Wind energy is among the world’s fastest-growing sources of energy. During the last decade, wind energy growth rates worldwide averaged about 30 percent annually. In the last three years, the U.S. and Texas wind energy markets also have experienced a rapid expansion of capacity. In 2007, for example, U.S. wind power capacity grew by 43 percent, while Texas’ rose by 57 percent.
This growth has been driven by a variety of factors including government subsidies and tax incentives, improved technology, higher fossil fuel prices and investor concerns about potential federal action to reduce carbon emissions, which could make electricity from fossil fuels more expensive.
Wind power is an abundant, widely distributed energy resource that has zero fuel cost, zero emissions and zero water use. Wind’s challenges are largely related to its variable nature – wind speed and direction can change by the season, day, hour and minute. For electricity grid operators the variability of wind – sometimes too much wind is blowing and at others too little – makes it difficult to integrate wind into a grid that was not designed for fluctuations. Moreover, surplus wind power cannot be stored, given current technology.
Many Texas landowners have willingly leased their lands to wind developers, but others oppose the industry. The siting of wind turbines can be problematic, due to opposition to their appearance, noise and potential hazard to wildlife. Some landowners complain that without a permitting process for wind projects, they have no way to protect their property rights.
Transmission is another significant hurdle, since the best sites for wind energy development often are far away from urban centers and the wire networks that provide them with power. Some landowners object to transmission lines traversing their ranches and farms, claiming they will lower their property values. Other critics say that wind energy, like other forms of alternative energy, is not really economically viable without substantial government subsidies and incentives.
Still, wind power can provide economic value to some property owners. Property owners leasing land for wind turbine development receive a steady income (although landowners with transmission towers and lines passing through their land receive only a one-time payment). And wind projects, like other energy projects, create construction and operation jobs and expand the local property tax base.
For centuries, people have used the wind to sail ships, grind grains, run small sawmills and pump water from wells. Today, however, wind power increasingly is used to generate electricity.
Rural areas used small windmills to produce electricity in the early years of the twentieth century. The widespread electrification of rural areas in the 1930s led to a decline in the use of windmills for this purpose. In the 1970s, however, an oil shortage led to renewed interest in renewable energy sources, including wind energy. Lower fossil fuel prices during much of the late 1980s and 1990s made wind energy less competitive and slowed its growth.
Wind power came back strongly in 1999, spurred by factors such as government incentives, growing environmental concerns, improved wind turbine technology, declining wind energy costs, and energy security concerns. Among the most significant factors behind the growth of utility-scale wind energy is the federal production tax credit, currently 2 cents per kilowatt-hour (kWh). More recently, higher fossil fuel costs and the expectation of future carbon regulation have also contributed to the growth of wind energy.
Wind can be used to provide mechanical energy; Texas ranchers still use windmills to provide well water for cattle. But wind’s ability to generate electricity without using water is by far its most important and promising aspect.
In West Texas, where wind is abundant and water is in short supply, desalination systems powered by wind can be used to develop brackish water sources for consumption. Wind also can be used to power desalination plants along coastal areas. These desalination plants require a constant power supply and use a lot of electricity.
According to NREL, wind energy can supply 20 percent of the nation’s electricity by 2030.
Wind turbines convert the wind’s kinetic energy into mechanical power that a generator, in turn, converts into electricity. There are two main types of wind turbines, the horizontal-axis and vertical-axis models. Most modern wind turbines have a horizontal axis, with blades resembling airplane propellers. Vertical-axis units have blades that resemble an eggbeater’s. Horizontal-axis units account for almost all utility-scale turbines – 100 kilowatts or several megawatts – in the U.S. and other countries.
Both small and large wind turbines can be used to generate electricity. Small turbines with a capacity to generate less than 10 kilowatts of electricity typically are used to power single homes or farms in remote or “off-grid” locations. Intermediate-sized systems, with a capacity of between 10 and 250 kilowatts, can power a village or a cluster of homes and buildings. Large, utility-scale turbines can generate several megawatts and usually are grouped together into power plants often called “wind farms,” and connected to the electrical utility grid; their power is sold to utility customers.
Demand for wind turbines has outstripped global supply. The total development timeline of a wind farm, from initial wind assessment through construction, can require from two to five years and involves many steps. Wind developers must locate sites and negotiate lease options that provide the wind company with a sufficient amount of time to allow for wind measurement, land surveys and studies including avian, environmental, geotechnical, foundation and soil tests to determine if the site is suitable for development.
A wind energy lease is different from an oil and gas lease because it involves only the land surface and not the mineral rights. The average term of a wind energy lease can range from 30 to 50 years, but typically is about 35 years. These long lease periods reflect the fact that creating a wind farm is a complex and expensive project with costs that can run into the hundreds of millions of dollars. Furthermore, wind turbines have a lifespan of more than 20 years. Wind farms are large and often encompass land from several landowners, thus requiring separate leases from each. In West Texas, most wind farms range from 2,000 acres to more than 100,000 acres.
The wind farm development and pre-construction phase involves numerous steps, such as title research, permitting, financing, equipment purchases, the development of a power sales strategy and connection to the electrical grid. The construction phase consists of assembly and installation of the wind turbines, transmission lines, substations, roads and other improvements as required. The operational phase of power production typically lasts about 25 years. However, the operational phase may be “repowered” with new equipment, as has been done recently in California, where wind projects have replaced equipment originally installed in the early 1980s.
While wind farms may extend over thousands of acres, the wind turbines themselves occupy only a small percentage of the land – generally 3 to 8 percent (one to two acres per turbine, mostly for the unit itself and associated service roads). This allows farmers and ranchers to use most of the land for other activities. The land occupied is often referred to as the wind turbine’s “footprint.”
A wind farm also requires substantial acreage for open space between turbines, however, to maximize their efficiency in capturing the wind and to avoid turbulence that can impede airflow. The size of the turbine, land characteristics – plains, hills, ridges, plateaus and mountains – and the direction of the prevailing winds determine the distance needed between wind turbines and turbine rows. One study noted that on a flat site with a single prevailing wind, each turbine requires 26.7 acres, while a site with two prevailing winds requires 59 acres per turbine. At present, neither the federal government nor the state has any spacing regulations for wind turbines.
The kinetic energy of moving air provides the motive force that turns a wind turbine’s generator. The wind turns the rotor blades; this motion spins a drive shaft that in turn spins the turbine of a generator to make electricity. A gear box located along the drive shaft increases speed to match generator requirements and optimize power generation. Some wind turbines have a large generator and no gearbox. Longer rotor blades mean a larger “rotor swept area,” the total area covered by spinning blades, increasing the energy that can be captured and generating more electricity.
Other factors including wind speed, the height of the wind turbine and air temperature also determine power output. The stronger the wind, the more power is available. A doubling of wind speed increases power output by a factor of eight. Utility-scale wind farms generally require a minimum annual average wind speed of 13 miles per hour.
Wind turbines often are located along hilltops and mountain ridges because a five-fold increase in the height of a wind turbine above the prevailing terrain can result in twice as much wind power. While actual wind characteristics are site-specific, in general, raising the height of a wind turbine increases available wind power. Air temperature also affects wind power generation, with cold, relatively dense air generating about 5 percent more power than hot air.
Today’s wind industry has increased output and reduced generation costs by building taller wind turbine towers with longer blades. Both wind turbine size and output have increased steadily since the early 1980s. At that time, the tallest wind turbines were about 56 feet tall; today, some of the larger wind turbines reach heights of nearly 400 feet. The output of wind turbines also has increased steadily, rising from 50 kW in the early 1980s to 500 kW in the mid 1990s and more than 3 MW in 2006.
Most wind turbines currently planned for installation in West Texas wind farms are 1 MW to 2.3 MW units. Again, a 1 MW wind turbine can generate electricity for about 230 Texas households.
Capacity factor is a measure of the energy production of a power plant. Since wind is variable, blowing strongly at times and not at all at others, a wind turbine’s capacity factor compares actual power produced over time with the power that would be produced by the same turbine operating at maximum output 100 percent of the time. For example, wind turbines at most locations run about 65 percent to 80 percent of the time, but during some of this time they generate at less than full capacity, further lowering their capacity factor.
Utility-scale wind turbines typically operate with a capacity factor ranging from 25 to 40 percent, though they may exceed these amounts during windy months. A recent U.S. Department of Energy (DOE) study noted that taller wind turbines, improved siting and improvements in wind turbine technology all have contributed to continuing improvements in capacity factors. For example, DOE found that capacity factors for projects installed before 1998 average 22.5 percent, compared to 36 percent for those installed in 2004 and 2005.
In Texas, the average capacity factor of wind farms installed in 2004 through 2005 is 39 percent, compared to 32 percent for projects installed between 2000 and 2001 and 19.6 percent for those installed before 1998. The West Texas wind farms that generate power for the city of Austin’s utility company, Austin Energy, have capacity factors ranging from 35 percent to 40 percent.
Sometimes wind production can drop suddenly. On February 26, 2008, wind production in the Electric Reliability Council of Texas (ERCOT) dropped from over 1700 MW down to 300 MW within a three hour period. Traditional power plant operators, who would normally provide more power on short notice, failed to provide power as promised. ERCOT was able to avoid blackouts by asking large industrial customers to cut back on power use. These demand-response customers get reduced electric rates in exchange for cutting power on short notice.
Too little wind is a problem on some days, but on other days heavy winds can generate too much power. When the wind blows hard and wind turbines produce more electricity than the grid can accommodate, the producers in West Texas shut down the wind turbines.
Another measure, the availability factor, gauges the reliability of power plant equipment. This measure is expressed as a percentage of the year in which the power plant is available to produce electricity. Like most complex devices, wind turbines are out of service at times, either for maintenance and repairs (a scheduled outage) or when they break down unexpectedly (unplanned outages). Wind turbine technology has improved over the last two decades, and today’s machines can have an availability factor of more than 98 percent. In comparison, the availability of large coal and nuclear plants average in the 90 to 95 percent range.