Our Renewable Future

Chapter 4. Transportation: The Substitution Challenge

Liquid petroleum is the world’s dominant energy source. Oil is energy-dense, portable, and easily moved by pipeline and tanker—characteristics that have made it very well suited as a transportation fuel. Further, during the twentieth century it was amazingly cheap. During the 1980s, for example, a barrel of oil, which contains 1700 kWh of energy (the equivalent of over 10 years of hard human labor[1]), cost a mere $35 in inflation-adjusted dollars. Cheap transport energy helped fuel globalization, one of the most significant economic trends of the past few decades. Today, transportation accounts for 41 percent of U.S. energy end use, and over 95 percent of that transportation runs on oil.[2] The vast majority of cars still burn oil-derived fuels, as do airplanes, ships, trucks, and rail locomotives.

Trade depends upon the transport of raw materials and finished products. While movement of money can be effected electronically and almost instantaneously, the physical economy that money symbolizes requires wheels, roads, rails, rudders, landing strips—and the oil that lubricates and fuels transportation. Because oil has been so plentiful and cheap for the past century, we have globalized the production of most of the goods we depend upon. In 2011, US ports took in $1.73 trillion in goods, 80 times the value of all US trade 50 years ago.[3]

WEB Image 4-1 combine harvesters
Combine harvesters in 1902 and 2014. (Credit, top: Robert N. Dennis, New York Public Library. Credit, bottom: (cc-by) Martin Pettitt, via flickr.)

In addition, oil is critical to our industrial food system. While much food-system oil use is for the transport of farm inputs and outputs, disesel and gasoline are also used to power tractors and other on-farm machinery. Of the ten calories of industrial energy used to grow, transport, process, refrigerate, and cook the average calorie of food in the United State, 21 percent come from oil. [4]

If the transition to renewables is to succeed, it must address these systemic dependencies on liquid fuels. As we will see, there are efforts underway to do this, but enormous challenges remain.


WEB Image 4-2 bus batteries
Overhead and undercarriage recharging for electric buses. (Credit, top: Oliver.auge, via Wikimedia commons). Credit, bottom: Spsmiler, via Wikimedia commons.)

Since solar and wind energy technologies produce electricity, an obvious solution to our oil dependency for transport is to electrify transportation. There are currently roughly 750,000 road-legal electric vehicles (EVs) globally, and the rate of growth in the market is a spectacular 76 percent annually.[5] The United States has seen a growth rate of 69 percent annually, with about 300,000 vehicles now running on batteries.[6] At this growth rate (roughly a doubling every year), the EV market in the United States could grow to equal the size of the current auto fleet in just a decade—though almost no one expects that to happen, as about half the gasoline-powered cars now in service will still be operational in ten years, and the vast majority of automobiles still being sold have conventional combustion engines.

However, electric cars suffer from the inherent inefficiency of all personal motorized, road-based transport: the need to move a one- to two-ton vehicle in order to transport a few hundred pounds worth of people. The vehicles themselves represent large amounts of embodied energy, up to 75 MWh each.[7] That means today’s global fleet of over one billion automobiles represents roughly 50 million GWh of energy, many times the total amount of renewable electricity produced in 2014.[8] Further, road building and maintenance also require energy: a meta-study at the University of Washington found that a reasonable estimate of total energy consumption for these purposes is between 0.5 and 1 GWh per lane-kilometer of paved roadway.[9] The vast majority of this energy expenditure is in the form of oil. With 65 million kilometers of paved roads worldwide, that represents up to 65 million GWh of energy, as much as is embodied in the world’s cars.[10]

The strategy of electrifying transport entails both opportunities and obstacles. The opportunities start with the fact that 1 kWh of energy will propel a typical electric car 2.94 miles, compared to 0.83 miles for a similarly sized gasoline-powered vehicle.[11] That’s because electric motors are very efficient compared to internal combustion engines.

However, the heart of the electric vehicle is its battery. And as we’ve already seen, though battery technologies are subject to innovation, falling prices, and increasing storage per unit of weight, nevertheless even the best theoretical battery has very low energy density compared to petroleum-based fuels. The result is that batteries work best in small, light vehicles. While a large majority of vehicles on the road are used to move people, 99.9 percent of the total weight being transported on US roads (not counting the vehicles themselves) is goods that we consume.[12] But large, heavy vehicles such as trucks, tractors, and cargo ships require batteries too heavy to be practical in most instances, particularly if they are traveling long distances. Meanwhile, battery-powered aviation (except in the case of one- or two-passenger aircraft) is simply not an option.

Many railways are already electrified (in 2012 half of all rail tonnage worldwide was carried by electric traction), and this is accomplished without batteries: electricity is distributed to locomotives via an overhead line or a third rail. Electrified rail offers the advantages of greater efficiency and lower operating costs as compared to diesel power, though initial capital costs for electrification are relatively high. A few companies have proposed building electrified highways in order to bring similar efficiencies to electric trucking, though once again infrastructure costs would be significant.[13]

Electric city buses and streetcars that draw their power from overhead wires have been in use for over a century. A new generation of battery-powered electric buses (some of which recharge at bus stops) is gaining in popularity—including on some bus rapid transit (BRT) systems, which feature high-capacity buses operating on dedicated corridors, with off-board fare collection, and station platforms level with the bus floor. Electric long-haul buses and electric trains would work for intercity trips. Electric vans running as jitneys or taxis, electric car-shares,[14] and small electric buses would be ideal for interneighborhood trips.

Over the short run, transport electrification is likely to take the form of increasing numbers of electric cars, though that trend may ultimately be limited by the inherent inefficiencies of personal automobile ownership and operation, and by the need for oil in building and repairing highways. Electrified rail could theoretically replace trucking and domestic aviation and move many more people than it currently does; however, the United States—with its decrepit existing passenger rail networks—is at a distinct disadvantage in this regard. Substantially expanding electric rail and buses will entail up-front infrastructure costs, embodied energy, and time for build-out. This implies a multidecade effort involving large initial subsidies, presumably from government.


Where batteries are unsuited for transport (e.g., in heavy road vehicles, ships, and aircraft), why not use biofuels? These are renewable fuels made from food crops, agricultural and forestry residue, or algae, usually in the forms of ethanol, biodiesel, or methanol. The United States already produces 14.34 billion gallons of ethanol and 1.27 billion gallons of biodiesel annually,[15] roughly 10 percent of the gasoline consumed in 2014.[16] Could these fuels be used to run the nation’s fleets of aircraft, ships, and trucks?

Since aviation, which represents 2 percent of global carbon emissions,[17] cannot be electrified, let’s focus on this industry. The direct use of ethanol or biodiesel in current aircraft engines is impractical because these fuels do not have the right chemical characteristics (biodiesel, for example, tends to become highly viscous at low temperatures). However, the aviation industry is engaged in ongoing experimentation with several chemical pathways toward a replacement fuel, the most common of which is based on refining oils extracted from plants or from algae. Airlines were approved to use up to a 50 percent blend of such fuels in 2011, and both test and commercial flights have been successful. Research continues into an alcohol-based pathway, as well as through synthetic biology. Virgin Atlantic has announced its intention to test an alternative aviation fuel reputed to have half the carbon footprint of the standard jet kerosene from a production process that ferments carbon monoxide–rich gases from industrial steel production into ethanol, with further conversion to jet fuel using the LanzaTech process.[18]

The challenges for the growth of the biofuels industry are environmental tradeoffs, cost, scalability, and energy profitability. These challenges are all closely interrelated. Let’s examine each of them, then return to a consideration of the task of running global aviation on a renewable replacement for refined petroleum.

Environmental Tradeoffs

Nearly all commercial biofuels are currently produced from food crops. In the United States, most ethanol is produced from corn, while Brazil grows sugar cane for this purpose. Global feedstocks for biodiesel include soybeans, palm oil, and jatropha. Producing these fuel crops has led to controversies about the diversion of land from growing food for people to making fuel for vehicles. Soil degradation, water use, and biodiversity loss are also implied in current agricultural biofuels production. Tellingly, in April 2015 the European Parliament voted to limit the use of crop-based biofuels due to impacts on food prices, hunger, forest destruction, land consumption, and climate change.[19]


New methods of biofuels production from agricultural and forestry residues, or from algae, have been proposed as solutions to the environmental tradeoffs of current biofuels production. However, so far these second- and third-generation biofuels have proven too costly to produce commercially. The US Energy Independence and Security Act of 2007 established cellulosic (second-generation) biofuel mandates for succeeding years; each year, the Environmental Protection Agency has had to waive most of that mandate due to the inability of the industry to profitably produce sufficient fuel. Actual 2015 cellulosic biofuel production was less than 100 million gallons, compared to the original 3 billion gallon mandate.[20] Biodiesel from algae (a so-called third-generation biofuel) has likewise proven more difficult to produce profitably than was forecast by the industry, with the break-even price currently stuck at about $7.50 per gallon.[21]


Unless and until second- and third-generation biofuels become commercially viable, the production of biofuels will continue to depend upon the conversion of land from forest or food production to fuel production. Given a growing human population with growing food requirements, as well as increasing concerns regarding loss of biodiversity, expansion of biofuels production beyond current levels creates unacceptable pressures for further conversion of land to dedicated fuel use.

In the case of jet fuel, the capacity to produce renewable alternatives based on plant-based oils is severely limited by the sheer magnitude of global jet fuel consumption. In 2012, global jet fuel use totaled about 250 million metric tons,[22] while worldwide production of all edible oils reached 161 million metric tons.[23] Given refining losses, diversion of even all global food oils to jet fuel production could supplant only about half of total current jet fuel use.

Energy Profitability

The last of these four factors heavily influences the previous two, and is generally acknowledged as the greatest economic hurdle to expanded use of biofuels. Calculated energy returned on energy invested (EROEI) figures for corn ethanol production in the United States range from less than 1:1 to 1.8:1—which falls below a proposed 3:1 threshold of economic viability for an energy resource.[24] Ethanol from sugar cane in Brazil is calculated to have an EROEI of 3.6:1 to 4:1, but when made from Louisiana sugar cane in the United States, where growing conditions are worse, the EROEI is closer to 1:1.[25] Distillation is highly energy intensive, and even more so in the case of cellulosic ethanol because the initial beer concentration is so low (~ 4 percent compared to 10–12 percent for corn). This dramatically increases the amount of energy needed to boil off the remaining water. At absolute minimum, 15,000 BTU of energy are required in distillation alone per gallon of ethanol produced (current corn ethanol plants use about 40,000 BTU per gallon).[26] This sets the limit on EROEI. If distillation were the only energy input in the process, and it could be accomplished at the thermodynamic minimum, then EROEI would be about 5:1. But there are other energy inputs to the process, and distillation is not at the thermodynamic minimum.

Soybean biodiesel currently returns 3.6 to 4 times the energy that is used to produce it, if co-products are credited.[27] Palm oil biodiesel appears to have the highest energy profitability, but it also has the highest environmental impacts. The EROEI of algal biodiesel has not been accurately calculated, since production is not yet occurring at an industrial scale; however, the high currently calculated break-even market price for the fuel suggests very low energy profitability.

When a great deal of energy has to be invested in an energy production process (in the case of ethanol, that investment includes plowing, seeding, fertilizing, harvesting, transporting, and distilling), this also implies greenhouse gas emissions. Thus, in the United States the use of ethanol to replace gasoline may reduce overall emissions only minimally.

Now, let’s return to our discussion of the potential for biofuels in aviation. Clearly, it is physically possible to manufacture such fuels. However, doing so at the scale required to support the industry at its current size, without unacceptable environmental and social impacts, and at a sufficient energy profit so that fuels are affordable without massive financial and energy subsidies, will pose challenges at every step along the way.


WEB Image 4-3 Honda_FCX_platform
Hydrogen tanks in a Honda PCX prototype. (Credit: Morio, via Wikimedia commons.)

Hydrogen may be a practical fuel for transport uses in marginal applications. Toyota has unveiled the first mass-market hydrogen car, and California has already installed a tiny network of hydrogen fueling stations, which it promises to expand. Energy futurists have long predicted a “hydrogen economy,” and some would say we are finally seeing the very first glimmer of its dawn.

Why has there been such a long wait? It turns out there are many potholes on the hydrogen highway. The first is the problem of getting the hydrogen: most commercial hydrogen is currently made from natural gas. Making hydrogen from water using renewable electricity implies substantial energy losses, with current hydrolysis systems averaging 65 percent efficiency.[28] Other production routes, such as biological water splitting, fermentation, solar thermal water splitting, and biohydrogen, are being researched, but none has achieved commercialization.[29]

The next hurdle is storage. Because hydrogen has a very low energy density per unit of volume, hydrogen-powered airplanes would need to carry compressed hydrogen in large storage containers that would add substantially to the size of the aircraft. Further, tanks will inevitably tend to leak, because the hydrogen atom is the smallest of all atoms and can eventually work its way through just about any material used to contain it.

Moreover, since conversion of energy is never 100 percent efficient, converting energy from electricity (e.g., from solar or wind) to hydrogen for storage before converting it back to electricity for final use will inevitably entail losses.

The problems with hydrogen are substantial enough that many analysts have concluded that its role in future energy systems will be limited (we are likely never to see a “hydrogen economy”), though for some applications it may indeed make sense. Energy storage using hydrogen fuel cells achieves a higher energy stored on investment (ESOI) ratio than battery storage (though not as high as pumped geological storage),[30] so some utility companies may end up using hydrogen storage systems for overgeneration in preference to battery banks.

Some 100 percent renewable energy plans are counting on cryogenic hydrogen to entirely solve the aviation problem, but the challenges of significantly replacing oil as a transport fuel with hydrogen are such that these should be viewed with caution. Not just fuel systems, but entire airplanes would need to be redesigned, with fuel tanks four times larger than today’s, and the fuel would likely be significantly more expensive.

Hydrogen is also being considered as a fuel for ships.[31] In shipping, the requirement for larger fuel tanks might not pose as much of a problem as in aviation; but high fuel costs would perhaps be even more of a burden, to which the inevitable leakage of hydrogen during long voyages at sea could only add.

Natural Gas

The challenges of finding renewable substitutes for liquid fuels have led some analysts to consider the use of compressed natural gas (CNG) as a bridge fuel to a renewable future. It could have the advantage of producing somewhat less greenhouse gas emissions; and CNG could fuel long-haul trucks, earth-moving and mining equipment, buses, and tractors. Indeed, some conversions are already under way (thousands of natural gas–fueled buses are already on the road, and FedEx and UPS are currently refitting their truck fleets).

However, natural gas is of course a fossil fuel. That ensures the dual problems of continuing greenhouse gas emissions and depletion of the resource base. The latter may be decisive from an economic standpoint. Conventional wisdom holds that the United States has an abundance of natural gas as a result of the opening of shale reservoirs with hydrofracturing. However, nonshale natural gas production in the United States is in steady decline, and research at Post Carbon Institute suggests that a peak in domestic shale gas production is likely later this decade, followed by ongoing production declines.[32] A full repeat of the recent US shale gas boom elsewhere in the world is unlikely due to a lack of infrastructure, expertise, and favorable private mineral rights ownership regimes. According to the independent German analytic organization Energy Watch Group, global natural gas production is likely to peak during the next decade.[33]

If renewable energy sources like solar and wind replace natural gas for power generation, this could free up some natural gas for the transport sector. However, even this reallocated resource availability would be temporary, perhaps providing a window of another decade in the United States—roughly the amount of time required to fully build out the new fleet of CNG vehicles. Thus, by the time the latter were ready to go in full force, their fuel supply would be uncertain. Natural gas could prove to be a bridge to nowhere.

Sails and Kites

Shipping consumes only 7.4 percent of oil used annually,[34] but it accounts for 90 percent of global trade, which in its current form would wither almost instantly in the absence of petroleum. Would it be possible to maintain high levels of trade using wind power, via sails or kites?

Kites can be flown at altitudes of 100–300 meters (330–980 feet), where winds are much stronger than at water surface; thus they receive a far higher thrust per unit area than conventional mast-mounted sails. SkySails, a Hamburg-based company, currently sells equipment to propel cargo ships, large yachts, and fishing vessels with sail-kites. Ships using the system maintain use of their oil-fueled motors, making them hybrid vehicles. The use of kite sails is estimated to reduce fuel consumption by 10 to 35 percent.[35] As use of the technology expands and evolves, and as ships are redesigned to use it more effectively, efficiencies may improve.

A more intensive return to wind-based ocean transport is being proposed by the Sail Transport Network,[36] which promotes both short-haul local deliveries using small boats in places such as the Puget Sound in Washington State, and major cargo deliveries using sail-powered or sail-assisted vessels. Meanwhile British wind power company B9 has tested the design elements of a planned 100-meter, 3000-ton carbon-neutral freighter that uses 60 percent wind power, relying on three computer-operated 55-meter masts supplemented by a biogas engine converting food waste into methane. B9 sees the plan as working best on small freighters.

A return to wind would almost certainly entail slower averaged speeds. However, in order to lower running costs during periods when oil prices are high, cargo ships are already accustomed to reducing speed to 12–15 knots, which makes them slower than the sail-powered clipper ships of the late nineteenth century.[37] Sailing boats also have to wait for the right winds, tides, and currents. In addition, they may need retractable masts if they have to go under bridges. Many of the seafaring skills of our ancestors may have to be rediscovered or relearned.[38]

Summary: A Less Mobile All-Renewable Future

The most likely adaptive strategy for the transport sector as we move toward an all-renewable future will entail all of the above. Farmers will likely use site-made biofuels to power agricultural machinery. Most cars will run on batteries, a few on fuel cells. CNG will be used for large vehicles until natural gas becomes too expensive or until engineers come up with a better option. Ships will employ more kites and sails. At least some aircraft will burn expensive, sophisticated biofuels, again until engineers find a better solution—if there is one.

This is not a very satisfying conclusion, for several reasons.

First, the electrification of transport (directly or via hydrogen or batteries) will put a significant extra burden on solar and wind technologies, requiring them to power not only much of the posttransition electricity sector but a large portion of the transport sector as well.

Second, we have not addressed the embodied energy in transport—the energy used in manufacturing cars and trucks; in building ships, locomotives, and aircraft; as well as in making roads, airports, rails, docks, and terminals. As problematic as it is to replace the operational energy for transport with renewable substitutes, the challenge for supplying manufacturing energy is probably even greater (as we will see in the next chapter).

Because oil is economically crucial and hard to replace, and because oil is leading the EROEI decline of fossil fuels, more and more energy investment capital will have to go toward maintaining essential existing oil-based energy usage systems, just as massive new investment is needed for renewable energy capacity, energy storage, and grid upgrades. The ballooning need for new investment just for current systems is confirmed (but probably seriously underestimated) in the 2014 International Energy Agency World Energy Investment Outlook report, which concludes that “meeting the world’s growing need for [mostly fossil] energy will require more than $48 trillion in investment over the period to 2035.”[39]

Thus, due to rising oil production costs and declining returns on investment, and the concomitant need to deploy often problematic or limited and expensive alternatives, society will probably become less mobile as the energy transition picks up speed. Even though big container ships use very little energy to move a ton of freight long distances, global trade of material goods will likely decline rather than inexorably grow. Nonproductive use of oil—the operation of personal vehicles and tourism for the middle class—will fare far worse. The implications of liquid fuel substitution limits for industrial agriculture are especially worrisome in a world of continually expanding human population. For transport, trade, and agriculture, renewable energy options exist—as we have seen—but they tend to be slower, more expensive, or supply constrained.


previous      next