ABSTRACT
The two most promising carbon-free and renewable energy resources are solar and wind energy, but these forms of energy are intermittent and therefore can meet only a portion of primary energy demand. With plug-in hybrid powertrains and advances in battery technology, it may be practical to displace a substantial portion of the liquid fuels that would otherwise be used in automobiles, but batteries are unlikely to be viable as a replacement of carbon-based fuels for long-distance passenger travel or for freight transportation, and certainly not as a replacement for carbon-based fuels for air travel. Furthermore, wind and solar energy often do not coincide with the locations of large energy demand. Geothermal energy can provide continuous heating and electricity production, but the resource is limited at a global scale and is not renewable. It could nevertheless provide a non-negligible fraction of present-day energy demand for many centuries. Biomass can provide continuous electricity generation and heat, can be easily stored and can be converted to liquid fuels (ethanol and biodiesel fuel in particular) that could be produced for transportation uses. However, the biomass energy potential is limited, it does not represent the most efficient use of limited land resources for the production of energy and some emissions of air pollutants will remain (particularly in transportation uses).
