Publishers version - DTU Orbit

1.2.3 Summary of sodars
Most of today commercially available sodars are still build on “pre wind energy era” antenna
design and processing technology, which do not in particular address nor support the high
accuracy demands required within wind energy and resource assessment studies of today. The
consequence is that most – if not all – of the available sodars today still exhibit insufficient
accuracy to be accepted by the wind energy industry and society as an accurate RS tool for
precise and “bankable” wind energy investigations.
Although some improvements seem to have occurred in accuracy since our first 2005 WISE
sodar investigation, it is still not this author’s belief that sodars as they come will be able
to meet the high accuracy demands of the wind energy society in the future unless a major
quantum jump can be demonstrated in their overall performance at high wind speed, neutral
atmospheric stratification, and at present wind turbine hub heights (> 100 m).
At Risø DTU we see two venues for further research along which improved accuracy of
sodarsmayhappen:Oneistoswitchtofullybi-staticpulsedorCWbasedsodarconfigurations,
however cumbersome, and the other is to take advantage of the immense, fast and cheap
embeddable processing power set to our disposal from the information technology industry
today, and apply these for enhanced on-line real time signal processing.
1.3 Part II: RS of wind by light (lidars)
1.3.1 Introduction to lidars
The motivation and demand in the wind energy market for wind lidars are similar to those
of wind sodars. At a continuously increasing rate today wind turbines are being installed on,
offshore, in hilly and forested areas, and even in complex or mountainous terrain. At the same
time, as the turbines gets bigger and more powerful, they also reach higher and higher into
the atmospheric flow, and thereby also into hitherto unknown wind and turbulence regimes
– on as well as offshore.
The industry’s traditionalmethod for performing accredited and traceable measurements of
power performance is to mount a single accurately calibrated cup anemometer at hub height
two to four rotor diameters upwind in front of the turbines on a tall meteorological mast. IEC
61400-12-1 describes the accepted standard for power performance verification (power curve
measurement) and prescribes measurements of power production correlated with wind speed
measurements from a cup anemometer located at hub height in front of the wind turbine 2–4
rotor diameters upstream.
With turbines becoming bigger correspondingly high meteorology masts equipped with
wind speed instrumentation becomes progressing more cumbersome and expensive to install,
especially in mountainous and complex terrain. As wind turbines rotor planes reaches 120 m
in diameter or more it is evident that the incoming wind field over the entire rotor planes is
not measured representatively from a single cup anemometer mounted at hub height.
Accurate measurements of the inflow of today’s huge wind turbines will require multi-point
multi-height wind measurements within the entire rotor plane, to characterize the wind speed
and wind shear over the entire rotor plane. Research activities addressing detailed rotor plane
inflowandwakesisongoingat RisøDTUinconnectionwiththeestablishmentofnewresearch
infrastructure based on wind lidars, see Windscanner.dk and Mikkelsen (2008).
1.3.2 Wind RS methodologies
RS measurement methodologiesforwind energyapplicationsare todaycommerciallyavailable
and encompass various measurement techniques that include sound based sodars, laser based
lidars and satellite borne scatterometry. The application range for wind measurements are
also plentiful, and encompass for example:
1. Wind turbine power performance verification – Establishment of new RS based measure-
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