Turbine Types and Efficiency

A Pelton wheel the size of a car can produce 400 MW from a thin jet of high-pressure water. A wind turbine needs a rotor 150 meters across to produce 10 MW from low-pressure air. Both are turbines, but the fluid they work with dictates completely different designs.

Turbine design is governed by the working fluid's density and pressure. Steam turbines handle high-pressure, moderate-density steam at 3,600 RPM. Multiple stages of blades extract energy progressively as steam expands. Efficiency: 85-90% (isentropic). Gas turbines handle hot combustion gases at extremely high speed (3,000-15,000 RPM), with compressor, combustor, and power turbine on a single shaft. Efficiency: 35-42% (thermal). Water turbines come in three types matched to head (height of water fall): Pelton (high head, 300-1,800 m) uses impulse from a water jet; Francis (medium head, 40-600 m) uses a combination of impulse and reaction; Kaplan (low head, 2-40 m) uses adjustable blades like a propeller. Water turbine efficiency: 90-95%.

Water turbines are the most efficient because water is incompressible, dense, and can be directed precisely. Steam and gas turbines lose efficiency to compressibility, heat transfer, and blade-tip losses.

Worked Example

A hydroelectric site has 200 m of head and 50 m³/s flow rate. A Francis turbine (93% efficient) is selected.

• Calculate available power. P = ρghQ = 1,000 x 9.81 x 200 x 50 = 98.1 MW (gross).
• Apply turbine efficiency. 98.1 x 0.93 = 91.2 MW (electrical output).

Why wasn't a Kaplan turbine selected for this site?

Kaplan turbines are optimized for low head (2-40 m). At 200 m, a Francis turbine matches the pressure and flow conditions. Turbine selection follows the fluid conditions, not preference.

Turbine type and efficiency are determined by the physical properties of the working fluid, not by brand or vintage.

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