Ocean Power
Tidal Energy
Some of the oldest ocean energy technologies use tidal power. All coastal areas experience two high tides and two low tides over a period of slightly more than 24 hours. For those tidal differences to be harnessed into electricity, the difference between high and low tides must be more than 16 feet (or at least 5 meters). However, there are only about 40 sites on Earth with tidal ranges of this magnitude.
Tidal Energy Technologies
Tidal energy technologies include barrages or dams, tidal fences, and tidal turbines.
Tidal energy technologies include barrages or dams, tidal fences, and tidal turbines.
Barrages or Dams
A barrage or dam is typically used to convert tidal energy into electricity by forcing water through turbines, which activate a generator. Gates and turbines are installed along the dam. When the tides produce an adequate difference in the level of water on opposite sides of the dam, the gates are opened. The water then flows through the turbines. The turbines turn an electric generator to produce electricity.
A barrage or dam is typically used to convert tidal energy into electricity by forcing water through turbines, which activate a generator. Gates and turbines are installed along the dam. When the tides produce an adequate difference in the level of water on opposite sides of the dam, the gates are opened. The water then flows through the turbines. The turbines turn an electric generator to produce electricity.
Tidal Fences
Tidal fences look like giant turnstiles. They can reach across channels between small islands or across straits between the mainland and an island. The turnstiles spin via tidal currents typical of coastal waters. Some of these currents run at 5–8 knots (5.6–9 miles per hour) and generate as much energy as winds of much higher velocity. Because seawater has a much higher density than air, ocean currents carry significantly more energy than air currents (wind).
Tidal fences look like giant turnstiles. They can reach across channels between small islands or across straits between the mainland and an island. The turnstiles spin via tidal currents typical of coastal waters. Some of these currents run at 5–8 knots (5.6–9 miles per hour) and generate as much energy as winds of much higher velocity. Because seawater has a much higher density than air, ocean currents carry significantly more energy than air currents (wind).
Tidal Turbines
Tidal turbines look like wind turbines. They are arrayed underwater in rows, as in some wind farms. The turbines function best where coastal currents run between 3.6 and 4.9 knots (4 and 5.5 mph). In currents of that speed, a 49.2-foot (15-meter) diameter tidal turbine can generate as much energy as a 197-foot (60-meter) diameter wind turbine. Ideal locations for tidal turbine farms are close to shore in water 65.5–98.5 feet (20–30 meters) deep.
Tidal turbines look like wind turbines. They are arrayed underwater in rows, as in some wind farms. The turbines function best where coastal currents run between 3.6 and 4.9 knots (4 and 5.5 mph). In currents of that speed, a 49.2-foot (15-meter) diameter tidal turbine can generate as much energy as a 197-foot (60-meter) diameter wind turbine. Ideal locations for tidal turbine farms are close to shore in water 65.5–98.5 feet (20–30 meters) deep.
Wave Energy
Wave energy technologies extract energy directly from surface waves or from pressure fluctuations below the surface. Renewable energy analysts believe there is enough energy in ocean waves to provide up to 2 terawatts of electricity. (A terawatt is equal to a trillion watts.)
Wave energy technologies extract energy directly from surface waves or from pressure fluctuations below the surface. Renewable energy analysts believe there is enough energy in ocean waves to provide up to 2 terawatts of electricity. (A terawatt is equal to a trillion watts.)
However, wave energy cannot be harnessed everywhere. Wave power-rich areas of the world include the western coasts of Scotland, northern Canada, southern Africa, and Australia as well as the northeastern and northwestern coasts of the United States. In the Pacific Northwest alone, it is feasible that wave energy could produce 40–70 kilowatts (kW) per 3.3 feet (1 meter) of western coastline.
Wave Energy Technologies
Wave energy can be converted into electricity by offshore or onshore systems.
Wave energy can be converted into electricity by offshore or onshore systems.
The Pelamis device is a semi-submerged, articulated structure, 120m long and 3.5m in diameter, that is composed of cylindrical sections linked by hinged joints. The wave-induced motion of these joints is resisted by hydraulic rams, which pump high-pressure fluid through hydraulic motors via smoothing accumulators. The hydraulic motors drive electrical generators to produce electricity. Power from all the joints is fed down a single umbilical cable to a junction on the sea bed. Several devices can be connected together and linked to shore through a single seabed cable. Depending on the wave resource, machines will on average produce 25-40% of the full rated output.Source: Wind & Hydropower Technologies Program, U.S. Department of Energy, Energy Efficiency and Renewable Energy
Offshore Systems
Offshore systems are situated in deep water, typically of more than 131 feet (40 meters). Sophisticated mechanisms—such as the Salter Duck—use the bobbing motion of the waves to power a pump that creates electricity. Other offshore devices use hoses connected to floats that ride the waves. The rise and fall of the float stretches and relaxes the hose, which pressurizes the water, which, in turn, rotates a turbine.
Offshore systems are situated in deep water, typically of more than 131 feet (40 meters). Sophisticated mechanisms—such as the Salter Duck—use the bobbing motion of the waves to power a pump that creates electricity. Other offshore devices use hoses connected to floats that ride the waves. The rise and fall of the float stretches and relaxes the hose, which pressurizes the water, which, in turn, rotates a turbine.
Specially built seagoing vessels can also capture the energy of offshore waves. These floating platforms create electricity by funneling waves through internal turbines and then back into the sea.
Onshore Systems
Built along shorelines, onshore wave power systems extract the energy of breaking waves. Onshore system technologies include:
Built along shorelines, onshore wave power systems extract the energy of breaking waves. Onshore system technologies include:
•Oscillating Water Columns: Oscillating water columns consist of a partially submerged concrete or steel structure that has an opening to the sea below the waterline. It encloses a column of air above a column of water. As waves enter the air column, they cause the water column to rise and fall. This alternately compresses and depressurizes the air column. As the wave retreats, the air is drawn back through the turbine as a result of the reduced air pressure on the ocean side of the turbine.
•Tapchans: Tapchans, or tapered channel systems, consist of a tapered channel that feeds into a reservoir constructed on cliffs above sea level. The narrowing of the channel causes the waves to increase in height as they move toward the cliff face. The waves spill over the walls of the channel into the reservoir, and the stored water is then fed through a turbine.
•Pendulor Devices: Pendulor wave-power devices consist of a rectangular box that is open to the sea at one end. A flap is hinged over the opening, and the action of the waves causes the flap to swing back and forth. The motion powers a hydraulic pump and a generator.
Biomass Technologies
Biomass technologies break down organic matter to release stored energy from the sun.
Biofuels are liquid or gaseous fuels produced from biomass. Most biofuels are used for transportation, but some are used as fuels to produce electricity. The expanded use of biofuels offers an array of benefits for our energy security, economic growth, and environment.
Current biofuels research focuses on new forms of biofuels such as ethanol and biodiesel, and on biofuels conversion processes.
Ethanol
Ethanol—an alcohol—is made primarily from the starch in corn grain. It is most commonly used as an additive to petroleum-based fuels to reduce toxic air emissions and increase octane. Today, roughly half of the gasoline sold in the United States includes 5%-10% ethanol.
Biodiesel
Biodiesel use is relatively small, but its benefits to air quality are dramatic.
Biodiesel is produced through a process that combines organically-derived oils with alcohol (ethanol or methanol) in the presence of a catalyst to form ethyl or methyl ester. The biomass-derived ethyl or methyl esters can be blended with conventional diesel fuel or used as a neat fuel (100% biodiesel).
Biomass Resources
Biomass resources include any plant-derived organic matter that is available on a renewable basis. These materials are commonly referred to as feedstocks.
Biomass Feedstocks
Biomass feedstocks include dedicated energy crops, agricultural crops, forestry residues, aquatic crops, biomass processing residues, municipal waste, and animal waste.
Dedicated energy crops
Herbaceous energy crops are perennials that are harvested annually after taking 2 to 3 years to reach full productivity. These include such grasses as switchgrass, miscanthus (also known as elephant grass or e-grass), bamboo, sweet sorghum, tall fescue, kochia, wheatgrass, and others.
Short-rotation woody crops are fast-growing hardwood trees that are harvested within 5 to 8 years of planting. These include hybrid poplar, hybrid willow, silver maple, eastern cottonwood, green ash, black walnut, sweetgum, and sycamore.
Agricultural Crops
Agricultural crops include currently available commodity products such as cornstarch and corn oil, soybean oil and meal, wheat starch, and vegetable oils. They generally yield sugars, oils, and extractives, although they can also be used to produce plastics as well as other chemicals and products.
Agriculture Crop Residues
Agriculture crop residues include biomass materials, primarily stalks and leaves, that are not harvested or removed from fields in commercial use. Examples include corn stover (stalks, leaves, husks, and cobs), wheat straw, and rice straw. With approximately 80 million acres of corn planted annually, corn stover is expected to become a major feedstock for biopower applications.
Forestry Residues
Forestry residues include biomass not harvested or removed from logging sites in commercial hardwood and softwood stands as well as material resulting from forest management operations such as pre-commercial thinning and removal of dead and dying trees.
Aquatic Crops
There are a variety of aquatic biomass resources, such as algae, giant kelp, other seaweed, and marine microflora.
Biomass Processing Residues
Biomass processing yields byproducts and waste streams that are collectively called residues and have significant energy potential. Residues are simple to use because they have already been collected. For example, the processing of wood for products or pulp produces unused sawdust, bark, branches, and leaves/needles.
Municipal Waste
Residential, commercial, and institutional post-consumer waste contains a significant proportion of plant-derived organic material that constitute a renewable energy resource. Waste paper, cardboard, wood waste, and yard waste are examples of biomass resources in municipal waste.
Animal Waste
Farms and animal-processing operations create animal wastes that constitute a complex source of organic materials with environmental consequences. These wastes can be used to make many products, including energy.
Some biomass feedstocks, such as municipal waste, are found throughout the United States. Others, such as energy crops, are concentrated in the eastern half of the country. As technologies develop to more efficiently process complex feedstocks, the biomass resource base will expand.
Collecting Gas from Landfills
Landfills can be a source of energy. Organic waste produces a gas called methane as it decomposes, or rots.
Methane is the same energy-rich gas that is in natural gas, the fuel sold by natural gas utility companies. It is colorless and odorless. Natural gas utilities add an odorant (bad smell) so people can detect seeping gas, but it can be dangerous to people or the environment. New rules require landfills to collect methane gas as a pollution and safety measure.
Wind Energy
Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are modified by the earth's terrain, bodies of water, and vegetative cover. This wind flow, or motion energy, when "harvested" by modern wind turbines, can be used to generate electricity.
The terms "wind energy" or "wind power" describe the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity to power homes, businesses, schools, and the like.
Global maps of average wind speed help researchers determine where to develop wind energy. Wind turbines (high-tech windmills) can generate power in places far from power plants and without an electricity grid�but planners need to know where there is sufficient wind for the turbines to operate efficiently. A team at NASA�s Langley Research Center developed these maps, and maps of solar insolation, and provide them free of charge. Private companies are using these data to design, build, and market new technologies for harnessing this energy.
How Wind Power Is Generated
The terms "wind energy" or "wind power" describe the process by which the wind is used to generatemechanical power or electricity. Wind turbines convert the kinetic energy in the wind into mechanical power. This mechanical power can be used for specific tasks (such as grinding grain or pumping water) or a generator can convert this mechanical power into electricity to power homes, businesses, schools, and the like.
Wind Turbines
Wind turbines, like aircraft propeller blades, turn in the moving air and power an electric generator that supplies an electric current. Simply stated, a wind turbine is the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity.
Wind Turbine Types
Modern wind turbines fall into two basic groups; the horizontal-axis variety, like the traditional farm windmills used for pumping water, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor. Most large modern wind turbines are horizontal-axis turbines.
Turbine Components
- Anemometer: Measures the wind speed and transmits wind speed data to the controller.
- Blades: Most turbines have either two or three blades. Wind blowing over the blades causes the blades to "lift" and rotate.
- Brake: A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.
- Controller: The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they might be damaged by the high winds.
- Gear box: Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speeds from about 30 to 60 rotations per minute (rpm) to about 1000 to 1800 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes.
- Generator: Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.
- High-speed shaft: Drives the generator.
- Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
- Nacelle: The nacelle sits atop the tower and contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.
- Pitch: Blades are turned, or pitched, out of the wind to control the rotor speed and keep the rotor from turning in winds that are too high or too low to produce electricity.
- Rotor: The blades and the hub together are called the rotor.
- Tower: Towers are made from tubular steel (shown here), concrete, or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.
- Wind direction: This is an "upwind" turbine, so-called because it operates facing into the wind. Other turbines are designed to run "downwind," facing away from the wind.
- Wind vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.
- Yaw drive: Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don't require a yaw drive, the wind blows the rotor downwind.
- Yaw motor: Powers the yaw drive.
- other equipment, including controls, electrical cables, ground support equipment, and interconnection equipment.
Turbine Configurations
Wind turbines are often grouped together into a single wind power plant, also known as a wind farm, and generate bulk electrical power. Electricity from these turbines is fed into a utility grid and distributed to customers, just as with conventional power plants.
Wind Turbine Size and Power Ratings
Wind turbines are available in a variety of sizes, and therefore power ratings. The largest machine has blades that span more than the length of a football field, stands 20 building stories high, and produces enough electricity to power 1,400 homes. A small home-sized wind machine has rotors between 8 and 25 feet in diameter and stands upwards of 30 feet and can supply the power needs of an all-electric home or small business. Utility-scale turbinesrange in size from 50 to 750 kilowatts. Single small turbines, below 50 kilowatts, are used for homes, telecommunications dishes, or water pumping.
Advantages and Disadvantages of Wind-Generated Electricity
A Renewable Non-Polluting Resource
Wind energy is a free, renewable resource, so no matter how much is used today, there will still be the same supply in the future. Wind energy is also a source of clean, non-polluting, electricity. Unlike conventional power plants, wind plants emit no air pollutants or greenhouse gases. According to the U.S. Department of Energy, in 1990, California's wind power plants offset the emission of more than 2.5 billion pounds of carbon dioxide, and 15 million pounds of other pollutants that would have otherwise been produced. It would take a forest of 90 million to 175 million trees to provide the same air quality.
Cost Issues
Even though the cost of wind power has decreased dramatically in the past 10 years, the technology requires ahigher initial investment than fossil-fueled generators. Roughly 80% of the cost is the machinery, with the balance being site preparation and installation. If wind generating systems are compared with fossil-fueled systems on a "life-cycle" cost basis (counting fuel and operating expenses for the life of the generator), however, wind costs are much more competitive with other generating technologies because there is no fuel to purchase and minimal operating expenses.
Environmental Concerns
Although wind power plants have relatively little impact on the environment compared to fossil fuel power plants, there is some concern over the noise produced by the rotor blades, aesthetic (visual) impacts, and birds and bats having been killed (avian/bat mortality) by flying into the rotors. Most of these problems have been resolved or greatly reduced through technological development or by properly siting wind plants.
Supply and Transport Issues
The major challenge to using wind as a source of power is that it is intermittent and does not always blow when electricity is needed. Wind cannot be stored (although wind-generated electricity can be stored, if batteries are used), and not all winds can be harnessed to meet the timing of electricity demands. Further, good wind sites are often located in remote locations far from areas of electric power demand (such as cities). Finally, wind resource development may compete with other uses for the land, and those alternative uses may be more highly valued than electricity generation. However, wind turbines can be located on land that is also used for grazing or even farming.
All renewable energy (except tidal and geothermal power), and even the energy in fossil fuels, ultimately comes from the sun. The sun radiates 100,000,000,000,000 kilowatt hours of energy to the earth per hour. In other words, the earth receives 10 to the 18th power of watts of power. About 1 to 2 per cent of the energy coming from the sun is converted into wind energy. That is about 50 to 100 times more than the energy converted into biomass by all plants on earth.
After reaching 1,000 MW of wind energy in 1985, it took more than a decade for wind to reach the 2,000-MW mark in 1999. Since then, installed capacity has grown fivefold. Today, U.S. wind energy installations produce enough electricity on a typical day to power the equivalent of over 2.5 million homes.
