Chapter 24: Energy and Water

Transcription

1 Chapter 24: Energy and Water Goals of Period 24 Section 24.1: To describe the Earth s water cycle Section 24.2: To explain hydroelectric power generation Section 24.3: To discuss climate change and water supply In this period, we discuss an essential ingredient to life on Earth: water. the practical utilization of sources of energy derived from solar energy such as water power, wind power, and biomass. We continue with some more direct conversions and utilizations of solar energy such as solar heating and generating electricity using solar cells. As a practical application of solar energy, we discuss the design and operation of a so-called solar home The Water Cycle How is it that solar energy powers the Earth s weather and climate? One very important factor is the water cycle. Solar energy, incident on the Earth s oceans, rivers, lakes, and soil evaporates water. The water vapor rises due to convection. As the vapor rises, it cools and condenses on small dust particles in the air to form clouds. These clouds may be made of water droplets or, at higher elevations, ice crystals. Under the right conditions, the water precipitates as rain or snow to the Earth's surface. Some precipitation falls into the oceans. If precipitation falls on land, the force of gravity may cause the water may to flow downhill as rivers to the oceans. Energy of water flowing in rivers, water power, is a valuable source of energy. Clouds are an integral part of the water cycle and play an important role in the energy balance of the Earth. Clouds have two competing effects. The first is a cooling effect, since clouds reflect incoming sunlight. The second is a warming effect, since clouds prevent infrared radiation from escaping into space; as noted earlier, water vapor is a greenhouse gas. In class, you will trace possible movements of water molecules through the water cycle. Some water may move by evaporation from the Earth s surface into the atmosphere, condense, and precipitate back to Earth as described above. Some water maybe locked in glacial ice at high latitudes or in tropical glaciers. Other water may remain underground as ground water. The possible paths of a water molecule through the Earth s water cycle are complex. For this reason, the movement of a water molecule through the water cycle may take over 250 years! As water moves through the water cycle, it may become contaminated by pollutants from minerals and organisms in the soil, lakes, and rivers, and oceans. Contaminants are removed when water evaporates (changes phase from liquid to gas) 229

2 or sublimates (changes phase from solid to gas). Some particulate matter may be filtered from water as it percolates through rock and sand. In class you will see the net effect of contaminants added to and removed from water as it moves through the water cycle Energy from Water Power Kinetic Energy from Water Power On a relatively small scale, water power has been used for centuries and played a very important role in the industrial revolution early in the 19th century. Early uses of water power included grinding grains, such as wheat and corn, into flour by rotating heavy stone mill wheels. Water power also provided mechanical energy for machines like lumber-yard saws. In such uses, a small river was dammed and diverted to a water wheel. The gravitational potential energy of the water was converted into kinetic energy as water falling onto the spokes of a large wheel caused the wheel to spin. The spinning motion of a shaft connected to the water wheel was ideal for powering stationary machines like saws. Electrical Energy from Water Power In modern times, to generate electricity on a relatively large scale, a river is dammed to form an artificial lake. The same underlying physics principles used for milling grain are applied: gravitational potential energy from the lake water is converted into kinetic energy as the water falls onto the blades of a turbine, causing it to spin. The spinning turbine is connected to a generator. The kinetic energy of the spinning turbine is used to spin large magnets near coils of wire, producing an electric current. Thus, the gravitational potential energy of the falling water is converted into electrical energy. Water power is presently a very popular and reliable source of electricity for many places around the world. It depends on the water cycle, which is powered by solar energy. For this reason, it might seem that water power is environmentally safe. This may not be true since the ecology above and below the dam may be disturbed or even destroyed. The reservoir formed by the dam may also provide water for human needs such as irrigation; water is thus removed from the previously natural river. On the other hand, the use of water power does not release carbon dioxide or other potentially damaging gasses into the atmosphere as does burning fossil fuels to produce electricity. Electrical Energy from Tidal Energy In addition to the many hydroelectric power plants built along rivers, a tidal powered generating plant along the ocean shore can use tides to turn turbines. This method of generating electricity from water power uses tidal energy. There is a familiar periodic rise and fall of the sea level called the tide. The cycle time for a tide is about twelve hours; along most ocean shores there are two high-tides every day. It requires energy to affect the tides; a rising water level implies an increase in gravitational energy of the water. From where does the tidal energy come? 230

3 Tidal energy comes from the gravitational interactions between the Earth and moon and, to a much smaller extent, between the Earth and the Sun. Because in relation to tides the Sun s gravitational force is much less than the moon s force, tidal energy is not considered to originate from solar energy. The moon s gravitational force causes the oceans, which are an easily deformed fluid, to form two bulges, one on either side of the Earth. As the Earth spins on its axis, land bordering the oceans (the shore) passes through both bulges each day; thus the familiar cycle of two high tides and two low tides per day. How can such energy be utilized to generate electricity? When the tide is rising, there is a flow of water from the ocean into the harbor. Obviously, there must be a flow of water from the harbor back to the ocean when the tide is ebbing. If the harbor is large enough, there can be a significant flow of water throughout the day. Now, if the opening of such a harbor is dammed, water entering or leaving the harbor may be diverted to a water turbine to generate electricity. This method has been used in France. The technology involved in harnessing tidal power is very similar to that involved in harnessing water power provided by rivers. Note that one major adverse effect of harnessing tidal energy by damming a harbor is that it alters the local ecology. Electrical Energy from Water Currents The use of power from water currents can be compared to the use of tidal power. Except for the subtle difference of how the water is set in motion, the concepts and technologies are very similar. Hydrokinetic power generation uses the energy of ocean currents and waves and flowing rivers to turn submerged turbines. This method allows the production of electricity, albeit on a smaller scale, without the environmental damage from damming a river or a harbor. Rivers with strong currents, such as the Niagara River upstream or downstream from Niagara Falls, are good candidates for hydrokinetic power. Current plans are for the installation of 875 submerged turbines in the Niagara River. Electrical Energy from Geothermal Energy Chapter 9 examined the use of shallow water geothermal energy for heating and cooling. In moderate climates, ground water at least 10 feet below the Earth s surface remains at a constant temperature of about 55 O F (13 O C) year round. During summer, fans blowing across coils of this cool water can help cool a building. During winter, heat pumped from this 55 O F water can help warm a building. However, deeper into the Earth s interior, temperatures are much hotter than 55 O F as evidenced by volcanoes, geysers and hot-springs. Actually, most of the Earth consists of molten rock; only a relatively thin crust has cooled and solidified. This thin crust, only few miles thick, is very effective in insulating us from the hot molten rock or magma. Only in geologically active areas does magma come close to the surface or, in the case of volcanoes, reach the surface. Nevertheless, the interior of the Earth is a great reserve of thermal energy. 231

4 This geothermal energy is the result of radioactive decay of unstable nuclei within the Earth. The thermal energy released from these fission reactions raises the temperature of the Earth s core to 9,000 O F (5,000 O C). As thermal energy is conducted from the core through the Earth s mantle, heat is dissipated to turn water in the crust into steam. The result is that hot water and steam can be vented onto the Earth s surface and atmosphere in hot springs, such as the Old Faithful Geyser in Yellowstone National Park in Wyoming. In some volcanically active areas, such as Iceland, geothermal energy provided by natural hot-springs is used to generate electricity and to heat homes and other buildings. To produce electricity, the thermal energy from hot water and steam is used as the heat source for the steam pressure needed to turn turbines, which spin magnets near coils of wire. Electricity from geothermal energy has the advantage of producing electricity without the drawbacks of using fossil fuels. Its availability is limited to areas of geologic activity. Another drawback, however, is that some gasses such as sulfur dioxide, which smells like rotten eggs, are released in such operations Climate Change and Water Resources Glacial Retreat The melting (retreat) of glaciers has been well documented. Tropical mountain, mid-latitude, and high latitude glaciers are retreating at an increasing rate, and some glaciers have already disappeared. Snowpack accumulates on glaciers during the winter months and experiences melting during the summer. For a stable glacier, the snow in winter is balanced by the melting in summer. Over time, the snowpack solidifies into ice. A retreating glacier is one in which summer melting exceeds winter snow addition, with a net result of loss of ice. Mid-latitude glaciers are located in mountain ranges in the northern hemisphere between the Tropic of Cancer and the Arctic Circle, primarily the North American Rocky Mountains and Pacific Coast ranges, the European Pyrenees and Alps, and the Indo- Asian Himalayas. Mid-latitude glaciers in the southern hemisphere between the Tropic of Capricorn and the Antarctic Circle are located in the South American Andes and in New Zealand. Nearly all glaciers in the mid-latitudes are retreating. Tropical mountain glaciers, such as those on Mt Kilimanjaro in Africa, are retreating as well. In the last one hundred years, the glacial ice on Mt. Kilimanjaro has decreased by 75% and is expected to disappear entirely within the next ten years. Glaciers and ice sheets in the high latitude regions, within the Arctic and Antarctic Circles are also experiencing unprecedented melting. In addition to causing a rise in sea level, what effect will the disappearance of glaciers have on our use of water resources? 232

5 Impact of Glacial Retreat for Hydroelectric Energy Summer melt water from glaciers provides crucial fresh water for drinking, crop irrigation, aquatic plant and animal life, and hydroelectric energy. Regions located in the rain shadow of mountain ranges, such as the east side of the Andes Mountains, are particularly dependent on glacial melt water. As warm, humid air meets a mountain range and is forced upward, the air cools and water vapor condenses into precipitation, which falls on the windward side of the range. Drier air descends the opposite side of the range, which results in an arid rain shadow region. In South America, millions of people depend on the hydroelectric power produced when melt-water swollen rivers are dammed. It is estimated that the majority of the Andean glaciers will have disappeared by A similar situation exists in the European Alps and the Pacific Northwest mountain ranges, when melt water is widely used for hydroelectric generation. Without this relatively pollution-free source of hydroelectric power, other fuels must be used to generate electricity. Glacial Lake Dams Initially, as glaciers melt, increased volumes of melt water will exit into the current mountain drainage. In mid-latitude mountain ranges, particularly the Himalayas and the Andes, many earthen dams hold back melt water into glacial lakes. As glacial melting progresses, an increased volume of melt water will be released into these lakes. Should these earthen fail under the increased pressure of more water, populations downstream could be devastated. An estimated 100 such dams are currently susceptible to causing such flooding. Agriculture As glaciers disappear, melt water will be greatly diminished. Less water for agricultural irrigation will cause decreases in crop yield, particularly in the rain shadow of mountain ranges. South America is especially vulnerable to reduced melt water, as many irrigation lakes are currently filled by glacial melt water. In addition, many fresh water plants and animals used as food sources are dependent on the glacial melt water rivers and streams. Drinking Water Once glacial melt water is no longer readily available, an estimated one to two billion people worldwide will likely face a shortage of fresh water. The options for providing sources of fresh water for this population include piping in fresh water, often from great distances. Salty ocean water can be put through a desalination process involving reverse osmosis or distillation. Finally waste water, also known as gray water can be purified and recycled. However, all these alternatives for providing fresh water for this population require an expenditure of money and energy. In effect, the loss of fresh water from melting glaciers adds to the energy demands on the planet. 233

6 Period 24 Summary 24.1: Solar energy drives the water cycle. Water from oceans and lakes evaporates when radiation from Sun warms the water surface. Water vapor rises, is cooled, and condenses on dust particles, forming clouds. When clouds become saturated, precipitation falls as rain or snow. Precipitation eventually runs back into lakes and oceans and the cycle repeats. 24.2: Water power can be used to generate electricity by using the gravitational potential energy of the water to turn a turbine. The spinning turbine moves large magnets near coils of wire. Wind power can be used to turn generate electricity when windmill blades turn turbines. In tidal plants, water flows in during high tide and is trapped behind gates. As the tide recedes, the trapped water is left at a higher level than the surrounding ocean. The trapped water returning to the ocean spills over the turbines, turning them. Geothermal generation of electricity taps into the thermal energy of decaying radioactive nuclei in the Earth s core. As heat moves to the outer layers of the Earth, the resulting hot water and steam can be used to generate electricity. 24.3: The melting of glaciers, particularly mountain glaciers in the mid-latitudes and tropics, will produce fresh water shortages for billions of people currently dependant on glacial melt water for drinking water, irrigation, and hydroelectric power. Alternatives for providing fresh water to this population will require large expenditures of money and energy. 234