The Rankine cycle is the steam power loop used in many power plants to turn heat into mechanical work and then electricity. In renewable energy systems, the heat can come from geothermal reservoirs, burning biomass, concentrated sunlight, or recovered waste heat from industry. The same basic machine layout can work with different heat sources because the cycle depends on heating, expanding, cooling, and pumping a working fluid.
Understanding this loop helps students see how thermal energy becomes useful electrical energy.
Key Facts
- The four main Rankine cycle devices are pump, boiler or heat exchanger, turbine, and condenser.
- Pump: low-pressure liquid is compressed to high pressure using a small amount of work.
- Boiler: heat is added at high pressure, turning water into steam or another vapor.
- Turbine: high-pressure vapor expands and does work, often spinning a generator.
- Condenser: low-pressure vapor releases heat and becomes liquid again.
- Ideal efficiency can be estimated by η = Wnet / Qin, where Wnet = Wturbine - Wpump.
Vocabulary
- Rankine cycle
- A thermodynamic cycle that converts heat into mechanical work by evaporating, expanding, condensing, and pumping a working fluid.
- Working fluid
- The substance that moves through the cycle and carries energy, such as water, steam, or an organic fluid.
- Turbine
- A rotating machine that extracts energy from expanding vapor and converts it into shaft work.
- Condenser
- A heat exchanger that cools vapor until it changes back into liquid.
- Thermal efficiency
- The fraction of heat input that becomes net useful work or electricity.
Common Mistakes to Avoid
- Thinking the pump creates the electricity. The pump only raises the pressure of the liquid, while most useful work comes from the turbine.
- Forgetting that the cycle is closed. In most Rankine systems, the working fluid is reused again and again rather than being thrown away after one pass.
- Assuming all heat becomes electricity. Some heat must be rejected in the condenser, so the efficiency is always less than 100 percent.
- Mixing up boiler and condenser roles. The boiler adds heat to make high-pressure vapor, while the condenser removes heat to make liquid.
Practice Questions
- 1 A Rankine cycle turbine produces 850 kJ of work per kg of steam, and the pump requires 20 kJ per kg. What is the net work output per kg?
- 2 A plant receives 2400 kJ of heat per kg in the boiler and produces 720 kJ of net work per kg. What is the thermal efficiency as a percent?
- 3 A geothermal Rankine plant uses hot underground water to heat a separate working fluid in a heat exchanger. Explain why separating the geothermal fluid from the working fluid can help protect the turbine and keep the cycle running reliably.