Personal Transportation in the 21st Century and Beyond

Danny J. Krebs

Petroleum Production and Consumption

The original world endowment of conventional petroleum is generally estimated to be less than three trillion barrels, with about a third of that resource having already been pumped [1]. With the world consuming about 26 billion barrels of oil each year, only about 75 years worth of conventional oil remains in the ground. Getting at the last trillion barrels will be a lot harder than getting the first trillion, so production rates will soon decline. One recent study concluded that oil production will peak in 2014 and that by the year 2050 ninety percent of the recoverable oil on the planet will have been pumped [2]. More optimistic studies forecast a gradual global decline by 2020 or later. Many believe that oil from shale, tar sands, and heavy crude can provide additional petroleum to last well into the 22nd century, but only at increasingly higher prices, and greater environmental peril. The $440 billion in oil payments by the US in 2008 was the largest transfer of wealth in human history. In 2011 we will almost certainly import more than half-a-trillion dollars worth of petroleum. US imports of petroleum account for about 2.4 percent of our GDP and about one third of our balance of payments deficit. In addition to these figures we must also consider the cost of our military posture in the Middle East.

But is petroleum the only energy source able to satisfy our transportation needs? In this article I examine how our transportation system must change to adjust to the realities declining petroleum production, with a particular view to examining possible alternative fuels for private automobiles.

Efficiency and Fuel Options

I believe that we do not have an energy problem so much as a transportation fuel problem. While petroleum provides about 94% of our transportation energy, the total energy derived from petroleum is significantly less than the energy that we derive from coal and natural gas. The mechanical energy necessary for our transportation sector is less than half the useful energy delivered by electric utilities to customers [3]. We rely on petroleum as the least expensive way to derive liquid fuels for transportation. Liquid fuels are generally preferred for vehicles because they store energy more compactly than gaseous fuels or batteries. But gasoline internal combustion engines are far from ideal power sources for transportation. The Carnot efficiency for a gasoline internal combustion engine is about 37%, but actual gasoline engines are only about 20% efficient.

When petroleum fuels are burned, we recover energy stored millions of years ago by photosynthesis of carbon dioxide from the atmosphere, that is, we are tapping into and perturbing the world’s carbon cycle. Gasoline is the primary fuel for cars and light trucks in the US and diesel fuel is the primary fuel for heavy transport. Liquefied Petroleum Gas (LPG) is a mixture of propane and butane which is liquid at room temperature if compressed. It is a by-product of petroleum refining, and so is plagued by the same supply issues as other petroleum fuels. Natural gas, which is predominantly methane, the simplest hydrocarbon, is an excellent fuel, can be readily burned in internal combustion engines, contributes least to global warming, and is about two-and-one-half times cheaper per unit of energy than gasoline. World proven reserves of natural gas are roughly equivalent to a trillion barrels of petroleum with much of the proven reserves located in the Middle East and Russia. Natural gas can be liquefied for international shipment in cryogenic tanker vessels. There are terminals in the US for receiving liquified natural gas, but only about 1% of our natural gas is imported this way. Vehicles store natural gas as a compressed gas. I have personal experience of driving a natural gas car on a 300 mile round trip without having to use the gasoline backup tank. Some public figures advocate much more use of natural gas in US transportation. Completely replacing the gasoline consumed in the US with natural gas would cut our estimated domestic reserve of natural gas from 90 years worth to about 50 years worth.

Alcohols like methanol, ethanol and butanol are also potential replacements to gasoline. Methanol has the disadvantages of lower energy content, higher volatility, and toxicity, but is more easily produced. Ethanol is the most developed non-fossil fuel, but its energy content is about 39% less than gasoline, it cannot be transported in pipelines designed for gasoline, and its production has an effect on food prices. Butanol is generally compatible with gasoline infrastructure, but is more difficult to produce than ethanol or methanol.

Hydrogen has received a great deal of attention as a potential alternative fuel. Produced from water using electrolysis, hydrogen can fuel internal combustion engines directly, or power fuel cells to make electricity for electric motors. When the hydrogen is burned in an engine or utilized in a fuel cell, water is produced, thereby reversing the electrolysis reaction. Because the electrolysis process is currently too costly, hydrogen is commercially produced from natural gas. Technical improvements to the electrolysis process are being developed, as are biological and solar/catalytic approaches that could also enable hydrogen production from water [4]. While this sounds promising, but there are significant problems with hydrogen. Hydrogen manufacture requires other energy sources, such as electricity, solar energy, or natural gas. Hydrogen does not liquefy at reasonable temperatures, so bulk distribution would probably need to be done in the gaseous state. Distribution is problematic due to the low volumetric energy content and high reactivity of hydrogen [5]. Despite the drawbacks, there is considerable allure to the prospect of a "hydrogen economy. Honda will lease 200 hydrogen-fuel-cell vehicles to California residents for $600 per month, and has developed home refueling stations that plug into domestic natural gas lines. The Honda vehicle stores its hydrogen in 5000 psi tanks and has an advertised range of 200 miles.

Using ammonia as a fuel has attracted some adherents. Complete combustion of ammonia yields only water and nitrogen, so in principle, ammonia engines can be non-polluting. Production methods for ammonia either require methane or large inputs of electrical energy. Another problem with ammonia is its toxicity. The permissible exposure limit is 35 parts per million, and extremely high levels of exposure can result in death [6]. Nonetheless, one study has concluded that the risks of ammonia transport are no greater than the risks of transporting gasoline or LPG [7].

What about synthetic fuels?

Gasoline, diesel fuel, natural gas, jet fuel, ethanol, methanol, butanol, and hydrogen can all be manufactured synthetically. Chemical methods of manufacturing liquid fuels from coal have been known since the 1920’s. The Fischer-Tropsch process, or similar methods, can be used to synthesize liquid fuels from coal, tar sands, or biomass. The current worldwide production capacity of synthetic fuels is about 240,000 barrels per day, equivalent to about 0.3% of the world crude oil production of about 70 million barrels per day. Germany produced up to 120,000 barrels per day of synthetic fuel during World War II. The US had an active synthetic fuels program in the early 1950’s with a plant in St. Louis, Missouri, producing 1.5 million gallons of synthetic gasoline from coal between 1949 and 1953. The program was de-funded by Congress in 1953, partly as a result of lobbying by the National Petroleum Council. A recent study concluded that a 50,000 barrel per day, coal-to-synthetic-diesel plant could produce a return on investment of almost 20% [8]. One drawback to these processes is that they typically consume other finite resources like coal. While the US has about 250 years worth of coal reserves at current consumption rates, conversion to a coal-based transportation system would put a strain on reserves and greatly damage efforts to limit greenhouse-gas emissions.

What about Biofuels?

In a sense, biofuel technology converts solar energy into fuel using biological processes. There is enough solar energy falling on an area 40 miles by 40 miles to substitute for the energy expended by all the cars in the US, so it is not totally crazy to search for practical biofuel technologies. Biofuel technology has the additional advantage that it need not contribute to greenhouse gas emissions and may actually serve to reduce them. Biofuel technology can be lumped into three general approaches: (1) crop based, which uses only the kernel from the plant; (2) cellulosic, which uses the whole plant; and (3) photo-bioreactor, which grows simple organisms in ponds or containers. The best developed biofuel is ethanol, which has reduced the Brazilian need for gasoline by an approximate factor of two. The US ethanol program, which is based on ethanol production from corn, produced about 10 billion gallons in 2009, or about 2.6% of the US consumption of gasoline. Although this program has many detractors, it has reduced importation of crude oil to some extent. Both the US Navy and Air Force have aggressive programs to develop biofuel mixtures for jet aircraft and ships

The technology for producing large quantities of ethanol or methanol from cellulosic materials like switch grass or wood chips is not mature. One approach involves breaking the cellulose down into glucose with mild acids, enzymes, or fungi; followed by fermentation. A company in Canada produced about 150,000 gallons of ethanol from straw using an enzyme process in 2009. Despite significant funding from the Department of Energy and private investors, a plant to synthesize ethanol from wood chips recently closed its doors without producing any ethanol [9].

The production of liquid fuels from micro-algae has attracted some adherents, chiefly because the processes appear to be scalable. The Department of Energy estimates that the US requirement for liquid fuels could be satisfied by dedicating 15,000 square miles of land to micro-algae farming, which is less than one-seventh the land currently dedicated to corn production. Unfortunately, production of fuel from micro-algae is not cost competitive; the ponds or bioreactors are difficult to keep clean and harvesting is problematic.

Electric "fuel Options

Electricity is a serious contender as an alternative fuel, particularly for urban commuting. Electric motors can have efficiencies close to 90%. Therefore, the amount of energy that must be stored by a fuel cell or battery-powered vehicle is about a factor of four less than the energy that is required for an internal combustion engine vehicle (although not necessarily in volume or weight). A distribution system for electrical energy already exists, and the per-mile fuel cost for electricity is about a factor of four to five times less than for gasoline. The electrical generating capability of the US would not have to undergo a drastic expansion to accommodate electric cars, partly because charging can be done off-peak.

A number of types of electric vehicles are now available. Hybrid vehicles provide a way to recapture some of the energy that would otherwise be lost in braking, and apply it to the next cycle of acceleration. Because of the power boost provided by the electric motor, the gasoline engine can be smaller than it would otherwise need to be, and smaller engines require less fuel. The Toyota Prius and some other hybrids have designs that allow the electric motors to contribute power over a wide range of vehicle speeds. Other hybrids use a simpler, but less beneficial scheme that utilizes the electric motor only at low speeds. Currently, hybrid vehicles use nickel-metal-hydride batteries, which are better developed and less expensive than lithium ion batteries.

The weight of batteries has traditionally been a problem for electric vehicles. The EV1 built by GM and Honda in the 1990’s had 1200 pounds of lead acid batteries, which was almost half the weight of the vehicle. Lead-acid batteries store only 35 watt-hours per kilogram. Lithium-ion batteries achieve about 150 watt-hours per kilogram at the cell level. The Chevrolet Volt uses about 400 pounds of lithium ion batteries to achieve its 40 mile (electric only) range. Other battery technologies, such as lithium-air and lithium-sulfur are theoretically capable of storing up to 5000 watt-hours per kilogram [10]. But those batteries are in a very early stage of development. It is possible that greatly improved battery technology will be available for cars in the future. At current gasoline prices the fuel cost for an electric vehicle is about three to five times cheaper than a gasoline powered vehicle with similar characteristics.

Plug-in hybrids allow some energy to be stored in the battery from a charging station or normal household plugs. After-market kits for adapting the Prius for plug-in operation are available. GM calls the Volt an extended-range electric vehicle; its electric motors and batteries are sufficient to support electric-only operation for a significant distance. The EPA rating for gasoline-only operation of the Volt is 37 miles per gallon. Nonetheless, owners who only occasionally take more than short trips could see an effective gas mileage of over 200 miles per gallon. The keys to success for these vehicles in the market place will be reliability and acquisition costs.

If the motor-generator system in the extended range electric vehicle is replaced with additional batteries, one then has an all-electric vehicle (EV). Tesla Motors in California has been producing a high performance all-electric sports car since 2008. That vehicle is one of the fastest accelerating production cars in the world: zero to 60 mph in 3.9 seconds. It has a range of 236 miles and costs about $101,500, mainly due to the high cost of its lithium-ion batteries. In 2011 Tesla will introduce a sedan that will cost around $50,000 and have a range of 300 miles. Nissan is introducing an all-electric vehicle called the Leaf that will cost about $25,000 after a $7,500 federal subsidy is deducted from the cost. The Leaf has an advertised range of 100 miles.

At this point, all-electric vehicles do not make a lot of economic sense for most people. Even hybrid owners are not likely to recoup the difference in initial cost from fuel savings. With improvements in battery performance, reductions in battery cost, and likely rises in the cost of gasoline, all-electric vehicles will soon become cost competitive for cars and light trucks.

Summary and Outlook

Synthetic fuels and electric vehicles could help us to avoid the worst consequences of diminishing oil supplies and contribute to reducing carbon emissions. The availability of petroleum from tar sands and oil shale will allow us some "breathing room. Natural gas could also provide some relief from petroleum shortfalls, but domestic supplies are finite and probably should be preserved for other uses.

The technical alternatives to petroleum fuels are all problematic, and it is not clear when or if the hoped-for breakthroughs will occur. The one thing that we can most readily do to reduce petroleum imports is conservation. Since 1980 the average horsepower of American light vehicles has doubled and the fuel economy has remained relatively constant [11]. While this is a remarkable engineering achievement, one must ask what improvements in fuel economy would have been possible if the average horsepower had not doubled. The Corporate Average Fuel Economy (CAFE) regulations enacted by Congress in 1975 were largely ineffective in improving the average fuel economy of American vehicles. Those regulations counted SUV’s as light trucks and then promulgated very modest improvements for the "light truck category. In 2007 more aggressive standards were put in to place by Congress. In 2009 the Obama administration proposed even tougher CAFE standards: 39 mpg and 30 mpg for cars and light trucks respectively by 2016. As much as many of us prefer large vehicles or high performance cars, our preference for those vehicles is costly, both personally and collectively. A few decades ago we were able to satisfy personal and business needs without "Super-Duty pickups, SUV’s, and "sport sedans with 300-plus horsepower engines. There are a number of options for improving the fuel economy of gasoline driven vehicles at reasonable costs [12].

Shifting to a hydrogen-based transportation system seems unlikely. There appear to be too many issues with hydrogen for it to be a viable fuel in the 21st century. Despite likely advances in battery technology, liquid fuels will continue to be necessary for heavy transport. The power and energy requirements for trucks and locomotives are too great to contemplate replacement with battery technology. One hopes that synthetic fuels can eventually be available for heavy transport and air travel. Synthetic fuels from micro- algae, genetically modified bacteria, or normal crops are attractive possibilities, but the costs will be high and the past failures in this area have been many. Whatever approach is taken, synthetic fuels are likely to be more expensive than petroleum fuels.

With planning and foresight, civilization can survive the depletion of petroleum resources. It is up to the next few generations to manage the transition to a low-petroleum world economy. The U.S. is particularly vulnerable to disruptions in petroleum supply because our dispersed geography, our current infrastructure, and our mindset of expecting cheap fuel to be available indefinitely. There are those who see efforts at moving the US toward conservation and alternative fuels as naive or unpatriotic. They advocate more domestic production to lessen dependence on foreign sources. Increasing domestic production can lessen our dependence on foreign oil in the short term, but only exacerbates the long-term problem. Transitioning to a low-petroleum transportation sector will not be easy, but it is the only long-term solution. I hope that I have conveyed how difficult it will be. By doing good technical development and laying the groundwork now, we can leave an appropriate legacy for future generations.


[1] Thomas S. Ahlbrandt, Global Resource Estimates from Total Petroleum Systems (AAPG Publication ISBN 0-89181-267-5, 2005). Different values can be found in other sources; Wikipedia, for example, cites a "proven reserve of 1.2 trillion barrels for the top 17 national reserves.

[2] Ibrahim Sami Nashawi, Adel Malallah and Mohammed Al-Bisharah, "Forecasting World Crude Oil Production Using Multicyclic Hubbert Model, Energy Fuels, 24(3) 1788–1800 (2010); Michael Moyer, "How Much is Left?, Scientific American, 30(3), September 2010.

[3] Energy Information Administration, Annual Energy Review 2009.

[4] A. Regalado, "Reinventing the Leaf, Scientific American 303(4), October 2010.

[5] R. Muller, Physics for Future Presidents, (W.W. Norton and Company, 2008), pp. 302-304.

[6] Airgas Material Safety Data Sheet No. 001003, May 2011.

[7] N. J. Duijm et al, Safety Assessment of Ammonia as a Transport Fuel, Riso Report No. 1504(EN), Risø National Laboratory, Roskilde, Denmark, February 2005.

[8] L. Van Bibber, Baseline Technical and Economic Assessment of a Commercial Scale Fischer-Tropsch Liquids Facility (DOE/NETL-2007/1260, April 9, 2007, p. 5).

[9] David Biello, "The False Promise of Biofuels, Scientific American 305(2), August 2011. As an example of some of the research being conducted in this area, see

[10] J. Markoff, "Pursuing a Battery So Electric Vehicles Can Go the Extra Mile, New York Times, 14 September 2009.

[11] U.S. Environmental Protection Agency, Light-Duty Automotive Technology, Carbon Dioxide Emissions, and Fuel Economy Trends: 1975 Through 2009, November 2009.

[12] Ben Knight, "Better Mileage Now, Scientific American 302(2), February 2010.

Danny J. Krebs
Department of Physics, Saginaw Valley State University

Danny Krebs is retired from the NASA Goddard Space Flight Center in Greenbelt, MD where he was a lead engineer for space flight laser and detector systems. Previously he was at McDonnell Douglas Electronic Systems Company in St. Louis, where he did research leading to the first all-solid-state, pulsed high power laser. He has a B.S. in Engineering Physics from the Colorado School of Mines, M.S. degrees in Engineering Management and Physics, and a Ph.D. in Physics, all from the University of Missouri-Rolla (now Missouri University of Science and Technology). He has taught physics at Saginaw Valley State University as both an Adjunct Professor and full time faculty.

This paper is adapted from a talk presented by the author to the Torch Club International Annual Convention, 25 June 2011.

These contributions have not been peer-refereed. They represent solely the view(s) of the author(s) and not necessarily the view of APS.