Changes in the potential for wind energy generation due to terrain modification of the boundary-layer flow

Abstract

In situ measurements and a wind tunnel experiment are used to define the degree of wind speedup in the boundary-layer flow over the top of a small mesa. Measurements of the speedup of the wind relative to that observed over level terrain at the same height above the ground are presented as a function of distance from the windward edge of an escarpment. These results are shown for neutral as well as stable atmospheric conditions. It was found that the wind tunnel measurements, in situ measurements, and existing two-dimensional models of the terrain modification of escarpment flow agreed well for neutral conditions but differed considerable when stable conditions existed at the mesa site. The average speedup factors at a mesatop location, which had a directionally-weighted distance of 2.5 times the mesa height away from the edge of the escarpment, were 1.58 at 5 m, 1.50 at 10 m, and 1.36 at 20 m above the surface of the mesa. The depth of the zone which appeared to be modified by the mesa was on the order of half the height of the mesa. The main speedup occurred at the windward edge of the mesa and decreased exponentially over the first 50 m. Within the flow zone modified by the mesa, the ratio of the power available at a given height over the mesa to that at the same height above the surface over level terrain (the wind power ratio) ranged from approximately 4 at a distance equal to half the height of the mesa away from the windward edge of the mesa to a value of 3 at a distance equal to 2 mesa heights away from the windward edge. At 4 mesa heights from the windward edge of the mesa, the wind power ratio was just slightly greater than 1. Elevating a wind energy system above the relatively shallow layer modified by the mesa (50 m) would result in a wind power ratio with a value between 2 and 3. This is a numerically smaller wind power ratio than that found within the modified zone but the actual wind energy density above the modified layer would be greater since the largest increases of wind power with height are found near the surface.