Full steam ahead!

With global demand for electricity exploding, steam turbines have a hot future ahead

By Morand Fachot

Steam turbines, first introduced in the late 19th century, have been greatly in demand ever since for electricity generation, marine propulsion and in industry. They are responsible for producing some 80% of the world's electricity, from fossil and nuclear fuels as well as from certain renewable sources, and are likely to continue generating most of it in the foreseeable future.

Cutaway of Siemens SST-500 GEO steam turbine for geothermal power plants (Photo: Siemens)

Well-established technology

Steam turbines, which use heat derived from burning fossil fuels (coal or oil), from nuclear reactors and from biomass and other renewable sources to drive generators, were first introduced in the 1890s.

Developed from an initial 1884 design by British engineer Charles Parsons, steam (and hydraulic) turbines rapidly became the main source of electricity generation. Steam turbine generator technology has evolved considerably over the years and the flexibility of steam turbines enables them to be used for a wide range of applications, operating in combination with different energy sources and other systems to improve their overall efficiency.

Flexible / multiple applications

Most of the electricity produced in the world today is generated by steam turbines. Their capacity can range from 50 kW to several hundreds of MWs for large power plants. Unlike gas turbines, in which heat is a by-product of power generation, steam turbines generate electricity from a derivative of heat (steam).

This enables them to operate with a wide range of fuels including fossil fuels, biomass and nuclear energy, and in a variety of installations not necessarily primarily designed for electricity generation. These may include CHP (combined heat power) systems, in which steam is extracted from the turbine and is used for heating entire districts or is converted to other forms of thermal energy including hot or chilled water, as well as in industrial installations like refineries, chemical plants or pulp and paper mills. In the last category, inexpensive fuels such as residual refinery oils, wood chips, hog fuel (a mix of coarse chips of bark and wood fibre) and other by-product fuels are burnt to power steam turbines, maximizing energy use and reducing the quantity of waste products.

From natural forces…

Steam turbines are found throughout the energy generation chain, including in the harnessing of renewable energies where natural heat from the earth and the sun is converted in geothermal power plants and in CSP (concentrated solar power) installations.

Steam turbines make possible the generation of electricity from geothermal sources (see article on geothermal power in this e-tech). When superheated (i.e. from 240°C – 300°C) dry steam is present it can be used directly in steam turbines to produce electricity. However, when only hot water rather than dry steam is available, excess water must be removed from steam in flash plants or steam must be produced using a heat exchanger (see article on geothermal energy in this e-tech for details). Using superheated steam from the earth directly in steam turbines presents a number of challenges as solid particles and corrosive chemicals are likely to be present, leading to the likely early deterioration and corrosion of turbine blades, and requiring specially designed turbines.

Steam turbine technology aligns with electricity production using the three most common CSP technologies: parabolic trough, solar power tower and linear Fresnel systems (see article on CSP in this e-tech for details). In parabolic trough plants, a heat transfer fluid (e.g. synthetic thermal oil or molten salt) is heated by focused sunlight and is pumped through heat exchangers to produce steam. In the other installations, focused sunlight is used to boil water directly, producing steam which is fed into steam turbines to generate electricity.

…or fossil fuels and nuclear sources

Most of the electricity generated by steam turbines comes from heat resulting from the combustion of fossil fuels, such as coal and oil, or from nuclear reactors. This will remain the case for decades; the challenge will be to achieve cleaner, more efficient power generation from fossil fuels.

According to the IEA (International Energy Agency) world electricity demand will increase by more than two-thirds from 2011 to 2035. Coal's share of electricity production, which currently stands at 41%, will fall to 33% and nuclear energy will make up between 12% and 18%, depending on how the various IEA scenarios set out evolve. However, when taking into account the growing importance of renewables such as geothermal and CSP, that are also used to power steam turbines, it is clear that these will continue to provide most of the world's electricity.

Steam turbines installed in nuclear power plants present other characteristics and are of a different design to those powered by fossil fuels, but their operating principle is similar.

Improving efficiency

Steam turbine manufacturers produce models that can be used with different types of fuels and in different applications, with some modifications. For instance, leading producers of steam turbines offer machines derived from their families of thermal power plant steam turbines for use in CSP and geothermal power installations, following modifications to optimize their operation under different conditions and with alternative heat sources.

Increasing the efficiency of gas-powered thermal power plants is possible using CCP (combined cycle package) configurations, where one or more steam turbine(s) is (are) installed to use residual heat (exhaust) from a gas turbine.

Gas turbine packages have thermal efficiency ratings of 35% – 40% on their own. However, if used in CCP configuration the additional steam turbine(s) can raise the overall thermal efficiency rating to over 60%.

Likewise, in power plants using steam turbines, the residual heat from high-pressure turbines can be used in lower pressure turbines to increase thermal efficiency.

Huge global market

According to a December 2013 GlobalData report, “Steam Turbines in Thermal Power, 2013”, the global market for steam turbines, which benefits from the expansion of electricity demand, new installations and retrofitting of older ones, and was worth USD 14,1 billion in 2013, is set to grow at a 4,3% CAGR (compound annual growth rate) to reach USD 19 billion by 2020.

In its turn, the global steam turbine MRO (maintenance, repair and overhaul) services market is forecast to nearly double in size between 2012 and 2017, growing at a 9,6% CAGR to reach USD 35,4 billion.

International Standards matter

Steam turbines have been installed for over a century, the technology is mature and International Standards prepared by TC (Technical Committee) 5: Steam turbines, have contributed to the expansion of the sector.

These Standards concern specifications, as well as acceptance tests related to the accuracy of various types and sizes of turbines and of speed control systems. Standards for thermal verification tests of retrofitted steam turbines are also important since "today the market in retrofitting older existing machines or components is comparable to that of the new machines of any size", according to TC 5 data.

TC 5 has also published a Technical Specification for steam purity, a highly important factor in minimizing the risk of turbine failure due to corrosion or loss of efficiency or output. However, this specification does not apply to "geothermal plants in which the turbine is fed direct from the geothermal sources", as steam from these sources may contain naturally occurring solid particles and corrosive chemicals.

Manufacturers have highlighted the importance of IEC International Standards for steam turbines in their documents, stressing in particular that their test results "met or exceeded requirements set by IEC Standards".

siemens_sst_500_geo Cutaway of Siemens SST-500 GEO steam turbine for geothermal power plants (Photo: Siemens)
arabelle ST for nuclear plant_Alstom Alstom Arabelle steam turbine for nuclear power plant (Photo: Alstom)
GE-D17_steam turbine High-efficiency combined cycle D-17 GE steam turbine (Photo: General Electric)