CO2 Expander to Improve
System Efficiency
P. Binneberg, C. Norris, F. Rinne
Sanden Technical Centre (Europe) GmbH, Germany
Automotive Alternate Refrigerant Systems Symposium 2003
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CO2 Expander to Improve System Efficiency
! Introduction and History
! CO2 Cycle Simulation
! Summary
Automotive Alternate Refrigerant Systems Symposium 2003
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How can Energy be recovered?
• During the expansion device throttling
process, a significant amount of pressure
energy is lost
Typical CO2 Cycle
Heat Rejection
(high pressure)
Com
pres
sion
Energy
Loss due to
Expansion
Device
Throttling
Process
(low pressure)
Evaporator
Cooling
• If all this energy could be recovered for a
CO2 system, then 45% of the compressor
work could be recovered (note: for R134a
the theoretical recoverable energy is only
21%)
• SANDEN is considering developing a low
cost expander/compressor that could
recover this “lost” energy that could
realistically improve the total system
efficiency by 25-30%
Automotive Alternate Refrigerant Systems Symposium 2003
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Modified existing equipment is not a
practical way forward, a special design
for automotive application is required!
History
Author
Expander type
Design
Application
Medium
Maurer (FH Gießen)
axial piston and
gear expander
modified hydraulic
devices
automotive air
conditioning
CO2
Heyl, Quack (TU of
Dresden)
free piston
expander
own design
heat pump
CO2
double sided swash
plate compressor/
expander
own design
commercial
application
CO2
Preisner, Huff
(University of
Maryland)
scroll expander
modified SANDEN
scroll compressor
automotive air
conditioning
CO2
Smith (University
College London)
screw expander
own design
power recover
water vapour
Carrier
screw expander
own design
refrigeration
system
R404a
screw, roots, scroll
devices
AMR380 (Aisin),
OA1040 (Opcon),
G40 (VW)
fuel cell
air
Heidelck, Kruse
(University of
Hannover)
Pischinger (RWTH
Aachen)
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Definition of Efficiencies
ηis =
∆hcomp ,is
∆hcomp ,real
ηvol ,comp =
ηvol ,exp =
Vcomp ,real
Vcomp ,theor
Vexp ,theor
Vexp ,real
Isentropic efficiency
Volumetric efficiency compressor
Volumetric efficiency expander
Automotive Alternate Refrigerant Systems Symposium 2003
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CO2 Cycle Simulation
•
Baseline Cycle
QC
•
Baseline Cycle with IHX
QC
2
2
3
3, 4
1
PComp
4
PComp
6
6,1
Q0
5
•
Expander Cycle
QC
Q0
5
•
Expander Cycle with IHX
QC
2
2
3
3, 4
QC
1
PComp
PComp
4
PExp
6
PExp
5
6,1
PExp
5
Q0
2
3
8
4
PComp
Q0
1
9
5
7
6
•
Q0
Expander Cycle with Intermediate Pressure (CIP)
Automotive Alternate Refrigerant Systems Symposium 2003
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Baseline Cycle
condenser/gas cooler
QC
2
3, 4
compressor
PComp
expansion valve
6,1
5
Q
COP = 0
PComp
Q0
evaporator
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Boundary Conditions for Baseline Cycle
•
Fixed system conditions:
t5
t6 = t1
ηis
ηvol
0°C (3485 kPa)
10°C
0,7 (compressor)
0,9 (compressor)
CarbonDioxide
17000
•
Variable system conditions:
20°C
12000
t3= t4
t2
10°C
P [kPa]
temperature gas
cooler outlet (tGC,out = t3)
discharge pressure (pd)
1 ,2
30°C
0°C
7000
2,
2
t1= t6
t5
2000
0
100
200
300
kJ
/k
gK
-10°C
400
500
600
700
h [kJ/kg]
Automotive Alternate Refrigerant Systems Symposium 2003
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COP Baseline Cycle
5
4
Optimum
discharge
pressure
tGC, out
32°C
50°C
35°C
55°C
40°C
45°C
60°C
65°C
COP
3
2
1
0
7000
8000
9000 10000 11000 12000 13000 14000 15000 16000
pd [kPa]
ps = 3485 kPa / t6 = t1 = 10°C / parameter = tGC, out
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Baseline Cycle with IHX
condenser/gas cooler
QC
internal
heat
exchanger
(IHX)
3
compressor
PComp
1
6
4
2
∆tmin,IHX = 5K = t3 – t1
expansion valve
5
COP =
Q0
PComp
Q0
evaporator
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Boundary Conditions for Baseline Cycle IHX
Fixed system conditions:
t5
t6
∆tmin,IHX
ηis
ηvol
0°C (3485 kPa)
10°C
5 K ( = t3 – t1)
0,7 (compressor)
0,9 (compressor)
CarbonDioxide
17000
30°C
20°C
t4
t2
t3
10°C
0°C
7000
-10°C
t5
2000
0
100
200
300
t6
400
t1
500
2,
2
kJ
temperature gas
cooler outlet (tGC,out = t3)
discharge pressure (pd)
12000
/k
gK
Variable system conditions:
P [kPa]
•
1 ,2
•
600
700
h [kJ/kg]
Automotive Alternate Refrigerant Systems Symposium 2003
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COP Baseline Cycle with IHX
5
4
Optimum
discharge
pressure
tGC, out
32°C
50°C
35°C
55°C
40°C
45°C
60°C
65°C
COP
3
2
1
0
7000
8000
9000 10000 11000 12000 13000 14000 15000 16000
pd [kPa]
ps = 3485 kPa / t6 = 10°C / ∆tmin,IHX = 5 K /parameter = tGC, out
Automotive Alternate Refrigerant Systems Symposium 2003
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COP Comparison Baseline Cycle w/o and w IHX
5
COP
4
tGC, out
At higher temperature the
IHX is required.
35°C
50°C
3
65°C
35°C IHX
2
50°C IHX
65°C IHX
1
0
7000
10000
13000
16000
discharge pressure [kPa]
Automotive Alternate Refrigerant Systems Symposium 2003
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Expander Cycle
condenser/gas cooler
QC
2
3, 4
compressor
PComp
expander
PExp
6,1
5
Q0
COP =
PComp − PExp
Q0
evaporator
Automotive Alternate Refrigerant Systems Symposium 2003
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Boundary Conditions for Expander Cycle
Fixed system conditions:
t5
t6 = t1
ηis
ηis
ηvol
0°C (3485 kPa)
10°C
0,7 (compressor)
17000
0,85 (expander)
0,9 (comp. and expander)
CarbonDioxide
30°C
1 ,2
•
20°C
t3= t4
t2
10°C
0°C
7000
-10°C
2000
0
2,
2
kJ
temperature gas
cooler outlet (tGC,out = t3)
discharge pressure (pd)
12000
/k
gK
Variable system conditions:
P [kPa]
•
t1= t6
t5
100
200
300
400
500
600
700
h [kJ/kg]
Automotive Alternate Refrigerant Systems Symposium 2003
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COP Expander Cycle
5
tGC, out
4
32°C
50°C
35°C
55°C
40°C
45°C
60°C
65°C
COP
3
2
1
Optimum discharge pressure
0
7000
8000
9000 10000 11000 12000 13000 14000 15000 16000
pd [kPa]
ps = 3485 kPa / t6 = t1 = 10°C / parameter = tGC, out
Automotive Alternate Refrigerant Systems Symposium 2003
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Expander Cycle with IHX
condenser/gas cooler
QC
internal
heat
exchanger
(IHX)
3
PExp
5
COP =
Q0
PComp − PExp
PComp
compressor
1
∆tmin,IHX = 5K = t3 – t1
4
expander
2
6
Q0
evaporator
Automotive Alternate Refrigerant Systems Symposium 2003
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Boundary Conditions for Expander Cycle with IHX
Fixed system conditions:
0°C (3485 kPa)
10°C
5 K ( = t3 – t1)
0,7 (compressor)
0,85 (expander)
0,9 (comp. and exp.)
CarbonDioxide
17000
30°C
0°C
7000
/k
gK
-10°C
kJ
temperature gas
cooler outlet (tGC,out = t3)
discharge pressure (pd)
t2
10°C
P [kPa]
Variable system conditions:
t3
20°C
12000
•
t4
2000
0
t5
100
200
t6
300
400
t1
500
2,
2
t5
t6
∆tmin,IHX
ηis
ηis
ηvol
1 ,2
•
600
700
h [kJ/kg]
Automotive Alternate Refrigerant Systems Symposium 2003
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COP Expander Cycle with IHX
5
4
Optimum
discharge
pressure
tGC, out
32°C
50°C
35°C
55°C
40°C
45°C
60°C
65°C
COP
3
2
1
0
7000
8000
9000 10000 11000 12000 13000 14000 15000 16000
pd [kPa]
ps = 3485 kPa / t6 = 10°C / ∆tmin,IHX = 5 K / parameter = tGC, out
Automotive Alternate Refrigerant Systems Symposium 2003
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COP Comparison Expander Cycle w/o and w IHX
5
For the expander cycle the IHX
decreases COP -> no IHX required.
tGC, out
COP
4
35°C exp.
50°C exp.
3
65°C exp.
35°C exp. IHX
2
50°C exp. IHX
65°C exp. IHX
1
0
7000
10000
13000
16000
discharge pressure [kPa]
Automotive Alternate Refrigerant Systems Symposium 2003
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COP Baseline w / Expander Cycle w/o IHX
5
The expander cycle without IHX
has the best COP.
tGC, out
COP
4
35°C base IHX
50°C base IHX
65°C base IHX
3
35°C exp.
2
50°C exp.
65°C exp.
1
0
7000
10000
13000
16000
discharge pressure [kPa]
Automotive Alternate Refrigerant Systems Symposium 2003
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Expander Cycle with Intermediate Pressure
condenser/gas cooler
QC
2
3
PExp
expander
4
HP
compressor
PComp
8
LP
compressor
1
9
5
7
expansion valve
Q0
6
Q0
COP =
PComp , HP + PComp , LP − PExp
evaporator
Automotive Alternate Refrigerant Systems Symposium 2003
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Boundary Conditions for CIP
Fixed system conditions:
t6
t7
ηis
ηis
ηvol
•
0°C (3485 kPa)
10°C
0,7 (HP compressor)
0,85 (expander, LP comp.)
0,9 (comp. and expander)
20°C
40°C
t2
t3
Variable system conditions:
temperature gas
cooler outlet (tGC,out = t3)
discharge pressure (pd)
intermediate pressure (pi)
t5=t9=f(pi)
CarbonDioxide
17000
12000
P [kPa]
•
7000
t5
t4
t6
2000
0
100
200
t1
t9
t8
t7
300
400
60°C
500
600
h [kJ/kg]
Automotive Alternate Refrigerant Systems Symposium 2003
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COP Cycle with Intermediate Pressure
5
4
Optimum discharge pressure
tGC, out
COP
35°C
3
40°C
2
50°C
60°C
1
0
7000
10000
13000
16000
discharge pressure [kPa]
ps = 3485 kPa / t7 = 10°C / parameter = tGC, out
Automotive Alternate Refrigerant Systems Symposium 2003
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COP Comparison Expander Cycle / CIP
5
The CIP has an advantage
in COP (5-8 %).
tGC, out
COP
4
35°C exp.
50°C exp.
60°C exp.
3
35°C CIP
2
50°C CIP
60°C CIP
1
0
7000
10000
13000
16000
discharge pressure [kPa]
Automotive Alternate Refrigerant Systems Symposium 2003
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Swept Volume Ratio - Expander Cycle
8,5
7,5
tGC, out
6,5
32°C
50°C
35°C
55°C
40°C
45°C
60°C
65°C
Vcomp / Vexp
5,5
4,5
Optimum discharge
pressure
3,5
2,5
1,5
0,5
7000
8000
9000 10000 11000 12000 13000 14000 15000 16000
pd [kPa]
ps = 3485 kPa / t6 = t1 = 10°C / parameter = tGC, out
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Conclusion - Volume Ratio
! Advantage:
• To reach the optimum discharge pressure
only small changes in swept volume ratio are
required.
! Challenge:
• Identify possible expander/compressor
mechanisms
• Cost of system components to find an
optimum between COP and investment
Automotive Alternate Refrigerant Systems Symposium 2003
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Comparison COP
The CIP can improve the COP more than
60 % compared to the baseline cycle.
2,5
2,25
COP [-]
2
2,08
1,5
1,51
1,39
1
Hot climate
conditions
0,5
0
baseline
cycle
tev
tev, out
tGC, out
baseline
cycle IHX
expander
cycle
CIP
0°C (3485 kPa)
10°C
50°C
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Swept Volume of the Main Compressor
The CIP needs
approximately 25 %
smaller HP compressor
compared to the
baseline cycle.
Swept volume [%]
110
100
100,0
97,8
97,0
90
80
Hot climate
conditions
70
75,6
60
baseline
cycle
tev
tev, out
tGC, out
baseline
cycle IHX
expander
cycle
CIP
0°C (3485 kPa)
10°C
50°C
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Summary
Advantages:
Baseline Cycle
Baseline Cycle
with IHX
Expander Cycle
CIP
good at lower
temperature
better COP at higher
temperature than
baseline cycle
can eliminate the IHX
hermetically design for
LP compressor and
expander → no shaft
seal, no leakage
similar COP like CIP
lower pressure ratio
→ better compressor
performance
probably lower
investment costs than
CIP
smaller compressor
swept volume for the
high pressure side
simplest cycle
compact design
possible → it can be
mounted close to the
evaporator → shorter
pipes → smaller losses
Disadvantages:
compressor needs a
bigger swept volume
than the CIP to reach
the same cooling
capacity
compressor needs a
bigger swept volume
than the CIP to reach
the same cooling
capacity
compressor needs a
bigger swept volume
than the CIP to reach
the same cooling
capacity
high pressure ratio
high pressure ratio
high pressure ratio
worse COP at higher
temperature
worse COP at lower
temperature
longer pipes to the
evaporator → losses
Automotive Alternate Refrigerant Systems Symposium 2003
one compressor in
addition is required
→ additional costs
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Outlook
SANDEN is working on
expander systems to fulfil the
specific requirements for
automotive applications.
⇒ update 2004
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