Today, the majority of U.S. oil consumption is accounted for by the transportation sector, which includes more than 220 million privately owned vehicles. Volatile oil prices, challenges in maintaining stability in global energy supplies and evolving and more stringent environmental policies have made alternative energy vehicles a plausible solution to reducing the nation’s dependence on imported petroleum.
While uber-efficient fuel cell vehicles utilizing hydrogen are still in development phase, several types of alternative energy vehicles, such as alternative fuel and hybrid electric vehicles are now available on the market. A primary and continuing challenge for the research and development of alternative fuel vehicles involve keeping the cost of these cutting-edge technologies low enough to be commercially viable.
Some of the earlier efforts to develop cleaner, more energy efficient vehicles started with research into alternative, non-petroleum based fuels produced from sources other than crude oil. These fuels include natural gas and ethanol, as well as Fischer-Tropsch diesel, made from the conversion of gaseous fuels or coal into liquid fuels, and biodiesel made from renewable sources. The U.S. Department of Energy (DOE) continues to work on overcoming technical barriers to expand markets for alternative fuels.
Use of Alternative Fuels in Vehicles
Biodiesel can be employed in traditional compression ignition engine (diesel) vehicles in B5 (5%) or in some cases B20 (20%) blends. Ethanol, which is an alcohol fuel used as a substitute for gasoline can be used in up to a 10% blend of ethanol (E10), which is covered under warranty by every auto manufacturer that sells vehicles in the U.S. for every make and every model of vehicle. E85 (85% ethanol, 15% gasoline) is for use in flexible fuel vehicles (FFVs) which can operate on gasoline or any blend of ethanol up to 85%. An FFV contains a single fuel tank, fuel system and engine. Due to the corrosive nature of ethanol, the engine and fuel system must be slightly adapted. The vehicle also needs a sensor in the fuel line to read the fuel mixture and control the fuel injection and timing, which are dependent on the fuel blend.
Bi-fuel
Another type of alternative energy vehicle includes the bi-fuel vehicle. In contrast to the FFV, this vehicle contains two separate fuel systems. One system is reserved for gasoline or diesel and the other for liquefied propane gas (LPG) or compressed natural gas (CNG). Pressurized tanks must be used to store both LPG and CNG, which cannot be pumped into the gasoline tank. Despite the bi-fuel vehicle being a cleaner, more economical alternative to a gasoline or diesel vehicle, its need for two separate fuel systems and a gaseous fuel storage tank renders it more expensive and less spacious compared to other vehicles.
Hybrid and Plug-In Hybrid Vehicles
Hybrid electric vehicles and Plug-in Hybrids are clean, quiet, and have the ability to use electricity from the grid to displace the petroleum used for transportation. As grid power becomes cleaner over time with increased wind and solar power development, the benefits of a reduction in greenhouse gases (GHGs) and an increase in air quality will be further realized. Hybrids utilize both internal combustion engines (ICE) and electric motors. The ICE engine produces power through repeated, controlled explosions that press down pistons connected to a rotating crankshaft. The rotating force is transmitted to the vehicle’s wheels. The hybrid’s electric motor is energized by a battery, which produces power via chemical reaction. The battery is recharged by a generator that is driven by the ICE.
Some hybrid vehicles use regenerative braking systems that capture the energy from deceleration, store it, and then convert it to electricity to help propel the vehicle, which ultimately increases overall efficiency. Ultracapacitors are also used in various hybrids to extend the life of the vehicle’s on-board battery system, as they are better suited to capturing high power from regenerative braking and releasing it for initial acceleration.
Hybrids can have a parallel design, a series design, or both. The parallel design hybrid has an energy conversion unit and electric propulsion system that are connected to the vehicle’s wheels. The primary engine is used during highway driving, and the electric motor gives extra power during periods of high demand, like hill climbs or acceleration. A series design hybrid uses a primary engine that is connected to a generator, which produces electricity. The electricity charges the batteries, which power an electric motor to power the wheels. Some hybrid electric vehicles can use a combination of both designs. The series design configuration is used at low speeds while the parallel design configuration is used for highway driving and acceleration.
Hybrid electric technology has made further strides in combating the main problem with electric vehicles by expanding their range. Adding extra batteries and a way to charge it externally, or “plug-in” the vehicle have afforded hybrids the ability to drive most of their daily mileage on clean, less expensive electricity. An easily refillable fuel tank adds security for longer trips. Some hybrids have been converted to plug-ins, and many automobile manufacturers are working on making more plug-in models available.
However, plug-in hybrids can be problematic, as their batteries are costly and extremely heavy. The Department of Energy’s National Renewable Energy Laboratory (NREL) has been extensively researching thermal management, modeling and systems solutions for energy storage technology. The NREL feels that even at today’s battery costs, plug-ins may be able to repay their costs within a few years. NREL is also researching improved power electronics, which are critical to hybrid efficiency, conducting analyses to illustrate the economic viability of plug-ins and identify key areas for improvement, and looking into the future possibilities of reversible plug-ins, which would allow vehicle owners and local utility companies to utilize the extra electrical storage capacity in the vehicle batteries to meet peak residence and business demand for electricity.
Fuel cell
While hybrids have gained a solid foothold in the auto industry, fuel cell technology and fuel cell vehicles (FCV) are starting to make ground. Fuel cells produce electricity through a chemical reaction between hydrogen and oxygen yet release no detrimental emissions. FCVs can generate their own electricity using hydrogen or other fuel changeable to hydrogen. In FCVs, hydrogen is generally stored as pressurized gas in onboard fuel tanks. Similar to hybrid models, the electricity supplies a storage battery, which powers the electric motor. In fact, FCVs resemble hybrids because both of their electric batteries are charged by a separate onboard system. The NREL feels this emphasizes the importance of advancing current hybrid technology and will ultimately help reduce petroleum consumption and encourage clean, environmentally safe transportation methods. However, it may take a few more years before FCVs are as cost-competitive as traditional gasoline vehicles and even hybrids.
Research
The DOE has been working to improve the technology of alternative energy vehicles through their FreedomCAR and Vehicle Technologies Program and their 21st Century Truck Partnership. They have been conducting research on advanced automotive and truck technologies that will provide the transition to hydrogen-powered fuel cell vehicles.
Sources: National Renewable Energy Laboratory (NREL), U.S. Department of Energy (Energy Efficiency and Renewable Energy), U.S. Environmental Protection Agency and National Defense Council Foundation