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Choice of a Battery Type for Electric Vehicles
6.8kg of hydrogen for a 3-L, 1,500-kg vehicle with a driving range of 560km is 340L at 25MPa, and 160L at 52MPa. A typical gas tank volume for such a vehicle is 70L. Thus the limited energy storage capacity of hydrogen and the lack of an infrastructure to supply it makes it necessary to develop a process to extract hydrogen from gasoline.
The Daimler-Chrysler experimental fuel-processing technology converts gasoline into hydrogen, carbon dioxide (CO2), and water (H2O) in a multistage chemical reaction process. The five stage processing components consist of the following:
Fuel Vaporizer By applying heat, liquid gasoline is converted to gases to ensure low pollution. The vaporized gas during combustion passes on to the next stage. Partial Oxidation (POX) Reactor Vaporized fuel is combined with some air in a Partial Oxidation reactor, producing H2 and CO. Water-Gas Shift Steam as the catalyst converts most of the CO to harmless CO2 and additional H2. Since CO is harmful to both, excessive inhalation and the fuel cell. Thus the concentration of CO must be reduced to less than 10ppm. Preferential Oxidation (PROX) In the PROX, the injected air reacts with the remaining CO. With steam as the catalyst the preferential oxidation process results in production of CO2 and hydrogen-rich gases. Fuel Cell Stack The hydrogen gas, combined with air, produces electricity to move the vehicle with virtually no pollution—with the emission of water vapor.
The greatest challenges facing the changes in transportation are the lack of understanding of the broad range of consequences of environmental pollution and reliance on IC engine based transportation. In addition, the lack of confidence in the alternate fuel technology is the key deterrent of commercialization of the alternative fuel based technology transportation.
The increase in the hydrogen program expenditures over the past decade can be summarized in Table 1–4. The increase in the annual expenditure demonstrates a significant promise in the fuel cell based vehicles for both commercial and domestic passenger vehicles.
CHOICE OF A BATTERY TYPE FOR ELECTRIC VEHICLES
VRLA battery designs operate successfully in partially closed environments. They do not require as much floor space as their flooded lead-
Table 1–4 Spending between 1992 and 2000 for the hydrogen program.
Fiscal Year Expenditure (million $) 1992 1.4 1993 3.8 1994 9.5 1995 10 1996 14.5 1997 15 1998 18 1999 20 2000 25
acid type counterparts. In addition, they certainly do not require as much maintenance. As they continue to decrease in size, they are improving in energy density and cost.
NiMH batteries are also termed environmentally friendly and continue to improve both in energy density and cost.
Li-ion batteries are capable of storing up to three times more energy per unit weight and volume than the conventional Pb-acid and NiMH batteries. This is approximately three-times voltage level of 3.5V. Because of the high-energy characteristics, Li-ion batteries find widespread applications including aerospace, EV, and hybrid EV designs. However, the scaling of the consumer Li-ion cells is necessary.
While evaluating battery suitability for unique applications, it is important to understand a variety of battery characteristics, including the energy/power relationship (Ragone Plot), battery and cell impedance as a function of temperature, pulse discharge capability as a function of both temperature and load, and battery charge/discharge characteristics.
The self-discharge rate of the solid-state Li-ion battery is fairly low—5% of the capacity per month, compared to the 15% for the VRLA battery and 25% for NiMH battery. There is no memory effect in the solid-state Li-ion battery as is the case in the NiMH and the VRLA battery. The battery cycle life is superior to the NiMH and VRLA batteries. In the case of the NiMH battery, the cycle life typically drops to 80% of the rated capacity after 500 cycles at the C-rate (one hour charge followed by a one hour discharge). Solid-state Li-ion batteries can achieve more than 1,200 cycles before reaching 80% of their rated capacity.
Figure 1–1 Life cycle of a Li-ion EV cell.
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% Capacity
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0 100 200 300 400 500 600 700 800 # of Cycles
Cycle Life of a Li-ion EV Cell
BatteryCapacity 5 hours C/5 Ahr 65 Energy Density 5 hours Whr/l 270 Specific Energy 5 hours W/kg 115 Power Density 30sec 80% DOD W/l 435 Specific Power 30sec 80% DOD W/kg 180 Cycle Life 80% of Capacity 700 Rate Capability Cap@C/1/Cap@C/5 % 80 Charge Time Hr 4–5
Li-ion batteries, particularly solid-state batteries are efficient at charge-discharge rates other than the C-rate. In addition, the liquid Liion batteries are not suited for use in EVs owing to safety reasons while the solid-state batteries are well suited for high-rate applications.
Solid-state Li-ion batteries allow for the development of virtually any size batteries. In addition, the batteries can be stacked into efficient multicell configurations. From a cost perspective, the solid-state Li-ion battery uses a relatively inexpensive metal oxide that is fabricated in sheet form to allow inexpensive battery production. Electrodes, electrolyte, and foil packaging—all on a continuous-feed roll—are sandwiched together into finished batteries in one integrated process. By comparison, the liquid Li-ion battery cells require a cumbersome
winding and canning process. Thus in comparison, solid Li-ion batteries will be easily mass-produced at less than a $1 per Whr. The NiMH battery, after years of improvements, is being produced at approximately the same cost, $1 per Whr.
The solid Li-ion batteries are safer to produce than the liquid Li-ion batteries because the solid polymer electrolyte is both nonvolatile and leak-proof. There is no chance for the Li-ion battery cell to be breached leading to an electrolyte leak.
Future developments of solid-state Li-ion batteries go into full-scale commercial production. Efforts to enhance the energy density and rate capability of the next generation batteries are already underway. The U.S. Advanced Battery Consortium (USABC) is funding research to improve the ion conductivity of solid-state electrolytes.
A large number of characteristics of the Li-ion battery are favorable for EV applications. These include:
• High gravimetric and volumetric energy densities • Ambient temperature operation • Long life cycle (See Figure 1–1) • Good pulse power density
The LiMn2O4 oxide based Li-ion battery is:
• Considerably cheaper • More environmentally benign • Less toxic than LiCoO2 and LiNiO2 based batteries
In addition, the LiMn2O4 based EV batteries demonstrate a cycle life between 700 to 1,000 cycles before the capacity of the battery drops down to 80% of its initial capacity under room temperature conditions. Table 1–5 summarizes the developing Li-ion battery chemistry and characteristics.
The next generation design efforts include:
• to further extend the battery service life to 10 years • to cut the battery costs significantly
