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Wind energy: on-shore, large scale

Besides conventional hydropower, onshore large-scale wind energy is presently the cheapest form of renewable energy. At locations with a good level of wind resource, it can be cost competitive with some forms of traditional/thermal power production. Global wind energy capacity has been growing very rapidly over the past decade. In 2009, newly installed capacity was more than 34GW (GWEC, 2010) representing more than 25% of total new generation capacity globally. In 2009 wind energy contributed approximately 1.8% of worldwide electricity demand, a percentage that has doubled since 2006 (Wiser and Bolinger, 2010).

Introduction top

Wind energy is actually a form of solar energy; the temperature differences caused by the sun shining on the earth act, along with other factors, to cause large bodies of air, winds, to move across the face of the planet.  As a result of these factors the highest wind speeds, and thus renewable resource, is found at larger latitudes, however there are also many localised regions with good wind speeds for electricity generation closer to the equator.

The conversion of the kinetic energy in these winds into electrical power is known as wind energy.  There are a number of ways in which this conversion can be done; however after a period of experimentation and development beginning primarily in the seventies (the use of wind power for grain grinding and water pumping dates back as far as 500 A.D.) one design has come to dominate the market.  This is known as the horizontal axis wind turbine (HAWT) with its archetypal three-bladed rotor as seen in the figure above.

A large wind turbine primarily consists of a main supporting tower upon which sits a nacelle (the structure containing the mechanical to electrical conversion equipment).  Extending from the nacelle is the large rotor (three blades attached to a central hub) that acts to turn a main shaft, which in turn drives a gearbox and subsequently an electrical generator (Fig 1). In addition to this there will be a control system, an emergency brake (to shut down the turbine in the event of a major fault) and various other ancillary systems that act to maintain or monitor the wind turbine.

Cut-away view of a typical wind turbine (source ZF, 2010)

Figure 1: Cut-away view of a typical wind turbine (source ZF, 2010)

Modern wind turbines have main towers that are typically 50 to 100 metres high supporting rotors with a similar range of diameters. Inside the tower there is a mechanism that ensures that the nacelle/rotor faces into the wind (i.e. is yawed correctly) to give maximum generation and maintain symmetric loads on the three blades and drive shaft. Generally the three blades are constructed from composites which provide a relatively high strength (required due to the large bending moments they experience) whilst maintaining a low weight and size given their length. Modern designs have a relatively low rotational speed in the order of 10 revolutions per minute (partly due to the desire to keep noise levels low) and thus typically require a gearbox to increase the speed of the drive shaft to match the rated generator speed. While most wind turbines use gearboxes (or indirect drive systems) there also exist direct drive configurations whereby the generator is coupled directly to the slow moving rotor.  These types of designs do not require gearboxes and thus avoid the reliability issues that have been known to trouble certain gearbox designs. However the larger generator size that is required in order to obtain the correct generation frequency faces its own challenges in regards to construction and cost.

Even for indirect systems, differences exist in the type of generator that can be used.  Older designs tend to be classed as ‘fixed speed’ meaning that the rotor always rotates at the same speed under all wind conditions.  For a number of reasons many modern turbines use generators that allow for variable speed generation whereby the rotational speed is optimised to the incoming wind speed and the generator provides output at a range of frequencies.  The resulting fluctuations in voltage and frequency are corrected by power electronics in order to provide electricity suitable for export to the grid. The advantages of this approach include reduced harmful torque fluctuations into the gearbox, increased conversion efficiency, the ability to continue operation during a grid disturbance and the ability to provide reactive power.  These last two are increasingly being demanded of wind parks by transmission system operators.  Further improvements to the level of energy capture are obtained by most modern turbines by changing the angle of the blades.  This ‘variable pitch’ system rotates the blades about their own axes so that for changing wind conditions the optimum efficiency is achieved. The system also acts to control the turbine, angling the blades ‘out of the wind’ during periods of high wind speed to prevent damage and providing the primary method for disabling the device.

In spite of continuing advances in turbine technology, there is an inherent physical limit as to the amount of energy in the wind that can be extracted.  A theoretically perfect (yet infeasible to construct) wind turbine could only ever extract 59 percent of the available energy, also known as the Lanchester-Betz limit. Modern turbines reach a conversion efficiency of approximately 50 percent, close to this theoretical limit and very close to the practical limit that is imposed by the drag of the blades. Nevertheless there is a significant body of ongoing global R&D into construction methods/materials for larger turbines, conversion efficiency refinements, lower cost components and improved reliability.

Feasibility of technology and operational necessities top

Resource

The first requirement when considering the possibility for wind energy is the identification of a suitable site that has a high level of resource; i.e. it is windy.  More specifically it should be windy at the height above the ground at which the rotor will be situated.  The surface friction of the earth’s surface, local topology and surface cover means that wind speeds are lower near the ground than they are higher up. 

In order to quantify the different levels of resource they are grouped into classes based on the yearly average energy available ranging from 1 to 7; with class 1 having low wind levels and class 7 having very high.  Many studies of global resource levels consider that for a site to be economically feasible it should have a resource/wind level of higher than class 3; corresponding to an average wind speed of more than 7 metres per second at 50m height (or approximately 5.6m/s at 10m height).  However in any given region it is understandably preferable to develop those sites that have the highest level of resource first as these will provide the best economic return.

Even at good sites there will be many times when a wind turbine is operating below its rated power (its nameplate or nominal capacity) or producing no power at all because of a lack of wind.  This means that although a turbine may be rated at, for example, 2MW it will produce a certain percentage of the theoretical power it could have produced had it operated continuously; this percentage is the capacity factor.  For onshore wind turbines this capacity factor varies between sites depending on the amount and consistency of the wind. In Europe capacity factors are in the order of 20 to 30 percent, in China on average approximately 23 percent, in India around 20 percent while in the US roughly 30 percent (IPCC, 2010).

Specialised software programs can be used to identify potential sites that could be suitable for wind energy development.  These programmes take historical wind data from local monitoring stations and lower resolution wind data from satellites to interpolate the estimated wind resource across an area or region of interest.  The software attempts to take into account the local topology and surface friction/roughness to estimate the available resource. It also typically allows planning issues to be identified by comparing wind resource to existing infrastructure and land uses.  This type of software is useful to identify possible sites, but is typically insufficient with regards to data reliability. To allow a project to proceed, actual measurements of the wind speed must be taken at the proposed site.

Because the wind speed is constantly changing it is necessary to take measurements over an extended period of time in order to gain a good understanding of the available resource and also to provide assurance of returns to investors such as banks.  Although there may be historical data available nearby from a neighbouring meteorological station it is common to take local measurements at the site of a proposed wind farm over the period of one year to observe seasonal variations. This is typically done with a large ‘mast’ or pole that is erected at the site and which has wind measurement equipment mounted on it with data recorders.  Once the wind speeds are determined it is then possible to make a stronger business case the for proposed wind farm.

Location

Until now the vast majority of wind turbines have been situated onshore, however along with the ease of access and construction that this provides, there are also planning issues with existing land users that must be checked and resolved.

In addition to requiring an appropriate/economically-feasible level of wind any site for a wind farm must also account for both certain technical requirements and also the needs of a number of possible stakeholders including:

  • A relatively smooth surrounding topology and surface cover (to minimise turbulence in the wind that can be harmful to the turbine over time)
  • Access to grid infrastructure for power export
  • Local inhabitants and concerns about noise, shadow flicker (the shadow of blades causing visual disruption) and visibility.
  • Vicinity to roads, power-lines, or other structures in case of tower collapse (considered very low risk but a requirement in certain countries) or ice forming on the blades and being thrown (in cold climates only)
  • Vicinity to protected nature areas and bird migration routes
  • Possibility of radar interference (sometimes raised as an issue due to neighbouring airports or military facilities)

As mentioned above, specialist software is typically used to map these constraints and determine possible locations for the installation of onshore wind turbines.

Technical Requirements

The technical capability required to support the deployment of large onshore wind energy largely depends on the level of localisation that is desired in the long term.  The fabrication and assembly of modern wind turbines has become a highly specialized industry dominated by a relatively small number of global players.  In certain countries that foresee a high level of wind energy installation there may be potential for local fabrication of certain components in partnership, under licence or behalf of an international company, for example blades or towers which have relatively lower technical demands than the drive-train conversion equipment. Equally, there is potential for other more technical aspects of the fabrication to be done locally should the demand and skill set allow it.  There has for example been strong political support for localising wind turbine manufacturing in China over the past couple of years.

On the contrary it is likely that early deployment of wind energy in most countries which do not have a large experience with wind energy will be based on imported components using local labour for parts of the installation only.  In these instances the local equipment requirements are for suitable cranes and construction equipment for the building of the tower foundations as well as road improvements and construction associated with the transport of the equipment on large trucks.

Ongoing maintenance of a wind turbine requires a relatively high level of experience and familiarity with the equipment and is often carried out by the OEM supplier during the initial years while the devices are under warranty, but could then pass into the hands of local teams who have received the necessary training.

Grid integration

Grid integration of wind energy has been a topic of substantial discussion, which is why it is discussed separately here. The primary perceived problem with wind energy is related to the intermittency of supply.  The variability of wind on any given day, week or month means that the amount of power that is produced can change accordingly.  In the short term wind levels and thus power generation can be estimated/forecast from meteorological reports with a reasonable degree of accuracy.  However this does not solve the issue that on days when there is little or no wind, an alternate form of generation is required, requiring additional backup capacity that would not be needed for traditional base-load power stations.

To date there have been a large number of studies of the integration of wind energy into electricity networks.  IPCC (2010) provides a good summary of the related literature which broadly concludes that at levels of penetration of up to 20 percent of supply the effects of variability and associated costs are relatively low but not insignificant.  The report also discusses the issue of transmission costs which are more of a problem for wind compared to traditional power stations due to the often large geographic distribution and remote nature of the resource. Again these costs are found to be moderate but not insignificant.  It is worth noting that the integration studies mentioned, mostly consider large electrical systems in industrialised countries; in smaller or less developed countries where the electrical generation system is less diversified and extensive, the effects/costs of wind variability are not so clear and would need to be studied further, should high levels of wind penetration be planned.

Legal/Regulatory

There is extensive experience in many countries with permitting and planning frameworks for wind parks including in many developing countries. Generally, the following are necessary:

  • A lease/payment-scheme for the area of interest for deployment of the wind park.  This land may be government owned which makes negotiations potentially straightforward, but in many instances wind parks are deployed on agricultural land and can coexist with the existing agricultural practices.  In areas where the land is owned by a small number of stakeholders direct payments to these parties are often arranged.  In a country where a larger number of small-scale land users might own an area of interest, this may present problems and should be considered in the development of a wind park; payments to communities may assist in overcoming this issue.
  • Appropriate environmental permits based on an environmental impact assessment (EIA) that can take between 1-2 years depending on the level of baseline data demanded by the permitting authority and the sensitivity of the area.
  • Permits from local/district planning officials that control the use of land within a region. These have been known to cause problems/delays/cancellations in instances where local communities/councils are not in favour of erecting wind turbines.
  • Grid connection agreement / power purchase agreement with the relevant body to ensure distribution and a market for the resulting electricity.

Social Acceptance

It can generally be said that the level of acceptance of wind parks onshore is high if appropriate measures are taken to ensure the limited noise and shadow effects do not affect local communities.  In certain instances there have been objections to projects on the basis of people disliking the sight of the wind park or because it could affect tourism or nature values in a region.  The occasional strong objection to local wind parks, especially in the UK, has become known under the acronym NIMBY (or Not In My Backyard). Only through consultation and meetings with stakeholders can the awareness of local communities be improved and potential local benefits made clear.

Status of the technology and its future market potential top

Wind energy is presently in a phase of rapid market deployment and represents an ever increasing share year of year of new global installed generation capacity. Additional globally installed wind capacity was 45 GW during 2012 increasing total cumulative installed capacity to over 282 GW (GWEC, 2013). 2012 was another record year with wind power adding more capacity than any other renewable technology. 

 

illustration © climatetechwiki.org

Figure 2: Total cumulative installed wind power capacity 1996-2012 (Source: GWEC, 2013).

 

Concentration remained in the top 10 countries, which accounted for around 85% of the global capacity, however the market has begun to broaden. As can be seen from figure ? below, cumulative installed capacity is dominated by China (26.7%), USA (21.2%) and Europe with Germany leading the way having a 11.1% share of the market.

illustration © climatetechwiki.org

Figure 3: Top 10 countries by cumulative installed capacity at year end 2012 (Source: GWEC, 2013).

The world’s top 10 turbine manufacturers captured 77% of the global market 2012. Vestas (Denmark), the top manufacturer since 2000, surrendered its lead to GE Wind, which secured top spot due mainly to the strong U.S. market. Other top companies were United Power, Sinovel, and Mingyang (all China); Gamesea and Goldwind.

The forecast beyond 2014 continues to be strong with many countries still having a low penetration of wind energy but excellent levels of resource.  Total onshore world resource estimates vary widely due to different assumptions on the availability of land for installation and the available wind resource.  IPCC (2010) summarise literature with a range of 20,000 to 53,000TWh or approximately 7,600 to 20,000GW of installed capacity.  A growing proportion of this increase in onshore capacity is likely to come from developing countries due to potentially lower constraints on land availability and the lower cost of onshore wind power compared to offshore wind energy.  For industrialised nations, a significant proportion of the growth in wind energy in the longer term is forecast to come from offshore wind.

Contribution of the technology to protection of the environment top

Although wind energy has a net positive impact on climate change mitigation (see below) local environmental impacts must also be considered. The most well publicised potential issue is the impact that wind turbines can have on bird and bat populations due to collisions.  IPCC (2010) provides a good summary of the specific studies that have looked at the number of fatalities of these species.  There is a strong argument that the number of recorded fatalities, while site specific, is relatively low compared to other anthropogenic causes of bird and bat deaths such as cars, collisions with buildings, feral cats and transmission lines.

In terms of other ecological effects related to the installation, the turbines have a relatively small environmental footprint and are often constructed on agricultural or brown-field sites, which limits their impact on local habitats or ecosystems. In instances where they are being installed in more pristine environments, a more rigorous environmental impact assessment may be required.

Climate top

The renewable nature of wind energy, the large available resource and the relatively advanced nature of the technology mean that it has the potential to make a significant contribution to climate change mitigation. By acting to displace generation from thermal power plants onshore wind energy can prevent the emission of roughly 2,000 tonnes of CO2 per year per megawatt of installed wind capacity (assuming it replaces coal and is located at a reasonable wind energy site).

Although there is some amount of carbon used in the manufacture of the devices, studies have shown that the payback period (the time it takes for the wind energy to offset the emissions associated with its fabrication and installation) is relatively low, typically in the order of 6 months or less (IPCC, 2010).

Financial requirements and costs top

Though the cost of wind energy has declined significantly since the 1980s, in most regions of the world, policy measures are required to make wind energy economically attractive (IPCC, 2010). However in areas where prices of conventional electricity supply are high due to imported fuels or other factors but which have a good wind resource, wind power can be economically competitive without subsidies. 

The LCOE for wind power plants varies considerably dependent upon the location and attainable full-load hours. As Figure 4 below shows this range for on-shore developments can be from 60 euro/MWh up to 115 euro/MWh (80 to 155 USD/MWh).

illustration © climatetechwiki.org

Figure 4: Levelized cost of electricity for onshore and offshore wind power, 2012 (Source: Fraunhofer ISE, 2012)

 Relative to their 2008 peak, wind turbine prices have fallen by up to 20-25% in the western markets and as much as 35% in China, but have stabilised in 2012. Operation and maintenance costs of wind farms have also fallen further increasing the cost-competitiveness of wind power generation (REN21, 2013).

illustration © climatetechwiki.org

Figure 5: Installed cost distribution for onshore and offshore wind power plants (IPCC, 2010)

Clean Development Mechanism market status top

[this information is kindly provided by the UNEP Risoe Centre Carbon Markets Group]

Project developers of large-scale wind projects in the CDM pipeline mainly apply the following CDM methdology:

ACM 2: "Consolidated baseline methodology for grid-connected electricity generation from renewable sources”

CDM projects based on wind represent 17.3% of all CDM projects in the pipeline. Recent years have shown a tendency towards a more widening geographical dispersal of CDM wind projects, indicating that countries other than India and China observe the CDM as a tool to support wind projects.

Overview over wind projects in the CDM

Figure 6: Overview over wind projects in the CDM (source: UNEP Risoe CDM/JI Pipeline Analysis and Database, February 1st 2010)

 Example CDM project

Title: “KL Rathi Steels 1.5 MW Wind Power Project at Kutch District” (CDM Ref. No. 2706)
M/s KL Rathi Steels is a closely held Public Limited company, engaged in making TOR steel bars and steel wires. The work of the company takes place in the village of Chapraula, on the outskirts of Ghaziabad, near Delhi. M/s KL Rathi Steels have installed one 1.5 MW wind turbine generator (WTG) of Suzlon make, in Abdasa, Kutch District, Gujarat.
Project investment: USD 2,200,000
Project CO2 reduction over a crediting period of 7 years: 27,100 tCO2e
Expected CER revenue (USD 10/CER): USD 271,000

References top

EWEA (European Wind Energy Association) 2009. Wind at work, January  2009; available from http://www.ewea.org/fileadmin/ewea_documents/documents/publications/Wind_at_work_FINAL.pdf

Fraunhofer ISE 2012. Levelized Cost of Electricity: Renewable Energies. May 2012.

GWEC (Global Wind Energy Council), 2010. Global Wind 2009 Report. Global Wind Energy Council, Brussels, Belgium, 68 pp.

IEA (International Energy Agency) 2009. World Energy Outlook 2008. International Energy Agency, Paris, France, 569 pp.

IEA (International Energy Agency) 2010, Projected Costs of Generating Electricity – 2010, available from: http://www.iea.org/Textbase/npsum/ElecCost2010SUM.pdf

IPCC 2010. Special Report on Renewable Energy Sources and Climate Change Mitigation.

REN21. 2012. Renewables 2013 Global Status Report (Paris: REN21 Secretariat).

ZF, 2010. ZF Signs Contract to Supply Wind Turbine Gear Boxes, available from: http://www.zf.com/corporate/en/press/press_releases/press_release.jsp?newsId=21752488

Wiser, R. & M. Bolinger 2010. 2009 Wind Technologies Market Report. US Department of Energy, Washington, DC, USA

Author affiliation:

Energy research Centre of the Netherlands (ECN), Policy Studies