Wind Power Basics
The sun is the ultimate source of wind energy. As the sun heats the surface of the earth, the air above it warms and rises upwards into the atmosphere. However, the sun's heat is uneven. In some places, such as near the poles where the heat from the sun is much less that at the equator, the air remains cold and stays close to the surface. In addition, land masses heat up faster than do the seas and they also cool faster. As warmer air rises it creates a partial vacuum and the cooler heavier air flows in to fill the void. This sun induced flow of air over the earth's surface is what we call wind. Eventually all the wind's energy is converted into diffused heat from the friction with land masses and the atmosphere.
Wind power is the conversion of wind energy into a more useful form of energy. Examples are the use of wind turbines to create electricity, or wind mills to grind crops or pump water. The first wind turbine was constructed during the late 1800s in Ohio. This wind mill had one hundred and forty five blades and the energy generated by this device was used to charge batteries. Top
World Wide Wind Capacity
As shown in the chart at the left, new world wide installed wind capacity was doubling every three years until 2010 according to the Global Wind Energy Council (GWEC). During 2010 and 2011 there was a growth slow down as only a few more Giga-Watts (GW) of wind capacity were installed. Growth resumed in 2012 with roughly 45 GW being installed.
2013 was a disapointment as only 35.5 GW were installed, down about 10 GW from 2012. Only 1.1 GW were in installed in the US, down from 13.1 GW in 2012. The drop-off can be attributed to the late extension of the Production Tax Credit (PTC) and Investment Tax Credit (ITC). In total there were 318 Giga-watts of cumulative wind capacity installed worldwide at the end of 2013. (That is 2.3 times the amount of PV solar installed.) Worldwide, wind power generates approximately 3% of the world's electricity. At the end of 2013, China had the largest installed base, 92.0 GW, followed by the US and Germany. China also installed the largest amount of new capacity in 2013 with 16.1 GW.
AS of the end of 2012, Denmark has the highest level of wind power penetration at 30%, followed by 16% in Portugal and Spain. In Germany, whose 30 GW of wind capacity is the 3rd highest in the world, the wind penetration is 11 percent. An amazing statistic, in 2012 Ontario, Canada's grid carried more electricity from wind than from coal for the first time.
The United States remains second only to China, with 61,000 total megawatts of wind capacity, enough to power more than 14 million U.S. homes. As with the rest of the world, the US generates only about 3% of its energy from wind power. However, wind energy is very popular with Americans. A Harris poll in 2010 found 87 percent of Americans want more wind energy. A wind farm that is one tenth of the area of the state of Nevada could supply the United States with enough energy to completely replace all fossil fuels.
The lack of a long term US energy policy has resulted in many utilities not being eager to enter wind power purchase agreements. US renewable energy programs have been authorized for only one year in advance whereas the lead times to purchase land, obtain permits, and install equipment and transmission lines takes many years. Some states such as Texas and Clifornia, have taken it upon themselves to vigorously promote wind power. The 12,400 megawatts in Texas and the 5,800 megawatts in California would rank them sixth and eleventh on the world wind power list. Driven by state renewable energy goals, 14 states have installed over one GW of wind power, and a total of 37 states now have some utility scale wind power installed within their borders. Six states are estimated to have wind power exceeding 10% of their total energy production. Top
US Wind Resources
Not all areas of the US are suitable for wind generated power as wind turbines do not operate at wind speeds less than about 10 mph. For a wind farm, wind speeds should average 20 mph, or better yet 30 mph. As can be seen on the map at the left, large areas (shown in white) of the southwest and southeast do not qualify for wind farms. The midwest and Texas are the best areas in the US for a wind farm. The state of South Dakota at the end of 2011 generated 22% of its energy from wind, followed by Iowa at 20% and Minnesota at 15%. As mentioned above, in terms of total installed capacity, Texas is number one with over 12,400 MW of capacity followed by 5,800 in California.
On October 28th, 2010 strong winds in Texas pushed wind generated electricity to a remarkable 25% of the total electricity produced in the state that day. The Roscoe Wind Farm in Texas is the world's largest wind farm at 782 Mega-watts capacity. It consists of 627 wind turbines manufactured by General Electric, Mitsubishi, and Siemens. The project spans four Texas counties and covers an area the size of Manhattan. Top
Cost Of Wind Power
Capital Costs - How much does a large wind turbine cost? The average size of a utility scale turbine is 2 mega-watts and costs about $3.5 million installed, which is $1,750 a kilo-watt. Wind turbines exhibit significant economies of scale, meaning larger units cost less per kilo-watt than smaller units. Residential size turbines cost less overall, but are more expensive on a kilo-watt basis. Wind turbines under 100 kilo-watts cost roughly $3,500 to $5,000 per kilo-watt of capacity. That means a 10 kilo-watt unit (the size needed to power an average home) would cost $35,000 to $50,000. There are no fuel costs for wind energy, so the major cost factors are the initial capital cost followed by some turbine maintenance costs.
Levelized Cost Of Wind Power - The Levelized Cost Of Electricity (LCOE) is the break even price of electricity from a utility source over its entire lifetime. It is an economic assessment of the overall cost of an energy generating system including the initial design and investment, operating costs, maintenance cost, and cost of fuel.
|Energy Plant Type||Lifetime Cost ¢ per Kwh|
The table on the left lists the estimated cost of electricity by several different sources. No subsidies are included in the calculations. The table is from a paper by noted energy cost expert Ken Zweibel of George Washington University (GWU) in Energy Policy.
AS one can see, wind power is second only to natural gas. However including subsidies, wind power is competitive with natural gas right now, especially when carbon footprints are taken into account. Of all the renewable sources of energy, wind power is the most cost effective at this time.
Modern wind turbines are huge machines. As shown on the left, a typical modern turbine is 95 meters (312 feet) high (longer than a football field) and 70 meters (230 feet) in blade width. A Boeing 747 could fit inside the blade path. The higher the turbine from the ground, the better are the wind conditions. Wind turbines require unobstructed local wind, which is usually found on mountain tops, open prairies, and at open sea.
All wind turbines have a "rated" wind speed at which its generator will be able to produce at its full capacity, known as "nameplate capacity". The "rated" wind speed is usually about 30 miles per hour (mph). Each turbine also has a "cut-in" wind speed at which it will begin producing power, normally about 10 mph. At 10 mph, however, the power output is just above zero.
Power output increases in a cubic relationship to wind speed. That is, if the wind speed doubles, the power output is able to increase eightfold (2 x 2 x 2). If the average wind speed of a site is 15 mph -- one half of the rated wind speed -- the average power output will be one eighth of the nameplate capacity. So as the wind speed slows, the resultant power output falls exponentially. This is a very important point when locating (siting) a turbine.
On the other hand, at a wind speed of 55 mph or more, the wind turbine must shut itself down for safety sake. A major consideration is that the tip of the blade travels faster than the center of the blade as it has further to travel in the same amount of time as the hub. Tip of blade speeds can reach 200 mph in very high performance turbines. Top
The large GE turbine housing, shown at the left, that contains the shafts, gear box, generator, controller, etc. is called the nacelle, pronounced "na'-sell". Nacelles can be very large indeed, large enough for a helicopter to land on. The nacelle of the GE 1.5 MW turbine weighs 56 tons, the blades weigh 36 tons, and the whole unit weighs 163 tons. The foundation is usually about 40 feet in diameter, 7 feet deep, and filled with about 1,200 tons of rebar and concrete. The towers are made of steel or concrete and are very large in diameter in order to support the rotor and nacelle.
The blades are curved similar to an airplane wing. When the wind blowsaround them, the wind on one side has to travel further than the wind on the other side creating a pressure differential which causes a "lift" to spin the blades. The blades are curved similar to an airplane wing. When the wind blows around them, the wind on one side has to travel further than the wind on the other side creating a pressure differential which causes a "lift" to spin the blades. The blades are "pitched" (rotated) to control their speed. The blades are attached to a hub, and the blades and the hub together are called the rotor.
The rotor spins the low speed shaft about 18 revolutions per minute (rpm), not nearly fast enough to generate electricity. However, inside the nacelle is a gear box that increases the revolutions by a factor of about 100 to roughly 1800 rpm on the high speed generator shaft. 1800 rpm is plenty fast for electricity generation. The gear box is a heavy, costly part of the turbine.
On the tail end of the nacelle is a wind vane and anemometer. The wind vane communicates the exact direction of the wind to the controller which directs the yaw drive to aim the shaft head-on into the wind. The anemometer measures the wind speed and sends that information to the controller as well. The controller starts up the turbine at about 10 mph by changing the pitch of the blades and releasing the brake. The controller shuts down the turbine at about 55 mph by again changing the pitch of the blades and applying the brake. Wind turbines do not operate in winds above 55 mph because such high speeds could cause damage. Top
Wind Energy Challenges:
|Avg. Wind Speed||Capacity Factor|
The capacity factor is the "actual average output" of a wind turbine (or other generating device) divided by the turbines "rated capacity" or "nameplate capacity". Normally the rated capacity is calculated at a wind speed of about 30 mph. The average power produced is approximately related to wind speed by the cube of the actual average wind speed divided by rated capacity. For example, at an average wind speed of 15 mph, 1/2 the rated speed, the resultant power is one eighth the rated output (1/2 x 1/2 x 1/2 = 1/8). The table on the left shows the drop off in power for a given turbine design with a decrease in wind speed. What is the lesson from this table? "Siting", the placement of wind farms, is incredibly important as power falls off exponentially as wind speed falls. Wind farms should only be "sited" where there is an above average, constant supply of wind.
Public figures want to support "green energy" because it is popular with voters. However, placement of the wind farm is super critical. Places that have good wind power seasonally may not be good sites for a whole year. Wind power sites must have at least an "average wind supply of 20 mph" to make economical sense. Wind speeds vary from year to year, from season to season, with the time of day, and with the turbine height above ground. There are many sites with "good" wind in spring and fall, but the sites selected for wind farms must have a "good wind average" for all seasons. Planning a wind farm, one must accurately know its wind pattern year round. As manufacturers have improved the efficiency of their products over time, the average US capacity factor has crept up from 30% to 36% in the last 10 years. The best turbines in "good" locations now have an an average capacity factor of 40%. Top
Managing Wind Variability
Electricity generated by wind is highly variable, both on an hourly basis and seasonally. This is especially true in the spring (April) and fall (October) when wind conditions can change very quickly. During winter the wind is rather strong and steady. During summer wind is weak and unchanging. When the wind suddenly begins to blow in a wind power region, a power surge is encountered on the grid. Likewise when the wind suddenly dies, there is a large power drop. If the wind suddenly dies, the load has to be taken up by traditional coal and gas plants. Because these traditional plants take considerable time to power up, they must be run in "spinning standby", i.e. running but not producing any electricity. When wind is only 2% of the supply, variability is not a problem as 2% can be easily covered by existing standby capacity (usually about 5%) reserved for grid emergencies. In 2006, President Bush and the Department Of Energy (DOE) proposed that by 2030, the US should obtain 20% of its electricity from wind power.
As wind approaches 20% or more of total capacity, managing the variability becomes much more serious as much more capacity must be kept spinning or alternative back up arrangements must be made with adjacent providers. In the past few years there have been two major US studies of this problem - the Eastern Wind Integration and Transmission Study (EWITS) and the Western Wind and Solar Integration Study (WWSIS). Both of these studies were multi-year efforts consisting of huge simulations using real wind data streams, prices of fuel, and actual placements of wind installations and fossil fuel generators. Both studies re-affirmed that 20% penetration of electricity demand by wind is technically feasible with only minor improvements in current practices. Penetration of 30% requires new transmission lines to be built and much more careful forecasting of wind conditions down to the 10 minute interval level. However, achieving 30% wind penetration would reduce emissions of carbon dioxide by up to 45%, nitrogen oxides by up to 50%, and sulfur dioxide by 30% in addition to reducing our dependence on fossil fuels. At 30% penetration, electricity operating costs would be reduced by approximately 40% with only a modest investment in new transmission capacity. Top
Wind Curtailment, is a major issue for wind farms. Curtailment is reducing electrical generation at a wind farm below what it is capable of producing, sometimes referred to as dumping power into the ground. Curtailment occurs during very windy conditions when the turbines generate more electricity than demand requires (mainly during night time hours) or more than transmission lines can safely handle. This is not as much of a problem at a conventional fuel plant because when a generator is turned off, fuel costs are also cut off. In a wind plant most of the costs of generation are incurred up front (sunk costs). A wind operator wants to run his equipment as much as possible to return a profit on his investment. When a plant is curtailed, that wind is lost forever and the operator is forgoing his profit. The capacity factor for a wind farm on average is about 36% and "normal" curtailment might run 2% (except in Texas where it runs 8.5%) reducing the actual capacity factor to 34%. However, from a financial perspective that is a 5.6% reduction in revenue.
In the US Texas has the biggest curtailment problem as it has the largest installed base of wind farms exceeding 12,400 MW of power capacity. Curtailment in Texas used to run about 8 to10% in the very windy winter and spring months. Texas has recently added wind transmission capacity of 18,000 MW. That gives current wind farm operators a nice return on their investment. Top
Spinning turbine blades maime and kill birds and bats. Especially vulnerable are large birds of prey that like to fly in the same sorts of places that developers like to construct wind towers. Fog, a common situation on mountain ridges, aggravates the problem for all birds.
Guidelines from the US Fish and Wildlife Service state that wind turbines should not be built near wetlands or other known bird or bat concen-tration areas. Likewise turbines should not be placed in areas with a high incidence of fog or low cloud ceilings. It is illegal in the US to kill migratory birds. In April, 2011 Minneapolis based utility Xcel cancelled a 150 MW wind project in North Dakota following concerns over the effect on local bird populations. The US Fish and Wildlife service had informed Xcel that a number of bird species were at risk from the project.
Wind farms with large rotating blades are noisy. A 35 meter blade revolving at 18 rpm travels about 150 mph at the tip. Every time the rotor passes the tower, the compressed air makes a deep resonating "thump". It is a relentless rumble like the sound of thunder of an approaching storm. The penetrating low frequency noise, a thudding vibration, travels much further than "normal traffic" noise. The noise from a turbine can be heard up to a mile away from the site. One developer official was quoted as saying, "Wind turbines do not make good neighbors." Remote rural sites are required for wind farms.