In modern society, everything from transportation to commerce to food supply is heavily dependent on the availability of cheap and plentiful energy supplies. In the past few years many have realized that the traditional sources of energy — oil and gas — are in limited supply and that we need to prepare for the approaching production maxima. It is in the interest of national economic security to investigate alternative sources of transportation energy before the extraction of existing supplies becomes prohibitively expensive. This meta-study investigates a number of potential fuels and their sources, including: • agricultural solutions - ethanol (corn and cellulosic), • agricultural solutions - biodiesel, • unconventional refining techniques such as coal-to-liquid, • oil shale retorting and tar sand processing, • traditional petroleum sources. The concentration in the current study is on transportation needs, although it is recognized that building space conditioning and electricity consumption are also significant demands for energy. The results are reported for land use, water use, input-to-output energy ratio, and carbon emissions for each fuel cycle and source. Data are given for the cases of 10, 25, and 50 percent displacements of the 2012 predicted transportation energy needs (i.e., the equivalent of 430 million gallons of gasoline per day). Cradle to grave findings indicate that some novel fuels cannot substitute for conventional fuels without consuming more water or land and emitting more greenhouse gases than fuels in use today. The most sustainable direction for the US transportation fuels sector is suggested.
City-integrated photovoltaics sustainably satisfy urban transportation energy needs