Harvesting the Sun: A Biological Approach

HARVESTING THE SUN: A BIOLOGICAL APPROACH J. SHEN-MILLER* And pluck till time and times are done . . . The golden apples of the sun. [William Butler Yeats, "The Song of Wandering Aengus"] The exploitation of natural reserves by man and the squandering of energy in our daily living will soon leave an earth barren of riches. On the day when we use up our last drop of oil, what will the landscape be? Will it be fields of luscious green trees and plants, or will it be a bleak scene of nuclear plants? A happy medium would be a combination, and now is the time for an interdisciplinary collaboration between biologists and engineers to seek and apply other resources, resources such as the energy ofthe sun, in fuel production. The annual radiation that reaches the earth's surface is 3 x 1021 kj, which is four orders of magnitude greater than the total global energy consumption of 1970 [I]. The United States seems to see no great urgency in developing this renewable resource, but I hope this view will change. It cannot be emphasized enough that this is the time to give this matter a high priority and substantial funding. According to one estimate, the times. for average depletion rate of global coal, oil, and gas reserves are 150, 50, and 49 yr, respectively [2]. Judging from the demand for energy, with the assumption ofan increasing consumption per capita, the fossil fuel reserves of the United States have a depletion time of 500 yr [3, 4]. Nuclear power is most likely the major practical hope, but the development of reactors and other energy sources (geothermal, wind, etc.) will take time, and alternate approaches are just as essential for supplementing energy needs. (Even the diehard nuclear enthusiasts do not wish to put all our eggs in the nuclear basket.) Tapping of solar energy for fuel is just such an alternative. This article *I received support for this work from the U.S. Energy Research and Development Administration as a member of the Division of Biological and Medical Research, Argonne National Laboratory, Argonne, Illinois 60439. I am grateful to Dr. D. Miller, Total Systems , for stimulating discussions and for furnishing some ofthe current references, and to Drs.J. F. Thomson, Argonne National Laboratory, and R. S. Bandurski andJ. W. Hanover of Michigan State University for critical comments. Perspectives in Biobgy and Medicine ¦ Autumn 1977 | 77 will cover only the biological harvest from the sun; photoelectric and photochemical approaches are other means of harvest [5, 6]. Green plants, converting energy from the sun, synthesize cellulose, which is the most abundant organic compound on earth; about 1011 tonnes (1 tonne is equivalent to 2,204 lb, or 1.102 ton) are produced annually [I]. Cellulose is a clean fuel which does not add accumulated Co2, sulfur, and radionuclides into the atmosphere [7, 8], and it is a renewable resource. The major sources of cellulose come from agricultural , forest, and aquatic production, most of which are disseminated into agricultural, industrial, and municipal wastes. The contribution of individual sources ofcellulosic waste is seemingly minute, a fraction ofa percent of the total energy demand; however, adding them together could amount to 8% ofthe total estimated U.S. energy demand for 2000 and a monetary value of $48.3 billion (assuming an average retail cost for 2000 to be $2.84/10* kj [9]). The above estimate is based on the cellulosic waste alone. If large-scale "energy farming" is employed and research (basic and applied) in this area receives greater support, the increase in the supply of cellulose will be manyfold greater. Although nuclear power plants play a dominant role in fuel development, presently they contribute only 3% of the total energy supply in the United States [9]. In this article, I will discuss the various cellulosic sources, gross fuel production from each source, some ofthe fuel production processes and problems, and the necessary areas of biological research. Cellulosic Sources AGRICULTURAL Crop and crop residues constitute a sizable amount of the total cellulosic material generated each year. Among the high-yield crops, in terms of biomass production, are corn, sugarcane, sorghum, etc. The corn crop yields about 14...