Hydro and thermal power sources

Hydropower

Principle: Water stored at a height has potential energy. It flows through a penstock to a turbine, which turns an alternator. Power:

$$P = \eta\rho gQH\ \text{W}$$

where $Q$ = flow (m³/s), $H$ = net head (m), $\eta$ = overall efficiency.

 Dam/Intake -> Headrace -> Surge tank -> Penstock
     -> Turbine --[Generator]--> Tx -> Grid
     -> Tailrace -> River

Major features

• Fuel is free; very low running cost; renewable and non-polluting.
• High efficiency (85–90 %) and long life (50+ years).
• Quick start and load change: useful for peak load and frequency control.
• High initial cost and long construction time; site depends on geography; often far from load centres.
• Output varies with season (run-of-river plants drop in dry season).
• Examples: Kali Gandaki A (144 MW), Upper Tamakoshi (456 MW).

Thermal (steam) power

Principle: Fuel (coal, oil, gas) is burned in a boiler; the heat turns water into high-pressure steam, which expands in a steam turbine coupled to an alternator, then is condensed and pumped back (Rankine cycle). Chemical energy → heat → mechanical → electrical energy.

 Fuel -> [Boiler] -> steam -> [Turbine]--[Gen]-> Grid
            ^                    |
       [Feed pump] <- [Condenser] <- cooling water

Major features

• Lower initial cost and shorter construction; can be built near loads or fuel source.
• High running cost due to fuel; overall efficiency only 30–40 %.
• Pollution: CO₂, SO₂, NOx, ash; needs chimneys, precipitators and ash disposal.
• Needs large amounts of cooling water.
• Slow start; best for base load.