Energy from the biological conversion of solar energy

Abstract

Trees and other forms of vegetation are well designed for the collection and storage of solar energy. Moreover, photosynthetic organisms show enormous diversity and are well adapted for a wide range of environments. Biomass is convertible to liquid and gaseous fuels by a number of established processes, and this paper examines the possible contribution of biomass to world energy demands. The maximum efficiency of solar energy conversion in plant production is 5-6 %, but plants grown under usual field conditions do not achieve this degree of conversion. The highest yielding crops convert solar energy into plant material with an efficiency of 1-2%, but the average yields of the major crops and forests indicate considerably lower efficiencies. The average efficiency of solar energy conversion on a global scale is estimated as about 0.15 %. The energy content of the annual biomass residues in Australia and U.S.A. is equal to about one-quarter of the primary energy use in those countries, but only about one-third of the residues are considered to be readily recoverable. A number of high yielding crops are examined as potential fuel crops. Energy inputs for growing non-irrigated crops in Australia are estimated to amount to 7-17 % of the solar energy stored in the total crop biomass. Irrigation adds considerably to the energy cost of producing crops. The overall energy efficiency of fuel production from biomass varies from 20 to 58%, depending on the nature of the biomass and the process used to produce liquid or gaseous fuel. A recent estimate by an Australian committee of the potential contribution of biomass to liquid fuel production in Australia is presented. It is concluded that biomass will not be able to provide a substantial fraction of the world’s energy demand, although it can make a useful contribution.