Herbaceous energy crop development: recent progress and future prospects

Oil prices and government mandates have catalyzed rapid growth of nonfossil transportation fuels in recent years, with a large focus on ethanol from energy crops, but the food crops used as first-generation energy crops today are not optimized for this purpose. We show that the theoretical efficiency of conversion of whole spectrum solar energy into biomass is 4.6–6%, depending on plant type, and the best year-long efficiencies realized are about 3%. The average leaf is as effective as the best PV solar cells in transducing solar energy to charge separation (ca. 37%). In photosynthesis, most of the energy that is lost is dissipated as heat during synthesis of biomass. Unlike photovoltaic (PV) cells this energetic cost supports the construction, maintenance, and replacement of the system, which is achieved autonomously as the plant grows and re-grows. Advances in plant genomics are being applied to plant breeding, thereby enabling rapid development of next-generation energy crops that capitalize on theoretical efficiencies while maintaining environmental and economic integrity.