SOLAR THERMAL POWER! GEEN 4830 – ECEN 5007!3. Brief Review of Basic Thermodynamic Topics! Manuel A. Silva Pérez ! email@example.com !
Contents } Thermodynamic Laws } First and Second Law Efficiencies } Thermodynamics of Heat Engines1 GEEN 4830 – ECEN 5007 07/07/11
First Law of Thermodynamics } Energy } of a system } Work } Heat } Energy is conserved in any non-relativistic process } For a closed system: U = Q +W Where U : internal energy (J) Q: heat (J) W: work (J)2 GEEN 4830 – ECEN 5007 07/07/11
First Law of Thermodynamics} For an open system: UCV = Q + W + (mi·hi) – (mo·ho)} For a stationary open system: 0 = Q +W + (mi·hi) – (mo·ho) Where Subscripts and superscripts: U : internal energy (J) CV: Control volume h: specific enthalpy (J/kg) i: input m: mass o: output Q: heat (J) W: work (J) 3 GEEN 4830 – ECEN 5007 07/07/11
First Law efficiencies} Ratio of useful energy output to input energy of a device Example. For a steam turbine cycle N= -Wdelivered/Qinput} First Law Efficiencies can be > 100 %! 4 GEEN 4830 – ECEN 5007 07/07/11
Second Law of Thermodynamics } Different forms of energy have different quality } The 2nd Law of >Thermodynamics provides a means of assigning a quality index to energy: exergy or availability } Work is the most valuable form of energy } The quality of Heat depends on temperature } The quality of thermal energy depends on the state of the system5 GEEN 4830 – ECEN 5007 07/07/11
Second Law efficiencies} Ratio of useful exergy output to input exergy of a device Example. For a steam turbine cycle X= -Wdelivered/(Qinput·(1-Tc/Th)) Where Tc and Th are the heat sink and heat source temperatures, resp.} 2nd Law efficiencies are always ≤ 1 6 GEEN 4830 – ECEN 5007 07/07/11
Heat engines} Heat engines produce mechanical work (shaft work) from heat} The maximum 1st Law efficency (Carnot cycle efficiency) for a heat engine is N= 1-Tc/Th} The 2nd Law efficiency of the Carnot Cycle is X= 1 7 GEEN 4830 – ECEN 5007 07/07/11
Carnot Cycle} 2 isentropic processes + 2 isothermal processes} Maximum 1st and 2nd Law efficiencies} Cannot be realized in practice 9 GEEN 4830 – ECEN 5007 07/07/11
Brayton Cycle} 2 isentropic + 2 isobaric processes} Normally operated as an open cycle} Working fluid is a gas (air)} Efficiencies depend on the pressure ratio} Normally operate at high temperatures 10 GEEN 4830 – ECEN 5007 07/07/11
Rankine Cycle} 2 isentropic + 2 isobaric processes} Working fluid is water/steam (phase changes)} Operating temperatures limited by materials 11 GEEN 4830 – ECEN 5007 07/07/11
Combined Cycle} Brayton + Rankine} Heat input to Rankine is gas turbine exhausts} High efficiencies 12 GEEN 4830 – ECEN 5007 07/07/11
Stirling Cycle} 2 isothermal + 2 isochoric processes} Working fluid is gas (H2, He)} High operating temperatures} High efficiency 13 GEEN 4830 – ECEN 5007 07/07/11
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