Steam and Gas Turbines
Combining steam and gas turbines into one power generation system has been a tremendous boom for multiple industrial sectors. As Pirsh & Sage (n.d.). point out, “the major advantages advanced for such cycles were the improvement in overall cycle efficiency and the reduction in capital costs,” (p. 39). A combined steam and gas turbine provides the best of both worlds by minimizing heat loss and maximizing energy output gains. Energy companies like GE have been actively promoting the use of combined steam and gas turbines; while others like ANSYS have been dedicated to the development of cutting-edge combined turbine systems for use in multiple sectors. In addition to their obvious applications in commercial and private power generation, combined steam and gas turbines also have rich potential for use in naval and maritime situations.
Gas and steam combined turbines are therefore available in a wide range of configuration for use in different scenarios. Nuclear power plants are gravitating toward the use of combined-cycle turbines in lieu of steam-only generators. Efficiency is the primary goal of both gas and steam turbines; and combined-cycle turbines provide the best possible solution. Combined steam and gas turbines “incorporate lower fuel usage and reduced carbon emissions,” which characterizes efficient energy production (Gas and steam Turbines,” n.d.). According to one source, “a combination of gas and steam turbine leads to efficiencies of over 50%,” (“Start Your Engines: Gas Turbines,” n.d.). When power stations can afford heat-force coupling systems, energy efficiency can be as high as 90% — meaning nearly no energy loss (“Start Your Engines: Gas Turbines,” n.d.). Boss (n.d.) points out that “the trend toward higher gas turbine firing and exhaust temperatures has made reheat combined-cycles common,” (p. 1). Called Advanced-Combined Cycles, GE’s STAG is a primary example. As with standard combined steam-gas cycle systems, the waste or run-off energy is harnessed and fed back into the system: herein are the major benefits of a combined steam and gas turbine. Quantifiable cycle efficiencies are illustrated in the following diagram (“8.7 Combined Cycles in Stationary Gas Turbine for Power Production,” n.d.):
The net efficiency rate of 58% for combined cycle systems is substantiated in the literature (Kehlhofer, Hannemann, Stirnimann, & Rukes, 2009). “It is only a question of a few years until 60% will be overcome,” (p. 3). Thus, new and breakthrough technologies are operating at even greater efficiency with lower energy loss and lower cost of production.
Combined steam and gas turbines work on fairly simple principles. “The warm exhaust air of the gas turbine is…used for the production of steam for a steam turbine,” (“Start Your Engines: Gas Turbines,” n.d.). The following diagram illustrates one type of combined steam-gas turbine configuration:
Diagram from “8.7 Combined Cycles in Stationary Gas Turbine for Power Production,” (n.d.). This type of configuration is called a gas turbine-steam combined cycle power plant. The next diagram here shows the schematic of the overall heat engine, “which can be thought of as composed of an upper and a lower heat engine in series,” (8.7 Combined Cycles in Stationary Gas Turbine for Power Production,” n.d.). Heat inputs are indicated in these diagrams, and the outputs are also indicated. Heat exchange cycles are the hallmark of a combined steam and gas turbine. The benefits of each cycle are combined into a highly effective combined cycle that takes the energy loss of gas and fuels the hungry steam cycle.
Both diagrams reveal the ways lost energy is re-introduced into the system. The higher temperature cycle is known as the topping cycle. The waste that the topping cycle produces is re-introduced into the system but at a lower temperature. This is called the bottoming cycle (Kehlhofer, Hannemann, Stirnimann, & Rukes, 2009). Gas turbines are referred to in terms of the Brayton cycle, whereas the Rankine refers to the combined cycle. Because two thermal cycles are used in one plant generator, the efficiency is greater than it would be with only one cycle. The complementary nature of steam and gas turbines makes them ideal for a variety of purposes. Gas is generally used at the topping cycle stage; water/steam is used for the bottoming stage (Kehlhofer, Hannemann, Stirnimann, & Rukes, 2009). In the future it may be possible or likely to replace liquids other than water at the bottoming stage for even greater overall efficiency (Kehlhofer, Hannemann, Stirnimann, & Rukes, 2009). However, water is clean and inexpensive; whereas any chemical liquid proposed for use might be neither clean nor inexpensive (Kehlhofer, Hannemann, Stirnimann, & Rukes, 2009).
The gas component of the combined cycle compresses air, and can be used with natural gas. Gas turbines reached the peak of their popularity in the 1970s. As technology improved and allowed for the use of higher temperatures in the cycle, it was possible to develop combined steam and gas cycles. Gas turbines have widespread application in aviation.
“There are two basic types of steam turbines: impulse and reaction. Impulse turbines are used for smaller power-generation applications. Reaction turbines are used for industrial-scale power generation,” (Steam and Gas Turbines Information, n.d.). Condensing and non-condensing cycles in steam turbines also make them unique and different from gas and combustion generators. These issues are taken into account when developing technology for combined steam and gas turbine systems. Issues such as pressure, exhaust, and pump speed are also taken into account. Variables such as manual vs. automatic operation; low vs. high-power operating conditions, pressure ratios, and speed are also factors that go into the engineering of combined steam and gas turbines.
Combined cycle instruments developed by GE and ANSYS can be matched the suit the needs of individual power plants or vehicles such as cargo ship or airplane. “Flexibility is incorporated to allow the steam turbine design to be optimized for site-related parameters such as process extractions and condenser pressure,” (Boss, n.d.). There will also be potential for development of modular systems that can be configured and re-configured according to the changing needs of power stations or propulsion systems.
References
“8.7 Combined Cycles in Stationary Gas Turbine for Power Production,” (n.d.). Retrieved online: http://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node67.html
Boss, M. (n.d.). Steam turbines for STAG Combined-Cycle power systems. Retrieved online: http://site.ge-energy.com/prod_serv/products/tech_docs/en/downloads/ger3582e.pdf
Brain, M. (n.d.). How power grids work. How Stuff Works. Retrieved online: http://science.howstuffworks.com/environmental/energy/power1.htm
“Chapter 6: Turbines, Generators, and Power Plants,” (n.d.). Energy Story. Retrieved online: http://www.energyquest.ca.gov/story/chapter06.html
“Gas and Steam Turbines.” ANSYS. Retrieved online: http://www.ansys.com/Industries/Industrial+Equipment+&+Rotating+Machinery/Gas+&+Steam+Turbines
“Inner workings of a combined cycle power plant,” GE. Retrieved online: http://www.ge-flexibility.com/power-generation-basics/how-combined-cycle-works/index.html
Kehlhofer, R., Hannemann, F., Stirnimann, F. & Rukes, B. (2009). Combined-Cycle Gas & Steam Turbine Power Plants. Tulsa: PennWell.
Pirsh, E.A. & Sage, W.L. (n.d.). Combined steam turbine: Gas turbine supercharged cycles employing coal gasification. Retrieved online: http://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/14_2_TORONTO_05-70_0039.pdf
“The Role of Turbines in Power Generation,” (n.d.). Retrieved online: http://www.dieselserviceandsupply.com/Turbines.aspx
“Start Your Engines: Gas Turbines” (2000). Retrieved online: A combination of gas and steam turbine leads to efficiencies of over 50%.
“Steam and Gas Turbines Information.” Retrieved online: http://www.globalspec.com/learnmore/electrical_electronic_components/power_generation_storage/steam_gas_turbines
“Turbines and Support Systems,” (n.d.). Retrieved online: http://www.nucleartourist.com/systems/tb.htm
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