# Making electricity: thermal, nuclear, hydro, wind and solar: Gas turbines and diesel engines Source page: https://www.materialinsights.org/learn/energy/making-electricity/p2/ [Understanding Energy](https://www.materialinsights.org/learn/energy/) / [Module 7: Making electricity: thermal, nuclear, hydro, wind and solar](https://www.materialinsights.org/learn/energy/making-electricity/) Part 2 of 5 ## Gas turbines and diesel engines Steam is not the only way to turn fuel into motion. Gas turbines let the burning gases themselves spin the blades, and diesel engines push pistons with the force of combustion. Smil calls these two machines the prime movers of globalisation: they fly the planes, drive the ships, back up the grids and balance the wind and sun. ### 2.1 A gas turbine burns fuel in a stream of compressed air A gas turbine has three parts on one shaft. At the front a compressor, a series of fans with ever smaller blades, squeezes incoming air to 20 to 40 times atmospheric pressure. In the middle, fuel is sprayed into that air and burned continuously in a combustor, raising the gas temperature to well above 1,400 C. At the back the hot, high-pressure gas expands through a turbine, which takes energy out of the flow. Roughly half or more of the turbine's work goes straight back to drive the compressor; the rest is the useful output. What happens to that useful output defines the machine. In a jet engine the hot gas leaves through a nozzle and produces thrust. In a modern high-bypass turbofan, a huge front fan pushes most of the air around the hot core, which gives large thrust with less fuel and noise. In a power station the turbine instead turns a generator, and in pipelines it drives the compressors that keep natural gas moving across thousands of kilometres. Frank Whittle in Britain and Hans von Ohain in Germany invented the jet engine independently in the late 1930s; the first jet aircraft flew in 1939. Since then gas turbines have become the lightest powerful engines ever built. A large aero engine weighs a few tonnes and produces tens of megawatts, well under a tenth of a gram per watt, far lighter than any piston engine of similar output. On the ground, the property that matters most is speed. A gas turbine can go from cold to full power in minutes, and smaller aeroderivative units in under ten. That makes them ideal for meeting evening peaks and covering for sudden dips in wind or solar. Their weakness on their own is efficiency: a simple-cycle unit wastes its very hot exhaust, so it converts only about 35 to 42% of the fuel to electricity. Case #### Why turbines took over new US capacity Smil points out that by around 1990 close to half of all new generating capacity ordered in the United States was gas turbines. The reasons were practical: units come in factory-built modules, can be installed in a year or two rather than the five or more of a coal plant, need little land and no coal yard, emit far less sulphur and particulates, and can be started on short notice. - Smil, Two Prime Movers of Globalization: The History and Impact of Diesel Engines and Gas Turbines (2010) · via Smil Mastery Course, M7 'Gas turbines: power for flight and grids' - Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020) · via Smil Mastery Course, M7 'Gas turbines: power for flight and grids' ### 2.2 Combined cycle recovers the turbine's hot exhaust A simple gas turbine throws away exhaust gas at around 550 to 650 C. That is hotter than the steam in many older coal plants, which is an obvious invitation. In a combined-cycle gas turbine plant, usually shortened to CCGT, the exhaust passes through a heat recovery steam generator, a boiler with no burner of its own, which raises steam for a conventional steam turbine. The two turbines together make about half again as much electricity as the gas turbine alone from the same fuel. The arithmetic is simple. Suppose the gas turbine converts 40% of the fuel's energy into electricity. Of the remaining 60, perhaps 50 is still available in the hot exhaust. If the steam cycle converts about 40% of that, it adds another 20 units, for a total near 60%. The best new plants now reach a little above 62% under test conditions, the highest efficiency of any fossil-fuelled generator. High efficiency and a cleaner fuel together give CCGTs the lowest emissions among fossil plants. Natural gas releases about 56 kg of CO2 per gigajoule against about 95 for coal, and the plant needs less fuel per kilowatt-hour. The result is roughly 0.35 to 0.4 kg of CO2 per kilowatt-hour, well under half a coal plant's figure, though leaks of methane upstream can erode part of the advantage. The weak spot is fuel. A CCGT's economics depend heavily on gas prices, and gas is traded internationally as LNG. After the price spike that began in 2021, IRENA found that by 2025 a new CCGT in a high-gas-price market such as Italy, Germany or Japan produced electricity at close to USD 100 per MWh, against USD 50 to 60 where gas is cheap. India's gas fleet stood at about 25 GW until April 2025, when roughly 5 GW of units that had sat idle for years, because imported gas was too dear for Indian buyers, were written off, leaving about 20 GW. The survivors are now valued mainly as flexible backup for summer evening peaks, when solar output fades and air conditioners are still running. Process flowchart Natural gas through a combined-cycle plant Inputs **Natural gas**100 units of fuel energy Process steps 1 **Compressor and combustor** air squeezed, gas burned at over 1,400 C 2 **Gas turbine and generator** about 38-40 units of electricity 3 **Heat recovery steam generator** exhaust at about 600 C → steam 4 **Steam turbine and generator** about 20 more units of electricity 5 **Condenser and stack** about 40 units leave as low-temperature heat Outputs **Electricity**about 58-62 units Source: Smil, Two Prime Movers of Globalization: The History and Impact of Diesel Engines and Gas Turbines (2010), via Smil Mastery Course, M7 'How thermal power plants work' and 'Gas turbines: power for flight and grids' - Smil, Two Prime Movers of Globalization: The History and Impact of Diesel Engines and Gas Turbines (2010) · via Smil Mastery Course, M7 'How thermal power plants work' - [IRENA, Renewable Power Generation Costs in 2025 (2026)](https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2026/Jul/IRENA_TEC_RPGC_in_2025_2026.pdf (executive summary)) · web - Deccan Herald (Bloomberg), 'India shrinks its gas fleet as idle plants become unusable' (20 May 2025) · CEA capacity 25.2 GW in March and 20.1 GW in April 2025, https://www.deccanherald.com/india/india-shrinks-its-gas-fleet-as-idle-plants-become-unusable-3548996 ### 2.3 Diesel engines win where efficiency and compact fuel matter most Rudolf Diesel's idea, patented in the 1890s, was to compress air so hard that it becomes hot enough to ignite fuel injected into it, with no spark. High compression is the key to efficiency, just as a wide temperature gap is for steam. Early diesels were roughly twice as efficient as petrol engines, and the largest modern marine engines convert more than half of their fuel's energy into shaft work, the best of any combustion engine. Those marine engines are the giants of the machine world. Slow-speed two-stroke designs stand more than 10 metres tall, have up to 14 cylinders with bores near a metre and piston strokes over 2.5 metres, and turn a single propeller at roughly 100 revolutions a minute or less with about 80 MW. Container ships, bulk carriers and tankers, freight locomotives, lorries, tractors, combine harvesters and excavators all rely on diesels. Smil argues that diesel-powered container shipping cut the cost of moving goods across oceans so far that freight became a small part of the price of most manufactured goods, which made the Asian manufacturing boom possible. For electricity, diesel's role is smaller but important. Diesel generator sets, or gensets, provide backup to hospitals, data centres, telecom towers and apartment blocks, and primary power on islands and in remote mines. India has a very large installed base of such gensets, a legacy of unreliable grid supply; as the grid has improved and rooftop solar with batteries has become cheaper, many now run only for minutes a year. Electricity from small gensets is expensive. A small set turns a litre of diesel into roughly 3 to 3.5 kWh, so at about USD 1 a litre the fuel alone costs some USD 300 per MWh (calculated), and part-load running makes it worse. Smil warns that diesel will not leave heavy transport quickly. Batteries for one Asia-Europe voyage would take a large share of a container ship's cargo space. Ammonia and methanol are being trialled, but burned in modified diesels. In the data #### Weight per unit of power Prime movers differ hugely in how much machine is needed per watt. A big jet engine needs under 0.1 g per watt. A modern car petrol engine needs roughly 1 g per watt. A large marine two-stroke diesel needs well over 20 g per watt because it is built to run continuously for decades on poor fuel. A steam power plant, counting boiler, turbine and condenser, is heavier still. Light weight is why turbines fly and diesels sail. - Smil, Two Prime Movers of Globalization: The History and Impact of Diesel Engines and Gas Turbines (2010) · via Smil Mastery Course, M7 'Diesel engines: the movers of global trade' - Smil, How the World Really Works: A Scientist's Guide to Our Past, Present and Future (2022) · via Smil Mastery Course, M7 'Diesel engines: the movers of global trade' ### See it in the table [Gas Natural gas](https://www.materialinsights.org/energy/natural_gas/)[NH3 Ammonia](https://www.materialinsights.org/chemicals/ammonia/)[MeOH Methanol](https://www.materialinsights.org/chemicals/methanol/)[DC Data centres](https://www.materialinsights.org/energy/e_data_centres/) ### Check your understanding A gas turbine converts 40% of its fuel to electricity, and a steam cycle on its exhaust recovers 40% of the 50 units of heat still in that exhaust. What is the combined efficiency? 40 units from the gas turbine plus 40% of 50, or 20 units, from the steam turbine gives 60 units from 100 of fuel. What makes a diesel engine more efficient than a petrol engine? Squeezing the air much harder before combustion raises the efficiency of the cycle, in the same way a wider temperature gap helps a steam plant. Why are gas turbines prized on grids with a lot of solar power? Fast start-up and ramping let gas turbines fill the evening gap when solar fades or cover sudden drops in wind. [← Part 1: Steam: the workhorse of electricity](https://www.materialinsights.org/learn/energy/making-electricity/p1/)[Mark read · Part 3: Nuclear electricity →](https://www.materialinsights.org/learn/energy/making-electricity/p3/) ## Other sections - [Module overview](https://www.materialinsights.org/learn/energy/making-electricity/index.md) - [Steam: the workhorse of electricity](https://www.materialinsights.org/learn/energy/making-electricity/p1/index.md) - [Nuclear electricity](https://www.materialinsights.org/learn/energy/making-electricity/p3/index.md) - [Hydro and wind: electricity from moving water and air](https://www.materialinsights.org/learn/energy/making-electricity/p4/index.md) - [Solar electricity and the 2025 mix](https://www.materialinsights.org/learn/energy/making-electricity/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/energy/making-electricity/review/index.md)