MODULE 7 – Solar & Heat Energy
May 27, 2009.
Refer to pp. 102-107, 483-487, 523, 530-555 in the Prentice Hall Earth Science Book for background information during this module.
Lesson 7.01 – Energy Sources
Standard: ES4.a Students know the relative amount of incoming solar energy compared with Earth’s internal energy and the energy used by society.
ES9.a Students know the resources of major economic importance in California and their relation to California’s geology.
INTRODUCTION
There’s no doubt that we live in the age of fossils fuels. These non-renewable resources supply nearly 90 percent of the world’s energy. But that can’t last forever. At the present rates of consumption, the amount of recoverable fossil fuels may last only another 170 years. As the world population soars, the rate of consumption will climb as well. This will leave fossil fuel reserves in even shorter supply. In the meantime, the burning of huge quantities of fossil fuels will continue to damage the environment. Our growing demand for energy along with our need for a healthy environment will likely lead to a greater reliance on alternate energy sources.
INSTRUCTION
Solar Energy
Solar energy is the direct use of the sun’s rays to supply heat or electricity. Solar energy has two advantages: the “fuel” is free, and it’s non-polluting. The simplest and perhaps most widely used solar energy systems are passive solar collectors such as south-facing windows. As sunlight passes through the glass, objects in the room absorb its heat. These objects radiate the heat, which warms the air.
More elaborate systems for home heating use an active solar collector. These roof-mounted devices are usually large, blackened boxes covered with glass or plastic. The heat they collect can be transferred to areas where it is needed by circulating air or liquids through piping. Solar collectors are also used to heat water for domestic and commercial needs. For example, solar collectors provide hot water for more than 80 percent of Israel’s homes.
There are a few drawbacks to solar energy. While the energy collected is free, the necessary equipment and installation is not. A supplemental heating unit is also needed when there is less solar energy—on cloudy days or in the winter—or at night when solar energy is unavailable. However, over the long term, solar energy is economical in many parts of the United States. It will become even more cost effective as the prices of other fuels increase.
Research is currently underway to improve the technologies for concentrating sunlight. Scientists are examining a way to use mirrors to track the sun and keep its rays focused on a receiving tower. Figure 9 shows a solar collection facility with 2000 mirrors that was built near Barstow, California. This facility heats water in pressurized panels to over 500°C by focusing solar energy on a central tower. The superheated water is then transferred to turbines, which turn electrical generators.
Figure 9 Solar One is a solar installation used to generate electricity in the Mojave Desert near Barstow, California.
Another type of collector, shown in Figure 10, uses photovoltaic (solar) cells. They convert the sun’s energy directly into electricity.
Figure 10 Solar cells convert sunlight directly into electricity. This array of solar panels is near Sacramento, California. Applying Concepts What characteristics would you look for if you were searching for a location for a new solar plant?
Nuclear Energy
Nuclear power meets about 7 percent of the energy demand of the United States. The fuel for nuclear plants, like the one in Figure 11, comes from radioactive materials that release energy through nuclear fission. In nuclear fission, the nuclei of heavy atoms such as uranium-235 are bombarded with neutrons. The uranium nuclei then split into smaller nuclei and emit neutrons and heat energy. The neutrons that are emitted then bombard the nuclei of adjacent uranium atoms, producing a chain reaction. If there is enough fissionable material and if the reaction continues in an uncontrolled manner, fission releases an enormous amount of energy as an atomic explosion.
Figure 11 Diablo Canyon Nuclear Plant Near San Luis Obispo, California Reactors are in the dome-shaped buildings. You can see cooling water being released to the ocean. Analyzing The siting of this plant was controversial because it is close to faults. Why would that be a cause for concern?
In a nuclear power plant, however, the fission reaction is controlled by moving neutron-absorbing rods into or out of the nuclear reactor. The result is a controlled nuclear chain reaction that releases great amounts of heat. The energy drives steam turbines that turn electrical generators. This is similar to what occurs in most conventional power plants.
At one time, energy experts thought nuclear power would be the cheap, clean energy source that would replace fossil fuels. But several obstacles have slowed its development. First, the cost of building safe nuclear facilities has increased. Second, there are hazards associated with the disposal of nuclear wastes. Third, there is concern over the possibility of a serious accident that could allow radioactive materials to escape. The 1979 accident at Three Mile Island in Pennsylvania made this concern a reality. A malfunction in the equipment led the plant operators to think there was too much water in the primary system. Instead there was not enough water. This confusion allowed the reactor core to lie uncovered for hours. Although there was little danger to the public, the malfunction resulted in substantial damage to the reactor.
Unfortunately, the 1986 accident at Chernobyl in Ukraine was far more serious. In this case, the reactor went out of control. Two small explosions lifted the roof of the structure, and pieces of uranium spread over the surrounding area. A fire followed the explosion. During the 10 days that it took to put out the fire, the atmosphere carried high levels of radioactive material as far away as Norway. Eighteen people died within six weeks of the accident. Thousands more faced an increased risk of death from cancers associated with the fallout.
Wind Energy
According to one estimate, if just the winds of North and South Dakota could be harnessed, they would provide 80 percent of the electrical energy used in the United States. Wind is not a new energy source. People have used it for centuries to power sailing ships and windmills for grinding grains.
Following the “energy crisis” brought about by the oil embargo of the 1970s, interest in wind power and other alternative forms of energy grew. In 1980, the federal government started a program to develop wind-power systems, such as the one shown in Figure 12. The U.S. Department of Energy set up experimental wind farms in mountain passes with strong, steady winds. One of these facilities, at Altamont Pass near San Francisco, now operates more than 7000 wind turbines. In the year 2000, wind supplied a little less than one percent of California‘s electricity.
Figure 12 These wind turbines are operating near Palm Springs, California.
Some experts estimate that in the next 50 to 60 years, wind power could meet between 5 to 10 percent of the country’s demand for electricity. Islands and other isolated regions that must import fuel for generating power are major candidates for wind energy expansion.
The future for wind power looks promising, but there are difficulties. The need for technical advances, noise pollution, and the cost of large tracts of land in populated areas are obstacles to development.
Hydroelectric Power
Like wind, moving water has been an energy source for centuries. The mechanical energy that waterwheels produce has powered mills and other machinery. Today, the power that falling water generates, known as hydroelectric power, drives turbines that produce electricity. In the United States, hydroelectric power plants produce about 5 percent of the country’s electricity. Large dams, like the one in Figure 13, are responsible for most of it. The dams allow for a controlled flow of water. The water held in a reservoir behind a dam is a form of stored energy that can be released through the dam to produce electric power.
Figure 13 Glen Canyon Dam and Lake Powell on the Colorado River As dam operators release water in the reservoir, it passes through machinery that drives turbines and produces electricity.
Although water power is a renewable resource, hydroelectric dams have finite lifetimes. Rivers deposit sediment behind the dam. Eventually, the sediment fills the reservoir. When this happens, the dam can no longer produce power. This process takes 50 to 300 years, depending on the amount of material the river carries. An example is Egypt’s Aswan High Dam on the Nile River, which was completed in the 1960s. It is estimated that half the reservoir will be filled with sediment by 2025.
The availability of suitable sites is an important limiting factor in the development of hydroelectric power plants. A good site must provide a significant height for the water to fall. It also must have a high rate of flow. There are hydroelectric dams in many parts of the United States, with the greatest concentration in the Southeast and the Pacific Northwest. Most of the best U.S. sites have already been developed. This limits future expansion of hydroelectric power.
Geothermal Energy
Geothermal energy is harnessed by tapping natural underground reservoirs of steam and hot water Hot water is used directly for heating and to turn turbines to generate electric power. The reservoirs of steam and hot water occur where subsurface temperatures are high due to relatively recent volcanic activity.
In the United States, areas in several western states use hot water from geothermal sources for heat. The first commercial geothermal power plant in the United States was built in 1960 at The Geysers, shown in Figure 14. The Geysers is an important source of electrical power for nearby San Francisco and Oakland. Although production in the plant has declined, it remains the world’s premier geothermal field. It continues to provide electrical power with little environmental impact. Geothermal development is now also occurring in Nevada, Utah, and the Imperial Valley of California.
Figure 14 The Geysers is the world’s largest electricity-generating geothermal facility. Most of the steam wells are about 3,000 meters deep.
Geothermal power is clean but not inexhaustible. When hot fluids are pumped from volcanically heated reservoirs, the reservoir often cannot be recharged. The steam and hot water from individual wells usually lasts no more than 10 to 15 years. Engineers must drill more wells to maintain power production. Eventually, the field is depleted.
As with other alternative methods of power production, geothermal sources are not expected to provide a high percentage of the world’s growing energy needs. Nevertheless, in regions where people can develop its potential, its use will no doubt grow.
Tidal Power
Several methods of generating electrical energy from the oceans have been proposed, yet the ocean’s energy potential still remains largely untapped. The development of tidal power is one example of energy production from the ocean.
Question/Answer
Q Is power from ocean waves a practical alternative energy source?
A It’s being seriously explored now. In November 2000, the world’s first commercial wave power station opened on the Scottish island of Islay. It provides power for the United Kingdom. The 500-kilowatt power station uses an oscillating water column, in which incoming waves push air up and down inside a concrete tube that is partly under the ocean’s surface. Air rushing in and out of the top of the tube drives a turbine to produce electricity. If the facility succeeds, it could open the door for wave power to become a significant contributor of renewable energy in some coastal areas.
Tides have been a power source for hundreds of years. Beginning in the 12th century, tides drove water wheels that powered gristmills and sawmills. During the seventeenth and eighteenth centuries, a tidal mill produced much of Boston’s flour. But today’s energy demands require more sophisticated ways of using the force created by the continual rise and fall of the ocean.
Tidal power is harnessed by constructing a dam across the mouth of a bay or an estuary in coastal areas with a large tidal range. The strong in-and-out flow that results drives turbines and electric generators. An example of this type of dam is shown in Figure 15.
Figure 15 A At low tide, water is at its lowest level on either side of the dam. B At high tide, water flows through a high tunnel. C At low tide, water drives turbines as it flows back to sea through a low tunnel. Analyzing Concepts Why is a large tidal range (difference in water level between high and low tide) needed to produce power?
The largest tidal power plant ever constructed is at the mouth of France’s Rance River. This tidal plant went into operation in 1966. It produces enough power to satisfy the needs of Brittany—a region of 27,000 square kilometers—and parts of other regions. Much smaller experimental facilities have been built near Murmansk in Russia, near Taliang in China, and on an arm of the Bay of Fundy in Canada.
Tidal power development isn’t economical if the tidal range is less than eight meters or if a narrow, enclosed bay isn’t available. Although the tides will never provide a high portion of the world’s ever-increasing energy needs, it is an important source at certain sites.
PRACTICE
- Take notes on the above information.
- Click here to watch a video about energy sources.
ASSESSMENT
- Turn in your notes.
- Take the 7.01 Quiz