Meteorological Observations for Energy Siting

John Merrill,URI GSO

November 3, 2011

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In the wind resource assessment portion of the project we arefunded to acquire and use instrumented towers and acoustic windprofilers to gather data on winds. Data above the surface arescarce, as a general rule. We will also prepare an archive ofobservations, both at and above the surface, and make themavailable in user-friendly form through the Web interface discussedearlier.

Towers are usually utilized to acquire longer-term records, whileacoustic profilers are often used for brief deployments at multiplesites.

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Instrumented Towers - characteristics and sites

Two instrumented towers are to be installed in this project. A sitehas been identified adjacent to the Bay Campus of URI. Ourintention is to continue operation of this site into the indefinitefuture; it will serve as a baseline and point of comparison. Avertically-resolved wind and stability record extending over multipleyears will be valuable.

The second tower site is to be in Westerly, adjacent to the TownTransfer Station (near the capped landfill site). We are workingwith the WCRPC on this site.

The pictures that follow are from a site in Tiverton, whereequipment similar to what we have in mind is currently in use.

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• Modern towers areactually masts. 60 m isjust below the FAA limit.

• Anemometers, vanes andT/RH instruments areused at 40, 50, 60 m.

• Wind and stability profilesfit through the data canbe extrapolated withadequate accuracy.

• Paired booms allowanemometers to beupwind of the tower nomatter the direction.

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The two towers we will deploy supplement towers already inplace. An instrumented tower is in use at a site in Ninigret Park, inCharlestown. Another is in use at a town-owned site in Tiverton.In each case, vertically-resolved wind data are available, at heightsup to 50 or 60 m.

Together, data from these four towers will provide a transectacross the state of Rhode Island, in the area near the coast whereprior work suggests the available wind resource may be interesting.

There will be an overlap of the period of these observations. Itwould be desirable to extend the time frame of the observations sothat a year or more of observations would be available at the foursites, but it is not clear that this is feasible.

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• Acoustic profilers samplewind, volume-averaged,from 40 - 200 meters in20 m increments.

• They are much easier todeploy, requiring onlylimited cleared area andhaving few impacts.

• Their easy deploymentwill be enhanced byplacing the instrument ona utility trailer.

• Our intention is tocontinue using sodarsafter this project ends.

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Questions are often asked about comparison of sodar-observedwinds and winds measured on towers. The agreement is excellentas long as you take into account the fact that these instrumentsmeasure different things. Anemometers and vanes yield estimatesof the wind, speed and direction, at a particular height. Sodars, incontrast, estimate the wind, speed and direction, in a volumeabout as high as the vertical resolution of the data.

The greater height to which sodar data extend must be balancedagainst the fact that the fraction of time over which acceptablewind estimates are available decreases with height. Turbulencecharacteristics also sensed by acoustic profilers are not as useful invertical extrapolation as are static stability data.

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Our project will utilize two sodars. In their initial use they will bedeployed near instrumented towers. The aim is to “anchor” thesodar profiles to the in situ anemometer data, and to extend therange of available wind estimates above the top of the towers. Thecolocated observations will be carried out for between a few weeksand three months.

Later, the sodars will be moved to a series of sites, at whichprofiles will be observed for a period of months at each location.

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• These curves illustrate thedistribution of wind vs.height.

• The marked asymmetrynearer the surface givesway to a more nearlynormal curve.

• Despite reduced datareturn, the wind estimatesabove 120 m arestatistically sound.

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• The mean and variability of winds vs. time can be shown in abox and whisker plot.

• Here near-surface winds at the state airport are shown in thisform.

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• Especially when looking at winds above the surface, thedistribution is more informative.

• The preference is for time-resolved data; hourly averages arepreferred over even daily averages.

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⇒ finis ⇐

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