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(Ingenieurwerkstatt Energietechnik). This wind index is based on

the actual energy yields of a large number of wind turbines across

Germany. Current data for 25 regions is published monthly and the

index is subject to continuous adjustment.

For investors and operators, this has the consequence that a

predicted energy yield is probably incorrect – i.e. too high – if an

obsolete index is used in the calculation. Numerous studies in which

the energy yield of an existing wind farm was re-calculated have

shown that the current index also tends to signicantly overrate the

energy yield. Given this, exact measurements are imperative for

energy yield prediction.

Thorough reviews frequently reveal that expert reports clearly

overrate energy yields compared to the available empirical data

on energy yield, and thus upgrade sites’ apparent yield. Overrating

energy yield can generally be attributed to a conict of interest.

Given this, some companies have established internal classication

systems for the reports available to them. These classication systems

categorise such reports according to their trustworthiness.

Meeting concerns over sustainability

Even though most commercial forests are monocultures and total

forest area is increasing annually across Germany, the extent of

deforestation required must always be given consideration within

the scope of forest wind farms. For example, the largest forest wind

farm currently operating in Bavaria is Fasanerie. The project’s ve

2 MW Enercon E-82 machines, each with a rotor diameter of

82 metres, were connected to the grid in early 2011. The turbines

have hub heights of 138 metres. Each machine requires roughly one

hectare of forestry area. On average, an area of between 1350 m

and

2400 m

is required per turbine. This total area comprises the

area needed for the foundation, the crane and turbine assembly.

Generally, the existing network of forestry roads can be used for the

development of the wind-farm site.

Prior consultation with forestry companies helps to prevent

negative impacts on the regional fauna. According to the Bavarian

state forest enterprise in Regensburg, the small clearings caused by

forest wind farms reduce the density of the forest area and have a

positive ecological impact on the forest ecosystem. TÜV SÜD has

proved that energy yields are not enhanced by further deforestation

beyond the areas required for the turbines. Further deforestation

leads to higher wind speeds at ground level, but not at hub height.

Furthermore, the required deforestation may be compensated

for by afforestation in other areas. Bank guarantees for the cost of

the nal dismantling and decommissioning of the wind turbines also

ensure that the original condition will be restored at the end of the

service life. In the case of complete dismantling, both turbines and

foundations will be removed and the ground relled. At a forest wind-

farm such as Fasanerie, the area can then be reforested.

Greater investment security

For protable and reliable wind-farm operation, all stakeholders

need to protect themselves against potential risks by conducting

a comprehensive due diligence review. Following the successful

selection of a site, safe and reliable wind-farm construction must be

ensured in compliance with the laws and regulations on pollution

control, nature and landscape conservation.

In this context, both operators and investors benet from third-

party wind-farm certication. Wind-farm due diligence assesses the

economic framework conditions and calculates wind-farm operation

costs and maximum yields. In addition, it reviews pollution forecasts,

permits, contracts and approvals.

The expansion of onshore capacities in commercial forests offers

major opportunities and may contribute signicantly and cost-

effectively to strengthening decentralised electricity supply. As in the

planning of the Fasanerie wind farm, local administrations, residents,

land owners and authorities should be involved from an early stage.

The project’s success has shown that thorough co-ordination and

consideration of the interests of all stakeholders places wind farm

operations on a protable and environmentally sound footing.

Author Details

Peter Herbert Meier is head of wind assessment at TÜV SÜD Industrie

Service.

e-mail: peter-h.meier@tuev-sued.de

This article is available on-line. To comment on it or forward it to a

colleague, visit: www.RenewableEnergyWorld.com

WIND DEVELOPMENT

Locating wind farms in forest areas was only made economically feasible in

recent years by rapid progress in wind turbine technology

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WIND TECHNOLOGY SUPPLEMENT - MAY 2011

BUILDING FOR THE FUTURE

By Dr Thomas Mestl

FACING THE

CHALLENGES

In its new Technology Outlook to 2020, DNV’s Research and Innovation unit presents its vision for future developments in

wind turbine technology and looks in detail at the grid transmission systems that will deliver that power.

TECHNOLOGY AND TRANSMISSION

In the next 10 years or so, technology trends onshore and offshore are

expected to change signicantly. Capacity onshore is typically in the

range of 2.3–3 MW, constrained by the size (width, height, and length)

that can be transported by truck. It’s unlikely that future onshore

turbines will become signicantly larger, but they will, however, be

smarter, and may be tailored for each specic location.

Today’s offshore turbines are basically slightly modied onshore

versions. Towards 2020, two different trends will emerge in offshore

wind power: turbines will increase in size and they will be installed

further from shore. In 2009, the average distance from shore for

European wind farms was 12.8 km, but the planned UK Dogger Bank

site is 125–195 km offshore.

Advanced control

Optimum control is key to smooth and safe running of a wind turbine

and the latest systems can measure loads in each of the blades, and

are used to smooth out loads in turbulent wind conditions. The same

measurements can be used to calculate fatigue effects and can shut

down the turbine if damage is critical. The data can also allow the

operating strategy to be altered to maximise energy yield.

Operating individual turbines in wind farms with different

operating rules requires smart sensor technology and complex control

algorithms. Improved data transfer capabilities and decision support

systems will enable centrally located centres to optimise operations.

In addition, wind turbines use blades that can be twisted (pitched)

to suit the wind speed and to maintain the desired output. Faster

responses could be obtained if the blade itself were to twist when the

loads on it increase. This can be achieved by designing the turbine

blades with some degree of sweep (like a curved sword).

Another solution is to orient bres in the blade so that they twist

slightly whenever bent. It is also possible to design ‘smart blades’ with

active controls, i.e. blades that change their aerodynamic properties

according to the measured loads.

More sophisticated blade designs are anticipated within the

next decade. The challenge will be to prove that new designs, with

limited eld history, function satisfactorily over their operating life.

Key aspects will be robustness and fatigue strength.

Fixed and oating foundations

At present, offshore turbines are limited to shallow water

(20–30 metres), and most use a single, tubular, monopole foundation.

For deeper waters, various ‘jacket’ structures that comprise several

footings and are similar to (but smaller than) offshore oil and gas

installations, will be developed. Optimised platform types and better

understanding of loads and foundation design, will increase the viable

water depths to about 50 metres.

Towards the end of the next decade, oating platforms will be

used for wind turbines, enabling them to operate in almost unlimited

water depths and where winds are best. Prototypes are being tested

Novel foundations will allow turbines to stand further offshore

WeserWind

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WIND TECHNOLOGY SUPPLEMENT - MAY 2011

and several concepts are on the drawing board. One challenge is the

need for complex dynamic cables to enable connection to the grid.

As oating wind turbines require deep water for connecting/

mating of the nacelle to the support structure, they will require new

installation methods. However, several areas with high wind energy

potential, including the US and Asia, have shallow water close to

shore, and therefore the mating operation will either have to occur at

site, requiring complex and costly offshore operations, or a completely

different installation method must be developed.

One new concept is to transport fully assembled turbines

horizontally, by barge, from the fabrication yard to the offshore

site. Once on site, the barge tilts 90 degrees through a ballasting

operation, to release the turbine in a vertical position. Provided

that challenges related to the up-ending and release of turbines

are resolved, horizontal installation will be commercially available

by 2020.

Direct drive

Most turbines use a gearbox to increase the generator speed, but they

are prone to failure and increase the mass of the turbine. A number of

manufacturers have replaced gearboxes with a single, large-diameter

generator which, combined with a converter, can be connected to

the grid but this is not without its own challenges. Both the cost and

weight of this option currently exceed the more conventional gearbox

designs. However, this approach is very promising, particularly if

permanent magnets become cheaper and more powerful, lighter

materials are introduced, and converters become more versatile.

Direct drive options will become cost-competitive towards 2020, and

are likely to become dominant.

T&D

Within a decade, many countries, especially in the EU, will experience

signicant challenges in managing variable output from wind and

solar plants. Managing the uncertainty of renewables will be a key

issue in the future design and operation of power systems. A long-

term sustainable solution calls for inter-regional transmission highways

(or supergrids), intraday markets, demand response, harmonised grid

codes, and bulk and distributed energy storage.

Meeting EU 2020 renewable energy targets will entail more than

50 GW of wind power in and around the North Sea region. The winds

there are highly correlated, in which moving wind fronts simultaneously

hit large areas, leading to steep ramps of several gigawatt-hours.

Transmission enables wide area balancing through cross-border

power exchange, greater market pools, and subsequent smoothing.

However, transmission projects can take up to 10–15 years to complete

due to long concession times and frequent public opposition.

Electricity markets today are designed for dispatchable generation

under relatively low load uncertainty. Going from the present ‘day-

ahead’ to ‘two-hour ahead’ forecasting could cut uncertainty by 50%,

a market evolution which is ongoing in many countries.

Another important measure is grid code requirements for

variable renewables. Conventional power plants above a certain size

are obliged to provide the full range of ancillary services, including

frequency response, up/down regulation, voltage and reactive

power regulation, and fault ride through. Although these services are

essential for system stability, smaller power plants, such as wind and

solar, are exempt from providing many of them. An increasing amount

of renewable energy will thus challenge the power system.

Many variable-output plants are now obliged to provide voltage

and reactive power regulation. Down-regulation of wind power to

avoid overproduction is also being used. However, up-regulation

of wind or solar will require continuous operation below available

capacity. This measure is not in use today, but might be implemented

in systems with a very high share of wind and solar power by 2020.

As large amounts of renewable energy will be connected to grids

in many parts of the world, a signicant increase in transmission

capacity is essential. Supergrids, dened as wide-area transmission

networks, connecting large geographical areas into a single, unied

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WIND TECHNOLOGY SUPPLEMENT - MAY 2011

system, enable trading of high volumes of electricity over long

distances and the leverage of production variability. In Europe,

supergrids could enable the transmission of offshore wind in the

North Sea, along with solar energy in the Sahara, to load centres in

mainland Europe. In the US, the three non-synchronous areas can

be more closely interconnected to improve security of supply and to

facilitate integration of renewables.

The drivers for such grids are bulk transmission of power from

production sites to load centres, large-scale integration of variable

renewables, and lower use of peak power plants. Supergrids will

use technologies such as high voltage direct current (HVDC), ultra

high voltage alternating current (UHVAC), high temperature low

sag conductors (HTLS), and exible alternating current transmission

system units (FACTS).

Operating the supergrids

The successful operation of supergrids will involve stakeholders

from different states and countries to a substantially greater extent

than at present. A single operator should be responsible and will

ensure increased utilisation of generation assets on, for example,

a US or European level. This can be achieved by interconnections

enabling levelling of loads due to different consumption patterns and

time zones. However, an overlaying supergrid introduces potential

dangers in terms of single outages that could affect power systems on

an unprecedented scale. Therefore, this calls for the development of

robust system controls, including the increased use of load-shedding,

real-time monitoring and self-restoration.

By 2020, large offshore wind farm projects may be sited hundreds

of kilometres from shore and produce power in the range of several

gigawatts. Transmitting the output back to shore will require HVDC.

The challenges of developing offshore HVDC grids towards 2020 are

substantial, and related to AC/DC converter technology, development

of meshed DC networks and DC circuit breakers, offshore substations,

and subsea developments. Interconnection of countries with different

regulatory regimes will also be a major obstacle to overcome.

In order to accommodate the EU 2020 renewable target, up to

40 GW of offshore wind power could be installed in the North Sea,

requiring grid investments in the order of €11–28 billion.

The US Department of Energy estimates an offshore wind

potential for the United States of 54 GW within 2030. In the US, the

Atlantic Wind Connection project is one of the rst steps towards

interconnecting multiple offshore wind power plants using an offshore

transmission grid.

In addition to technological challenges, regulatory issues such as

operation of the offshore grid, cost coverage, and market coupling

are major obstacles for interregional offshore grid developments.

Converters are used in HVDC systems, either to rectify or to invert

the current. Voltage source converters (VSC) have been commercially

available since 1997, but suffer twice the energy losses and carry a

fth of the capacity of traditional line commutated converters (LCC).

On the other hand, their compact design means that VSCs are feasible

for offshore platforms. Unlike LCCs, VSCs can be connected to weak

or passive AC networks (low short circuit capacity) such as wind power

plants and offshore oil & gas installations (loads). In addition, VSCs

provide voltage control and black start capabilities.

VSC technology will pave the way for multi-terminal DC networks,

enabling interconnection of wind parks to shore and trading

links between countries. Within 2020, VSC energy losses will be

comparable with those of LCCs (about 0.5 % at both terminals).

Two-terminal (point-to-point) HVDC connections have been

installed in many parts of the world, connecting asynchronous

systems – systems with different frequency levels – and being used

for bulk power transmission. An integrated offshore grid will require

further development of multi-terminal HVDC (MTDC) technology.

MTDC will reduce the necessary number of converter stations,

and therefore platform space offshore, and reduce subsequent

energy losses. However, a MTDC system is very sensitive to DC faults;

without a DC circuit breaker, the entire MTDC system would be shut

down to clear a fault. Towards 2020, some smaller offshore MTDC

networks, without DC breakers, will arise on a national level. Inter-

regional offshore MTDC networks will not emerge until after 2020 due

to the lack of inter-regional frameworks and long lead times.

In order to provide a comparable level of redundancy and

reliability to today’s AC networks, MTDC networks will require DC

breakers capable of clearing DC faults within milliseconds. The

intrinsic nature of AC results in fairly simple circuit breakers, breaking

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the current when it is close to zero. In order to be able to clear a DC

fault, a DC circuit breaker must be able to break full power as there is

no natural zero current crossing. DC breakers for HVDC are currently

not commercially available. DC breaker prototypes for 2000A DC

currents and 500 kV DC voltage have been successfully tested on

an existing MTDC scheme. Solid-state, hybrid circuit breakers and

forced commutation are possible solutions for DC breakers. For the

DC breakers to add value, stringent requirements demonstrating high

reliability of the breaker itself must be met.

Exploration of deep and ultra-deep water, oil, and gas elds, as

well as government requirements for lower emissions through power

supply from shore, will result in more electrical equipment being

placed subsea. Voltage levels will typically increase from a couple

of kilovolts to tens of kilovolts, and power capacities in the range

of tens of megawatts. A major advantage of subsea power supply is

reduction or elimination of topside equipment.

When locating land based equipment on seabed, one solution

is air or gas lled modules with atmospheric pressure bringing

along weight challenges, while another is modules lled with uid

challenging electric insulation and power electronics.

Towards 2020, numerous substations, of increasing complexity

and size, will be located offshore and will contain large AC/DC

converters, DC and AC bus bars, and high voltage equipment,

including transformers. Today, AC/DC converters have been placed

on offshore oil and gas platforms, and some have been constructed

for offshore wind.

However, with existing capacity not exceeding 400 MW, they

are far below the capacity necessary for large-scale wind integration

(1–2 GW). Scaling up from today’s rather simple, seabed-xed

constructions, to large structures, both oating and seabed-xed,

with 2 GW capacity requires further reductions in equipment size,

while costs must also be kept at a reasonable level.

Author Details

Dr Thomas Mestl is a project manager at DNV. The DNV Research and

Innovation unit is headed by Elisabeth Harstad.

e-mail: thomas.mestl@dnv.com and elisabeth.harstad@dnv.com

This article is available on-line. To comment on it or forward it to a

colleague, visit: www.RenewableEnergyWorld.com

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WIND TECHNOLOGY SUPPLEMENT - MAY 2011

TESTING WIND

By Andrew Williams

EUROPEAN TURBINE

TESTING ADVANCES

Manufacturers can now experiment with new designs and certify their existing products at a new testing ground for a dozen

wind turbines in the Netherlands.

EUROPE’S LARGEST TEST SITE OPENS

In February, a new wind turbine prototype testing and certication

site, the largest of its kind in Europe, opened its doors for business.

Located on the leafy campus of Wageningen University and Research

Centre in the Netherlands, Test Site Lelystad will test the upcoming

generation of wind turbines. The purpose-built 3000 acre (1200 ha)

site can accommodate 12 multi-megawatt turbines.

Day-to-day management is contracted out to Dutch company

Ecofys, which created Ecofys Wind Turbine Testing Services (WTTS)

for the purpose. Lelystad has already entered into agreements with

several companies eager to make use of its services. So eager, in fact,

that the site is already fully booked for the next seven years.

The rst turbine was scheduled to be operational by the end of

March 2011, with a further seven turbines expected to follow during

the course of the year. ‘The last four turbines will be erected in 2012,’

says Michiel Müller, site manager and managing director of Ecofys

WTTS. ‘In addition several meteorological masts will be erected.’

The rst operational turbine to be tested will come from locally

based STX Windpower, followed by a further batch of turbines from

Lagerwey, EWT, Suzlon and Leitwind. Other turbines from Eneco,

Zeshoek and Wageningen UR are expected to be installed and used

for long-term programmes, mainly focused on O&M strategies.

Lelystad has 12 positions. Six are for machines up to a maximum

200-metre tip height, while six can take a tip height of 150 metres.

The installed capacity can total up to 30 MW.

Lelystad offers the full range of measurement services required

by the IEC for the certication of wind turbine prototypes, including

wind measurements, site calibration, power performance, power

quality, mechanical load and sound emissions testing and grid

code certication. The results of each measurement exercise are

subsequently veried by one of the main certication bodies.

Testing and certication

For manufacturers, the site offers comprehensive turbine

certication and accreditation services. Earlier this year, Ecofys

WTTS nalised a collaboration agreement with the German

measurement and testing institute Windtest Grevenbroich,

enabling it to provide the full portfolio of accredited measurement

services required for IEC and MEASNET compliance certication.

Before making use of the Lelystad facility, manufacturers must

sign a seven-year contract with both Ecofys WTTS and Windtest

Grevenbroich and agree to subject their technology to an extensive

testing and measurement programme. Companies may then renew the

contract and agree to a further testing and measurement programme,

or leave the site, allowing other rms to be contracted. According to

Müller, while some companies focus on type certication – with two

or more prototypes being measured – others are interested in more

extensive research programmes that focus, for example, on improving

blade performance or investigating generator modications.

‘In practice, the manufacturers dene their own measurement and

test programme. Specic measurement contracts are signed between

the turbine manufacturer, Ecofys WTTS and Windtest Grevenbroich

GmbH,’ explains Müller. ‘According to this contract, met mast(s) are

erected, measurement equipment installed and campaigns executed.’

Test Site Lelystad is one of the results of a ve-year-old initiative

between Wageningen UR and Ecofys’ parent company Eneco to

develop renewable energy innovations on the grounds of Wageningen

UR. Eneco subsequently asked Ecofys to take over the test site and

in October 2010 the relevant contract was signed, paving the way for

construction to begin on the rst turbine and access roads.

For Müller, sites such as Lelystad are very important in driving

wind energy development further. To meet increasingly ambitious

targets, turbine technology must undergo continual improvement

and innovation, he says. ‘I think a test site like this is essential for the

development of the wind energy sector and consequently for working

towards a fully sustainable energy supply,’ he concludes.

Author Details

Andrew Williams in a freelance correspondent.

e-mail: rew@pennwell.com

This article is available on-line. To comment on it or forward it to a

colleague, visit: www.RenewableEnergyWorld.com

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DESIGN LIMITS

By Jos Beurskens

DESIGN LIMITS

AND SOLUTIONS

The UpWind research project has explored the potential design limits for wind turbines, concluding that machines as large

as 20 MW are feasible with the development of a radical new design philosophy. Here we present an extract of the report.

REACHING THE 20 MW TURBINE

Signicant additional research efforts in wind energy are needed if

the European Commission’s goals for wind power are to be achieved.

It will mean delivering EWEA’s ‘high’ scenario: 265 GW of wind power

capacity, including 55 GW offshore, by 2020; 400 GW, of which

150 GW is to come from offshore, by 2030; and 600 GW, with

350 GW offshore, by 2050, meeting 50% of the EU‘s electricity

demand. Going offshore implies not only new technologies but also

an upscaling of wind turbine dimensions and wind farm capacities as

well as new electrical infrastructure. Underlying all research activities

is a focus on reducing the cost of energy.

The industry is taking two parallel pathways towards cost

reduction: rst, incremental innovation, looking at cost reductions

through economies of scale resulting from increased market

volumes of mainstream products, with a continuous improvement

of manufacturing and installation methods and products; secondly,

breakthrough innovation, focused on creating innovative products,

including signicantly upscaled dedicated offshore turbines, to be

considered as new products.

Funded under the European Union’s Sixth Framework Programme

(FP6), the UpWind project has been exploring both pathways. The

UpWind project initiators realised that wind technology disciplines

were rather fragmented, in that no integrated veried design

methods were available; that essential knowledge was still missing

in high priority areas, for example in external loads; that measuring

equipment was still not accurate or fast enough; and that external

factors were not taken into consideration in minimising cost of energy

(grid connection, foundations, wind farm interaction and so on).

Optimum technology?

Rather than pursue one single ‘optimum’ technology, UpWind

explored various high-potential solutions and integrated them with a

view to the potential reduction of cost of energy. An optimised wind

turbine is the outcome of a complex function combining requirements

in terms of efciency of electricity production, reliability, access,

transport and storage, installation, visibility, support to the electricity

network, noise emission, cost, and so forth. UpWind‘s focus was the

wind turbine as the essential component of a wind power plant. Thus

external conditions were only investigated if the results were needed

to optimise the turbine conguration.

In search of the 20 MW wind turbine

UpWind established that ‘the same, but bigger’ would simply

not work. Starting with a reference 5 MW wind turbine it initially

extrapolated this reference design to upscale it to 10 MW, and then

on to 20 MW.

Testing blade lightning protection – entirely new testing concepts will be

required for a 120 metre blade LM Wind Power

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This virtual 20 MW design was unanimously assessed as almost

impossible to manufacture, and uneconomic. It would weigh

880 tonnes on top of a tower making it impossible to store today at

a standard dockside, or to install offshore with the current installation

vessels and cranes.

The support structures able to carry such mass, placed at

153 metres height, are not possible to mass manufacture today.

The blade length would exceed 120 metres, making it the world‘s

largest ever manufactured composite element, which cannot be

produced as a single piece with today‘s technologies. The blade

wall thickness would also exceed 30 cm, which puts constraints

on the heating of inner material core during the manufacturing

process, and the blade length would also require new types of

bres in order to resist the loads.

UpWind’s ‘lighthouse’ approach to methodology

This 20 MW concept was drawn up to provide values and behaviour

that could be a starting point for optimisation. UpWind then

developed innovations to enable this basic design to be signicantly

improved, and therefore enable development of a potentially

economically sound design.

A means of doing this was adopting the lighthouse – a virtual

concept design of a wind turbine in which promising innovations,

either mature or embryonic, are incorporated. The lighthouse is

not a pre-design of a wind turbine that will actually be realised, but

a concept from which ideas can be drawn by industry for product

development. One example is a blade made from thermoplastic

materials, incorporating distributed blade control, including a control

system, the input of which is partly fed by LIDARs.

The 20 MW innovative turbine

While the 20 MW design was unfeasible due to issues of weight

and corresponding loads, the future large-scale wind turbine system

drawn up by the UpWind project is smart, reliable, accessible,

efcient and lightweight.

It analysed wind turbine materials, optimising the micro-structure

of blade materials to develop stronger and lighter blades. Fatigue

loading also needed to be reduced so that longer and lighter blades

may be built. And the aerodynamic and aeroelastic qualities of the

models were signicantly improved. Signicant knowledge was

gained on load mitigation and noise modelling.

UpWind demonstrated that advanced blade designs could

alleviate loads by 10%, by using more exible materials and fore-

bending the blades in the second work programme (WP). After

reducing fatigue loads and applying materials with a lower mass to

strength ratio, a third essential step is needed: the application of

distributed aerodynamic blade control, requiring advanced blade

concepts with integrated control features and aerodynamic devices.

Fatigue loads could be reduced by 20%–40% (WP2). Various devices

can be utilised to achieve this, such as trailing edge aps, (continuous)

camber control, synthetic jets, micro tabs, or exible, controllable

blade root coupling. Within UpWind, prototypes of adapting trailing

edges, based on piezo-electrically deformable materials and SMA

(shape memory alloys) were demonstrated (WP1B.3).

However, the control system only works if both hardware and

software are incorporated in the blade design. Thus advanced

modelling and control algorithms need to be developed and applied.

This was investigated in WP1B3.

Further reducing loads requires advanced rotor control strategies

(WP5). These control strategies should be taken into account in the

design of offshore support structures (WP4). The UpWind project

demonstrated that individual pitching of the blades could lower

fatigue loads by 20%–30%. Dual pitch as the rst step towards a

more continuous distributed blade control (pitching the blade in two

sections) could lead to load reductions of 15%. In addition, the future

smart turbine will use advanced features to perform site adaptation of

its controller in order to adapt to local conditions (WP5).

Advanced control strategies are particularly relevant for large

offshore arrays, where UpWind demonstrated that 20% of the power

output can be lost due to wake effects between turbines.

Optimised wind farm layouts were proposed, and innovative

control strategies were developed, for instance lowering the power

output of the rst row (thus making these wind turbines a bit more

transparent for the air ow), facing the undisturbed wind, allowing for

higher overall wind farm efciency (WP8).

Control and maintenance strategies require load sensors, which

were adapted and tested within UpWind. To avoid sensor failures

causing too much loss of energy output, loss of sensor signals was

incorporated into the control strategies (WP5) and a strategy was

developed to reduce the number of sensors. The fatigue loading

on individual wind turbines can be estimated from one heavily

instrumented turbine in a wind farm if the relationship of fatigue

loading between wind turbines inside a wind farm is known. The so-

called Flight Leader Concept 4 was developed in WP7.

FATIGUE LOADINGS ON INDIVIDUAL

TURBINES CAN BE ESTIMATED FROM

ONE HEAVILY INSTRUMENTED TURBINE

Those load sensors can be Bragg sensors – tested and validated

within the project (WP7). UpWind demonstrated the efciency and

reliability of such sensors, and assessed the possibility of including

optical bres within the blade without damaging the structure (WP3).

However, using sensors implies the rotor is only reacting to loading

phenomenon. As a result of system inertia, the load will be partly

absorbed. A step further is to develop preventative load alleviation

strategies by detecting and evaluating the upcoming gust or vortex

before it arrives at the turbine. A nacelle-mounted LIDAR is able to do

this (WP6), and can be used as an input signal for the individual blade

pitching, or in distributed blade control strategies (WP5).

In recent years, UpWind has been a focal point for LIDAR

development, and has considerably helped the market penetration

of LIDAR technologies. Although LIDARs are still considerably more

expensive than SODARs, for instance, their technical performance,

and potential, is substantial. UpWind demonstrated that LIDARs are

sufciently accurate for wind applications (WP1A2). LIDARs can be

used for the power curve estimation of large turbines, for control

DESIGN LIMITS

RENEWABLE

ENERGY

WORLD

RENEWABLE

ENERGY

WORLD

Renewable Energy World - magazine - 5/6-2011

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