The Rankine cycle is the basic thermodynamic model for many steam power plants, including coal, nuclear, biomass, and some solar thermal plants. It explains how heat energy is converted into mechanical work and then into electrical energy. The cycle uses water because it is inexpensive, safe, and has useful phase change properties.
Understanding the Rankine cycle helps engineers improve efficiency, reduce fuel use, and design reliable power systems.
In a Rankine cycle, liquid water is pressurized by a pump, heated in a boiler until it becomes high pressure steam, expanded through a turbine to produce work, and condensed back into liquid water. On a T-s diagram, heat addition and rejection appear as paths across temperature and entropy, while ideal pump and turbine processes are nearly vertical. Superheating raises steam temperature before the turbine to increase work output and reduce moisture.
Reheating expands steam in stages, adding heat between turbine sections to improve efficiency and protect turbine blades.
Key Facts
- Basic loop: pump → boiler → turbine → condenser → pump.
- Thermal efficiency: η = W_net / Q_in = (W_turbine - W_pump) / Q_in.
- Turbine work per unit mass: w_t = h_in - h_out.
- Pump work per unit mass for an incompressible liquid: w_p ≈ v(P_out - P_in).
- Boiler heat input per unit mass: q_in = h_boiler out - h_boiler in.
- Condenser heat rejection per unit mass: q_out = h_condenser in - h_condenser out.
Vocabulary
- Rankine cycle
- A thermodynamic cycle that models how steam power plants convert heat into mechanical work using a boiler, turbine, condenser, and pump.
- Boiler
- A device that adds heat to pressurized water to produce high temperature steam.
- Turbine
- A machine that extracts work from expanding steam and usually drives an electric generator.
- Condenser
- A heat exchanger that removes energy from exhaust steam so it changes back into liquid water.
- T-s diagram
- A temperature versus entropy graph used to visualize heat transfer, phase change, and efficiency in thermodynamic cycles.
Common Mistakes to Avoid
- Ignoring pump work, because it is small but not always zero. For accurate efficiency calculations, subtract pump work from turbine work to find net work.
- Confusing boiler pressure with turbine inlet temperature, because both affect cycle performance differently. Pressure changes the saturation conditions, while superheating mainly raises steam temperature and enthalpy.
- Drawing the turbine expansion as a horizontal line on a T-s diagram, because an ideal turbine is approximately isentropic. The ideal path should be nearly vertical with constant entropy.
- Assuming the condenser wastes useful work directly, because it actually rejects heat to return steam to liquid. The condenser is needed so the pump handles liquid water instead of low density vapor.
Practice Questions
- 1 A Rankine cycle has turbine work of 950 kJ/kg and pump work of 12 kJ/kg. If the boiler adds 2600 kJ/kg of heat, calculate the net work and thermal efficiency.
- 2 Steam enters a turbine with h_in = 3450 kJ/kg and exits with h_out = 2300 kJ/kg. The pump work is 8 kJ/kg and the mass flow rate is 20 kg/s. Calculate the net power output in MW.
- 3 Explain why superheating steam before it enters the turbine can increase power plant efficiency and reduce damage to turbine blades.