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When I ask engineers of all ages their least favorite class in college, thermodynamics usually comes out on top. But in real life, when thermodynamics conflicts with hopes, wishes and desires, thermodynamics usually wins. Enter combined heat and power (CHP). Although there are differing definitions, CHP is generally understood to be the beneficial use of exhaust heat from a heat engine, such as a gas turbine (GT) or gas engine, for domestic or industrial heating, drying, chemical conversion, refining, food processing or other thermal energy application. In heat engines, almost all of the energy losses (i.e., the inefficiencies of the engine) are converted into hot exhaust gas. Since most heat engines operate between 20% to 45% efficiency, most of the remaining 55% to 80% of the energy is theoretically available as thermal energy. That means that a lot of hot air is available for CHP. One of the somewhat misleading features of CHP applications is that efficiency appears to be very high. Quoted efficiencies often exceed the Carnot limit which is the theoretical highest possible efficiency for any heat engine. But how can relatively simple CHP machines achieve better than 80% efficiencies, while the most state-of-the-art combined cycle plants barely ...
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