Ask An Expert: Why are wind turbines the size they are

By The Daily Journal (Contact) | The Daily Journal

Published Thursday, May 1, 2008

ditor’s note: This week’s question comes from the reader comment section of our online edition edition where poster Mel asks about the size of wind turbines. We turn to Bryan D. Morlock, P.E. , a consultant and enginner at Otter Tail Power for the answer.

Question: Can anyone tell me why the turbines have to be the size that they are? Years ago they generated electricity and pumped water with a small regular windmill, that didn't clobber birds as it spun around, while standing 1,000 ft. high with blades 800 ft. long.

The answer is that it all comes down to economics. Obviously, wind generators are removing power from the air to generate electricity. Basically, the amount of power in the wind is related to the air density and the speed of the wind. The formula is:

P = (air density x wind speed3)

2

The key component here is the wind speed because the speed of the wind is cubed, or multiplied times itself three times. Wind speed increases as you go higher. The wind speed at 100’ is approximately 26% greater than it is at 20’ above the ground. Since the wind speed is cubed to determine the wind power, a 100’ level will have double the wind power compared to a 20’ level (1.26 times 1.26 times 1.26). Doubling the wind speed will increase the wind power by 8 times (2 times 2 times 2). So wind tower height makes a large difference in the performance of the wind generator and the cost of the electricity that it produces.

A wind turbine is expensive to construct, so the more energy that can be obtained out of the machine, the cheaper the cost of the energy will be. A wind turbine will generally not operate at speeds below 8 or 9 mph. Since the wind speed increases with height, a wind generator at a higher elevation will operate more hours of the year than one at a lower elevation since the higher generator will see the minimum operating speed more often. In general, as long as the value of the increase in electricity output is greater than the cost of building a higher tower, it is more economic to build tall wind generators.

In the 1930’s and 1940’s there were farms that did not have access to the electric grid. They did use electricity, which came from batteries typically stored in the basement and recharged by a small wind generator. These systems were DC (direct current) systems such as you have in a vehicle. A DC wind generator can operate at whatever speed the wind blows the blades at, within some maximum limit. Wind turbines connected to today’s electric grid must generate AC (alternating current) electricity and therefore operate at a fixed speed. The wind generator cannot operate slower or faster than the 60 cycle frequency of the AC system. This can create a problem, and usually does for a wind generator. Wind speed is not constant, but contains gusts and sudden speed changes. With the old DC systems the blade speed could change and some of the stress caused by a wind gust would be converted to electricity. With today’s fixed speed AC machines, virtually none of the wind gust gets converted to electricity as the blade speed cannot change. Thus the force of the wind gust gets converted mostly to mechanical stress on the wind generator, the turbine blades, and the tower structure. Turbulence from wind gusts and other sources create mechanical stresses that cause many of the maintenance and failure problems with wind turbines. Minimizing physical stress is very important.

Most wind turbines today (even the arctic models) will shut down at a temperature of -22º F. to -26º F. to reduce the potential mechanical stresses. Estimates are given that the wear and stress on the components at those extreme temperatures is about 50 percent greater than at warmer temperatures.

There are power electronics systems available today that will allow a wind generator to generate AC electricity, which then is converted to DC electricity, and is then converted back to AC electricity to match the frequency of the electric grid. These systems are very expensive and can only be cost justified on the large utility scale generators. Most large utility scale wind turbines will use this technology. This variable speed technology breaks the link between the fixed frequency of the electric grid and the speed of the turbine blades. It allows more of the power contained in a wind gust to be converted to electricity and less to mechanical stress.

So why not just use a DC electrical system? In the early years of electricity the utility electric systems were DC. The primary use of electricity was for lighting and DC light bulbs were available. But AC electricity is more efficient and can be provided with equipment that is physically smaller. The output from a DC machine would require a much larger generator than the same output from an AC generator. This is the same reason that automobile manufacturers switched from using generators in cars in the 1950’s to the alternators that they use today. The alternator is much more efficient and can provide the same output from a smaller unit. An alternator generates AC, which is then converted to DC for use in the vehicle. Within a relatively short period of time in the development of the electric industry, systems were converted to AC.

So it all comes down to the cost of the electricity from the generator. Here’s an example to illustrate the difference. A large utility scale wind generator might cost about $2,100 per kilowatt of nameplate rating to install. In a good wind installation this generator might operate at a 40 percent capacity factor. Capacity factor is defined as the average output divided by the maximum output over a period of time. On a tall tower at a good wind site in North Dakota, 40 percent is an achievable level. And let’s assume that with maintenance, insurance, property tax, etc. that the electricity from this unit will cost about 6.5 cents per kilowatt-hour. With the federal production tax credit (PTC) a wind owner receives, that brings the cost down to about 4.5 cents per kilowatt-hour. Now let’s consider a smaller unit on a much shorter tower. Smaller units are more expensive on a per kilowatt basis, but let’s use the same cost of $2,100 per kilowatt. A small unit at a low height will typically provide a 25 percent to 30 percent capacity factor. The construction cost of a wind turbine makes up possibly 75 percent of the cost of the wind generation. With the reduced output of our smaller unit, now the cost goes to almost 8.7 cents per kilowatt-hour without the PTC and 6.7 cents or higher per kilowatt-hour with the PTC. The PTC is currently 2.0 cents per kilowatt-hour, but the total dollars received will be much less with the smaller unit. The federal PTC is currently set to expire at the end of 2008. If it is not renewed by Congress it will have a severe negative effect on the wind industry in the United States.

There are also economies of scale to consider. Wind developers generally consider that a 40,000 kilowatt to 50,000 kilowatt wind farm is necessary to lower costs further through economy of scale. It takes about 16 semi loads to bring in a crane capable of assembling a large turbine. Building a larger wind farm helps to spread that cost over many more kilowatt-hours, lowering the average price.

The unit recently built along I-94 near Fergus Falls is about 39 kilowatts in size. The generating units recently installed by Otter Tail Power Company are 1,500 kilowatts in size. This is in contrast to the unit recently built along I-94 near Fergus Falls, which is about 39 kilowatts in size. Their tower height is 80 meters (262 feet) to the hub, or center of the turbine blades. Blade lengths will vary from site to site, but a typical blade length might be 100 feet to130 feet, so the tip of the blade as it spins reaches nearly 400 feet at maximum and 130 feet at minimum. Larger units will have longer blades and smaller units will have shorter blades. The wind characteristics at a site will also play a factor in determining optimum blade length. The goal is to optimize the amount of power extracted from the wind, so blade efficiency is an important consideration. The theoretical limit on blade efficiency is 59 percent. A wind turbine cannot remove all of the speed from the wind, or the air will begin to pile up on the blade. If that happens, then new wind cannot reach the blade and it will stop turning. Blade efficiency is well below the 59 percent level, possibly closer to half of that amount. Surprising as it may sound, turbine blades do kill insects. The buildup of dead insects on the turbine blades will degrade efficiency.

There have been some issues with bird kills in the mountain passes in California. These have been alarming to many because the birds are typically raptors (eagles, hawks, etc.), which may be endangered. Measures have been taken to increase the visibility of the blades and cameras have been used to study the bird strikes. One interesting aspect is that many of the raptor deaths appear to be younger birds, equivalent in age to human teenagers. Researchers have speculated the possibility, based on videotape evidence, that these raptors may be playing a teenage game of “chicken” by intentionally flying through the moving blades. One other change that has been made is the move to smooth tubular towers rather than lattice type towers. Lattice towers can provide spots for birds to roost while the tubular towers do not, creating more exposure to rotating turbine blades.

In any event, siting requirements for wind farms typically involve environmental studies that will consider migratory bird flyways, local habitat of species including endangered species, etc. to minimize the bird impacts. In our area bird kills tend to be minimal and do not involve endangered species. The one other significant incident that has attracted attention in recent years is an installation out east that appears to be in a bat flyway, which has resulted in a number of bat deaths.

There is some data on bird kills available from the American Wind Energy Association. The following graph demonstrates the available data on the causes of bird kills. No matter how extensively wind is developed in the future, bird deaths from wind energy are unlikely to ever reach as high as 1 percent of those from other human-related sources such as hunters, house cats, buildings, and autos. (House cats, for example, are believed to kill 1 billion birds annually in the U.S. alone.) Wind is, quite literally, a drop in the bucket. Still, areas that are commonly used by threatened or endangered bird species should be regarded as unsuitable for wind development. The wind industry is working with environmental groups, federal regulators, and other interested parties to develop methods of measuring and mitigating wind energy's effect on birds.

One final consideration with turbine siting also relates to wind turbulence. As previously mentioned, wind turbulence in the form of gusts can cause increased maintenance and failures in wind turbines. Turbulence can be increased because of trees, buildings, even the type of vegetation on the ground. Some general rules of thumb are that a building can create turbulence for a distance equal to twice its height in front of the building, twenty times its height on the downwind side of the building, and at an altitude twice the height of the building. Trees and shelterbelts will have a similar effect. So locating wind turbines in areas free from obstructions can be critical to the life of the wind generator. What this means is that locating a small wind generator near every house or on every farm may be problematic due to trees and buildings.

Anyone seeking further information can access the American Wind Energy Association website at www.awea.org.

The estimated change in wind speed with height is known as the 1/7 power law. In essence, wind speed can be assumed to vary with the 1/7th power of the height. It is somewhat more complicated than simply calculating the power in the wind.

Today’s wind turbines will typically reach maximum output at about 27-28 mph. At that speed the blades then begin to adjust their pitch so that they do not remove too much power from the wind and burn out the generator. A wind turbine will typically go into complete shutdown mode at about 55 mph to prevent a massive mechanical failure.

Have a question you’d like answered? Email Daily Journal editor Jeff Hage at jeff.hage@fergusfallsjournal.com and we’ll find you an answer.