EVS24
Stavanger, Norway, May 13 - 16, 2009
Electrochemical Capacitors as Energy Storage in Hybrid- Electric Vehicles: Present Status and Future Prospects
Andrew Burke, Marshall Miller
University of California- Davis, Institute of Transportation Studies, CA 95616 afburke@ ucdavis. edu
Abstract
The development of electrochemical capacitors ( ultracapacitors) has continued since the early 1990s. Activated microporous carbon and hybrid carbon devices from a number of developers world- wide have been tested and evaluated for use in hybrid vehicles of various types. The test data indicate that the useable energy density of the activated carbon devices is about 5 Wh/ kg and that of the hybrid carbon devices is 10- 12 Wh/ kg. The power capability of the carbon/ carbon devices can be very high (> 2000 W/ kg for a 95% high power pulse); the power capability of the hybrid carbon devices are significantly lower being 500- 1000 W/ kg for a 95% pulse. This means that the P/ E ratio of the hybrid carbon devices is much lower than the carbon/ carbon devices and as a consequence, it may be difficult to take full advantage of the higher energy density of the hybrid carbon devices in some applications. Simulation results for various types of hybrid vehicles are presented. The results for micro- hybrids are particularly interesting and surprising, because of the large fuel economy improvements predicted. The improvements were about 40% on the FUDS and ECE- EUD cycles and 20% on the Federal Highway and US06 cycles using the carbon/ carbon ultracapacitor units. The improvements were significantly less using the hybrid carbon units because of their lower round- trip efficiencies.
Keywords: electrochemical capacitors, hybrid vehicles, control strategy, fuel economy
1 Introduction
It is well recognized that the future development and successful marketing of hybrid- electric vehicles of various types are highly dependent on the performance and cost of the energy storage technologies available. There seems to be high confidence that the performance and cost of the other mechanical and electrical components in
hybrid- electric drivelines are suitable for hybrid vehicle applications, but there remains considerable uncertainty regarding the energy storage technologies. Whether a particular energy storage technology is suitable for use in a particular type of hybrid depends both on its characteristics and the requirements for energy storage in the vehicle design. This paper is concerned with both the requirements for energy storage in various types of hybrid vehicles and the characteristics of
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the energy storage devices being developed. In the paper, both charge sustaining and plug- in hybrids will be considered and the role of ultracapacitors in providing the energy storage assessed.
2 Hybrid vehicle energy storage requirements
From a vehicle performance point- of- view, energy storage requirements are defined in terms of the peak power ( kW) and energy storage capacity ( Wh or kWh). The vehicle designer is also interested in the weight and volume of the energy storage unit which follows once the energy and power densities of the technologies are known. It is important to recognize that the energy capacity and the peak power refer to the useable energy capacity and the useable peak power from the energy storage unit for the particular application of interest. By “ useable’ is meant the “ quantity” that can be utilized from the storage unit consistent with other system constraints such as the effect of round- trip efficiency on peak power, depth of discharge on energy capacity and cycle life, and maximum charge voltage on energy capacity, cycle life and safety. These further considerations in most cases result in storage unit performance that is significantly less than one would infer from the name- plate ratings given by the manufacturer of the batteries or ultracapacitors.
A second difficulty in quantifying the peak power and energy requirements are that the useable power and energy requirements can be highly dependent on the control strategy linking operation of the engine and electric drive system. In the case of a charge sustaining hybrid, the useable energy required can vary from 100- 300Wh depending on how often and at what power level the engine is used to recharge the energy storage unit ( References 1- 2). In the case of the plug- in hybrids, the peak power requirement depends on the blending strategy of the electric motor and engine when the vehicle is operating in the “ all- electric” or charge depleting mode ( References 3- 4).
Energy and power requirements for selected hybrid vehicle designs and operating strategies are shown in Table 1 for a mid- size passenger car. Requirements are given for both charge sustaining and plug- in hybrids. These requirements can be utilized to size the battery and ultracapacitors in the vehicles when the characteristics of the energy storage devices are known. In some of the vehicle designs considered in Table 1, ultracapacitors are used to provide the peak power rather than batteries. The objective of this paper is the evaluation of ultracapacitor technology ( present and future) to assess whether these vehicle applications of ultracapacitors appear to be feasible and attractive.
Consider first charge sustaining hybrid vehicles. In this case, the energy storage unit is sized by both useable power ( kW) and energy storage ( Wh) requirements. For batteries, the key issues are the power requirement and the minimum useable energy consistent with high cycle life for shallow cycles. The total energy stored in the battery unit is of secondary importance as far as operation of the vehicle is concerned, but it has a strong effect on the weight, volume, and cost of the unit. For ultracapacitors, the key issue is the minimum energy ( Wh) required to operate the vehicle in real world driving because the energy density characteristics of ultracapacitors are such that the power and cycle life requirements will be met in most cases if the unit is large enough to met the energy requirement. This is certainly the case for
Table 1: Energy storage requirements for various types of hybrid- electric vehicles
Type of hybrid driveline
System
voltage
V
Useable energy storage
Maximum pulse power at 90- 95% efficiency kW
Cycle life
( number of cycles)
Useable depth- of- discharge
Plug- in
300- 400
6- 12 kWh battery
100- 150 Wh
ultracapacitors
50- 70
2500- 3500
deep
60- 80%
Charge sustaining
150- 200
100- 150 Wh
ultracapacitors
25- 35
300K- 500K
Shallow
5- 10%
Micro-
hybrid
45
30- 50 Wh
ultracapacitors
5- 10
300K- 500K
Shallow
5- 10%
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 2
carbon/ carbon devices using microporous carbons, but may not be the case for advanced hybrid ultracapacitors having lower power capability. This question is considered in some detail later in the paper.
For plug- in hybrid vehicles, the energy requirement of the ultracapacitors is essentially the same as for a charge sustaining hybrid and the power requirement is determined by the power rating of the electric drive system in the vehicle. As discussed in Reference 5, the use of ultracapacitors in plug- in hybrids will become more likely with advanced batteries having high energy density. This is the case because it is unlikely that the power density of the advanced batteries will increase sufficiently along with their energy density.
3 Ultracapacitor characteristics and test data
3.1 Cells
3.1.1 Carbon/ carbon double- layer devices
There are presently commercially available carbon/ carbon ultracapacitor devices ( single cells and modules) from several companies – Maxwell, Ness, Batscap, LS Cable, Nippon Chem- Con, and Power Systems ( References 6- 9). All these companies market large devices with capacitance of 1000- 5000 F. These devices are suitable for high power vehicle and stationary applications. The performance of the various devices is given in Table 2. The energy densities ( Wh/ kg) shown correspond to the useable energy based on constant power discharge tests from V0 to ½ V0. Peak power densities are given for both matched impedance and 95% efficiency pulses. For most applications with ultracapacitors, the high efficiency power density is the appropriate measure of the power capability of the device. For the large devices, the energy density for most of the available devices is between 3.5- 4.5 Wh/ kg and the 95% power density is between 800- 1200 W/ kg. In recent years, the energy density of the devices has been gradually increased for the carbon/ carbon ( double- layer) technology and the cell voltages have increased to 2.7V/ cell using acetonitrile as the electrolyte.
The highest performance carbon/ carbon devices tested at UC Davis were from ApowerCap in the Ukraine. These devices are packaged in a laminated pouch ( see Figure 1). A summary of the test data for the device is shown in Table 3. The ApowerCap device has an energy density of about 5.5 Wh/ kg and a power density for a 95% efficient pulse of 2500 W/ kg.
Figure 1: The ApowerCap 450F carbon/ carbon device
3.1.2 Hybrid carbon/ pseudo- capacitive devices
There has been considerable development of hybrid devices that utilize pseudo- capacitive, non- double- layer mechanisms for electrical charge storage. Most of these devices use carbon in one electrode and psuedo- capacitive or Faradaic materials in the other electrode ( Reference 10- 12). The most highly developed of these hybrid devices use microporous carbon in the negative and graphitic carbon in the positive electrode. As indicated in Table 2, these devices are being developed by Power Systems and Fuji/ JSR Micro. These devices are soft packaged in laminated pouches ( see Figure 2). Test data for the Fuji/ JSR devices are given in Table 4. Note that the energy density of the devices is 11- 12 Wh/ kg and the peak power is 900- 1000 W/ kg for a 95% efficient pulse. The JSR devices are in the early stage of commercialization.
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 3
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Table 2: Summary of the performance characteristics of ultracapacitor devices
Device
V
rated
C
( F)
R
( mOhm)
RC
( sec)
Wh/ kg
( 1)
W/ kg
( 95%)
( 2)
W/ kg
Match. Imped.
Wgt.
( kg)
Vol.
lit.
Maxwell*
2.7
2885
.375
1.08
4.2
994
8836
.55
.414
Maxwell
2.7
605
.90
.55
2.35
1139
9597
.20
.211
ApowerCap**
2.7
55
4
.22
5.5
5695
50625
.009
---
Apowercap**
2.7
450
1.4
.58
5.89
2574
24595
.057
.045
Ness
2.7
1800
.55
1.00
3.6
975
8674
.38
.277
Ness
2.7
3640
.30
1.10
4.2
928
8010
.65
.514
Ness ( cyl.)
2.7
3160
.4
1.26
4.4
982
8728
.522
.38
Asahi Glass
( propylene carbonate)
2.7
1375
2.5
3.4
4.9
390
3471
.210
( estimated)
.151
Panasonic
( propylene carbonate)
2.5
1200
1.0
1.2
2.3
514
4596
.34
.245
EPCOS
2.7
3400
.45
1.5
4.3
760
6750
.60
.48
LS Cable
2.8
3200
.25
.80
3.7
1400
12400
.63
.47
BatScap
2.7
2680
.20
.54
4.2
2050
18225
.50
.572
Power Sys.
( activated carbon, propylene carbonate) **
2.7
1350
1.5
2.0
4.9
650
5785
.21
.151
Power Sys.
( graphitic carbon, propylene carbonate) **
3.3
3.3
1800
1500
3.0
1.7
5.4
2.5
8.0
6.0
486
776
4320
6903
.21
.23
.15
.15
Fuji Heavy Industry- hybrid ( AC/ graphitic Carbon) **
3.8
1800
1.5
2.6
9.2
1025
10375
.232
.143
JSR Micro
( AC/ graphitic carbon)**
3.8
1000
2000
4
1.9
4
3.8
11.2
12.1
900
1038
7987
9223
.113
.206
.073
.132
( 1)
Energy density at 400 W/ kg constant power, Vrated - 1/ 2 Vrated
( 2)
Power based on P= 9/ 16*( 1- EF)* V2/ R, EF= efficiency of discharge
* Except where noted, all the devices use acetonitrile as the electrolyte
** all device except those with ** are packaged in metal containers, these devices are in laminated pouches
Table 3: Test data for the Pouch packaged APowerCap device
Constant current discharge data 2.7V - 0
Current A
Time sec
Capacitance F
Resistance mOhm
10
120.5
450
-----
20
60.3
453
-----
40
30
453
-----
80
14.7
452
1.4
120
9.6
455
1.4
160
7.1
456
1.3 Constant power discharges data 2.7 – 1.35V
Power W
W/ kg *
Time sec
Wh
Wh/ kg
12.5
219
95.5
.332
5.82
22
385
54.9
.336
5.89
41.5
728
28.8
.332
5.82
80.5
1412
14.6
.326
5.72
120
2105
9.1
.303
5.31
* weight of device - 57 gm as tested
Table 4: Test data for the JSR 2000F cell
Constant Current discharge 3.8V – 0V
Current ( A)
Time ( sec)
C( F)
Resistance ( mOhm) **
30
102.2
2004
---
50
58.1
1950
---
80
34.1
1908
---
130
19.1
1835
2.0
200
11.1
1850
1.9
250
8.2
1694
1.84
** resistance is steady- state value from linear V vs. time discharge curve
Constant Power discharges 3.8V – 2.2V
Power ( W)
W/ kg
Time( sec)
Wh
Wh/ kg *
Wh/ L *
102
495
88.3
2.5
12.1
18.9
151
733
56
2.35
11.4
17.8
200
971
40
2.22
10.8
16.9
300
1456
24.6
2.05
10.0
15.7
400
1942
17
1.89
9.2
14.4
500
2427
12.5
1.74
8.5
13.3
* based on the weight and volume of the active cell materials
Cell weight 206 gm, 132 cm3
Pulse resistance test
3.2 Modules
For vehicle applications, the cells are connected in series to form higher voltage modules. The module voltage utilized depends on the application and varies from 16V to about 60V. The characteristics of modules ( Reference 6, 13) from several companies are summarized in Table 5. The electrical characteristics ( capacitance and resistance) of the module follow directly from the cell characteristics. Note, however, that the weight and volume of the modules are significantly greater than the cells alone with packaging factors of .6-. 7. All the modules being marketed utilize balancing circuits for each cell to prevent over- voltage of the cell and to minimize cell- to- cell variability during cycling. For this reason, it is best to base the energy storage and power capacity of the modules on the cell weight and volume, but to include the packaging factors in determining the weight and volume of the ultracapacitor unit to be installed in a vehicle. When the energy storage ( Wh) and power requirements ( kW) are given for a particular application, the data given in Tables 2 and 5 can be used to determine the characteristics of the ultracapacitor unit for use in a vehicle.
Current ( A)
Pulse ( 5sec)
mOhm
100
2
200
1.9
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4 Applications for hybrid vehicles
It is of interest to evaluate ultracapacitor unit designs for the hybrid vehicle applications shown in Table 1. The ultracapacitor devices/ technologies used in this design exercise are those consistent with the data shown in Tables 3 and 4, which have been designated as carbon/ carbon ( Table 3) and hybrid carbon ( Table 4). The carbon/ carbon device has an useable an energy density of 5 Wh/ kg and a high power density of 2500 W/ kg for a 95% efficient pulse. The hybrid carbon device has a high energy density of 11- 12 Wh/ kg and a modest power of 1038 W/ kg for a 95% efficient pulse. The characteristics of the ultracapacitor units using the two technologies for each of the applications are given in Table 6.
4.1 Micro- hybrids
The ultracapacitor units for the micro- hybrid vehicle are quite small and light and provide the power required for this application. It would appear that the carbon/ carbon devices are the best choice for this application as they are well developed at the present time, have proven cycle life, and will be lower cost. The higher energy density of the hybrid carbon devices is not needed for this application.
4.2 Charge sustaining hybrids
The data given in Table 6 indicate that this is a difficult application for ultracapacitors because it requires both relatively high energy storage ( Wh) and high power ( kW). This application seems to need the higher energy density of the hybrid carbon devices, but their power capability for high efficiency pulse power is less than would be optimum. The power requirement of 35 kW selected for this application is relatively high and some charge sustaining hybrids have been designed with significantly lower electric motor power ( ex. the single shaft Honda hybrids with 10- 15 kW). The carbon/ carbon units would function satisfactorily in the high power, charge sustaining hybrids, but their volume and weight would not be much different than batteries.
4.3 Plug- in hybrids
In the plug- in hybrid, the ultracapacitor would be used to assist the energy storage battery ( 12 kWh) in providing the power when the vehicle was operating in the charge depleting mode. As discussed in Reference 5, it makes most sense to use ultracapacitors with high energy density batteries which do not have sufficiently high power capability to provide the total power alone. For the designs given in Table 6, it is assumed the battery energy density is 200 Wh/ kg and that the battery will provide one- third of the total power of 70 kW
Table 5: Ultracapacitor module characteristics
Module * Weight/ Volume kg/ lt. Cell capacitance F Voltage Wh ( Wh/ kg) Power ( kW) ( 90% effic.) Weight packaging factor Volume packaging factor Ness ( 194 F) 18.5/ 20.9 48 43/ 2.1 19.1 .655 .36 Ness ( 100F) 9.1/ 7.22 48 22.5/ 2.47 10.8 .769 .692 Maxwell ( 145 F) 13.5/ 13.4 48 36/ 2.7 14.5 .627 .484 Maxwell ( 430F) 5.0/ 4.85 16 11.8/ 2.36 4.8 .564 .445 Asahi Glass 280F 3.75/ 2.95 16 7.65/ 2.04 2.1 .528 .422 Power Systems 4.4/ 4.8 32 11/ 2.5 2.5 .573 .375 Power Systems 7.2/ 8 59 20/ 2.78 4.7 .642 .413 EPCOS 29/ 24 56 49/ 1.7 16 .5 .48 EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 6
Table 6: Ultracapacitor units for hybrid vehicle applications
Vehicle design
ultracap
energy stored
Wh
ultracap
peak power
kW
system
voltage
V
No. of
cells
Capac- itance
F
Max. power
90% eff.
kW
Weight ( kg) /
volume ( L)
cells unit*
Micro- hybrid
30
6
48
Carbon/ carbon
18
2550
> 10
6 / 4.6
9/ 9.2
Hybrid carbon
14
2000
5.7
2.8/ 1.8 4.3/ 3.6
Charge sustaining hybrid
150
35
200
Carbon/ carbon
80
2865
> 50
30/ 23
46/ 46
Hybrid carbon
65
2000
26
13/ 9
20/ 18
Plug- in
Hybrid
12 kWh bat. **
200
45
300
Carbon/ carbon
120
2200
> 100
40/ 30
61/ 60
Hybrid carbon
84
2000
36
18/ 12
28/ 24
* Packaging factors: weight .65 volume .5
** Energy density of the battery in the Plug- in hybrid - 200 Wh/ kg,
Vehicle electric use 156 Wh/ km
requiring a battery power density of about 400 W/ kg. Combining ultracapacitors with the battery will also make thermal management of the battery less demanding and extend the battery cycle life as it will not be subjected to the high current pulses which are taken by the ultracapacitor unit. The results shown in Table 6 indicate that this application also needs the high energy density of the hybrid carbon device and that the power density of that device is marginally high enough. Note that even using the hybrid carbon devices, the weight of the ultracapacitor unit will be about one- third that of the battery unit. The combined weight of the two energy storage units is likely to be about 100 kg for a plug- in hybrid with an all- electric range of about 50 miles
5 Computer simulations of hybrid vehicles using ultracapacitors
Simulations of mid- size passenger cars using the ultracapacitors in micro- hybrid, charge sustaining, and plug- in hybrid powertrain designs were performed using the Advisor vehicle simulation program. All the powertrains were in the same vehicle having the following characteristics: test weight 1660 kg, Cd =. 3, AF = 2.25 m2 , RRCF =. 009. The engine map used in the simulations was for a Ford Focus 2L, 4- cylinder engine. The rated engine power was 120 kW for the conventional ICE vehicle and the micro- hybrid and 110 kW for the charge sustaining and plug- in hybrids. The electric component sizes/ power are given in Table 6. All the hybrids use the single- shaft approach similar to the Honda Civic hybrid.
The simulation results are summarized in Table 7 for a conventional ICE vehicle and each of the hybrid designs. Results are given for fuel usage in terms of both L/ 100 km and mpg for various driving cycles. It is clear from Table 7 that large improvements in fuel usage are predicted for all the hybrid powertrains using ultracapacitors for energy storage. The simulation results will be discussed separately for each hybrid design.
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Table 7 Summary of the vehicle fuel economy simulation results using ultracapacitors for various driving cycles L/ 100 km / mpg Driveline type Energy storage type Voltage and weight cells ( kg) EM Peak kW FUDS HWFET US06 ECE- EUDC ICE baseline 10/ 23.8 6.9/ 34.4 9.6 24.7 9.7/ 24.6 Micro- HEV Lead- acid/ ultracaps 48 Carbon/ carbon 6 kg 6 5.7/ 41.7 5.3/ 44.7 7.8/ 30.6 5.9/ 40.2 Hybrid carbon 3 kg 6 7.3/ 32.8 6.3/ 38.0 8.9/ 26.7 7.1 33.4 Charge sustaining hybrid Ultracaps 200 Carbon/ carbon 30 kg 35 5.4/ 43.8 5.0/ 47.9 7.1/ 33.6 5.5/ 43.2 Hybrid carbon 13 kg 35 5.8/ 40.9 5.2/ 45.8 8.0/ 29.9 5.8/ 41.3 Plug- in hybrid 12 kWh Li battery and ultracaps 300 70 kW with 45 kW from ultracaps Carbon/ carbon 40 kg 45 5.5/ 43.2 5.0/ 47.7 7.0/ 33.9 5.5/ 42.9 Hybrid carbon 18 kg 45 5.8 41.2 5.2/ 46.2 7.9/ 30.2 5.8/ 41.2
5.1 Micro- hybrids
The results for the micro- hybrids are particularly interesting and surprising, because of the large fuel economy improvements predicted. These improvements were about 40% on the FUDS and ECE- EUD cycles and 20% on the Federal Highway and US06 cycles using the carbon/ carbon ultracapacitor units. The improvements were significantly less using the hybrid carbon units because of their lower round- trip efficiencies.
In the micro- hybrid designs, the rated engine power used was the same as that in the conventional ICE vehicle in order that the performance of the hybrid vehicle when the energy storage in the ultracapacitors is depleted would be the same as the conventional vehicle. The ultracapacitors were used to improve fuel economy with only a minimal change in vehicle acceleration performance. The control strategy used was to operate on the electric drive when possible and to recharge the ultracapacitors when the engine was operating. As shown in Figure 1, this resulted in a large improvement in average engine efficiency from 19% in the ICE vehicle to 30% in the micro- hybrid even though the electric motor had a peak power of only 6 kW.
Additional computer simulations were made for higher motor power ( up to 12 kW) and larger ultracapacitor energy storage ( up to 50 Wh). It was found that the improvements in fuel economy were only marginally greater. Using a motor power of 3 kW reduced the fuel economy improvement on the FUDS by more than 50%. Note from Table 7 that the fuel economy improvements using the carbon/ carbon ultracapacitors were for all the cycles greater than those using the hybrid carbon devices. This was the case because the round- trip efficiencies for the carbon/ carbon units were 95- 98% and those of the hybrid carbon units were 75- 90% for the various driving cycles. As noted previously, the hybrid carbon devices had higher energy density, but even though their power density for 95% efficiency was relatively high ( 1050 W/ kg), it was not proportionally higher – that is twice as high- as the carbon/ carbon devices with lower energy density. These results show clearly that it is essentially to develop high energy EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 8
Engine operating efficiency for the ICE vehicle-
average engine efficiency .19 Engine operating efficiency for the micro- hybrid -
average engine efficiency .30
Figure 1 A comparison of engine efficiencies for a conventional ICE vehicle and a micro- hybrid on the FUDS cycle using carbon/ carbon ultracapacitor
density ultracapacitors with proportionally higher power density; otherwise their use in vehicle applications will be compromised.
5.2 Charge sustaining hybrids
The fuel economy simulation results for charge sustaining hybrids are also shown in Table 7 for a mid- size passenger car using both carbon/ carbon and hybrid carbon ultracapacitors. Using the carbon/ carbon ultracapacitor unit, the fuel savings are about 45% for the FUDS and ECE- EUD cycles and about 27% for the Federal Highway and US06 cycles. These improvement values are higher than for the micro- hybrid, but not by as large a factor as might be expected. The prime advantage of the high power electric driveline in the charge sustaining hybrid is that it yields large fuel economy improvements even for high power requirement driving cycles like the US06. The fuel economy improvements using the hybrid carbon ultracapacitor unit are not much less ( 5- 10%) than those with the carbon/ carbon unit even though the round- trip efficiency of the hybrid carbon unit is only 85- 90% compared to 98% for the carbon/ carbon unit. Since the volume of the hybrid carbon unit is relatively small - 43% of that of the carbon/ carbon unit, it appears that the charge sustaining hybrid application is a better application for the present hybrid carbon technology than the micro- hybrid application.
5.3 Plug- in hybrids
The plug- in hybrid vehicle studied is one that utilizes a high energy density battery ( 200 Wh/ kg) and ultracapacitors that would provide two- thirds of the power to a 70kW electric motor. The electric energy use of the vehicles in the charge depleting mode ( engine off) is assumed to be 156 Wh/ km resulting in a charge depleting range of 60 km ( 38 mi.) for 80% DOD of the battery. The fuel economy results shown in Table 7 are for vehicle operation in the charge sustaining mode after the energy battery ( 12 kWh) has been depleted. As expected in this mode, the operation of the plug- in hybrid vehicle is essentially the same as previously discussed for the charge sustaining hybrid. Hence the hybrid carbon ultracapacitor unit would also be suitable for the plug- in hybrid. The combined weight of the cells in the battery and hybrid carbon ultracapacitors would be 78 kg for a plug- in hybrid with an all- electric range of about 40 miles. The combined weight using the carbon/ carbon ultracapacitors would be 100 kg. Using a high power lithium- ion battery with an energy density of 100 Wh/ kg without ultracapacitors, the weight of the battery cells alone would be 120 kg. Hence for plug- in hybrids combining a battery with ultracapacitors may be an attractive design option.
6 Summary and conclusions
The development of electrochemical capacitors ( ultracapacitors) has continued since the early 1990s. Activated microporous carbon and hybrid carbon devices from a number of developers EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 9
world- wide have been tested and evaluated for use in hybrid vehicles of various types. The test data indicate that the useable energy density of the activated carbon devices is about 5 Wh/ kg and that of the hybrid carbon devices is 10- 12 Wh/ kg. The power capability of the carbon/ carbon devices can be very high (> 2000 W/ kg for a 95% high power pulse); the power capability of the hybrid carbon devices are significantly lower being 500- 1000 W/ kg for a 95% pulse. This means that the P/ E ratio of the hybrid carbon devices is much lower than the carbon/ carbon devices and as a consequence, it may be difficult to take full advantage of the higher energy density of the hybrid carbon devices in some applications.
The simulation results for micro- hybrids are particularly interesting and surprising, because of the large fuel economy improvements predicted. The improvements were about 40% on the FUDS and ECE- EUD cycles and 20% on the Federal Highway and US06 cycles using the carbon/ carbon ultracapacitor units. The improvements were significantly less using the hybrid carbon units because of their lower round- trip efficiencies.
The fuel economy simulation results for charge sustaining hybrids show that for the carbon/ carbon ultracapacitor unit the fuel savings are about 45% for the FUDS and ECE- EUD cycles and about 27% for the Federal Highway and US06 cycles. These improvement values are higher than for the micro- hybrid, but not by as large a factor as might be expected. The prime advantage of the high power electric driveline in the charge sustaining hybrid is that it yields large fuel economy improvements even for high power requirement driving cycles like the US06. The fuel economy improvements using the hybrid carbon ultracapacitor unit are not much less ( 5- 10%) than those with the carbon/ carbon unit even though the round- trip efficiency of the hybrid carbon unit is only 85- 90% compared to 98% for the carbon/ carbon unit. Since the weight/ volume of the hybrid carbon unit is relatively small - 43% of that of the carbon/ carbon unit, it appears that the charge sustaining hybrid application is a better application for the hybrid carbon technology than the micro- hybrid application.
The plug- in hybrid vehicle simulated is one that utilizes a high energy density battery ( 200 Wh/ kg) and ultracapacitors that would provide two- thirds of the power to a 70kW electric motor. The combined weight of the cells in the battery and hybrid carbon ultracapacitors would be only 78 kg for a plug- in hybrid with a charge depletion electric range of about 40 miles. The combined weight using the carbon/ carbon ultracapacitors would be 100 kg. Using a high power lithium- ion battery with an energy density of 100 Wh/ kg without ultracapacitors, the weight of the battery cells alone would be 120 kg. Hence for plug- in hybrids combining a battery with ultracapacitors may be an attractive design option.
References
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[ 2]
Burke, A. F., Batteries and Ultracapacitors for Electric, Hybrid, and Fuel Cell Vehicles, IEEE Journal, special issue on Electric Powertrains, April 2007
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Axsen, J., Burke, A. F., and Kurani, K., Batteries for Plug- in Hybrid Electric Vehicles ( PHEVs): Goals and State of the Technology ( 2008), Report UCD- ITS- RR- 08- 14, May 2008
[ 4]
Burke, A. F. and Van Gelder, E., Plug- in Hybrid- Electric Vehicle Powertrain Design and Control Strategy Options and Simulation Results using Lithium- ion Batteries, paper presented at EET- 2008 European Ele- Drive Conference, Geneva, Switzerland, March 12, 2008 ( paper on the CD of the proceedings of the conference)
[ 5]
Burke, A. F., Miller, M., and Van Gelder, E., Ultracapacitors and Batteries for Hybrid Vehicle Applications, 23rd Electric Vehicle Symposium, Anaheim, California, December 2007 ( paper on CD of proceedings)
[ 6]
Burke, A. F. and Miller, M., Supercapacitor Technology- Present and Future, Advanced Capacitor World Summit 2007, San Diego, California, July 14- 16, 2007
[ 7]
Burke, A. and Miller, M., Supercapacitors for Hybrid- electric Vehicles: Recent Test Data and Future Projections, Advanced Capacitor World Summit 2008, San Diego, California, July 14- 16, 2008
[ 8]
Burke, A. F. and Miller, M., Tests of New Ultracapacitors and Comparisons with Lithium- ion Batteries for Hybrid vehicle Applications, Proceedings of the 16th International Seminar on EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 10
Dr. Marshall Miller is a Senior Development Engineer at the Institute of Transportation Studies at the University of California, Davis. He is the Director of the Hydrogen Bus Technology Validation Program which studies fuel cell and hydrogen enriched natural gas buses. He also supervises testing in the Hybrid Vehicle Propulsion Systems Laboratory where he does research on fuel cells, advanced batteries, and ultracapacitor technology. His overall research has focused on advanced environmental vehicles and fueling infrastructure to reduce emissions, greenhouse gases, and oil usage. He received his B. S. in Engineering Science and his M. S. in Nuclear Engineering from the University of Michigan. He received his Ph. D. in Physics from the University of Pennsylvania in 1988.
Double- layer Capacitors and Hybrid Storage Devices, Deerfield Beach, Florida, December 2007
[ 9]
Burke, A. F., Considerations for Combinations of Batteries and Ultracapacitors for Vehicle Applications, Proceedings of the 18th International Seminar on Double- layer Capacitor and Hybrid Energy Storage Devices, Deerfield Beach, Florida, December 2008
[ 10]
Burke, A. F., Materials Research for High Energy Density Electrochemical Capacitors, published by the Materials Research Society in their Symposium Series- Spring 2008 Symposium JJ
[ 11]
Yoshio, M., Megalo- Capacitance Capacitor and Management System, Proceedings of the Second International Symposium on Large Ultracapacitor ( EDLC) Technology and Applications, Baltimore, Maryland, May 16- 17, 2006
[ 12]
Hatozaki, O., Lithium- ion Capacitor ( LIC), Proceedings of the 16th International Seminar on Double- layer Capacitor and Hybrid Eergy Storage Devices, Deerfield Beach, Florida, December 2006
[ 13]
Burke, A. F. and Miller, M., Supercapacitor Technology- Present and Future, proceedings of the Advanced Capacitor World Summit 2006, San Diego, California, July 2006
Authors
Andrew Burke, Research faculty, ITS- Davis. Ph. D., 1967, Princeton University. Since 1974, Dr. Burke’s research has involved many aspects of electric and hybrid vehicle design, analysis, and testing. He was a key contributor on the US Department of Energy Hybrid Test Vehicles ( HTV) project while working at the General Electric Research and Development Center. He continued his work on electric vehicle technology, while Professor of Mechanical Engineering at Union College and later as a research manager with the Idaho National Engineering Laboratory ( INEL). Dr. Burke joined the research faculty of the ITS- Davis in 1994. He directs the EV Power Systems Laboratory and performs research and teaches graduate courses on advanced electric driveline technologies, specializing in batteries, ultracapacitors, fuel cells and hybrid vehicle design. Dr. Burke has authored over 80 publications on electric and hybrid vehicle technology and applications of batteries and ultracapacitors for electric vehicles.
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 11