A Primer on Wind Power

Photo rights: The Guardian

The disruption of air as a result of wind turbines results in cloud formations in Horns Rev wind farm, Denmark – the world’s first large scale offshore wind farm. Photo rights: The Guardian

Wind power is a critical technology in the battle to replace fossil-fuel based power with renewable energies. Wind is one of the truly renewable, clean sources of energy we have at our disposal – it’s limitless, abundant across the planet, and doesn’t produce any harmful emissions when harnessed for generating electricity.

As a resource, there is estimated to be about a million gigawatts (GW) of potential energy available from total land coverage (1). Offshore wind potential tells a similar story – European resources within 30km of the coast alone would capable of supplying the total demand for electricity of the European Union (1). The World Wind Energy Association’s Mid 2014 Report estimated that by June this year there was approximately 336 GW of installed wind power, with 360 GW expected by the end of 2014 (for a reminder on energy units see references; 2).

Wind is ubiquitous, but it’s also a considerably mercurial resource, subject to massive variation. Wind’s geographic distribution and strength is determined by the planet’s weather systems and while these are predictable to some extent, they represent a critical factor in wind power’s capacity to deliver energy in a reliable fashion – an important point of discussion we’ll return to.

Wind power could never be a singular energy solution for the world, but its contributions to utility grids is enormous, and above all else, like solar, wind power represents a clean energy solution that holds potential in every corner of the planet. Be sure to check out this presentation of global wind flows and you’ll quickly get an impression of the ubiquity of wind.

Wind flows off the coast of Northern European.

Powerful wind flows off the coast of Northern European represent a massive resource that the wind power industry in Europe taps into. This image shows near real-time wind flows at the time of capture on 07/10/14, using the site Natural Earth.

The recent decades have seen an enormous growth in the amounts of wind power installed around the planet – doubling roughly every three and a half years over the last decade (2). Fortunately trajectories for continued growth don’t show signs of this trend lessening on a global level, although installation in forerunning countries is levelling out. While there is a clear concentration of wind power in the US, Europe and China, this only goes to demonstrate the enormous potential for growth in other regions where wind is equally as plentiful.

 

Alta Windfarm, California, US.  Photo rights: www.energy.ca.gov

Wind turbines at the world’s largest wind farm, the Alta Wind Energy Center, California, US. Image rights: Terra-Gen Power

A Brief History of Wind Power

Wind power of various descriptions has an ancient history. The earliest examples of harnessing the wind to drive machines date back several thousand years; and windmills were a common feature of landscapes in the Middle Ages, used to drive water pumps and in food production alike. The first electricity-generating wind turbines arrived in the late nineteenth century in Scotland. Denmark, a country which is currently leading the world’s wind power industries, had several thousand windmills powering machines by the beginning of the twentieth century. The arrival of cheap high-tensile steel during the 1930s allowed for the industry to move closer towards industrial levels of energy output, and the first megawatt wind turbine was installed in 1941 in the United States. Utility grids have been supplied with electricity generated from wind power since that same time in the United States, and 1951 in Scotland.

Nevertheless, the massive rise in fossil-fuel power during the latter half of the twentieth century largely overshadowed the wind power industry, to the extent that it was marginalised as a niche source of energy for farms and remote areas, receiving little significant interest or investment for larger scale projects.

That picture has changed dramatically in the last few decades. The rise of wind power as a source of energy on industrial scales can be traced back to Denmark in the 1970s when anti-nuclear sentiments and the energy crisis prompted the state to look toward renewables as a source for future energy needs. It was there were models for small cooperative-based wind turbines can be seen to have been established – bringing with them demonstration of the feasibility and advantages of wind power as a source of electricity. Beginning in the 1980s and especially the 1990s, things took off at pace; motivated largely by the growing realisation that a carbon-fuelled world carried with it massive consequences for the planet’s health, development of clean, renewable energies gained political and economic momentum from which wind power industries benefitted. To the present day and there are well over 200,000 turbines in use in 103 countries around the world; 17.6 GW of new installations  were completed in the first half of 2014 alone (2). Given that wind power often displaces coal-fired power plants, it’s reckoned that with each kilo watt hour (kWh) of wind power displaces about 1 kg of carbon dioxide being released as a result of conventional fossil fuel burning (1).

 

Physics of Wind Power

The process of converting the wind’s kinetic energy into electricity is actually quite straight forward. As a wind turbine’s rotors (also referred to as blades) turn, they convert wind energy into low speed rotational energy. The profile of each blade is very similar to the wing of an aeroplane, and actually harnesses the same force, ‘lift’, to turn. Due to their shape, as wind passes over a blade an air pressure differential is created: on one side of the blade, the wind produces an increase in pressure; on the other side, a pocket of lower pressure; together these forces cause the blades to rotate.

Even in high winds the rotors turn relatively slowly, typically between 12-20 RPM – a speed not sufficient to generate electricity. Therefore gears are used to convert low speed rotational energy into high speed rotations of around 1800 RPM that is then used to generate electricity. The massive gear ratio involved in this conversion means that the gear boxes are subject to forces that result in considerable wear; as such, their regular maintenance is a important aspect to wind turbine upkeep and longevity. Research efforts have gone into producing turbine designs that do not feature a gearbox. While this is a less common practice, and only possible if using a direct drive turbine, the technology has been demonstrated.

A profile view of a rotor blade on a 2MW turbine.  Image rights: www.berlinwind.com

A profile view of a rotor blade on a 2MW turbine. Image rights: Berlin Wind.

 

Wind as a Highly Variable Resource

It’s commonly noted that a fundamental disadvantage of wind power is the intermittency with which it generates electricity. It goes without saying that wind isn’t a constant – and it’s massive variance certainly does present challenges to researchers in the wind power industry who are seeking out more efficient ways to deliver steady amounts of electricity from wind power. Before we present some solutions of this sort, it’s important to conceptualise wind as the mercurial resource it is.

Firstly and foremost, wind itself, the flow of atmospheric air, is a force driven by a multitude of factors – the two principle factors being atmospheric pressure differentials (largely a result of temperature variation across the planet), and to a lesser degree the rotation of the Earth (the Coriolis effect). These factors combine to determine wind flows, their strength and direction. Just as water carries a force as it flows, so too does wind. The force of wind however, is much lower than water because its density is far less – hence why hydro power can yield much larger amounts of energy than wind can.

So how much wind is required to begin generating electricity? Well, not a great deal actually – 5m/s is generally cited as the minimum operating wind speed; at which, a single turbine can produce around 2457 MWh (1). Note also that wind speeds of about 5 m/s can typically be found in-land on all five continents. Offshore, wind speeds are generally higher, and for example 8 m/s is common in European coastal waters.

But as a force moving through a volume, wind power increases with the cube of the wind speed. As a result, even very slight changes in wind speed can have a very large affect on how much power a wind turbine can produce. Annual energy production from a turbine facing a wind speed of 7m/s can produce 5629 MWh and jumps to 6725 MWh at 8 m/s (1). On a larger scale, in Denmark for instance, power output can drop from 4100 MW to zero in a matter of hours depending on changes in the wind.

The energy yield of a wind turbine is calculated in kWh per square meter of its rotor sweep (measured by the area through which rotor blades pass). Consequently, if the diameter of the rotor blades is doubled, the power increases by a factor of four. If the wind speed then doubles, power increases by a factor of eight. Energy yields per square meter of rotor area have doubled since 1990, and the Danish Energy Agency estimated in 2008 that the most productive machines delivered about 1500 kWh per square meter of rotor area.

A solution to the variability of wind has been found in the integration of several wind farms into utility grids – by preparing appropriate infrastructure to support several wind farms feeding into a common electrical grid, if one wind farm isn’t reaching capacity, then another one which is, can be providing electricity. The sum result of integrated grid systems is threefold: first, wind variation is averaged out, ensuring that enough electricity is present within the system to ensure energy is supplied when demand is high; second, the need for reserve stores of energy is mitigated; and third, net productivity of wind power within a grid is higher and therefore lowers costs per kWh.

 

Construction of Wind Turbines

Naturally wind turbine designs vary depending on its type and the manufacturer’s methods – but the description that follows provides at least a basic insight on typical construction of a horizontal turbine. Construction and design can be considered in relation to four aspects: the foundations, the tower, the nacelle (which houses turbine machinery such as generators and gears), and the rotors.

Image rights:

Construction of a wind turbine. Image rights

Taken together, the foundations and tower are a critical element in construction of a wind turbine. Not only must they support the weight of the nacelle and rotors, but also absorb the considerable loads that result from powerful winds. The exact design of foundations is determined by two factors: one, the specifications of the turbine (its size, and the winds it’s predicted to work with) to ensure that loads and strains are appropriately accommodated; and two, the topography of the land (we’ll leave offshore construction aside for now, other than to say that most are found in waters no deeper than 20m). Foundations are invariably built from concrete reinforced with a steel lattice. The lower tower section is bolted to steel left protruding from the surface of the foundation. Towers themselves are constructed from pre-fabricated concrete sections, which are fastened together during construction. Steel (stay-) cables run down through the inside of the concrete wall for added reinforcement.

offshore-wind-farm-construction

A rotor blade attachment is guided into the hub at the top of a tower. Image rights: Fluor.

Wind turbine rotors are largely constructed from a mixture of synthetic materials, reinforced with fiberglass and other carbon fibers. Unlike steel or aluminium, synthetic fibers fatigue little over time, and represent a more durable material suitable for coping with strains that rotors must endure.

While most turbines feature three blades, some machines have two. Either way, they connect to a cast steel hub which turns in response to the rotors, directing energy from the rotors to the generator.

A ‘nacelle’ is the name used for the large component situated behind the rotors that houses the machinery of the turbine: its generator, brakes, rotor shaft, and various sensors, controls and electronics. Brakes are required to ensure that rotation during high wind speeds doesn’t damage the turbine machinery.

Large collections of large turbines are referred to as wind farms, and vary considerably in scale. The largest offshore wind farm is the London Array, off the coast of England – it contains 175 Siemens turbines with a capacity of 630 MW, generating electricity for close to half a million homes (some 900,000 tonnes of CO2 emissions are avoided as consequence of this). The largest onshore wind farm is the Alta Wind Energy Center, in California; it currently holds 490 turbines spread over some 3200 acres of land; its current capacity is 1320 MW, but it is still growing in size.

Installation of onshore wind turbines is particular quick – a 50-MW wind farm with twenty turbines can be completed on timescale of 18 months to two years. However most of that time is used for planning, modelling winds found at the site and obtaining construction permits; the wind farm itself can be built in less than six months (3).

Offshore construction of a 6 MW turbine at the Gunfleet Sands III demonstration project. Each turbine can supply up to 6000 European homes with clean electricity. Image rights: Siemens AG

 

Innovation in Wind Power

The exponential deployment of wind power around the world has been driven by research and innovation that has produced wind turbines that can generate far greater amounts of electricity. Just as we see with solar technology, as energy yields have increased, wind power has become far more cost-effective and feasible. While the fundamental technology behind way power has remained relatively unchanged, it’s largely the scaling up of wind-turbines (both capacity and rotor diameter), optimising efficiency and refinement in their mechanical workings that has been the focus of innovation.

Several decades ago, wind turbines were considerably smaller than those we are used to seeing today. Turbines with a 15-20m rotor diameter, producing 50-100 kW, were typical in the 1980s. Whereas modern utility-scale onshore wind turbines with a capacity of about 3 MW or more, feature a rotor diameter of between 60-80m and stand 80m tall (1). Larger still are offshore wind turbines, with an output up to 6 MW and rotor diameters of around 120m. This upscaling has been enabled through the advent of novel materials that can used to build larger turbines that remain highly stable, and have an operational life of around twenty years.

Inherent to the characteristics of wind, larger machines generate more energy for two reasons: one, because if the rotors are located higher from the ground they catch stronger winds; and two, because larger rotors create a larger sweep area and therefore absorb more energy from the wind. Vestas’  V164 holds the record as the turbine with the largest capacity – it is rated to 8 MW, and is also the world’s tallest at 220 m – it was introduced in 2014. Already though, several companies are developing 10 MW turbines.

Increasing capacity this much represents a massive accomplishment, and it’s central to the rise in prominence of wind power – it has not only lowered the cost of electricity generated by wind power, but it’s also made the technology a feasible source of significant amounts of energy using fewer resources (land and materials). To illustrate this point, consider that an average household in the US uses about 10,000 kWh of electricity a year; since one megawatt of wind energy can generate from 2.4 to more than 3 million kWh annually, a single wind 1 MW turbine generates enough electricity for up to 300 households (3).

Minimising quantities of production materials is a major consideration for wind industries. Lighter and more flexible constructions are certainly at the heart of the research. Key turbine components which are subject to this line of research include the towers, generators, blades, and foundations. Although larger machines require more materials, their much greater capacities, coupled with a twenty year lifespan, means they remain cost-effective investments.

Aerodynamic modelling is a major feature in the wind power industry. It’s used to design and refine rotor blades to ensure they maximise their use of wind’s energy; as well as to choose turbine configurations best suited to optimise wind farm performance. Modelling also ensures that machines are designed and built to withstand the massive stresses they are subjected to, so as to reduce maintenance as much as possible, and increase the lifespan of the turbine.

Modelling of wind effects and turbulence within a wind farm. The figure shows a three-dimensional model of the flow in a simulated wind-farm. The blue regions show a volume rendering of low-velocity, turbulent wind regions. Image rights: NCSA (National Center for Supercomputing Applications).

Sophisticated computer modelling has also been used in recent years to study the effects of wind turbulence in wind farms with the aim of optimising net performance. The motion of turbine rotors can create turbulence in the wind that disturbs the efficiency of turbines positioned in this turbulent wake. When considered over several large rows as in a wind farm, these effects can add up significantly. Modelling of these wind dynamics has allowed industry to build smarter, more efficient wind farms in which each turbine can adjust its performance to ensure that net efficiency of the entire wind farm is optimised (for example). At the same time, smarter configurations and adjusting performance can reduce mechanic stress on those turbines located in the wake of others.

 

Recent Global Development

As of June 2014, an estimated 340 GW of electricity is generated annually from wind power around the world (2). World wind energy capacity has doubled about every three and a half years since 1990: total capacity at the end of 2011 was greater than 236 GW. Global wind capacity grew by 13.5% between mid-2013 and mid-2014, and in the first half of 2014 alone, 17.6 GW were installed (2). It should be noted that no other energy technology has ever grown with such pace.

Image rights: World Wind Energy Association

Worldwide increases in wind power capacities. Image rights: World Wind Energy Association, Mid 2014 Report.

Asia is the continent with most installed capacity, 119 GW or 36.9% of the global total; with Europe just behind at 36.7%. Though China has the highest installed wind capacity of any country (98.58 GW), Denmark has the highest per capita – it’s 4.85GW supplies some 34% of electricity consumption in the country, and was  750 W per capita by the end of 2012, compared to 400 W/cap in Germany, 190 W/cap in US, and 60 W/cap in China (all statistics: WWEA, Mid 2014 Report). The vast majority of wind power is located onshore, just 2% is offshore – a total of 7.4 GW by the end of 2013; around 90% of which is located off the coast of northern Europe (1).

Electricity_generated_from_renewable_energy_sources,_EU-28,_2002–12_YB14

European increases electricity generated by renewable energies, also shown is the proportion of electricity consumption by energy. Image rights: Eurostat.

 

Projections for Growth

Like all renewables, forecasts for the growth of wind power vary considerably and are subject to numerous conditions. The International Energy Agency’s New Policies Scenario suggests 587 GW global capacity by 2020. The European Wind Energy Association estimates 230 GW in Europe alone by 2020, and 300-350 GW by 2030. Relating to offshore specifically, projections of 80 GW installed by 2020, with 75% of this in Europe, are typical (1).

But to try and frame these projections with a degree of caution they deserve, consider the following two points. First, the European Commission’s 1997 estimation for wind power in 2010 was 40 GW (some 16 times that of the 1996 level); but that target was already met in 2005, and by the end of 2009, European capacity was already over 72 GW. Second, in 2009 the IEA published an estimate of installed capacity of global wind by 2030 to be 587 GW; but just two years later, in light of actual growth, the same amount was the forecasted capacity for 2020.

According to some of the more ambitious projections, a world total wind capacity in 2020 of 700 GW (World Wind Energy Association) or 750 GW (World Energy Council, 2013) has been estimated. The World Energy Council also estimates a further doubling to 1550 GW by 2030. But there are grounds to consider this a conservative estimate on account of the notion that such projections are largely predicated on trends in areas where wind growth has already been growing at pace – the top ten capacity countries (including the United States, China, India and several European countries) collectively account for some 85% of worldwide capacity and have driven the doubling of installed capacity every 3.5 years we noted earlier. The remaining 15% share is spread over 69 countries. This aspect of distribution means that developing countries represent a massive new arena that can drive the growth of wind power, alongside those top ten countries that we know to be committed to aggressive policies of growth.

So while the estimations vary somewhat, the overall trend for continued exponential growth is clear. As innovation continues and costs of wind power decline, the industry will increase its competitiveness with traditional power generation technologies too – making wind power an increasingly attractive option for renewable energy production.

It’s important to be mindful though that wind power installation is often subsidised by governments (lowering costs considerably) in order that they meet renewable energy targets. Subsidisation has been a major cause for the massive growth in wind power presented above, and it’s important to see such financial support and political will upheld if growth is to be maintained – this is a contentious issue however, and one we’ll have return to in the future.

Burbo Bank WindFarm England. Image Rights: The Guardian

Burbo Bank Wind Farm, England. Image Rights: The Guardian

 

Future Generations of Wind Power

There are several lines of technology and innovation that are sure to foster future generations of wind power. Floating wind turbines and smarter grid solutions represent two exciting prospects – they are introduced only briefly here.

In 2009, Siemens and Statoil launched the Hywind project, the world’s first floating wind turbine. The Hywind turbine can be installed in water depths between 120 and 700 meter and as such brings the potential to extend the placement of wind farms beyond shallow waters. A key advantage to this would be allowing access to high winds found in deeper waters.

Developing novel systems for distribution of energy is an important topic, applicable to the future development of many renewable energies. Whether it is moving energy through time and space (i.e. distribution) or improving our capacity to store energy, there are considerable benefits to be had. As an example, outputting of wind (and solar) energy by way of smarter grid-solutions, to a larger geographic area in order that we avoid locking-in wind power to the region where it is generated. This challenge is particularly attractive to wind power industries as it would overcome problems associated with wind resources being located at sea and coastal regions.

 

Conclusions

The development of wind power over the last two decades has been extraordinary. Wind power has become a frontrunner in the renewable energy market due to massive innovation which has enabled cost-effective and sustainable production of electricity. The potential for further growth remains great; the leading countries are still committed to expanding their wind power installations, while new markets are sure to emerge too and will benefit enormously from the latest generations of wind technologies.

In the near future, working to deliver the benefits of cheap wind power to inland areas through smarter grid solutions and integration, will prove a vital component in our ability to make better use of wind resources. Above all else, in its ubiquity and simplicity, wind power represents a clear and viable solution (though not a singular one) for combatting climate change and working towards cheap, clean energy production.

 

References & Resources

(1) World Energy Council (WEC), World Energy Resources 2013 Survey

(2) World Wind Energy Association (WWEA) Mid 2014 Report

(3) American Wind Energy Association (AWEA)

European Commission, renewable energy statistics via Eurostat

(Just a reminder: one kilowatt (kW) is equal to 1000 watts; one megawatt (MW) is equal to one million watts; one gigawatt is equal to one billion watts (or one thousand MW); and one terawatt (TW) is equal to one trillion watts. The world’s annual energy consumption is measured in the region of 18 TW).