Thermo 101

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10 Vapor Power Cycles

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Many power cycles, particularly those used in power plants for electricity generation, use a working fluid that undergoes liquid-to-vapor and vapor-to-liquid phase changes. These plants use an external heat transfer source, which can be from the combustion of fossil fuels (natural gas, oil, coal) or biomass materials, from nuclear energy release, from solar energy, from geothermal energy, from waste thermal energy, or from any other thermal source. The cycle to produce mechanical power output to drive an electrical generator is the same regardless of the thermal energy source used as input to the cycle. 

Engines operating on vapor cycles are usually closed cycles, and the working fluid is cycled among various devices. (This is not always the case. Steam locomotives, for example, often took liquid water from the engine tender. The water was boiled to produce high-pressure steam and then the steam was expanded in the engine pistons to produce work. The low pressure steam was simply exhausted from the pistons directly to the atmosphere rather than condensed and reused.)

The major vapor power cycle used today is the Rankine Cycle, which is the focus of this chapter. The Rankine Cycle has its roots in the steam age, when thermodynamic cycles first became widely used. These early thermodynamic cycles were used in steam engines to generate mechanical power from the combustion of fossil fuels (most notably coal) and wood. The application of the steam engine radically altered society in many ways. Prior to the steam engine, transportation speeds were typically not much faster than a person could walk and major inland transportation routes usually followed a waterway, such as a river, canal, or lake. Steam engines significantly increased the speed of boats and ships. Steam powered trains opened new inland transportation trade routes that did not depend on waterways. The earliest electric generating plants used hydropower to generate electricity and were only economically feasible in areas with easily exploitable hydro resources (i.e., where a significant volume of water underwent a significant elevation change over a short distance). The application of the steam engine to generate electricity brought electricity to the large populations living in areas without adequate hydro resources nearby, most notably the mid-West.