Almost all of the world's electricity starts with the same four machines in a closed loop. Follow the working fluid as a pump pushes it into a boiler, a flame turns it to high-pressure steam, a turbine spins electricity out of it, and a condenser cools it back to water to go round again.
The Rankine cycle is the practical engine cycle that powers most of the world's electricity. A working fluid — almost always water — repeatedly boils into steam and condenses back into liquid as it travels round four components: a pump, a boiler, a turbine and a condenser. It is the real-world answer to the ideal Carnot cycle you may have met.
From coal and gas plants to nuclear stations and concentrated solar towers, the same loop shows up again and again — only the heat source changes. Understanding it explains why every plant has a pair of cooling towers, why efficiency tops out well below 100%, and why the whole system is really a heat engine with a boiler bolted on.
Liquid water leaving the condenser is cold and at low pressure. The feed pump forces it up to the high pressure of the boiler, where it is ready to absorb heat. Pumping a liquid takes very little energy, which is what makes the cycle workable.
Inside the boiler the pressurised water is heated until it boils. The energy from burning fuel (or from a nuclear or solar source) turns it into high-pressure, high-temperature steam, full of energy that the rest of the cycle will turn into work.
The high-pressure steam expands through the turbine, pushing its blades and spinning the shaft. That shaft drives a generator, producing electricity. As the steam does work it expands and cools, losing pressure on the way through.
The low-pressure steam leaves the turbine and passes over cooling tubes in the condenser. Cooling water carries the leftover heat away — this is the natural plume you see above a plant's cooling towers — and the steam condenses back into liquid water.
The liquid is now ready to be pumped to the boiler again, and the cycle begins once more. It is truly a closed loop: the same water circulates round and round, while heat constantly enters at the hot end and leaves at the cold end.
It is the cycle real steam power plants run: a working fluid that repeatedly boils and condenses as it passes through four components — a pump, a boiler, a turbine and a condenser. It is the practical engine cycle behind most electricity generation.
Water is cheap, safe and easy to handle, and turning it into high-pressure steam lets the cycle absorb and release a lot of energy. The liquid-to-vapour change carries far more energy per kilogram than simply heating a gas, which is why steam is used almost everywhere.
Condensing the exhaust back into water drops its pressure to a small fraction of atmospheric pressure, which lets the turbine extract far more work. It also recovers the water so the same fluid can circulate around the loop again and again.
The Carnot cycle is the ideal, and far too slow to be practical. The Rankine cycle keeps the same idea — absorb heat when hot, reject heat when cold — but uses boiling and condensing to add and remove heat, and runs at a useful rate that makes real power.
Usually somewhere around 33% to 45%, depending on steam temperature and pressure. It can never reach 100% because some heat must always be dumped at the condenser, which is the practical face of the second law of thermodynamics.