Wind Data
Between August 2002 and August 2005 the Renewable Energy Research Lab at the University of Massachusetts performed a study of the Vinalhaven wind resource on a site just west of the North Haven Road, south of Seal Cove.
Graphs of average wind speed, direction, and frequency have been included below. For more information, all of the data and interim and final reports from the study can be found at: http://www.ceere.org/rerl/publications/resource_data/Vinalhaven/
Wind Data & Interpretation
Wind speed and direction are critical for power generation estimates. Since power generation varies with the cube of the wind velocity, a small increase in wind speed can yield a dramatic increase in power generation. (eg. doubling the wind velocity increases power output by a factor of 8; increasing velocity by 20% increases output by 75%.) Hourly, seasonal, and yearly wind velocity fluctuations will cause power generation fluctuations.
Wind Direction
The following “wind rose” shows the percent of time that the wind blew from a given compass direction on Vinalhaven in 2004. Notice how the predominant wind direction is NNW in the winter, SW in the spring and summer, and SW/NE in the fall. Though they vary somewhat year-to-year, these seasonal patterns persist.
Wind Speed
Although the wind may blow from the SW during the fall, spring and summer, the NW component is important because winter months are windier than fall, spring and especially summer months. The following graph shows the seasonal variation in wind speeds on Vinalhaven from September 2002 to July 2005. Notice that low wind speeds occur around July and peak wind speeds occur around December.
Altitude
Higher wind speeds typically occur at higher altitudes. This phenomenon is called wind shear. Below is a graph of 2004 monthly average wind speed as measured at 40m, and estimated at 80m, along with the associated power outputs that could be expected from a turbine at these two heights. The 80m wind speed averages were calculated using the Power Law Method (see “Wind Speed Estimation Methods” below). Notice how increases in average wind speed can dramatically increase power generation.
Wind Speed Estimation Methods
Accurate wind speed estimates are vital to power generation estimates. Ideally, wind speed is measured at the height of the proposed turbine. However, the UMass study measured the wind speed at a height of 40 meters. We intend to erect turbines that are taller than this, where the wind is expected to be stronger. The methodology for adjusting the wind data to account for the higher elevation of the turbines is summarized below.
Wind Shear Estimates and Wind Speed Adjustment
The wind speed analysis conducted by RERL UMass only measured wind speed at a single elevation: 40 meters. The current configuration for the Fox Islands Wind Project calls for turbines with hub heights of 80 meters. Thus, an estimate of the expected wind speed at this elevation is necessary.
The standard methodology for this estimation is to use the Power Law Method. This calculation uses an empirically-derived equation to estimate the wind speed at height Hy given the measured wind speed at height Hx. If the measured wind speed at Hx.is Vx then the estimated wind speed is given by the following formula:
Vy = Vx (Hy/Hx)a
Where:
Vy is the unknown wind speed at height Hy,
Vx is the known wind speed at height Hx,
a is the wind shear exponent.
On flat, open plains, a is generally in the range of .14. However, in woodlands and hilly areas it can be as high as .4*
Using data from a wind assessment performed on Swan's Island, which included anemometers at two different heights on a site that is very topographically similar to the site on Vinalhaven, wind shear estimates of between .32 and .36 have been estimated. In addition, the scientists at the UMass RERL have indicated that using a wind shear estimate of .3 is very reasonable for the Vinalhaven site.
Using a wind shear exponent of .3 for Vinalhaven yields an estimate for the average wind speed at 80 meters on the site of 6.5 meters per second. This average is 23% higher than the measured wind speed at 40 meters. Thus, in our economic analysis of the Vinalhaven project, each hourly wind speed measurement was multiplied by 1.23, and this was used to calculate the power that would be generated by the wind turbines.
* For data on wind shear exponents in various terrains, se Paul Gipe, Wind Power, 2004. p. 41
