How thermal power plants work: boiler, turbine, generator

A thermal power plant makes electricity in three steps: it releases heat from a fuel, uses that heat to drive a turbine, and lets the turbine spin a generator. Coal, lignite, gas, oil, biomass, waste and nuclear plants all follow this principle; only the heat source differs.

The basic steam cycle

Most thermal plants use a steam cycle, known to engineers as the Rankine cycle. In simplified form:

1. Boiler or steam generator. Fuel is burned (or, in a nuclear plant, fission produces heat). The heat turns water in a network of tubes into steam at high pressure and temperature.
2. Steam turbine. The steam expands through rows of turbine blades and makes the shaft rotate. Large turbines have high-, intermediate- and low-pressure sections.
3. Generator. On the same shaft, a generator converts the rotation into alternating current. In Europe it turns in step with the grid frequency of 50 hertz.
4. Condenser. After the turbine, the steam is cooled and condensed back into water, using a cooling tower, a river or the sea.
5. Feed pump. The water is pumped back into the boiler, and the cycle starts again.

A transformer at the plant raises the voltage for the transmission grid, which carries the power to substations and finally to the socket at home.

Why efficiency matters

Efficiency is the share of the fuel’s energy that ends up as electricity. Physics sets a limit: a heat engine can only convert part of its heat into work, and the rest has to be released at a lower temperature. In a steam plant, that rejected heat leaves through the condenser.

Engineers raise efficiency mainly by raising steam temperature and pressure. Modern ‘supercritical’ and ‘ultra-supercritical’ coal plants operate above the critical point of water, where it no longer boils in the usual sense. Older plants with lower steam conditions waste a greater share of their fuel. Higher efficiency also means less fuel and fewer emissions per kilowatt-hour.

Gas turbines and combined cycle

A gas turbine works differently from a steam turbine. A compressor squeezes air, fuel is burned in it, and the hot, expanding gas drives the turbine blades directly. Gas turbines start quickly, which makes them useful for covering peaks in demand.

Their exhaust is still very hot. A combined cycle gas turbine (CCGT) plant uses that exhaust in a heat recovery steam generator to raise steam for a second, steam-driven turbine. Two cycles share one fuel input, and the combination reaches the highest efficiencies among fossil plants. That is one reason why gas plants have replaced older coal plants in many European countries.

Combined heat and power

Even the best power-only plant rejects a large share of its heat. A combined heat and power (CHP) plant puts that heat to use, for example in a district heating network, in an industrial process or in a greenhouse. It produces a little less electricity than a power-only plant of the same size, but the total share of the fuel’s energy that is used is much higher.

CHP exists at every scale: from large city plants that heat tens of thousands of flats, down to small units in the basement of an apartment block or a hospital. In Germany these small units are known as Blockheizkraftwerke.

Biomass, waste and nuclear

Biomass and waste-to-energy plants use the same steam cycle as coal plants. Their fuels are more varied and often wetter, which affects boiler design and requires careful cleaning of the exhaust gas; see flue gas cleaning explained.

Nuclear plants produce heat by splitting uranium atoms in a reactor. In most designs the reactor heats water under pressure, which passes its heat to a separate steam circuit. From the turbine onwards, the equipment resembles any other steam plant, including the large cooling towers.

The role of thermal plants in a changing grid

For most of the twentieth century, thermal plants supplied the bulk of Europe’s electricity. Today wind and solar produce a growing share, and the role of thermal plants is shifting from constant output towards balancing: running when the wind is weak and the sun is down, and holding back when renewables are plentiful. How the shares have moved is shown in Europe’s electricity mix; official annual figures by fuel are published by Eurostat.

Flexibility now also comes from batteries, demand response and many small systems acting together. Read how that works in virtual power plants.

From the turbine hall to the balcony

A plug-in solar system has nothing in common with a steam turbine except the grid it feeds. A solar module converts light into direct current without moving parts, and a small inverter turns that into alternating current that matches the 50 hertz of the grid. How that works in detail is explained in how plug-in solar works.

For an overview of the system behind the socket, see the power and grid section.