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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'How does the wind blow behind wind turbines and in wind farms?' taking place on Tuesday, 11 March 2014 at 16:30-18:00. The meet-the-authors will take place in the poster area.

Dimitri Foussekis C.R.E.S., Greece
Co-authors:
Dimitri Foussekis (1) F P
(1) C.R.E.S., Pikermi, Greece

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Dimitri FOUSSEKIS has been working at CRES, the Greek Centre for Renewable Energy Sources for more than 20 years and is currently a Senior Research Engineer. His primary research interests lie in the fields of i) wind potential studies (member of MEASNET’s Expert Group for Site Assessment), ii) LIDAR and SODAR performance evaluation in complex terrains and iii) design and implementation of remotely controlled measurement systems for mission critical applications (wind farm monitoring, load and power performance measurements of wind turbines). He has more than 50 papers, presentations and announcements in scientific journals, conferences and workshops.

Abstract

Lidar measurements of hub-height winds over the North Aegean Sea

Introduction

The Aegean Sea is mainly characterized by numerous islands of complex topography and of various sizes, by deep waters, as well as, by strong winds. The waters depth is, up to now, the main preventing factor against the installation of offshore wind farms. On the other hand, the high wind potential can improve the projects return rates, therefore accurate knowledge of the wind inflow is crucial for the economic feasibility of offshore wind farms.

Approach

A meteorological mast and a lidar operated for four months in Samothraki, a north-Aegean island. This island offers a privileged location (a very long and narrow sort of peninsula) as shown in , simulating in a high degree, an offshore measurement location. The main wind direction of the site is characterized by a free streamline (both for the inflow and the wake) travelling only the open sea. The annual average wind speed of the site slightly exceeds 7.8m/s.
The main quantities investigated are the wind shear and veer. Results are in form of direct comparisons to those of NORSEWInD project (a reference project of an array of lidars and offshore masts, for the wind shear investigation, at fourteen different sites in the North and Baltic Seas [1]).
The established [2] lidar verification procedure was also performed versus the nearby calibrated, mast-mounted cup anemometers. Results show very high correlations for both the prevailed wind directions, of the order of 0.99 and very consistent regression coefficients (slope and offset) maintaining the same values for all the free wind direction sectors
Turbulence intensities levels, as measured by cup anemometers, were found around 10%. Given the opportunity of the 'clear' and flat landscape, comparison results are presented between cup and lidar turbulence intensities.
Flow inclination results confirm the average flow horizontality from practically all sectors .
An attempt to estimate flow stability is performed adopting the same approach as in NORSEWInd project, thus by using ‘profile methods’ for the calculation of the roughness length, the friction velocity and the logarithmic profile correction (psi-m).


Main body of abstract

The paper presents the first hub-height lidar measurements obtained in Greece for the Aegean Sea, in the framework of the development of a strategic environmental planning for offshore wind farms. The chosen site is practically governed by offshore wind conditions (due to the particular topographic conditions) and offers an excellent trade-off for the open-sea flow properties knowledge over financial cost.
The deployed wind lidar was a ZephIR unit from Natural Power of continuous wave type. The duration of the experiment was unfortunately interrupted after fourth months, due to a failure of a laser related component, requiring a system return to the manufacturer. Nevertheless, data collected covered a relatively extended range of temperatures (0-25degC) and of weather conditions (late summer to early winter). The total operational hours were 2899 and the filtered set of data collected were 1812 hours of valid measurements concurrently at all the measured heights (20m, 40m, 80, 120m and 150m).
The wind shear, estimated as the exponent coefficient α of the power law, has an average weighted value of 0.03, taking into account all the wind speeds above 4m/s. This shear value, being in a good agreement with the ones reported in NORSEWInD, contrasts with the value (0.20) used for load calculations and is among the main conclusions of the paper.
The diurnal variation of the wind shear can be considered stable and differs from the one measured in typical greek continental terrains which presents noticeable lower values around noon. An almost negligible effect of the temperature was noticed on the wind shear value, as examined per each wind direction and a narrow wind speed range.
Turbulence intensity measured by the cup anemometer was less than 10% in average with a slightly decreasing trend when moving to higher heights (as obtained by lidar results – ratios of relative values).
A detailed examination of the wind veer did not reveal any noticeable values, as shown in . Wind veer was studied separately for each wind direction sector and for wind speed bins of 1m/s. In fact, it is remarkable that the average values systematically remain below 3deg for a height difference of 130m, permitting the deduction that yaw misalignment issues should not occur at this site.
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Conclusion

The aim of this project was to reveal the vertical wind profile at a specific location which simulates in a high degree offshore wind conditions. A nearby meteorological mast was used to validate LIDAR’s results, through measurement comparisons at the height of the mast’s top anemometer.
Ground based vertically scanning LIDARs are becoming increasingly popular and have emerged as a useful wind resource assessment tool. Their deployment can reduce the costs associated with the installation of very high (>100m) traditional meteorological masts. In offshore conditions, the difference in the economics between sea platforms (permanent or floating) carrying a high meteorological mast and a lidar is even more exaggerated. Moreover, for today’s MW-size wind turbines, LIDARs provide a more representative picture of the wind flow, as they scan areas comparable to their rotor size, in the opposite of cup anemometers measuring at single point(s).
Cup anemometers are the today’s standard measuring devices for wind speed measurements, consequently lidars must prove that they can provide results of similar accuracy, in order to be adopted by the standards as standalone measurement devices. In the present experiment cup/lidar comparisons proved to be excellent with a correlation coefficient close to 0.99.
The most interesting results of this work concern the wind shear and veer of the open-sea inflow, since they are measured for the first time in Greece. Wind veer is practically negligible from 20m to 150m, confirming expectations, as well as, numerical models results. In contrary, a relatively broad range of shear values was recorded but the average value was found to be 0.03, when load calculations are performed with a value of another order (0.20). More similar investigations are needed concerning the wind shear in the marine atmospheric boundary layer, in order to close this gap and further optimize the design of offshore wind turbines.



Learning objectives
This work was the first successful attempt to measure wind profiles in the Aegean Sea, for the large rotors of offshore wind turbines. The obtained results provide i) an insight for the expected wind shear and veer values in offshore conditions and ii) an inter-comparison to those published for the North Sea. They constitute valuable input in the optimization of parameters used in the wind turbines design and load calculations.


References
1. “Offshore Vertical Wind Shear”, Final report on NORSEWInD’s project (task 3.1), August 2012.
2. IEA Wind, Annex15, “Ground-based vertically profiling remote sensing for wind resource assessment”, January 2013