Wind power

Wind power is one of the oldest renewable energy technologies.  The first wind-driven devices are recorded in what was then Persia in the 7th Century—although the Chinese may have invented similar devices several centuries earlier. The first machines, called panemones, were primitive devices consisting of a vertical rotating shaft turning a mill stone. The shaft was positioned inside a circular wall with an opening facing the prevailing wind.  Attached to the shaft were paddles that caught the wind on one side of the shaft facing the opening–turning the shaft. Lifting water and grinding grain were the earliest forms of work driven by the wind—applications that continued for over a thousand years.

In Asia and China in the 10th Century, windmills were being used for irrigation and drainage. By the 13th Century the machines were widely used in Europe. In low-lying Holland the machines were extensively used to pump water from the coastal polders.  The Dutch refined wind technology in several ways.  They invented a rudimentary aerofoil; they created the spoiler and the air brake; and they improved the overall efficiency of the machines.

The first windmills in America copied the European models. But by the middle of the 19th Century Daniel Halladay has begun to experiment with the design that eventually developed into the familiar multi-bladed water pumping machine still to be seen across the rural North American landscape. 

In August 1854, Halladay patented the first commercially viable windmill—Halladay’s Self-Governing Windmill. Halladay had been approached to work on the design by a local businessman, John Burnham. Burnham was involved in the pump business and understood that if a reliable source of power could be found to bring ground water to the surface, he could significantly increase his sales.

Halliday’s windmill in 1858

Halladay’s design allowed the windmill to automatically turn to face changing wind directions, and it regulated and maintained a uniform speed by changing the pitch of the fan blades—without human intervention.

In July of 1854, the New-York Tribune, described Halladay’s new windmill with its self-furling ‘sails’ “the wind wheel is ten feet, and it has been in operation for six months without a hand being touched to it to regulate the sails.” The article went on to detail the unique design stating that the windmill would stand still during a storm with high winds, the edge of the sail wings facing into the wind, and as the storm died down the wings would gradually resume their position to catch the breeze. The windmill had also successfully drawn water from a well 28 feet deep, moving it more than 100 feet to a small reservoir in the upper part of a barn. The cost of this new-fangled invention was only $50—but with the cost of the pumps and pipes running to an additional $25.

Halladay quickly formed the Halladay Windmill Company of Ellington, eventually moving the firm to South Coventry in Connecticut, and manufacturing in that town until 1863. Burnham then persuaded  Halladay to move the business to Batavia, Illinois–closer to the expanding Midwestern market and the growing number of water-thirsty locomotives powering across the country on an expanding network of railroads. Halladay’s US Wind Engine and Pump Company’s products sold in the thousands to this market, where wind power made it significantly easier to provide water for crop irrigation and livestock.

Over the course of the 19th and early 20th Century, more than six million multibladed water-pumping windmills are estimated to have been erected in North America.  Now made of metal, they were also among the first machines to be mass-produced.  Factories in the US and Germany exported them to South America, Africa, Australia, Japan and to nearly all the European countries [1]

The windmills that “made the American west”.

Although several million water-pumping wind mills were in operation during the early part of the 20th Century, it was the development and evolution of the airplane wing and propeller that stimulated the development of the modern high-speed machines.  And it was quickly recognized that the high rotational speeds were ideal for driving an electrical generator.

In the early 1920s, innovators were using aircraft propellers to build simple wind turbines to charge batteries that powered electric lights and the first electric appliances and radios.

In the 1930s, small wind machines that generated electricity came onto the north American market.  Between 1930 and 1960 thousands of wind powered turbines were sold and installed in many countries.  Production in the US slowed in the 1960s after the Rural Electrification Administration succeeded in providing American farms and rural homes with inexpensive electricity generated by a distant coal-fired power plant linked to a transmission system.

20 kW Jacobs wind turbine

By 1957, the Jacobs Wind Company in the US had sold over 30,000 wind turbines worldwide.  Larger experimental wind turbines followed.  In 1957, Johannes Juul in Denmark built a machine with three 24-meter blades that generated 200 kilowatts (kW) of electrical power.

The energy crisis in the 1970s triggered an intense interest in wind power in the US. More sophisticated science was brought to bear on the design and construction of the rotors, and in 1981, the first US wind farm started up in California at Altamont Pass.  But before that, a 2-megawatt (MW) machine had been constructed in Denmark, and the Danish company, Vestas, started production—soon followed by other Danish manufacturers.

In the 1980s, the first offshore machines were installed near the coast of Denmark; and by 1990 there were 46 windfarms generating power across the USA.

The modern era

By the turn of the 21st Century, total installed wind power capacity had reached 17,400 MW—which almost doubled two years later.  And then doubled again–reaching more than 59,000 MW in 2005 [2].

Twelve years later, in 2017, the global installed capacity for wind power had surged to nearly 539 GW (i.e. 539, 000 MW). China leads in terms of new installations, followed by Germany and the US, with India passing Brazil to rank fourth.  Other countries in the top ten were France, Turkey, the Netherlands, the UK and Canada. [3] On a per capita basis, Denmark, Sweden, Germany, Ireland and Portugal would be the top five countries.  Wind power has now become the least-cost option for new power generating capacity in an increasing number of markets.  At least, thirteen countries (mostly European but including Costa Rica, Nicaragua and Uruguay), now generate more than 10% of their electricity from wind. [4]

How does it work?

In principle, it’s simple enough. The wind turns the turbine blades—usually three of them. The blades are attached to a drive shaft running through a gear box that increases the rotational speed, and the gears connect to a generator that produces electricity.

Main elements of a large wind turbine

A modern megawatt-scale turbine is a sophisticated machine. The variable-pitch aerofoil blades are computer-controlled to extract the maximum amount of energy from the wind, and the machine is constantly monitored remotely to produce maximum performance, and to ensure that there are no problems with the gearbox and generator.  The diagram above shows the main elements of a modern wind turbine.[5]

The amount of power produced by a wind turbine obviously depends on how hard the wind blows.  But it also depends on the characteristics of the turbine. All wind turbines have a characteristic power curve that shows how much power the turbine will produce at different windspeeds.  The diagam below shows the power curve for a Vestas V80 2 MW wind turbine. [6].

Power curve for a Vestas V80 2MW turbine

The cut-in speed is the windspeed at which the blades begins to turn and the machine starts to produce electrical power. In this case, the Vestas V80 starts to generate power when the windspeed is about 3 meters/second (m/s). As the windspeed increases, the turbine produces more power until, at its rated power speed of 14 m/s, the machine is producing its full rated output—which in this case is 2000 kW.

If the windspeed increases above the machine’s rated power speed, the output remains constant until the windspeed becomes excessive—in which case the turbine shuts down. Called the cut-out speed, for the Vestas V800, this occurs at a windspeed of 25 m/s–which would be reckoned gale-force winds on the Beaufort scale.

Capacity factors

The capacity factor of a power technology is the fraction of time it is generating electricity at its rated output. For a nuclear power plant, capacity factors are generally very high—above 90 percent. But for solar and wind energy, their power output is intermittent. A wind turbine can never operate at its rated power 100 percent of the time, because for a significant period the windspeeds at the site will be outside the range of windspeeds at which the turbine produces its rated power. In addition, the physics of air flowing through a turbine limit the maximum possible efficiency to just under 60 percent.

This is critically important because it means that a wind turbine rated at 2 MW can never produce an amount of electricity equal to 2 MW  x 8760 hours a year = 17,520 MWh/year. It’s physically impossible given the variability of the wind regime and the physics of the air flow through the turbine blades.

The capacity factor for solar photovoltaic energy and wind power will always be low compared to fossil fuel-powered technologies and nuclear energy. But the cost of energy produced by a technology is not only determined by its capacity factor. Solar and wind energy technologies have zero fuel costs. This is the critical factor in the calculation of the cost of energy produced by renewable energy technologies compared to power generated from coal, oil, and natural gas—a calculation which shows  that electricity from wind turbines in good locations is now less costly than electricity from fossil fuels including natural gas. And most importantly, there are no emissions of carbon dioxide, methane, and the other greenhouse gases—a factor that would be crucial even if the cost of electrical power came out to be about the same.

Windspeed distribution

It’s obvious to everyone that the wind is highly variable. Wind speeds vary over not only the space of a few seconds, but also between night and day, and over the seasons. Moreover, knowing the average or mean windspeed at a particular place isn’t good enough to accurately calculate the power output of a specific wind turbine that is located at the site.  For this we need to know the windspeed distribution.

Wind speeds at a site proposed for a wind turbine need to be measured for at least a year (and preferably two or three) in order to get an idea of the wind speeds at the site and their variability over time.  Windspeeds can be measured by an anemometer, recorded, and graphed as a histogram—plotting the time the wind blows for every increment of speed between zero and say 25 m/s. The chart below shows the windspeed distribution for an actual site in the UK, recorded hourly over the period 2005 to 2007. In this observatory, the windspeed was measured in knots.

Windspeed distribution at Plymouth, UK, 2005-2007

The mean windspeed is 10.2 knots equal to 5.25 m/s, but the distribution of wind speeds is not symmetrical about the mean.  It is skewed towards the right. The most frequently occuring wind speed is around 5 knots or 2.6 m/s.[7]

To estimate the average power from a wind turbine over a period  of time, it is necessary take a wind turbine power curve, like the one shown above, and multiply the power at every increment of wind speed by the fraction of time the wind is blowing at that speed. In practice, this is calculated by employing a computer model of the wind turbine’s performance and the wind speed distribution.  To formulate the model, the windspeed distribution has to be approximated by a mathematical formula–a probability distribution function. We will not go into the mathematics here, except to note that software packages are available (like the WindPower program) that compute the energy produced by a wind turbine installed at a site with a specified windspeed probability distribution. For instance, the estimated annual output from the 2MW Vestas offshore wind turbine calculated by the WindPower program is shown below [8]

Annual energy produced by a Vestas 2 MW offshore wind turbine

So for a site where the mean windspeed is measured at 7 m/s (generally the minimum mean windspeed needed for a good wind power site), the 2MW Vestas turbine would generate about 5672 MWh of electricity—assuming no losses in power transmission.  The machine’s capacity factor at this site is estimated to be about 32 percent.  If cost data are available, the levelized cost of electricity from this turbine can then be calculated.

As wind turbines have become larger and more technically advanced, capacity factors have been trending upwards. This has been driven by not only by the increase in the average hub height, turbine rating, and rotor diameter, but also by the trends in resource quality in new projects. 

For onshore wind energy, capacity factors for new turbines are approaching 30 %.  For offshore wind turbines, capacity factors are higher—over 40%.  In both cases, the trend continues to be upwards—a sign of the continuing technical advances being made in the design and operation of these  machines.  

There are significant regional variations, however.  Capacity factors for onshore wind turbines are highest in the US: over 40 %, driven not only by technology improvements but also the trend towards the location of projects in areas with the best resources [9].

Capacity factors are useful for making quick estimates of wind farm power outputs and for cross checking manufacturers claims of superior performance. Let’s say we are looking at a new 20 MW offshore wind farm off the coast of the UK. Approximately how much electricity would you expect that wind farm to produce in a typical year?

Offshore wind turbines are now showing capacity factors of more than 40 percent. Taking this number as a ballpark figure, we would estimate the output as 0.4 x 20 MW x 8760 hours/year = 70,080 MWh/year.  A more accurate calculation would be based on the wind turbine’s power curve and the site’s windspeed probability distribution as discussed above. For a large windfarm the calculation is complex—because wind speed patterns and the windfarm’s power output are affected by the precise location of each turbine, and this needs to be modelled mathematically in order to obtain an accurate estimate of the windfarm’s energy output over the course of the year.

Relative height of the 12 MW Haliade-X offshore wind turbine

Wind turbines are becoming larger and more efficient.  In March 2018, General Electric (GE) unveiled a plan to develop the largest and most powerful offshore wind turbine to date. Called the Haliade-X, the nacelle of this mammoth machine will stand 260 meters above the waves.  Its three 107-meter-long blades will drive a generator producing 12 MW of electricity at its rated output—enough power for about 16,000 homes.  Although not yet in service (planned for 2021), GE estimates the turbine’s capacity factor at an impressive 63%–which is a good bit higher than offshore wind turbines currently in operation.[10]

Wind energy resources

The excellent Wind Atlas website shows mean windspeeds for regions across the globe in considerable detail.  The mean wind speed for a site is available at three different heights: 50, 100 and 200 meters. As a first cut, if one assumes a typical windspeed distribution for the indicated mean windspeed, it is possible to estimate the power output of any turbine if the power curve is known.  Even if only the rated power and the rated windspeed for the turbine are known, it is still possible to make a reasonably good estimate of the energy produced by the turbine for a given location.

On the east coast of North America from Virginia northward up to Newfoundland, wind resources are excellent.  The Great Lakes also look good [11].

Across the mid-west of the US, wind resources are excellent. In 2017, Texas alone added 2.3 GW, for a year-end total of 22.6 GW.  If the Lonestar state were a country, it would rank sixth worldwide for cumulative wind power capacity. Wind power accounted for nearly 15% of electricity generation in the state during 2017. Utility-scale wind power accounted for more than 15% of annual generation in eight additional states, more than 30% in four states (including Iowa, at 36.9%) and 6.3% of total US electricity generation. [12]

Not surprisingly, wind farms are being built and operated in areas with the best wind resources. The table shows the world’s largest wind farms operating in 2017, with a capacity greater than 500 MW. [13]

 

The largest onshore windfarm at Gansu in China is planned for a huge 20,000 MW total capacity, but reportedly has operational problems linked to a lack of demand for electricity in the region and the transmission of this amount of intermittent power [14].

Offshore windfarms tend to be smaller. The largest in 2017 was the London Array with a capacity of 630 MW. Only six offshore wind farms are above 500 MW–four of which are in UK waters. However, this disparity is changing: much larger offshore wind farms are in the works–nearly all of them planned for European waters. [15] 

In 2018, the 660 MW Walney Extension Offshore Wind Farm was powered up in the Irish Sea, approximately 19 km from the coast of Cumbria in the UK, and close to the original 367 MW Walney Offshore Wind Farm. The Walney Extension wind turbines will generate enough electricity to power more than 460,000 homes. The chart shows the world’s largest offshore windfarms in 2018. [16 ]

The cost of electricity

The Levelized Cost of Electricity from a wind power project is a function of the cost of installation, the quality of the wind resource, the technical characteristics of the wind turbines, operation and maintenance costs, the cost of capital, and the economic life of the project.  The LCOE therefore largely depends on four factors:

  • Capacity factor. This is the result of the interplay of several variables, among which are the characteristics of the wind energy resource, the technical characteristics of the turbine, and its operational availability.
  • Total installed cost. The cost of the turbine is generally the largest cost item. Offshore projects have higher installation and operation and maintenance (O&M) costs.
  • Operation and Maintenance. There are both fixed and variable costs associated with the operation and maintenance of the turbines and their ancillary equipment.  O&M can represent 20 to 25 percent of the LCOE.
  • Cost of capital.  The weighted cost of capital (WACC) is a major factor in the calculation of the LCOE.

During the period 1983 to 2016, the LCOE of onshore wind energy dropped by an average of 15% for each doubling of installed capacity.  The global weighted average value declined from USD 0.40/kWh in 1983 to USD 0.06/kWh in 2017 [17].  Moreover, the trend is still downward—and this is expected to continue as turbine designs improve, machines get larger and more efficient, and economies of scale continue to exert a downward impact on costs.

The LCOE of offshore wind projects is higher—but is also trending downwards. From 2010 to 2016, the global weighted average of offshore wind decreased from USD 0.17 to USD 0.14/kWh. This was made possible by improved technology that has allowed higher capacity factors that have more than offset the increase in installed costs because of the larger and heavier machines.  The prices awarded in auctions in 2016 and 2017 for projects coming online by 2020-2022 range from USD 0.10/kWh to as low as as USD 0.06/kWh [18]

The technology is rapidly evolving. In the US, wind farms are being ‘repowered’ as technology upgrades increase their energy production. As the largest windfarms are getting older, their owners are starting to ‘repower’ them with more efficient generators, new electronics, and longer lighter blades. In 2017, the US wind industry completed 15 repowering projects totalling 2136 MW. The upgrades extend the  life of the projects without having to build a new windfarm. Since the infrastructure and power purchase agreements are already in place, the increased energy output produces greater revenues from the same location. [19]

In Europe, repowering has become a billion-dollar industry. While most repowering involves the replacement of old turbines with fewer, larger, and more efficient and reliable machines, some operators are switching even relatively new machines for upgraded turbines and software improvements. [20]

European wind power is rapidly gaining ground over nuclear energy. Although Britain has eight operating nuclear plants, over the first three months of 2018, UK residents received more electricity from wind power than from nuclear energy—the first time that wind had overtaken nuclear in the UK.  One of the reasons reported for this increase in production was due to a new transmission line between Scotland and north Wales that opened in December 2017.  This allowed wind turbines to keep generating electricity, whereas in the past their output might have been curtailed once the grid they fed into became unable to accept more power [21]. This is a point worth noting—solar and wind energy, because of their intermittency—may require upgraded electrical transmission systems that can handle their characteristic unpredictability and variability. But technical advances and the falling costs of utility-scale storage batteries are increasingly solving the problem of intermittency.

Environmental impacts

There are essentially three types of environmental impact associated with the installation and operation of large wind turbines: noise, aesthetics, and wildlife mortality.

Wind turbines make noise, and for many people noise is bothersome and if loud enough, definitely annoying and stressful.

A comprehensive study of the noise issues and health impacts of wind turbines was conducted by Health Canada in 2014.  The study focused  on over 1500 families living within 600 meters of a wind turbine in Ontario and Prince Edward Island. The researchers investigated the prevalence of health effects or health indicators among people exposed to wind turbine noise (WTN) using both self-reported and objectively measured health outcomes. They also looked at low frequency noise and infrasound from wind turbines to see if there was an adverse community reaction.

The study could not find any statistical correlation between wind turbine noise and sleep disturbance, illness, stress and quality of life.

However, in some cases, wind turbines clearly annoy people that are too close to them. There was a statistical correlation between WTN and annoyance linked to noise, shadow flicker, blinking lights and visual impact.  In all cases, as expected, annoyance got worse with increasing exposure to noise levels. Community annoyance fell significantly at distances between 1 and 2 km from the turbines. Communities on Prince Edward Island living within 550 meters of a wind turbine were recorded as being ‘highly annoyed’.

Interestingly, annoyance was significantly lower among 110 participants who received personal benefit from the installation of the turbines: either rent, payment or other indirect benefits of having wind turbines in the area—such as some form of community improvement [22].

The takeaway from the Health Canada study is perhaps the obvious one that large machines with long rotating blades that make a noise can be annoying if you live too close to them. But it helps if you gain from their presence.  But clearly, wind turbines should not be located too close to communities. A rule of thumb would be that 1 km is the minimum, but 2 km is better.  Offshore wind projects would seem to have a definite advantage in this respect.

Noise is clearly an issue in other jurisdictions as well: in June 2018, a judge in Minnesota recommended that the Freeborn Wind Farm be denied an operating permit, saying the southern Minnesota project failed to show it could meet state noise standards.[23]

Aesthetically, some people just don’t like wind turbines. Whether by themselves on a hillside, grouped into a windfarm on land, or out to sea on the horizon, many people object to their presence and see wind turbines as an unattractive blot on the landscape. An eyesore. This objection may diminish as the turbines are located farther away from the observer’s viewpoint, but one suspects that some people are just never going to like wind turbines intruding on the landscape even if they are at a considerable distance.  Would they prefer a nuclear or gas-fired power plant with multiple cooling towers billowing steam?

This is the NIMBY syndrome: Not In My Back Yard.  But if you want electricity, the technology has to be in someone’s back yard. Generating megawatt-scale electrical power to light and heat hundreds of thousands of homes requires lots of big, noisy, machinery and infrastructure. It can’t go underground.  But if the electricity is generated by wind, all that machinery can go out to sea—which is more than fossil fuels and nuclear energy can do.

Birds and bats

Wind turbines are big machines and the tip of the long rotating blades is moving extremely fast. It is undeniable that wind turbines kill birds and flying mammals such as bats.  

But how big of a problem is it really?

The US Fish and Wildlife Service looked into this question a few years back and estimated the level of threat to birds from several human activities. The table shows their assessment—necessarily approximate but nonetheless instructive.

Common human-caused threats to birds in North America

It’s obvious that wind turbines are not the main problem.  Even oil pits kill more birds than wind turbines.  But cats have a lot to answer for.[24]

What about bats?

Unfortunately, wind turbines can kill lots of bats—perhaps as many as a million every year in the US.  Bats tend to congregate around tall trees and it is thought they mistake the turbines for trees. The fragile animals are frequently struck by the blades, but they can also be killed by the large and sudden changes in pressure caused by the swirling air.

Since bats only fly in relatively light winds—when turbines are not producing much power, one proposed solution is to raise the cut-in speed of turbines in areas where bats fly from 3 or 4 m/s up to  6 m/s.  A study by Bat Conservation International in 2010 showed that higher cut-in speeds reduced bat mortality by up to 90 percent [25]

If wind turbines kill a bat species that is listed as an endangered, the US Fish and Wildlife Service could order wind turbine operators to raise the turbine’s cut-in speed–which would reduce the wind farms’ power output and revenue and certainly push up the levelized cost of electricity.

Windfarms positioned several kilometres out to sea presumably would not kill any bats—so this is another argument in favour of offshore wind.


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Check out these publications for more information:

[1] See: The History of wind turbines by Zachary Shahan, available at //windenergyfoundation.org/about-wind-energy/history/
[2] All the numbers and historical information are from the History of Wind Turbines, Op.cit.
[3] See: Renewables 2017 Global Status Report. Renewable Energy Policy Network for the 21st Century (REN21). Available at : //www.ren21.net/gsr-2017/
[4] REN21 Renewables 2018 Global Status Report.
[5] See //www.energy.gov/eere/wind/how-do-wind-turbines-work
[6] WindPower Program Basic Concepts, accessed at www.wind-power-program.com
[7] Ibid
[8] The table is from the WindPower program computer run for the 2MW Vestas offshore turbine
[9] Renewable Power Generation Costs in 2017. Op.cit.
[10] See: GE announces Haliade-X, the world’s most powerful offshore wind turbine. At: //www.genewsroom.com/press-releases/ge-announces-haliade-x-worlds-most-powerful-offshore-wind-turbine-284260
[11] See the Wind Atlas website at //globalwindatlas.info
[12] REN21. Renewables 2018 Global Status Report. Op.cit.
[13] The list is from Wikipedia–which has a more comprehensive list. See: //en.wikipedia.org/wiki/List_of_onshore_wind_farms
[14] See: It Can Power a Small Nation. But This Wind Farm in China Is Mostly Idle. Accessed at: //www.nytimes.com/2017/01/15/world/asia/china-gansu-wind-farm.html
[15] See the Wikipedia article at: //en.wikipedia.org/wiki/List_of_offshore_wind_farm
[16] See the September 2018 Guardian article: World’s largest offshore windfarm opens off Cumbrian coast. At: //www.theguardian.com/environment/2018/sep/06/worlds-largest-offshore-windfarm-opens-cumbrian-coast-walney-extension-brexit
[17] Renewable Power Generation Costs in 2017. Op. cit.
[18] Renewable Power Generation Costs in 2017. Op.cit.
[19] See: Aging wind farms are repowering with longer blades, more efficient turbines. //insideclimatenews.org/news/27032018/wind-power-blades-capacity-clean-energy-technology-jobs-ge-siemens-leeward-midamerican-repowering
[20] REN 21. Renewables 2018 Global Status Report. Op.cit.
[21] Wind power overtakes nuclear for first time in UK across a quarter.  Access at: //www.renewableenergyworld.com/articles/pt/2018/05/wind-power-overtakes-nuclear-for-first-time-in-uk-across-a-quarter.html?
[22] See the study by Health Canada.  At : //www.canada.ca/en/health-canada/services/environmental-workplace-health/noise/wind-turbine-noise/wind-turbine-noise-health-study-summary-results.html
[23] See Judge’s ruling against Minnesota wind farm causes alarm for advocates. Accessed at: //www.startribune.com/judge-s-ruling-against-minnesota-wind-farm-causes-alarm-for-advocates/485312391/
[24] See: The impact of free-ranging domestic cats on the wild life of the United States, at //www.nature.com/articles/ncomms2380.  The estimated body count for birds killed by cats is between 1.3 and 4.0 billion birds each year—just in the US.  Also see the US Fish and wildlife Service report: Threats to birds.  Accessed at //www.fws.gov/birds/bird-enthusiasts/threats-to-birds.php 
[25] See: Wind turbines kill more than 600,000 bats a year. What should we do? At: //www.popsci.com/blog-network/eek-squad/wind-turbines-kill-more-600000-bats-year-what-should-we-do.  Also : Bat killings by wind energy turbines continue. At: //www.scientificamerican.com/article/bat-killings-by-wind-energy-turbines-continue/