Burton T. (et. al.) Wind energy Handbook

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vane. If possible one anemometer is mounted at the hub height of the proposed

wind turbines with others lower to allow wind shear to be measured. Measure-

ments are made over at least a 6 month period (although clearly the more data

obtained the more conﬁdence there will be in the result) and correlated with the

measurements made concurrently at the meteorological station.

There are a number of difﬁculties of using MCP (Lan dberg and Mortensen, 1993)

including:

• with modern wind turbines, high site meteorological masts are necessary and

these may themselves require planning permission;

• there may not be a suitable meteorological station nearby (within say 50–100 km)

or with a similar exposure and wind climate;

• the data obtained from the meteorological station may not always be of good

quality and may include gaps so that it may be time consuming to ensure that it

is properly correlated with the site data;

• it is based on the assumption that the previous long-term record provides a good

estimate of the wind resource over the lifetime of the wind farm.

Conventional MCP techniques assume that the wind direction distribution at the

site is the same as that of the meteorological station. Recent investigations (Addison

et al., 2000) suggest that this assumption is source of signiﬁcant error and a

correlation technique based on artiﬁcial neural networks is proposed. Using this

approach an improvement in predictive accuracy of 5–12 percent was obtained

although it should be emphasized that these advanced techniques are not common

industrial practice.

9.1.4 Micrositing

The conventional MCP technique is now well established and specially designed

site data loggers, temporary meteorological masts and software programs for data

processing are commercially available. The estimate of the long-term wind speeds

obtained from MCP may then be used together with a wind-farm design package

(e.g., WindFarmer, Windfarm) to investigate the performance of a number of

potential turbine layouts. These sophisticated programs take the wind distribution

data and combine them with topograp hic wind-speed variation and the effect of the

wakes of the other wind turbines to generate the energy yield of any particular

turbine layout. Two models commonly used to calculate the effect of topology and

surface roughness of the site and surro unding area are the Wind Atlas Analysis and

Application Program (WA

P) and MS-Micro/3 (the latest in a series of versions of

the MS3DJH/3R model) (Walmsley, Salmon and Tay lor, 1982, Walmsley, Taylor

and Keith, 1986). Other constraints such as turbine separation, terrain slope, wind

turbine noise and land ownership boundaries may also be applied. Optimization

techniques are then used to optimize the layout of the turbines for maximum

PROJECT DEVELOPMENT 515

energy yield of the site taking into account local wind speeds and wake effects. The

packages also have visualization facilities to generate zones of visual impact, views

of the wind farm either as wire frames (see Section 9.2.3) or as photomontages.

Figure 9.2 shows an energy map for a prospective wind farm site created with wind

farm design software. The areas on the top of the hills show the highest energy

density.

Figure 9.2 Example of Energy Map for a Prospective Wind farm Site Created with Wind

farm Design Software

0-1127 Wm

2

1127-1277 Wm

2

1277-1427 Wm

2

1427-1502 Wm

2

(Courtesy of Garrad Hassan www.garradhassan.co.uk) Based upon the Ordnance Survey

mapping on behalf of the Controller of Her Majesty’s Stationary Ofﬁce # Crown Copyright.

MC 100014737.

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WIND TURBINE INSTALLATIONS AND WIND FARMS

Publisher's Note:

Permission to 

reproduce this image 

online was not granted 

by the copyright holder. 

Readers are kindly 

requested to refer to 

the printed version of 

this article.

9.1.5 Site investigations

At the same time as wind speed data are being collected more detailed investiga-

tions of the proposed site may also be undertaken. These include a careful

assessment of existing land use and how best the wind farm may be integrated

with, for example, agricultural operations. The ground condi tions at the site also

need to be investigated to ensure that the turbine foundatio ns, access roads and

construction areas can be provided at reasonable cost. Local ground conditions may

inﬂuence the position of turbines in order to re duce foundation costs. It may also be

important to undertake a hydrological study to determine whether spring water

supplies are taken from the wind farm site and if the proposed foundations or cable

trenches will cause disrupt ion of the ground-water ﬂow. More detailed investiga-

tions of the site access requirements will include assessment of bend radii, width,

gradient and any weight restrictions on approach roads. Dis cussions are also likely

to continue with the local electricity ut ility concerni ng the connection to the

distribution network and the export of the wind farm power.

The planning application will require the preparation of an Environmental

Statement and the scope of this is generally agreed, in writing , with the civic

authorities dur ing the Project Feasibility Assessment.

9.1.6 Public consultation

Prior to the erection of the site anemometer masts the wind farm developer may

wish to initiate some form of informal public consultation. This is likely to involve

local community organizations, environmental societies and wildlife trusts. It may

also be appropriate to keep local politicians informed. Of course, the erection of

meteorological masts does not necessarily imply that the wind farm will be

constructed but, as they are highly visible struct ures, careful consultation is re-

quired to ensure that unnecessary public concern is allayed.

9.1.7 Preparation and submission of the planning application

Wind farms are recognized as having signiﬁcant environmental impact and it is

usual for an Environmental Statement to be required as a major part of the

application for planning permission. The preparation of the Environmental State-

ment is an expensive and time-consuming unde rtaking and usually requires the

assistance of various specialists. The purpose of a wind farm Environmental

Statement may be summarized:

• to describe the physical characteristics of the wind turbines and their land-use

requirements,

• to establish the environmental character of the proposed site and the surrounding

area,

PROJECT DEVELOPMENT 517

• to predict the environmental im pact of the wind farm,

• to describe measures which will be taken to mitigate any adverse impact, and

• to explain the need for the wind farm and provide details to allow the planning

authority and general public to make a decision on the application.

Topics cove red in the Environmental Statement will typically include the following

(BWEA, 1994).

Policy framework. The application is placed in the co ntext of national and regional

policy.

Site selection. The choice of the particular site that has been selected is justiﬁed.

Designated areas. The potential impact of the wind farm on any designated areas

(e.g., National Parks) is evaluated.

Visual and landscape assessment. This is generally the most important consideration

and is certainly the most open to subjective judgement. Hence it is usual to employ

a professional consultancy to prepare the assessment. The main techniques which

will be used include: zones of visual inﬂ uence (ZVI) to indicate where the wind

farm will be visible from, wireframe analysis which show the location of the

turbines from parti cular views, and photomontage production which are computer

generated images overlaid on a photograph of the site.

Noise assessment. After visual impact, noise is likely to be the next most important

topic. Hence predications of the sound produced by the proposed development are

required with special attention being paid to the nearest dwellings in each direction.

It may be necessary to establish the background noise at the dwellings by a series of

measurements so that realistic assessments can be made after the wind farm is in

operation.

Ecological assessment. The impact of the wind farm, includ ing its construction, on the

local ﬂora and fauna needs to be considered. This may well require site surveys at a

particular season of the year.

Archaeological and historical assessment. This is an extension of the investigation

undertaken during the site selection.

Hydrological assessment. Depending on the site, it may be nece ssary to evaluate the

impact of the project on water courses and supplies.

Interference with telecom munication systems. Alth ough wind turbines do cause some

interference with television transmis sion this is normally only a local effect and can

usually be remedied at mod est cost. Any interference with major point-to-point

communication facilities (e.g., microwave systems) or airﬁeld radar is likely to be a

much more signiﬁcant issue.

Aircraft safety. The proximity to airﬁelds or military training areas needs to be

considered carefully.

518 WIND TURBINE INSTALLATIONS AND WIND FARMS

Safety. An assessment is required of the safety of the site including the structural

integrity of the turbines. Particular local issues may include highway safety and

shadow ﬂicker.

Trafﬁc management and construction. The Environmental Statement a ddresses all

phases of the project and so both the access tracks and the increase in v ehicle

movements on the public roads need to be considered.

Electrical connection. There may be signiﬁcant environmental impact associated with

the electrical connection (e.g., the construction of a substation and new circuit).

Although this may be dealt with formally as a separate planning application it still

needs to be considered, particularly as any requirement to place underground any

long, high-voltage circuits will be very expensive.

Economic effects on the local economy, global environmental beneﬁts. It is common to

emphasize the ben eﬁt that the wind farm will bring both to the local economy and

to reduction in gaseous emissio ns.

Decommissioning. The assessment should also include proposals for the decommis-

sioning of the wind farm and the removal of the turbines at the end of the project.

Decommissioning measures are likely to involve the removal of all equipment

which is above ground and re storation of the surface of all areas.

Mitigating measures. It is obvious that the wind farm will have an impact on the

local environment and so this section details the steps that are proposed to mitigate

and adverse effects. This is likely to emphasize the attempts that have been made to

minimize visual intrusion and control noise.

Non-technical summary. Finally a non-technical summary is required and this may

be widely distributed to local residents.

9.2 Visual and Landscape Assessment

Modern wind turbines are large structures, sometimes over 100 m to the blade tip,

and must be sited in exposed locations of high mean wind speed to operate

effectively. The individual turbines must be spaced at at least 3–5 rotor diameters

and hence large wind farms are extensive. There is generally a trade-off between

energy yield and visibility of wind turbines and so visual and landscape assessment

is a key aspect of the Environmental Statement required for any wind-farm de-

velopment. The assessment of landscape and visual effects of a wind farm is

normally undertaken by a professional consultant specializing in such work, who

will attempt to quantify the impacts, but it is generally recognized that a degree of

subjective inte rpretation is required for such work.

The assessment process is, of course, iterative and will inﬂuence the design and

layout of the wind farm. However, it may be divided roughly into a number of

areas:

• landscape character assessment (including landscape policy and designation),

VISUAL AND LANDSCAPE ASSESSMENT 519

• design and mitigation,

• assessment of impa cts (including visibility and viewpoint analysis),

• shadow ﬂicker.

The landscapes within which wind farms are constructed vary widely and the

turbine layout is chosen to take account of the landscape character as well as wind

conditions. Thus in Figure 9.3 the tu rbines are located along the ﬁeld boun daries

while linear arrangements are shown in Figures 9.4 to 9.6 either along ridges or the

coast line.

It is also essential to consider the sociological aspects of the development. An

individual’s perception of a wind-farm development will be determined not only by

the physical parameters (e.g., wind turbine size, number, colour, etc.) but also by

his/her opinio n of wind energy as part of the energy supply (Taylor and Rand, 1991).

9.2.1 Landscape character assessment

The fundamental step in minimizing the visual impact of a wind farm is to identify

an appropriate site and ensure that the proposed development is in harmony with

the location. Many exposed upland areas are likely to be of high amenity and have

been designated as areas of signiﬁcant landscape value or even as National Parks.

Figure 9.3 Windfarm of Six 660 kW Turbines in Flat Terrain (Reproduced by permission of

Cumbria Wind Farms Ltd, Paul Carter)

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WIND TURBINE INSTALLATIONS AND WIND FARMS

Figure 9.4 Windfarm of 600 kW, 43 m diameter Wind Turbines, Tarifa, Spain (Reproduced

by permission of NEG MICON; www.neg-micon.dk)

Figure 9.5 Windfarm of 7 600 kW Wind Turbines Located Along Coast (Reproduced by

permission of Powergen Renewables/Wind Prospect Ltd www.windprospect.com)

VISUAL AND LANDSCAPE ASSESSMENT 521

Development of wind farms within a National Park is likely to be extremely

difﬁcult and it is also important to recognize that any view of the site from inside a

specially designated area will be considered to be important by the planning

authorities.

Once a potential site has been identiﬁed, a baseline study is undertaken to es-

tablish the landscape and visual context of the area. This will include a description

of the existing landscape based on ﬁeldwork observations and up-to-date maps. It

is necessary to describe the landform, landcover and land-use pattern s. The land-

scape character will be described and its quality evaluated together with the

location of potentially sensitive viewpoints.

Stanton (1994) suggests some characteristics of the wind farm visual appearance

which are considered to be desirable. The development should be sim ple, logical

and avoiding visual confusion. Although one landscape type is no more appro-

priate for a wind farm development than another this author considers that their

suitability for different types of development varies greatly. For example, ﬂat

agricultural land is considered suitable for either a small number of wind turbines

or large wind farms of similar regularly spaced machines while coastal areas are

considered appropriate for large numbers of wind turbines but the development

Figure 9.6 Cameron Ridge in California USA (Reproduced by permission of Renewable

Energy Systems Ltd www.res-ltd.com)

522

WIND TURBINE INSTALLATIONS AND WIND FARMS

should relate to the linear quality of the coastal space. Such view s are, of course,

open to debate but they do illustrate that it is essential to consider the character of

the landscape before a wind farm development is proposed.

Cumulative effects are also considered to be importan t and the impact of more

than one wind farm being visible in an area is a major consideration. Local features

such as buildings and hedges, which restrict views, may mean that a landscape has

the capability to accommodate additional wind turbines but detailed investigation

is required to establish this.

9.2.2 Design and mitigation

Wind turbine designers have recognized for some years that the overall form of the

structure of a large wind turbine has to be pleasing and this aspect of industrial

design is considered early in the development of a new machine. It is now generally

recognized that for aesthetic reasons three-bladed turbines are preferred (see

Section 6.5.6). Two-bladed rotors sometimes give the illusion of varying speed of

rotation, which can be disconcerting. In addition, for a similar swept area, a two-

bladed rotor will operate faster than one with three blades. There is considerable

evidence (Taylor and Rand, 1991, Gipe, 1995) that a slower speed of rotation is more

relaxing to the eye. This effect works to the advantage of the large modern wind

turbines, which operate typically at 30–35 r.p.m. There is some speculation that, in

the future, very large wind turbines designed speciﬁcally for remote offshore

installation may revert to two-bladed rotors to realize the engineering ben eﬁts of

this arrangement.

There would appear to be a difference between European and North American

practice concerning the acceptability of lattice steel towers. There are clearly

engineering beneﬁts associated with lattice towers particularly in terms of the costs

of the foundations (see Section 7.10.4), although the upper, tapered section of the

tower must be such as to provide adequate clearance for the blades. From a distance

and in certain light conditions lattice towers can disappear leaving only the rotors

visible and this effect is generally thought to be undesirable. In Europe, solid

tubular towers are generally preferred. Tower heights can vary greatly in response

to both wind conditions and planning constraints. In Northern Germany, very high

towers (.60 m) are used in order to maximize energy generation. Conversely, in

western parts of the UK, which have high mean wind speeds, tower heights are

reduced in order to minimize the area from which the wind farm is visible.

The appearance of the turbines can be inﬂuenced to some extent by their colour.

In the upland sites of the UK, the turbines will generally be seen against the sky and

so an off-white or mid-grey tone is gene rally considered to be appropriate. Where

the wind farm is seen against other backgro unds a colour to blend in with the

ground conditions may be more suitable. There is general agreement that the outer

gel-coat of the blades should be a matt or semi-matt ﬁnish to minimize reﬂections.

In the USA there have been a number of wind farms that used a variety of wind

turbines sometimes of quite different design (i.e., with differing numbers of blades,

direction of rotation, tower types etc). It is unlikely that such developments would

be acceptable in highly populated parts of Europe.

VISUAL AND LANDSCAPE ASSESSMENT 523

The layout and design of the wind farm is important in determining if planning

consent is to be received. The preliminary wind-farm layout will be determined by

engineering considerations (e.g., local wind speeds, turbine separation, noise, geo-

technical considerations, etc.) and will then be modiﬁed to take account of visual

and landscape impacts. On open, ﬂat land the turbines are often arranged in a

regular layout in order to provide a simple and logical visual image with maximum

power output. Alternatively on hill sites, or where there are signiﬁcant hedges or

ﬁeld boundaries, the turbine layout is arranged around these features. When

viewed from up to 1–2 km, wind turbines are considered to dominate the ﬁeld of

vision and so it is desirable that views within this distance can be minimized (e.g.,

by moving turbines to take make use of local screening features or by tree planting).

When turbines are seen one behind another there is an increase in visual confusion

and this ‘stacking’ effect is considered to be undesirable. Some viewpoints are likely

to be particularly important and it may be appropriate to arrange the turbines so

that these views of the wind farm are as clear and uncluttered as possible.

In addition to the wind turbines, there are a number of associated ancill ary

structures required. With smaller wind turbines, the local transformers may be

located adjacent to the towers, often in an enclosure to provide protection against

the weather and vandalism. However, the to wers of many large turbines are wide

enough to accommodate the transformers inside and this obviously reduces visual

clutter on the wind farm. There is often a requirement for a main wind farm

substation and some form of local control building. Engineering considerations

would indicate that this substation should be located in the middle of the wind

farm but, in order to reduce visual impact, it is often located some distance away

where it gives minimal visual intrusion. Within European wind farms all power

collection circuits are underground and it may also be appropriate, if rather

expensive, to use underground cable to make the ﬁnal connection to the local utility

system. Roads are required within the wind farm for construction and it is an

occasional requirement of the planning consent that they are re-vegetated after

commissioning. This of course can lead to very considerable expense if the road has

to be reinstated to allow a large crane to be used for maintenance or repair.

9.2.3 Assessment of impact

A major part of the Environmental Statement is the assessment of visual impact.

Two main techniques are used: (1) visibility analysis using zones of visual impact

(ZVI), and (2) viewpoint analysis using wire frames and photomontages.

Zones of visual impact show those areas of the surrounding country, usually up

to 10–20 km radius, from which a wind turbine, or any part of a wind turbine, in a

wind farm is visible. The ZVI is generated using compute r methods based on a

digital terrain model and shows how the local topology will inﬂuence the visibility

of the wind farm. Usually ZVI techniques ignore local landscape features such as

screening from trees and buildings. Also, weather conditions are not considered

and clear visibility is assumed. Figure 9.7 shows an example of a ZVI generated

using commercially available wind farm design software. The cumulative impact of

a number of wind farms may be calculated in a similar manner.

524 WIND TURBINE INSTALLATIONS AND WIND FARMS