With all the recent news around the Paris climate agreement and the US Clean Power Plan, one key component of the renewable energy puzzle that hasn't been addressed is the transportation sector. Transportation accounts for approximately 30% of the total energy consumed in the United States and, correspondingly, about 30% of the greenhouse gas emissions (see figure for 2013 greenhouse gas emission data from EPA). Yet very little has been said about how the country and world will transition from a fossil fuel based transportation system to renewables.
The EPA has identified 4 key opportunities
to reduce greenhouse emissions from transportation: 1) use of alternative fuels, 2) improvements in engine fuel efficiency, 3) improvements in operating practices, and 4) reduction in miles driven. To date, improvements in all these areas have involved marginal advancements as illustrated in the following figure which tends to show that advancements over the past 15 years have not kept pace with the additional miles driven.
More of the same won't be sufficient to achieve the reductions necessary in this sector. The solution with the most promise today for making a step change in results is the electric vehicle (EV). However, EVs have a number of critical challenges to overcome before they can be adopted in large enough quantities to make an impact. 1) There aren't very many models to choose from. Although most major manufacturers are now offering an EV model, that is compared to multiple models with gasoline engines. 2) They are expensive. Base prices range from about $22,000 for the Smart Electric Drive to more than $130,000 for the Tesla Model X. Even with $7,500 in federal tax credits, the cost is typically thousands more than similarly-sized gas-powered cars. Although the cost per mile to drive an EV, both in terms of power and maintenance cost, is significantly less than a gas powered vehicle, consumers typically make decisions based on initial cost. 3) They still have limited range. Typical range is 60-100 miles on a charge. The typical American drives less than 50 miles a day, but the restriction on being able to take a long trip limits flexibility. 4) They take a long time to charge. Using a typical 240-volt charger, it takes between 4.5 and 6 hours to fully charge an EV battery. Specialty charging facilities are coming on the market, but charging will still take many times longer than the 5 minutes to fill up at the gasoline pump. And 5) The charging infrastructure is still in its infancy as well. Any long trip will require careful planning and logistics, which are not things that Americans are used to or want to do.
Another concern at this time is that in most areas in the country, the electrical power that is generated to charge an EV battery results in more greenhouse emissions than would result from a comparable gasoline powered vehicle traveling the same distance. See Our World of Energy How much more environmentally friendly is an all-electric car than a gas powered car?
To have the greatest effect, the transition to an electrically powered transportation sector must be linked with the overall transition of the electrical power generation system to renewable energy sources.
And one final thought - there are certain segments of the transportation sector where conversion to renewable fuel isn't realistic with today's technology. In particular, aviation and shipping are likely to always be dependent on petroleum based fuel. Therefore, a 100% renewable energy future is probably unlikely, which supports OWOE's premise that the future will require an appropriate mix of all forms of energy.