)
- Wind: P = ½ ρ A v³ × η → P ∝ v³
- Hydro: P = η × ρ × g × h × Q
- Efficiency η = useful energy out ÷ energy in
- Fuel cell: 2H₂ + O₂ → 2H₂O + electrical energy
- Energy E = P × t (1 kWh = 3.6 MJ)
Worked examples
1. Sunlight of 1000 W/m² falls on a 1.5 m² panel with 20% efficiency. Find the electrical power.
P = 1000 × 1.5 × 0.20 = 300 W.
2. That panel gets this sunlight for 5 hours a day. How much energy in kWh?
E = 0.3 kW × 5 h = 1.5 kWh per day.
3. A wind turbine gives 200 kW at 6 m/s. Estimate its power at 12 m/s.
Speed doubles, power × 2³ = 8. P = 200 × 8 = 1600 kW (if the turbine can handle it).
4. Water falls 50 m through a turbine at 2 m³/s with 80% efficiency. Find the power (g = 9.8 m/s²).
P = 0.8 × 1000 × 9.8 × 50 × 2 = 784 000 W ≈ 0.78 MW.
5. A biogas plant gets 500 kJ from dung and gives 150 kJ of useful heat. Find its efficiency.
η = 150 ÷ 500 = 0.30 = 30%.
6. A town needs 10 MW day and night. Why can 10 MW of solar panels alone not do it?
Panels give 0 at night and less in clouds; their average output may be only about 20% of the peak. The town needs more panels plus storage, or other sources like wind, hydro or biogas.
Common mistakes
- Calling nuclear power renewable. It is low-carbon, but uranium is mined and runs out.
- Thinking a solar panel turns all the light into electricity. Typical efficiency is 15–22%.
- Thinking double wind speed gives double power. It gives 8 times (v³).
- Thinking renewable means no effect on nature. Dams flood valleys; panels and turbines need land and materials.