Biomass can also be converted to liquid fuels, especially methanol {methyl al­cohol) and ethanol (ethyl alcohol), which can then be used in internal combustion engines. How­ever, a major disadvantage of alcohol fuels, whether they are produced from biomass, natural gas, or coal, is that 30 to 40 percent of the energy in the starting material is lost in the conversion to alcohol.

It is also possible to convert biomass, particu­larly animal wastes, into biogas. Biogas, usually composed of a mixture of gases, can be stored and transported easily like natural gas. It is a clean fuel whose combustion produces fewer pollutants than either coal or biomass. In China, several million family-sized biogas digesters use microbial decom­position of household and agricultural wastes n> produce biogas that is used for heating and cooking (Figure 12-14). When biogas conversion is com­plete, the solid remains are removed from the di­gester and used as fertilizer.

Advantages of Biomass: Use Biomass is attractive as a source of energy because it reduces dependence on fossil fuels and because it can make use of wastes, thereby reducing our waste disposal prob­lem. For example, the Mesquite Lake Resource Recovery Project in southern Cali­fornia burns cow manure in special furnaces to gen­erate electricity for thousands of homes. The ma­nure is too salty and contaminated with weed seed to be used as fertilizer, so its use as an energy source helps solve the problem of its disposal.

Biomass is usually burned to produce energy, so the pollution problems caused by fossil fuel com­bustion, particularly carbon dioxide emissions, arc not completely absent in biomass combustion. However, the low levels of sulfur and ash produced

by biomass combustion compare favorably with the high levels produced when bituminous coal is burned. It is possible to offset the CO₂ that is re­leased into the atmosphere from biomass combus­tion by increasing tree planting. As trees photosynthesize, they absorb atmospheric CO_Z and lock it up in organic molecules that make up the body of the tree, thereby providing a carbon "sink". Thus, if bio-muss is regenerated to replace the biomass used, there is no net contribution of CO₂ to the atmos­phere and to global warming.

Disadvantages of Biomass Use: Some problems are associated with use of biomass, especially from plants. For one thing, biomass production requires land and water. The use of agricultural land for energy crops competes with the growing of food crops, so shifting the balance toward energy pro­duction might decrease food production, leading to higher food prices. For this reason, some scientists are interested in the commercial development of certain desert shrubs, which produce oils that could be used for fuel. The shrubs do not require prime agricultural land, although care would have to be taken to ensure that the desert soils were not de­graded or eroded by overuse.

As mentioned earlier, at least half of the world's population relies on biomass as its main source of energy. Unfortunately, in many areas peo­ple burn wood faster than they replant it. Intensive use of wood for energy has resulted in severe dam­age to the environment, including soil erosion, deforestation and desertification, air pollution, and degradation of water supplies.

Crop residues, another category of biomass that includes cornstalks, wheat stalks, and wood wastes at paper mills and sawmills, are increasingly being used for energy. At first glance, it may seem that crop residues, which normally remain in the soil after harvest, would be a good source of energy if they were collected and burned. After all, they're just waste material that will eventually decompose. As it turns out, however, crop residues left in the ground prevent erosion by helping to hold the soil.

in place. Also, their decomposition serves to enrich the soil by making the minerals that were originally locked up in the plant residues available for new plant growth. If all crop residues were removed from the ground, the soil would eventually be de­picted of minerals and its future productivity would decline. Forest residues, which remain in the soil after trees are harvested, fill similar ecological robs. The use of crop and forest residues for biomass would have to be carefully managed: some of the residues could he removed for energy, and the rest would have to be left in the soil to maintain soil fertility.

Wind Energy

Wind, which results from the warming of the atmos­phere by the sun, is an indirect form of solar energy in which the radiant energy of the sun is trans­formed into mechanical energy—the movement of air molecules. Wind is sporadic over much of the Earth's surface, varying in direction and magnitude; and, like direct solar energy, wind power is a highly dispersed form of energy. Harnessing wind energy to generate electricity has great potential, however, and it is likely that someday wind will have a greater role in supplying our energy needs. It is cur­rently the most cost-competitive of all forms of solar energy.

For many centuries people have used windmills to pump water, irrigate fields, and grind grain. The Dutch designs for large, slow windmills, which were developed by the 16th century, remained un­changed until wind was first used to generate electricity in the 19th century. Improvements in wind machine design in the 20th century have resulted in much greater efficiency, and further develop­ment of this new, highly sophisticated technology promises even more increases in efficiency.

Harnessing wind energy is most profitable in areas that receive fairly continual winds, such as islands, coastal areas, mountain passes, and grass­lands. The world's largest cluster of wind turbines is located on the northern side of the island of Oahu in Hawaii. In California mountain passes the number of wind farms, arrays of wind turbines, increased significantly during1980s; California currently produces 85 percent of the world's wind generated electricity. Other wind

farms operate in Denmark, the Netherlands, and India.

In the continental United States, the best loca­tions for large-scale electricity generation from wind energy are off the coasts of New England and the Pacific Northwest and on the western Great Plains. Some experts envision in the future large collections of wind turbines scattered across the Great Plains, supplying much of America's electri­cal needs.

The use of wind power does not cause major environmental problems (although one concern, currently under study, is reported bird kills). Be­cause it produces no waste, it is a clean source of energy. A major problem with increasing our use of wind power is aesthetics: wind machines detract from the beauty of the landscape. Fortunately, most of the locations that are most appropriate for large-scale wind power are not densely populated. Com­bining wind farms with cattle grazing, as is done in Altamont, California, for example, is a very pro­ductive and profitable use of land.

Hydropower

The sun's energy drives the hydrologic cycle: evap­oration from land and water and transpiration from plants, precipitation, and drainage and runoff. As water flows from higher elevations back to sea level, we can harness its energy. Unlike the sun's energy, which is highly dispersed, hydropower is a concentrated energy. The potential energy of water held back by a dam is converted to kinetic energy as the water falls over a spillway, where it turns turbines to generate electricity.

Currently, hydropower produces approximately one-fourth of the world's electricity, making it the form of solar energy in greatest use. Developed countries have already built dams at most of their potential sites, but this is not the case in many de­veloping nations. There—particularly in undevel­oped, unexploited parts of Africa and South America—hydropower is still a great potential source of electricity. However, even if all potential sites worldwide were used, the energy generated would be less than 15 percent of the total energy needed.

One of the problems associated with hydro-lower is that building a dam changes the natural low of a river. A dam is cause water to back up, flooding large areas of land and forming a reservoir, which destroys plant and animal habitats. Below he dam, the once-powerful river is reduced to an elative trickle. The natural beauty of the countryside is affected, and certain forms of wilderness rec­reation are made impossible or less enjoyable.

In arid regions, the creation of a reservoir be­hind a dam results in greater evaporation of water, because the reservoir has a larger surface area in contact with the air than the stream or river did. As a result, serious water loss and increased salinity of the remaining water may occur.

Dams destroy farmlands and displace people. When a dam breaks, people and property down­stream may be endangered. In addition, waterborne diseases such as schistosomiasis may spread throughout the local population. Schistosomiasis is a tropical disease, caused by a parasitic worm that can damage the liver, urinary tract, nervous system, and lungs. It is estimated that half the population of Egypt suffers from this disease, largely as a direct result of the Aswan Dam, built on the Nile River in 1902 to control flooding but used since 1960 to provide electrical power.

The ecological, environmental, and personal impacts of a dam may not be acceptable to the peo­ple living in a particular area. Laws have been passed to prevent or restrict the building of dams in certain locations. In the United States, for exam­ple, the Wild and Scenic Rivers Act prevents the hydroelectric development of 37 rivers.

Dams cost a great deal to build but are rela­tively inexpensive to operate. A dam has a limited life span, usually 50 to 200 years, because over time the reservoir fills in with silt until it cannot hold enough water to generate electricity. This trapped silt, which is rich in nutrients, is prevented from enriching agricultural lands downstream. For exam­ple, the gradual depletion of agricultural productiv­ity downstream from the Aswan Dam in Egypt is well documented.

Ocean Temperature Differences: In the future it may be possible to generate power using ocean tem­perature gradients, the differences in temperature at various ocean depths. There may he as much as a 24°C difference between warm surface water and very cold, deeper ocean water. Ocean temperature gradients, which are greatest in the tropics, are the result of solar energy warming the surface of the ocean.

Some people visualize giant power plants that would float on the ocean and make use of this tem­perature differential. Warm surface water would be pumped into the power plant, where it would heat a liquid, such as ammonia, to the boiling point. (Because liquid ammonia has a very low boiling point, —33.3"C, and the heat from Warm Ocean water would cause it to boil.) The ammonia steam would drive a turbine and thus generate electricity.

The ammonia would then be cooled and condensed back to liquid form by colder water drawn up from a depth of, say, 1,000 meters.'' The generation of electricity from ocean tempera­ture differences is known as ocean thermal energy conversion (OTEC).

To see if such as ideas was workable; a tiny OTEC plant was designed and operated near Ha­waii in 1979. Although, this plant is required enor­mous amounts of energy to operate (for example, to pump water), it was generated more energy than it used. Thus, we know that OTEC is technologically possible. However, the potential impact of bringing massive quantities of cold water to the surface in a tropical area needs to be considered carefully. Such water properties as dissolved gases, turbidity (cloud­iness), nutrient levels, and salinity gradients (differ­ences in salt concentrations) are bound to be al­tered along with the temperature, and these changes would probably have a profound effect on marine organisms. Even so, some experts believe that OTEC will have fewer environmental conse­quences than other sources of energy, such as nu­clear power.

Ocean Waves Ocean waves are produced by winds, which are caused by the sun, so wave energy is considered an indirect form of solar energy. Like other types of flowing water, wave power has the potential to turn a turbine, thereby generating elec­tricity. Norway, Japan, and several other countries are investigating the production of electricity from ocean waves (see Focus On: The Power of Waves on page 249). Norway's pilot wave power station at Tostestallen is testing several different ways of tap­ping wave energy. Although harnessing wave power is technologically feasible, more research will have to be done to make generation of electric­ity from wave motion practical.