Wind Energy Primer (draft)
1. How a large wind turbine worksWind turbines use the movement of the wind to move large wing-like blades that turn a rotor shaft which – not getting needlessly technical – spins magnets to generate alternating current (AC) in wire coils.
Usually, three blades (each currently about 50 yards long) are mounted on a hub that connects them to the rotor shaft. The blades work like airplane wings and can be pitched to modulate how much they move in the wind. This is done to maintain a steady rate of rotation through the range of wind speeds in which the turbine is active. A steady rotation rate is necessary to generate electricity that matches the wave frequency of the grid.
The hub is attached to one end of the “nacelle” – the housing for the gearbox and generator. When the wind is blowing adequately, the 100-ton blade and nacelle assembly is turned by motors to face it.
The rotation rate of the rotor blades is increased several times by a large gearbox – which requires hundreds of gallons of cooling and lubricating oils – to create a much faster spinning rate in the generator.
The generator requires power from the grid to work (if there’s a power outage, the wind turbines are out, too). As the wind rises toward the “cut-in” speed at which the turbine begins to operate, the generator works as a motor (using power from the grid) to start the blades spinning. As the wind speed continues to rise, the torque from the blades allows electricity to be produced (pushed out by the generator) rather than consumed (pushed in by the grid).
The cut-in wind speed is typically 7-9 mph. The amount of electricity generated increases as the wind rises in a cubic relation to the wind speed (i.e., increasing eight times with every doubling of wind speed): from none at the cut-in speed to full capacity at the “rated” wind speed, which is typically 25-35 mph.
When the wind reaches a speed of, typically, 55 mph, the blades are “feathered” (pitched so that they do not move in the wind) and may also be braked to prevent damage, and the turbine shuts down. This is the “cut-out” wind speed. The blades are not repitched to move in the wind until the wind drops to a speed of, typically, 45 mph, which is called the “cut-back-in” speed.
2. Electricity and the gridIn electricity, energy is power times time. Thus, a megawatt (MW) is a measure of power, or the rate of producing or using energy, and a megawatt-hour (MWh) is a measure of energy, representing 1 megawatt of power produced or used for 1 hour. For example, a 100-watt lightbulb burning for 10 hours would use 10 × 100, or 1,000 watt-hours of energy, which is 1 kilowatt-hour. Kilowatt-hours (kWh) are familiar as the unit used in your electric bill. The “kilo” prefix means “thousand”; “mega” (M) means “million”; 1 MWh equals 1,000 kWh equals 1,000,000 watt-hours.
If it were to operate at full capacity for all 8,760 hours of a calendar year, a 1-MW generator would produce 8,760 MWh of energy over the year. The actual amount it produces is called its load capacity, or capacity factor, which is expressed as a percentage of its rated capacity.
Thus if a generator were shut down for maintenance 10% of the time, its load capacity would be 90% and it would produce 7,884 MWh of energy annually per MW of rated, or installed, capacity. If it is a generator that is used only when very high levels of electricity are needed, it may be operated only 30% of the time. In which case it would produce 2,628 MWh annually per MW installed capacity.
Wind turbines are unable to respond to customer demand, their output depending on and varying in response to the wind instead. As a result, their average output over a year, their load capacity, may be anywhere between 10% and 40%, depending on the site. In North America, it ranges from 20-25% in the east to 30-40% in the great plains.
Because not all generators will be available all of the time, because generators or substations may be subject to catastrophic outages, and because of the high variability of demand, the grid includes a substantial amount of “excess” capacity to guarantee reliability, some of it operating in “spinning standby” mode. In the U.S. the total installed capacity is about two times the average electricity demand (or load). On most systems, the excess capacity is 20-30% higher than peak demand.
In 2002, the U.S. used a total of 3.66 billion MWh of electricity. That represented an average load, or rate of production and consumption of almost 418,000 MW and an annual per-capita use of 12,600 kWh. The load varies from a low base in the wee hours of the morning to very high peaks during weekdays, especially in the summer. Managers of the electric grid dispatch different power plants to provide base, intermediate, and peak loads as customer demand varies through the day.
3. How wind energy works on the electric gridBecause the electricity generated by wind turbines varies with the wind speed and cannot be called up when needed, it is not like other sources of energy on the grid. It cannot provide peak energy unless by chance the wind is up at the same time demand rises, and it cannot provide base load because it is not steady.
For this reason, wind energy does not reduce the need for other sources to supply reliable energy as needed.
Wind is more like a large customer that varies its burden on the grid in a substantial and unpredictable way.
When the wind rises and the turbines turn, the grid is usually required to accept that incoming energy. To maintain the balance between supply and demand, the grid must therefore reduce the energy production from another source. In theory, this is how wind energy would reduce greenhouse gas emissions and pollution: by allowing fossil fuel–fired generators to be used less. In practice, however, it isn’t so simple.
If the amount of wind energy entering the system is within certain tolerances, the grid manager may simply allow the voltage on the system to rise slightly. Or if the grid has hydro power, that is the likely source to be cut off by wind. Obviously, neither of these options would reduce fossil fuel use.
Only with much more wind on the system might it become necessary to reduce production from fossil fuel–fired plants. But that comes at a cost, because running such plants at lower capacity reduces their efficiency, requiring more fuel per megawatt-hour and burning it with more emissions. More frequent “ramping” or switching off and on also requires more fuel. It is like your car’s gas mileage in stop-and-go city traffic versus on the highway.
In addition, thermal plants that use steam to turn their turbines can take hours to warm up. They can’t be switched off if they might be needed again soon, which is always the case with wind, since it fluctuates minute to minute. So they are simply taken off line in terms of energy generation but continue to burn fuel to remain on standby.
In other words, the effect that wind energy on the grid has on fossil fuel emissions is not at all straightforward. It may not reduce fossil fuel use at all, because some plants must continue to burn fuel while on standby, and added inefficiency cancels much of the potential savings at others.
At best, wind energy has driven the replacement of coal-generated electricity with new natural gas–fired generation, which is much cleaner and creates less CO₂. But because they are used to balance the highly variable wind-generated electricity, they run much less efficiently – burning more fuel and creating more CO₂ – than they could if there were no wind on the grid.
A further aspect of wind energy on the grid is the burden on transmission. As an intermittent source whose output varies with the wind speed, a wind facility’s average annual output will be only 20-40% of its nameplate capacity. And even though three-fifths of the time the output will be less than its average, occasionally the output will be near capacity. Besides new transmission lines to connect the facility, the existing grid often has to be upgraded to handle those occasional surges of wind-generated power. Alternatively, many utilities around the world limit the amount of wind capacity they will allow, or they reserve the ability to turn the wind turbines off when the lines are too full.
4. Adverse impacts of industrial wind energyThese include: noise and vibration disturbance of wildlife; fragmentation and degradation of habitat; danger to birds, bats, and insects from the blades and their pressure vortices; water table and runoff problems from the foundations, clearance, and roads; noise and visual disturbance of human neighbors (from loss of enjoyment of one’s property to loss of sleep and in many cases serious health effects); shadow flicker; strobe lights; devalued property; destruction of rural and wild vistas; loss of recreational areas and wild refuge; interference with wireless communications; danger to small planes and helicopters; fire hazard; and so on.
5. Drivers of wind energy developmentFirst, wind energy provides tax avoidance in the United States through the 10-year federal production tax credit (which was 2.2 cents per kWh for facilities connected in 2012) and 5-year double-declining accelerated depreciation. These provide two-thirds of the capital value of an industrial wind turbine. State subsidies may provide another 10% of the cost with grants and tax breaks. Other countries, and some U.S. states on a small scale, provide a “feed-in tariff” that requires utilities to pay a specified (inflated) price for wind energy.
This taxpayer/ratepayer support allows the wind company to sell the energy to utilities at a competitive price.
In addition to selling the actual energy, the company may then sell an “equivalent” amount of renewable energy credits (RECs, or green tags) with which people can claim the alleged environmental benefit of or satisfy the obligation to buy wind energy even though everyone on the grid – or even on another grid – in fact used it equally.
The obligation to buy wind energy is enacted by legislators through renewable portfolio standards (RPSs), renewable energy standards (RESs), renewables obligations, and the like. Most of such requirements that a certain amount or percentage of electricity be obtained from renewable sources actually favor wind. For example, they usually don’t count large or preexisting hydro, and some of them even specify a certain amount of new wind. Thus, utilities are forced to buy into big wind whether or not it is practical or affordable for them, whether or not it means building new conventional plants to back it up or new remote high-capacity transmission lines.
All of these schemes guarantee markets and large profits for developers and their investors.