Electric Vehicles: Fuel Cells pdf

Electric Vehicles: Fuel Cells C Hochgraf, General Motors Fuel Cell Activities, Honeoye Falls, NY, USA Published by Elsevier B.V. Introduction – Why Fuel Cells? Hydrogen fuel cells are one of the most promising al- ternatives to internal combustion engine hybrids and pure battery electric power for propelling passenger vehicles. Compared to internal combustion engine hybrid ve- hicles burning hydrocarbon fuels, fuel cell vehicles offer three primary advantages. First, the fuel cell system produces no tank-to-wheel carbon dioxide emissions and no other harmful emissions such as oxides of nitrogen, carbon monoxide, or particulates. Second, the fuel cell system offers the potential for approximately 30% higher well-to-wheel energy efficiency. Third, the hydrogen fuel consumed by the fuel cell can be produced from a variety of renewable sources including carbon-free methods such as electrolysis of water. Compared to pure battery-run electric vehicles, the fuel cell vehicle offers three primary advantages. First, the fuel cell vehicle has more than twice the driving range of a vehicle using existing batteries. Second, it offers a much shorter re fueling time, enabling brief re- fueling stops on long trips. Third, at cold temperatures, the fuel cell system can warm up much faster than a battery and therefore produce full power in a shorter period of time. A fuel cell vehicle can be refilled with compressed hydrogen at a rate of 2.0 kg hydrogen per minute. To recharge a battery electric vehicle at an equivalent rate would require the battery and charger to handle 2.5 MW of power. Such a charger would be 400 times larger than that typically used for battery electric vehicles. At –30 1C, many high-energy lithium battery chem- istries cannot provide high power, that is, they cannot support discharge C-rates of 10 or more. To get full power capability, the battery would need to be warmed up. However, the time and energy required to accomplish this for a battery are significantly longer than for a well- designed fuel cell system. Improved, but not full, power capability can be obtained at À30 1C by using lower- energy-density chemistries such as those using nano- partic le lithium titanium oxide. The primary disadvantages of fuel cell systems, compared to gasoline hybrids, are the high present-day cost, shorter than required fuel cell stack life, poor en- ergy density of fuel storage, and lack of a widespread hydrogen fueling infrastructure. Fuel cell system cost, while higher than a gasoline hybrid, is projected to be lower than that of an equivalent full-range electric vehicle with advanced batteries. Cost is being lowered and durability increased through engineering develop- ment efforts. The development of higher-energy-density hydrogen storage is an area of active research. Several studies have shown that the cost in the near term of producing, distributing, and dispensing hydrogen for use in fuel cell electric vehicles is in the range of US$2–3 per gallon of gasoline equivalent (on a cost-per- kilometer basis, not including taxes). Hydrogen is pro- duced in large quantities for industrial uses including oil refining and fertilizer production. The economics of hydrogen production by steam methane reforming are well understood. The US Department of Energy is tar- geting long-term costs of US$1.0–1.5 per gallon of gas- oline equivalent. The primary challenges are the initial cost to deploy the fueling infrastructure and delays in getting approval to site dispensing stations due to the absence of uniform building codes and standards for hydrogen. Support of government policy is often cited as being essential to overcoming these challenges. Requirements of an Automobile Fuel Cell Powertrain The requirements for an automobile propulsion system have evolved over many decades. Consumers’ expect- ations for on-road vehicle propulsion are guided by ex- perience, which is almost entirely with internal combustion engines. Consumers expect vehicles to start instantly, accel- erate quickly, drive for long periods without refueling, refuel in a few minutes, be cost effective, last for decades, and be safe to operate and service. These high-level re- quirements are quantified in Table 1. Achieving the desired driving range is particularly challenging for the fuel cell vehicle because the energy storage density of compressed hydro gen gas is less than 1kW h L À1 at 70 MPa compared to gasoline’s energy storage density of greater than 8 kW h L À1 . As a result, the requirement for fuel cell efficiency is significantly higher than that of a gasoline engine. Packaging the propulsion system to fit into a standard vehicle’s dimensions and shape drives compact and lightweight design solutions for the fuel cell system. Powertrain Configuration A fuel cell powertrain consists of a fuel cell stack with balance of plant components for air supply, fuel control, 236 . Electric Vehicles: Fuel Cells C Hochgraf, General Motors Fuel Cell Activities, Honeoye Falls, NY, USA Published by Elsevier B.V. Introduction – Why Fuel Cells? Hydrogen fuel cells are. design solutions for the fuel cell system. Powertrain Configuration A fuel cell powertrain consists of a fuel cell stack with balance of plant components for air supply, fuel control, 236 . hydrogen fuel consumed by the fuel cell can be produced from a variety of renewable sources including carbon-free methods such as electrolysis of water. Compared to pure battery-run electric