Flow Distortion Study Completed for the Chesapeake Light Tower
With the completion of a flow distortion study, a first stage-gate has been passed in the Wind Program's evaluation of the Chesapeake Light Tower for possible use as a reference facility for offshore renewable energy. Such a facility does not currently exist in U.S. waters and is needed to validate new technologies for resource assessment and to support research and development to advance offshore wind and marine and hydrokinetic (MHK) energy. The Chesapeake Light Tower stands in open water approximately 13 miles east of Virginia Beach.
The centerpiece of the potential reference facility will be a new meteorological mast that extends to approximately 100 m above the sea surface. This is roughly hub height for wind turbines that are expected to be deployed offshore. The mast will include devices to measure winds and turbulence at multiple heights. It will also include other supplemental meteorological measurements. The plan is to affix the mast to the existing light tower to cost-effectively take advantage of the existing foundation.
With the light tower similar in profile to offshore oil platforms, one concern was whether its bulky cross section would disturb the wind blowing around it so that measurements on the mast would have unacceptable errors. It was not practical to make detailed wind measurements around the tower, therefore the Energy Department's Pacific Northwest National Laboratory (PNNL) managed a computational fluid dynamics modeling study executed by DNV Renewables that used a very high resolution numerical technique to examine how much and where the wind is deflected as it blows past the light tower structure. This method used architectural drawings of the tower to construct a numerical solid model of the light tower. This was placed, numerically, within a computational grid made up of closely spaced points in all three dimensions. As the wind blew (in the model) around the tower, its value was calculated at each of the grid points to provide a detailed assessment of the tower's effect.
The study showed, as expected, that for typical meteorological conditions offshore, wind speeds and directions are strongly affected in the immediate vicinity of the structure. Wake effects are pronounced for several hundred meters downwind as well. However, the study also shows that there should be little effect on the existing structure at heights of 50 m and higher above the sea surface. Moreover, disturbance of the wind does not extend far from the light tower in the crosswind directions. This is good news for the proposed use of the tower as a reference facility. It means that new technologies such as floating lidars could be placed for validation close to a meteorological mast on the tower. It also means that it is feasible to take advantage of an existing structure to greatly reduce the cost of an offshore reference facility.