# Steam turbines and thermal power plants: Science and limits

Why physics forces even the best plant to waste heat

Source page: https://www.materialinsights.org/energy/e_steam_turbines/science/

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

A thermal power plant burns fuel (or uses nuclear or geothermal heat) to boil a working fluid, expands the high-pressure vapour through a turbine to spin a generator, then condenses the vapour and pumps it back. This loop is the Rankine cycle, and it still makes most of the world's coal, nuclear, biomass and waste electricity.

#### The conversion, step by step
**Combustion in the boiler**
Chemical (coal, oil, biomass) → Heat in high-pressure steam · about 88% (thermal-oil boiler efficiency assumed for a biomass ORC reference plant; large coal boilers are similar)^[7](https://task33.ieabioenergy.com/wp-content/uploads/sites/33/2022/07/06Bini-Turboden.pdf)**Expansion through the steam turbine**
Heat in steam → Mechanical (shaft rotation) · turbine stage efficiency up to about 90%; but most of the heat must still be rejected in the condenser^[7](https://task33.ieabioenergy.com/wp-content/uploads/sites/33/2022/07/06Bini-Turboden.pdf)**Generator**
Mechanical → Electrical · up to 99% for large hydrogen- and water-cooled units^[10](https://arabellesolutions.com/our-technology/generators-for-nuclear-power-plants/gigatop-generators)**Whole plant, US coal fleet 2025**
Chemical (coal) → Electrical · about 32% (calculated from a heat rate of 10,647 Btu/kWh; higher heating value basis)^[1](https://www.eia.gov/electricity/annual/html/epa_08_01.html)**Whole plant, best in the world (Pingshan II, China)**
Chemical (coal) → Electrical · 49.37% net (lower heating value)^[2](https://www.powermag.com/chinas-pingshan-phase-ii-sets-new-bar-as-worlds-most-efficient-coal-power-plant)

#### The science

A heat engine takes heat from a hot source, turns part of it into work and must dump the rest to a cold sink. In a steam plant the boiler turns water into steam at very high pressure and temperature; the steam pushes on rows of turbine blades as it expands and cools; the condenser, cooled by river, sea or cooling-tower water, turns the low-pressure steam back into water so a pump can return it to the boiler at little energy cost. Raising the steam temperature and pressure (subcritical, then supercritical and ultra-supercritical designs) and reheating the steam part-way through the turbine raise efficiency. The rejected heat is why a typical coal plant wastes about two thirds of its fuel energy. An organic Rankine cycle uses the same loop with a heavy organic fluid that boils at lower temperature, suited to geothermal brine, biomass and waste heat; the Kalina cycle uses an ammonia-water mixture that boils over a range of temperatures to match the heat source better (the Husavik plant in Iceland was designed for 2 MW net from brine at about 120 C, but delivered about 1.8 MW). Stirling and thermoacoustic engines are sealed-gas heat engines: heat applied outside the engine makes a fixed charge of gas expand and contract, driving a piston or a sound wave.

#### Limits set by science

| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| **Carnot limit for an ultra-supercritical coal plant** | about 65% (calculated: 1 - 303 K / 873 K, for 600 C steam and a 30 C condenser)^[2](https://www.powermag.com/chinas-pingshan-phase-ii-sets-new-bar-as-worlds-most-efficient-coal-power-plant) | No engine running between these temperatures can convert more heat to work. Real plants lose more because heat is added over a range of temperatures, flue gas leaves warm and there is friction in the turbine. | Pingshan II reaches 49.37% net with 600 C-class materials and double reheat, about three quarters of the Carnot figure (calculated). |
| **Materials limit (steam temperature)** | 600 C-class steels in service; 700 C designs not yet mature^[2](https://www.powermag.com/chinas-pingshan-phase-ii-sets-new-bar-as-worlds-most-efficient-coal-power-plant) | Boiler tubes and turbine rotors must hold enormous pressure for decades at red heat without creeping. Going hotter needs expensive nickel alloys. | Pingshan II already beats the efficiency expected from 700 C plants by using double reheat instead of hotter steam. |
| **Carnot limit for low-temperature heat (organic Rankine)** | about 30% (calculated: 1 - 298 K / 423 K, for 150 C geothermal brine)^[7](https://task33.ieabioenergy.com/wp-content/uploads/sites/33/2022/07/06Bini-Turboden.pdf) | Low-grade heat has a small temperature gap to the surroundings, so the ceiling is low whatever the machine. | Biomass ORC units with hotter thermal oil reach nearly 20% gross and about 16-20% net (2010 vendor data); geothermal units are lower because their heat is cooler. |
| **Stirling and thermoacoustic engines** | 31.25% sunlight-to-grid (Stirling dish record, 2008); 30% heat-to-sound, about 41% of Carnot (thermoacoustic engine at about 725 C)^[8](https://newsreleases.sandia.gov/releases/2008/solargrid.html)^[9](https://sites.science.oregonstate.edu/~hetheriw/energy/topics/doc/thermo/heat_eng/Stirling_engine/thermoacoustic-Stirling.heat.engine.detailed.study.j.acous.soc.2000.pdf) | These engines can in principle approach Carnot, but heat must pass through metal walls, which limits power density and cost. | Both records are laboratory or prototype results; neither design is in mass commercial use. |

#### Sources

1. [Electric Power Annual, Table 8.1: Average Operating Heat Rate for Selected Energy Sources, 2015 through 2025](https://www.eia.gov/electricity/annual/html/epa_08_01.html) US Energy Information Administration, 2026
2. [China's Pingshan Phase II Sets New Bar as World's Most Efficient Coal Power Plant](https://www.powermag.com/chinas-pingshan-phase-ii-sets-new-bar-as-worlds-most-efficient-coal-power-plant) POWER Magazine, 2023
3. [State of the art of ORC technology for biomass plants (R. Bini, Turboden)](https://task33.ieabioenergy.com/wp-content/uploads/sites/33/2022/07/06Bini-Turboden.pdf) IEA Bioenergy Task 33 workshop, 2010
4. [Sandia, Stirling Energy Systems set new world record for solar-to-grid conversion efficiency](https://newsreleases.sandia.gov/releases/2008/solargrid.html) Sandia National Laboratories, 2008
5. [A thermoacoustic-Stirling heat engine: Detailed study (Backhaus and Swift), J. Acoust. Soc. Am. 107(6)](https://sites.science.oregonstate.edu/~hetheriw/energy/topics/doc/thermo/heat_eng/Stirling_engine/thermoacoustic-Stirling.heat.engine.detailed.study.j.acous.soc.2000.pdf) Los Alamos National Laboratory / Acoustical Society of America, 2000
6. [GIGATOP generators](https://arabellesolutions.com/our-technology/generators-for-nuclear-power-plants/gigatop-generators) Arabelle Solutions, 2025

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_steam_turbines/index.md)
- [Overview](https://www.materialinsights.org/energy/e_steam_turbines/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_steam_turbines/correlation/index.md)
- [Value chain](https://www.materialinsights.org/energy/e_steam_turbines/valuechain/index.md)
- [Unit economics](https://www.materialinsights.org/energy/e_steam_turbines/economics/index.md)
- [Engineering methods](https://www.materialinsights.org/energy/e_steam_turbines/methods/index.md)
- [Equipment and suppliers](https://www.materialinsights.org/energy/e_steam_turbines/equipment/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_steam_turbines/links/index.md)