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 Engines
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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)
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GEEN 4830 – ECEN 5007
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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)
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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 %!
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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 system
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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
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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
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GEEN 4830 – ECEN 5007
07/07/11

9. Relevant heat engines
} Carnot Cycle
} Brayton (Gas Turbine) Cycle
} Rankine (Steam Turbine) Cycle
} Stirling Cycle
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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
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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
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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
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GEEN 4830 – ECEN 5007
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13. Combined Cycle
} Brayton + Rankine
} Heat input to Rankine is gas
turbine exhausts
} High efficiencies
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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
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GEEN 4830 – ECEN 5007
07/07/11