Semester 3 / Engineering Thermodynamics /Power Cycles

Power Cycles

3 min read Updated Tue Apr 28 2026 07:56:31 GMT+0000 (Coordinated Universal Time)
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2 major application domains of engineering thermodynamics:

  • power generation
    thermal power generation such as coal power plants, nuclear power plants, natural gas power plants, geothermal solar thermal and combined heating and power applications (CHP systems).
  • refrigeration amd heat pumps
    From simple domestic level refrigerator or air conditioner to a large scale industrial refrigeration systems and AC systems. Heat pumps also vary from small domestic scale heat pumps to very large scale industrial heat pumps.

Assumptions

These are assumptions regarding ideal cycles.

  • No friction
  • Expansion & compression are quasi-equilibrium processes
  • Heat losses in pipes are negligible
  • Change in kinetic and potential energies are neglected

CHP system

Short for Combined Heat and Power systems. Power and heat are produced simultaneously.

Examples:

  • Gasoline or diesel engines
  • Jet propulsion systems

that heat may be for process heating or it could be for district heating. In addition to these electrical power generation applications, we can’t generation systems like internal combustion engines.

These are considered as heat engines because they convert heat into work. They are operating in thermodynamic cycles.

Power cycle

The thermodynamic cycles that modeled the operation of these power generation applications.

Practically very complex. Assumptions are made to simplify the analysis. Carnot cycle is the perfect theoretical cycle, which is the upper limit for operation. Ideal thermodynamic cycle lies in between the practical and the Carnot cycle and it is much closer to the practical operation.

2 types based on the working fluid:

Closed Cycle

Working fluid returns to initial state.

Open Cycle

New working fluid each cycle (real engines).

Analysis

Thermal efficiency

Denoted by η\eta.

η=Net work outputHeat input\eta = \frac{\text{Net work output}}{\text{Heat input}}

Heat rate

Inverse of thermal efficiency. Technically unitless, but BTU/kWh is used; the amount of heat required to produce 1 kilowatt hour of work output. BTU is short for British Thermal Units.

1 kWh=3412 BTU1\,\text{kWh} = 3412\,\text{BTU}

Common term among turbine suppliers to compare different steam turbine cycle.

Specific steam consumption

Flow rate of steam per kilowatt hour of work produced. Determines the sizing of the power plants.

Specific steam consumption=wnetwgrossNet work=wout−winputGross work=wout\begin{equation} \nonumber \begin{split} \text{Specific steam consumption} & = \frac{w_\text{net}}{w_\text{gross}} \\ \text{Net work} &= w_\text{out} - w_\text{input} \\ \text{Gross work} &= w_\text{out} \end{split} \end{equation}

Efficiency ratio

Efficiency ratio=Actual efficiencyRankine efficiency\text{Efficiency ratio} = \frac{\text{Actual efficiency}}{\text{Rankine efficiency}}

Isentrophic efficiency

For compressors

Isentropic work inputActual work input\frac{\text{Isentropic work input}}{\text{Actual work input}}

Actual work input is higher than isentropic work input.

For turbine

Actual work outputIsentropic work output\frac{\text{Actual work output}}{\text{Isentropic work output}}

Isentropic work output is bigger.