| Energy [PJ/year] | ||||
|---|---|---|---|---|
| Fuel | Sector | Device | EndUse | |
| Gas | Industry | Gas Turbine | Mechanical | 14.1 |
| Liquid fuel | Industry | Gas Turbine | Mechanical | 2.8 |
4 Gas turbine
Adapted from (Paoli and Cullen 2020)
Gas turbines convert chemical energy into work by means of the Bryton cycle . A simple cycle (SC) gas turbine (GT) is composed of: a compressor, which continuously compresses gasses from ambient condition to high pressure; a burner, where fuel is added to raise the temperature of the compressed air; an expander (turbine), where gasses are continuously expanded to a lower pressure. The work generated in the expansion is larger than the work required for the compression hence the engine produces power output. A defining characteristic for gas turbines is the processes of compression, heat addition and expansions, all occur continuously and simultaneously, but in different locations.
4.1 Characterisation
Gas turbines’ use can be divided in four categories: jet-propulsion, power generation, industrial use and microturbines. In jet engines, the work generated is converted to kinetic energy which is used to propel the aircraft. Technology developed for aircraft propulsion has been translated for land-based applications in what are called aeroderivative (AD) gas turbines. Aeroderivatives are used in industry for mechanical drive and in situ power generation. Common applications of industrial gas turbines are to provide mechanical power for compressors and pumps, for example in the oil and gas industry.
For power generation, large devices known as Heavy Duty (HD) gas turbines, are operated in combined cycle (CCGT) to maximise efficiency. They are also operated in Open Cycle (OCGT), as peaking power plants, with lower efficiency but higher ramp-up capabilities.
These all use axial turbomachinery and have power ratings ranging from a 2 MW to 500 MW. Microturbines, on the other hand, make use of radial turbomachinery and their power rating ranges from 3 kW to 2000 kW. Microturbines are not currently used widely due to their high cost and low efficiency.
4.2 Key issues that affect efficiency
Higher Turbine Inlet Temperatures lead to higher efficiency, but are limited by the capabilities of the cooling system and the maximum temperature that materials can withstand. Inlet temperatures have been increasing since the 1950s, although the rate of increase has slowed down.
Current turbine inlet temperatures are only achievable thanks to blade cooling technology. Blade cooling allows keeping blades below their metallurgical limit, while allowing the combustion gasses to exceed that temperature. The metallurgical limit of even the most advanced single crystal alloys is in the order of 1300 K.
Polytropic Efficiency is a measure of the efficiency of the compressor and turbine.
Higher Pressure Ratios give higher efficiencies. Typical values range from 12 to 37 for single-cycle land based machines but can be as high as 60 for jet engines.
4.3 Efficiency limits
According to Paoli and Cullen (2020), the current efficiency and efficiency limits for gas turbines used in industry are:
| Current \(\eta_D\) | 30–42% |
| BAT (Best Available Technology) | 43% |
| TEL (Technological Efficiency Limit) | 59–62% |
4.4 Final Energy used
This table shows the quantity of final energy \(F\) used in the UK in one year:
Note that this does not include the use of gas turbines for power generation, because we are looking at final energy consumption.