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Cu stp 03_basics td

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  • 1. SOLAR THERMAL POWER! GEEN 4830 – ECEN 5007!3. Brief Review of Basic Thermodynamic Topics! Manuel A. Silva Pérez ! silva@esi.us.es !
  • 2. Contents }  Thermodynamic Laws }  First and Second Law Efficiencies }  Thermodynamics of Heat Engines1 GEEN 4830 – ECEN 5007 07/07/11
  • 3. 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
  • 4. 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
  • 5. 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
  • 6. 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
  • 7. 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
  • 8. 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
  • 9. Relevant heat engines}  Carnot Cycle}  Brayton (Gas Turbine) Cycle}  Rankine (Steam Turbine) Cycle}  Stirling Cycle 8 GEEN 4830 – ECEN 5007 07/07/11
  • 10. 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
  • 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
  • 12. 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
  • 13. Combined Cycle}  Brayton + Rankine}  Heat input to Rankine is gas turbine exhausts}  High efficiencies 12 GEEN 4830 – ECEN 5007 07/07/11
  • 14. 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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