Chapter 8. Production of Power from Heat 고려대학교 화공생명 공학과

Introduction Production of Power –Energy from the sun : Photovoltaric cells –Kinetic Energy from atmospheric wind : windmills – Fossil fuels, atomic fuels Chemical Energy  Heat  Power (Electrical Work) Efficiencies are low (35 – 50 %) –Fuel cell Chemical Energy  Power (Electrical Work) Greater efficiency (85 %)

Use of Fossil fuels Steam Power Plant : Fossil fuel and nuclear Internal combustion engines –Otto engines –Diesel engines –Gas turbine –Jet engines

8.1 Steam Power Plant

Diagram of a real power plant

Steam Plant…

Carnot cycle Efficiency increases as T H increases T C decreases T S THTH TCTC Vaporization process in the boiler Reversible, adiabatic expansion of saturated vapor into two-phase Condensation Isentropic compression

Rankine Cycle Several steps are almost impossible for practical reasons –2  3 : Steams with liquid content causes erosion problems in turbine blades –4  1 : Pumping of gas-liquid mixture is difficult

Rankine Cycle : alternative standard for power plant Two modification from Carnot cycle –1  2 : Heating beyond vaporization –3  4 : Complete Condensation T S Heating of subcooled liquid (const P) Vaporization at const T and P Superheating Expansion (reduced moisture contents) Condensation Pumping of sat. liquid to boiler temperature Real path due to irreversibility

The Regenerative Cycles Higher efficiencies –Increased Boiler Temperature  Increased Boiler Pressure  Increased cost for construction –Lower condenser temperature  Lower condenser presure Stagewise preheating the feed water can improve efficiencies

Schematic Diagram of Regenerative Steam Plant

8.2 Internal – Combustion Engines Characterisitcs of Steam Power Plant –Large heat transfer surfaces –Thick walls to to withstand high T,P impose a limit on heat absorption. –Complicated structure Characteristics of Internal-Combustion Engines –Combustion are carried out within engine –Complex thermodynamic analysis –No working fluid undergoes a cyclic process

The Otto Engine No working fluid undergoes cyclic process –An imaginary cyclic engine with air as working fluid – Equivalent in performance to actual engine

Otto Engine Cycle Pressure Volume Air / fuel mixture fed into the engine Air / fuel mixture are compressed by the piston Combustion of fuel : So rapid so that the volume remains const. Adiabatic expansion Valve opened and exhaust gas vented Piston pushes remaining exhasut gases

Thermodynamic Analysis of Otto Engine Increasing the compression ration is to increase the efficiency of engine. (Proof ?) Using idealized Otto engine A B C D P V

Otto Engine- Analysis R increases : efficiency increases

Diesel Engine Difference from Otto Engine –Compression is high, combustion is initiated spontaneously –High compression ratio, high efficiency A B C D V P

The Gas Turbine Engine Gas Turbine Engine –Advantage of high T and P for internal combustion engine –Advantage of using turbine rather than reciprocating engine

Gas-Turbine Engine Adiabatic compression Combustion Expansion Cooling (Back to original state)

8.3 Jet Engines, Rocket Engines Turbojet Engine / Power Plant

Aircraft jet engines Liquid-Fuel Rocket Engine