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Site Characterization of Offshore Wind Energy Areas

By Rick Cole

Metocean surface-buoy system with associated data plots. (Credits: RDSEA and Down East Instrumentation; ADCP contour plot provided by the University of South Florida's Ocean Circulation Group; met and sea surface temperature plots provided by China's First Institute of Oceanography)
With the push for alternative energy well underway in the United States, programs are ramping up with plans to begin sending megawatts of power from the coastal ocean environment along various regions of the eastern seaboard back to the beach for distribution. Wind, wave and ocean-current energy programs are making considerable advances, with wind power out in front.

Wind is the world's leading source of renewable electricity, although still considered an emerging industry in the U.S. It could help decrease greenhouse gas emissions and revitalize economic sectors of the nation. More than 90 percent of offshore wind installations are in Europe, with nearly 4,600 megawatts of power generated as of mid-2012. Outside of Europe, only China and Japan have operational wind platforms offshore.

The U.S. has yet to install a single turbine along its coastal oceans. The U.S. Department of Energy's estimate of offshore wind resource potential from state and federal waters along the U.S. and Great Lakes coastlines is in excess of 4,000 gigawatts. In November 2010, U.S. Secretary of the Interior Ken Salazar announced the Smart from the Start Atlantic wind energy initiative to hasten the responsible development of wind energy on the Atlantic Outer Continental Shelf (OCS). In early 2011, the Bureau of Ocean Energy Management (BOEM) began conducting environmental assessments to identify regions of the OCS offshore the Mid-Atlantic states of New Jersey, Delaware, Maryland and Virginia most suitable for development as wind energy areas (WEAs).

Regulatory Requirements
BOEM's regulatory process consists of identifying suitable areas, issuing a lease to a company interested in developing a WEA, approving a site assessment plan (SAP) that includes meteorological (met) measurements, and approving a full construction and operation plan.

An SAP must contain a proposal of any plans to construct met towers or the deployment of buoy systems and must be preapproved by BOEM before site characterization activities begin. These activities include but are not limited to hazards, geophysical, geotechnical, archaeological, biological and metocean surveys.

The first of these U.S. commercial leases was granted in 2010 when BOEM approved the construction of the Cape Wind project off Cape Cod, Massachusetts, where an estimated 75 percent of the electricity needed for the Cape, Martha's Vineyard and Nantucket Island could be generated via wind energy.

A collection of measurements is necessary to analyze wind resource data and environmental conditions to determine a location's suitability for wind energy development and the impact on biologically sensitive habitats within the WEA. The instrumentation used provides full metocean support (surface and water column physics and atmospheric studies), along with biological sampling of the bird (avian) and bat (chiroptera) populations offshore, and migrating marine mammals (cetacean) within each WEA region.

Peripherally, ocean-current velocity and direction, and wave field spectra are measured, and hydrophones are mounted on the seafloor to record marine mammal presence, and sensors are placed on buoys to listen for bird and bat activity. These measurements are in conjunction with many other required surveys, including coastal habitats, benthic resources, air and water quality, economic conditions and recreational activities.

Wind and Atmosphere Measurements
The measurement of wind speed and direction is the most important requirement for estimating energy capture and the corresponding electrical output from a given model of wind turbine.

Met Towers. Wind speed has a functional relationship to elevation. Met towers are the most common method of measuring winds at height. Usually, a tower or mast stands 50 to 100 meters tall with multiple anemometers, temperature and pressure sensors spaced at predetermined levels above ground. Data need to be collected at a minimum of two levels (more data points are preferred) to determine the structure of and the turbulence and shear in the wind.

Offshore met towers are common in Europe. In the U.S., this method may prove expensive and difficult to permit due to construction and associated impacts on the marine environment. Oceanographic surface buoys can provide excellent single-point in-situ measurements at the sea surface, and subsurface sensors can also contribute data.

Sensors are mounted on buoy towers or masts, with wind sampling at 3 to 5 meters height off the surface. Data adjustments are made in post-processing to 10 meters, a metocean standard. Wind turbine hub height is nominally 80 to 100 meters, therefore, further extrapolation of the wind field aloft must be derived using wind-power assessments when using buoys alone.

Remote Sensing and Simulation. Correlation of these approximations can be done using remote sensing and modeling products (numerical simulation), such as what NOAA's National Centers for Environmental Prediction (NCEP) and the National Environmental Satellite, Data and Information Service (NESDIS) provide online. Until recently, buoy systems used conventional cup and vane or propeller and vane anemometers, which produce a frequency and voltage output proportional to wind speed and direction angle.

This technology is slowly being replaced by ultrasonic wind sampling, which compares the time a sonic pulse takes to travel from one transducer to another. This involves no moving parts, allowing for longer deployment time frames and decreasing system maintenance and calibration costs.

Sonic winds are calculated from the differences in time of flight along each axis (e.g., north to south versus south to north). Sensor heads must be mated with a compass to produce directional output. Normally incorporated with wind measurements is a suite of met sensors for air temperature, relative humidity, barometric pressure and radiation. Sea surface temperature completes the measurement loop. Sample rate varies depending on the sensor but usually averages minutes over each hour and is sent to a logger and then to a receiving station. These data contribute to understanding near-surface flux (air-sea heat exchange), extrapolation functions and atmospheric stability on location.

Lidar. In parallel to using surface-buoy applications, met towers and numerical models, lidar technology is now being tested offshore on fixed and moving platforms. Lidar is an optical instrument used in atmospheric research and meteorology that remotely measures 3D horizontal wind velocity and direction within a volume of air.

This form of wind profiling has been very successful on land and is used by the U.S. military and NASA regularly. Profiles above hub height are being reported by manufacturers, allowing for hard measurements to be made at and beyond turbine energy-source levels.

In Europe, lidar technology is run through considerable testing and calibrated against proven cup-vane-propeller and sonic anemometers mounted on tall masts. A permanent reference lidar is used to evaluate consistency between units. Successful system comparisons have been conducted on land alongside met towers to provide assessment of performance and reliability. Offshore deployments of lidar buoys and fixed mounted structures are now underway, showing excellent results in data analysis. To continue this article please click here.

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