measuring offshore winds by kb

8/7/2019 Measuring Offshore Winds by KB

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Indiana University

A Brief Review ofOffshore WindMeasuring TechnologyPREPARED FOR DR. R. J. BARTHELMIE, GEOG-G562,SPRING 2011

Kelly Boyd 4/6/2011


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GEOGG562 GraduateTermPaper KellyBoyd

According to Dr. Rebecca Barthelmie, a leading wind researcher from Indiana University

and the Ris National Laboratory, offshore wind generation has great potential for future growth

via technologies in large turbine power generation.

1

Given the possibilities of harvesting this

renewable energy, significant research is needed for optimal placement of wind turbine sites

across the worlds oceans to harness this power. However, obtaining measurements to determine

where wind turbine farms need to be placed is difficult given the complexities of their locations

over water and the remote distance from offshore.2 This paper will discuss three major forms of

technology, which currently measures offshore, and then address their feasibility in usage,

accuracy in measuring, their lifetime durability in the maritime environment and their cost to

implement/use the technology.

Further Background on Obtaining Offshore Wind-Measurements

In a 2008 Surveys in Geophysics report by Sempreviva, et. al., wind resource

assessments can be obtained by a two phase process, (i.) by an evaluation of wind resources at

the regional scale to locate possible wind farm sites and (ii.) a site specific evaluation of wind

climatology and vertical profiles of the winds locally.3 Both detailed analysis require obtaining

wind data of the location by eitherin situ or by remotely sensed data collection methods. By

todays standards, the best method of gaining regional wind assessment is by satellite remote

sensing observations and then by site-specific data collection directly in the environment to

verify the local wind.3 Once areas have been located by satellite, in situ data must be collected

for at least a year to confirm whether or not the area is feasible for wind turbine placement

(according to the Geophysics report it is recommended that five years of research should be done

to determine whether or not wind turbine placement is viable).3 The following three sections of

this paper will outline these three technologies by first taking a broad analysis in a phase one


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wind assessment by applying active and passive satellite remote sensing technology and then in

phase two by using LiDAR and meteorological masts to measure winds directly (in situ) to

confirm the wind.

First Phase: Gaining Wind Measurements from Active/Passive Satellite Technology

In current practice, if no climatological wind data is available or currently in place then

the next best option is to use satellite technology to gain remote wind measurements.4 According

to a 2007 SAT-WIND Project Report, there are five basic data collection options for collecting

wind data over the ocean from space. These collection methods options are through:

1.

Scatterometers2. Passive microwave3. Passive microwave polarimetric4. Synthetic Aperture Radar (SAR)5. Altimeter.5

1.) Scatterometer satellites use active

radar that transmit pulses from the satellite in

space to the surface of the ocean where the

pulse is reflected back to the satellite for

processing.5 This process takes place in a

fraction of a second. Once the pulse hits the

surface of the ocean it spreads out over an

area from 12.5 to 50 square kilometers to

measure roughness of the sea surface in 500 km to 1,100 km wide swaths per scan (depending on

the satellite).5 This type of technology can measure the direction and speed of the winds on the

surface of the ocean.5 The names of some of the current satellites in use that implements this

Images1A12.5by12.5kmQuickSCATmapshowing

windspeedanddirectionaroundDenmarkonthe9thMarch,2005.5


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technology include: theEuropean Remote Sensing Satellite- ERS, the QuickScatterometer-

QuickSCAT, and the Meteorological Operational Satellite- MetOp.5

2.) Passive microwave satellites ( such as the SSM/I- Special Sensor Microwave Imager

and theAdvanced Microwave Scanning Radiometer- AMSR) use microwaves emitting from the

earth to determine wind speed of air in the atmosphere but not wind direction.5

These type of

satellites measure the scattering of electromagnetic energy from the sun (brightness temperature

measured in Kelvin) by the earths atmosphere ten meters above the surface of the ocean using

electromagnetic measuring bandwidths.5 The swath paths of these satellites are typically 1,400

km.

5

The platform names for some of the satellites in use currently include the F-13, F-14, Aqua,

and Midori-2 satellites.5

As of 2010, the SSM/I sensor satellite has been replaced by the WindSat

Sensor on the Coriolis satellite.5

3.) Another type of passive microwave

measuring technology is the polarimetric satellite

WindSat. This is currently the only type of

satellite available. The WindSat measures the

vertical and horizontal electromagnetic

components of radiation from the sun hitting the

earths ocean surface waves.5 This satellite

measures winds in 840 km swaths at 25 km

resolutions.5 As of 2007, research was currently

being conducted for possibly using the satellite to measure wind directions using this satellite.5

4.) The fourth type of satellite, the Synthetic Aperture Radar (SAR), is currently the most

widely used and preferred satellite technology for measuring weather and offshore winds.6 The

Image2.AnexampleofaSARRADARSATimagetaken

onDec13,2001.Noticethewindspeedanddirectionare

notedbycolorcodedvectors.5


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SAR uses high-resolution radar images of the oceans surface to produce a 30x30 meter spatial

observation (see image 2).6 It can penetrate clouds and works by sending microwave pulses that

measure the intensity, time delay and frequency of the return pulse from the surface of the

ocean.6 Unlike most satellites, the signal from the SAR satellite looks sideways, not directly

down on the earths surface.5

This property allows for a wider or narrower swath paths

depending on the type of resolution needed for measuring surface winds.5 Wind speed and

direction can be measured using the SAR technology by observing and classifying wave height

and frequency.5 There are several different types and models of the SAR satellite. Some satellite

types include: the Canadian Synthetic Aperture Radar Satellite- Radarsat, theEuropean

Environmental Satellite- Envisat, and theAdvanced Land Observing Satellite- ALOS.5

Depending on the satellite and resolution needed for analysis, swaths can vary from 25-405 km

at resolutions between 0.5-250 km2.5

5.) Altimeter satellite sensors work the same way as aircraft altimeters do, by measuring

the distance between the ground and the air/spacecraft. It is very accurate at measuring distances

and is mainly used to provide surface height of ocean and land.5 Altimeters measure wind speed

by the scattering of the footprint illuminated at the surface (the wind scatters particles in the air

to distort the invisible light from the satellite).5 The resolution of these areas (footprints) can vary

from 10 to 25 km2.5 Altimeters can only measure wind speed and not direction.5 Examples of

some of the satellite platforms in use to measure wind speed include: the Jason-1, the ERS-2 RA,

and the GFO-1.5

Data for most of the satellites previously mentioned can be accessed online (for an example

of spatial data set, see images 1&2).7 As of Spring 2011, the wind data can be downloaded from

The Physical Oceanography Distributed Active Archive Center website at


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http://podaac.jpl.nasa.gov, and the Remote Sensing Systems website at

http://www.remss.com/. Using complex statistical analysis, an expert can determine the daily,

monthly, seasonal, and yearly mean wind speeds for an area using several satellite images.3 Best

practices suggest the use of several satellites to examine an area to avoid bias. 5 After careful

analysis, a study region can be selected to begin a more in depth analysis of the area by land

based technology.

Second Phase: Acquiring Wind Measurements from LIDAR Technology

Light detection and ranging (LiDAR) technology is a ground based remote sensing

technique that probes a volume of wind in the atmosphere by emitting electromagnetic radiation

that illuminates natural aerosols and detects the scattering of light that results from this

interaction.8 Simultaneously the LiDAR will receive, the return signal to analyzed and to

determine its mean Doppler shift frequency which is then used to obtain the line-of-sight wind

velocity.8 A typical LiDAR system will continuously rotate an eye-safe laser of energy through a

prism at pre-programmed heights in 3 second increments (see image 3a for a visual).9 ToImage3(a.)AnimageshowingaZephIRwindLiDARsviewpathfromthesensor.(b.)ApictureofLIDARsonanoceanplatform. 9


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determine wind direction LiDARs are equipped with a thermal wind sensor mounted on the top

of a small mast. This piece of equipment on the LiDAR can detect the slight temperature

differences of the air around the probe, which then determines the wind direction.9

A typical wind-measuring LiDAR will have a small mast connected to a large circular

cylinder unit that appears to resemble a large coat rack (see image 3b). LiDAR can measure

several wind variables with very accurate precision. The technology can measure mean wind

speed, the horizontal wind speed component, the vertical wind speed component, turbulence and

wind shear up to 100-200 meters.7 LiDAR can be used on the site of study to confirm desired

results from satellite data. LiDAR technology is predominately used to determine wind turbine

stability variables and to estimate energy production.8

LiDARs operate in many meteorological

conditions, from fog to heavy rainfall.9 In order to get accurate results, the LiDAR must be on

stable ground and should be tested with several cup anemometers at many heights to help

calibrate and to determine whether data collection issues will surface.8,9 Two levels of data are

stored by a LiDAR system; the speed and direction obtained from each three seconds of data at

one height, (representing the turbulence parameter) and then the actual ten minute average of the

good fit measure of the actual wind.9 Other than LiDAR, another ground based technology

available to wind experts is the use of meteorological masts.

Second Phase: Obtaining Wind Measurements Directly from Meteorological Masts

Meteorological masts (met masts/towers) are very large poles sticking out of the ocean

that have several anemometers measuring wind speeds at different height profiles. A typical met

tower has a lattice structure (to allow wind flow through the tower) and is typically 48 meters

tall.10 Anemometers are mounted on booms that are typically two meters long at various heights,

along the tower with one at the very top of the mast.9 Wind data is collected directly from the


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use very little power compared to a LiDAR.9 Finally, met masts can uplink data to satellites,

whereas LiDARs require data download to a flashdrive or nearby computer storage units.9

LiDARs have problems of their own when collecting offshore wind data. Some of the

disadvantages in collecting data include:

Measured wind heights are limited- that area measured by the LiDAR is limited dueto its operation. Since the LiDAR measures wind by light the error increases withheight from the LiDAR. 9 The equipment can typically only measure wind accuratelyat 150 meters and below. Over 300 meters the wind measurements have an unusuallylarge amount of error.9

Cloud Cover - can disrupt non-uniformed aerosols in the atmosphere when cloudcorrection algorithms are not instituted. Since clouds sometimes move faster than

aerosols, the backscatter of light can indicate irregularities in wind speedmeasurement.9

Power- as mentioned earlier, LiDARs require a lot of power. This fact can be achallenge when measuring winds remotely offshore away from power sources.9

LiDAR measuring equipment has many more advantages then disadvantages when

measuring offshore winds. For one, LiDARs are cheaper than met masts. According to the

ZephIR website, a typical LiDAR can cost anywhere between $250,000 and $500,000 dollars,

where met towers can cost well over a $1 million dollars.9 LiDARs require no other equipment

other than the unit itself. LiDARs can measure winds higher in the atmosphere than met mast.

They can also measure turbulence.9

LiDARs are as dependable if not better at recording wind than anemometers (see graphs

1 to 3 on the next page). The graphs show the anemometer measurements from three-met mast

compared to LiDAR measurements taken offshore from Denmark. With high R values these

LiDAR measurements are highly precise. Still LiDARs rely heavily on anemometers to make

sure measurements are accurate.9 LiDARs will never replace anemometer measurements.


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Feasibility, Accuracy, Durability and Cost of Satellites

Since governments and weather agencies around the world use these certain type of

satellites for synoptic scale forecasting, a similar analysis can be conducted on ocean surfaces to

measure wave height to determine wind speed and direction. Most government research satellites

allow for free data use by the public, for this fact the feasibility and cost of using satellite data to

measure wind variables is cheap and plausible for wind energy research.5

After careful calibration, most satellites are accurate in measuring wind speed and

direction. Scatterometers have a nominal accuracy of root mean square error (rmse) of 2 ms-1

within 3-20 ms-1 for instantaneous wind speeds and wind direction accuracies within 20 from

actual wind direction.5 The WindSat passive microwave satellite can measure instantaneous wind

from 3-30 ms-1 with a rmse of 2 ms-1.5 SAR satellites are the most accurate at measuring wind

speed and can have a rmse of 1.1 ms-1.5 The durability of satellites in space can vary depending

on their engineering, their solar activity exposure, and their usage at measuring phenomena as

Charts13:Depictthecorrelationbetweenanemometermeanwindspeedsand

LiDARmeanwindspeeds.Allarehighlycorrelated. 9


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well as other unpredictable anomalies (space dust, radiation, etc).13 Some disadvantages of

using satellite technology over land-based technology include:

Outside Meteorological Interference- The equipment is sensitive to rainfall. Theinterference of rain hitting the ocean surface can cause noise when measuring winds.The accuracy of the measurements can be skewed.14

Temporal Issues- Sometimes it takes satellites 6 plus hours to make another sweep of thearea. Others may only pass over an area 35 to 60 times a month. So instantaneous andconstant wind speeds cannot be measured for long periods of time.12

Selection bias- Preferential treatment for high wind data sets can be selected over lowwind speed data sets or the use of just one satellite to measure winds can drasticallyaffect results.15

Observation of offshore winds from satellites are still being refined and developed, but their

dependability at measuring offshore wind speeds is vital to determine offshore wind farm areas.

Offshore Wind Measuring Technology as a Whole

The future of these technologies will only get better with time, yet they are already helpful in

their development of offshore wind farm placement. Satellites can help determine broad study

areas on the planet, while met mast with anemometers and LiDARs measure local winds. All are

important in their function. The relatively new remote sensing technology of satellites and

LiDAR have a bright future ahead of them and with further research and development their

measuring capabilities will only get better.

In concluding, ocean wind measurements are nothing new to society. It has been taking

place for hundreds of years now, but only now with the advent of technology the precision and

accuracy of offshore wind measurement are just now beginning. These three sophisticated

methods for measuring offshore winds are all important to the wind energy industry. These

measurements help determine the industries future and in return their needs help drive innovation

and accuracy to help measure offshore winds.


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1Barthelmie, R. J. (2007). Wind Energy. Status Trends. The Geography Compass 1(3):275-301.

2

Barthelmie, R.J. (1993). Prospects for offshore wind energy: The state of the art and futureopportunities. British Wind Association, Department of Trade and Industry and WindEngineering Seminar.

3 Sempreviva A.M., Barthelmie R.J., and Pryor S.C. (2008). Offshore wind resourceassessment. Surveys in Geophysics 29 471-497.

4 Christiansen MB, Koch W, Horstmann J, Hasager CB, Nielsen M (2006). Wind resourceassessment from C-band SAR. Remote Sens Environ 105:6881.

5Hasager CB, Astrup P, Nielsen M, Christiansen MB, Badger J, Nielsen P, Srensen PB,

Barthelmie RJ, Pryor SC, Bergstroem H (2007). SAT-WIND project the Final report. Ris-R-1586(EN). Ris National Laboratory, Technical University of Denmark, Roskilde, Denmark, 131pp. http://www.risoe.dk/rispubl/reports/ris-r-1586.pdf

6 Maini, Anil Kumar., and Varsha Agrawal. (2011). Satellite technology: principles andapplications. Chichester, West Sussex, U.K.: Wiley.

7Geotimes.orgWebsite(2011).WindSAR.ImageretrievedonMarch15,2011.

http://www.geotimes.org/nov03/Wind_SAR.html

8Jaynes,D.,Jacquemin,J.,(2009).ExploringLIDARRemoteSensingTechnologyfor

OffshoreWindResourceMonitoringApplications.PublishedbyGarradHassan.

http://www.leosphere.com/file/jaynes_jacquemin_awea2009_offhore_poster_final.pdf

9A.Pea,C.B.Hasager,I.Antoniou,M.Courtney,S.E.GryningandT.Mikkelsen.(2008).

OffshoreWindProfilingUsingLiDARMeasurements.WindEnergyResearch10BarthelmieR.J.,HansenO.,EnevoldsenK.,HojstrupJ.,FrandsenS.,PryorS.C.,LarsenS.,MottaM.,andSanderhoffP.(2005).Tenyearsofmeteorologicalmeasurementsfor

offshorewindfarms.JournalofSolarEnergyEngineering127170176.

11TheEuropeanWindEnergyAssociation(2011).WindEnergyTheFacts:Measurement

offshore.VisitedonApril2,2011.http://www.windenergythefacts.org/en/partitechnology/chapter5offshore/windresourceassessmentoffshore/measurement

offshore.html

12RenewableUKWebsite(2011).RenewableUKFrequentlyAskedQuestions.Visitedon

April2,2011.http://www.bwea.com/ref/faq.html#howlong


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13NOAA(2011).SatellitesandSpaceWeather.VisitedonApril4th,2011.

http://www.swpc.noaa.gov/info/Satellites.html

14Accadia,C.,S.Zecchetto,A.Lavagnini,andA.Speranza,(2007).Comparisonof10m

windforecastsfromaregionalareamodelandQuickSCATscatterometerwindobservationsovertheMediterraneanSea.Mon.Wea.Rev.,135,19451960.

15BarthelmieR.J.andPryorS.C.(2003).Cansatellitesamplingofoffshorewindspeeds

realisticallyrepresentwindspeeddistributions?JournalofAppliedMeteorology428394.

measuring offshore winds by kb

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