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Alternative Cooling Processes 2
Hans Jonsson
Agenda• Expansion Cycles
- Joule Cycle with applications- Stirling Cycle- Hilsch Tube (Ranque Vortex Tube)
• Thermoelectric Processes- Peltier Process
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Expansion Cycles• Cycles utilizing a gaseous working media.• No phase change!• Usually used when large temperature lifts
are needed (i.e. for achieving low temperatures).
Expansion Cycles
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Joule Cycle 3Coefficient of performance, COP
( )( ) ( )[ ] =−−−⋅⋅
−⋅⋅=
−==
adbcp
abp
ExK
222 TTTTcm
TTcmEE
QE
QCOP&
&
&&
&
&
&
( )( ) ( ) =
η⋅−−η−−
=Exis,adKbis,c
abTTTT
TT
( )( ) ( )
( )( )( )
=
⎪⎪
⎭
⎪⎪
⎬
⎫
⎪⎪
⎩
⎪⎪
⎨
⎧
η⋅−−=
η⋅−=−
η−=−=
−−−−
=
Exis,adda
Exis,adad
Kbis,cbc
adbc
ab
TTTT
TTTT
TTTTesefficienci Isentropic
TTTTTT
Joule Cycle 4( )( )
( ) ( ) =η⋅−−η−
η⋅−−−=
Exis,adKbis,c
Exis,addb
TTTTTTTT
( )
( )
=
⎪⎪
⎭
⎪⎪
⎬
⎫
⎪⎪
⎩
⎪⎪
⎨
⎧
τ==→
τ==→=
κ−κ
κ−κ
121is,ad
121bis,c
ppTT:ad
ppTT:cb
processes Isentropic
( )( )( ) ( ) =
η⋅−⋅−η−⋅η⋅−−⋅−
=Exdis,adKbis,cb
Exdis,adb
TT1T1TTTTT11TT
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Joule Cycle 5( )( )
( ) ( ) =η⋅τ−⋅−η−τ⋅
η⋅τ−−⋅−=
ExdKb
Exdb11T1T
111TT
( )11COP2 −τ
=
( )( )( )
τη⋅−ηη⋅τ−−⋅−
⋅−τ
=ExdKb
Exdb
TT111TT
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If ηEx = ηK = 1, then
Joule Cycle 6
ηEx = ηK=
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Joule Cycle 7If air is used as the working media, heat exchangers can be omitted which gives better performance.
System A System B
Joule Cycle 8
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Joule Cycle - Applications• Cold stores• Cooling of aircraft
Cooling of aircraft• Cockpit + Cabin pressurization,
heating/cooling• Avionics cooling (electronics cooling)• Radar cooling• Windshield defrosting/defogging• Pressurization of fuel tanks• Military: G-protection, Canopy seal etc.
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Joule Cycle - Applications
Compressed air. High pressure and temperature
Compressor
Turbine
Heat exchangers
Condenser
Modern Fighter Aircraft
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Stirling cycle• Can achieve very
low temperatures• Has best
performance at large temperature lifts
• Ideal Stirling cycle has the same COP as the Carnot cycle
Hilsch Tube/Ranque Vortex Tube
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Vortex tubes
Dimension –length (mm)
Max. Refrigeration
(W)
Price(USD)
Small size 100 40 130
Medium size
150 190 170
Large size 300 1900 680
Hilsch Tube / Ranque Vortex Tube
Peltier process• Thermoelectric process• Give a very even temperature distribution
that can be controlled very accurately.• Utilize the Seebeck effect
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Seebeck effectIf temperatures T1and T2 are not the same, there will be a (small) voltage between the two solder points (Seebeck effect).Wires of two
different materials are soldered together.
Solder points
Peltier process
Peltier discovered that if you put a voltage across the two wires, the two solder points will be at different temperatures.
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Peltier process
Peltier process• The Seebeck effect is very dependent on the
material properties. In order to have a good performance, the materials should have:
• Low electric resistance• Low thermal conductivity
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Peltier process
To achieve a high Joffe number, R and K should tend to zero!
In order to describe the materials, a figure of merit has been defined:
e: Seebeck constant for the material combination
R: Electric resistance
K: Thermal conductivity
KReZ
2
⋅=
Joffe number, Z
Peltier process
1ZT1TTZT1
TTTCOP
m
21m
21
2opt,2 +⋅+
−⋅+⋅
−=
Tm: Mean temperature between T1 and T2
A corresponding Carnot efficiency can be defined as:
C,2
opt,2C COP
COP=η
Theoretically, an optimum COP could be found as: