Renewable Energy: An Overview, Exercises of Fossil Fuels

Download Renewable Energy: An Overview and more Exercises Fossil Fuels in PDF only on Docsity! What is Renewable Energy? Renewable energy uses energy sources that are continually replenished by nature—the sun, the wind, water, the Earth’s heat, and plants. Renewable energy technologies turn these fuels into usable forms of energy—most often elec- tricity, but also heat, chemicals, or mechanical power. Why Use Renewable Energy? Today we primarily use fossil fuels to heat and power our homes and fuel our cars. It’s convenient to use coal, oil, and natural gas for meeting our energy needs, but we have a limited supply of these fuels on the Earth. We’re using them much more rapidly than they are being created. Even- tually, they will run out. And because of safety concerns and waste disposal prob- lems, the United States will retire much of its nuclear capacity by 2020. In the mean- time, the nation’s energy needs are expected to grow by 33 percent during the next 20 years. Renewable energy can help fill the gap. Even if we had an unlimited supply of fos- sil fuels, using renewable energy is better for the environment. We often call renew- able energy technologies “clean” or “green” because they produce few if any pollutants. Burning fossil fuels, however, sends greenhouse gases into the atmos- phere, trapping the sun’s heat and con- tributing to global warming. Climate scientists generally agree that the Earth’s average temperature has risen in the past century. If this trend continues, sea levels will rise, and scientists predict that floods, heat waves, droughts, and other extreme weather conditions could occur more often. Other pollutants are released into the air, soil, and water when fossil fuels are burned. These pollutants take a dramatic toll on the environment—and on humans. Air pollution contributes to diseases like asthma. Acid rain from sulfur dioxide and nitrogen oxides harms plants and fish. Nitrogen oxides also contribute to smog. Renewable Energy: An OverviewCLEARINGHOUSE ENERGY EFFICIENCY ANDRENEWABLE ENERGY T OF ENER G YD E PA RTMEN U E N IT E D STAT S OFA E R IC A M This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory. The document was produced by the Information and Outreach Program at NREL for the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiency and Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL / DOE. The statements contained herein are based on information known to EREC and NREL at the time of printing. No recommendation or endorsement of any product or service is implied if mentioned by EREC. Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste DOE/GO-102001-1102 FS175 March 2001 A PV-system at the Pinnacles National Monument in California eliminates a $20,000 annual fuel bill for a diesel generator that pro- duced each year 143 tons of carbon dioxide—a greenhouse gas. N at io na l P ar k S er vi ce , N R E L/ P IX 04 92 4 Renewable energy will also help us develop energy independence and secu- rity. The United States imports more than 50 percent of its oil, up from 34 percent in 1973. Replacing some of our petroleum with fuels made from plant matter, for example, could save money and strengthen our energy security. Renewable energy is plentiful, and the technologies are improving all the time. There are many ways to use renewable energy. Most of us already use renewable energy in our daily lives. Hydropower Hydropower is our most mature and largest source of renewable power, pro- ducing about 10 percent of the nation’s electricity. Existing hydropower capacity is about 77,000 megawatts (MW). Hydro- power plants convert the energy in flowing water into electricity. The most common form of hydropower uses a dam on a river to retain a large reservoir of water. Water is released through turbines to generate power. “Run of the river” systems, how- ever, divert water from the river and direct it through a pipeline to a turbine. Hydropower plants produce no air emis- sions but can affect water quality and wildlife habitats. Therefore, hydropower plants are now being designed and oper- ated to minimize impacts on the river. Some of them are diverting a portion of the flow around their dams to mimic the natural flow of the river. But while this improves the wildlife’s river habitat, it also reduces the power plant’s output. In addition, fish lad- ders and other approaches, such as improved tur- bines, are being used to assist fish with migration and lower the number of fish killed. Bioenergy Bioenergy is the energy derived from bio- mass (organic matter), such as plants. If you’ve ever burned wood in a fireplace or campfire, you’ve used bioenergy. But we don’t get all of our biomass resources directly from trees or other plants. Many industries, such as those involved in con- struction or the processing of agricultural products, can create large quantities of unused or residual biomass, which can serve as a bioenergy source. Biopower After hydropower, biomass is this coun- try’s second-leading resource of renewable energy, accounting for more than 7,000 MW of installed capacity. Some utilities and power generating companies with coal power plants have found that replac- ing some coal with biomass is a low-cost option to reduce undesirable emissions. As much as 15 percent of the coal may be replaced with biomass. Biomass has less sulfur than coal. Therefore, less sulfur dioxide, which contributes to acid rain, is released into the air. Additionally, using biomass in these boilers reduces nitrous oxide emissions. A process called gasification—the conver- sion of biomass into gas, which is burned in a gas turbine—is another way to gener- ate electricity. The decay of biomass in landfills also produces gas, mostly methane, which can be burned in a boiler to produce steam for electricity generation or industrial processes. Biomass can also be heated in the absence of oxygen to chemically convert it into a type of fuel oil, called pyrolysis oil. Pyrolysis oil can be used for power generation and as a feed- stock for fuels and chemical production. Biofuels Biomass can be converted directly into liq- uid fuels, called biofuels. Because biofuels are easy to transport and possess high energy density, they are favored to fuel vehicles and sometimes stationary power generation. The most common biofuel is ethanol, an alcohol made from the fermen- tation of biomass high in carbohydrates. The current largest source of ethanol is corn. Some cities use ethanol as a gasoline addi- tive to help meet air quality standards for 2 Hydropower is our most mature and largest source of renewable power… A small-scale hydropower system in King Cove, Alaska, provides residents in this remote area with a less expensive source of electricity. D ua ne H ip p e, N R E L/ P IX 04 41 0 There are basically three types of solar collectors for heat- ing water: flat- plate, evacuated- tube, and concen- trating. The most common type, a flat-plate collector, is an insulated, weatherproof box containing a dark absorber plate under a transpar- ent cover. Evacu- ated-tube collectors are made up of rows of parallel, transparent glass tubes. Each tube consists of a glass outer tube and an inner tube, or absorber, covered with a coating that absorbs solar energy but inhibits heat loss. Concentrating collec- tors for residential applications are usually parabolic-shaped mirrors (like a trough) that concentrate the sun’s energy on an absorber tube called a receiver that runs along the axis of the mirrored trough and contains a heat-transfer fluid. All three types of collectors heat water by circulating household water or a heat-trans- fer fluid such as a nontoxic antifreeze from the collector to the water storage tanks. Col- lectors do this either passively or actively. Passive solar water-heating systems use natural convection or household water pressure to circulate water through a solar collector to a storage tank. They have no electric components that could break, a feature that generally makes them more reliable, easier to maintain, and possibly longer lasting than active systems. An active system uses an electric pump to circulate water or nontoxic antifreeze through the system. Active systems are usually more expensive than passive sys- tems, but they are also more efficient. Active systems also can be easier to retrofit than passive systems because their storage tanks do not need to be installed above or close to the collectors. Also, the moving water in the system will not freeze in cold climates. But because these systems use electricity, they will not function in a power outage. That’s why many active systems are now combined with a small solar-electric panel to power the pump. The amount of hot water a solar water heater produces depends on the type and size of the system, the amount of sun available at the site, proper installation, and the tilt angle and orientation of the collectors. But if you’re currently using an electric water heater, solar water heating is a cost-effective alternative. If you own a swimming pool, heating the water with solar collectors can also save you money. Solar Electricity Solar electricity or photovoltaic (PV) tech- nology converts sunlight directly into elec- tricity. Solar electricity has been a prime source of power for space vehicles since the inception of the space program. It has also been used to power small electronics and rural and agricultural applications for three decades. During the last decade, a strong solar electric market has emerged for powering urban grid-connected homes and buildings as a result of advances in solar technology along with global changes in electric industry restructuring. Although many types of solar electric sys- tems are available today, they all consist of basically three main items: modules that convert sunlight into electricity; inverters that convert that electricity into alternat- ing current so it can be used by most household appliances; and possibly or sometimes batteries that store excess elec- tricity produced by the system. The remainder of the system comprises equip- ment such as wiring, circuit breakers, and support structures. Today’s modules can be built into glass skylights and walls. Some modules resem- ble traditional roof shingles, but they gen- erate electricity, and some come with built-in inverters. The solar modules avail- able today are more efficient and versatile than ever before. In over 30 states, any additional power produced by a PV system, which is not being used by a home or building, can be fed back to the electric grid through a process known as net metering. Net meter- ing allows electricity customers to pay only for their “net” electricity, or the 5 Passive solar building techniques turn homes into huge solar collectors. The Four Times Square Building in New York City uses thin-film PV panels to reduce the building’s power load from the utility grid. A nd re w G or d on a nd F ox & F ow le A rc hi te ct s, N R E L/ P IX 09 05 2 amount of power consumed from their utility minus the power generated by their PV system. This metering arrangement allows consumers to realize full retail value for 100 percent of the PV energy produced by their systems. Grid-connected PV systems do not require batteries. However, some grid connected systems use them for emergency backup power. And of course in remote areas, solar electricity is often a economic alternative to expensive distribution line extensions incurred by a customer first connecting to the utility grid. Electricity produced by solar electric systems in remote locations is stored in batteries. Batteries will usually store electricity produced by a solar- electric system for up to three days. What type of system to purchase will depend on the energy-efficiency of your home, your home’s location, and your budget. Before you size your system, try reducing energy demand through energy- efficient measures. Purchasing energy-sav- ing appliances and lights, for example, will reduce your electrical demand and allow you to purchase a smaller solar-electric system to meet your energy needs or get more value from a larger system. Energy efficiency allows you to start small and then add on as your energy needs increase. Solar Thermal Electricity Unlike solar-electric systems that convert sunlight into electricity, solar thermal elec- tric systems convert the sun’s heat into electricity. This technology is used primar- ily in large-scale power plants for power- ing cities and communities, especially in the Southwest where consistent hours of sunlight are greater than other parts of the United States. Concentrating solar power (CSP) tech- nologies convert solar energy into electric- ity by using mirrors to focus sunlight onto a component called a receiver. The receiver transfers the heat to a conven- tional engine-generator—such as a steam turbine—that generates electricity. There are three types of CSP systems: power towers (central receivers), parabolic troughs, and dish/engine systems. A power tower system uses a large field of mirrors to concentrate sunlight onto the top of a tower, where a receiver sits. Molten salt flowing through the receiver is heated by the concentrated sunlight. The salt’s heat is turned into electricity by a conventional steam generator. Parabolic- trough systems concentrate the sun’s energy through long, parabolic-shaped mirrors. Sunlight is focused on a pipe filled with oil that runs down the axis of the trough. When the oil gets hot, it is used to boil water in a conventional steam generator to produce electricity. A dish/engine system uses a mirrored dish (similar in size to a large satellite dish). The dish-shaped surface focuses and con- centrates the sun’s heat onto a receiver at the focal point of the dish (above and cen- ter of the collectors). The receiver absorbs the sun’s heat and transfers it to a fluid within an engine, where the heat causes the fluid to expand against a piston to pro- duce mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity. Concentrating solar technologies can be used to generate electricity for a variety of applications, ranging from remote power systems as small as a few kilowatts (kW) up to grid-connected applications of 200 MW or more. A 354-MW power plant in South- ern California, which consists of nine trough power plants, meets the energy needs of more than 350,000 people and is the world’s largest solar energy power plant. Wind Energy For hundreds of years, people have used windmills to harness the wind’s energy. Today’s wind turbines, which operate dif- ferently from windmills, are a much more efficient technology. Wind turbine technology may look simple: the wind spins turbine blades around a central hub; the hub is connected to a shaft, which powers a generator to make electric- ity. However, turbines are highly sophisti- cated power systems that capture the wind’s energy by means of new blade designs or airfoils. Modern, mechanical drive systems, combined with advanced generators, con- vert that energy into electricity. Wind turbines that provide electricity to the utility grid range in size from 50 kW to 6 Wind energy has been the fastest growing source of energy since 1990… This dish/Stirling solar power system in Arizona is capable of producing 25 kW of electricity. B ill T im m er m an , N R E L/ P IX 08 98 2 7 Hydrogen is high in energy, yet its use as a fuel produces water as the only emission. 1 or 2 MW. Large, utility-scale pro- jects can have hun- dreds of turbines spread over many acres of land. Small turbines, below 50 kW, are used to charge bat- teries, electrify homes, pump water for farms and ranches, and power remote telecommunica- tions equipment. Wind turbines can also be placed in the shallow water near a coastline if open land is limited, such as in Europe, and/or to take advan- tage of strong, offshore winds. Wind energy has been the fastest growing source of energy in the world since 1990, increasing at an average rate of over 25 percent per year. It’s a trend driven largely by dramatic improvements in wind tech- nology. Currently, wind energy capacity amounts to about 2500 MW in the United States. Good wind areas, which cover 6 percent of the contiguous U.S. land area, could supply more than one and a half times the 1993 electricity consumption of the entire country. California now has the largest number of installed turbines. Many turbines are also being installed across the Great Plains, reaching from Montana east to Minnesota and south through Texas, to take advantage of its vast wind resource. North Dakota alone has enough wind to supply 36 percent of the total 1990 electricity consumption of the lower 48 states. Hawaii, Iowa, Min- nesota, Oregon, Texas, Washington, Wiscon- sin, and Wyoming are among states where wind energy use is rapidly increasing. Hydrogen Hydrogen is high in energy, yet its use as a fuel produces water as the only emission. Hydrogen is the universe’s most abundant element and also its simplest. A hydrogen atom consists of only one proton and one electron. Despite its abundance and simplicity, it doesn’t occur naturally as a gas on the Earth. Today, industry produces more than 4 tril- lion cubic feet of hydrogen annually. Most of this hydrogen is produced through a process called reforming, which involves the application of heat to separate hydro- gen from carbon. Researchers are develop- ing highly efficient, advanced reformers to produce hydrogen from natural gas for what’s called Proton Exchange Membrane fuel cells. You can think of fuel cells as batteries that never lose their charge. Today, hydrogen fuel cells offer tremendous potential to produce electrical power for distributed energy systems and vehicles. In the future, hydrogen could join electricity as an important “energy carrier”: storing, mov- ing, and delivering energy in a usable form to consumers. Renewable energy sources, like the sun, can’t produce energy all the time. But hydrogen can store the renew- able energy produced until it’s needed. Eventually, researchers would like to directly produce hydrogen from water using solar, wind, and biomass and bio- logical technologies. Ocean Energy The ocean can produce two types of energy: thermal energy from the sun’s heat, and mechanical energy from the tides and waves. Ocean thermal energy can be used for many applications, including electricity generation. Electricity conversion systems use either the warm surface water or boil the seawater to turn a turbine, which acti- vates a generator. The electricity conversion of both tidal and wave energy usually involves mechanical devices. A dam is typically used to convert tidal energy into electric- ity by forcing the water through turbines, activating a generator. Meanwhile, wave energy uses mechanical power to directly activate a generator, or to transfer to a working fluid, water, or air, which then drives a turbine/generator. Most of the research and development in ocean energy is happening in Europe. The 6-MW Green Mountain power plant in Searsburg, Vermont, consists of eleven 550-kW wind turbines. G re en M ou nt ai n P ow er C or p or at io n, N R E L/ P IX 05 76 8 NASA uses liquid hydrogen to launch its space shuttles and hydrogen fuel cells to provide them with electricity N A S A , N R E L/ P IX 03 81 4