The Rankine Cycle: The Steam Power Plant
Most of the world's electricity comes from a single loop of water turning to steam and back. The Rankine cycle threads water through four parts -- boiler, turbine, condenser, pump -- and the phase change is the trick that makes it efficient.
Drag the phase to follow the water around the ring. The boiler adds heat, the turbine makes work, the condenser rejects heat, and the pump returns the liquid. Whether the heat comes from burning coal, a nuclear reactor, or concentrated sunlight, only the source changes; this water loop itself stays almost the same.
On the T-s plane the cycle hugs the saturation dome. On a T-s diagram the heat traded in each step is the area beneath its path, so the strip the closed loop encloses is the net heat absorbed, which over one cycle equals the net work. Drag the boiler temperature; the loop grows, and the area it encloses -- the net work -- grows with it.
The net work is the turbine work minus the pump work. Because the pump only squeezes liquid, its work is tiny, so almost all the turbine output is kept. Because a liquid is almost incompressible, pushing water to high pressure barely shrinks its volume and costs next to nothing, whereas compressing a gas instead would eat a large share of the turbine's output. Drag the boiler pressure.
Carrying the steam above the dome -- superheating -- raises the average temperature heat is added at, lifting efficiency, and keeps the turbine exit drier. A wet exit lets water droplets batter and erode the fast-spinning turbine blades, so keeping the steam dry guards against this wear and extends the blades' life. Toggle saturated and superheated.
Three takeaways: four parts, η = WnetQH, and water is chosen because its phase change exchanges heat near constant temperature for almost no pumping cost. Next, gas cycles with no phase change: Otto and Diesel. A modern steam plant turns roughly 40% of its heat into electricity, and pairing it with a gas turbine in a combined-cycle plant pushes that past 60%.