Project Date:
June 13, 2008
Acquired: May 24, 2007 18:26 to June 13, 2008 10:48 (about 385 days)
Sample Rate: 5 minutes
Location: Havre de Grace, MD 21078
Rooftop (approx. 55 feet from grade)
Equipment Description:
Signet Marine 3-cup anemometer mounted atop an approximately 30 ft pole on the roof of a two story structure. The closest vegetation of relevant height was approximately 50 ft away to the north. Wind direction was not recorded.
Signal conditioning was through an Analog Devices (AD536AJO) True-RMS to DC signal converter with 100uF averaging capacitor yielding and averaging sample time of 2.5 seconds or a frequency of 0.4 Hz. The slowest frequency recordable from the anemometer was approximately twice this value (0.2 Hz). The output from the RMS to DC converter was fed to a non-inverting amplifier utilizing an LM324N (manufacturer unknown) quad operational amplifier. The resistors were chosen as to yield an overall voltage ratio (gain) of 3.
The voltage was sampled at 5 minute intervals (to facilitate 3 month download intervals) using a Lascar EL-USB-3 portable voltage data logger. The device has an approx. accuracy of +-1%.
Method
The raw voltage reading from the data logger was divided by 3 (non-inverting amplifier gain) to obtain the actual raw RMS voltage from the anemometer. The calibration constant given by Signet Marine was 1/8 Vrms per knot of wind speed, therefore the values were then divided by 0.125 to obtain wind speed in knots. The values were then scaled by 1.15077 to obtain miles per hour (a unit that more people are comfortable with). The result for the entire 385 day data set is shown in the images above.
The unweighted average of this data set was 1.818 mph. This average puts the site into the class 1 rating according to the American Wind Energy Association (www.awea.org). Generally, a class 2 or better site is required for a turbine to be economically feasible. Most industrial wind farms are installed in class 4 sites or better. Quoting from the National Renewable Energy Laboratory's Wind Energy Resource Atlas of the United States:
Each class represents a range of mean wind power density (in units of W/m2) or equivalent mean wind speed at the specified height(s) above ground. Areas designated class 3 or greater are suitable for most wind turbine applications, whereas class 2 areas are marginal. Class 1 areas are generally not suitable, although a few locations (e.g., exposed hilltops not shown on the maps) with adequate wind resource for wind turbine applications may exist in some class 1 areas.
The industry standard for calculating the amount of available wind energy at a given site is to use a probability distribution function to obtain the “effective average” speed. The typical probability distribution function is the Rayleigh distribution which is an asymmetric bell shape that skews toward the zero value, not the mean as in a normal (Gaussian) distribution. This effectively says that the wind is more likely to be blowing less than the mean (mode is less than mean) and that very high gust speeds tend to pull the average higher than it should be for effective electricity generation. The figure above shows a Rayleigh distribution for a good site (between class 5 and 6).
This Rayleigh probability distribution has a mean around 6.5 m/s (14.5 mph). It's evident how the Rayleigh distribution shifts the probability toward the zero so that the highest probability wind occurring at this theoretical site would be around 5-6 m/s. Notice also how there is a zero probably of the wind stopping. The figure below shows the Rayleigh distribution for the site in question. Note the high probability or near-zero wind speed.
Conclusions
The actual recorded average wind speed of 1.818 mph was much less than the 10 mph required to achieve class 2 status (and therefore be economically viable according to the Department of Energy). Also, the Rayleigh distribution shows that the most probable wind speed to occur (and, by extrapolation, the speed at which the wind spends most of the time) is more around 0 to 1.5 mph. Therefore, a small scale wind turbine is not recommended for this particular site.
The figure below shows the official National Renewable Energy Laboratory (www.nrel.gov) resource map for Maryland and Delaware. The site in question is located predominantly in a class 2 area. However, it should be noted that the NREL data is designed to represent “open plain” wind or, in mountainous areas, the very crest of the most prominent mountains (shaded areas). The local obstructions (trees, buildings, etc.) at this site have a very deleterious effect on the available wind resource.
Sources/Further Information:
American Wind Energy Association (www.awea.org)
National Renewable Energy Laboratory (www.nrel.gov)
Department of Energy – Energy Efficiency & Renewable Energy – Wind and Water (http://www1.eere.energy.gov/windandhydro/)
Danish Wind Industry Association (www.windpower.org)