This paper focuses on ultracapactors (electrochemical capacitors) as energy storage in vehicle applications and thus evaluates the present state-of-the-art of ultracapacitor technologies and their suitability for use in electric and hybrid drivelines of various types of vehicles. A key consideration in determining the applicability of ultracapacitors for a particular vehicle application is the proper assessment of the energy storage and power requirements. For hybrid-electric vehicles, the key issues are the useable energy requirement and the maximum pulse power at high efficiency. For a Prius size vehicle, if the useable energy storage is about 125 Wh and needed efficiency is 90-95%, analysis shown in this paper indicate that vehicles can be designed using carbon ultracapacitors (both carbon/carbon and hybrid carbon) that yield high fuel economy improvements for all driving cycles and the cost of the ultracapacitors can be competitive with lithium-ion batteries for high volume production and carbon prices of less than $20/kg. The use of carbon/carbon devices in micro-hybrids is particularly attractive for a control strategy (sawtooth) that permits engine operation near its maximum efficiency using only a 6 kW electric motor. Vehicle projects in transit buses and passenger cars have shown that ultracapacitors have functioned as expected and significant fuel economy improvements have been achieved that are higher than would have been possible using batteries because of the higher round-trip efficiencies of the ultracapacitors. Ultracapacitors have particular advantages for use in fuel cell powered vehicles in which it is likely they can be used without interface electronics. Development of hybrid carbon devices is continuing showing energy densities of 12 Wh/kg and a high efficiency power density of about 1000 W/kg. Vehicle simulations using those devices have shown that increased power capability in such devices is needed before full advantage can be taken of their increased energy density compared to carbon/carbon devices in some vehicle applications. Energy storage system considerations indicate that combinations of ultracapacitors and advanced batteries (Wh/kg>200) are likely to prove advantageous in the future as such batteries are developed. This is likely to be the case in plug-in hybrids with high power electric motors for which it may be difficult to limit the size and weight of the energy storage unit even using advanced batteries.

This report is concerned with the present status of emerging technologies that can be utilized in light-duty vehicles in the next five to ten years to significantly reduce their CO2 emissions. The emerging technologies considered are modern clean diesel engines and hybrid-electric powertrains using batteries. The status of each of these technologies is assessed based on available information and data from the literature. In addition, the present marketing situation for each technology is summarized and prospects for marketing the technologies in California assessed.

In the case of the modern diesel engines, it was found that such engines have much higher toque characteristics than gasoline engines of the same displacement and vehicles powered by diesel engines have higher fuel economy (mpg) by 40-50% than similar (same size and performance) vehicles powered with gasoline engines. This is the reason that diesel-powered vehicles have presently reached almost a 50% market share in Europe. The major problem with the diesel engine for use in California is that “so-called clean” diesel powered vehicles have NOx emissions that are still 10X higher than Tier 1 vehicles with gasoline engines and to meet the SULEV NOx standard, a reduction of at

least an additional factor of three is required. Whether this will be possible with diesel engines is uncertain.

Hybrid-electric passenger cars are currently being marketed in Japan and the United States by Toyota and Honda. These hybrids show large improvements (at least 30-50%) in fuel economy compared to conventional ICE vehicles of the same size and performance. All the hybrids meet SULEV emission standards and have been designated AT-PZEVs by CARB. The hybrids have been well received in the market with annual global sales approaching 100,000 in 2004. There seems to be no reason that hybrid sales will not continue to increase between 2005-2010 especially as the ZEV Mandate requirements become more demanding in 2008 and beyond.

The use of ultracapacitors in plug-in hybrid vehicles (PHEVs) with high energy density lithium-ion and zinc-air batteries is studied. Simulations were performed for various driving cycles with the PHEVs operating in the charge depleting and charge sustaining modes. The effects of the load leveling of the power demand from the batteries using the ultracapacitors are evident. The average and the peak currents from the batteries are lower by a factor of 2-3.

Proton Exchange Membrane fuel cell (PEMFC) technology is one of the most attractive candidates for transportation applications due to its inherently high efficiency and high power density. However, the fuel cell system efficiency can suffer because of the need for forced air supply and water-cooling systems. Hence the operating strategy of the fuel cell system can have a significant impact on the fuel cell system efficiency and thus vehicle fuel economy. The key issues are how the fuel cell back pressure and air flow through the fuel cell are controlled. One approach is fixed back pressure control. The other is optimum varying back pressure control. In both cases, the air flow stoichiometry is optimized. In this paper, a dynamic forward-looking vehicle model with a dynamic fuel cell system model is employed. The effects of different fuel cell system operation modes and different power split strategies on fuel economy of fuel cell hybrid vehicles are simulated. The simulation results of light duty vehicles on various driving cycles indicate a significant improvement in fuel economy for optimum varying back pressure operation compared to high fixed back pressure operation. For various fuel cell system operation modes, the load leveling control can significantly improve fuel economy on some aggressive driving cycles such as US06. The vehicle with a small fuel cell system becomes more efficient during low speed or low power demand driving by avoiding low fuel cell output power region.

The development of electrochemical capacitors (ultracapacitors) has continued since the early 1990s. Activated microporous carbon and hybrid carbon devices from a number of developers world-wide have been tested and evaluated for use in hybrid vehicles of various types. The test data indicate that the useable energy density of the activated carbon devices is about 5 Wh/kg and that of the hybrid carbon devices is 10-12 Wh/kg. The power capability of the carbon/carbon devices can be very high (> 2000 W/kg for a 95% high power pulse); the power capability of the hybrid carbon devices are significantly lower being 500-1000 W/kg for a 95% pulse. This means that the P/E ratio of the hybrid carbon devices is much lower than the carbon/carbon devices and as a consequence, it may be difficult to take full advantage of the higher energy density of the hybrid carbon devices in some applications. Simulation results for various types of hybrid vehicles are presented. The results for micro-hybrids are particularly interesting and surprising, because of the large fuel economy improvements predicted. The improvements were about 40% on the FUDS and ECE-EUD cycles and 20% on the Federal Highway and US06 cycles using the carbon/carbon ultracapacitor units. The improvements were significantly less using the hybrid carbon units because of their lower round-trip efficiencies.

Proton Exchange Membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other applications has been intensively developed in recent decades. Besides the fuel cell stack, air and fuel control and thermal and water management are major challenges in the development of the fuel cell for vehicle applications. The air supply system can have a major impact on overall system efficiency. In this paper a fuel cell system model for optimizing system operating conditions was developed which includes the transient dynamics of the air system with varying back pressure. Compared to the conventional fixed back pressure operation, the optimal operation discussed in this paper can achieve higher system efficiency over the full load range. Finally, the model is applied as part of a dynamic forward-looking vehicle model of a load-following direct hydrogen fuel cell vehicle to explore the energy economy optimization potential of fuel cell vehicles.

Proton Exchange Membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other applications has been extensively developed in recent decades. Besides the fuel cell stack, air and fuel control, and thermal and water management are major challenges in the fuel cell vehicle development. The air supply system can have a major impact on overall system efficiency. In this report, a fuel cell system model for optimizing system operating conditions was developed which includes the transient dynamics of the air system with varying back pressure. The model is scalable so that it can be used to simulate the operation of an arbitrary size (power) fuel cell. Finally, the model is applied as part of a dynamic forward-looking vehicle model of a load-following direct hydrogen fuel cell vehicle to explore the energy economy optimization potential of fuel cell vehicles.

This paper is concerned with the present status and future projections for emerging technologies that can be utilized in light-duty vehicles in the next five to ten years to significantly reduce their CO2 emissions. The emerging technologies considered are modern clean diesel engines and hybrid-electric powertrains using batteries and/or ultracapacitors for energy storage. Throughout the study, six classes of vehicles –compact passenger cars to large SUVs -were considered. For each vehicle class, computer simulations (Advisor 2002) and cost analyses were performed for conventional ICE and mild and full parallel hybrids using port- fuel injected and lean burn gasoline engines and direct- injection turbo-charged diesel engines to determine the fuel economy and differential costs for the various vehicle designs using the conventional gasoline PFI engine vehicle as the baseline. CO2 emissions (gmCO2/mi) for each driveline and vehicle case were calculated from the fuel economy values. On a percentage or ratio basis, the analyses indicated that the fuel economy gains, CO2 emissions reductions, and cost/price increases due to the use of the advanced engines and hybrid-electric drivelines were essentially independent of vehicle class. This means that a regulation specifying the same fractional reductions in CO2 emissions for all the vehicle classes would not favor one class over the others.

The results of the study were then used to calculate the increase in the CAFÉ standard (miles per gallon gasoline for the new car fleet) that was feasible using each of the emerging technologies and the associated vehicle price increase that would be incurred. It was determined that the CAFÉ standard could be increased to 38 mpg and 48 mpg using the PFI and lean-burn gasoline engines, respectively, in mild hybrids with an associated percentage price increase of 7-9%. The CAFÉ standard could be increased to 42 mpg and 52 mpg using the PFI and lean-burn gasoline engines, respectively, in full hybrids with an associated cost increase of 16-18%. The CAFÉ increases using the diesel engines in mild hybrids were close to those of gasoline engines in full hybrids. The vehicle price increases using diesel engines in hybrids were 17-23% compared to the baseline ICE vehicles using the PFI engine. The fleet CO2 emissions corresponding to the cited CAFÉ standards were 185-234 gmCO2/mi for the mild hybrids using gasoline engines and 170-208 gmCO2/mi for full hybrids. The corresponding values using diesel engines are189-216 gmCO2/mi. The CO2 emissions of the new car fleet for a CAFÉ standard of 27.5 mpg are 322 gmCO2/mi indicating that implementing the hybrid powertrain technologies in the new car fleet could reduce CO2 emissions of the fleet by 25-50%.

This report is concerned with the testing and evaluation of various battery chemistries for use in PHEVs. Test data are presented for lithium-ion cells and modules utilizing nickel cobalt, iron phosphate, and lithium titanate oxide in the electrodes. Cells with NiCoO2 (nickelate) in the positive electrode have the highest energy density being in the range of 100-170 Wh/kg. Cells using iron phosphate in the positive have energy density between 80-110 Wh/kg and those using lithium titanate oxide in the negative electrode have energy density between 60-70 Wh/kg. The power densities can vary over a wide range even for a given chemistry. In general, it is possible to design high power batteries (500- 1000 W/kg at 90% efficiency) for all the chemistries if one is willing to sacrifice energy density and likely also cycle life. The data indicate that high power iron phosphate cells can be designed without a significant sacrifice in energy density. When power densities greater than 2000 W/kg for lithium-ion batteries are claimed, it is for low efficiency pulses. For example, for an efficiency of 65%, the 15Ah EIG iron phosphate battery has a pulse power of 2330 W/kg rather than the 919 value for a 90% efficient pulse.

Cycle life data were not taken as part of the present study. However, cell cycle life data reported by Altairnano for their cells using lithium titanate oxide in the negative electrode indicate cycle life in excess of 5000 cycles for charge and discharge rates of 2C and greater. It seems likely that the cycle life of both titanate oxide and iron phosphate lithium batteries will be satisfactory for vehicle applications.

The cost of lithium batteries remains high ($500-1000 /kWh) when purchased in relative small quantities, but detailed cost modeling of batteries done at Argonne National Laboratory for the various chemistries indicate that in high production volume (greater than 100,000 packs per year), the costs to the OEMs of all chemistries can be in the range of $250-400/kWh depending on the battery size (kWh energy stored). The lithium titanate chemistry is projected to have the highest cost, but it also will have the longest cycle life.

R&D is continuing to increase the energy density of lithium-ion batteries. Proto-type cells presently being developed have energy densities in the range of 250-300 Wh/kg using layered metal oxides/spinels in the positive electrodes. Higher energy densities appear to be likely combining these electrodes with negative electrodes using composites of silicon oxides and carbon. R&D on electrically rechargeable Zinc-air cells is presently in progress. Energy densities in the range of 300-400 Wh/kg, 700-1000 Wh/L appear to be possible using the Zn-air chemistry. The power capability of the advanced batteries is uncertain at the present time.

The fuel cell powered vehicle is one of the most attractive candidates for the future due to its high efficiency and capability to use hydrogen as the fuel. However, its relatively poor dynamic response, high cost, and limited life time have impeded its widespread adoption. With the emergence of large supercapacitors (also know as ultracapacitors, UCs) with high power density and the shift to hybridization in the vehicle technology, fuel cell/supercapacitor hybrid fuel cell vehicles are gaining more attention. Fuel cells in conjunction with supercapacitors can create high power with fast dynamic response, which makes it well suitable for automotive applications. Hybrid fuel cell vehicles with different powertrain configurations have been evaluated based on simulations performed at the Institute of Transportation Studies, University of California-Davis. The following powertrain configurations have been considered: (a) Direct hydrogen fuel cell vehicles (FCVs) without energy storage (b) FCVs with supercapacitors directly connected in parallel with fuel cells (c) FCVs with supercapacitors coupled in parallel with fuel cells through a DC/DC converter (d) FCVs with fuel cells connected to supercapacitors via a DC/DC converter Simulation results show that hybridization of fuel cell vehicles with supercapacitors with load leveling control can significantly reduce the stress on fuel cells electrically and mechanically and benefit fuel economy of the vehicles. Compared to fuel cell vehicles without energy storages, fuel cell-supercapacitor hybridization achieved fuel economy increases of up to 28% on the FUDS cycle and up to 24% on the US06 cycle for mid-size passenger vehicles. In general, the maximum fuel economy improvements are greater using supercapacitors than batteries. The simulation results show that the power assist control strategy is better than load-level control for batteries because of the lower losses in the DC/DC converter and batteries, but load level control is better for supercapacitors. The best approach for hybridization of the fuel cell vehicles is to use supercapacitors with load leveled control as it greatly mitigates the stress on fuel cells and results in a near maximum improvement in fuel economy and fuel cell durability.