SOLAR THERMAL POWER!GEEN 4830 – ECEN 5007!

Manuel A. Silva Pérez!silva@esi.us.es!

3. Brief Review of Basic Thermodynamic Topics !

Contents

}  Thermodynamic Laws }  First and Second Law Efficiencies }  Thermodynamics of Heat Engines

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First Law of Thermodynamics

}  Energy }  of a system }  Work }  Heat

}  Energy is conserved in any non-relativistic process }  For a closed system:

ΔU = Q + W Where

U : internal energy (J) Q: heat (J)

W: work (J)

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First Law of Thermodynamics }  For an open system:

ΔUCV = Q + W + Σ(mi·hi) – Σ(mo·ho)

}  For a stationary open system: 0 = Q + W + Σ(mi·hi) – Σ(mo·ho)

Where U : internal energy (J)

h: specific enthalpy (J/kg) m: mass

Q: heat (J) W: work (J)

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Subscripts and superscripts: CV: Control volume

i: input o: output

First Law efficiencies

}  Ratio of useful energy output to input energy of a device

Example. For a steam turbine cycle

ηN = -Wdelivered/Qinput

}  First Law Efficiencies can be > 100 %!

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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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Second Law efficiencies

}  Ratio of useful exergy output to input exergy of a device

Example. For a steam turbine cycle

ηX = -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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Heat engines }  Heat engines produce

mechanical work (shaft work) from heat

}  The maximum 1st Law efficency (Carnot cycle efficiency) for a heat engine is

ηN = 1-Tc/Th

}  The 2nd Law efficiency of the Carnot Cycle is

ηX = 1

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Relevant heat engines }  Carnot Cycle }  Brayton (Gas Turbine) Cycle }  Rankine (Steam Turbine) Cycle }  Stirling Cycle

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Carnot Cycle }  2 isentropic processes + 2

isothermal processes }  Maximum 1st and 2nd Law

efficiencies }  Cannot be realized in

practice

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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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Rankine Cycle }  2 isentropic + 2 isobaric processes }  Working fluid is water/steam (phase changes) }  Operating temperatures limited by materials

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Combined Cycle }  Brayton + Rankine }  Heat input to Rankine is gas

turbine exhausts }  High efficiencies

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Stirling Cycle }  2 isothermal + 2 isochoric processes }  Working fluid is gas (H2, He) }  High operating temperatures }  High efficiency

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