The electrical generating efficiency of standard steam turbine power plants varies from a high of 37% HHV4 for large, electric utility plants designed for the highest practical annual capacity factor, to under 10% HHV for small, simple plants which make electricity as a byproduct of
delivering steam to processes or district heating systems.
Steam turbine thermodynamic efficiency (isentropic efficiency) refers to the ratio of power actually generated from the turbine to what would be generated by a perfect turbine with no internal losses using steam at the same inlet conditions and discharging to the same downstream pressure (actual enthalpy drop divided by the isentropic enthalpy drop). Turbine thermodynamic efficiency is not to be confused with electrical generating efficiency, which is the ratio of net power generated to total fuel input to the cycle. Steam turbine thermodynamic efficiency measures how efficiently the turbine extracts power from the steam itself. Multistage (moderate to high-pressure ratio) steam turbines have thermodynamic efficiencies that vary from 65% for small (under 1,000 kW) units to over 90% for large industrial and utility sized units. Small, single stage steam turbines can have efficiencies as low as 50%. When a steam turbine exhausts to a CHP application, the turbine efficiency is not as critical as in a power only condensing mode. The majority of the energy not extracted by the steam turbine satisfies the thermal load.
Power only applications waste the exhaust turbine steam energy in condensers.Table 1 summarizes performance characteristics for typical commercially available steam turbines and for typical boiler/steam CHP systems in the 500 kW to 15 MW size range.
5 Characteristics for “typical” commercially available steam turbine generator systems. Steam turbine data based on information from: TurboSteam, Inc for 500 kW and 3 MW; General Electric for 15 MW turbine.
6 Equipment cost includes turbine, gearbox, generator, controls, and switchgear; boiler and steam system costs are not included.
7 Installed costs vary greatly based on site-specific conditions; Installation costs of a “typical” simple installation were estimated to be 70% of the equipment costs.
8 The Isentropic efficiency of a turbine is a comparison of the actual power output compared to the ideal, or isentropic, output. It is a measure of the effectiveness of extracting work from the expansion process and is used to determine the outlet conditions of the steam from the turbine.
9 CHP electrical efficiency = Net electricity generated/Total fuel into boiler; A measure of the amount of boiler fuel converted into electricity.
10 Fuel input based on condensate return at steam outlet pressure and saturation temperature.
11 Total CHP efficiency = (Net electricity generated+Net steam to process)/Total fuel into boiler
12 Power/Heat Ratio = CHP electrical power output (Btu)/ useful heat output (Btu)
13 Net Heat Rate = (total fuel input to the boiler – the fuel that would required to generate the steam to process assuming the same boiler efficiency/steam turbine electric output (kW).
14 Effective Electrical Efficiency = (Steam turbine electric power output)/(Total fuel into boiler – (steam to process/boiler efficiency)). Equivalent to 3,412 Btu/kWh/Net Heat Rate.
Source: Technology Characterization: Steam Turbines