RKDF INSTITUTE OF

SCIENCE & TECHNOLOGY

A PROJECT REPORT

ON

“ADVANCES IN WIND ENERGY”

SUBMITTED FOR THE FULFILMENT OF THE MINOR PROJECT OF BACHELOR OF ENGINEERING [BRANCH – MECHANICAL ENGINEERING, VI SEM] UNDER THE KIND GUIDANCE OF PROF. M.K. CHOPRA HOD (MECHANICAL DEPARTMENT)

SUBMITTED BY:- BALRAM PANDEY DEVENDRA LODHI MANMOHAN SINGH

CONTENTS

TOPICS PAGE NO

INTRODUCTION TO WIND ENERGY

INTRODUCTION TO WIND ENERGY

Wind Energy and Wind PowerThe earth’s surface is made up of land and water, which absorbs heat from the sun at different rates .. 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.. Wind energy is renewable, clean and reduces greenhouse gas emissions.

The Earth is unevenly heated by the sun, such that the poles receive less energy from the sun than the equator; along with this, dry land heats up (and cools down) more quickly than the seas do. The differential heating drives a global atmospheric convection system reaching from the Earth's surface to the stratosphere which acts as a virtual ceiling. Most of the energy stored in these wind movements can be found at high altitudes where continuous wind speeds of over 160 km/h (99 mph) occur. Eventually, the wind energy is converted through friction into diffuse heat throughout the Earth's surface and the atmosphere

WIND POWER GENERATIONThe 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.

.Wind TurbinesWind 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 turbines today are up to the task of producing serious amounts of electricity. Turbines vary in size from small 1 kW structures to large machines rated at 1.6 MW. A popular sized machine in the U.S. today is a state-of-the-art 750 kW turbine that stands as tall as a 20-story building. With a good wind resource, this size turbine can produce 2 million kWh of electricity each year. That's enough energy to run 200 average American households.

Wind energy could theoretically meet global demand yet it provides just about two percent of power consumption. The sector started slowly, but is now one of the most mature of renewable energies.

Worldwide Importance and Future ProspectsGrowth in wind power is tremendous, with capacity more than doubling every three years. In 2009, global installed wind capacity reached around 160 Gigawatts, rising 40 GW on the previous year, according to the World Wind Energy Association, which estimates that by 2020 global capacity could reach 1900 GW. Since 2007, annual wind power additions in Europe have exceeded growth of any other power source.

In terms of actual electricity output, global wind power in 2009 generated 340 Terawatt hours per annum, equivalent to the total electricity demand of Italy and equaling two percent of global electricity consumption. Nine percent of Europe’s electricity demand is now being met by wind power, compared to just two percent in 2000.

But Europe no longer leads the market. Recent growth has been driven by Asia, which accounted for 40 percent of new additions in 2009, with China the main locomotive for the international wind industry followed by the United States. Even so, wind energy still accounts for less than one percent of China’s electricity supply.

Further growth will be driven mostly by other rapidly developing countries such as India, Brazil, and Mexico. At the same time, Europe and North America are accelerating offshore wind park development: eight new offshore wind farms were connected to the grid in Europe in 2009.

TIME TRAVEL WITH WIND ENERGY

Wind Power's Beginnings (1000 B.C. - 1300 A.D.)

The history of wind power shows a general evolution from the use of simple, light devices driven by aerodynamic drag forces; to heavy, material-intensive drag devices; to the increased use of light, material-efficient aerodynamic lift devices in the modern era. But it shouldn't be imagined that aerodynamic lift (the force that makes airplanes fly) is a modern concept that was unknown to the ancients. The earliest known use of wind power, of course, is the sail boat, and this technology had an important impact on the later development of sail-type windmills. Ancient sailors understood lift and used it every day, even though they didn't have the physics to explain how or why it worked.

The first windmills were developed to automate the tasks of grain-grinding and water-pumping and the earliest-known design is the vertical axis system developed in Persia about 500-900 A.D. The first use was apparently water pumping, but the exact method of water transport is not known because no drawings or designs -- only verbal accounts -- are available. The first known documented design is also of a Persian windmill, this one with vertical sails made of bundles of reeds or wood which were attached to the central vertical shaft by horizontal struts.

Grain grinding was the first documented wind mill application and was very straightforward. The grinding stone was affixed to the same vertical shaft. The mill machinery was commonly enclosed in a building, which also featured a wall or shield to block the incoming wind from slowing the side of the drag-type rotor that advanced toward the wind.

Vertical-axis windmills were also used in China, which is often claimed as their birthplace. While the belief that the windmill was invented in China more than 2000 years ago is widespread and may be accurate, the earliest actual documentation of a Chinese windmill was in 1219 A.D. by the Chinese statesman Yehlu Chhu-Tshai. Here also, the primary applications were apparently grain grinding and water pumping.

One of the most scenic and successful applications of windpower (and one that still exists), is the extensive use of water pumping machines on the island of Crete. Here, literally hundreds of sail

rotor windmills pump water for crops and livestock.

An early sail-wing horizontal-axis mill on the Mediterranean coast.

Windmills in the Western World (1300 - 1875 A.D.)

The first windmills to appear in western Europe were of the horizontal-axis configuration. The reason for the sudden evolution from the vertical-axis Persian design approach is unknown, but the fact that European water wheels also had a horizontal-axis configuration -- and apparently served as the technological model for the early windmills -- may provide part of the answer. Another reason may have been the higher structural efficiency of drag-type horizontal machines over drag-type vertical machines, which lose up to half of their rotor collection area due to shielding requirements. The first illustrations (1270 A.D.) show a four- bladed mill mounted on a central post (thus, a "postmill") which was already fairly technologically advanced relative to the Persian mills. These mills used wooden cog-and-ring gears to translate the motion of the horizontal shaft to vertical movement to turn a grindstone. This gear was apparently adapted for use on post mills from the horizontal-axis water wheel developed by Vitruvius.

As early as 1390, the Dutch set out to refine the tower mill design, which had appeared somewhat earlier along the Mediterranean Sea. The Dutch essentially affixed the standard post mill to the top of a multi-story tower, with separate floors devoted to grinding  grain, removing chaff, storing grain, and (on the bottom) living quarters for the windsmith and his family. Both the post mill and the later tower mill design had to be oriented into the wind manually, by pushing a large lever at the back of the mill. Optimizing windmill energy and power output and protecting the mill from damage by furling the rotor sails during storms were among the windsmith's primary jobs.

While continuing well into the 19th century, the use of large tower mills declined with the increased use of steam engines. The next spurt of wind power development occurred many thousands of miles to the west.

A steel-bladed water pumping windmill in the American Midwest (late 1800's)

Role of Smaller Systems

For hundreds of years, the most important application of windmills at the subsistence level has been mechanical water pumping using relatively small systems with rotor diameters of one to several meters. These systems were perfected in the United States during the19th century, beginning with the Halladay windmill in 1854, and continuing to the Aermotor and Dempster designs, which are still in use today.

The first mills had four paddle-like wooden blades. They were followed by mills with thin wooden slats nailed to wooden rims. Most of these mills had tails to orient them into the wind, but some were weather-vaning mills that operated downwind of the tower. Speed control of some models was provided by hinging sections of blades, so that they would fold back like an umbrella in high winds, an action which reduced the rotor capture area to reduce thrust. The most important refinement of the American fan-type windmill was the development of steel blades in 1870 (Figure 4). Steel blades could be made lighter and worked into more efficient shapes. They worked so well, in fact, that their high speed required a reduction (slow-down) gear to turn the standard reciprocal pumps at the required speed.

In the United States, the development of the “water-pumping windmill” was the major factor in allowing the farming and ranching of vast areas otherwise devoid of readily accessible water. Windpumps contributed to the expansion of rail transport systems throughout the world, by pumping water from water wells for the steam locomotives.]The multi-bladed wind turbine atop a lattice tower made of wood or steel was, for many years, a fixture of the landscape throughout rural America. When fitted with generators and battery banks, small wind machines provided electricity to isolated farms.

First Use of Wind for "Large-Scale" Generation of Electricity

(1875 -1977 A.D.)

The first use of a large windmill to generate electricity was a system built in Cleveland, Ohio, in 1888 by Charles F. Brush. The Brush machine was a postmill with a multiple-bladed "picket-fence" rotor 17 meters in diameter, featuring a large tail hinged to turn the rotor out of the wind. It was the first windmill to incorporate a step-up gearbox (with a ratio of 50:1) in order to turn a direct current generator at its required operational speed (in this case, 500 RPM.)

Despite its relative success in operating for 20 years, the Brush windmill demonstrated the limitations of the low-speed, high-solidity rotor for electricity production applications. The 12 kilowatts produced by its 17-meter rotor pales beside the 70-100 kilowatts produced by a comparably-sized, modern, lift-type rotor.

In 1891, the Dane Poul La Cour developed the first electrical output wind machine to incorporate the aerodynamic design principles (low-solidity, four-bladed rotors incorporating primitive airfoil shapes) used in the best European tower mills. The higher speed of the La Cour rotor made these mills quite practical for electricity generation. By the close of World War I, the use of 25 kilowatt electrical output machines had spread throughout Denmark, but cheaper and larger fossil-fuel steam plants soon put the operators of these mills out of business.

The Brush postmill in Cleveland, Ohio, 1888. The first use of a large windmill to generate electricity.

By 1920, the two dominant rotor configurations (fan-type and sail) had both been tried and found to be inadequate for generating appreciable amounts of electricity. The further development of wind generator electrical systems in the United States was inspired by the design of airplane propellers and (later) monoplane wings.

Small System Pioneers

The first small electrical-output wind turbines simply used modified propellers to drive direct current generators. By the mid-1920's, 1 to 3-kilowatt wind generators developed by companies like Parris-Dunn and Jacobs Wind-electric found widespread use in the rural areas of the midwestern Great Plains. (A 3-kilowatt Jacobs unit is shown, being adjusted by a cigarette-puffing M.L. Jacobs at Rocky Flats, Colorado in 1977.) These systems were installed at first to provide lighting for farms and to charge batteries used to power crystal radio sets. But their use was extended to an entire array of direct-current motor-driven appliances, including refrigerators, freezers, washing machines, and power tools. But the more appliances were powered by the early wind generators, the more their intermittent operation became a problem.

The demise of these systems was hastened during the late 1930s and the 1940s by two factors: the demand of farmsteads for ever larger amounts of power on demand, and the Great Depression, which spurred the U.S. federal government to stimulate the depressed rural economies by extending the electrical grid throughout those areas.

A lot is made of this development and how horrible it was for the government to intervene. But the farmers who were helped by the new electrical grids would share this feeling. And the growing demand for electrical power created by the wind generator, combined with the inability of the technology to adapt, helped make the situation inevitable. The early success of the Midwest wind turbines actually set the stage for the possibility of more extensive wind energy development in the future.

While the market for new small wind machines of any type had been largely eroded in the United States by 1950, the use of mechanical and electrical system continued throughout Europe and in windy, arid climates such as those found in parts of Africa and Australia.

M.L. Jacobs adjusting the spring-actuated pitch change mechanism on a Jacobs Wind-electric in 1977.

"Bulk" Power from Wind

The development of bulk-power, utility-scale wind energy conversion systems was first undertaken in Russia in 1931 with the 100kW Balaclava wind generator. This machine operated for about two years on the shore of the Caspian Sea, generating 200,000 kWh of electricity. Subsequent experimental wind plants in the United States, Denmark, France, Germany, and Great Britain during the period 1935-1970 showed that large-scale wind turbines would work, but failed to result in a practical large electrical wind turbine.

The largest was the 1.25 megawatt Smith-Putnam machine, installed in Vermont in 1941. This horizontal-axis design featured a two-bladed, 175-foot diameter rotor oriented down-wind of the tower. The 16-ton stainless steel rotor used full-span blade pitch control to maintain operation at 28 RPM. In 1945, after only several hundred hours of intermittent operation, one of the blades broke off near the hub, apparently as a result of metal fatigue. This is not surprising considering the huge loads that must have been generated in a structure that had a lot in common with a gigantic rotating erector set.

Figure 7. Palmer Putnam's 1.25-megawatt wind turbine was one of the engineering marvels of the late 1930's, but the jump in scale was too great for available materials.

The modern wind power industry began in 1979 with the serial production of wind turbines by Danish manufacturers Kuriant,Vestas, Nordtank, and Bonus. These early turbines were small by today's standards, with capacities of 20–30 kW each. Since then, they have increased greatly in size, with the Enercon E-126 capable of delivering up to 7 MW, while wind turbine production has expanded to many countries.

Between 1850 and 1970, over six million mostly small (1 horsepower or less) mechanical output wind machines were installed in the U.S. alone. The primary use was water-pumping and the main applications were stock watering and farm home water needs. Very large windmills, with rotors up to 18 meters in diameter, were used to pump water for the steam railroad trains that provided the primary source of commercial transportation in areas where there were no navigable rivers.

European Development

European developments continued after World War II, when temporary shortages of fossil fuels led to higher energy costs. As in the United States, the primary application for these systems was interconnection to the electric power grid.

In Denmark, the 200 kW Gedser Mill wind turbine operated successfully until the early 1960s, when declining fossil-fuel prices once again made wind energy made uncompetitive with steam-powered generating plants. This machine featured a three-bladed upwind rotor with fixed pitch blades that used mechanical windmill technology augmented with an airframe support structure. The design was much less mechanically complex than the Smith-Putnam design. In fact, it was not that far removed from Poul La Cour's 1920-era windmill (a fact that worked to its advantage.)

An airframe holding together the three blades of the "Gedser Mollen." Fiberglass later eliminated this design requirement.

In Germany, Professor Ulrich Hutter developed a series of advanced, horizontal-axis designs of intermediate size that utilized modern, airfoil-type fiberglass and plastic blades with variable pitch to provide light weight and high efficiencies. This design approach sought to reduce bearing and structural failures by "shedding" aerodynamic loads, rather than "withstanding" them as did the Danish approach. One of the most innovative load-shedding design features was the use of a bearing at the rotor hub that allowed the rotor to "teeter" in response to wind gusts and vertical wind shear. Hutter's advanced designs achieved over 4000 hours of operation before the experiments were ended in 1968. Post war activity in Denmark and Germany largely dictated the two major horizontal-axis design approaches that would emerge when attention returned to wind turbine development in the early 1970s. The Danes refined the simple, fixed pitch, Gedser Mill design, utilizing advanced materials, improved aerodynamic design, and aerodynamic controls to reduce some of its shortcomings. The engineering innovations of the light-weight, higher efficiency German machines, such as a teeter hinge at the rotor hub, were used later by U.S. designers. The development of modern vertical-axis rotors was begun in France by G.J.M. Darrieus in the 1920s. Of the several rotors Darrieus designed, the most important one is a rotor comprising slender, curved, airfoil-section blades attached at the top and bottom of a rotating vertical tube. Major development work on this concept did not begin until the concept was reinvented in the late 1960s by two Canadian researchers.

U.S. efforts with the Darrieus concept at SANDIA NATIONAL LABORATORIES began after the 1973 oil embargo, with the entry of the U.S. Federal Wind Energy Program into the cycle of wind energy development.

Hutter's wind turbines, like other German devices of the mid-20th century, were advanced for th

An operating Dutch windmill (1994) that features leading edge airfoil sections (at top right). The mechanism used to turn the rotor into the wind and the windows of the first-floor living quarters are easily seen.

A primary improvement of the European mills was their designer's use of sails that generated aerodynamic lift. This feature provided improved rotor efficiency compared with the Persian mills by allowing an increase in rotor speed, which also allowed for superior grinding and pumping action.

The process of perfecting the windmill sail, making incremental improvements in efficiency, took 500 years. By the time the process was completed, windmill sails had all the major features recognized by modern designers as being crucial to the performance of modern wind turbine blades, including 1) camber along the leading edge, 2) placement of the blade spar at the quarter chord position (25% of the way back from the leading edge toward the trailing edge), 3) center of gravity at the same 1/4 chord position, and 4) nonlinear twist of the blade from root to tip (Drees, 1977). Some models also featured aerodynamic brakes, spoilers, and flaps. The machine shown in Figure 10 (which was operating with two of its buddies pumping water about one meter up from one irrigation pond to another in the Netherlands in 1994) features leading edge airfoil sections.

These mills were the "electrical motor" of pre-industrial Europe. Applications were diverse, ranging from the common waterwell, irrigation, or drainage pumping using a scoop wheel (single or tandem), grain-grinding (again, using single or multiple stones), saw-milling of timber, and the processing of other commodities such as spices, cocoa, paints and dyes, and tobacco.

Finally Time for Wind?(1977 – TILL NOW)

During the years 1977-1986, the commercial wind turbine market evolved from domestic and agricultural applications of small machines in the 1 to 25 kilowatt size range to utility interconnected wind farm applications of intermediate-scale machines of 50 to 600 kilowatts. Wind farms in California made up the majority of wind turbine installations until the early 1990s. In California, over 17,000 machines, ranging in output from 20 to 350 kilowatts, were installed in wind farms between 1981 and 1990. At the height of development, these turbines had a collected rating of over 1,700 megawatts and produced over 3 million megawatt hours of electricity, enough (at peak output) to power a city of 300,000.

The World Market Catches Up

In northern Europe and Asia, on the other hand, wind turbine installations increased steadily through the 1980s and 90s. The higher cost of electricity and excellent wind resources in northern Europe created a small, but stable, market for single, cooperative-owned wind turbines and small clusters of machines. After 1990, most market activity shifted to Europe and Asia. Driven by high utility power purchase rates, the installation of 50-kW, then 100-kW, then 200-kW, then 500-kW and now 1.5 megawatt wind turbines by cooperatives and private landowners in the Netherlands, Denmark, and Germany has been particularly impressive. The installation of over 10,000 megawatts of European wind capacity has helped support a thriving private wind turbine development and manufacturing industry. Until recently, this contrasted with the United States where low utility rates (primarily due to abundant, under-priced natural gas imported from Canada) and threatened deregulation of the utility industry virtually strangled wind energy development. 

A large international market has long been predicted for small village power or "wind-hybrid" installations. Despite some promising pilot projects, the apparent interest of many countries and of many non-governmental organizations (NGOs), and significant commitments from several wind turbine manufacturers and U.S. research laboratories (including SANDIA NATIONAL LABORATORIES and the NATIONAL RENEWABLE ENERGY LABORATORIES) this market has yet to emerge.

In the 1990s, the California wind farm market began to be affected by the expiration or forced re-negotiation of attractive power purchase contracts with the major California utilities: Southern California Edison and Pacific Gas and Electric. And much of the existing inventory of 1980's wind turbines were really an albatross around the wind industry's neck.

Renewal was needed, and -- bouyed by "green power" initiatives in Colorado, Texas and elsewhere -- U.S. wind energy development resumed in 1999, with a much broader geographical base.

A variety of new wind projects were installed in the U.S. in the late '90s, including a cluster of ZOND Z-40 TURBINES operated for a utility in southwest Texas, a wind plant of 46 Vestas machines planned for Big Spring, Texas, a 10-megawatt wind plant in Northern Colorado, a number of plants in the upper midwest, and the "re-powering" of some projects in California. Some of these involve foreign machines manufactured in the U.S. There's a sense that the industry is finally on the move again, with over 2000 megawatts of new capacity planned for 2001 in the U.S. alone. Existing and planned U.S. projects can be explored using the wind project map maintained by the American Wind Energy Association. 

The cost of energy from larger electrical output wind turbines used in utility-interconnected or wind farm applications has dropped from more than $1.00 per kilowatt-hour (kWh) in 1978 to under $0.05 per kWh in 1998, and is projected to plummet to $0.025 per kWh when new large wind plants come on line in 2001 and 2002. The hardware costs of these wind turbines have dropped below $800 per installed kilowatt in the past five years, underpricing the capital costs of almost every other type of power plant.

It's difficult to accurately compare the costs of wind plants and fossil fuel plants because the cost drivers are so different. Low installed-cost-per-kilowatt figures for wind turbines are somewhat misleading because of the low capacity factor of wind turbines relative to coal and other fossil-fueled power plants. ( "Capacity factor" is simply the ratio of actual energy produced by a power plant to the energy that would be produced if it operated at rated capacity for an entire year.) Capacity factors of successful wind farm operations range from 0.20 to 0.35. These can be compared with factors of more than 0.50 for fossil-fuel power plants and over 0.60 for some of the new gas turbines.

However, the use of "capacity factor" is also misleading because wind has a "rubber" capacity factor that varies with the density of the wind resource. But that wind resource is constant for the life of the machine and is not subject to manipulation or cost increases. One reason why fossil fuels are so popular with investors is that many of the risks are passed on to consumers. Fossil fuel shortages result in an increase in revenues for investors, who are actually rewarded for:

1) speeding the depletion of a nonrenewable resource or2) not investing enough of their profits in support infrastructure, which (as we have seen in 2000-2001) drives up prices.

If a big oil coal or gas company could start charging for the wind, they would make sure that wind power development happened. In late 1996, with the purchase of Zond Systems by Enron (a now-defunct gas mining and distribution company), the possibility of this happening became very real. (Even though Enron proved to be a poor steward for the Zond technology, the subsequent purchase of what was one of the only viable Enron divisions by GE Energy in 2003 maintained U.S. visibility in the large wind turbine market.)

Lowering the Cost-of-Energy Bar

Since the late 1970's the U.S. cost goals for wind power has continued to be about $0.04 per kilowatt hour, despite inflation. Wind turbines have consistently been able to arrive at that level, but by the time they get there, another reduction in the cost of non-renewable fossil fuels has taken place and the bar is lowered further.

Cost per kilowatt hour figures of $0.04 or less (in 1998 dollars) are now commonly projected for advanced U.S. wind turbines in 17 mph or better wind regimes, where capacity factors of over 0.40 can be achieved. That means that the wind energy cost goals of 1980 --which seemed daunting or impossible at the time--have been met many times over. (This fact should be remembered by those doubting the achievability of recently refigured cost goals--which are now closer to $0.025/kWh.)

The lower cost of energy from these advanced turbines is partly a result of higher efficiencies and rotor loading made possible by improved rotor design, shedding of fatigue loads provided by teetered hubs and flexible structures, and other innovations such as variable speed operation. But reduced weight and material usage and high reliability are perhaps more important factors in the cost equation.

Costs of smaller systems vary widely, with installed costs from $2000 to $3000 per installed kilowatt. Energy costs for small turbines of $0.12 to $0.20 are still the norm in the U.S. market.

The German Enercon features a huge, low speed "ring" generator

Worldwide, there are 10 to 12 manufacturers of large, utility-scale systems, marketing 200kW to 3.0 MW systems of various configurations, including three-bladed machines with full-span pitch control and two-bladed, stall control machines with teetering hubs. News on these developments is available from the major industry magazine,THE WINDPOWER MONTHLY. 

The more advanced configurations (from an aerodynamic standpoint at least) have been developed under the U.S. Department of Energy Advanced Turbine Program. 

European manufacturers like Tacke, Micon, Vestas, and Enercon have commercialized turbines with more conventional rotors, but featuring such important innovations as low speed generators and complete variable speed systems incorporating advanced power electronics. Recently, GE Energy (which purchased the wind division of defunct Enron) has adopted the European design philosophy in the U.S., with its merger of the technical expertise of Zond and Tacke. 

One of the latest innovations being investigated in the U.S. and Europe is the addition of a hinge at the nacelle-tower attachment, allowing the turbine to "nod" up and down in response to turbulence and wind shear (the difference in wind speed at the top and bottom of the  rotor disk). This configuration has been tested at Riso and promises substantial reductions in rotor and drive-train loads and in control system costs. A model intended for commercial development operated in California for several years and has been investigated by the National Wind Technology Center. However, such innovations may not be necessary for wind to meet its cost goals for several years. 

The result of recent mergers is that, in 2001, there is a virtual internationalization of the wind turbine industry and research community. As recent as 1995, pundits like Paul Gipe could claim that the Europeans' use of smaller machines with conventional aircraft airfoils meant that low tech had beaten high tech in the wind business. In 2001, with European wind turbine power ratings pushing 2 megawatts, Denmark's Riso Laboratories touting its new wind turbine airfoil designs (modeled closely after pioneering activities in the U.S.), and the U.S. company Enron marketing machines from both the U.S. and Europe, there is really very little difference between European and U.S. technology. The last remaining major area of controversy is the issue of two versus three blades for

large wind turbines. Theoretically, a two-bladed machine should be less expensive and more efficient than a three-bladed one. But considerable refinements are still needed to offset the greater stability and lower per-blade loads of three-bladed designs. And the optical illusion of speed fluctuations and out-of-plane rotation associated with two-bladed machines makes them less attractive to some onlookers. Time will tell if one design will win out or if both will be able to exist in specific applications.

Based on the mid-1980's ESI-80, the 2-bladed, AWT 26-meter machine was a contemporary expression andrefinement of the Hutter design philosophy. Whether or not such designs will ever be widely commercialized is uncertain.

The Future Is Now

In the near future, wind energy will be the most cost effective source of electrical power. In fact, a good case can be made for saying that it already has achieved this status. The actual life cycle cost of fossil fuels (from mining and extraction to transport to use technology to environmental impact to political costs and impacts, etc.) is not really known, but it is certainly far more than the current wholesale rates. The eventual depletion of these energy sources will entail rapid escalations in price which -- averaged over the brief period of their use -- will result in postponed actual costs that would be unacceptable by present standards. And this doesn't even consider the environmental and political costs of fossil fuels use that are silently and not-so-silently mounting every day.

The major technology developments enabling wind power commercialization have already been made. There will be infinite refinements and improvements, of course. One can guess (based on experience with other technologies) that the eventual push to full commercialization and deployment of the technology will happen in a manner that no one can imagine today. There will be a "weather change" in the marketplace, or a "killer application" somewhere that will put several key companies or financial organizations in a position to profit. They will take advantage of public interest, the political and economic climate, and emotional or marketing factors to position wind energy technology (developed in a long lineage from the Chinese and the Persians to the present wind energy researchers and developers) for its next round of development.

TYPES OF WIND TURBINE 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.

Horizontal Axis Wind Turbines: (HAWT)

Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.

Since a tower produces turbulence behind it, the turbine is usually positioned upwind of its supporting tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted forward into the wind a small amount.

Downwind machines have been built, despite the problem of turbulence (mast wake), because they don't need an additional mechanism for keeping them in line with the wind, and because in high winds the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since cyclical (that is repetitive) turbulence may lead to fatigue failures, most HAWTs are of upwind design.

11 x 7,5 MW E126 Estinnes Windfarm, Belgium, July 2010, one month before completion, with unique 2 part blades.

Modern wind turbines

Turbines used in wind farms for commercial production of electric power are usually three-bladed and pointed into the wind by computer-controlled motors. These have high tip speeds of over 320 kilometres per hour (200 mph), high efficiency, and low torque ripple, which contribute to good reliability. The blades are usually colored light gray to blend in with the clouds and range in length

from 20 to 40 metres (66 to 130 ft) or more. The tubular steel towers range from 60 to 90 metres (200 to 300 ft) tall. The blades rotate at 10-22 revolutions per minute. At 22 rotations per minute the tip speed exceeds 300 feet per second (91 m/s). A gear box is commonly used for stepping up the speed of the generator, although designs may also use direct drive of an annular generator. Some models operate at constant speed, but more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system. All turbines are equipped with protective features to avoid damage at high wind speeds, by feathering the blades into the wind which ceases their rotation, supplemented by brakes.

Vertical Axis Wind Turbines: (VAWT)

Vertical axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically. Key advantages of this arrangement are that the turbine does not need to be pointed into the wind to be effective. This is an advantage on sites where the wind direction is highly variable, for example when integrated into buildings. The key disadvantages include the low rotational speed with the consequential higher torque and hence higher cost of the drive train, the inherently lower power coefficient, the 360 degree rotation of the aerofoil within the wind flow during each cycle and hence the highly dynamic loading on the blade, the pulsating torque generated by some rotor designs on the drive train, and the difficulty of modelling the wind flow accurately and hence the challenges of analysing and designing the rotor prior to fabricating a prototype.

With a vertical axis, the generator and gearbox can be placed near the ground, hence avoiding the need of a tower and improving accessibility for maintenance. Drawbacks of this configuration include(i) wind speeds are lower close to the ground, so less wind energy is available for a given size

turbine, and (ii) wind shear is more severe close to the ground, so the rotor experiences higher loads.

Air flow near the ground and other objects can create turbulent flow, which can introduce problems associated with vibration, such as noise and bearing wear which may increase the maintenance or shorten the service life. However, when a turbine is mounted on a rooftop, the building generally redirects wind over the roof and this can double the wind speed at the turbine. If the height of the rooftop mounted turbine tower is approximately 50% of the building height, this is near the optimum for maximum wind energy and minimum wind turbulence. It should be borne in mind that wind speeds within the built environment are generally much lower than at exposed rural sites.

Features:

No massive tower required so construction costs are lower. No Yaw device required. Less noise than HAWT models. Higher airfoil pitch angle improves aerodynamics. Lower wind startup speeds. Larger swept area than HAWT's with the same radius. No turning to face the wind so VAWT's are ideal for turbulent

conditions.

SUBTYPES

Darrieus wind turbine 

"Eggbeater" turbines, or Darrieus turbines, were named after the French inventor, Georges Darrieus. They have good efficiency, but produce large torque ripple and cyclical stress on the tower, which contributes to poor reliability. They also generally require some external power source, or an additional Savonius rotor to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades which results in greater solidity of the rotor. Solidity is measured by blade area divided by the rotor area. Newer Darrieus type turbines are not held up by guy-wires but have an external superstructure connected to the top bearing.

Giromill

A subtype of Darrieus turbine with straight, as opposed to curved, blades. The cycloturbine variety has variable pitch to reduce the torque pulsation and is self-starting. The advantages of variable pitch are: high starting torque; a wide, relatively flat torque curve; a lower blade speed ratio; a higher coefficient of performance; more efficient operation in turbulent winds; and a lower blade speed ratio which lowers blade bending stresses. Straight, V, or curved blades may be used.

Savonius wind turbine 

These are drag-type devices with two (or more) scoops that are used in anemometers, Flettner vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always self-starting if there are at least three scoops. They sometimes have long helical scoops to give a smooth torque.

Darrieus wind turbine of 30 m in the magdalen Islands.

Advantages of Vertical axis turbine

Omni directional—accepts wind from any direction.

Components can be mounted at ground level providing ease in service and establishing lighter weight towers.

Can theoretically use less material to capture the same amount of wind.

Disadvantages of Vertical axis turbine

Rotors generally near ground where wind is poorer. Centrifugal force stresses blades. Poor self starting capabilities. Requires support at top of turbine rotor. Requires entire rotor to be removed to change the bearings.

Overall poor performance.

TURBINE DESIGN AND CONSTRUCTION

Wind turbines are designed to exploit the wind energy that exists at a location. Aerodynamic modeling is used to determine the optimum tower height, control systems, number of blades and blade shape.

Wind turbines convert wind energy to electricity for distribution. Conventional horizontal axis turbines can be divided into three components.

The rotor component, which is approximately 20% of the wind turbine cost, includes the blades for converting wind energy to low speed rotational energy.

The generator component, which is approximately 34% of the wind turbine cost, includes the electrical generator, the control electronics, and most likely a gearbox (e.g.planetory gearbox,adjustable-speed drive or continuously variable transmission) component for converting the low speed incoming rotation to high speed rotation suitable for generating electricity.

The structural support component, which is approximately 15% of the wind turbine cost, includes the tower and rotor yaw mechanism.

A 1.5 MW wind turbine of a type frequently seen in the United States has a tower 80 meters high. The rotor assembly (blades and hub) weighs 48,000 pounds (22,000 kg). The nacelle, which contains the generator component, weighs 115,000 pounds (52,000 kg). The concrete base for the tower is constructed using 58,000 pounds (26,000 kg) of reinforcing steel and contains 250 cubic yards of concrete. The base is 50 feet (15 m) in diameter and 8 feet (2.4 m) thick near the center.

Turbine Components

Horizontal turbine components include:

blade or rotor, which converts the energy in the wind to rotational shaft energy; a drive train, usually including a gearbox and a generator; a tower that supports the rotor and drive train; and other equipment, including controls, electrical cables, ground support equipment, and

interconnection equipment.

Wind Turbine diagram

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 turbines range in size from 50 to 750 kilowatts. Single small turbines, below 50 kilowatts, are used for homes, telecommunications dishes, or water pumping.

Unconventional wind turbines

One E-66 wind turbine at Windpark Holtriesm, Germany, carries an observation deck, open for visitors. Another turbine of the same type, with an observation deck, is located in Swffam, England. Airborne wind turbines have been investigated many times but have yet to produce significant energy. Conceptually, wind turbines may also be used in conjunction with a large vertical solar updraft tower to extract the energy due to air heated by the sun.

Wind turbines which utilise the Magnus effect have been developed.

Small wind turbines

A small wind turbine being used in Australia.

Small wind turbines may be as small as a fifty-watt generator for boat or caravan use. Small units often have direct drive generators, dirct current output, aeroelastic blades, lifetime bearings and use a vane to point into the wind.

Larger, more costly turbines generally have geared power trains, alternating current output, flaps and are actively pointed into the wind. Direct drive generators and aeroelastic blades for large wind turbines are being researched.

Wind turbine spacing

Wind turbines are best spaced 15 to 25 times the rotor diameter apart. This has been concluded by research conducted by Charles Meneveau of the Johns Hopkins University. On most windturbine farms, a spacing of 7 times the rotor diameter is often upheld, but this has been shown to be too little.

Electricity generation

Typical components of a wind turbine (gearbox, rotor shaft and brake assembly) being lifted into position

In a wind farm, individual turbines are interconnected with a medium voltage (often 34.5 kV), power collection system and communications network. At a substation, this medium-voltage electric current is increased in voltage with a transformer for connection to the high voltage electric power transmisssion system.

The surplus power produced by domestic microgenerators can, in some jurisdictions, be fed into the network and sold to the utility company, producing a retail credit for the microgenerators' owners to offset their energy costs.

Grid management

Induction generators, often used for wind power, require reactive power for excitation so substations used in wind-power collection systems include substantial capacitor banks for power factor correction. Different types of wind turbine generators behave differently during transmission grid disturbances, so extensive modelling of the dynamic electromechanical characteristics of a new wind farm is required by transmission system operators to ensure predictable stable behaviour during system faults. In particular, induction generators cannot support the system voltage during faults, unlike steam or hydro turbine-driven synchronous generators. Doubly-fed machines generally have more desirable properties for grid interconnection. Transmission systems operators will supply a wind farm developer with a grid code to specify the requirements for interconnection to the transmission grid. This will include power factor, constancy of frequency and dynamic behavior of the wind farm turbines during a system fault.

Capacity factor

Since wind speed is not constant, a wind farm’s annual energy production is never as much as the sum of the generator nameplate ratings multiplied by the total hours in a year. The ratio of actual productivity in a year to this theoretical maximum is called the capacity factor. Typical capacity factors are 20–40%, with values at the upper end of the range in particularly favourable sites. For example, a 1 MW turbine with a capacity factor of 35% will not produce 8,760 MW·h in a year (1 × 24 × 365), but only 1 × 0.35 × 24 × 365 = 3,066 MW·h, averaging to 0.35 MW. Online data is available for some locations and the capacity factor can be calculated from the yearly output.

Unlike fueled generating plants, the capacity factor is limited by the inherent properties of wind. Capacity factors of other types of power plant are based mostly on fuel cost, with a small amount of downtime for maintenance. Nuclear plants have low incremental fuel cost, and so are run at full output and achieve a 90% capacity factor. Plants with higher fuel cost are throttled back to follow load. Gas turbine plants using natural gas as fuel may be very expensive to operate and may be run only to meet peak power demand. A gas turbine plant may have an annual capacity factor of 5–25% due to relatively high energy production cost.

In a 2008 study released by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy, the capacity factor achieved by the wind turbine fleet is shown to be increasing as the technology improves. The capacity factor achieved by new wind turbines in 2004 and 2005 reached 36%.

Penetration

Wind energy "penetration" refers to the fraction of energy produced by wind compared with the total available generation capacity. There is no generally accepted "maximum" level of wind penetration. The limit for a particular grid will depend on the existing generating plants, pricing mechanisms, capacity for storage or demand management, and other factors. An interconnected electricity grid will already include reserve generating and transmission capacity to allow for equipment failures; this reserve capacity can also serve to regulate for the varying power generation by wind plants. Studies have indicated that 20% of the total electrical energy consumption may be incorporated with minimal difficulty.These studies have been for locations with geographically dispersed wind farms, some degree of dispatchable energy, or hydropower with storage capacity, demand management, and interconnection to a large grid area export of electricity when needed. Beyond this level, there are few technical limits, but the economic implications become more significant. Electrical utilities continue to study the effects of large (20% or more) scale penetration of wind generation on system stability and economics.

At present, a few grid systems have penetration of wind energy above 5%: Denmark (values over 19%), Spain and Portugal (values over 11%), Germany and the Republic of Ireland (values over 6%). But even with a modest level of penetration, there can be times where wind power provides a substantial percentage of the power on a grid. For example, in the morning hours of 8 November 2009, wind energy produced covered more than half the electricity demand in Spain, setting a new record. This was an instance where demand was very low but wind power generation was very high.

Variability and intermittency

Electricity generated from wind power can be highly variable at several different timescales: from hour to hour, daily, and seasonally. Annual variation also exists, but is not as significant. Related to variability is the short-term (hourly or daily) predictability of wind plant output. Like other electricity sources, wind energy must be "scheduled". Wind power forecasting methods are used, but predictability of wind plant output remains low for short-term operation.

Because instantaneous electrical generation and consumption must remain in balance to maintain grid stability, this variability can present substantial challenges to incorporating large amounts of wind power into a grid system. Intermittency and the non-dispatcable nature of wind energy production can raise costs for regulation, incremental operating reserve, and (at high penetration levels) could require an increase in the already existing energy demend management, load shedding, or storage solutions or system interconnection with HVDC cables. At low levels of wind penetration, fluctuations in load and allowance for failure of large generating units requires reserve capacity that can also regulate for variability of wind generation. Wind power can be replaced by other power stations during low wind periods. Transmission networks must already cope with outages of generation plant and daily changes in electrical demand. Systems with large wind capacity components may need more spinning reserve (plants operating at less than full load).

Pumped-storage hydroelecticity or other forms of grid energy storage can store energy developed by high-wind periods and release it when needed. Stored energy increases the economic value of wind energy since it can be shifted to displace higher cost generation during peak demand periods. The potential revenue from this arbitrage can offset the cost and losses of storage; the cost of storage may add 25% to the cost of any wind energy stored, but it is not envisaged that this would apply to a large proportion of wind energy generated. The 2 GW Dinorwig pumped storage plant in Wales evens out electrical demand peaks, and allows base-load suppliers to run their plant more efficiently. Although pumped storage power systems are only about 75% efficient, and have high installation costs, their

low running costs and ability to reduce the required electrical base-load can save both fuel and total electrical generation costs.

In particular geographic regions, peak wind speeds may not coincide with peak demand for electrical power. In the US states of California and Texas, for example, hot days in summer may have low wind speed and high electrical demand due to air conditioning. Some utilities subsidize the purchase of geothemalheat pumps by their customers, to reduce electricity demand during the summer months by making air conditioning up to 70% more efficient; widespread adoption of this technology would better match electricity demand to wind availability in areas with hot summers and low summer winds. Another option is to interconnect widely dispersed geographic areas with an HVDC “Super grid”. In the USA it is estimated that to upgrade the transmission system to take in planned or potential renewables would cost at least $60 billion.

In the UK, demand for electricity is higher in winter than in summer, and so are wind speeds.Solar power tends to be complementary to wind. On daily to weekly timescales, high pressure areas tend to bring clear skies and low surface winds, whereas low pressure areas tend to be windier and cloudier. On seasonal timescales, solar energy typically peaks in summer, whereas in many areas wind energy is lower in summer and higher in winter. Thus the intermittencies of wind and solar power tend to cancel each other somewhat. The Institute for Solar Energy Supply Technology of the Uneversity of Kassel pilot-tested a combined power plant linking solar, wind, biogas and hydrostorage to provide load-following power around the clock, entirely from renewable sources.

A report on Denmark's wind power noted that their wind power network provided less than 1% of average demand 54 days during the year 2002. Wind power advocates argue that these periods of low wind can be dealt with by simply restarting existing power stations that have been held in readiness or interlinking with HVDC. Electrical grids with slow-responding thermal power plants and without ties to networks with hydroelectric generation may have to limit the use of wind power.

Three reports on the wind variability in the UK issued in 2009, generally agree that variability of wind needs to be taken into account, but it does not make the grid unmanageable; and the additional costs, which are modest, can be quantified

Capacity credit and fuel saving

Many commentators concentrate on whether or not wind has any "capacity credit" without defining what they mean by this and its relevance. Wind does have a capacity credit, using a widely accepted and meaningful definition, equal to about 20% of its rated output (but this figure varies depending on actual circumstances). This means that reserve capacity on a system equal in MW to 20% of added wind could be retired when such wind is added without affecting system security or robustness. But the precise value is irrelevant since the main value of wind (in the UK, worth 5 times the capacity credit value) is its fuel and CO2 savings.

According to a 2007 Stanford University study published in the Journal of Applied Meteorology and Climatology, interconnecting ten or more wind farms can allow an average of 33% of the total energy produced to be used as reliable, baseload electric power, as long as minimum criteria are met for wind speed and turbine height.

THE AERODYNAMICS OF THE WIND TURBINE

The three bladed rotor is the most important and most visible part of the wind turbine. It is through the rotor that the energy of the wind is transformed into mechanical energy that turns the main shaft of the wind turbine.

BASIC THEORYAerodynamics is the science and study of the physical laws of the behavior of objects in an air flow and the forces that are produced by air flows.The front and rear sides of a wind turbine rotor blade have a shape roughly similar to that of a long rectangle, with the edges bounded by the leading edge, the trailing edge, the blade tip and the blade root. The blade root is bolted to the hub. The radius of the blade is the distance from the rotor shaft to the outer edge of the blade tip. Some wind turbine blade have moveable blade tips as air brakes, and one can often see the distinct line separating the blade tip component from the blade itself. If a blade were sawn in half, one would see that the cross section has a streamlined asymmetrical shape, with the flattest side facing the oncoming air flow or wind. This shape is called the blade’s aerodynamic profile.

THE AERODYNAMIC PROFILE

The shape of the aerodynamic profile is decisive for blade performance. Even minor alterations in the shape of the profile can greatly alter the power curve and noise level. Therefore a blade designer does not merely sit down and outline the shape when designing a new blade. The shape must be chosen with great care on the basis of past experience. For this reason blade profiles were previously chosen from a widely used catalogue of airfoil profiles developed in wind tunnel research by NACA (The United States National Advisory Committee for Aeronautics) around the time of the Second World War. The NACA 44 series profiles were used on older Bonus wind turbines (up to and including the 95 kW models).

This profile was developed during the 1930’s, and has good all-round properties, giving a good power curve and a good stall. The blade is tolerant of minor surface imperfections, such as dirt on the blade profile surface. The LM blades used on newer Bonus wind turbines (from the 150 kW models) use the NACA 63 profiles developed during the 1940’s. These have slightly different properties than the NACA 44 series. The power curve is better in the low and medium wind speed ranges, but drops under operation at higher wind speeds. Likewise this profile is more sensitive with regard to surface dirt. This is not so important in Denmark, but in certain climate zones with little rain, accumulated dirt, grime and insect deposits may impair and reduce performance for longer periods. The LM 19 blades, specifically developed for wind turbines, used on the Bonus 500 kW, have completely new aerodynamic profiles and are therefore not found in the NACA catalogue. These blades were developed in a joint LM and Bonus research project some years ago, and further developed and wind tunnel tested by FFA (The Aerodynamic Research Institute of The Swedish Ministry of Defence).

THE AERODYNAMICS OF A MAN ON A BICYCLETo fully describe the aerodynamics of a wind turbine blade could appear to be rather complicated and difficult to understand. It is not easy to fully understand how the direction of the air flow around the blade is dependent on the rotation of the blade. Fortunately for us, air constantly flows around everyday objects following these very same aerodynamic laws. Therefore we can start with the aerodynamics of an air flow that most of us are much more familiar with: A cyclist on a windy day.

The diagrams (next page) show a cyclist as seen from above. The diagrams are perhaps rather sketchy, but with a good will one can visualize what they represent. The diagram (A) on the left, illustrates a situation, during which a cyclist is stationary and can feel a side wind “v” of 10 meters per second (m/s) or roughly 22 mph (this is known as a fresh breeze). The wind pressure will attempt to overturn the cyclist. We can calculate the pressure of the wind on the windward side of the cyclist as roughly 80 Newton per square meter of the total side area presented by the cyclist against the wind. Newton, or N for short, is the unit for force used in technical calculation. 10 N is about 1kg/force (Multiply by 0.2248 to obtain lbf.). The direction of the force of the wind pressure is in line with the wind flow. If we consider that a normal sized cyclist has a side area facing the wind of about 0.6 square meters, then the force F from the pressure of the wind will be 0.6 x 80 N =app. 50 N/m2.In the center drawing (B) our cyclist has started out and is traveling at a speed “u” of 20 km/hour, equivalent to about 6 meters/second, still with a side wind ‘v” of 10 m/s. We can therefore calculate the speed of the resulting wind “w” striking the cyclist, either mathematically or by measurement on the diagram as 12 m/s. This gives a total wind pressure of 100 N/m2. The direction of the wind pressure is now in line with the resulting wind, and this will give a force “F’ on the cyclist of about 60 N/m2. In the right hand drawing (C) the force of the wind pressure “F” is now separated into a component along the direction of the cyclist’s travel and into another component at a right angle to the direction of travel. The right angled force “Fv" will attempt to overturn the cyclist, and the force “Fm” along the axis of travel gives a resistance that slows down the cyclist’s forward motion. The size of “Fm” is about 30 N/m2. This is the resistance force that the cyclist must overcome. A beginner, unused to cycling, may wonder why the wind has changed direction and a head wind is felt on reaching speed. This beginner might well ask “ How can it be that I felt a side wind when I was at rest and standing still, could the wind have possibly changed its direction? “ But no, as any experienced cyclist unfortunately knows, head wind is an integral component of movement itself. The wind itself has not turned. The head wind is a result of speed, the faster one travels the more wind resistance one experiences. Perhaps, as a famous Danish politician once promised his voters, that if elected he would insure favorable tailwinds on the cycle-paths, things may change in the future. However we others have learnt to live with the head winds resulting from our own forward movement, whether we run, cycle or go skiing.

WIND TURBINE BLADES BEHAVE IN THE SAME WAYReturning to the wind turbine blade, just as in the situation for the cyclist, we can observe the aerodynamic and force diagrams in two different situations.

,

when the wind turbine is stationary and when it is running at a normal operational speed. We will use as an example the cross section near the blade tip of a Bonus 450 kW Mk III operating in a wind speed “v” of 10 m/s. When the rotor is stationary, as shown in drawing (A) below, the wind has a direction towards the blade, at a right angle to the plane of rotation, which is the area swept by the rotor during the rotation of the blades. The wind speed of 10 m/s will produce a wind pressure of 80 N/m2 of blade surface, just like the effect on our cyclist. The wind pressure is roughly in the same direction as the wind and is also roughly perpendicular to the flat side of the blade profile. The part of the wind pressure blowing in the direction of the rotor shaft attempts to bend the blades and tower, while the smaller part of the wind pressure blowing in the direction of the rotation of the blades produces a torque that attempts to start the wind turbine. Once the turbine is in operation and the rotor is turning, as is shown in the center diagram (B), the blade encounters a head wind from its own forward movement in exactly the same way as the cyclist does. The strength of head wind “u” at any specific place on the blade depends partly on just how fast the wind turbine blade is rotating, and partly how far out on the blade one is from the shaft. In our example, at the normal operating speed of 30 rpm, the head wind “u” near the tip of the 450 kW wind turbine is about 50 m/s. The “meteorological” wind “v” of 10 m/s will thus give a resulting wind over the profile of about 51 m/s. This resulting wind will have an effect on the blade surface with a force of 1500 N/m2. The force “F” will not be in the direction of the resulting wind, but almost at a right angle to the resulting wind.

In the drawing on the right (C) the force of the wind pressure “F” is again split up into a component in the direction of rotation and another component at a right angle to this direction. The force “Fa” at a right angle to the plane of rotation attempts to bend the blade back against the tower, while the force “Fd” points in the direction of rotation and provides the driving torque. We may notice two very important differences between the forces on the blade in these two different situations and forces on the cyclist in the two corresponding situations. One difference is that the forces on the blade become very large during rotation. If vector arrows illustrating the forces in the diagrams were drawn in a scale that was indicative of the sizes of the different forces, then these vector arrows of a wind turbine in operation would have been 20 times the size of the vector arrows of the same wind turbine at rest. This large difference is due to the resulting wind speed of 51 m/s striking a blade during operation, many times the wind speed of 10 m/s when the wind turbine is at rest. Just like the cyclist, the blade encounters head wind resulting from its own movement, however head wind is of far greater importance on a wind turbine blade than for a cyclist in motion. The other important difference between a wind turbine blade and a cyclist is that the force on the blade is almost at a right angle to the resulting wind striking the profile. This force is known as the lift and also produces a small resistance or drag. The direction of this lift force is of great importance.

A cyclist only feels the wind resistance as a burden, requiring him to push down extra hard on the pedals. However with a wind turbine blade this extra wind resistance will act as a kind of power booster, at least in the normal blade rotational speed range. The reason for this difference is due to the blades streamlined profile, which behaves aerodynamically completely differently as compared to the irregular shaped profile of a man on a bicycle. The wind turbine blade experiences both lift and drag, while a cyclist only experiences drag.

LIFTLift is primary due to the physical phenomena known as Bernoulli’s Law. This physical law states that when the speed of an air flow over a surface is increased the pressure will then drop. This law is counter to what most people experience from walking or cycling in a head wind, where normally one feels that the pressure increases when the wind also increases. This is also true when one sees an air flow blowing directly against a surface, but it is not the case when air is flowing over a surface.

One can easily convince oneself that this is so by making a small experiment. Take two small pieces of paper and bend them slightly in the middle. Then hold them as shown in the diagram and blow in between them. The speed of the air is higher in between these two pieces of paper than outside (where of course the air speed is about zero), so therefore the pressure inside is lower and according to Bernoulli’s Law the papers will be sucked in towards each other. One would expect that they would be blown away from each other, but in reality the opposite occurs. This is an interesting little experiment, that clearly demonstrates a physical phenomenon that has a completely different result than what one would expect. Just try for yourself and see.

The aerodynamic profile is formed with a rear side, that is much more curved than the front side facing the wind. Two portions of air molecules side by side in the air flow moving towards the profile at point A will separate and pass around the profile and will once again be side by side at point B after passing the

Profile’s trailing edge. As the rear side is more curved than the front side on a wind turbine blade, this means that the air flowing over the rear side has to travel a longer distance from point A to B than the air flowing over the front side. Therefore this air flow over the rear side must have a higher velocity if these two different portions of air shall be reunited at point B. Greater velocity produces a pressure drop on the rear side of the blade, and it is this pressure drop that produces the lift. The highest speed is obtained at the rounded front edge of the blade. The blade is almost sucked forward by the pressure drop resulting from this greater front edge speed. There is also a contribution resulting from a small over-pressure on the front side of the blade.

Compared to an idling blade the aerodynamic forces on the blade under operational conditions are very large. Most wind turbine owners have surely noticed these forces during a start-up in good wind conditions. The wind turbine will start to rotate very slowly at first, but as it gathers speed it begins to accelerate faster and faster. The change from slow to fast acceleration is a sign that the blade’s aerodynamic shape comes into play, and that the lift greatly increases when the blade meets the head wind of its own movement.

The fast acceleration, near the wind turbine’s operational rotational speed places great demands on the electrical cut-in system that must “capture and engage “ the wind turbine without releasing excessive peak electrical loads to the grid.

THE CHANGE OF FORCES ALONG THE BLADE

The drawings previously studied, mainly illustrate the air flow situation near the blade tip. In principle these same conditions apply all over the blade, however the size of the forces and their direction change according to their distance to the tip. If we once again look at a 450 kW blade in a wind speed of 10 m/s, but this time study the situation near the blade root, we will obtain slightly different results as shown in the drawing above.

In the stationary situation (A) in the left hand drawing, wind pressure is still 80 N/m2 . The force “F” becomes slightly larger than the force at the tip, as the blade is wider at the root. The pressure is once again roughly at a right angle to the flat side of the blade profile, and as the blade is more twisted at the root, more of the force will be directed in the direction of rotation, than was the case at the tip. On the other hand the force at the root has not so great a torque-arm effect in relation to the rotor axis and therefore it will contribute about the same force to the starting torque as the force at the tip.

During the operational situation as shown in the center drawing (B), the wind approaching the profile is once again the sum of the free wind “v” of 10 m/s and the head wind “u” from the blade rotational movement through the air. The head wind near the blade root of a 450 kW wind turbine is about 15 m/s and this produces a resulting wind “w” over the profile of 19 m/s. This resulting wind will act on the blade section with a force of about 500 N/m2.

In the drawing on the right (C) fore is broken down into wind pressure against the tower “Fa”, and the blade driving force “Fd” in the direction of rotation. In comparison with the blade tip the root section produces less aerodynamic forces during operation, however more of these forces are aligned in the correct direction, that is, in the direction of rotation. The change of the size and direction of these forces from the tip in towards the root, determine the form and shape of the blade.

Head wind is not so strong at the blade root, so therefore the pressure is likewise not so high and the blade must be made wider in order that the forces should be large enough. The resulting wind has a greater angle in relation to the plane of rotation at the root, so the blade must likewise have a greater angle of twist at the root. It is important that the sections of the blade near the hub are able to resist forces and stresses from the rest of the blade. Therefore the root profile is both thick and wide, partly because the thick broad profile gives a strong and rigid blade and partly because greater width, as previously mentioned, is necessary on account of the resulting lower wind speed across the blade. On the other hand, the aerodynamic behavior of a thick profile is not so effective.

Further out along the blade, the profile must be made thinner in order to produce acceptable aerodynamic properties, and therefore the shape of the profile at any given place on the blade is a compromise between the desire for strength (the thick wide profile) and the desire for good aerodynamic properties (the thin profile) with the need to avoid high aerodynamic stresses (the narrow profile).

As previously mentioned, the blade is twisted so that it may follow the change in direction of the resulting wind. The angle between the plane of rotation and the profile chord, an imaginary line drawn between the leading edge and the trailing edge, is called the setting angle, sometimes referred to as “Pitch”.

WHAT HAPPENS WHEN THE WIND SPEED CHANGES?The description so far was made with reference to a couple of examples where wind speed was at a constant 10 m/s.We will now examine what happens during alterations in the wind speed. In order to understand blade behavior at different wind speeds, it is necessary to understand a little about how lift and drag change with a different angle of attack. This is the angle between the resulting wind “w” and the profile chord. In the drawing below the angle of attack is called “a” and the setting angle is called “b”. The setting angle has a fixed value at any one given place on the blade, but the angle of attack will grow as the wind speed increases.

The aerodynamic properties of the profile will change when the angle of attack “a” changes. These changes of lift and drag with increasing angles of attack, are illustrated in the diagram above used to calculate the strength of these two forces, the lift coefficient “CL” and the drag coefficient “CD”. Lift will always be at a right angle to the resulting wind, while drag will always follow in the direction of the resulting wind.We will not enter into the formulas necessary to calculate these forces, it is enough to know that there is a direct connection between the size of “CL” and the amount of lift. Both lift and drag abruptly change when the angle of attack exceeds 15-20 degrees. One can say that the profile stalls. After this stalling point is reached, lift falls and drag increases. The angle of attack changes when the wind speed changes.

To further study these changes, we can draw diagrams, shown to the right, illustrating three different wind speeds “v” (5, 15 and 25 m/s) from our previous cross section, this time near the blade tip of a 450 kW wind turbine. This situation is rather convenient as the setting angle “b” near the wing tip is normally 0 degrees.The head wind from the movement “u” is always the same, as the wind turbine has a constant rotational speed controlled by the grid connected generator (in these situations we do not consider the small generator used on certain small wind turbines). The free air flow “v” has three different values and this gives three different values of the resulting wind “w” across the profile. The size of “w” does not change very much, from 50 m/s at a wind speed of 5 m/s to 52 m/s in a 25 m/s wind. The reason for this relatively minor change is due to the dominating effect of the head wind.

However, the angle of attack “a” between the resulting wind and the chord of the blade changes from 6 degrees at a wind speed of 5 m/s to 16 degrees at 15 m/s to 27 degrees at 25 m/s. These changes are of great importance for determining the strength of the aerodynamic forces.Studying the diagram showing the lift coefficient “CL” and the drag coefficient “CD” we may note the following:

At a wind speed of 5 m/s (A), the angle of attack is 6 degrees. The lift coefficient is 0.9 and the coefficient of drag is 0.01. Lift is therefore 90 times greater than drag, and the resultant force “F” points almost vertically at a right angle to the mean relative wind “w”

At a wind speed of 15 m/s (B), the profile is almost about to stall. The angle of attack is 16 degrees. The lift coefficient is 1.4 and the coefficient of drag is 0.07. Lift is now 20 times drag.

At a wind speed of 25 m/s (C), the profile is now deeply stalled, the angle of attack is 27 degrees, the lift component is 1.0 and the component of lift is 0.35. Lift is now 3 times greater than drag. We can therefore note the following:

During the change of wind speed from 5 to 15 m/s there is a significant increase in lift, and this increase is directed in the direction of rotation. Therefore power output of the wind turbine is greatly increased from 15 kW to 475 kW.

During the change of wind speed from 15 to 25 m/s, there is a drop in lift accompanied by an increase in drag. This lift is even more directed in the direction of rotation, but it is opposed by drag and therefore output will fall slightly to 425 kW.

THE STALL PHENOMENAThe diagrams showing the components of lift and drag illustrate the result of stall.Lift diminishes and drag increases at angles of attack over 15 degrees. The diagrams however do not illustrate the reasons for this stall phenomena. A stall is understood as a situation during which an angle of attack becomes so large that the air flow no can longer flow smoothly, or laminar, across the profile. Air looses contact with the rear side of the blade, and strong turbulence occurs. This separation of air masses normally commences progressively from the trailing edge, so the profile gradually becomes semi-stalled at a certain angle of attack, but a full stall is first achieved at a somewhat higher angle. From the diagram showing the lift and drag components, one can estimate that the separation at the trailing edge starts at about 12 degrees, where the curve illustrating lift starts to fall. The profile is fully stalled, and the air flow is separated all over the rear side of the blade at about 20 degrees. These figures can greatly vary from profile to profile and also between different thicknesses of the same profile.

When the stall phenomena is used to restrict power output, as in all Bonus wind turbines, it is important that blades are trimmed correctly. With the steep lift curve, the angle of attack cannot be altered very much, before maximum output also changes, therefore it is essential that the angle of the blade is set at the correct value.

One cannot alter the different angles on the blade itself, once the form, shape and blade molding has been decided upon and fabricated. So we normally talk about calibrating the tip angle. Not because the blade tip has any special magical properties, but we can place a template at the tip, which allows us to make measurements using a theodolite. Adjusting of the tip angle can therefore be understood as an example of how the angle of the total blade is adjusted.Of importance for power output limitation is also the fact that in practice lift and drag normally behave exactly as would be expected from the theoretical calculations. However this is not always the case. Separation can often occur before expected, for instance due to dirt on the leading edges, or it can be delayed if the air flow over the profile for some reason or other, is smoother than usual. When separation occurs before expected, the maximum obtainable lift is not as high as otherwise expected and therefore maximum output is lower. On the other hand, delayed separation can cause continuous excessive power production output.

Accordingly profile types chosen for our blades have stable stall characteristics with little tendency to unforeseen changes. From time to time, however, it is sometimes necessary to actively alter the stall process. This is normally done by alteration to the leading edge, so that a small well-defined extra turbulence across the profile is induced. This extra turbulence gives a smoother stall process. Turbulence can be created by an area of rougher blade surface, or a triangular strip, fixed on the leading edge.

This stall strip acts as a trigger for the stall so that separation occurs simultaneously all over the rear side. On a wind turbine blade, different air flows over the different profile shapes, interact with each other out along the blade and therefore, as a rule, it is only necessary to alter the leading edge on a small section of the blade. This altered section will then produce a stall over the greater part of the blade. For example, the Bonus 450 kW Mk III turbine, is usually equipped with a 0.5 meter stall strib, which controls the stall process all over the 17 meter long blade.

SUMMARYThe main points as described in this article can be shortly stated in the following:

The air flow around a wind turbine blade is completely dominated by the head wind from the rotational movement of the blade through the air.

The blade aerodynamic profile produces lift because of its streamlined shape. The rear side is more curved than the front side.

The lift effect on the blade aerodynamic profile causes the forces of the air to point in the correct direction.The blade width, thickness, and twist is a compromise between the need for streamlining and the need for strength.

At constant shaft speed, in step with the grid, the angle of attack increases with increasing wind speed. The blade stalls when the angle of attack exceeds 15 degrees. In a stall condition the air can no longer flow smoothly or laminar over the rear side of the blade, lift therefore falls and drag increases.

PROS AND CONS OF WIND ENERGY

With the rising costs of traditional energy, alternate sources of energy are being looked into. Wind Energy is one such alternative source of energy. Here are some pros and cons of Wind Energy.

Advantages

Wind energy is fueled by the wind, so it's a clean fuel source. Wind energy doesn't pollute the air like power plants that rely on combustion of fossil fuels, such as coal or natural gas. Wind turbines don't produce atmospheric emissions that cause acid rain or greenhouse gasses.

Wind energy relies on the renewable power of the wind, which can't be used up. Wind is actually a form of solar energy; winds are caused by the heating of the atmosphere by the sun, the rotation of the earth, and the earth's surface irregularities.

Wind energy is one of the lowest-priced renewable energy technologies available today, costing between 4 and 6 cents per kilowatt-hour, depending upon the wind resource and project financing of the particular project.

Wind turbines can be built on farms or ranches, thus benefiting the economy in rural areas, where most of the best wind sites are found. Farmers and ranchers can continue to work the land because the wind turbines use only a fraction of the land. Wind

power plant owners make rent payment. The greatest advantages of Wind Energy are that it is widely distributed, cheap, and

also reducing toxic gas emissions. Wind Energy is also advantageous over traditional methods of creating energy, in the sense that it is getting cheaper and cheaper to produce wind energy. Wind Energy may soon be the cheapest way to produce energy on a large scale.

Along with economy, Wind Energy is also said to diminish the greenhouse effect.

Also, wind energy generates no pollution. Wind Energy is also a more permanent type of energy. The wind will exist till the time the sun exists, which is roughly another four billion years. Theoretically, if all the wind power available to humankind is harnessed, there can be ten times of energy we use, readily available.

One other advantage of wind energy that it is readily available around the globe, and therefore there would be no need of dependence for energy for any country. Wind energy may be the answer to the globe's question of energy in the face of the rising petroleum and gas prices.

Wind costs are much more competitive with other generating technologies because there is no fuel to purchase and minimal operating expenses.

Disadvantages

However, there are some disadvantages for wind energy, which may put a dampener in its popularity. Though the costs of creating wind energy is going down, even today a large number of turbines have to be built to generate a proper amount of wind energy.

Wind can never be predicted. Even the most advanced machinery may come out a cropper while predicting weather and wind conditions. Since wind energy will require knowledge of the weather and wind conditions on long term basis, it may be a bit impractical. 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). Therefore, in areas where a large amount of wind energy is needed, one cannot depend completely on wind.

Many potential wind farms, places where wind energy can be produced on a large scale, are far away from places for which wind energy is best suited. Therefore, the economical nature of wind energy may take a beating in terms of costs of new substations and transmission lines.

Wind Energy is non-dispatchable. This may also put a spanner in depending upon wind power as a primary energy supplier. Wind energy depends upon the wind in an area and therefore is a variable source of energy. The amount of wind supplied to a place and the amount of energy produced from it will depend on various factors like wind speeds and the turbine characteristics. Some critics also wonder whether wind energy can be used in areas of high demand.

Even though the cost of wind power has decreased dramatically in the past 10 years, the technology requires a higher 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),

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.

CHALLENGES AND LIMITATIONS

Challenges

Wind power must compete with conventional generation sources on a cost basis. Depending on how energetic a wind site is, the wind farm may or may not be cost competitive. Even though the cost of wind power has decreased dramatically in the past 10 years, the technology requires a higher initial investment than fossil-fueled generators.

Good wind sites are often located in remote locations, far from cities where the electricity is needed. Transmission lines must be built to bring the electricity from the wind farm to the city.

Wind resource development may compete with other uses for the land and those alternative uses may be more highly valued than electricity generation.

Although wind power plants have relatively little impact on the environment compared to other conventional power plants, there is some concern over the noise produced by the rotor blades, aesthetic (visual) impacts, and sometimes birds have been killed by flying into the rotors. Most of these problems have been resolved or greatly reduced through technological development or by properly siting wind plants.

Limitations

The limitations are both theoretical and technological. Of all the wind energy that a wind turbine sees, the theoretical maximum amount of energy that can be converted into electricity is 60%. For 100% efficiency, the wind speed behind the turbine blades would have to be zero. Other limitations include the amount of power output a wind turbine can have. The power produced by a turbine is proportional to the wind velocity cubed. This means, at low wind speeds (i.e. 4 m/s), there is a significant drop-off in the power produced versus moderate wind speeds (i.e. 7 m/s). Also, the power produced is proportional to the swept area, or another way of saying the same thing, is its proportional to the blade length squared. Clearly, infinite length blades are unrealistic. Therefore finite blade lengths have to be used, which gets into the technological limitations. Some of the technological limitations are:

Limitations on Blades: Additional limitations involve the wind turbine blades. The most common method for producing wind turbine blades is fiberglass. This involves cutting multiple sheets of fiberglass to the shape of the blade and molding them with resin between each fiberglass layer. Small imperfections develop in the surface of the blade as the resin cures which can lead to premature failure during operation.

Limitations on Structure: At high wind speeds, the turbines become unsafe to operate. One of the main reasons for this is the vibrations caused by the high velocities. Because of this, the power output of a turbine must be limited to keep the turbine from being overloaded and/or from a catastrophic failure.

Limitations on Transmission: The modern electricity transmission grid is not so modern. Built for the energy needs 100 years ago, the grid is not suited to transport electricity for long distances, which are usually required for wind turbines since high wind speeds are often found in less populated areas. The problem is that transmission lines and the connections between them are too small for the amount of power companies want to squeeze through them.

Limitations on Energy Storage: Since wind turbines rely on wind to generate electricity, a calm day is a bad thing. Currently the capability to store extra electricity when it's not needed is virtually zero. This severely limits the practically of wind turbines as main stream power producers since they cannot provide a constant source of electricity.

WIND POWER FORECASTING

A wind power forecast corresponds to an estimate of the expected production of one or more wind turbines (referred to as a wind farm) in the near future. By production is often meant available power for wind farm considered (with units kW or MW depending on the wind farm nominal capacity). Forecasts can also be expressed in terms of energy, by integrating power production over each time interval. Forecasting of the wind power generation may be considered at different time scales, depending on the intended application:

• from milliseconds up to a few minutes, forecasts can be used for the turbine active control. Such type of forecasts are usually referred to as very short-term forecasts

• for the following 48–72 hours, forecasts are needed for the power system management or energy trading. They may serve for deciding on the use of conventional power plants and for the optimization of the scheduling of these plants. Regarding the trading application, bids are usually required during the morning of day d for day d+1 from midnight to midnight. These forecasts are called short-term forecasts

• for longer time scales (up to 5–7 days ahead), forecasts may be considered for planning the maintenance of wind farms, or conventional power plants or transmission lines. For the specific case of offshore wind farms maintenance costs may be prohibitive, and thus an optimal planning of maintenance operations is of particular importance.

For the last two possibilities, the temporal resolution of wind power predictions ranges between 10 minutes and few hours (depending on the forecast length). Lately, most of the works for improving wind power forecasting solutions have focused on using more and more data as input to the models involved, or alternatively on the providing of reliable uncertainty estimates along with the traditionally provided predictions.

Reason for wind power forecasts

In the electricity grid at any moment balance must be maintained between electricity consumption and generation - otherwise disturbances in power quality or supply may occur. Wind generation is a direct function of wind speed and, in contrast to conventional generation systems, is not easily dispatchable. Fluctuations of wind generation thus receive a great amount of attention. Variability of wind generation can be regarded at various time scales. First, wind power production is subject to seasonal variations, i.e. it may be higher in winter in Northern Europe due to low-pressure meteorological systems or it may be higher in summer in the Mediterranean regions owing to strong summer breezes. There are also daily cycles which may be substantial, mainly due to daily temperature changes. Finally, fluctuations are observed at the very short-term scale (at the minute or intra-minute scale). The variations are not of the same order for these three different timescales. Managing the variability of wind generation is the key aspect associated to the optimal integration of that renewable energy into electricity grids.

The challenges to face when wind generation is injected in a power system depend on the share of that renewable energy. It is a basic concept, the wind penetration which allows one to describe the share of wind generation in the electricity mix of a given power system. For Denmark, which is a country with one of the highest shares of wind power in the electricity mix, the average wind power penetration over the year is of 16-20% (meaning that 16-20% of the electricity consumption is met wind energy), while the instantaneous penetration (that is, the instantaneous wind power production compared to the consumption to be met at a given time) may be above 100%.

The Transmission System Operator (TSO) is responsible for managing the electricity balance on the grid: at any time, electricity production has to match consumption. Therefore, the use of production means is scheduled in advance in order to respond to load profiles. The load corresponds to the total electricity consumption over the area of interest. Load profiles are usually given by load forecasts which are of high accuracy. For making up the daily schedule, TSOs may consider their own power production means, if they have any, and/or they can purchase power generation from Independent Power Producers (IPPs) and utilities, via bilateral contracts or electricity pools. In the context of deregulation, more and more players appear on the market, thus breaking the traditional situation of vertically-integrated utilities with quasi local monopolies. Two main mechanisms compose electricity markets. The first one is the spot market where participants propose quantities of energy for the following day at a given production cost. An auction system permits to settle the electricity spot price for the various periods depending on the different bids. The second mechanism is the balancing of power generation, which is coordinated by the TSO. Depending on the energy lacks and surplus (e.g. due to power plant failures or to intermittence in the case of wind power installations), the TSO determines the penalties that will be paid by IPPs who missed in their obligations. In some cases, an intra-day market is also present, in order to take corrective actions.

General methodology

There exists today a wealth of methods for short-term prediction of wind generation. The simplest ones are based on climatology or averages of past production values. They may be considered as reference forecasting methods since they are easy to implement, as well as benchmark when evaluating more advanced approaches. The most popular of these reference methods is certainly persistence. This naive predictor — commonly referred to as ‘what you see is what you get’ — states that the future wind generation will be the same as the last measured value. Despite its apparent simplicity, this naive method might be hard to beat for look-ahead times up to 4–6 hours ahead

Advanced approaches for short-term wind power forecasting necessitate predictions of meteorological variables as input. Then, they differ in the way predictions of meteorological variables are converted to predictions of wind power production, through the so-called power curve. Such advanced methods are traditionally divided into two groups. The first group, referred to as physical approach, focuses on the description of the wind flow around and inside the wind farm, and use the manufacturer's power curve, for proposing an estimation of the wind power output. In parallel the second group, referred to as statistical approach, concentrates on capturing the relation between meteorological predictions (and possibly historical measurements) and power output through statistical models whose parameters have to be estimated from data, without making any assumption on the physical phenomena.

Prediction of meteorological variables

Wind power generation is directly linked to weather conditions and thus the first aspect of wind power forecasting is the prediction of future values of the necessary weather variables at the level of the wind farm. This is done by using Numerical Weather Prediction (NWP) models. Such models are based on equations governing the motions and forces affecting motion of fluids. From the knowledge of the actual state of the atmosphere, the system of equations allows to estimate what the evolution of state variables, e.g. temperature, velocity, humidity and pressure, will be at a series of grid points. The meteorological variables that are needed as input for wind power prediction obviously include wind speed and direction, but also possibly temperature, pressure and humidity. The distance between grid points is called the spatial resolution of the NWPs. The mesh typically has spacing that varies between few kilometers and up to 50 kilometers for mesoscale models. Regarding the time axis, the forecast length of most of the operational models today is between 48 and 172 hours ahead, which is in adequacy with the requirements for the wind power application. The temporal resolution is usually between 1 and 3 hours. NWP models impose their temporal resolution to short-term wind power forecasting methods since they are used as a direct input.

Predictions of meteorological variables are provided by meteorological institutes. Meteorologists employ atmospheric models for weather forecasts on short and medium term periods. An atmospheric model is a numerical approximation of the physical description of the state of the atmosphere in the near future, and usually is run on a supercomputer. Each computation starts with initial conditions originating from recent measurements. The output consists of the expected average value of physical quantities at various vertical levels in a horizontal grid and stepping in time up to several hours after initiation. There are several reasons why atmospheric models only approximate reality. First of all, not all relevant atmospheric processes are included in the model. Also, the initial conditions may contain errors (which in a worse case propagate), and the output is only available for discrete points in space (horizontal as well as vertical) and time. Finally, the initial conditions age with time - they are already old when the computation starts let alone when the output is published. Predictions of meteorological variables are issued several times per day (commonly between 2 and 4 times per day), and are available few hours after the beginning of the forecast period. This is because some time is needed for acquiring and analyzing the wealth of measurements used as input to NWP models, then run the model and checks and distribute the output forecast series. This gap is a blind spot in the forecasts from an atmospheric model. As an example in the Netherlands, KNMI publishes 4 times per day expected values of wind speed, wind direction, temperature and pressure for the period the between 0 and 48 hours after initialization of the atmospheric model Hirlam with measured data, and then the period before forecast delivery is of 4 hours.

Many different atmospheric models are available, ranging from academic research tools to fully operational instruments. Besides for the very nature of the model (physical processes or numerical schemes) there are some clear distinctive differences between them: time domain (from several hours to 6 days ahead), area (several 10.000 km² to an area covering half the planet), horizontal resolution (1 km to 100 km) and temporal resolution (1 hour to several hours).

One of the atmospheric models is the High Resolution Limited Area Model, abbreviated HiRLAM, which is frequently used in Europe. HiRLAM comes in many versions, that’s why it is better to speak about "a" HiRLAM rather than "the" HiRLAM. Each version is maintained by a national institute such as the Dutch KNMI, the Danish DMI or Finnish FMI. And each institute has several versions under her wing, divided into categories such as: operational, pre-operational, semi operational and for research purposes.

Other atmospheric models are UKMO in the UK, Lokalmodell in Germany, Alladin in France (Alladin and Lokalmodell are also used by some other country’s within Europe), and MM5 in the USA.

Here is an example of wind forecasting done in Denmark.

Physical approach to wind power forecasting

Meteorological forecasts are given at specific nodes of a grid covering an area. Since wind farms are not situated on these nodes, it is then needed to extrapolate these forecasts at the desired location and at turbine hub height. Physical-based forecasting methods consist of several sub-models which altogether deliver the translation from the wind forecast at a certain grid point and model level, to power forecast at the site considered. Every sub-model contains the mathematical description of the physical processes relevant to the translation. Knowledge of all relevant processes is therefore crucial when developing a purely physical prediction method (such as the early versions of the Danish Prediktor). The core idea of physical approaches is to refine the NWPs by using physical considerations about the terrain such as the roughness, orography and obstacles, and by modeling the local wind profile possibly accounting for atmospheric stability. The two main alternatives to do so are:

(i) to combine the modeling of the wind profile (with a logarithmic assumption in most of the cases) and the geostrophic drag law for obtaining surface winds;

(ii) to use a CFD (Computational Fluid Dynamics) code that allows one to accurately compute the wind field that the farm will see, considering a full description of the terrain.

When the wind at the level of the wind farm and at hub height is known, the second step consists in converting wind speed to power. Usually, that task is carried out with theoretical power curves. However, since several studies have shown the interest of using empirically derived power curve instead of theoretical ones, theoretical power curves are less and less considered. When applying a physical methodology, the modeling of the function which gives the wind generation from NWPs at given locations around the wind farm is done once for all. Then, the estimated transfer function is consequently applied to the available weather predictions at a given moment. In order to account for systematic forecasting errors that may be due to the NWP model or to their modeling approach, physical modelers often integrate Model Output Statistics (MOS) for post-processing power forecasts.

A company is using wind sensors on 300 cell phone towers to predict 95% of the 9GW wind power in Texas with an accuracy of 10 minutes.

Statistical approach to wind power forecasting

Statistical prediction methods are based on one or several models that establish the relation between historical values of power, as well as historical and forecast values of meteorological variables, and wind power measurements. The physical phenomena are not decomposed and accounted for, even if expertise of the problem is crucial for choosing the right meteorological variables and designing suitable models. Model parameters are estimated from a set of past available data, and they are regularly updated during online operation by accounting for any newly available information (i.e. meteorological forecasts and power measurements).

Statistical models include linear and non-linear models, but also structural and black-box types of models. Structural models rely on the analyst’s expertise on the phenomenon of interest while black-box models require little subject-matter knowledge and are constructed from data in a fairly mechanical way. Concerning wind power forecasting, structural models would be those that include a modeling of the diurnal wind speed variations, or an explicit function of meteorological variable predictions. Black-box models include most of the artificial-intelligence-based models such as Neural-Networks (NNs) and Support Vector Machines (SVMs). However, some models are ‘in-between’ the two extremes of being completely black-box or structural. This is the case of expert systems, which learn from experience (from a dataset), and for which prior knowledge can be injected. We then talk about grey-box modeling. Statistical models are usually composed by an autoregressive part, for seizing the persistent behavior of the wind, and by a ‘meteorological’ part, which consists in the nonlinear transformation of meteorological variable forecasts. The autoregressive part permits to significantly enhance forecast accuracy for horizons up to 6–10 hours ahead, i.e. over a period during which the sole use of meteorological forecast information may not be sufficient for outperforming persistence.

Today, major developments of statistical approaches to wind power prediction concentrate on the use of multiple meteorological forecasts (from different meteorological offices) as input and forecast combination, as well as on the optimal use of spatially distributed measurement data for prediction error correction, or alternatively for issuing warnings on potentially large uncertainty.

Uncertainty of wind power forecasts

Predictions of wind power output are traditionally provided in the form of point forecasts, i.e. a single value for each look-ahead time, which corresponds to the expectation or most-likely outcome. They have the advantage of being easily understandable because this single value is expected to tell everything about future power generation. Today, a major part of the research efforts on wind power forecasting still focuses on point prediction only, with the aim of assimilating more and more observations in the models or refining the resolution of physical models for better representing wind fields at the very local scale for instance. These efforts may lead to a significant decrease of the level of prediction error.

However, even by better understanding and modeling both the meteorological and power conversion processes, there will always be an inherent and irreducible uncertainty in every prediction. This epistemic uncertainty corresponds to the incomplete knowledge one has of the processes that influence future events. Therefore, in complement to point forecasts of wind generation for the coming hours or days, of major importance are to provide means for assessing online the accuracy of these predictions. In practice today, uncertainty is expressed in the form of probabilistic forecasts or with risk indices provided along with the traditional point predictions. It can be shown that any decision related to wind power management and trading cannot be optimal without accounting for prediction uncertainty. For the example of the trading application, studies have shown that reliable estimation of prediction uncertainty allows wind power producer to significantly increase their income in comparison to the sole use of an advanced point forecasting method. Other studies of this type deal with optimal dynamic quantification of reserve requirements, optimal operation of combined systems including wind, or multi-area multi-stage regulation. More and more research efforts are expected on prediction uncertainty and related topics.

WIND POWER IN INDIAThe development of wind power in India began in the 1990s, and has significantly increased in the last few years. Although a relative newcomer to the wind industry compared with Denmark or the US, India has the fifth largest installed wind power capacity in the world.In 2009-10 India's growth rate is highest among the other top four countries.

As of 31 Dec 2010 the installed capacity of wind power in India was 13065.37 MW , mainly spread across

Tamil Nadu (4906.74 MW),

Maharashtra (2077.70 MW),

Gujurat (1863.64 MW),

Karnataka (1472.75 MW),

Rajasthan (1088.37 MW),

Madhya Pradesh (229.39 MW),

Andhra Pradesh (136.05 MW),

Kerala (27.75 MW),

Orissa (2MW),

West Bengal (1.1 MW)

And other states (3.20 MW). It is estimated that 6,000 MW of additional wind power capacity will be installed in India by 2012. Wind power accounts for 6% of India's total installed power capacity, and it generates 1.6% of the country's power.

India is the world's fifth largest wind power producer, with an annual power production of 8,896 MW.

The worldwide installed capacity of wind power reached 157,899 MW by the end of 2009. USA (35,159 MW), Germany (25,777 MW), Spain (19,149 MW) and China (25,104 MW) are ahead of India in fifth position. The short gestation periods for installing wind turbines, and the increasing reliability and performance of wind energy machines has made wind power a favored choice for capacity addition in India.

Suzlon, as Indian-owned company, emerged on the global scene in the past decade, and by 2006 had captured almost 7.7 percent of market share in global wind turbine sales. Suzlon is currently the leading manufacturer of wind turbines for the Indian market, holding some 52 percent of market share in India. Suzlon’s success has made India the developing country leader in advanced wind turbine technology.

State-level wind power

There is a growing wind energy installations in the number of the states across the India.

Tamil Nadu (4906.74 MW)

India is keen to decrease its reliance on fossil fuels to meet its energy demand. Shown here is a wind farm in Muppandal, Tamil Nadu.

Tamil Nadu is the state with the most wind generating capacity: 4906.74 MW at the end of the March 2010. Not far from Aralvaimozhi, the Muppandal wind farm, the largest in the subcontinent, is located near the once impoverished village of Muppandal, supplying the villagers with electricity for work. The village had been selected as the showcase for India's $2 billion clean energy program which provides foreign companies with tax breaks for establishing fields of wind turbines in the area.

Wind turbiness in Tamil Nadu

In february 2009, Shriram EPC bagged INR 700 million contract for setting up of 60 units of 250 KW (totaling 15 MW) wind turbines in Tirunelveli district by Cape Energy. Enercon is also playing a major role in development of wind energy in India. In Tamil Nadu, Coimbatore and Tiruppur Districts having more wind Mills from 2002 onwards,specially, Chittipalayam, Kethanoor, Gudimangalam, Poolavadi, Murungappatti (MGV Place), Sunkaramudaku, KongalNagaram, Gomangalam, Anthiur are the high wind power production places in the both districts.

Maharashtra (2077.70 MW)

Maharashtra is second only to Tamil Nadu in terms of generating capacity. Suzlon has been heavily involved. Suzlon operates what was once Asia's largest wind farm, the Vankusawade Wind Park (201 MW), near the Koyna reservoir in Satara district of Maharashtra.

Gujarat (1863.64 MW)

Samana &sadodar in jamanagar district is set to host energy companies like China Light Power (CLP) and Tata Power have pledged to invest up to 8.15 billion ($189.5 million) in different projects in the area. CLP, through its India subsidiary CLP India, is investing close to 5 billion for installing 126 wind turbines in Samana that will generate 100.8 MW power. Tata Power has installed wind turbines in the same area for generating 50 MW power at a cost of 3.15 billion. Both projects are expected to become operational by early next year, according to government sources. The Gujarat government, which is banking heavily on wind power, has identified Samana as an ideal location for installation of 450 turbines that can generate a total of 360 MW. To encourage investment in wind energy development in the state, the government has introduced a raft of incentives including a higher wind energy tariff. Samana has a high tension transmission grid and electricity generated by wind turbines can be fed into it. For this purpose, a substation at Sadodar has been installed. Both projects are being executed by Enercon Ltd, a joint venture between Enercon of Germany and Mumbai-based Mehra group.

ONGC Ltd has commissioned its first wind power project. The 51 MW project is located at Motisindholi in Kutch district of Gujarat. ONGC had placed the EPC order on Suzlon Energy in January 2008, for setting up the wind farm comprising 34 turbines of 1.5 MW each. Work on the project had begun in February 2008, and it is learnt that the first three turbines had begun production within 43 days of starting construction work. Power from this 308 crore captive wind farm will be wheeled to the Gujarat state grid for onward use by ONGC at its Ankleshwar, Ahmedabad, Mehsana and Vadodara centres. ONGC has targeted to develop a captive wind power capacity of around 200 MW in the next two years.

Karnataka (1472.75 MW)

There are many small wind farms in Karnataka, making it one of the states in India which has a high number of wind mill farms. Chitradurga, Gadag are some of the districts where there are a large number of Windmills. Chitradurga alone has over 20000 wind turbines.

The 13.2 MW Arasinagundi (ARA) and 16.5 MW Anaburu (ANA) wind farms are ACCIONA’S first in India. Located in the Davangere district (Karnataka State), they have a total installed capacity of 29.7 MW and comprise a total 18 Vestas 1.65MW wind turbines supplied by Vestas Wind Technology India Pvt. Ltd.

The ARA wind farm was commissioned in June 2008 and the ANA wind farm, in September 2008. Each facility has signed a 20-year Power Purchase Agreement (PPA) with Bangalore Electricity Supply Company (BESCOM) for off-take of 100% of the output. ARA and ANA are Acciona’s first wind farms eligible for CER credits under the Clean Development Mechanism (CDM).

ACCIONA is in talks with the World Bank for The Spanish Carbon Fund which is assessing participation in the project as buyer for CERs likely to arise between 2010 and 2012. An environmental and social assessment has been conducted as part of the procedure and related documents have been provided. These are included below, consistent with the requirement of the World Bank's disclosure policy.

Rajasthan (1088.37 MW)

Gurgaon-headquartered Gujarat Fluorochemicals Ltd is in an advanced stage of commissioning a large wind farm in Jodhpur district of Rajasthan. A senior official told Projectmonitor that out of the total 31.5 mw capacity, 12 mw had been completed so far. The remaining capacity would come on line shortly, he added. For the INOX Group company, this would be the largest wind farm. In 2006-07, GFL commissioned a 23.1-mw wind power project at Gudhe village near Panchgani in Satara district of Maharashtra. Both the wind farms will be grid-connected and will earn carbon credits for the company, the official noted.] In an independent development, cement major ACC Ltd has proposed to set up a new wind power project in Rajasthan with a capacity of around 11 mw. Expected to cost around 60 crore, the wind farm will meet the power requirements of the company's Lakheri cement unit where capacity was raised from 0.9 million tpa to 1.5 million tpa through a modernisation plan. For ACC, this would be the second wind power project after the 9-mw farm at Udayathoor in Tirunelvelli district of Tamil Nadu. Rajasthan is emerging as an important destination for new wind farms, although it is currently not amongst the top five states in terms of installed capacity. As of 2007 end, this northern state had a total of 496 mw, accounting for a 6.3 per cent share in India's total capacity.

Madhya Pradesh (229.39 MW)

In consideration of unique concept, Govt. of Madhya Pradesh has sanctioned another 15 MW project to MPWL at Nagda Hills near Dewas. All the 25 WEGs have been commissioned on 31.03.2008 and under successful operation.

Kerala (27.75 MW)

The first wind farm of the state was set up at Kanjikode in Palakkad district. It has a generating capacity of 23.00 MW. A new wind farm project was launched with private participation at Ramakkalmedu in Idukki district. The project, which was inaugurated by chief minister V. S. Achuthanandan in April 2008, aims at generating 10.5 MW of electricity.

The Agency for Non-Conventional Energy and Rural Technology (ANERT), an autonomous body under the Department of Power, Government of Kerala, is setting up wind farms on private land in various parts of the state to generate a total of 600 mw of power. The agency has identified 16 sites for setting up wind farms through private developers. To start with, ANERT will establish a demonstration project to generate 2 mw of power at Ramakkalmedu in Idukki district in association with the Kerala State Electricity Board. The project is slated to cost 21 crore. Other wind farm sites include Palakkad and Thiruvananthapuram districts. The contribution of non-conventional energy in the total 6,095 mw power potential is just 5.5 per cent, a share the Kerala government wants to increase by 30 per cent. ANERT is engaged in the field of development and promotion of renewable sources of energy in Kerala. It is also the nodal agency for implementing renewable energy programmes of the Union ministry of non-conventional energy sources.

West Bengal (1.10MW)

The total installation in West Bengal is just 1.10 MW as there was only 0.5 MW addition in 2006-2007 and none between 2007–2008 and 2008–2009

Bengal - Mega 50 MW wind energy project soon for country.

Suzlon Energy Ltd plans to set up a large wind-power project in West Bengal Suzlon Energy Ltd is planning to set up a large wind-power project in West Bengal, for which it is looking at coastal Midnapore and South 24-Parganas districts. According to SP Gon Chaudhuri, chairman of the West Bengal Renewable Energy Development Agency, the 50 MW project would supply grid-quality power. Gon Chaudhuri, who is also the principal secretary in the power department, said the project would be the biggest in West Bengal using wind energy. At present, Suzlon experts are looking for the best site. Suzlon aims to generate the power solely for commercial purpose and sell it to local power distribution outfits like the West Bengal State Electricity Board (WBSEB).

Suzlon will invest around 250 crore initially, without taking recourse to the funding available from the Indian Renewable Energy Development Agency (Ireda), said Gon Chaudhuri. He said there are five wind-power units in West Bengal, at Frazerganj, generating a total of around 1 MW. At Sagar Island, there is a composite wind-diesel plant generating 1 MW. In West Bengal, power companies are being encouraged to buy power generated by units based on renewable energy. The generating units are being offered special rates. S Banerjee, private secretary to the power minister, said this had encouraged the private sector companies to invest in this field.

Utilization

Despite the high installed capacity, the actual utilization of wind power in India is low because policy incentives are geared towards installation rather than operation of the plants. This is why only 1.6% of actual power production in India comes from wind although the installed capacity is 6%. The government is considering the addition of incentives for ongoing operation of installed wind power plants.

Projects in India

India's Largest Wind power production facilities (10MW and greater)

Power Plant Producer Location StateTotal

Capacity (MWe)

Vankusawade Wind Park

Suzlon Energy Ltd. Satara Dist. Maharashtra 259

Cape ComorinAban Loyd Chiles Offshore Ltd.

Kanyakumari Tamil Nadu 33

Kayathar Subhash Subhash Ltd. Kayathar Tamil Nadu 30Ramakkalmedu Subhash Ltd. Ramakkalmedu Kerala 25Muppandal Wind Muppandal Wind Farm Muppandal Tamil Nadu 22Gudimangalam Gudimangalam Wind Farm Gudimangalam Tamil Nadu 21

Puthlur RCI Wescare (India) Ltd. PuthlurAndhra Pradesh

20

Lamda Danida Danida India Ltd. Lamda Gujarat 15

Chennai MohanMohan Breweries & Distilleries Ltd.

Chennai Tamil Nadu 15

Jamgudrani MP MP Windfarms Ltd. DewasMadhya Pradesh

14

Jogmatti BSES BSES Ltd.Chitradurga Dist

Karnataka 14

Perungudi Newam Newam Power Company Ltd. Perungudi Tamil Nadu 12Kethanur Wind Farm Kethanur Wind Farm Kethanur Tamil Nadu 11

Hyderabad APSRTCAndhra Pradesh State Road Transport Corp.

HyderabadAndhra Pradesh

10

Muppandal Madras Madras Cements Ltd. Muppandal Tamil Nadu 10Poolavadi Chettinad Chettinad Cement Corp. Ltd. Poolavadi Tamil Nadu 10

Shalivahana WindShalivahana Green Energy. Ltd.

Tirupur Tamil Nadu 20.4

*…..WIND FARMS…..*A wind farm is a group of wind turbines in the same location used for production of electric power. A large wind farm may consist of several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other purposes. A wind farm may also be located offshore.

Design

A wind farm is a group of wind turbines in the same location used for production of electric power. Individual turbines are interconnected with a medium voltage (usually 34.5 kV) power collection system and communications network. At a substation, this medium-voltage electrical current is increased in voltage with a transformer for connection to the high voltage transmission system.

A large wind farm may consist of several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other purposes. A wind farm may be located offshore to take advantage of strong winds blowing over the surface of an ocean or lake.

As a general rule, wind generators are able to be used better if wind speed is 10 mph (16 km/h or 4.5 m/s) or greater. An ideal location would have a near constant flow of non-turbulent wind throughout the year, with a minimum likelihood of sudden powerful bursts of wind. An important factor of turbine siting is also access to local demand or transmission capacity.

Usually sites are preselected on basis of a wind atlas, and validated with wind measurements. Meteorological wind data alone is usually not sufficient for accurate siting of a large wind power project. Collection of site specific data for wind speed and direction is crucial to determining site potential in order to finance the project. Local winds are often monitored for a year or more, and detailed wind maps constructed before wind generators are installed.

The wind blows faster at higher altitudes because of the reduced influence of drag. The increase in velocity with altitude is most dramatic near the surface and is affected by topography, surface roughness, and upwind obstacles such as trees or buildings. Typically, the increase of wind speeds with increasing height follows a wind profile power law, which predicts that wind speed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34%.

ONSHORE WIND FARMS

Onshore wind power refers to the construction of wind farms on land surfaces to generate electricity from wind. The world's first wind farm – consisting of 20 wind turbines rated at 30 kilowatts each – was installed on the shoulder of Crotched Mountain in southern New Hampshire in December, 1980.

Many of the largest operational onshore wind farms are located in the USA. As of November 2010, the Roscoe Wind Farm is the largest onshore wind farm in the world at 781.5 MW, followed by the Horse Hollow Wind Energy Center (735.5 MW). The largest wind farm under construction is the 800 MW Alta Wind Energy Center in the USA. The largest proposed project is the 20,000 MW Gansu Wind Farm in China.

Onshore turbine installations in hilly or mountainous regions tend to be on ridgelines generally three kilometers or more inland from the nearest shoreline. This is done to exploit the topographic acceleration as the wind accelerates over a ridge. The additional wind speeds gained in this way can increase the amount of energy produced because more wind is going through the turbines. Great attention must be paid to the exact positions of the turbines (a process known as micro-siting) because a difference of 30 m can sometimes mean a doubling in output.

World's largest onshore wind farms

Wind farm

Currentcapacity

(MW) Country

Biglow Canyon Wind Farm 450 USA

Buffalo Gap Wind Farm 523.3 USA

Capricorn Ridge Wind Farm 662.5 USA

Dabancheng Wind Farm 500 China

Fowler Ridge Wind Farm 599.8 USA

Horse Hollow Wind Energy Center 735.5 USA

Panther Creek Wind Farm 458 USA

Roscoe Wind Farm 781.5 USA

Sweetwater Wind Farm 585.3 USA

OFFSHORE WIND FARMS

Offshore wind power refers to the construction of wind farms in bodies of water to generate electricity from wind. Better wind speeds are available offshore compared to on land, so offshore wind power’s contribution in terms of electricity supplied is higher. Offshore wind turbines—which generate electricity from the typically stronger winds that blow over the seas—figure prominently in the ambitious long-term goals set forth by the German government and industry experts.

Offshore wind turbines are being used in a number of countries to harness the energy of the moving air over the oceans and convert it to electricity. Offshore winds tend to flow at higher speeds than onshore winds, thus allowing turbines to produce more electricity. Much of this potential energy is near major population (and energy load) centers where energy costs are high and land-based wind development opportunities are limited.

Because the potential energy produced from the wind is directly proportional to the cube of the wind speed, increased wind speeds of only a few miles per hour can produce a significantly larger amount of electricity. For instance, a turbine at a site with an average wind speed of 16 mph would produce 50% more electricity than at a site with the same turbine and average wind speeds of 14 mph.

Offshore wind facilities today are generally developed and operated as follows. Once a suitable place for the wind facility is located, piles are driven into the seabed. For each turbine, a support structure and a tower to support the turbine assembly, to house the remaining plant components, and to provide sheltered access for personnel are attached to the piles. After the turbine (generally a three-bladed rotor connected through the drive train to the generator) is assembled, wind direction sensors turn the nacelle (a shell that encloses the gearbox, generator, and blade hub) to face into the wind and maximize the amount of energy collected. Wind moving over the blades makes them rotate around a horizontal hub connected to a shaft inside the nacelle. This shaft, via a gearbox, powers a generator to convert the energy into electricity.

Offshore wind turbines are also bigger than onshore turbines (to take advantage of the steadier offshore winds and economies of scale). A typical onshore turbine installed today has a tower height of about 60 to 80 meters, and blades about 30 to 40 meters long; most offshore wind turbines are at the top end of this range. Offshore turbines installed today are generally between 2 and 4 MW, with tower heights greater than 200 feet and rotor diameters of 250 to 350 feet. Turbines of up to 5 MW are being tested.

Transport of Wind-Generated Energy

Undersea collection cables connect multiple turbines in the wind facility and transport the electricity from them to a transformer where the combined electricity is converted to a high voltage for transmission via undersea cables to a substation. There the electricity is connected to the onshore electricity grid. Alternative approaches, such as using the wind to produce hydrogen (through the hydrolysis of desalinated seawater), which would be shipped to shore for later use, are also being investigated.

Environmental Considerations

Potential impacts on the environment that may occur during construction, operations, and decommissioning of offshore wind facilities are highlighted below.

Marine life-- Foundations can act as artificial reefs with a resultant increase in fish populations from the new food supply. These increases in fish population may also have stimulating effects on bird populations in the area, which could cause collisions between birds and towers or rotors.

Migrating birds-- Besides potential collisions (bird strikes), it is possible that the birds would need to consume more energy to avoid collisions and maintain their orientation when navigating around the turbines. Tower illumination may also cause navigational disorientation for birds.

Interference with navigation for endangered and threatened species-- Electromagnetic fields created by the electric cables running from the turbines and underwater noises and vibrations could affect orientation and navigational ability.

Potential alteration of natural environments and diminution of habitats-- Underwater support pilings, anchoring devices, scour-protection materials, and electromagnetic fields could cause a decrease in benthic communities, alter natural environments, and possibly affect migration patterns.

Emissions-- Each unit of electricity generated from the wind that saves a unit generated from fossil fuels, which will help reduce greenhouses gases, pollutants, and waste products that result from fossil fuel use.

Marine traffic, recreation, and other sea space uses-- It is possible that wind turbine energy plants may disrupt air traffic control and maritime radar systems, and that facilities siting could affect recreation and other sea space uses.

Visual impacts from towers, rotating turbine blades and navigation and aerial warning lights.

Noise impacts from rotating turbine blades

OFFSHORE WIND FARM.

ONSHORE WIND FARM

LATEST TECHNOLOGIES IN WIND ENERGY

What’s happening right now?Wind power has already made its way into the pantheon of renewable energy sources as it’s clearly exhibited by the following numbers, which says that by the end of July, 2010, worldwide nameplate capacity of wind-powered generators was 175 GW. Now compare this to total capacity in the year 2000, which was only 17.4 GW. Energy production was 340 TWh by 2010, which is about 2 percent of worldwide electricity usage and has doubled in the past three years. Well, this is all well and good, except for the fact that electricity generated from wind power can be highly variable at several different timescales: from hour to hour, daily, and even seasonally. In response to this type of variation, now we are looking forth to integrate systems that can harness winds at much higher altitudes.

Trends:

1. MARS – A wind turbine that is up in air

At the first instance, it may look like a giant blimp, but in actuality MARS (Magenn Air Rotor System) is a lighter-than-air tethered wind turbine that rotates about a horizontal axis in response to wind, generating clean electrical energy. The inflated system uses a three dimensional structure (unlike the two dimensional blades of conventional turbines) kept afloat in the air by helium. The mechanism can harness winds at a level of 600 ft to 1000ft and even nocturnal jet streams, while the resultant energy captured is transferred down to earth by 1000ft long tether cables.

2. Joby Energy tests high-altitude wind turbine prototype

A prototype by Joby Energy epitomizes innovation at its best. Basically a flying platform with an array of wind propellers (much like an aircraft without cockpit), the propellers lift up the contraption to a certain level, and once steadily deployed in the air, the whole system moves in rapid circular motion. This motion pattern maximizes the face exposure to wind streams, thus allowing it to collect the energy and transmitting it back to the surface by use of cables. The system has a capacity of 30 KW, but if successful, the company could even manufacture a 100 KW version.

3. Energy starved New York could soon get powered by jet stream winds

Jet streams can be described as fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. Now researchers from Carnegie Institution and California State University have identified New York City as one of those locations that have the potential to immensely benefit from harnessing energy from jet streams. Basically the system calls for energy harnessing kites to be flown high up at an altitude of more than 30,000 ft, where they can transfer the collected energy from the existing jet streams back to the earth by their tethered cables. If the system works successfully, then according to the scientists it can produce ten times more energy as conventional wind turbines, with more than 400 MW of electricity generation.

4. Kite farm offers parasailing while producing energy for the adventurous sort

Imagine you are parasailing over the deep blue sea, with your adrenaline pumping high and the glorious wind waving at your visage, but on a different system, the whole world is benefiting from your little tryst with adventure. Now that would be good feeling, and that is exactly what Colombian landscape architecture studio Paisajes Emergentes plans on doing. The proposition calls for an energy generating kite farm, on a public beach, in Abu Dhabi, under the Land Art Generator Initiative. There would be around 200 kites tethered across a 60-meter grid of flexible posts, and with an advanced wind belt generator, each of them (the kites) can produce roughly 6,200 kilowatt-hours a year, which is enough to power three energy-efficient homes. So in a modest estimate, this innovative ‘adventure’ wind farm can power around 600 homes!

5. Makani Power to develop energy harvesting kites

California-based high altitude wind company Makani Power wants to develop a 1MW prototype of an energy-harvesting kite, which can be flown at high altitudes of more than 600 m (20,000 ft). Such kites can make use of steadier wind velocities to generate more power than conventional wind turbines. Moreover, to add to that ’sci-fi’ flavor, robots will fly these kites while enabling them to keep afloat for a longer period.

6. NASA researchers envisions airborne wind turbines for renewable energy

In NASA’s reverie, wind turbines would not take up earth’s spaces, but rather ‘float’ in the free space above earth. NASA aerospace engineer Mark Moore believes that the idea in itself has great potential, as wind speed is more consistent and its velocity much higher and steadier at higher altitudes. The resultant energy can be transmitted back to earth via nanotube cables.

The concept:We have already talked about the variations in magnitude of wind energy that we can come across while harnessing it at surface level. In order to counter these variations, many ingeniously conceived technologies look forth to incorporate mechanisms that can harness winds at higher altitudes, as wind at this height becomes steadier, more persistent, and of higher velocity. Moreover, theoretically the power available in wind increases according to the cube of velocity (the velocity-cubed law), hence assuming other parameters remaining the same, doubling a wind’s velocity gives us 2×2x2=8 times the power (or thrice velocity gives us 27 times the power). Known as high altitude wind power (HAWP), the mechanisms involve capturing the power of winds high in the sky and transmitting it back to earth surface by use of tether and cable technology.

The advantages:We already know that the wind energy generated at high altitudes can be multiple times the magnitude of the energy generated at ground level. However, other than that, such technologies would totally forego the usage of comparatively costly, cumbersome and easily damageable ground wind turbines. That in effect would also nullify the adverse conditions of noise pollution and even the visual burden on natural landscaping, all of which are generally associated with conventional wind turbines.

The impact:Statistically, we have already seen the rise of the capacity of wind power by 10 times in the last 10 years. Now considering HAWP technologies are still at their nascent stage, and given their enormous potential, we certainly have a great chance to witness an exponential increase in wind power output, once the system advances beyond its present state. Like California-based HAWP company, Makani Power believes that capturing a small fraction of the global high altitude wind energy would be sufficient to power the entire planet!

7. WIND NEST

WindNest, designed by Trevor Lee and Clare Olsen, combines aesthetics with clean energy generation. The installation is designed for Site #2 in Abu Dhabi, between Saadiyat Island and Yas Island. WindNest was one of the entries in The Land Art Generator Initiative (LAGI) Competition 2010. It’s partly rooted to the ground and partly floating in the open air, trapping both the Sun and the wind energy.

WindNest is a multi-stranded structure maintaining the ecological balance all around its vicinity. It’s equipped with a network of windsock turbines that keep track of the wind movement and thereby energy generation through it.Solar fabric covering the windsock is used to harness solar energy. Wind energy is harnessed by the turbines fitted to windsocks. Lightweight materials are used throughout the project’s lifecycle. For the nested elements, hand-woven, natural materials are used. Carbon rods are used for the foundation in the ground. Teflon fibers are used for the network, which are UV resistant and light in weight.