Electric Power from Rice Paddy Fields

Abstract

One of the largest innovations in the twentieth century is the use of petroleum (and other fossil fuels, such as, coal, and natural gases) for the energy source that supports the human society. Petroleum is an easy-to-use, energy-intensive fuel as well as a material for a variety of chemical products. It is produced by pumping-up from natural underground reservoirs. In 2006, petroleum and other fossil fuels support 86% of the total energy supply for the human society (Energy Information Administration 2007), indicating that they are currently indispensable for us. Recently, however, humans face problems associated with the use of fossil fuels. One of such problems is the limitation in fossil fuels stored in our planet. It is known that fossil fuels (as the name indicates) had been mostly generated by long-term geochemical reactions with ancient plants and algae as original resources and accumulate in underground reservoirs. Such reactions may also occur currently, while the recent consumption of fossil fuels is much faster than that, resulting in the rapid decrease in the amounts of fossil fuels in underground reservoirs. In addition, a large fraction of petroleum stored in easily accessible reservoirs is considered to have already been consumed, and engineers predict that the cost for producing petroleum will dramatically increase in the 21th century. Another problem results from the combustion of fossil fuels; this process generates carbon dioxide that is released and accumulates in the air, resulting in the green-house effect and global warming. According to an assessment report in 2007 by the Intergovernmental Panel on Climate Change (IPCC), global surface temperature increased 0.74 ± 0.18°C during the 20th century (IPCC 2007). This increase corresponds to an increase in the carbon dioxide concentration in the atmosphere (from 0.03% to 0.037%) in the 20th century (Global Warming Art 2007). We therefore consider that the use of fossil fuels is such a process that converts underground carbons into carbon dioxides and release them into the atmosphere. Under such circumstances, energy sources alternative to fossil fuels are strongly desired for supporting human activities in the 21th century, particularly those that are renewable and not associated with the global warming. The primary important is the solar energy. It has been estimated that the amount of solar energy that strikes the Earth every hour (~4.3×1020 J) is approximately equal to the total amount of energy consumed by human society every year (Donohue & Cogdell 2006). Hence, global energy needs can be substantially satisfied even with a small fraction of the available solar energy, and the use of photovoltaic solar cells (Fahrenbruch & Bube 1983) is currently expanding. We also consider that biomass is another important renewable energy source. Biomass includes all biologically synthesized