Types of renewable energy sources

Renewable energy sources are naturally replenished and do not run out. The main types used for electricity are:

1. Hydropower

• Potential energy of water at a height drives a turbine: $P = \eta\rho gQH$.
• Types: storage, run-of-river, pumped storage; large, small, micro (<100 kW).
• High efficiency (85–90 %), dispatchable (storage type), long life; but high initial cost and seasonal flow variation.
• Nepal's main source (e.g. Upper Tamakoshi 456 MW).

2. Solar energy

• Solar PV: Cells convert sunlight directly into DC, inverted to AC.
• Solar thermal (CSP): Mirrors concentrate sunlight to raise steam for a turbine.
• No fuel, modular, little maintenance; but intermittent (only daytime) and needs land or storage.

3. Wind energy

• Wind turns rotor blades coupled to a generator: $P = \frac12\rho A v^3 C_p$.
• Clean and cheap per kWh at good sites; output varies with wind speed, noise and visual impact.

4. Biomass and biogas

• Burning wood, agricultural residue, bagasse, or using biogas from digesters to run engines or boilers.
• Uses waste; carbon-neutral if replanted; low energy density and fuel collection problem.

5. Geothermal energy

• Heat from the earth's interior produces steam to drive turbines.
• Continuous (base load); only at volcanic/hot-spring sites.

6. Tidal and wave energy

• Rise and fall of tides or ocean waves drive turbines.
• Predictable, but only on coastlines; high cost.

7. Others

• Fuel cells using green hydrogen, ocean thermal energy conversion (OTEC).