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A Review of Energy Storage System for Hybrid Electric Vehicles

Nitin Singh, Rahul Kumar, Satendra Pratap Singh and Shashank Srivastava

Abstract--Hybrid electric vehicles (HEV) have emerged as a promising technology that uses energy storage system to reduce petroleum consumption in the vehicle fleet. However, there is a broad spectrum of HEV designs with greatly varying costs and benefits. In particular, cost of energy storage system, vehicle performance attributes and driving habits greatly influence the relative values of HEV. This paper presents a review of energy storage technologies developed and implemented in HEVs so far. Energy storage system is one of the most significant components which affect the performances of HEV. The importance of integration of different technologies of energy storage methods in HEV are addressed and challenges in the energy storage methods of HEV based on performance and feedback by user are also discussed in this paper.

Index Terms—Battery, Energy Storage System (ESS), Hybrid Electric Vehicles(HEV), Ultracapacitor.

Introduction

In recent years, a significant interest in hybrid electric vehicle (HEV) has arisen globally due to the pressing environmental concerns and skyrocketing price of oil. Representing a revolutionary change in vehicle design philosophy, hybrid vehicles surfaced in many different ways. As a product of advanced design philosophy and component technology, the maturing and commercialization of HEV technologies demand extensive research and developments.

A hybrid electric vehicle (HEV) [1] is a type of hybrid vehicle and electric vehicle which combines a conventional internal combustion engine (ICE) propulsion system with an electric propulsion system. The presence of the electric power train is intended to achieve either better fuel economy than a conventional vehicle or better performance. A hybrid vehicle drive usually consists of one or two power trains as this will reduce the complexity of the system.

There are various types of HEV, and their functions depend on their architecture [2]. Basic components used in HEV’s are electric motor, power electronics converters, energy storage system, electrical sensor and internal combustion engine (ICE) as its main component [3].The main challenges in selecting HEV are range selection, fuel economy, cost optimization [5], [17] & controlling of vehicle parameters [18], [19].

This paper presents a review of HEVs, fundamentals, topologies, types and energy storage system of HEVs. The various energy storage systems like battery, ultracapacitors &

fuel cells are studied and their suitability in HEV’s are discussed [6]-[51]. The history [1]-[8] of HEVs, the present scenario & the future expectations from HEV [9] are also discussed in this paper.

Development OF HEVS

The concept of a hybrid electric vehicle is almost as old as the automobile itself. The primary purpose in development of HEV was to assist the ICE to provide an acceptable level of performance rather than decreasing fuel consumption. The first hybrid vehicles reported were shown at the Paris Salon of 1899. These were built by the Pieper (Fig.1) [1]. The Pieper vehicle was a parallel hybrid with a small air-cooled gasoline engine assisted by an electric motor and lead–acid batteries. It is reported that the batteries were charged by the engine when the vehicle coasted or was at a standstill.

[pic]Fig.1. Henri Pieper's 1905 Hybrid Vehicle Patent Application

The first series hybrid vehicle was introduced at the Paris Salon of 1899 and was derived from a pure electric vehicle commercially built by the French firm Vendovelli and Priestly. Frenchman Camille Jenatzy presented a parallel hybrid vehicle at the Paris Salon of 1903.

H. Krieger, built the second reported series hybrid vehicle in 1902. His design used two independent DC motors driving the front wheels. They drew their energy from 44 lead–acid cells that were recharged by a 4.5 hp alcohol spark-ignited engine coupled to a shunt DC generator.

Other hybrid vehicles, both of the parallel and series type, were built during 1899 to 1914. Dr. Victor Wouk was recognized as the modern investigator of the hybrid electric vehicle movement in 1975, along with his colleagues, he built a parallel hybrid version of a Buick Skylark.

The researchers’ focus was drawn by the electric vehicle, of which many prototypes were built during the 1980s. The hybrid electric vehicle concept drew great interest during the

1990s when it became clear that electric vehicles would never achieve the objective of saving energy. The Ford Motor Corporation initiated the Ford Hybrid Electric Vehicle Challenge, which drew efforts from universities to develop hybrid versions of production automobiles.

Architecture of Hybrid Electric Vehicle

The architecture of a hybrid vehicle [14], [15] is defined as the connection between the components that define the energy flow routes and control ports. Based on architecture HEVs can be classified into four types (i) Series Hybrid (ii) Parallel Hybrid (iii) Series – Parallel Hybrid and (iv) Complex Hybrid.

A. Series Hybrid Electric Drive Trains

A series hybrid drive train (Fig. 2a) is a drive train where two power sources feed a single electric motor that propels the vehicle. The most commonly found series hybrid drive train is the series hybrid electric drive train. The unidirectional energy source is a fuel tank and the unidirectional energy converter is an engine coupled to an electric generator. The output of the electric generator is connected to an electric power bus through an electronic converter (rectifier). The bidirectional energy source is an electrochemical battery pack, connected to the bus by means of power converters [14]-[18].

B. Parallel Hybrid Electric Drive Trains

A parallel hybrid drive train (Fig. 2b) is a drive train in which the engine supplies its power mechanically to the wheels like in a conventional ICE-powered vehicle. It is assisted by an electric motor that is mechanically coupled to the transmission. The powers of the engine and electric motor are coupled together by mechanical coupling [14]-[18].

C. Series-Parallel Hybrid Electric Drive Trains

In series-parallel HEVs (Fig. 2c), the configuration incorporates the features of both series & parallel HEVs, but involving an additional mechanical link compared with the series hybrid and also an additional generator compared with parallel hybrid[14]-[18].

D. Complex Hybrid Drive Trains

Complex Hybrid Drive Trains (Fig. 2d), possess more than two electric motors, energy consumption and performance are optimized, multimode operation capability. It seems to be identical to series-parallel hybrid, since the generator and electric motor are both electric machinery. However, the key difference is due to the bidirectional power flow of electric motor in complex hybrid while in series-parallel the power flow of generator is unidirectional [14]-[18].

Present Status Of HEVs

Various hybrid electric vehicles have been manufactured by major locomotive manufacturers such as motorcycles, cars, trucks, and military vehicles. Based on the level of electric power and the function of the electric motor (mode of operation), the present HEVs can be classified as:

A. Full Hybrid

It is called as strong hybrid; it is a vehicle that can run on engine or battery or combination of both. Commercially Toyota Prius, Ford Escape Hybrid, and Ford Fusion Hybrid are examples of full hybrids, as these cars are running on battery power alone [7]-[20].

B. Mild Hybrid

It is a vehicle that cannot be driven solely on its electric motor, because the electric motor does not have enough power to propel the vehicle on its own. Mild hybrids only include some of the features found in hybrid technology, and usually have limited fuel consumption savings, up to 15 percent in urban driving and 8 to 10 percent overall cycle. Mild hybrids have comparatively smaller batteries and small motor/generator, which allows manufacturers to reduce cost and weight [7]-[20].

C. Micro Hybrid

It uses a limited-power EM as a starter alternator, and the ICE insures the propulsion of the vehicle. The EM helps the ICE to achieve better operations at startup. Due to fast dynamics of EMs, micro hybrid HEVs employ a stop-and-go function, which means that the ICE can be stopped when the vehicle is at a standstill (e.g., at a traffic light). Fuel economy improvements are estimated to be in the range of 2%–10% for urban drive cycles [7]-[20].

Energy Storage System

Energy storage system (ESS) is an important part of HEV. ESS is mainly used for storage of electrical energy in form of charges. The electrical energy storage units must be sized so that they store sufficient energy (kWh) and provide adequate peak power (kW) for the vehicle to have a specified acceleration performance and the capability to meet appropriate driving cycles. The main aim for ESS is to meet appropriate cycle and calendar life requirements storing sufficient energy to satisfy the range requirement in world living. Different type of energy storage systems use mainly either battery or ultracapacitor for storing energy and driving electric vehicle[21]-[51].

In case of battery powered HEV, the battery is sized to meet the specific range of vehicle. The weight and volume of battery can be easily calculated from energy consumption and energy density discharged over specific test cycle. In reference to ultracapacitors, it must be noted that their power and life cycle characteristics be significantly better than high power batteries because the energy density of capacitors will be significantly less than that of batteries. In recent developments ultracapacitors use pseudocapacitive or battery like materials in one of the electrodes with micro porous carbon in other electrode to increase energy density of devices.

The proper selection of energy storage system in different cases depends on some parameters which are listed below:

• High specific energy ( KWh/ kg) and energy density (KWh/ L)

• High specific power ( KW/ kg) and power density (KW/ L)

• Fast charging and deep discharging capabilities

• Long cycle and service lines

• Self discharging rate and high charging efficiency

• Safe and cost effectiveness

• Maintenance should be negligible

• Environmentally sound and recyclable

The main question to be answered becomes “Can HEV’s be made affordable?” one major factor that makes HEV affordable are cost and range. To tackle the range , the development of advance ESS such that Ni-MH, Zn/ Air , Li-Ion and ultracapacitors are in progress. To tackle the cost, efforts are being made to improve various HEV subsystems such as motors, power converters, electronic controllers, energy management units, ESS, battery chargers and other HEV auxiliaries as well as HEV system level integration and optimization.

[pic]

(a) Series Hybrid

[pic]

(b) Parallel Hybrid

[pic] (c) Series- parallel Hybrid

[pic] (d) Complex hybrid

Fig 2. Four Common Architectures of HEVs.

In order to see the development trends of var ious ESS for HEV’s , a survey has been made w.r.t the number of papers published related to HEV’s. After many years of development , HEV technologies are becoming popular. Many advanced technologies are employed to extend the driving range and reduce the cost . The employment of advanced valve – regulated lead acid (VRLA) battery, Ni-MH battery, Li-Ion battery and ultracapacitors to improve the HEV’s energy storage.

BATTERY

Battery is an electrochemical conversion device that has all of its chemicals stored inside, and it converts those chemicals into electricity. Battery is a cell or connected group of cells that converts chemical energy into electrical energy by reversible chemical reaction and that may be recharged by passing a current through it in the direction opposite to that of its discharge. At the present time and in the foreseeable future, batteries have been agreed to be the major energy source for HEVs. Most of the battery powered and hybrid vehicles tested and marketed to date (2006) have used nickel metal hydride (Ni-MH) batteries. The development of lithium–ion batteries has progressed to the state that strong consideration is being given to the use of those batteries in both electric and hybrid vehicles[21]-[27]. Much of the recent battery development has been concerned with high power batteries for HEVs.

The battery output power is given by

[pic] (1)

Where, Pba is Battery output power

Pbs is internal power of battery

Voc is open circuit voltage

R is internal resistance

It is to be noted that Pbs < 0 discharges the battery.

The battery efficiency in conventional vehicles is in general high because usually the battery operates at low discharge rates and only from time to time at high rates. In HEV high transients (especially in urban driving) are typical due to power boosting and frequent ICE cranking. In these cases, state of charge (SOC) estimation based on a simple current integration leads to significant errors. Fig. 3 shows equivalent circuit of battery[42].

[pic]

Fig. 3 Equivalent Circuit of Battery

The efficiency of the battery is given by

[pic] (2)

Where Ri has a range of 8-20 mohm for 55 Ah, 12V, lead-acid battery. At low discharge rate (e.g. 12.5 A), Rload ≈ 1 ohm, efficiency of the battery is 99.1%. In case of high loads (e.g. 200 A), Rload ≈ 0.05 ohm, efficiency of the battery is 83.3%. The efficiency of the battery decreases as the supply current increases.

Those viable HEV batteries consist of the VRLA, Ni-Cd, Ni-Zn, Ni-MH, Zn/Air, Al/Air, Na/S, Na/NiCl ,Li-Polymer, and Li-Ion types.Some of their important parameters, including specific energy, energy density, specific power, cycle life, and projected cost with respect to the US Advanced Battery Consortium (USABC)’s goals, are shown in Table. Table summarizes the key features, including advantages, disadvantages, and potentiality, of the aforementioned batteries [31]-[35].

It can be found that those batteries with near-term high potentiality are the VRLA, Ni-Cd, and Ni-MH. Since the features of the Ni-MH are superior to those of the Ni-Cd, except maturity, the Ni-Cd is being superseded by the Ni-MH. Actually, some manufacturers used to produce the Ni-Cd for HEV applications have redirected their efforts to the Ni-MH.

Thus, in near term, the VRLA is still popular due to its maturity and cost effectiveness, whereas the Ni-MH is attractive because of its good performances. On the other hand, those batteries with midterm high potentiality include the Ni-Zn, Zn/Air, Na/NiCl , Li-Polymer, and Li-Ion. The Li-Ion has been identified by many battery manufacturers to be the most promising midterm EV battery. Its key obstacle is high initial cost, which should be greatly reduced upon mass production [45]-[48].

The Zn/Air is also very promising because of its excellent specific energy and fast mechanical refueling. However, this mechanically rechargeable battery cannot accept energy

resulting from regenerative braking. Since the major drawback of the Ni-Zn, namely, short cycle life, is being alleviated in recent development, it may have the potential to compete with the Ni-MH in midterm. The Na/NiCl is relatively the acceptable high-temperature battery for HEV applications. It is promising in midterm provided that the battery performances can be further improved. The Li-Polymer has demonstrated to exhibit good performances for HEV applications. It is promising in midterm provided that more battery manufacturers are involved to accelerate its research and development. Table 1 shows characteristics of various types of batteries in HEV.

The nature of HEVs is governed mainly by the characteristics of the batteries used. The weight and volume of the batteries is usually a deciding factor. For a car, usually the battery weight could range from 300 to 600 Kg and they require a 200 to 300 Liters of adjoining volume. Recapturing kinetic energy with regenerative braking during deceleration can neutralize negative effect of battery weight for urban driving. Hence, HEV will be always preferred for urban driving with a limited drive range and for slow to medium speeds. For long distance driving, electric vehicle will not be able to match the performance of conventional IC engine in terms of distance traveled in a single charge and quick refueling time [46]-[51].

Table 1 Characteristics of various types of batteries used in HEV

|Battery |

|Technology |

|Energy/weight |30-40 Wh/Kg |

|Energy/volume |60-75 Wh/Liter |

|Power/weight |180 Wh/Kg |

|Efficiency |70 to 90 % |

|Energy/$ |5 to 8 Wh/$ |

Nickel Metal Hydride (Ni-MH) battery is a type of rechargeable battery where hydrogen absorbing alloy is used

for the anode. They have high energy/weight and energy/volume ratios compared to Lead-acid batteries, hence they are very compact but also very expensive. Table 3 typical values of battery efficiency, energy/weight, energy/volume ratios for Ni-MH batteries from manufacturer’s specification sheets.

Table 3 Ni-MH battery specifications

|Ni-MH Batteries |

|Energy/weight |30-80 Wh/ Kg |

|Energy/volume |140-300 Wh/Liter |

|Power/weight |250-1000 W/ Kg |

|Efficiency |60 to 70 % |

|Energy/$ |1.25 to 2 Wh/$ |

Zinc-air batteries (or sometimes referred as Zinc-air fuel cells) are non-rechargeable electrochemical batteries powered

by oxidation of Zinc with oxygen from the air. The cell comprises of central static replaceable anode comprising of electrochemically generated Zinc particles in a potassium hydroxide solution. Water and oxygen from the air react at the cathode and form hydroxyls which migrate into the zinc paste and form zincate at which point electrons are released that travel to the cathode. The zincate decays into zinc oxide and water is released back into the system. Zinc-air batteries have properties of fuel cells as well as batteries. The zinc is the fuel and the rate of the reaction can be controlled by controlling the

air flow, and used zinc/electrolyte paste can be removed from the cell and replaced with fresh paste [40]. Table 3 shows Zinc-air battery specifications. These batteries also have very high energy densities and are relatively inexpensive compared to Ni-MH batteries. Table 4 gives the key features of HEV batteries.

Table 3 Zinc-Air battery specifications

|Zinc-air batteries |

|Energy/weight |200 Wh/ Kg |

|Energy/volume |220 Wh/Liter |

|Power/weight |200 W/ Kg |

|Efficiency |50 to 60 % |

|$/ KW |$ 205/ - per kW |

Table 4 Key features of HEV batteries

|Types of Battery|Key advantages/ disadvantages for HEV |Potentiality |

| |applications | |

|VRLA |mature, low cost, fast rechargeable, |near-term |

| |high specific power / low specific |very high |

| |energy | |

|Ni-CD |mature, fast rechargeable, high |near-term |

| |specific power/ high cost, low specific|high |

| |energy | |

|Ni-Zn |high specific energy, high specific |mid-term |

| |power, low cost/ short cycle life |high |

|Ni-MH |high specific energy, high specific |near-term very |

| |power, fast rechargeable / high cost |high |

|Zn/Air |mechanically rechargeable, low cost, |near-term |

| |very high specific energy / very low |low |

| |specific power, cannot accept | |

| |regenerative energy | |

|Na/S |high specific energy, high specific |mid-term |

| |power / high cost, safety concerns, |moderate |

| |need of thermal management | |

|Na/NiCl2 |high specific energy / high cost, need |mid-term |

| |of thermal management |high |

|Li-Polymer |very high specific energy, high |mid-term |

| |specific power / weak low-temperature |high |

| |performance | |

|Li-Ion |very high specific energy, very high |mid-term |

| |specific power / high cost |very high |

A. Ultracapacitor

Because of frequent start/stop operation of HEVs, the discharge profile of the battery is highly variable. The average

power required from the battery is relatively low while the peak power of relatively short duration required for acceleration or hill-climbing is much higher. In fact, the amount of energy involved in the acceleration and deceleration transients is roughly 2/3 of the total amount of energy over the entire vehicle mission in the urban driving. Therefore, based on present battery technology, the design of batteries has to carry out the tradeoffs among the specific energy, specific power, and cycle life [27]-[48].

Ultracapacitor can be used as an auxiliary energy source. In the foreseeable development of the ultracapacitor, it is not practical for it to be used as the sole energy source for HEVs because of its exceptionally low specific energy. Nevertheless, there are a number of advantages that can be resulted from using the ultracapacitor as an auxiliary energy source. The promising application is the so-called battery and ultracapacitor hybrid energy system for HEVs. Hence, the specific energy and specific power requirements of the HEV battery can be decoupled, thus affording an opportunity to design the battery that is optimized for the specific energy and cycle life with little attention being paid to the specific power.

Due to the load levelling effect of the ultracapacitor, the high-current discharge from the battery is minimized so that the available energy, endurance, and life of the battery can be significantly increased. Moreover, compared to the battery, the ultracapacitor can provide much faster and more efficient energy recovery during regenerative braking of HEVs, as well as operating at a very low temperature. Therefore, as a combined effect of load leveling and efficient energy recovery, the vehicle range can be greatly extended. Notice that system integration and optimization should be made to coordinate the battery, ultracapacitor, electric motor, and power converter. The power converter and corresponding controller should take care both the electric motor and ultracapacitor.

Ultracapacitors for vehicle applications have been under development since about 1990. Most of the development has been on double-layer capacitors using microporous carbon in both of the electrodes. From the outset of that work, the twin goals were to achieve an energy density of at least 5 Wh/kg for high power density discharges [28]. The life cycle goal was at least 500000 deep discharge cycles. In order to justify the development of ultracapacitors as a distinct technology separate from high power batteries, it is critical that their power and life cycle characteristics be significantly better than the high power batteries because the energy density of the capacitors will be significantly less than that of batteries. Recently, there has been considerable research [27]–[51] on ultracapacitors that use pseudocapacitive or battery-like materials in one of the electrodes with micro porous carbon in the other electrode. This is being done to increase the energy density of the devices.

There are presently commercially available carbon/carbon ultracapacitor devices (single cells and modules) from several companies VMaxwell, Ness, EPCOS, Nippon Chem-Con, and Okamura Power Systems [26]–[30]. All these companies market large devices with capacitance of 1000–5000 F. These devices are suitable for high power vehicle applications. The performance of the various devices is given in Table 2. The energy densities (Wh/kg) shown correspond to the useable energy from the devices based on constant power discharge tests from V0 to 1/2 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 2.7 V/cell using acetonitrile as the electrolyte. Table 5 shows various types of ultracapacitors.

The present performance of ultracapacitors is suitable for use in mild HEVs using either engines or fuel cell as the primary energy converter. By mild hybrid is meant designs in which the power rating of the engine or fuel cell is large enough to provide satisfactory vehicle performance even if the energy storage unit is depleted. The ultracapacitor unit would be sized based on the energy storage requirement (75–150 Wh). The power density capability of ultracapacitors is such that the maximum power capability of the(75–150 Wh unit will exceed the electrical power requirement for the driveline system.

Table 5 Characteristics of Ultracapacitors

Device |VRated |C(F) |R(mohm) |RC(sec) |Wh/kg |W/kg

(95%)

(2) |W/kg

Match.

Imped. |Wgt.

(kg) |Vol.

lit. | |Maxwell |2.7 |2800 |.48 |1.4 |4.45 |900 |8000 |.475 | .320 | |Ness |2.7 |10 |25.0 |.25 |2.5 |3040 |27000 |.0025 |.0015 | |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 |2.7 |5085 |.24 |1.22 |4.3 |958 |8532 |.89 |.712 | |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 | |Panasonic |2.5 |1791 |.30 |.54 |3.44 |1890 |16800 |.310 |.245 | |Panasonic |2.5 |2500 |.43 |1.1 |3.70 |1035 |9200 |.395 |.328 | |EPCOS |2.7 |3400 |.45 |1.5 |4.3 |760 |6750 |.60 |.48 | |Okamura Power Sys. |2.7 |1350 |1.5 |2.0 |4.9 |650 |5785 |.21 |.151 | |ESMA |1.3 |10000 |.275 |2.75 |1.1 |156 |1400 |1.1 |.547 | |

Conclusion

In this paper, various energy storage systems are discussed with technical and commercial viability for hybrid electric vehicle (HEV) applications. Various types of batteries and ultracapacitors are discussed and compared taking in account of various parameters. The following conclusions can be drawn from the results of study.

1) The energy density and power density characteristics of both batteries and ultracapacitors technologies are sufficient for the design of attractive HEVs.

2) Battery powered vehicles using lithium ion batteries can be designed with ranges up to 240 km with reasonable size battery packs. The acceleration performance of these vehicles would be comparable or better than ICE vehicles.

3) Charge sustaining, engine powered HEVs can be designed using either batteries or ultracapacitors with fuel economy improvements of 50% and greater. The

largest fuel economy improvements can be achieved in full hybrids using down-sized engines and relatively

large electric motors. These vehicles would use batteries (Ni-MH or Li ion) that are sized by the power demand and are shallow discharged at an intermediate state-of-charge.

4) Mild HEVs can be designed using ultracapacitors having energy storage capacity of 75-100 Wh. The fuel economy improvement with ultracapacitors is 10-15% higher than with the same weight of batteries due to higher efficiency of the ultracapacitors and more efficient engine operation.

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