Power Cycle Performance Expressions

Thermodynamic (or thermal) efficiency is obtained using the first law of thermodynamics and is calculated as the net work produced divided by the heat energy consumed. It is generically expressed in percent as:

Net work output

Heat energy input

In the English system, work output is commonly expressed as either hp (shaft output) or kWe (electric generator output). Heat energy input is typically given as Btu/h. Alternative English and SI expressions for thermal efficiency are listed in Table 2-1.

To express efficiency in terms of fuel energy input (e.g., to a steam boiler serving turbine-driven equipment), boiler efficiency must be included in the expression. Thus, the fuel utilization efficiency, ^tf, would be expressed as:


Heat rate is the inverse of thermal efficiency or the amount of energy input required (heat added) to generate shaft work. It is expressed as:

Heat rate =

Heat energy input Net work output

In English units, heat rate is commonly expressed as Btu per horsepower-hour (Btu/hp-h) or Btu per kilowatt-hour (Btu/kWh). Alternative English unit and SI expressions are listed in Table 2-1.

Fuel rate is the heat rate expressed in terms of units of fuel added (or input) per unit of work unit output. Fuel units are commonly expressed on a mass-unit basis as pounds (lbm), tons, or kilograms (kg) or on a volumetric-unit basis as gallons (gal), liters (L), barrels, cubic feet (ft3 or cf), or cubic meters (m3 or cm). Fuel rate is expressed as:

, Fuel input

Net work output

For operation on natural gas, Equation 2-12 becomes:

Fuel (gas) rate = Btuinput'hp-h = cf/hp-h (2-l2a) Btu/cf

In SI units, the corresponding expression would be in m3/kWhm.

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