Ugrás a tartalomhoz

Electric Vehicles

Gyuláné Vincze, Gergely György Balázs

Budapest University of Technology and Economics Department of Electric Power Engineering

Chapter 9.  Drives of electric and hybrid-electric cars

Chapter 9.  Drives of electric and hybrid-electric cars

One special group of electric vehicles is electric cars, which were aimed to give an alternative against internal combustion engines from the beginning, but the competition is unbalanced. The biggest advantages of internal combustion engines are their high energy density fuel (diesel oil, petrol, PB-gas) which is available in big amount, the possibility to use additional power (sometimes too high), and the simplicity of refilling. Nowadays it became obvious that we cannot postpone actions to lower pollution in big cities, replace oil, and increase use of renewable energy sources. To decrease pollution, one of the most important steps to take is to replace internal combustion engines with electric or hybrid-electric drives.

Development of electric solutions can be found in every vehicle types, from mopeds through passenger  cars and trucks, till city buses. Nowadays, the direction of the development is that electric solutions should reach the same or almost the same running and comfort behaviors as regular for internal combustion solutions. Earlier developments were aimed to build lightweight, small, low-power, less comfort mini electric cars (LEV, SULEV Light, Superlight Electric Vehicle). These cars were planned to be used for short-length urban transport, shopping and going to work. One special category is vehicles for parks, environmental places and closed places, where pollution is prohibited so only vehicles with pure electric drive and energy storage are permitted.

Advantages of electric drives vs. internal combustion engines

Most of the drives are electric in everyday life activities and industry. Only one area exists where electric drive did not win, and this area is road transport. Break-through is not easy in this filed although electric drives have several advantages here also.

Advantages from environmental and energetics viewpoints:

  1. There is no pollution during operation;

  2. Less noise generated;

  3. Efficiency of energy conversion is much higher.

  4. In case of electric drive, during stops in urban traffic, which happens often, energy consumption can be minimal. No energy is needed for  no-load (compare with the no-load running consumption).

  5. Energy regenerating braking can be solved; kinetic energy during slowing down can be used for electric energy generation. With regeneration, 20…30 % energy saving can be reached because of frequent acceleration and deceleration in urban traffic.

Advantages from vehicle design viewpoint:

  1. Properties and characteristics of electric drives can be varied freely with electronic devices, they can be fitted to user demands in every respect.

  2. Torque-speed limit characteristics of almost all kinds of electric drives can be set to meet the ideal characteristics of traction. There is no need for gearshift between the electric motor and wheels. In contrast, internal combustion drives have to use variable mechanical gears, because their torque-speed characteristics are different than traction characteristics.

  3. Multi-motors or axle-box motors can be used. Common control of multi motors has no technical problem.

  4. With using several, low power motors the design is easier, especially for low board vehicles.

  5. Axle-box motor drive can be implemented only with electric drive.

  6. With multi-motors energy regeneration is easier, electric and traditional mechanical (hydraulic or pneumatic) brake can be combined for more  wheels.

  7. Possible to use per-wheel controls to prevent spinning-off and slippage.

From this summary we see that electric drive can be an ideal solution for urban and public road transport with respect to environmental and energetics aspects. The bottleneck to re-build vehicles to electric ones is the problem of energy storage on board. Development directions of electric cars are also restricted by electric energy supply.

Main development directions of electric drive vehicles :

Electric cars, literally, are the first three ones in the list above. The main characteristic of electric cars is that they have no internal combustion engines, drive is strictly electric. Pure electric car is where energy generation, charging and storing are also electric and pollution of the vehicle is zero. Vehicles having fuel cells use electric storage too to store electric energy but refilling is not electric,  pollution is not zero, stack gas appears depending on the fuel (hydrogen, methanol). Section 8.1 deals with electric cars.

Contrarily, in hybrid-electric vehicles internal combustion engines and one or more electric motors can be found. Pollution is decreased with this hybrid solution but is does not become zero. Drive of the wheels is purely electric or combined with internal combustion drive. In these vehicles batteries or ultra-capacitors are used to store electric energy temporarily, but the main energy source is fuel stored in tank. In PHEV vehicles electric network charge is also used for refilling energy. Fuel consumption and pollution is determined by the internal combustion engine. One of the main design aim of hybrid cars is to decrease consumption and pollution while improve running behaviours. Nowadays new terms like full, middle and mild hybrid cars are used, which gives the ratio between total power and electric power used for traction. Section 8.2 deals with hybrid cars.

Electric cars

In electric cars, purely electric drive is used, for which electric energy source and supply network is needed on-board. We can distinguish three groups based on the type of the energy source:

Electric motor drive has to be chosen and designed so that its M-ω characteristic is suitable for traction needs and no gearshift (variable mechanical gear) is needed. Mechanical characteristic of an electric drive for a given F-v traction characteristic can be seen in Figure 2.5. Design and selection is introduced in Section 2.3. Estimation of F-v traction force can be based in Figure 1.4, where minimal specific traction demand  is summarized for urban vehicles.

Almost every types of electric drives introduced in Section 2.4 can be used in electric cars. The following four tables demonstrate this. In Table 8.1 and 8.2, data of cars with  batteries realized as specific products and prototypes are described. Data of fuel cell electric cars are summarized in Table 8.3 and 8.4 (Source: M. H. Westbrook:  The Electric Car. UK University Press, Cambridge. 2005.)

Third rows of the tables show the drive type used in the cars:

  1. Separately excited DC drive (described in Section 4.1.5) is used more and more rarely. Usual structure of this drive is introduced in Section 4.2.5, one of the possible electric circuitries can be seen in Figure 4.19. Formerly, multistep switch controlled series excited DC drive was also used for electric car drives. Switches were used to change series resistance like vehicles described in Section 4.2.1, or terminal voltage was changed by varying serial and parallel connections of battery cells.

  2. Voltage source inverter-fed 3-phase induction (AC induction) motor drive with field-oriented control is used often, described in Section 5.2.

  3. Permanent magnet (PM) sinusoidal field synchronous motor drive with voltage source inverter is often used in electric cars, introduced in Section 6.1.2.

  4. Brushless DC drive with voltage source inverter is used for lower-power electric cars, especially for wheel hub motors, described in Section 6.2. Three- and five-phase types of this drive are developed and applied by several manufacturers.

  5. Pilot cars with SRM switched reluctance motor drive exist but are not described here.

Table 8‑1. Technical data of electric cars with batteries (till February 2001).

Manufacturer

Citroen

Daihatsu

Ford

GM

Honda

Nissan

Nissan

Peugeot

Renault

Toyota

Model  name

AX/Saxo

Electrique

Hijet EV

Th!nk

City

EV1

EV Plus

Hypermini

Altra EV

106 Electric

Clio

Electric

RAV 4

Drive type

Separately

excited DC

PM Synchron

3-phase

induction

3-phase

induction

PM Synchron

PM Synchron

PM Synchron

Separately

excited DC

AC

Induction

PM Synchron

Battery type

NiCd

NiCd

NiMH

NiMH

Li-ion

Li-ion

NiCd

NiCd

NiMH

Max power

O/P (kW)

20

27

102

49

24

62

20

22

50

Voltage (V)

120

114

343

288

345

120

114

288

Battery energy capacity (kWh)

12

11,5

26,4

15

32

12

11,4

27

Charging connector

Conductive

Inductive

Conductive

Inductive

Inductive

Conductive

Conductive

Top speed

(km/h)

91

100

90

129

129

100

120

90

95

125

Claimed max

Range (km)

80

100

85

130

190

115

190

150

80

200

Charge time

7

7

5-8

6

6-8

4

5

7-8

10

Table 8‑2. Technical data of electric cars with batteries (till February 2001, cont.).

Manufacturer

BMW

Daimler

Chrysler

Daimler

Chrysler

Fiat

Ford

GM

Lada

Mazda

Mitsubishi

Toyota

Model  name

BMW

Electric

Zytek Smart EV

A-Class

Electric

Secento

Elettra

e-KA

Impuls 3

Rapan

Roadster-EV

Libero

E-com

Drive type

PM Synchron

Brushless DC

3-phase

induction

3-phase

induction

3-phase

induction

2x3phase

induction

Separately

excited DC

AC

induction

PM Synchron

Battery type

NaNiCl2

NaNiCl2

NANiCl2

Lead-acid

Li-ion

NaniCl2

NiCd

NiCd

NiCd

NiMH

Max power

O/P (kW)

45

30

50

30

65

42

30

19

Voltage (V)

289

216

286

192

288

Battery energy capacity (kWh)

29

13

30

13

28

26

Charging connector

Conductive

Inductive

Top speed

(km/h)

130

97

130

100

130

120

90

130

130

100

Claimed max

Range (km)

155

160

200

90

150

150

100

180

250

100

Charge time

8 (75%

boost charge 40min)

7

8 (80% in 4h)

6

6-8

8

8

Table 8‑3. Technical data of fuelcell electric cars.

Manufacturer

Daimler

Chrysler

Daimler

Chrysler

Ford

Ford

GM

GM

Honda

Mazda

Model  name

NECAR 5

Commander

SUV

P2000

HFC

Th!ink Focus FCV

Opel

Zafira

Opel Zafira

HydroGen 1

FCV-V3

Demo-FCEV

Drive type

3-phase

induction

3-phase

induction

3-phase

induction

3-phase

induction

PM synchron

3-phase

induction

Power surce

Fuel-cell+

methanol

reformer or

H storage

Fuel-cell+

methanol

reformer+

battery

Fuel-cell +

H storage

Fuel-cell +

H storage

Fuel-cell +

methanol reformer  or

H storage

Fuel-cell +

H storage

Fuel-cell +

H storage +supercap

Fuel-cell +

H storage

+supercap

Max power

O/P (kW)

55

70

67

67

80

89

60

65

Voltage (V)

330

255

315

Top speed

(km/h)

145

128

128

120

145

130

90

Claimed max

Range (km)

450

160

160

640

400

177

170

Date for

production

2004

2004?

2004

2004

2004

2003

Table 8‑4. Technical data of fuelcell electric cars (cont.).

Manufacturer

Mitsubishi

Nissan

Peugeot/Citroen

Renault/Volvo

Euro Project

Toyota

VW

VW

Model  name

Fuel-cell EV

FCV

Partner

Fever

FCEV

Bora Hymation

Sharan

Drive type

PM Synchron

Synch wound rotor

PM Synchron

3phaseinduction

Power surce

Fuel-cell+

reformer

Fuel-cell+

reformer

Fuel-cell +

reformer or

H storage

Fuel-cell +

H storage +

NiMH battery

Fuel-cell +

methanol reformer

Fuel-cell +

H storage

Fuel-cell +

reformer

Max power

O/P (kW)

30

50

89

Voltage (V)

FC 90 System 250

Top speed (km/h)

120

125

140

Claimed max

Range (km)

400

500

350

Date of production

2005

2004/5

2003/4

2003

Battery fed electric cars called “Puli” were manufactured in Hungary, Hódmezővásárhely, with series excited DC drive, 10 pieces of 6V/240Ah lead-acid batteries, 65 km/h max speed and 60-100 km range. In contrast, Tesla-Roadster luxury car was developed in 2007, with inverter fed AC induction drive, Li-ion batteries, 130km/h max speed and 400 km range.

From the tables above, we can see that supply voltage can be varied in a wide range 114-330V, both for battery or fuel cell fed vehicles. Load current can be reduced if supply voltage is increased for a drive with certain power need. Optimal selection of voltage and current is influenced by the energy source and the type of drive. Vehicles exist where voltage used for drive is different that the voltage of the energy source, in this case DC/DC converter is needed. Such a vehicle can be seen in Table 8.4, where fuel cell voltage is 90V and drive voltage is transferred to 250V.

Energy supply for electric cars

Electric energy needed for electric cars is determined by the drive, mainly, but energy needed for auxiliaries can also be high.

Auxiliaries in traditional vehicles with internal combustion engines are fed by auxiliary battery with 6, 12 or 24V output voltage, directly or through a power electronic circuit. Controlled charging of this auxiliary battery is realized with a generator driven by the engine.

In contrast, generator cannot be found in electric cars. As we use the same auxiliaries, low-voltage auxiliary battery can also be found in electric vehicles. This battery must be charged by the main circuit. The typical structure of the main circuit for electric cars can be seen in Figure 8.1. The main power source can be battery, fuel cell alone or with a secondary energy storage device.

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Figure 8-1. Typical structure of the main circuit for electric cars

Auxiliaries used in traditional cars induce special consequences. Auxiliaries are designed so that there is no need to connect their negative pole to the battery; the negative pole is realized by the body of the car. If the DC/DC charger shown in Figure 8.1 is not isolated, then the negative pole of the main circuit will also be at body potential. There are electric cars where main voltage is divided to 50-50% and “grounding” is at the middle. In this case the circuit is asymmetric, with respect to the auxiliary battery, but voltage is reduced to ±Umain/2 for electric shock protection. In modern high-power vehicles, auxiliaries with alternating current may exist, in this case DC/AC inverters are connected usually to the main circuit (see Figure 8.1).

Electric cars with battery

Types of batteries selected by the manufacturers may vary, as can be seen in the fourth rows of Tables 8.1 and 8.2. Energy storages used in cars were acid or lead batteries formerly. Nowadays they are used only for auxiliaries and they are closed (gel) or valve regulated lead acid (VRLA) types. There were times when a lot of electric cars were manufactured with NiCd batteries, but their usage was prohibited by the new environment protection provisions, because Cd is dangerous waste. Instead, Nickel-Metalhydrid (NiMH) type batteries were developed with almost the same parameters. Main data of these three traditional types are summarized in the first three columns of Table 8.5. Data of Lithium based batteries can be found in the fourth column for comparison. These data are getting better and better as development continues.

Table 8‑5. Battery types

Battery type

Lead-acid

Nickel-Cadmium,

NiCd

Nickel-Metalhydrid,

NiMH

Lithium-ion and

Lithium-ion polymer

Operational temperature

-10…55ºC

-40…50ºC

-40…50ºC

-45…85 ºC

Electrolyte

Liquor of sulfuric acid

Liquor of alkali solution

Liquor of alkali solution

Organic electrolyte or polimer

Non-operative voltage

2,1V

1,35V

1,35V

3,5V

Energy-storing capacity/unit

30…45Wh/kg

40…55Wh/kg

50…80Wh/kg

100…250Wh/kg

Power/unit

100…200W/kg

180…260W/kg

180…250W/kg

300…800W/kg

Lifetime

300…850 cycle

600…1000 cycle

600…1000 cycle

500…1200 cycle

Batteries listed here can be operated in normal temperature which ensures their use for general purposes. They can be used without additional devices in electric cars.

A possible promising type for electric vehicles was NaNiCl2 battery (called “Zebra”), with high efficiency and 90-100Wh/kg energy storing capacity. Its main disadvantages are its complexity and high working temperature (300-350ºC).

Li-based batteries gave a breakthrough with respect to their application in vehicles, especially lithium-ion and lithium-polymer types.

Lithium-ion (Li-ion) technology is based on movement of lithium ions. During charge ions drift to the negative carbon-based electrode, while they drift to the positive metal-oxide electrode during discharge. Organic solvent with conductive additions is used as electrolyte. Li-based batteries were first developed in the 80s. They used metallic lithium and could overheat during overload which led to explosion and melt. Nowadays, batteries use several compounds or additional materials as lithium ion sources (e.g. yttrium) so lithium is bound securely. Despite of the dangers, a lot of manufacturers develop Li-ion batteries, because their electric and energy storage properties are the best. Energy storage capacity (Ah) is about twice as of NiMH batteries which come from the doubled cell voltage. (Cell voltage is ≈3,5V when fully charged). Even, discharged cell can provide about 3 V comparing to 1-1,35V for NiCd or NiMH batteries. More advantages are their relative light weight and that no crystals appear during operation.

Li-polymer battery is a promising development, too. Its main advantage is that no, or very few electrolyte is used, they use special polymer to separate anode and cathode. This fact can produce thin and flexible cells, no thick-wall container is needed to protect environment against electrolyte. But, shorter lifetime and longer charging time is expected.

These batteries can be compared by several viewpoints, like: operational temperature, energy storage capacity, specific power, lifetime, energy efficiency, production cost, robustness, maintenance.

Energy storing capacity per unit (specific energy), which is 30-170Wh/kg for present batteries, is the most important for vehicle application. This nominal value is given by the manufacturers for 25°C operational temperature and constant nominal current discharge. Energy available for real use can vary and depends on temperature, overload, deterioration etc. About 15kWh energy is needed to operate 1t mass vehicle for 100 km. This means that 300 kg of batteries with 50Wh/kg energy storing capacity would be needed but half would be enough when using 100Wh/kg energy storing capacity. So improving energy storing capacity is very important.

Power per unit (specific power) is also an important parameter. It shows how much P=ui momentary dynamic electric power load the battery can bear. Based on this data and the voltage, we can estimate how much overcurrent can be permitted, how much is the allowed charging current and whether boost charging is acceptable.

Ragone-diagrams are often used to compare different battery types. The diagram shows energy storage capacity vs specific power, often comparing to other energy storages.

Battery energy storage is based on series connected battery cells. Batteries used in vehicles can be operated without additional devices, no or very little (periodical) maintenance is needed, except type NaNiCl2 („Zebra”). The main circuit of an inverter-fed battery car can be seen in Figure 8.2.

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Figure 8-2. Main circuit diagram of inverter-fed car with battery.

As shown earlier, the negative pole of the main battery or the central point is on the same potential as the car body. Charger for the low-voltage auxiliary battery is connected to the mainbattery.

Energy efficiency of the selected main battery is important for vehicles application, which describes that what percentage of the filled in energy can be taken out. The energy efficiency is:

In the expression index ch index means voltage and current during charge time and dc during discharge time. Discharge value u dc of output voltage u main is always lower than no-load voltage in Table 9.5 (u dc <U o), and charging value is higher (u ch >U o). Main circuit voltage u main depends on several factors, like current, charge and deteriorationstate, environmental temperature. Typical change in output voltage vs charge degree is shown in Figure 8.3.a, where the parameter is the current of the battery. The limit of discharge is set by final discharge voltage U end, which can be even zero for some battery types. Charging is limited by permitted maximal value Umax.

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Figure 8-3. Characteristics of batteries, a.) voltage change vs charge degree, b.) output capacity vs overload and c.) temperature vs current.

Charge degree is marked with SOC (State of Charge) value, which gives how much capacity is available to reach final discharge state comparing to the nominal capacity of the accumulator.

Nominal capacity of the battery is the amount of charge available during nominal discharge time t n with nominal discharge current I n at 25°C temperature, K n =I n t n, which is given by the well-known Ah (Ampere-hour) value. t n nominal discharge time can vary, for vehicle batteries is it 5 hours typically, but can be 10, 20 hours, too. If the discharge current is higher than the nominal value, for example I/I n =2, than discharge time decreases to t dc <(t n /2) value, this means that actual output capacity decreases in case of overload. Capacity change is shown in Figure 8.3.b. The amount of output charge decreases also if the temperature of the battery is lower than 25°C, this change is shown in Figure 8.3.c.

Capacity of batteries is strictly connected to energy storage capacity, for which nominal value is E n =K n U n =I n U n t n, where U n is nominal voltage measure on the poles at nominal current, which is lower than no-load voltage U n <U o. Output voltage changes during operation, and depends on charging degree as well as load current, as can be seen in Figure 8.3.a.

Charge that can be got during time tx with discharge current idc is . The output energy for this time period is .

Usable energy also depends on udc discharge voltage, but energy storing capacity depends on overload and temperature, similarly as for capacity shown in Figure 8.3.b and 8.3.c.

Current I n of battery feeding the main circuit (its K n nominal capacity) must be set so that it should provide the designed nominal power P n =U n I n. The designed power is the sum of the power required by the drive and the auxiliary devices. During design, we have to consider that the battery should provide i>I n current because of the dynamic requirements of the vehicle drive. It could be overloaded while accelerating and could recuperate decelerating energy with regenerating braking (dashed direction in Figure 8.2). Regenerated energy can reach even 20-30% of used energy for urban vehicles. Energy saving is the highest in urban vehicles because in this case they brake and stop often. To reduce dynamic load of batteries, an ultracapacitor can be used as additional energy storage combined with the main circuit (Section 8.1.3).

One of the main components of running cost of battery vehicles is the life-cycle of the accumulator. This means the maximal value of charge-discharge cycles that a battery can endure. This value specifies how often the whole battery set should be replaced. If the max number of charges is 1000 and the car is used and recharged every day, then the lifetime is three years.

Disadvantages of using batteries are the following:

  1. Relatively low specific energy storing capacity;

  2. Frequent charge required, while boost charge is hard to realise;

  3. Relatively short lifetime;

  4. Hard to measure the remaining available energy;

  5. Used batteries have to be gathered and recycled.

Disadvantages show that the most promising application of battery vehicles is urban transport. Range available with one charge is relatively short because of the low energy storage capacity of the batteries. One more problem arises. Not only the drive but all the other equipments  are electric, this means that lower comfort should be used to reduce consumption. Every comfort equipment, especially air conditioning shortens the range of the vehicle.

Because of low energy storage, the vehicle should be charged often and charging adapter and protection circuits must be used.

Several solutions exist to charge the main accumulator:

  1. High capacity boost charging stations;

  2. Slow charge from consumer electric network (during night);

  3. Slow charge at work parking lots;

  4. Special parking lots with high-frequency power transmission;

  5. With solar cell built onto the vehicle (additional charging with solar cell);

  6. With additional treadle operation generator built into the vehicle.

From the list above we can see that there are two main charging methods: fast (boost) and slow. Slow charging is better for increasing lifetime. Boost charging means heavy stress for batteries. In this case additional slow charge cycles are also preferred. From this we can conclude that the connection to the charging network should be available for several charging methods.

Boost charge would be optimal if recharge time could be fast (10…15 mins) enough comparing to internal combustion engines. This means a lot of problems. One big problem is that charging current must be much higher than nominal current i t >I n (t n /t t ). If nominal charging time is t n =5 hours for an accumulator, and charging time is expected to be t t=10 minutes then charging current should be 30 times than nominal (t n /t=30). Another problem is that high-power charging stations should be installed for boost charging. For example, 90 kW charging power is required to charge fully a 15kWh energy storing battery in 10 minutes. Besides, both the battery and the station must be secured and protected.

Opportunities to reduce boost charge power:

  1. increasing charge time to an acceptable value,

  2. partial boost charge to 40-50% of full capacity.

There are charging stations where boost charge is available. Shape and handling of filler head is similar to petrol ones, only the filler is connected to the electric charger with a cable. Energy required for recharge is transmitted with special high frequency transformers. Insulated energy transmission, which is important for safety, can be guaranteed with inductive coupling. The structure of a boost charger can be seen in Figure 8.4.

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Figure 8-4. Typical structure of a boost charging device.

Boost charger consists of a network filter, high frequency power supply and filler head. The AC/AC converter provides one-phase, regulated, 30-70kHz frequency AC voltage and charging current control. Primary coil installed in the filler head and ferrite-core secondary coil installed in the vehicle create high-frequency coupled transformer. Secondary voltage is transformed to rectified DC with an AC/DC converter.

During slow charge at night or in parking lot charging current demand is similar to nominal current, i ch <(2-3)I n. Acceptable charging time can be 5-8 hours for night and during work time, and can be shorter for other parking lots. During night, charging should be operated with normal household electric network. The charger itself can be outside or inside the vehicle, or divided as shown in Figure 8.4.  The simplest one is the charger inside the vehicle, which needs additional space and weight but can be connected conductively (with simple industrial plug) to the network. Newer developments target to create high-frequency inductive charging at parking lots. This charging method is similar to the one that can be seen in Figure 8.4 but inductive connection is realized not with a filler head. Primary coil of the transformer is flat inside the parking lot and the car should be stopped so that the coupling between the two coils be the best.

Solar cell and treadle generator additional charging is used in hobby vehicles. In both cases it is important that electric circuit should prevent energy consumption of the chargers. For example, solar cells should not be energy consumers when there is no light for normal operation (in dark). Additional charging electronics always include a rectifier diode which prevents that the direction of the charging current changes. In solar cell chargers electronics and control is set to give the best efficiency for the solar cell and to provide continuous current. This ensures maximal output power during different light intensity.

Battery management is used when batteries are connected to microcontroller based state monitoring, protection and signal electronics. The main roles of the management are:

  1. to monitor temperature of the battery,

  2. to monitor voltage difference between cells or cell groups, and start balancing if required,

  3. to monitor charging state of the accumulator.

Equalizing charging voltage of battery cells can improve the charge efficiency and lifetime of batteries for both boost charge and the time after charge. If the batteries are connected in serial during operation, then charging is also made in serial, with controlling voltage and charging current of the charger. Voltage is not uniform on the cells, especially during boost charge. There are battery cells where voltage is lower or higher than average. This difference worsens the use of the whole system. This can be a big problem during boost charge, because some cells can be over-charged while others are underfed. Capacity of the system decreases because of the underfed cells and lifetime shortens because of the over-charge. There are special control systems to provide voltage equalization. In Figure 8.5.a., voltage higher than acceptable is decreased with shunt circuits. Current on the shunt means loss in the system.

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Figure 8-5.Voltage equalizing of batteries a./ with shunt circuits, b./ chain connection. c./ Operational circuit diagram for chain connection.

Figure 8.5.b shows an almost loss-less solution. In the chain, EQ circuits compare voltages on two neighbor cells. If voltages are different, then they control the difference of the charging currents. This idea can be seen in Figure 8.5.c. If u 1 > u 2, then transistor T1 opens and i 1 < i 2. In this circuit only the current difference causes loss on resistance R EQ. Potential divider consists of two resistances R, and provides reference signal.

It is important to calculate the charging state of the main battery in electric vehicles, just like to measure the level of petrol in petrol-driven cars. We have to know how much the „remaining” energy is in the accumulator, what range can be reached without recharge. Momentary available energy is measured by a relative available energy referred to the nominal capacity of the battery in the literature. This value is called SOC (State of Charge), in percent.

There are several methods to calculate charge state:

  1. measuring consumed charge ( òidt ) and comparing it with calculated capacity coming from the characteristics of the battery,

  2. measuring consumed energy ( òuidt ) and comparing it with calculated energy storage capacity coming from the characteristics of the battery,

  3. capacity calculated from voltage measurement, calculated from the response to dynamic (rectangle shape) load change,

  4. capacity calculated from impedance measurement, calculated from the response to superposed sinus voltage signals.

All of the methods require a lot of calculations, and we have to take into account the temperature and lifetime state of the battery.

Instead of a battery, ultracapacitor (super-capacitor) can also be used.

Ultracapacitor is a new and important product nowadays. It is a special capacitor which can take and provide extra high power.

Usually, energy stored in a capacitor C charged to voltage U can be calculated as W=CU2/2. The capacity of the capacitor is C=εrε0A/δ, where ε r is relative permittivity of the dielectric, ε 0=8,85∙10-12F/m is the permittivity of vacuum, A is area of capacitor plates, δ is thickness of dielectric. Traditional capacitors have only about 0,1 Wh/kg relative (specific) energy storage even for the best polyethylene dielectric.

U ltra c apacit or is a two-layer capacitor made by special electro-chemical technology where dielectric thickness δ is extremely small, sometimes in the range of μm. Because of this, very high, 500-1500F capacitors can be made with low loss and long lifetime. Relative energy storing capacity is much higher than that of the traditional capacitors, in the range of 5 Wh/kg, but much lower than the energy storing capacity of batteries (50…150Wh/kg). Voltage permitted on the poles of the ultracapacitor is low (3-5 V) so several serias-connected cells are required, similar to the batteries. The plates of ultracapacitors can be flat or scrolled. Dielectric used between the plates can be carbon-metal composite, foam carbon, activated synthetic monolithic carbon, polymer carbon film, metaloxide etc. Ultracapacitors are manufactured by several companies, like ESMA, ELIT, NESS, PowerCache, SAFT, etc.

In several applications not the energy storage capacity of the ultracapacitor is used, but its high peak-power input and output capacity, for a short time period (impulse regime). Momentary power of a capacitor is p=ui, where u is voltage of the capacitor, i is charging or discharging current. Even 2,5kW/kg unit (specific) power is possible momentarily, depending on the type of the ultracapacitor. Direction of current can be charging, in this case capacitor consumes power, or discharging, when it generates power.

One of the most important application field of ultracapacitors is in electric cars. There are experiments where they are used for main energy supply, but they are used as  secondary and temporary energy storage more frequently.  Using it we can prevent the primary energy source from peak loads.

Fuel cell electric cars

The development of fuel cell electric cars is very important and several car manufacturers deal with it, as can be seen in Table 9.3. and 9.4. There is example for fuel cell bus as well.

Comparing fuel cell and battery energy sources

In Fuel Cell Electrical Vehicles (FCEVs) fuel cell, likebattery, serves as DC energy source. Main electric circuit and vehicle drive are also similar (Figure 8.1.), only voltage U main is generated by a fuel cell. But, some important points are different for fuel cells sources:

  1. Fuel cell is electro-chemical converter, it cannot store electric energy.

  2. Fuel cell, like internal combustion engine, works with fuel, so electric energy can be produced only if there is enough fuel and operating conditions are fulfilled.

  3. Storing and refilling fuel is more complex than even for internal combustion engines because fuel used in FCEVs is hydrogen usually which is stored as high-pressure gas or liquid, or methanol reformed. Secure handling and storing of hydrogen is an important challenge nowadays.

  4. Operating intensity of a fuel cell in normal operation regime changes as electric consumption changes. Fuel cells can follow the dynamic stress required for acceleration with delay in basic configuration.

  5. Fuel cell energy source cannot utilize regenerated energy from braking; its current direction cannot change.

  6. Because of the last two disadvantages, fuel cell energy source alone cannot be used in vehicles; it has to be extended with an energy storage device. This can be electric storage, battery or ultracapacitor (as can be seen on Table 9.3 and 9.4) or flywheel mechanical storage. Section 9.1.3. deals with these devices. Nowadays, Ovonics Company develops fuel cell combined with metalhydrid hydrogen storage which enable regeneration and eliminated delay with chemical energy storing.

  7. Operation of fuel cell has to be controlled and monitored continuously; its control and auxiliaries are complicated. Starting the operation of the cell, its cooling and monitoring fuel level has to be provided separately.

  8. Fuel cells are sensitive for ambient temperature when starting; there are problems when starting them below -4°C.

  9. Pollution of fuel cell vehicles is not zero, even when using pure hydrogen. Besides water resulting from the burning of hydrogen, nitrogen-oxides can appear if air and not pure oxygen is used. When using methanol fuel, carbon dioxide is also appear, as secondary product.

Fuel cell energy source for cars

Fuel cell is an environment friendly electro.chemical power source. There are several types of fuel cells:

  1. AFC (Alkaline Fuel-Cell) with traditional alkaline,

  2. PEMFC (Proton Exchange Membrane Fuel-Cell), with polymer membrane electrolyte,

  3. MHFC (Metal Hydrid Fuel-Cell), PEMFC combined with metal-hydrid hydrogen storage,

  4. PAFC (Phosphoric Acid Fuel-Cell),

  5. MCFC (Molten Carbonate Fuel-Cell),

  6. SOFC (Solid Oxid Fuel-Cell) with zirconium ceramic,

  7. high pressure fuel cells.

From these types, PEMFC is used in vehicles, where operating temperature is about 70-80°C, operating pressure is 1-10bar. Its handling is the best for vehicle application. Theoretical and practical structure of such a cell can be seen in Figure 8.6.

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Figure 8-6. a: Theoretical structure of a PEM cell.

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Figure 8-6. b: Practical structure of a PEM cell.

In the cell, chemical reaction is realized with proton change while electron flow (signed as e-) is closed through the external circuit. Proton exchange membrane, coated with platinum or graphite, is surrounded by anode and cathode which are porous plates. Coating intensifies the chemical reaction. Yellow indicates oxygen, blue indicates hydrogen admissions. The secondary product, water, appears on the cathode.

The fuel cell energy source is built up from these flat PEM cells, connected in serial. The structure of this fuel cell is shown in Figure 8.7.

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Figure 8-7. Theoretical structure of a fuel cell energy source.

In no-load operation the output voltage is u=U 0 (switch K is open, load current is zero, i=0).In load operation i>0 and output voltage u<U 0. If current increases, voltage decreases non-linearly. Fuel cell energy source, likebattery, is built up from several cells connected in series because the cell voltage is low, in the range of 1V. Output voltage depends on the number of the cells (N), quality of fuel feeding, operational temperature and pressure, and load current i. Load current depends on the active surface of the cells and the maximal current density q[A/cm2]. Maximal current density can be in the range of 0,5…1A/cm2 for a typical cell.

The changing of voltage when current or temperature changes is important for vehicle application. This depends on the quality of cells connected in series. Dependency of voltage u c on load current density q is shown in Figure 8.8, based on the measurements in National Energy Laboratory.

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Figure 8-8. Voltage vs load for a single PEM fuel cell.

The figure shows the output power of a PEM cell at 70°C operational temperature for different feeding modes. The two upper curves show pure hydrogen feeding, the upper one with compressed air, the lower one without air compression. The lowest curve stands for reformed hydrogen feeding which includes carbon-monoxide as well. From the figure we can conclude that voltage changes in a wide range. Voltage drop can reach 40-50% at maximal load.

Temperature dependency of output voltage for a fuel cell is indicated in Figure 8.9. The figure shows u-i load measurement at different load temperatures T(C°) for a 5 kW PEMFC built from 75 cells connected in series.

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Figure 8-9. Output voltage vs temperature for a PEM cell.

On the figure we can see that temperature dependency of output voltage is significant, it can change in the range of 43-74V caused by the load and the temperature together.

Electric power available from fuel cell is the product of its current i and output voltage u c , p=u c i. Unit (specific) power density for the electrode surface is u c q, its maximal value is 0.5…0.7W/cm2 per cell. There is another indicator value for power density which is power per volume expressed in liters W/ℓ.

Technical data for measuring and comparing electric properties of fuel cells are the following:

  1. no-load voltage;

  2. load capacity per unit, maximal current density;

  3. maximal power density;

  4. recovery time of the fuel cell;

  5. operational conditions (temperature and pressure);

  6. operational efficiency.

Transient behavior of a fuel cell is characterized by its recovery time. In the fuel cell, the development of the electrochemical processes and the changing of the intensity of these processes cannot happen without delay. As load changes, it needs time to stabilize the processes and create a new operating point, and this time depends on the fuel feeding. Recovery time is given for the transient process where load changes from no-load to 90% of maximal load current, until the new operating point is reached. Recovery time is not the same as the time needed for starting the operation of the system.

Operational temperature is important for two things. On one hand, we have to know the minimal temperature where the fuel cell can be started, and what temperature is where chemical processes can operate with maximal efficiency, on the other, which is required for optimal operation.

Efficiency of the fuel cell: quotient of output electric energy during a given time, and heat energy from the burning of fuel for this time.

Output electric energy for time Δt is the integral of instantaneous power:

8‑1

If mass of fuel  Δ m fc is required for this output power during time Δ t, and energy storing capacity for mass unit w fc[Wh/kg] is known, then efficiency of fuel cell is

8‑2

Efficiency of fuel cells is relatively high comparing to other electric energy sources: 50-70%.

Energy source includes auxiliary devices, like cooler, feeder, pressure controller etc. besides the fuel cell itself (Figure 8.10.).

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Figure 8-10. Fuel cell with auxiliary devices.

Efficiency of the whole fuel cell energy source is lower than the value (8.2) because fuel cell has to supply energy for auxiliary electric devices (Figure 8.10.). Output power for vehicle traction is p ki =u(i-i aux )=u main (i-i aux ), part of the momentary output power generated p aux =ui aux is required for auxiliary consumption, which decreases the overall efficiency. Its impact is important in low-load operation when output current i is low and is comparable with auxiliary current i aux.

Control methods for fuel feeding:

  1. Type A feeding, when fuel (hydrogen) has constant pressure and flows through. Fuel cell consumes fuel as required for operation, and unnecessary, not-burned fuel is fed back to the feeding side. Output power of the fuel cell can be set by the pressure (velocity) of the air flowing in. If the amount of air increases, then hydrogen consumption and the intensity of chemical processes inside the cell also increase.

  2. Type B feeding, when the amount (and pressure) of the hydrogen is controlled, pressure of the air is not controlled and it is plentifully available. This type is similar to the feeding of petrol or gas injected internal combustion engines.

Nowadays, there are complete fuel cell energy sources for vehicle and other applications.

As an example, structure of elements of the Xcellsis product type XCS-HY-75, hydrogen fed fuel cell is shown in Figure 8.11.

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Figure 8-11. Elements of type XCS-HY-75 fuel cell.

Fuel is high pressure (10bar) compressed gas hydrogen at normal ambient temperature, stored in a vessel. Feeding of fuel is type A. The device consists of modules:

  1. The heart of the device is a PEMFC fuel cell which operates at 1-4 bar pressure and 70-85ºC temperature. Ambient temperature allowed during operation is 5-40ºC, during storage is 10-40ºC.

  2. Air compression module compresses inlet filtered air to the required pressure before air gets into humidifier. Humidifier module waters the inlet hydrogen and air with deionized water taken from the cooling system. Both preparing processes improve the efficiency of the chemical process.

  3. Pressure controller module controls fuel pressure from 10bar storage pressure to 1-4 bar operating pressure. Not used fuel is refilled. Hydrogen valve in the refilling loop opens if security or other emergency issues arise.

  4. Water steam condenser module reuses part of the water resulted from fuel cell.

  5. Cooling and heat exchanger module controls operating temperature of fuel cell.

  6. An auxiliary voltage regulator module is also part of the system. Auxiliary battery can be operated from this DC/DC converter.

The whole system is controlled by a microcontroller. Central microprocessor unit controls information and data flow, and monitors the device. The system can be connected to external controller through this unit. The load and efficiency characteristic for a 68kW and 250V nominal voltage fuel cell can be seen in Figure 8.12.

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Figure 8-12. Load and efficiency characteristic of type XCS-HY-75 fuel cell.

250 V nominal voltage can be measured on the output poles  at about 270A load current. Output voltage is higher than 250V with lower load, its maximal value is 450V. Drive system must be designed to tolerate 450V without damage.

As can be seen from the efficiency characteristic in Figure 8.12., 50% efficiency (its catalog data) can be reached only in a narrow range at 20…30kW. Efficiency worsens outside this range.

The mass, volume and energy consumption of the auxiliary modules is significant comparing to the main fuel cell module. Noise of air compressor can also be high.

The size of Xcellsis product type XCS-HY-75 fuel cell (applicable in ekectric car also) is 1770x950x300mm, its recovery time is 1s. Its picture can be seen in Figure 8.13.

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Figure 8-13. Type XCS-HY-75 fuel cell

Transient behavior of a fuel cell: It is important for every energy source devices; what pulse load it can bear and how transient processes pass off. The following oscillogram (Figure 8.14.) shows the transient behavior of a 30kW 70V purely hydrogen fed fuel cell made by Nuvera Fuel Cells Europe, which demostrates the time functions of the typical values of the fuel cell for load step change from 50A to 550A.

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Figure 8-14. Transient characteristics of Nuvera Fuel Cells.

During load change, the control system of the fuel cell changes the pressure of inlet air. Mass flow can increase with delay after increasing pressure. The new load state sets in after about 1 second, while output voltage decreases to 64V from about 82V. Current of air compressor is relatively high (about 50A) comparing to the current of the fuel cell, as can be seen in the figure. Because this is the main part of the auxiliary consumption, current of fuel cell increases to about 600A during the process.

Application of PEMFC fuel cell in electric vehicles

The theorethical structure given in Figure 8.10 is not complete, if PEM cell is used, two problems arise:

  1. Pulse type, dynamic load change during acceleration would not be fulfilled,

  2. Regeneration of energy would not be fulfilled as PEM cell cannot change its current direction.

To overcome these problems, the PEM fuel cell has to be extended with temporary energy storage. This storage can be battery, ultracapacitor or flywheel mechanical energy storage. Nowadays MHFC type fuel cells are developed where fuel cell is combined with metal-hydrid hydrogen storage which realizes chemical energy storage.

In vehicles, combined energy source with battery  or ultracapacitor is used most often.

a./ Fuel cell combined with battery energy storage can be seen in Figure 8.15. Battery is connected to the output of the fuel cell with a DC/DC converter. Figure also shows a possible electric circuit for this DC/DC conversion (Buck-Boost), L1-C elemets have filter function. The circuit is similar if ultracapacitor is used instead of battery for secondary energy storing.

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Figure 8‑15. Fuel cell combined with battery energy storage.

With appropriate control we can reach that fuel cell works in optimal condition and with minimal fuel consumption during most of the normal operation, and battery takes the extra stress from pulse load change. Regeneration can be realized with combined energy source. About 40% fuel consumption saving can be reached with combined energy source and optimal load distribution, comparing to pure fuel cell feeding, taking into account the regeneration capacity of the accumulator. Economic operation in transient mode can be realized with correct control strategy.

Load distribution between the fuel cell and battery can be controlled with the DC/DC converter. Operating states of the combined energy source are:

  1. steady state:  iload≈const. and iload≈iFC,

  2. during acceleration, when iload>iFC is required, battery can provide additional current, iload=iFC+idown ,

  3. purely battery operation is possible, for example during starting of the fuel cell or malfunction: iload=idown (iFC=0).

  4. if load is lower than average iload<iFC, battery can be charged: iFC=iload+iup,

  5. regeneration is possible during brake state of the vehicle: ibreak=iup(iFC=0).

Battery must be designed to bear current peaks higher than average and to store the resulting required energy. As battery can be with lower nominal power than total power required, it has to be protected with SOC (State of Charge) control.

A FCEV application example is a vehicle with combined fuel cell and battery developed by Toyota (Toyota Motor Corporation, Aichi, Japan). The PEMFC cell used is 90 kW, hydrogen fed, air mass controlled, with air compression and air humidifier. Secondary energy storage is made by 6.5Ah NiMH batteries with air cooling. Drive is realized with 80kW maximal power, water cooled permanent magnet synchronous motor with current vector control introduced in Section 6.1. Motor is fed by a water cooled inverter. The circuit can be seen in Figure 8.16.

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Figure 8-16. Circuit diagram of fuel cell combined with battery energy storage.

Control system of the vehicle consists of three parts: electric drive control block, load distribution controller, and fuel cell controller. Theoretical scheme of the control is shown in Figure 8.17. (On the figure, index B stands for battery, v for vehicle speed, ω is angular velocity of the motor, m is torque of the motor.)

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Figure 8-17. Control diagram of fuel-cell combined with battery energy storage.

As shown in the figure, torque of the motor (ma) can be set in two ways. One method is direct torque signal controlled by accelerating and brake pedal, as in traditional vehicles. Another method is selectable speed controlled operation (tempomat) when motor torque is set by output signal of the speed controller.

In continuous traction operation required traction power P req =m a ω can be calculated from required traction torque m a >0. Load distributer controls the feeding of hydrogen and air of the fuel cell and sets reference voltage uvr of voltage regulator DC/DC converter connected to the battery based on this calculation. In steady state, controlled voltage is set so that current of energy storing battery is almost zero. In this case the required power is provided by the fuel cell with iFC current. Control strategy of the voltage can be seen in Figure 8.18.

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Figure 8-18. Setting output voltage level for combined energy source.

If the SOC of the battery is too low, then reference voltage u vr is reduced to increase current of fuel cell until it can provide charge for battery as wheel as driving the vehicle.

During acceleration, when load pulse appears, load distributor controller increases fuel feeding and realizes “additional acceleration” control with voltage regulator (decreases u vr). Battery provides additional current and power for traction temporarily until fuel cell gets into the new operating point. Load distribution control provides protection for both fuel cell and battery.

In electric regeneration brake operation mode the torque reference signal of the motor is negative, m r =-m brake . In this case the load distributor controls the operation point of the fuel cell to zero load, switches off fuel feeding and increases voltage reference signal u vr. Energy flow reverses with the DC/DC converter. Energy is fed back to the battery and it is charged. Energy regeneration to the fuel cell isnot possible.

The inner set-up of the vehicle is shown in Figure 8.19.

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Figure 8-19. Set-up of a Toyota FCEV fuel cell car.

b./ Fuel cell source combined with chemical energy storage is MHFC (Metal Hydrid Fuel-Cell) where fuel cell is extended with metal-hydride hydrogen storage. This solution is developed by Ovonic Fuel Cell Company.  

Fuel cell used in MHFC for vehicle application is similar to PEMFC solid polymer-electrolyte cell but the material of the cathode and anode is different. Metal-hydrid is used instead of porous material used in anode of PEMFC and metal-oxide in the cathode. MHFC can be made by less expensive materials and can work without platina catalyzer but metal-hydrid coating increases its mass and unit (specific) power is also less.

Metal-hydrid coating on the anode can store hydrogen temporarily, depending on its material and mass, and this storing is realized with atomic bond. If there is metal-hydrid stored hydrogen in the cell, then starting and operation without normal feeding can be realized. MHFC bears both advantages of fuel cell and battery. It can regenerate energy (with opposite current direction) as long as metal-hydrid can store generated hydrogen. From these comes its name “regenerative fuel cell”.

The main differences between the operation of PEMFC and MHFC fuel cells are:

  1. PEM cell can provide energy only if fuel feeding is continuous and available every moment, and other operating conditions (pressure, temperature, correct feeding operation etc.) are fulfilled. Amount of feeding  fuel and air can be changed only with time delay which limits the dynamic load of the electric output. Direction of output current cannot be changed.

  2. MHFC cell can use stored hydrogen as long as its storing capacity is available. This provides fast response to dynamic load change without delay. It can generate electric energy without momentary external fuel feeding for a certain time period. This new construction provides energy regeneration ability, it can produce hydrogen from electric energy. If there is a regenerative electric energy on the output, i.e. the direction of the current changes, it fills its hydrogen storage, as long as its capacity is reached.  Electric output power available is not so sensitive to starting and operational temperature, pressure and feeding. Output voltage decreases slowly during operational hours and its lifetime is longer than for PEM cells. Characteristic of output current vs. voltage is similar to PEM cells but lower current density is allowed for MHFC cells.

Advantages of MHFC fuel cells:

  1. If the metal-hydrid hydrogen storage is full then energy source can start practically without delay.

  2. It reacts fast to dynamic load change; it can reach the new operating point practically without any delay.

  3. It can tolerate pulsating  current load more than PEM cells.

  4. The direction of the output current can be changed for a certain time period so it can provide regenerating brakeoperation.

Electric cars with multiple energy storage

Electric cars are often equipped with multiple energy storage combining the advantages of the combined solutions. We give two examples for cars with multiple energy storage devices.

Battery car combined with ultacapacitor utilizes the relatively high energy storing capacity of battery and higher unit power of ultracapacitor at the same time. Main energy supply is the battery of which energy storing capacity limits the range of the vehicle. Secondary energy source is ultracapacitor of which power determines the maximal current of pulse load change, i.e. the dynamics of acceleration and regenerative braking. With appropriate circuit and control, ultracapacitor can take the pulse load instead of the battery. With this, the lifetime of the battery can be extended. Energy stored in the ultracapacitor during regenerating brake can be used for traction, for example for next accelerating the vehicle. The scheme of battery combined with ultracapacitor in electric car can be seen in Figure 8.20.

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Figure 8-20. Scheme of battery combined with ultracapacitor in electric car.

In the figure, continuous line indicates current direction in traction mode and dashed line shows the current direction during braking. DC/DC converter has to be able to control energy flow in both directions, often buck-boost circuit is used for that. Buck-boost conversion can distribute load between battery and ultracapacitor in all operatingmodes, traction or braking. This circuit enables controlled bi-directional energy flow even if the voltage of the ultracapacitor is lower than that of the battery or if it is higher. Based on the measurement data from NREL Laboratory shown in Figure 8.21., we can compare the load currents of battery systems with or without ultracapacitor, during one load cycle. (Internet: http://www.ctts.nrel.gov)

Blue line indicates battery load in case of pure NiMH battery supply (positive is the direction of charging current, negative is for discharge). Red line shows what current the ultracapacitor can take and green line indicates the moderated pulse load of the battery, comparing to the original blue line.

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Figure 8-21. Load measurements of battery combined with ultracapacitor.

In an battery combined with flywheel energy storage device the flywheel drives a separate electric generator. The flywheel must rotate in order to supply energy. Speed-up of the flywheel can be made with external or internal energy source. External source can be at one end of the rail in case of a rail-guided vehicle, for example, where flywheel can be revved up before going back to traffic and this can provide the necessary energy. Internal source is required for cars. Rev up can be made with battery or with regeneration energy during brake. Kinetic energy store in the flywheel can be used for accelerating the car. Two flywheels rotating in opposite direction with aligned shafts have to be used in order to avoid decrab problems caused by precession effect.

Flywheel energy storage is a cylinder or disk shape rotating mass with very high rotational speed. Kinetic energy stored is W=Θω2/2, where Θ is the inertial of the rotating mass, ω is the rotational speed. Because of the quadratic dependence, rotational speed should be set the highest acceptable. Mechanical strength limits the maximal rotational speed. Stored energy can be as high as 2,8kWh/kg which is similar to the specific energy of fossil fuels. Advantage of the flywheels is that it can be converted to electric energy with higher efficiency (η~90%) than the chemical energy of traditional fuels. Disadvantage is that the flywheel has to be revved up and energy storing has loss over time. The main cause of the loss is bearing friction and windage. To reduce them, flywheels are often rotated in vacuum and electromagnetic levitating bearing is used instead of traditional bearings. According to the literature, there are flywheels with 200000 1/min (ω=20940rad/s) rotating speed where annual loss is less than 20%. Flywheels are expensive and complex solutions. The scheme of battery combined with flywheel energy storing in electric vehicle can be seen in Figure 8.22.

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Figure 8-22. Battery combined with flywheel energy storing in electric vehicle.

Flywheel device is connected to the main circuit with DC/DC converter which controls bi-directional energy flow. Flywheel can store energy if it rotates. During rev up electric motor M/G operates in motor mode and gets energy from the main circuit. When providing energy, M/G operates in generator mode and its current from DC/DC converter helps to supply energy for the traction. Optional capacitance C helps absorb high current peaks.