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Think of all the sunlight that falls across the United States. If we could capture even a small fraction of the energy it contains, we would be well on our way to energy independence and a cleaner environment. Modern photovoltaic and other solar collection systems are a first step on that path. But nature offers us another opportunity - to literally harvest the sun - and humans have been using it ever since they first discovered fire.

Through the process of photosynthesis, plants store sunlight in the form of sugars, starches, and other complex carbohydrates. For most of human history it was customary to recover this energy limply by burning the plant residues for heat, and this is still the main energy use to which these and other biomass (plant and animal) wastes are put, in the United States and elsewhere. But, as important as they are, residues alone have only limited long- term energy potential, and combustion is not the cleanest or most efficient way to use them. Over the past few years, therefore, agricultural researchers have focused on developing special crops designed exclusively for conversion to energy, and engineers and scientists have been perfecting new methods for using them more cleanly and efficiently and in a wider variety of applications, including electricity and transportation.

With these advances, power crops now have the potential to satisfy a significant portion of America's energy needs, while at the same time revitalizing rural economies, providing energy independence and security, and achieving important environmental benefits. Indeed, farming communities of the future could very well be entirely self-sufficient when it comes to energy, using locally grown crops and residues to make fuels for their cars and tractors and to generate heat and electricity for their homes.

The special energy, or "power," crops grow extremely fast, naturally regenerate after harvesting, and have a high energy content per pound, even though they require fewer chemical pesticides and fertilizers and less irrigation than most food crops. Some trees, for example, such as poplars and willows, may grow up to 40 feet high in the seven or eight years between harvests. Perennial prairie grasses such as switchgrass may reach heights of eight feet every year. And perennial tropical grasses such as energy cane grow faster still.

Some annual crops, including corn and sorghum, also have a high energy content, but they are less sustainable to grow than trees and perennial grasses, since annual replanting requires more intensive management and higher chemical inputs. the same is true of the major oilseed crops, such as soybeans and sunflowers. A more productive and sustainable oil crop with great promise for the future is micro-algae, tiny aquatic plants with the potential to grow extremely quickly under very adverse conditions - the hot, shallow, saline water found in some lakes in the desert southwest.

Until recently, plants' solid form meant that burning them was the most practical way to release their energy. But burning limits the range of applications in which plants can be used as fuel, and, unless it is carefully controlled, can be inefficient and polluting. Fortunately, though, plants and other forms of biomass actually contain the very same elements - principally carbon and hydrogen - found in fossil fuels, and, with modern technology, it is possible to convert them from solid to liquid or gaseous form.

For example, heating, but not burning, biomass in an oxygen-deficient atmosphere can produce crude oils, tars, and solids, which can then be refined into combustible liquids and gases, such as methanol, syngas (a mixture of carbon monoxide and hydrogen), or pure hydrogen. Alternatively, plant matter can be fermented with yeast to produce ethanol, or literally digested by microbes to produce biogas (a mixture of carbon dioxide and methane). Finally, chemical treatment of plant oils yields biodiesel, functionally almost identical to petroleum diesel. The choice of which process to use depends on the type of biomass to be converted and the form of fuel desired.

Once it has been converted into liquid or gaseous form, biomass can be used in a wide range of power systems. One of the most important uses for syngas or biogas is direct combustion in advanced modern gas turbines to run a generator to produce electricity. This process generates about twice as much electricity as simply burning the raw biomass to produce steam for a steam turbine, and it is much cleaner. Alternatively, a biomass-derived methanol or hydrogen can be used in a fuel cell - an exceptionally clean, extremely efficient power system - which can be large enough to power businesses or factories or small enough to power cars. Another option is to use any of the plant-derived liquid fuels in internal combustion engines for transportation. In fact, gasoline blended with ethanol made from corn is already helping to meet clean air requirements in some parts of the country, and sum bus fleets are already operating on biodiesel.

In theory, power crops could be grown on a very large scale and, used in the ways described, could displace significant proportions of current fossil fuel use. In the future, for example, it might be possible to harvest power crops on up to 200 million acres of cropland - without irrigation and without displacing food crop production - with a probable yield of around 1 billion tons of biomass every year. This quantity of biomass should be capable of satisfying about half of current US electricity or two-thirds of current US gasoline demand.

With thoughtful practice and management, power crops raised on this scale could bring important environmental improvements to air, soil, water, and habitat diversity. Because power crops recycle carbon from the atmosphere, using them to replace fossil fuels could reduce carbon emissions substantially and help mitigate climate change. Some fossil fuel is still needed to grow, harvest, and transport power crops, and to provide the heat necessary for their conversion, but researchers are focusing on reducing the amount of input energy required and ensuring that it comes from plants or other renewable sources to begin with. With best practices, net carbon reductions could be 90 percent or more, and sulfer dioxide emissions could be reduced by up to 70 percent, helping to prevent the formation of acid rain.

The deep roots of power crops can improve the physical, chemical, and biological quality of soil degraded by overuse in supporting annual row crops. Deep-rooted crops can increase the soil's organic content, improve its nutrient-retention capacity, aid in aeration, and facilitate water penetration and retention. Reduced chemical use also helps to protect water supplies from poisons and excessive aquatic plant growth. Finally, power crops create greater habitat diversity than annual row crops, and attract a wider variety of species, especially birds and small mammals.

To realize the potential environmental benefits of using power crops for energy, however, it is vital that farmers manage these crops sustainable - by minimizing the use of chemicals, heavy machinery, and irrigation. And if unforeseen problems become apparent with the production or use of power crops, growers and users should be prepared to modify their practices accordingly.

Power crops can also bring significant economic benefits. For growers, power crops are less expensive to grow than food crops, since they require fewer chemicals and less labor and management. Formers can purchase expensive tree-harvesting equipment collectively, and can cut and bale grasses with machinery they already own. Power crops also offer farmers greater income stability: yields are more reliable than those of food crops, and growers can avoid the fluctuations of the commodity markets by entering into long-term contracts to supply energy producers such as power companies and ethanol refiners.

The versatility of power crops provides energy producers with great flexibility in terms of when, where, and how to use the resource, allowing them to tailor their operations precisely to the power or transportation needs of the local community. In some cases, farmers may even choose to convert power crops to energy themselves, for their own use or to sell to others. Either way, because power crops create jobs locally in every sector from agriculture to energy production, they have the potential to boost the economies of whole communities, especially in rural areas.

Power crops also provide security against a number of circumstances that are likely - perhaps certain - to increase the costs of conventional fuels in the future. Since power crops are a domestically grown, renewable fuel, they will be unaffected by unrest in oil-producing regions of the world, and they will not run out. And since they produce fewer pollutants than fossil fuels, they will be less susceptible to the imposition of environmental costs on dirty fuels.

Whether our nation realized the full potential of power crops will depend on a variety of interacting factors, especially the economics of agricultural and energy production, concerns over the environment and global climate change, and the political will of our country's decision makers. Right now, biomass is generally cost-effective only in certain situations - where residues are available at a low or negative cost and when it is possible to take advantage of the by-product or co-products of primary energy processes. Power crops themselves are not quite cost-effective yet - either for farmers to grow or for energy producers to use - mainly because the balance of subsidies is tipped in favor of food crops and fossil fuels and because the environmental benefits of biomass are not formally valued.

To change this picture, over the past few years the Department of Energy has operated a number of programs both to conduct primary research into power crop production and to provide funds to help private industry commercialize promising conversion technologies. The federal government also provides a few tax credits for biogas, biopower, and ethanol. Unfortunately, these programs and incentives are of limited scope and duration, are vulnerable to political pressure, and together amount to only a tiny fraction of the corresponding support for the fossil fuel and nuclear power industries.

These programs have revealed the potential contributions that power crops could make to the nation's energy mix, but they are not sufficient, in themselves, to realize that potential. The choice of whether or not to take that next step lies with the American people and the value we place on having a robust rural economy, a strong degree of energy independence, and a clean environment.

 

(Reprint, Nucleus Magazine, Winter 1995-96 Edition. Paul Jefferiss is energy research coordinator for UCS.)

 

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