Making electricity: thermal, nuclear, hydro, wind and solar: Steam: the workhorse of electricity

1.1 A steam plant turns fuel into a spinning shaft in four steps

The cycle at the heart of a thermal power station is called the Rankine cycle, after the Scottish engineer who described it in the 1850s. It has four stages, and the same water goes round and round.

First, a pump pushes water at high pressure into a boiler. Second, burning fuel heats the boiler tubes until the water turns into very hot, high-pressure steam. Third, that steam rushes through a turbine, a shaft carrying rows of curved blades, and as the steam expands and cools it pushes the blades round. The turbine shaft is coupled to a generator, a large rotating magnet inside coils of copper wire, which turns mechanical rotation into alternating current. Fourth, the spent steam leaves the turbine and enters a condenser, a large chamber of tubes carrying cold water, where it turns back into liquid.

The condenser is not an afterthought. Turning steam back into water creates a near vacuum at the turbine exit, and that low back pressure is what lets the steam expand fully and do as much work as possible. The price is that the heat removed in the condenser has to go somewhere, usually into a river, the sea or the air through a cooling tower.

The heat source does not matter to the turbine. In a coal plant it comes from a furnace; in a biomass plant from burning wood pellets or bagasse; in a nuclear plant from fission in a reactor core; in a concentrated solar plant from mirrors focusing sunlight. The back end is shared, which is why the Energy table's 'Steam turbines and thermal power plants' tile covers so many fuels.

The physics that limits the whole arrangement is the second law of thermodynamics. A heat engine can only convert part of its heat into work, and the share it can convert rises as the gap between the hot steam temperature and the cold condenser temperature widens. That single rule explains most of what follows.

• Jaffe and Taylor, The Physics of Energy (2018) · via Smil Mastery Course, M7 'How thermal power plants work'
• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M7 'How thermal power plants work'

1.2 Most of the fuel's energy leaves as warm water, not electricity

Follow 100 units of coal energy through an ordinary plant. About 10 leave up the chimney with the hot flue gases. A few more are used inside the plant to grind coal, run fans and drive pumps. Roughly 35 to 40 leave as electricity. The largest single share, about half, is dumped as low-grade heat at the condenser.

That condenser heat is the reason thermal plants are built beside rivers, lakes and coasts, or carry giant hyperbolic cooling towers. A once-through plant pulls in cold water, warms it by several degrees and returns it. A plant with wet cooling towers instead evaporates part of its water to carry the heat into the air, which uses less water in total but consumes more, because the evaporated water is gone.

For India this is not abstract. Many coal plants sit in water-stressed basins, and summer heatwaves have forced some to cut output when intake water ran short or became too warm to cool the condenser well. Warmer cooling water raises the condenser temperature, narrows the temperature gap and so lowers efficiency, exactly when demand for cooling is highest. Dry, air-cooled condensers avoid most of the water use but cost more and lose a little efficiency.

The same balance sheet also explains emissions. Every unit of electricity from a 36% efficient coal plant requires nearly three units of coal energy. Since coal releases close to 95 kg of CO₂ per gigajoule of heat, the plant emits roughly 0.9 to 1.0 kg of CO₂ per kilowatt-hour sent out, and more if the coal is wet and ash-laden. Raising efficiency from 36% to 45% cuts fuel and CO₂ per kilowatt-hour by a fifth, which is why steam conditions matter so much.

Finally, the waste heat is waste only because it is at a low temperature, around 30 to 40 C. It is too cool to make more electricity but can be useful for heating if there is a customer nearby, which is the logic of cogeneration.

• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M7 'How thermal power plants work'
• Smil, Energy at the Crossroads: Global Perspectives and Uncertainties (2003) · via Smil Mastery Course, M7 'How thermal power plants work'

1.3 Hotter steam and reused heat push efficiency up

Because the second law rewards a wider temperature gap, the main lever for better steam plants has been to make the steam hotter and more pressurised. Older subcritical units run at about 16 to 17 MPa and 540 C and reach roughly 33 to 38%. Above about 22 MPa water no longer boils in the usual sense: it turns smoothly from liquid to a dense fluid with no bubbles. Plants designed for these supercritical conditions, and the ultra-supercritical ones running at around 600 C or more, reach 42 to 47%. The barrier to going higher is metallurgy: boiler tubes and turbine blades must hold their strength for decades at red heat, which needs costly nickel-rich alloys.

India's coal fleet shows the progression. Most units built before about 2010 are subcritical units of 200 to 500 MW. Since then new units have mostly been supercritical or ultra-supercritical, typically 660 or 800 MW each, because the extra efficiency pays for itself in fuel saved over a 30 to 40 year life.

The second lever is to stop wasting the condenser heat. In combined heat and power, steam is drawn off at a higher temperature than a pure power plant would allow and supplied to a factory or a district heating network. The plant makes somewhat less electricity, but the useful output of electricity plus heat can reach 80 to 90% of the fuel's energy. Indian sugar mills do this with bagasse, as do many paper mills and refineries.

Thermal plants have one more virtue that efficiency figures do not show. They are dispatchable: an operator can raise or lower output on command, day or night, whatever the weather. As wind and solar take a bigger share of the grid, that flexibility, and the spinning mass of their large generators which steadies grid frequency, becomes a service in its own right. Coal units are slow to start and dislike frequent cycling, so the most flexible thermal role usually falls to gas.

• Smil, Energy at the Crossroads: Global Perspectives and Uncertainties (2003) · via Smil Mastery Course, M7 'How thermal power plants work'
• Smil, Power Density: A Key to Understanding Energy Sources and Uses (2015) · via Smil Mastery Course, M7 'How thermal power plants work'

See it in the table

Check your understanding

In an ordinary coal plant, which single stream carries away the largest share of the coal's energy?

Why do supercritical and ultra-supercritical plants burn less coal per kilowatt-hour?

A heatwave warms a river used to cool a coal plant. What happens?