Chapter 9: Energy Sources

Transcription

1 Chapter 9: Energy Sources Energy Sources (Fig. 9.1) 1) Nonrenewable energy sources: The resources that are not replaced by natural processes or those whose rate of replacement is so slow as to be non-effective. Ex. Fossil fuels and nuclear energy. Energy sources commonly used in industrialized nations are fossil fuels: oil (34.4%), coal (24.4%), and natural gas (21.1%), which counts 80% of the world s consumption. 2) Renewable energy resources (13.5%): The energy sources that can be regenerated by natural processes. Ex. Solar, geothermal, and tidal energy. Some forms of renewable energy are also called perpetual energy. CO 2 + H 2 O + hv Biomass (chemical energy) + O 2 Energy is stored in plants as wood, starch, oil, or other compound. Resources and Reserves 1) Resources: The naturally occurring substances can be utilized by people but may not be economic. 2) Reserves: The deposits from which materials can be extracted profitably with existing technology under present economic conditions. Reveres < resources because the concept of reserves is an economic idea and is only loosely tied to the total quantity of a material present in the world. (Fig 9.2) The total amount of a resource such as coal or oil changes only by the amount used each year. The amount of a reserve changes as technology advances, new deposits are discovered, and economic conditions vary. Countries often restate the amount of their reserves for political reasons. It s possible for large increases in the amount of reserves, while the total amount of the resource falls. No fuel will be produced: (i) If the cost of removing and processing a fuel is greater than the fuel s market value, no one is going to produce it. (ii) If the amount of energy used to produce, refine, and transport a fuel is greater than its potential energy, the fuel will not be produced. A net useful energy yield is necessary to exploit the resource. If new technology or changing prices permit the profitable removal of some fossil fuels that are currently not profitable, those resources will be reclassified as reserves. Fossil-Fuel Formation Differences in formation of coal, oil, and natural gas: 1) Coal: Primarily used for electric power generation and other industry uses. Accumulation of plant material in fresh water swamp collected under water, decay was inhibited, and a spongy mass of organic materials formed geological changes these organic deposits were submerged by sea and covered by sediment the weight of plant material and the weight of the sediment on top of it compressed it into coal. Qualities of coal: depending on the amount of time the organic matter has been subjected to geological processes. Peat, lowest-grade fuel, due to high water content, not considered as a fuel. Peat deposits: a recent collection of organic material that are little changed and are 90% H 2 O, 5% C and 5% volatile materials. In Ireland, Latvia, and parts of Russia, peat is cut, dried, and used as fuel. Lignite, low grade-coal, least desired form: Peatlike materials that have been subjected to some compression and heating are converted to a harder form of low-grade coal known as lignite, containing ~40% H 2 O and higher proportion of burnable materials. 1

2 Bituminous (soft) coal: Given sufficient thinkness of overlying sediment, heat from the Earth, and the passing of geologic time, the lignite would have changed into bituminous coal, containing ~3% H 2 O. Anthracite, a hard-grade coal, most desirable, furnishes more energy, cleanest burning, not common, and more expensive: containing ~96% coal. Oil and Natural Gas (Fig 9.5) Dead, microscopic marine organisms accumulated on the ocean bottom buried by sediments and their breakdown released oil droplets gradually the muddy sediment formed rock called shale, containing dispersed oil drops extraction from shale is difficult because the oil is not concentrated concentrated oil form when a layer of porous sandstone formed on top of the shale and an impermeable layer of rock formed on top of the sandstone. Oil does not exit as a liquid form but within sandstone pores, where water and gas pressure force it out of the shale. If the rock layers were folded by geological forces, the accumulations of oil are more likely to occur. (Fig. 9.6: World Oil Reserves in 2004) If the heat generated with the Earth reached high enough temperatures, natural gas would have formed along with or instead of oil, as the organic material changed to lighter, more volatile hydrocarbons than those found in oil. The most common natural gas is CH 4. When pumping natural gas from a well, water, liquid hydrocarbons, and other gases may be present. (Fig. 9.7: World natural Gas Reserves) Issues related to the Use of Fossil Fuels World s commercial energy: 80% are from fossil fuels (~24% coal, ~35% oil, ~21% natural gas). Coal use: Two extraction methods Surface mining or strip mining: Removing the material located on top of a vein of coal, called overburden. Coal is usually mined if the overburden is < 100 m (or 328 ft) thick. They are efficient and profitable. Advances in the methods and better equipments: surface mining activity increases from 30% in 1970 to more than 60% now. Underground mining: If overburden is thick, the surface mining becomes too expensive. The deeply buried coal seam can be reached by (a) vertical shaft, (b) drift mine. (Fig. 9.9) 1) Health and Safety Issues: Coal mining is the most dangerous jobs in the world. Black lung disease: A respiratory condition results from accumulation of fine coal-dust particles in the miners lungs. The coal particles inhibit the exchange of gases between the lungs and the blood. The health costs and deaths benefits related to black lung disease are an indirect cost of coal mining. 2) Transportation Issues: Coal is bulky, shipping presents a problem. The most economical way to use coal is near where it is produced. Rail shipment is the most economic way of transportation but it requires the expense of constructing and maintaining the tracks. 3) Environmental Issues: Surface mining disrupts the landscape: the topsoil and overburden are moved to access the coal. It s possible to reclaim the area after mining operation is completed. (Fig. 9.10). Coal mining and transport generate a great deal of dust: tt the loading and unloading sites, it causes local air pollution. If boat or railroad car used to transport coal, there is an expense to clean them before other types of goods can be shipped. Coal contain S as it came from the protein of the original plants. S associates with acid mine drainage, which occurs when combined with the action of O 2, certain bacteria, and H 2 O to form H 2 SO 4, which can be washed into the streams, which become too acidic and only allow certain algae and bacteria to survive. (Fig. 9.12) The red color of the river is a common characteristic of acid mine drainage. 2

3 Air pollution from coal burning releasing millions of metric tons into air. Coal-burning power plants are the main source of this pollution. Acid rain is another problem: S SO 2 (g) or SO 3 (g) H 2 O(g) H 2 SO 4. CO 2 causes global warming: C CO 2 (g) H 2 O(g) H 2 CO 3. Oil Use: (Fig. 9.13) Oil, difficult to find, has several characteristics that make it a better energy source than coal and as the fuel for automobiles: 1) Characteristics: Its extraction causes less environmental damage than coal mining. It s a more concentrated source of energy as it burns with less pollution. It can be moved easily through pipes. 2) Extraction Methods: Difficulty: removing it from the ground. If the water or gas pressure associated with a pool of oil is great enough, the oil is forced to the surface when a well is drilled. a) When the natural pressure is not great enough, the oil must be pumped to the surface. b) Current technology is only allowed 1/3 of the oil in the ground to be removed. c) The secondary recovery methods are used to recover more oil. d) Secondary recovery methods include (i) Pumping water or gas into the well to drive the oil out. (ii) Starting a fire in the oil-soaked rock to liquefy thick oil. 3) Processing Crude Oil (Fig. 9.14) a) A barrel of crude oil produces slightly less than half a barrel of gasoline. b) Refining process: Distillation cracking (producing more volatile chemicals). 4) Environmental Issues: Liquid are much easier to transport than solid or gas. Pipelines are the primary methods by which oil is transported on continents. a) Problems associated with transportation: Leak and spill. b) All of the extraction, transportation, and refining activities create opportunities for accidental or routine releases that may cause air or water pollution. Nearly 60% of oil pollution in the oceans results in routine shipping operation. c) The evaporation of oil products and the incomplete burning of oil fuel contribute to air pollution. (Ch. 16) Natural Gas Use 1) Extraction methods: Similar to those used in oil. 2) Transport methods: a) If wells are too far from consumers to make a pipeline, the natural gas is burned as a waste product like the ones in Middle East, Mexico, Venezuela, and Nigeria. b) Tankers have been used to transport liquefied natural gas from the area of production to an area of demand. The concern is accidents may cause tankers to explode. A safer transport method is to convert natural gas to methanol and transport in that form. 3) Environmental Issues: a) Local odor problem. b) Global warming: Burning causes less CO 2 produced than oil and coal. c) Primarily used for heating; secondary used for fertilizers and manufacture petrochemicals. Renewable Sources of Energy: 13.5% of total energy consumption in 2003 (Fig. 9.17) 1) Hydroelectric power: 2%. a) Hydroelectric power plant (Fig. 9.18): Water released from reservoir flows through a turbine, spinning it, which in turn activates a generator to produce electricity. b) Hydroelectric power plant does not necessary requires a large dam. If the streams have steep gradients and a constant flow of water, hydroelectricity can be generated without a dam. c) A pumped storage plant, another type which can even store power, uses two reservoirs, one located at a higher elevation than the other. During periods of low demand for electricity,, energy is stored by reversing the turbines and pumping water from lower to the upper 3

4 reservoir. The stored water can later be released to run the turbine to generate electricity as it flows back to lower reservoir. d) More than 35 nations have > 2/3 of their electricity obtained from falling water. In South and Central America, 65% electricity are from hydroelectric power, compared to 44% in developing world as a whole. Norway gets 99% of electricity and over 60% of all lits energy from hydroelectricity.. e) Potential for Additional Hydropower: (i) The best area for developing hydroelectric power is mountainous regions and large river valleys. Canada, U.S., Europe, and Japan: already developed most of hydroelectric power Only 5% developed in Africa: Kariba in East Africa, Aswan on the Nile, Akosombo in Ghana. (ii) In 2006, China completed a huge hydroelectric dam, known as Three Gorgee Dam, on Yangtze River. It s the largest hydroelectric dam in the world, producing 18,000 megawatts in 2~3 years. f) Environmental Issues: (i) Construction causes environmental and social problems: impacts must be weighed against the environmental impacts of alternative energy sources. (ii) Hydroelectric power plants do not contribute air pollution, acid rains, and global warming. (iii) The most obvious impact is flooding of vast areas of land, much of it previously forested or used for agriculture. Also it displaced many people. (iv) Damming a river can alter the amount and quality of water in the river downstream of the dam as well as prevent fish from migrating upstream to spawn. The impacts can be reduced by requiring minimum flows downstream of a dam and by creating fish ladders that allow fish to move upstream past the dam. (v) Silt is trapped by a dam and deposited on the bed of the reservoir, which in turn reduces the water that can be stored and used for electrical generation. (vi) Bacteria present in decaying vegetation can change mercury, present in the rock, into a form that is soluble in water, cumulating in fish and possessing health hazard to those who depend on these fish for food. 2) Tidal Power: (Fig. 9.19) a) Tides, daily rise and fall of ocean levels relative to the coastlines, are a result of gravitational force of the moon and sun as well as the revolution of the Earth. b) Certain coastal regions experience higher tides than others, which is a result of amplification of tides caused by local geographical features such as bays and inlets. c) The higher the tides, the more electricity can be generated from a given site and the lower the cost of electricity produced. d) About 40 sites have this magnitude of tidal range. About 3000 gigawatts. Due to constraints, only 2% or 60 gigawatts is recovered for electricity generation. e) Technology for Obtaining tidal Energy: (i) The technology used in converting tidal energy to electricity is similar to the ones used I n traditional hydroelectric power plants. (ii) Requiring a dam or barrage across a tidal bay or estuary. Building such dam is expensive. The best tidal site is a bay with narrow opening, thus reduces the length of dam required. (iii) When the difference in the elevation of the water on the different sides of the barrage is adequate, the gates are open, which allows water flows into the turbines, turning an electric generator to produce electricity. f) Environmental issues: (i) Not causing air pollution, global warming. (ii) Changing tidal flows damage a bay or estuary could result in negative impacts on aquatic and shoreline ecosystems, as well as navigation and recreation. (iii) The impacts depend on the local geography. (iv) New style tidal power generator is used at Dalupiri Ocean Power plant in Philippines: a submerged turbine or hydro turbine, similar to underwater windmill, to generate electricity. 4

5 The hydro turbines have slow-moving blades, pose little danger to marine life, and will not impede the flow of water and silt. A protective fence prevents large marine mammals from approaching the turbines. 3) Geothermal Power: (Fig. 9.20) a) Linked to geologically active areas where heat from the Earth can reach the surface through thinner crust. b) Located in countries around the circum-pacific Ring of Fire, mid-ocean ridges (Iceland), continental rift zones, and other hot spots. c) More than 130 generating plants are operating in 12 other countries. The Philippines, Italy, Mexico, Japan, New Zealand, and Iceland produce sizable amounts of electricity by geothermal methods. d) Technology for Obtaining Geothermal Energy: (i) By drilling well to obtain the steam, which then is used to power electrical generator. At present, geothermal energy is practical only in areas where this hot mass is near the surface. (ii) It s used directly in heating. Iceland: half used to produce electricity and half is used for heating. e) Environmental Issues: Some environmental problems of H 2 S(g): rotten egg odor, an unpleasant form of air pollution. f) Heat Pumps: (i) Pipes placed within the Earth can extract heat and transfer it to a home, which requires the expenditure of energy through an electric-powered device. (ii) Obtaining heat in this manner is less expensive than other traditional sources. (iii) The use conserves fossil fuels and does not contribute to global warming. 4) Wind Power: (Fig. 9.21) (a) Warmer air is less dense and rises; cooler air flows in to take its place. (b) Because wind is variable, the energy generated by turbine varies. This means, the electrical energy from wind must be coupled with other, more reliable sources of energy. (c) Location can be a problem: Dakotas have the strongest winds, they are remote from energy-using centers and large losses in the amount of electricity would occur as the power is transmitted through electric lines. d) Europe is the leader in the amount of installed capacity. It s easy to install, which can increase in capacity annually. e) Future development: a push for energy deregulation and concerns about smog, acid rain, and global warming are driving policy makers to require utilities to sell electricity from renewable sources. Texas and Dakotas alone have enough wind to power the nation, but that s not likely to happen as most of U.S. wind sources remain untapped. f) Environmental issues: (i) Wind generators do have some negative effects: A moving blade is a hazard to birds and produce a noise that some find annoying. (ii) Vibrations from the generators can cause some structural problems. (iii) Some people consider the sight of a large number of wind generators to be visual pollution. 5) Solar Energy: (a) The energy received from each day is 600 times greater than the amount of energy produced each day by all other energy sources. (b) Due to the differences in the availability of sunlight, some parts of the world are more suited to the use of solar energy than others. (c) Solar energy is utilized in three ways: (i) In a passive heating system: (Fig. 9.23) The sun s energy is converted directly into heat for use at the site where it is collected. The early use is to dry food or clothes and to evaporate water to produce salt. The passive solar heating design is called direct gain: a sunspace (like a greenhouse) is built on the south side of a building. As sunlight passes through the glass, it warms the sunspace. Proper ventilation allows the heat to circulate into the building. A thick trombe 5

6 wall made with material that absorbs a lot of heat is painted black, facing south. A pane of glass or plastic glazing, installed a few centimeters in front of the wall, helps hold in heat. The wall heats up slowly during the day and it gives off its heat inside the building when it cools gradually during the night. The design also provides daylighting, which uses the natural sunlight to brighten a building s interior. To lighten north-facing rooms and upper levels, a clerestory (i.e. a row of windows near the peak of the roof) is often used along the light to bounce throughout the building. Too much solar heating and daylighting can be a problem during hot summer months. A design with overhangs can shade windows when sun is high in the summer. Sunplaces can be cooled off from the rest of the building. (ii) In an active heating system: (Fig. 9.24) It requires a solar collector, a pump, and a system of pipes to transfer the heat from the site of production to the area to be heated. The sun s energy is converted into heat, but the heat must be transferred from the collection area to the place of use like domestic water or pool or space heating, which requires careful selection of components and proper sizing. photovoltaic cell (iii) The sun s energy can be used to generate electricity via (Fig. 9.25) (PV, a low-voltage DC device with storage batteries), which may be used to transmit along normal transmission lines. Thin-film solar cells use layers of semiconductor materials only a few micrometers thick, which makes it possible for solar cells to double as rooftop shingles. Some solar cells are designed to operate with concentrated sunlight. The performance of a solar cell is measured in terms of its efficiency at turning sunlight into electricity. Only sunlight of certain energies will work efficiently to create electricity. (Fig. 9.26) Biomass Conversion All biomass is produced by green plants that convert sunlight into plant material through photosynthesis. Biomass is still the predominant form of energy used by people in the less-developed countries, accounting ~14% of world energy use. 1) Major types of biomass: four distinct sources of biomass energy (i) Fuel wood: The major energy source in the less-developed countries. It s not practical to transport because it s bulky and low level of energy compared to equal amount of coal or oil, it is used locally. Most of this energy is used in forest product industries (lumbering, paper mill). (ii) Solid waste: A major source of biomass and other burnable materials produced by society. (Fig. 9.27) It makes burning solid waste sense when the cost of waste disposal is taken into account. Though energy from solid waste is expensive, it can deduct the savings on landfill costs from the cost of energy. ~ 80% of solid waste is combustible: the waste must first be sorted to separate the burnable organic material from the inorganic material. The sorting is most economically done by the person who produces the waste. The trash must be gathered by compartmentalized collection trucks. To be used as energy source, it requires a large volume and a dependable supply. (iii) Crops residues and animal waste: The straw and stalks left on the field are collected and used for energy. Animal dung is dried and burned or processed in anaerobic digesters to provide a burnable gas. (iv) Energy Plantation: Forest plantations, sugarcane, corn, sugar beets, grains, kelp, etc. Two main factors determine whether a crop is suitable for energy use: Good energy crops have a very high yield of dry material per unit of land. A high yield reduces land requirements and lowers the cost of producing energy from biomass. 6

7 The amount of energy that can be produced from a biomass crop must be more than the amount of energy required to grow the crop. (v) Biomass conversion technologies: Direct combustion and cogeneration: Direct combustion is the primary way to use biomass for cooking, space heating, etc. Biomass is processed as pellets or bariquettes at high temperature and very high pressure before its using. Large scale operations, comparable efficiency to the fossil fuels, are used in industry but the burners require special design to handle the higher moisture content of biomass. The cost to generate electricity from biomass depends on the type of technology used, the size of power plant, and the cost of biomass fuel. Ethanol production: Ethanol is produced by fermentation. Alcohol provides 50% of Brazil s automobile fuel. In U.S., corn is used fro ethanol production and is blend with gasoline to produce gasohol. Anaerobic digestion: Anaerobic digestion is the decomposition of wet and green biomass, through bacterial action in the absence of O 2 to produce CH 4 and CO 2 known as biogas. Some occurs in municipal solid waste buried in landfill sites and the biogas eventually escapes to the atmosphere. The most efficient application of this technology involves anaerobic digestion of animal waste and plant residues in smaller digesters. (Fig. 9.29) Pyrolysis: A thermochemical process for converting solid biomass to a more useful fuel. Biomass is heated in the absence of O 2, or partially combusted with a limited O 2 to produce a hydrocarbon-rich gas mixture, an oil-like liquid, and a carbon-rich solid residue, which is usually charcoal. The process of carbonization is very slow and inefficient in these kilns, and the sophisticated kilns are replacing the traditional ones. The pyrolytic residue or bio-oil produced can be easily transported and refined into a series of products. The process is similar to refine crude oil. Gasification is a form of pyrolysis, carried out with more air and at high temperatures, to optimize the gas production. The resulting gas is known as the producer gas, a mixture of CO, H 2, CH 4, CO 2 and N 2. During World War II, Australia and Germany used producer gas to power vehicle. Environmental issues: Use of biomass as an energy source has many significant impacts on environments. It results in destroying much of forest land in Asia and Africa and has hastened the rate of desertification in these regions. Burning wood is a source of air pollution by producing high amounts of particulates and smoke, which causes many respiratory illness. Burning solid waste presents additional problems as the solid waste contains a mixture of materials, including treated paper and plastic, which generates pollutants not found in other forms of biomass. Using biomass energy destroys the biodiversity, transforming natural ecosystem into single-species energy plantation, of a region. ( monoculture ) (vi) Energy Conservation: A way of reducing the need for additional energy and saves money for the consumer. Energy efficiency improvements have significantly reduced the need for additional energy source. U.S. is much more energy efficient than it was 25 years ago. The amount of energy wasted through poorly insulated windows and doors alone is about as much energy as the U.S. received from the Alaskan pipeline each year. Improved automobile efficiency, better mass transit, and increased railroad use for passenger and freight traffic are simple and readily available means that contribute to energy saving. 7

8 Highly efficient fluorescent light bulb gives the same amount of light as regular incandescent light bulbs for 25% of the energy and produce less heat. Low-emissive glass for windows reduces the amount of heat entering a building while allow light to enter. Others are like automatic dimming devices or automatic lightshutoff devices. Poorly designed, energy-inefficient buildings and machines can be produced inexpensively. The short-term cost is low but the long-term cost is high. Are Fuel Cells and a Hydrogen Economy in the Future? Hydrogen economy: the shift from fossil fuels to hydrogen fuel. All fuel cells are fueled by some form of hydrogen. The primary difference between fuel cells and batteries is that fuel cells do not need to be charged; while batteries require a long recharging period and release toxic heavy metals when disposed. The most common fuel cell (Fig. 9.32) is the proton exchange membrane (PEM) fuel cell, whose specific reactions are (i) Pressurized H 2 enters the fuel cell and comes in contact with Pt catalyst that causes the H 2 to split into H +, known as proton and electron. (ii) A proton exchange membrane allows protons but not electrons to flow through it. The electrons instead flow through an electric circuit to do work. (iii) The protons flow through the membrane and recombine with the electrons that have passed through the circuit and with oxygen to form water. The difficulty in using hydrogen fuel cell is that it s difficult to obtain pure hydrogen, which is highly flammable, difficult to store and not available to consumers as readily as oil-derived fuels. It s been used in spacecrafts since 1960 due to their size, nontoxic emissions, and provided pure drinking water for the crews. Significant prospects for future development of fuel cells are in the automobile industry. (Fig. 9.33). There are 30 buses with fuel cells used since 2003 in Europe. 8