Among the energy storage technologies, the viable energy sources for EV and HEV applications are rechargeable electrochemical batteries (loosely called batteries), fuel cells, ultrahigh-capacitance capacitors (loosely called ultracapacitors) and ultrahigh-speed flywheels, as shown in Figure 4.5. Among them, the batteries, capacitors and flywheels are energy storage systems in which electrical energy is stored by electrical charging, whereas the fuel cell and engine are energy genera- tion systems in which electricity or motive energy is generated by chemical reac- tion. The hydrogen/methanol and gasoline undergo irreversible chemical or electrochemical reactions to drive the vehicle such that they cannot absorb any braking energy. The ultracapacitor, ultrahigh-speed flywheels, high power and high energy lithium-ion (Li-Ion) batteries, nickel-metal (Ni-Metal) battery, and valve- regulated lead-acid (VRLA) battery are high-power sources, while the metal/air battery, gasoline and fuel cell systems are high-energy sources (Ghoniem, 2011).
The battery has been identified as the major energy source for EV and HEV applications, because of its technological maturity and reasonable cost. The fuel cells, ultracapacitors and ultrahigh-speed flywheels are potential energy sources for EV and HEV applications. The introduction of high performance Li-Ion batteries into EVs and HEVs increased BEVs driving range from 160 km to 250–300 km.
The energy density of Li-Ion batteries is close to twice that of the nickel-metal hydride (Ni-MH) battery. However, it is still much less than the corresponding values of combustion engines using gasoline or diesel. There is no single energy
source that can fulfil the performance requirements of the EV. Some energy sour- ces can deliver high energy, while some energy sources can deliver high power.
The ultracapacitor and ultrahigh-speed flywheel have much higher charging and discharging power density compatible with regenerative braking at high speeds, and power surge during fast acceleration.
4.3.1 Batteries
The battery refers to the rechargeable electrochemical battery. The battery has to be intrinsically tolerant of abuse conditions such as overcharge, short circuit, crush, fire exposure, mechanical shock and vibration. Battery cells are connected in series and/or parallel in a vehicle battery system, thus, cells’ state of charges (SoCs) have to be balanced to prevent undercharge and overcharge. The operating temperature range of the battery is wide such that active thermal management systems are needed (Chan and Chau, 2001). The EV needs batteries with high specific power for quick charge and with high specific energy for long driving range. Table 4.1 shows some
Ultracapacitor
Ultrahigh-speed flywheel
Methanol
Gasoline
Fuel cell
VRLA Metal/air
High energy Li-Ion
5 10 20 50 100 200 500 1k 2k 3k
10 20 50 100 200 500 1k 2k 5k 10k
Ni-Metal
Specific energy (Wh/kg) (in logarithmic scale)
Specific power (W/kg) (in logarithmic scale)
High power Li-Ion
Figure 4.5 Specific energy and specific power of energy sources for vehicle application
Hybridization of energy sources for electric and hybrid vehicles 103
United States Advanced Battery Consortium (USABC) key goals for batteries in EV applications (USCAR, 2015).
The plug-in HEV (PHEV) needs batteries with high specific power but the amount of onboard energy source varies with the targeted pure electric driving range. Table 4.2 shows some USABC key goals for batteries in PHEV applications with the pure electric driving range at 20–40 miles (USCAR, 2015).
The viable batteries for vehicle applications includes the VRLA, nickel-zinc (Ni-Zn), Ni-MH, zinc/air (Zn/Air), aluminium/air (Al/Air), sodium/sulphur (Na/S), Table 4.1 Typical USABC goals for batteries in EV applications
Parameters Unit System level
Discharge specific power at 80% DoD*for 30 s W/kg 470 Regenerative specific power at 20% DoD†for 10 s W/kg 200
Power density W/L 1000
Onboard energy capacity kWh 45
Specific energy at C/3 discharge rate Wh/kg 235
Energy density at C/3 discharge rate Wh/L 500
Calendar life year 15
Cycle life to 80% DoD‡ cycle 1000
Operating temperature oC 30 toþ52
Selling price USD/kWh <125
Normal recharge time H 7
High recharge rate min 15 (80%DSoC)
Notes:
*The higher the degree of discharge (DoD), the lower the battery’s discharging power. The discharging power at 80% DoD can effectively indicate the battery’s characteristics during discharging.
†The lower the DoD, the lower the battery’s recharging power. The recharging power at 20% DoD can effectively indicate the battery’s recharging power during regenerative braking.
‡The higher the DoD in cyclic applications, the lower the battery’s cycle life. The cycle life in 80% DoD can effectively indicate the battery’s cycle life.
Table 4.2 Typical USABC goals for batteries in PHEV applications
Parameters Unit PHEV-20 PHEV-40
Reference equivalent electric range mile 20 40
Peak pulse discharge power for 10 s kW 37 38
Peak pulse discharge power for 2 s kW 45 46
Peak regenerative power for 10 s kW 25 25
Available energy for charge depleting mode at 10 kW discharge rate
kWh 5.8 11.6
Available energy for charge sustaining mode kWh 0.3 0.3
Cold cranking power at30C kW 7 7
Calendar life at 30C year 15 15
Maximum system weight kg 70 120
Maximum system volume L 47 80
Maximum operating voltage V 420 420
Operating temperature C 30 toþ52 30 toþ52
sodium/nickel chloride (Na/NiCl2), lithium metal-polymer (LiM-Polymer) and Li- Ion batteries. The specific energy and specific power of these batteries are shown in Figure 4.5. These batteries are classified into lead-acid (Pb-Acid), nickel-based, metal/air, sodium-beta and ambient-temperature lithium batteries, as shown in Figure 4.6 (Ko¨hler, 2009; Gutmann, 2009).
4.3.1.1 Pb-Acid batteries
The Pb-Acid battery has been a successful commercial product for over a century.
The Pb-Acid battery is mature and low cost. It has a nominal cell voltage of 2 V, specific energy of 35 Wh/kg, energy density of 90 Wh/L and specific density of 200 W/kg. In the sealed Pb-Acid battery, a special porous separator is employed in the cell such that the evolved oxygen is transferred from the negative electrode to the positive electrode and then combines with hydrogen to form water. Thus, it provides a definite advantage of maintenance-free operation. Moreover, the immobilization of the gelled (Gel) electrolyte or absorbed electrolyte with absorptive glass mat separators allows the battery to operate in different orientations without spillage. The sealed Pb-Acid battery is so-called VRLA battery (Rand and Moseley, 2009).
The VRLA battery has maintained its prime position for more than a century, there are a number of advantages contributing to this outstanding position: proven technology and mature manufacturing, low cost, high cell voltage, good high-rate performance that are suitable for vehicle applications, good low temperature and high temperature performances, high energy efficiency (75–80%) and available in a variety of sizes and designs. The VRLA battery’s specific energy and energy density are relatively low, typically, 35 Wh/kg and 70 Wh/L. Its self-discharge rate is relatively high at about 1% per day at 25C.
Advanced Pb-Acid batteries with improved performance are being developed for vehicle applications. Improvements of the VRLA battery in specific energy
Nickel-based Ni-Zn
Ni-MH
Al/Air Zn/Air
Na/S Na/NiCl2 LiM-Polymer Li-Ion Lead-acid
VRLA
Sodium-beta Ambient- temperature lithium Metal/air
Bipolar cell UltraBattery
Figure 4.6 Classification of vehicle batteries
Hybridization of energy sources for electric and hybrid vehicles 105
over 40 Wh/kg and energy density over 80 Wh/L with the possibility of rapid recharge have been attained. The bipolar VRLA battery and UltraBatteryTM are promising Pb-Acid batteries for vehicle applications (Wong and Chan, 2013).
4.3.1.2 Nickel-based batteries
There are many kinds of electrochemical batteries using nickel oxyhydroxide as the active material for the positive electrode, including the nickel-cadmium (Ni-Cd), Ni-Zn and Ni-MH. Among them, the Ni-MH battery has been well accepted for EV and HEV applications because of its proven technology and good performance.
The Ni-Zn battery is still under development. The Ni-Zn battery has high specific energy and low material cost; however, it has not achieved any commercial importance because of the short life in the zinc electrode (Cairns, 2009). The Ni-Zn battery nominally operates at 1.6 V and has energy and power densities of 60 Wh/
kg, 120 Wh/L and 300 W/kg. It uses zinc as the negative electrode and nickel oxyhydroxide as the positive electrode. The electrolyte is an alkaline potassium hydroxide solution. The Ni-Zn battery has the advantages of higher specific energy and power than the Ni-Cd battery, high cell voltage (the highest of the nickel-based family), non-toxicity (more environmental friendliness than the Ni-Cd), tolerance of overcharge and overdischarge, capable of high discharge and recharge rates, and wide operating temperature (from 20 C to 60 C). However, the major and serious drawback of the Ni-Zn battery is its short cycle life (about 300 cycles). It is mainly due to the partial solubility of zinc species in the electrolyte.
The Ni-MH battery has been on the market since 1992. Its characteristics are similar to the Ni-Zn battery. The principal difference between them is the use of hydrogen, absorbed in a metal hydride, for the active negative electrode material in the Ni-MH battery (Hariprakashet al., 2009). The Ni-MH battery has a nominal voltage of 1.32 V and attains specific energy of 65–110 Wh/kg for EV applications and 45–60 Wh/kg for HEV applications. It operates in a temperature from20C to þ45 C. A number of battery manufacturers, such as GM Ovonic, GP, GS, Panasonic, SAFT, VARTA and YUASA, have actively engaged in the develop- ment of this battery for HEVs.
4.3.1.3 Metal/air batteries
The rechargeable metal/air batteries include the electrically or mechanically rechargeable Zn/Air battery and the mechanically rechargeable Al/Air battery.
These metal/air batteries have very high specific energy and energy density (as high as 600 Wh/kg and 400 Wh/L for Al/Air) and they are low cost, environmental friendly. In addition, those mechanically rechargeable batteries have two distinct advantages which are very essential for EV applications, namely, fast and con- venient refuelling (comparable to petrol refuelling with a few minutes) and cen- tralized recharging and recycling (most efficient and environmentally sound use of electricity). The disadvantages associated with rechargeable metal/air batteries are low specific power (at most 105 W/kg for Zn/Air), narrow operating temperature window, carbonation of alkaline electrolyte due to carbon dioxide in air and evolution of hydrogen gas from corrosion in electrolyte.
The Zn/Air battery has been developed as an electrically and mechanically rechargeable battery. Although both of them have been applied to EV applications, the mechanically rechargeable battery is more favourable (Haas and Van Wese- mael, 2009). The electrically rechargeable Zn/Air battery nominally operates at 1.2 V and has the specific energy of 180 Wh/kg, energy density of 160 Wh/L and specific power of 95 W/kg. The mechanically rechargeable Zn/Air battery avoids the use of bidirectional air electrode and the shape change problem. Hence, it can offer a higher specific energy of 230 Wh/kg and a higher specific power of 105 W/kg.
The depleted zinc negative electrode cassettes can be replaced robotically by a mechanically refuelling system at a fleet servicing point or at a public service station.
The discharged fuel is then electrochemically recharged at central facilities.
A mechanically rechargeable Zn/Air battery was developed for field test. A 160- kWh Zn/Air battery was installed and tested in a Mercedes-Benz 180E van in 1994.
The driving range at a constant speed of 64 km/h was 689 km.
The Al/Air battery has a nominal voltage of 1.4 V. The Al/Air battery with a saline electrolyte is attractive only for low power applications. On the other hand, the alkaline Al/Air battery offers high specific energy and energy density of 250 Wh/kg and 200 Wh/L, respectively, and is suitable for high power applications.
Nevertheless, the corresponding specific power is low. Because of its exceptionally low specific power, the Al/Air battery is seldom used as the sole energy source for EVs and it is commonly used in conjunction with other batteries in a HES system.
The battery developer Phinergy developed a silver-based catalyst that only allows oxygen from the ambient air into the positive cathode. The oxygen then combines with the liquid electrolyte, releasing the latent electrical energy stored in the aluminium anode. In one Citroe¨n C1 prototype EV, the Phinergy demonstrated that 25 kg of aluminium cells can deliver a total energy of 100 kWh, which gives a 600-plus mile range.
4.3.1.4 Sodium-beta batteries
The sodium-beta battery refers to the Na/S and Na/NiCl2 batteries, which have liquid sodium as one reactant and beta-alumina ceramic as the electrolyte. The Na/
S battery operates at 300–350 C with a nominal cell voltage of 2 V, specific energy of 170 Wh/kg, energy density of 250 Wh/L and specific power of 390 W/kg.
In Na/NiCl2battery, the active materials are molten sodium for the negative electrode and solid nickel chloride for the positive electrode. In addition to the beta- alumina ceramic electrolyte as used in the Na/S, there is a secondary electrolyte, namely, sodium-aluminium chloride, in the positive electrode chamber. The sec- ondary electrolyte conducts sodium ions from the primary beta-alumina electrolyte to the solid nickel chloride positive electrode (Sudworth and Galloway, 2009).
The Na/NiCl2 battery operates at 155–350C with a nominal cell voltage of 2.58 V. Based on the battery configuration, the battery pack performance parameters are the specific energy of 86–120 Wh/kg and specific power of 150–300 W/kg.
Comparing with the Na/S battery, the Na/NiCl2battery has higher open circuit cell voltage, wider operating temperature, safer products of reaction (less corrosive than molten Na2Sx) and better freeze–thaw durability (smaller temperature difference).
Hybridization of energy sources for electric and hybrid vehicles 107
The AEG Zero Emission Battery Research Activity (ZEBRA) has been the major developer of the Na/NiCl2 battery. The ZEBRA battery, namely, Z12, offered a specific energy of 103 Wh/kg and a specific power of 180 W/kg.
4.3.1.5 Ambient-temperature lithium batteries
There are a number of approaches being taken in the design of rechargeable ambient-temperature lithium batteries. One approach is to use metallic lithium for the negative electrode and a solid inorganic intercalation material for the positive electrode. The electrolyte can be a solid polymer, leading to name as the LiM- Polymer battery. Another approach is the use of a lithiated carbon material as the negative electrode such that lithium ions move forth and back between the positive and negative electrodes during cycling. The ‘rocking-chair’ movements of lithium ions lead to name as the Li-Ion battery (Yamaki, 2009).
4.3.1.5.1 LiM-Polymer batteries
The LiM-Polymer battery uses lithium metal and a transition metal intercalation oxide (MyOz) for the negative and positive electrodes, respectively. A thin solid polymer electrolyte (SPE) is used, which offers the merits of improved safety and flexibility in design (Kobayashiet al., 2009). By using a lithium foil as a negative electrode and vanadium oxide (V6O13) as a positive electrode, the Li/SPE/V6O13
cell is a typical LiM-Polymer battery. It operates at a nominal voltage of 3 V and has the specific energy of 155 Wh/kg, energy density of 220 Wh/L and specific power of 315 W/kg. The advantages are high cell voltage (3 V), very high specific energy and energy density (155 Wh/kg and 220 Wh/L, respectively), very low self- discharge rate (about 0.5% per month) and capability of fabrication in a variety of shapes and sizes. However, its low temperature performance is weak.
4.3.1.5.2 Li-Ion batteries
Since the commercialization of the Li-Ion battery by Sony Energytec in 1990, the Li-Ion battery has been considered to be the most promising rechargeable battery of the future. The Li-Ion battery has already gained acceptance for HEV applications.
The specific energy of Li-Ion battery was 98 Wh/kg in 1990 and increased to 195 Wh/kg in 2008 (Yamaki, 2009).
The Li-Ion battery consists of two electrodes, namely, a porous separator impregnated with electrolyte and two current collectors. Lithium cobalt oxide (LiCoO2) typically serves as an active electrode material for the positive electrode.
The negative electrode is usually made of lithiated carbon or graphite (LiC6).
Electrodes are electrically isolated by the separator, where the space between them is filled by electrolyte. Copper foil is used for the negative current collector and aluminium for the positive current collector.
4.3.1.5.3 Vehicle applications
The Li-Ion battery can be made from different advanced positive electrode and negative electrode materials. The mature positive electrode materials are LiCoO2, LiMn2O4, LiFePO4, lithium nickel manganese cobalt (NMC) oxide (LiNiMnCoO2) and lithium nickel cobalt aluminium (NCA) oxide (LiNiCoAlO2). The mature
negative electrode materials are graphite and titanate. Table 4.3 shows the potential Li-Ion batteries for vehicle applications.
4.3.1.6 Batteries for EV and HEV applications
The battery is the most significant factor of commercialization of EVs and HEVs.
Developments of vehicle batteries are continued and accelerated to meet require- ments of EVs and HEVs, namely, safety, high specific energy, high specific power, short recharge time, long life cycle and low cost. The mature and promising bat- teries for EVs and HEVs are VRLA, Ni-MH and Li-Ion batteries. The specific power and specific energy of the batteries are shown in Figure 4.5 and their features are compared in Table 4.4. The VRLA battery is still popular for mild HEVs and low cost EVs due to its maturity and cost effectiveness. The Ni-MH battery is mainly used in HEVs. The Li-Ion batteries are the promising battery in the future.
The advanced Li-Ions batteries have demonstrated the potentials of improvements in specific power, specific energy, charge rate and safety.
4.3.2 Fuel cells
The fuel cell generates electrical energy rather than stores it. The fuel cell EV can achieve a long driving range comparable to an ICEV. The hydrogen for fuel cells
Table 4.3 Advanced Li-Ion batteries
Positive electrode
Negative electrode
Manufacturers Key feature
LiCoO2 Graphite Sony Mature
LiMn2O2 Graphite NEC, GS, Yuasa, LG High power NCA/NMC Graphite SAFT, Samsung, Sanyo, Evonik High energy LiFePO4 Graphite A123, Valence Tech, BYD Highly stable LiMn2O4 Titanate Toshiba, Enerdel High discharge rate
Table 4.4 Key features of promising batteries
Type Pb-Acid Nickel-based Lithium
Feature VRLA Ni-MH Li-Ion Li-Titanate
Specific energy (Wh/kg) 30–40 60–70 160 70–90
Cycle life at 100% DoD (cycle) 50–80 300–500 500–750 25,000
Safety Fire hazard Fire hazard Fire hazard Safest
Charge time (0–90% SoC) (h) 8 2 2 0.1
Operating temperature (C) 10 to 60 0–40 0–40 40 to 70
Environmental impact Toxic Low Minimal Minimal
Memory effect Very low Moderate None None
Power delivery Good Moderate Moderate High
Manufacturability Easy Adequate Easy Easy
Maintenance Moderate Moderate Moderate Moderate
Market position High volume Modest Good Rising
Cost Low Tied to Ni Moderate Moderate
Hybridization of energy sources for electric and hybrid vehicles 109
can be refilled in a much shorter time than battery recharging (except for those mechanically rechargeable ones). Fuel cell’s lifetime is generally much longer than that of batteries, and they generally require less maintenance than batteries.
Fuel cells are generally classified by the type of electrolyte, namely, acid, alkaline, molten carbonate, solid oxide and solid polymer. Instead of using hydro- gen as the fuel, carbon monoxide and methanol have also been adopted by some fuel cells. However, the by-product of these fuel cells becomes carbon dioxide, rather than plain water. The molten carbonate and solid oxide fuel cells suffer from very high-temperature operation, respectively, over 600 C and 900 C, making them practically difficult to be applied to EVs. The acid fuel cells, alkaline fuel cells (AFCs) and solid polymer fuel cells (SPFCs) are technically possible for EV applications.
4.3.2.1 Acid fuel cells
The acid fuel cell is generally characterized by the ionic conduction of hydrogen ions, and platinum or platinum alloys as electrocatalysts for both the anode and cathode. In the early development of acid fuel cells, many different acids were investigated to be the electrolyte, such as the sulphuric acid, hydrofluoric acid and phosphoric acid.
Finally, the phosphoric acid won this competition because of its attractive features of stable operation at temperatures up to at least 225C, reasonably high conductivity at temperatures above 150C, as well as efficient rejection of product water and waste heat at the operating temperature. The phosphoric acid fuel cell (PAFC) is the only acid fuel cell ready for applications. The PAFC generally operates at 150–210C and at atmospheric or slightly higher pressures, offering the power density of 0.2–0.25 W/
cm2. Its projected life can be over 40 thousand hours. The major disadvantage of the PAFC is its dependence on noble metal electrocatalysts.
4.3.2.2 AFCs
The AFC can adopt low-cost non-noble metal or oxide electrocatalysts, such as nickel for the anode and lithiated nickel oxide for the cathode, to provide reason- able performances. The lower working temperature further enhances the AFC to be more attractive than the PAFC for EV applications. However, there are two major challenges for widespread applications of the AFC. First, since it generally operates at less than 100C, the water rejection and heat removal must be design well to maintain its efficiency and reliability. Second, carbon dioxide must be completely removed from the inlet hydrogen and air before their entry into the cell. Even a small amount of carbon dioxide is sufficient to carbonate the electrolyte and form solid deposits in the porous electrode.
4.3.2.3 Proton exchange membrane fuel cells
The proton exchange membrane fuel cell (PEMFC), also named as the SPFC, uses a solid polymer membrane as the electrolyte. This membrane is sandwiched between two platinum-electrocatalysed porous electrodes, namely, the anode and cathode.
The PEMFC works at a lower temperature than the PAFC, it operates at 50–100 C and at atmospheric or slightly higher pressures. With low platinum