With environmental regulations toughening, alternative fuels and vehicles increased their prevalence in the US during 1999.
Fig. 1 shows a general diagram of some alternative fuel and vehicle choices. Each alternative has advantages and disadvantages.
A serious search for alternative fuels to replace petroleum-based fuels in the US began after the Organization of Petroleum Exporting Countries embargo of 1973. By the late 1990s, interest in alternative fuels came from a desire both to reduce the country`s dependence on petroleum and to have cleaner air.
Clean air goals include reducing greenhouse gases and ozone precursors. Greenhouse gases include carbon dioxide (CO2) and methane; ozone precursors include nitrogen oxides (NOx) and volatile organic compounds (VOCs).
CO2 emissions are directly related to fuel consumption. The use of oxygenated gasolines helps reduce emissions of carbon monoxide (CO), which inhibits the delivery of oxygen to the lungs. Ozone, NOx, and particulate matter (PM) are lung irritants that also can hinder breathing.
The combination of advanced engine technologies and reduction in sulfur had the potential to reduce conventional vehicle NOx emissions by 50-75% and PM by 80%. Although US conventional fuels were headed for major makeovers in 1999, concerns for clean air boosted interest in alternative fuels.
According to the US Environmental Protection Agency 1997 National Air Quality and Emission Trends Report, on-road vehicles contributed 57% of total CO emissions, 30% of NOx emissions, and 27% of VOC emissions in 1997.
Alternative transport fuels include coal-derived fuels, compressed natural gas (CNG), liquefied natural gas (LNG), liquefied petroleum gas (LPG), biodiesel, ethanol, methanol, hydrogen, and electricity.
The largest barriers to market entry for alternative fuels are cost-competitiveness and infrastructure. Other considerations include emissions performance, driving performance, fuel range, reliability, storage, and safety.
Legislation
The Clean Air Act Amendments (CAAA) of 1990 and the Energy Policy Act (EPACT) of 1992 did the most to outline future fuel specifications in the US. The California Air Resources Board (CARB) also set fuel standard trends with regulations often more strict than those of the federal government.
CAAA set air emission goals for US vehicles and air quality goals for US regions. Tier 1 set exhaust emission standards for CO, hydrocarbons (HCs), NOx, and PM for light-duty vehicles and trucks beginning with model year 1994. Tier 2 will reduce pollutant emissions for 2004 and 2007 model-year vehicles and reduce gasoline sulfur content.
Sulfur reduces the effectiveness of catalysts in emission-control systems of advanced technology vehicles. Catalyst quality affects HC, CO, and NOx emissions.
EPACT encourages the use of alternative fuels with tax incentives and requires government purchases of alternative fuel vehicles (AFVs).
In 1990, CARB decided that beginning in 1998 its Low Emission Vehicle Program (LEVP) would require 2% of all vehicles sold in California, or about 20,000 vehicles, to have zero emissions. Only hydrogen and battery-powered electric vehicles (EVs) meet this standard.
In early 1996, however, when it was apparent that EV production was not as far along as expected, CARB postponed a compliance date to 2003, when 10% of every car sold in California was to be a zero-emissions vehicle.
Alternate fuel trends
The Energy Information Administration`s Annual Energy Outlook 1999 predicted that gasoline market share would decline from 65% of petroleum consumed for transportation in 1997 to 61% in 2020.
Table 1 shows the estimated fuel consumption of US vehicles between 1992 and 2000. This table considers only CNG, LNG, LPG, ethanol, methanol, and electricity as alternative fuels.
Alternative fuels, excluding ethanol in gasohol and methyl tertiary butyl ether (MTBE), made up a very small portion of the entire fuels slate. They were expected to make up about 0.2% of total transportation fuel consumption in 2000.
E85 (85% ethanol with 15% gasoline) consumption had by far the largest expected average annual increase between 1998 and 2000, 35%/year. Much of the growth was expected to take place in fuel-flexible vehicles.
Consumption growth rates for CNG, LNG, and electricity were close behind ethanol. Each of these fuels were expected to experience average annual increases of 20% between 1998 and 2000. Gasoline and diesel were each expected to increase about 2%/year between 1998 and 2000.
The costs of AFVs are consistently higher than those of their gasoline-fueled counterparts. The Fuels Report, June 1999, published by the California Energy Commission, compared operation and purchase costs for AFVs available in California (Table 2).
While CNG, LPG, and electricity cost less than their energy equivalents of gasoline, higher vehicle costs account for total higher AFV costs. In contrast, total costs for ethanol and methanol Ford Tauruses are higher mostly as a result of higher fuel costs.
Electric vehicles cost much more than the other fueled vehicles.
CNG, LNG
Natural gas is touted as a cleaner fuel than gasoline and diesel because it combusts more completely. Dedicated natural gas vehicles have no evaporative emissions under normal operating conditions. Nonmethane hydrocarbon exhaust emissions are 40-60% less than their gasoline counterparts.
Because methane is a greenhouse gas, however, natural gas is not considered ideal. CNG and LPG produce less CO2 than other fuels for the amount of energy they provide.
CNG is natural gas that has been compressed and stored in 3,000-5,000 psi fuel cylinders. It is dispensed and stored in vehicles at 2,400-3,600 psi.
LNG is made by refrigerating natural gas below -260° F. LNG contains more energy per unit of volume than does CNG, which means vehicles using it require fewer stops for fueling.
As a vehicle fuel, LNG is not as popular as CNG, however, because of its refrigeration needs. Also, CNG can be delivered by regular pipeline and compressed at the refueling station; LNG must be transported as a liquid, which rules out pipeline transportation.
Fuel costs for natural gas are 40-50% lower than those of energy-equivalent gasoline. And maintenance costs are lower for gas-fueled than they are for gasoline-powered vehicles.
The main disadvantages of using natural gas are weight and space for storage, limited driving ranges, and lack of infrastructure. Although the pipeline infrastructure for natural gas is mature, the fuel requires costly refueling stations.
Also, total CNG vehicle and operation costs are as much as 6¢/mile more than gasoline counterparts (Table 2).
In 1996, Barwood Inc., a taxicab company in Kensington, Md., incorporated a number of dedicated CNG vehicles, based on Ford`s Crown Victoria, into its fleet. Emissions results from this experience documented superior nonmethane HC and CO emission, as compared to vehicles burning reformulated gasoline (RFG), and comparable NOx emissions performance (Fig. 2).
LPG
LPG has 45% less smog-forming potential than gasoline and emits less CO and HC emissions. Like natural gas vehicles, LPG vehicles emit about the same NOx levels as gasoline vehicles.
The US transportation grade of LPG is 95% propane and 5% butane. Although two thirds of LPG consumed in the US come from refineries, imports, or natural gas associated with crude production, LPG is considered an alternative fuel.
Advantages of LPG over gasoline are its high octane number, improved cold starting, and fuel price. Like CNG and electricity, LPG costs less than the energy equivalent of gasoline, but vehicle costs outstrip this advantage.
Like for natural gas, the pipeline and terminal infrastructure for propane is advanced. The cost of a fueling station is less expensive than for a CNG station.
Conversion of a gasoline vehicle to an LPG one costs $1,700-2,500, compared with $3,000-4,000 for a CNG conversion.
Disadvantages are limited availability, added weight and space of storage tanks, and the cost of retrofitted engines. Should LPG demand increase, the US supply of LPG would have to be supplemented by LPG imports.
Biodiesel
Biodiesel, a substitute for diesel, is typically blended at a 20 vol % with 80 vol % low sulfur diesel fuel. By itself, biodiesel has essentially no sulfur and emits less particulate, smoke, HC, and CO than conventional diesel. NOx emissions are similar to diesel, however.
The fuel can be produced by combining vegetable oil, animal fats, or microalgal oil with an alcohol (such as methanol) to form fatty esters. The alcohol-ester mixture is separated, and excess alcohol is recycled. The esters are then cleaned and purified.
An advantage of biodiesel is that it can use the existing diesel infrastructure. Unlike CNG and LPG, its use requires little modifications to engines and fueling stations. Highway mileage is essentially the same as conventional diesel.
Safety advantages of biodiesel include a higher flash point, very low toxicity if ingested, and biodegradability. Users must consider material compatibility, cold temperature tolerances, and shelf life, however.
Production costs for biodiesel are very high. Depending on the feedstock, costs range from $2.50/gal to $6/gal.
Ethanol, ETBE
Ethanol can be used as both an oxygenate additive for enhancing octane in a 10% blend with gasoline (E10) and a gasoline substitute (E85).
The renewable characteristic of ethanol distilled from corn makes it unique among other fuel alternatives. Its oxygen content associates it with high octane and low HC and CO tailpipe emissions. As well, it contains no aromatics or olefins.
In concentrations less than 85 vol %, ethanol adversely increases the Reid vapor pressure (rvp) of gasoline, which leads to increased VOC evaporative emissions.
A report issued by the National Research Council (NRC) in May 1999 said that neither ethanol nor MTBE appeared to have much impact on reducing smog, although both oxygen additives reduce some pollutants from motor vehicle emissions.
Ethanol proponents see the potential to replace lost MTBE volume with ethanol if MTBE use is banned from gasoline, as seemed possible in 1999. Ethanol is generally viewed as less toxic than MTBE and thus less likely to contaminate water supplies.
Controversy surrounds the production costs of ethanol. The Federal Highway Bill of 1998 extended the ethanol tax credit, which was scheduled to expire in 2000, to 2007. The 54¢/gal credit makes ethanol, which costs about twice as much to produce as gasoline, and its derivative ethyl tertiary butyl ether (ETBE) feasible for fuel use. The bill specified 1¢/gal reductions in 2001, 2003, and 2005.
Opponents pointed out that ethanol is not competitive with other fuels without the tax credit. Transportation, storage, and production costs are high.
The production of ethanol from cellulosic biomass has viability, although costs in 1999 were high. Cellulosic biomass can be urban, mill, forest, and agricultural wastes, some of which would be free of cost.
Depending on the feedstock and technology advances, the DOE estimated, producing ethanol from cellulosic biomass would cost 80¢-$1.50/gal in 2000 and 40-95¢/gal by 2010.
Ethanol can also be reacted with isobutylene to produce ETBE, another fuel oxygenate. ETBE does not have ethanol`s high volatility and water solubility problems. Production costs of ETBE and distances from major ethanol-producing areas to major refining centers limit its use.
Methanol, MTBE
Methanol can be a straight fuel, a blend for gasoline, or a component of MTBE.
It is the lead contender for the standard fuel for fuel-cell vehicles. Methanol fuel-cell vehicles promise lower CO, NOx, VOC, and PM emissions than gasoline cars.
Methanol and isobutylene react to produce MTBE, the most popular oxygenate used in RFG. Advantages of MTBE include low cost, high octane, and low volatility.
The future of MTBE, however, became clouded in March 1999 when California Gov. Gray Davis issued an executive order to eliminate it from the gasoline pool by Dec. 31, 2002. MTBE had been detected in groundwater and drinking water in California.
In one sense, methanol is safer than gasoline in an accidental fire because it burns cooler. The flame, however, is difficult to detect in bright sunlight.
The fuel contains about half the energy of gasoline per gallon, which means cars fueled by it get fewer miles to the gallon.
According to the American Methanol Institute (AMI), the existing gasoline infrastructure can be modified to supply methanol at a cost of about $30,000/station. The cost to make methanol available conveniently at one in ten gasoline stations nationwide is about $600 million.
Creating a hydrogen infrastructure to serve fuel-cell vehicles could cost as much as $1 trillion.
Hydrogen
Hydrogen can be used in both internal combustion engines (ICEs) and fuel cells. Intensive modifications are required for use in an ICE.
Whether used in an ICE or a fuel cell, energy efficiency is greater with hydrogen than gasoline.
A hydrogen ICE produces no HC, no sulfur compounds, and only a limited amount of NOx. A hydrogen fuel cell produces zero emissions-only heat and water.
Hydrogen can be used in compressed or liquefied states. Future developments promise unique chemical manners to store hydrogen, such as gas-solid adsorption or metal hydrides.
The major problems with hydrogen are adequate on-board storage space on vehicles, capital costs of fueling infrastructure, safety considerations, and fuel costs.
Hydrogen is also difficult and costly to liquefy. When pressurized in tanks, hydrogen has lower energy content than natural gas.
Producing hydrogen from natural gas is the cheapest way to produce hydrogen, but the resulting fuel is still twice the cost of that of gasoline.
How fuel cells work
Inside a fuel-cell EV, the fuel-cell system consists of four parts: a fuel storage container, a reformer, the fuel-cell stack, and an electric motor.
Fuel cells combine hydrogen and oxygen (from air) to generate electrical power, water, and heat. A cell consists of an electrolyte sandwiched between a platinum-catalyzed cathode and anode.
For transportation, the three most popular cells are the phosphoric acid fuel cell (PAFC), proton-exchange membrane fuel cell (PEMFC), and direct methanol fuel cell (DMFC).
Fuels for fuel cells can be natural gas, ethanol, methanol, hydrogen, or even conventional gasoline or diesel. If the fuel is not hydrogen, a catalytic reformer is required to convert the fuel into a hydrogen-rich gas.
A DMFC does not require a reformer, however, because the anode catalyst selectively withdraws hydrogen from methanol.
Fuel cell pros, cons
Because the fuel-cell process avoids the combustion of fuel, the technology is efficient and produces little emissions and little noise. The hydrogen fuel-cell efficiency is about 40%, while diesel engine efficiencies are about 25-32%.
Once in operation, the mechanical reliability of a fuel cell is high because it has very few moving parts.
Fuel-cell vehicles (FCVs) have some serious hurdles, however. Infrastructure, cost, size, weight, performance, and fuel range are key hurdles.
Will the predominant feed for on-board fuel-cell systems be gasoline, diesel, natural gas, ethanol, methanol, or just plain hydrogen? Energy companies in 1999 were competing for this market. As well, car companies had differing ideas. The answer to this question will settle uncertainties associated with infrastructure.
According to a statement by DaimlerChrysler in 1998, fuel-cell systems were 10 times too expensive for consumers. Technology improvements and mass production were expected to lower these costs.
One of the performance problems associated with FCVs is cell efficiency decreases at higher specific power requirements. Enlarging the fuel-cell design gives higher efficiency, but this leads to weight and size problems.
Start-up times for fuel-cell vehicles also need to be improved. Other technical challenges involve prevention of membrane dehydration and higher catalyst tolerance to CO.
Reforming technology emissions and efficiencies will make FCVs more viable. If the reformer is 50% efficient, the total fuel system may have an efficiency of only 20-30%.
In March 1999, DaimlerChrysler AG, unveiled its newest fuel-cell vehicle, NECar 4 (new electric car), which is powered with liquid hydrogen (Fig. 3). According to DaimlerChrysler, the car can go up to 90 mph and has a fuel range of 280 miles. The company plans to place fuel-cell vehicles in limited production by 2004.
Other fuel-cell experimenters included General Motors Corp. (GM), Ford Motor Co., Toyota Motor Corp., Honda Motor Corp., Nissan Motor Corp., Volkswagen, Volvo, International Fuel Cells, and Ballard Power Systems Inc.
Flexible fuel reformers, vehicles
To bridge the gap between conventional and nonconventional fuels, Arthur D. Little, Cambridge, Mass., developed a proprietary flexible fuel reformer under a 5-year program sponsored by the DOE in 1998.
The ADL proprietary reformer technology can convert gasoline, ethanol, methanol, or natural gas to hydrogen.
Another technology bridge is the fuel-flexible vehicle (FFV). FFVs can run on either alcohol or conventional gasoline, or a combination of the two. As the infrastructure for alcohols was not well developed in 1999, these cars can use the gasoline infrastructure until an alcohol infrastructure is established.
Alcohols used in FFVs are E85 or M85 (85% methanol and 15% gasoline).
Battery-powered EV
Battery-powered EVs emit 10-20 times less HC and CO emissions than gasoline. NOx emissions are almost nonexistent if the electric utilities are fueled with hydropower, nuclear power, or natural gas. Coal-based power plants, however, contribute more significantly to NOx emissions.
Disadvantages of EVs include driving range, battery disposal, and cost.
EVs are ideal where short, planned distances are possible. Charged battery ranges are about 120 miles, depending on driving conditions and driving speed.
The disposal of the batteries from EVs will generate hazardous waste, which must be considered when looking at EVs.
Battery-operated EVs have a steep cost barrier to overcome. The California Energy Commission estimated that it costs 10-20¢/mile more to lease and operate an electric vehicle than to own and operate a similar gasoline vehicle (Table 2).
One advantage of EVs, however, is good acceleration from a standing stop. Electric motors produce peak torque regardless of engine revolutions or vehicle speed.
GM, Ford, DaimlerChrysler, Toyota, Honda, and Nissan were among companies involved in development of battery-powered EVs in 1999.
Hybrid vehicles use both batteries and a small gasoline or diesel engine to supplement the battery. Although they are not zero-emission vehicles, they have fewer emissions than dedicated conventionally fueled vehicles.
In December 1999, Honda was to offer the first gasoline-electric hybrid vehicle available to the US. According to the company, the Honda Insight produces 84% fewer HCs and 50% less NOx than a typical car. The car has the best EPA mileage ratings in history, rated at 61 mpg city and 70 mpg highway.
Toyota brought the world`s first mass-produced, hybrid vehicle to market with its Prius gasoline-electric compact car in Japan in December 1997. It was to go on sale in the US and Europe in 2000.
GM, Ford, DaimlerChrysler, Nissan, Isuzu Motors Ltd., and Volvo were also among those involved in developing hybrid EVs.
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DaimlerChrysler AG introduced its NECar 4 (new electric car) in March 1999. The car uses liquid hydrogen to power a fuel cell. Photo courtesy of DaimlerChrysler; Fig. 3.
