Even though wind power is now a maturing industry, for developers there are still pitfalls to be avoided if a project is to be successful. Ton van de Wekken sets out the four most important considerations.

Ton van de Wekken, KEMA Nederland BV, Netherlands

Between 1980 and 2000 governmental awareness of wind energy was mainly in Denmark and Germany, where a large number of wind turbines were manufactured and installed. Nowadays, most European governments are well aware of the potential of wind energy.

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Indeed, during the past 20-25 years the implementation of wind projects has changed dramatically. Initially, for instance, stand-alone wind turbines of 100-500 kW were quite common, while the development of wind farms was rare. Since the development of incentives and planning regimes in many European countries over the past five to ten years, most installations are in farms of 25 turbines or more. Currently, the installed power of turbines ranges from 750 kW to 3 MW and farms totalling 50 MW are not uncommon. Furthermore, turbines of 4.5-6 MW are available as prototypes and test specimens but are not yet being commercially exploited. Modern wind turbines have rotor diameters of 55-100 m with hub heights that generally vary from 0.9 -1.25 times the rotor diameter. Most manufacturers offer turbines with two or three rotor diameters corresponding to low, medium, and high or offshore wind conditions, for which they use large, standard or small rotors, respectively.

The costs of onshore wind ranges from €55-100 ($86-156) per MWh, depending on the wind resource. However, for most locations, wind energy is not cost-effective, and incentives are a prerequisite to make a wind farm profitable.

Inevitably, offshore wind farms are more expensive to develop than those onshore – requiring about double the initial investment and double the operational costs – because of the extra costs of construction, transportation to site and interconnection. However, the wind resource and the installed capacity of offshore farms are generally greater, and there is less environmental impact, especially in terms of noise pollution.

The time span from first initiative to final commissioning of a wind farm is subdivided into a development period and a construction period. The duration of the construction period is predictable: about one year for a small-to-medium farm of 15 MW or less, and at 1-2 years for a large wind farm. The duration of the development period is less predictable, especially the time required to obtain planning consent, which, depending on the mandatory procedures and the number and kind of objections raised, may vary from approximately six months to more than five years. The technical life span of a wind turbine is 20 years.

Generally, the development and operation of a wind farm can be subdivided into four phases:

  • Initiation and feasibility (concluded by go/no-go)
  • Pre-building (concluded by go/no-go)
  • Building
  • Operation and maintenance.

Initiation and feasibility

In this phase, the basic parameters of the project are determined, for example the number and type of turbines and the total installed power. Potential sites are assessed and compared, taking into account the wind resource available and the proximity of suitable grid connections. The main issue is the distance to the nearest medium- or high-voltage substation with an adequate feed-in capacity and the cost of installing this interconnection. Construction of this connection may require a lengthy planning cycle and involve a significant cost. The limitations of regional, national and local planning regulations must also be considered at this stage. At the end of this phase the least suitable sites will have been eliminated and a decision made whether to proceed with further study of the remaining sites or to abandon the project.

Wind farms require large sites. Depending on the rotor diameter, the required mutual separation is 300-500 m with a similar separation distance from dwellings and commercial buildings to limit noise nuisance and provide a safety zone. Even for a medium-sized wind farm that comprises, say, five 2 MW machines, a substantial land area is required. Generally speaking, potential wind farm sites are preferably open areas of flat land or high or hilly areas because buildings, trees and other obstacles lead to a lowering of the wind speed. Obviously, sites should have high and recurrent wind resources.

Having selected the site, the next step is to assess the local long-term wind climate by reference to existing data or by long-term monitoring. The objective is to eliminate all sites that may be unsuitable – read ‘unprofitable’ – in the long term. If the site-screening process does not identify any prohibitive limitations, the feasibility study may proceed.

The wind resource assessment, and the consequent estimation of the yearly energy yield, is of crucial importance since it determines the project yield. The energy available from the wind is proportional to the cube of the wind speed. Based on local wind-speed data from meterological stations, a local wind atlas of the planned wind farm can be determined. It is necessary to use at least one full year of wind data to take into account seasonal variations.

The European Wind Atlas shows that Scandinavia, the UK, Ireland and the Atlantic coastline of the European continent have the best wind conditions for development. This wind atlas is combined with a contour map of the area to determine the wind speed at a specific site and height. The estimated wind distribution results in a yearly energy yield that represents the gross income of the wind farm.

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Table 1 shows the gross energy yield that corresponds to average wind speeds of 7-8.5 m/s. As an example, the yearly gross energy yield of a 10 MW wind farm, assuming a 7 m/s average wind speed at the given hub height, wind speed distribution accoring to the Weibull distribution function, shape factor 2 and no noise reduction requirements is 27 GWh, equivalent to 2700 full load hours or a utilization factor of 31 per cent.

Planning requirements of local authorities

The wind farm site has to meet planning and regulatory requirements. In most countries, wind turbines may not rotate above roads, railway tracks and waterways, and a minimum clearance from public infrastructure must be observed such as facilities for transport, storage or processing of hazardous goods, and residential, commercial or public buildings.

In northern countries and countries with a continental climate, specific attention has to be paid to the possibility of icing. Ice that develops on rotating rotor blades can be thrown long distances, potentially causing injury and damage, and planning authorities and regulatory bodies may require an additional risk analysis if the site is subject to icing.

There may also be a zoning plan that prohibits wind turbines or limits the maximum height of structures. Under such circumstances, the relevant authorities should be approached to investigate the possibility of obtaining permission at the earliest possible stage.

In most European countries, wind turbines must also be certified according to the relevant national or international safety standards. Manufacturers have to demonstrate conformance by the production of a valid type certificate.

For any proposed wind farm, the following should be considered:

  • Check municipal zoning plan on competing activities and maximum building height
  • Mutual distance between wind turbines of 400 m
  • There are to be no buildings and as few obstacles as possible within 300-500 m

Authorities or concerned parties may request a risk analysis if other activities are to occur within 400-500 m of the turbines.

Pre-building phase

Having established the basic installation parameters, the predicted availability of wind, any planning constraints and connection possibilities, the task during the pre-building phase is to confirm details and draw up agreements so that project finance can be secured. To enable this to happen, a power purchase agreement (PPA) must be negotiated and suppliers of equipment and contractors selected. It is during this stage that any assumptions made during the start-up phase are re-examined and justified to avoid expenditure on a non-viable project.

Based on the wind resource assessment the most promising wind farm locations are studied in more detail. WasP, Wind farmer and WindPRO are well known computer models to calculate energy yield. The modelling takes into account not only wind speed and direction distributions, but also the geography and terrain. For instance, a steep slope will cause higher winds at the hill top. Such details in the modelling ensure that wind turbines are optimally sited. Generally, the layout is optimized for exposure to the prevailing wind direction.

The gross energy yield of a wind farm is determined by the local wind distribution and siting of the turbines. To calculate the net yield, anticipated losses must also be determined, including those associated with wake, grid and availability.

Downstream of the turbine rotor, in the so-called wake, the wind speed is lower than the undisturbed wind speed, resulting in a somewhat reduced performance of other turbines sited in this area, and the mutual distance between the wind turbines has to meet the requirements of the manufacturer. Another, more serious, consequence may be damage to primary structural parts caused by the wake of wind turbines sited upwind. The minimum distance depends on the siting with regard to the prevailing wind direction. For turbines sited perpendicular to the prevailing wind direction, the mutual separation distance has to be at least four times and preferably five times the rotor diameter. It ißs common practice to estimate the wake losses as being in the range of 3-4 per cent of the gross energy yield.

Grid losses are defined as the electrical losses between the wind turbine switchgear and public grid connection. Electrical losses are typically in the range of 2-3 per cent of the gross yield.

The availability of a wind turbine is defined as the percentage of time that the turbine is either in operation or available for operation should wind conditions permit and the grid connection be available. The technical availability of the turbine is 97 per cent or higher, based on data from modern operational wind farms. Assuming 3 per cent wake and grid losses respectively and 97 per cent availability, the gross energy yield of 27 000 MWh becomes a net yield of 24 500 MWh.

Planning procedures and environmental issues

The wind farm must comply with all relevant environmental regulations. This may require a number of studies of, for example, the effects on birds, animals and plant life during the construction and use phases. Key parameters include noise, visual impact and safety, and most planning authorities also demand safety and risk assessment studies.

Wind turbines produce noise, mostly caused by the rotor blades and drive train, and the noise impact of wind turbines on the environment is one of the major planning issues. The distance to nearby residential buildings has to be sufficient to ensure that the noise level at the house front is below the statutory limit. The visual impact of a wind farm is also an important planning consideration. Wind farms require open, often elevated, sites and are consequently highly visible from a distance. Many of the potentially most productive sites are in areas of great natural beauty, where planning regulation can be very restrictive. Shadow flickering on dwellings and offices due to the periodic – about once per second – passage of the rotating blades across the sun can be very annoying for the occupants, although it is not regulated by law.

Grid connection

Each of the turbines of the farm is connected to a local grid, operating at a medium voltage of 10-20 kV to minimize losses. Usually a ring connection is used to provide a degree of redundancy. Since the generators operate at, typically, less than 1 kV, a transformer is required to step the voltage up to the grid voltage, and these may be housed in the nacelle or at the base of the tower. The bulk connection equipment required depends on the operating voltage of the public grid at the connection point. A transformer may be required in addition to switching, metering and protection equipment.

In some locations, there may be no suitable connection point, or the available connection point may have insufficient capacity. In this case, the cost of work to extend or reinforce the grid specifically for the wind farm has to be borne fully by the wind farm developer.

Selection of suppliers

Depending on the working practices of the wind farm developer, the tendering process may be public or restricted to a shortlist of pre-qualified or preferred suppliers. In either case, a tender enquiry document (TED) is required that contains all the relevant information. As a minimum, the TED should include basic information such as the project time plan and planning status, information required from bidders, such as financial, technical and operational information, contractual issues, scope of supply, technical specifications, maintenance and repair conditions, and insurance and warranty agreements.

With regard to the scope of supply, it may be decided that, by sub-contracting, a single contractor is responsible for delivering a turnkey wind farm including the turbines, foundations, access roads, grid and grid connection. Alternatively, the wind farm developer may manage the individual subcontracts. The latter may have cost advantages, but the project developer becomes the main contractor and is responsible for the integration commissioning, either internally or by appointing a contractor for this phase of the work. With turnkey delivery by one main contractor, responsibility is clearer.

In order to avoid any problems on warranties, the contractor should be required to state formally that the delivered wind turbines are fit for purpose for the site as described by the developer.


The building phase includes all activities from commencement of the works up to handover of the operational wind farm to the operator. Once all the initial stages have been concluded and the status reviewed, the manufacturing and build stage can commence.

Wind turbines are large and heavy, so suitable site access for transport is required. On-site storage and on-site assembly work will require space at the base of each tower of approximately 80 m x 50 m. In addition, heavy lifting equipment will be required on site, with a 2 MW turbine requiring a crane of capacity 600 tonnes for hoisting into position tower parts, nacelle and rotor.

Many of the best potential wind farm sites are in areas that have restrictive planning regulations
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For a small- and medium-size wind farm the time between placing a purchase order and shipment from the factory is six to nine months. In the meantime, the civil engineering work, including access roads, turbine foundations and substation, and the electrical infrastrture is built. The time required for rotor assembly and construction of the main structure takes 2-3 working days per wind turbine. Once the turbines are installed, about seven to ten working days are needed to complete the installation work, commission the system and make the connection to the grid.

Consequently, from the time all the material is on site, the construction time for a small- or medium-sized wind farm takes only two to three months.

It is common practice that the contractor assigns a number of so-called ‘hold and witness points’ for the client as part of the quality control process during production and construction. These points allow the wind farm owner to audit the progress and quality of work, including verification that the components conform to the specifications. Hold and witness points are mostly planned immediately following a project milestone and are often linked to staged payments against the contract. They typically include the start of component production, factory acceptance test (FAT) of components ready for shipment, site acceptance test (SAT) of components delivered to site and several inspections during building on site.

Following completion of the building and installation period and before handover of the wind farm, an overall inspection and commissioning of the works is carried out. Commissioning inspections are performed by representatives of the contractor and the final owner, with participation from the local network operator. The commissioning may involve an elaborate testing and monitoring plan, but the main objective is to verify that the system is complete, correctly installed and functioning properly.

Normally a commissioning procedure is formulated in co-operation with all the parties involved. It is quite common that a number of defects are discovered during the first commissioning inspection, resulting in a defect or ‘snag’ list. Actions to clear this list are decided between the parties and, if serious, may result in a second commissioning inspection being required. Approved commissioning and handover is usually related to final project payment.

Operation and maintenance

Starting from the date of handover, the owner is responsible for the daily operation of the wind farm. Also, warranty and maintenance contracts become valid from that date. The technical and economic life span of a wind farm is anticipated as 20 years.

During normal operation, it is not necessary to man the site. SCADA equipment allows remote monitoring via a modem and phone line of the performance and condition of the turbines. The main function of the daily operator is to verify regularly that the wind farm is in the optimum condition and performing according to expectations, and to ensure that maintenance and repairs are carried out in accordance with the contract and within a reasonable time.

Common warranties for the first five years following handover are those on delivered goods, including repairs and modifications, availability of individual wind turbines and wind farm. No warranty on actual performance can be given because the exact wind supply cannot be predicted.

If the availability or performance is below the warranted value, the difference between actual and warranted values has to be settled by the supplier. Some suppliers offer warranties of up to 8–12 years, or at least for a period comparable to the financing period. Insurance cover is required for third party liability, machine breakdown – for example material flaws, lightning strikes, fire, vandalism, actions of maintenance engineer and/or operator – and insurance for business interruption for unproductive days following a breakdown.

Modern wind turbines require a preventive maintenance service twice a year. Work includes inspection and testing of the control and safety devices, repair of small defects and replacement or replenishment of consumables such as gearbox lubrication. The gearbox is the most vulnerable component and therefore the subject of special interest during maintenance. Oil samples are taken at regular intervals and analyzed for signs of degradation. Filters are replaced and gears are inspected for damage.

The number of repairs required varies widely between different wind turbines and wind farms. On average, three to four corrective actions requiring a visit by a service engineer are required for each turbine. The mean downtime per failure is 2-4 days and the causes are equally divided between mechanical and electrical problems.

Although not formally admitted by manufacturers, it is common practice that a major overhaul is carried out on wind turbines after 10-12 years of operation. The overhaul includes cleaning of and repair work to the rotor blades and refurbishment of the drive train, including replacement of bearings and, if necessary, replacement of gearbox parts.

This article was first published in the March/April 2008 issue (p.74-83) of our sister publication Renewable Energy World.