A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turbine CCGT plant. The same principle is also used for marine propulsion, where it is called a combined gas and steam COGAS plant. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs. The principle is that after completing its cycle in the first engine, the working fluid the exhaust is still hot enough that a second subsequent heat engine can extract energy from the heat in the exhaust. Usually the heat passes through a heat exchanger so that the two engines can use different working fluids.
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Example 5. Combined Brayton-Rankine cycle. Find:a Sketch of the components flow diagram, and processes in the T-s diagram. Se pide:a Esquema de componentes y diagrama T-s de los procesos. Sketch of Brayton - Rankine combined cycle , and their T-s processes.
We start assuming isentropic expansion, since there is no specific data on that. Using Antoine's fitting or looking up in the steam tables :. B pv T puexp A 1 kPa exp Now we have to solve the gas turbine. As above, we assume isentropic compression andexpansion, since there is no specific data on that. The mass flow rate of air involved, mGT, isobtained from the work delivered by the gas turbine:with a result of: 1 Now we need more heat to vaporisethe greater amount of steam: Q m h3 h2 1.
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Example 5. Combined Brayton-Rankine cycle
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