Wind Resource - Wind Energy: Renewable Energy and the Environment - page 59

Over 1,000 MW have been installed in offshore wind farms in Europe, because in general wind
speeds are higher and onshore land has a high value. The United States is considering offshore wind
farms near Cape Cod, Massachusetts, and the Gulf Coast of Texas. Wind farm developers have
expressed interest in offshore in the Great Lakes and on both coasts. A program for offshore wind
resource assessment has began at NREL [20]. The maps will extend from coastal areas to 90 km off-
shore and have a horizontal resolution of 200 m. The final maps of wind speed and power will be for
50 m height, and the model data will be modified with data taken at sites on the coast and offshore.
4.4.1 T EXAS G ULF C OAST
The ocean wind data were used to calculate wind speed and power (10 m height) for 0.25° pixels
for a 5 by 5° area (longitude, 25-30 N, and latitude, 93-98 W) for the period of 1988-1994 [21].
The average wind speed is around 5 m/s, and further out in the Gulf of Mexico it is above 6 m/s
( Figure 4.6 ) . December and January are the high months, with June being the low wind month.
The National Renewable Energy Laboratory and the Texas State Energy Conservation Office
have a cost share project to produce high-quality and validated offshore wind resource maps [22].
These data include the near-shore region not covered by the ocean wind data. Maps of mean annual
wind speed at 10 to 300 m and mean annual wind power potential at 50 m are available. There are
class 3 winds along the northern third of the Gulf Coast, then class 4 winds with a region of class
5 winds from Corpus Christi almost to the border with Mexico. NREL also plans to use the data
to analyze the offshore wind shear plus other wind characteristics for offshore wind turbine design
and performance. The state has control of the land for a distance of 16 km from the coast and is
interested in leasing areas for wind farms.
4.4.2 W ORLD
The European Wind Atlas also has offshore winds ( Figure 4.7 ) . The Predicting Offshore Wind
Energy Resources project [23] aimed to assess the offshore wind power potential in European Union
waters, taking into account coastal effects and highlighting those sea areas where hazardous wind
or wave conditions exist. These estimates can then be used to pinpoint areas, which are favorable
for siting a wind farm. More detailed monitoring can then be undertaken to improve the initial wind
power estimates at selected sites.
4.5 INSTRUMENTATION
An anemometer is a device for measuring airflow. There are a number of measuring devices for
wind speed: pitot tube, cup, vane, propeller, hot wire, hot film, sonic, and laser Doppler anemome-
ters. The common devices are the cup and propeller anemometers, since they are cheaper. However,
their response times to changes in wind speed are slower. Wind turbines also have a response time
to changes in wind speed, so cup anemometers are adequate for determining the wind energy poten-
tial. Sonic and hot wire anemometers have the advantage of no moving parts and no response time
in contrast to mechanical sensors. However, their higher cost has kept them from much penetration
into the wind resource assessment market.
An anemometer can be obtained to measure the amount of wind that has passed, a wind run.
From the wind run, the average wind speed can be calculated for the time period. An anemometer
can also be obtained to measure the fastest mile, the maximum wind speed.
Previously, meters and strip charts, which give analog outputs, were used. However, analyzing
strip chart data becomes quite tedious, and the time resolution is fairly coarse unless the paper feed
rate is large. Today the major difference is the availability of microprocessors for sampling, storing,
and even analyzing data in real time. Also, personal computers alleviate most of the problems in
analyzing large amounts of data.