Diploma Thesis

Einfluß der Topographie auf das Windfeld und die Leistung von Windkraftanlagen

Matthias Hierteis (08/1998-02/1999)

Support: Thomas Foken

The objective of the presented diploma thesis is the analysis of possible effects of complex terrain on different wind parameters such as normalized turbulence characteristics, turbulence intensity, drag coefficient and the slope of the wind vector as well as their potential effects on the power production by a wind energy conversion system (WECS). Therefore extensive measurements of meteorological quantities with special regard to the wind turbulence were conducted on a 50 m high tower at the wind energy testing station Dlouhá Louka in the Krušné Hory (Iron mountains), Czech Republic, from 29.8. to 26.9.1998. Simultaneously power performance measurements took place at a stall regulated WECS about 100 m east of the mast with a rated power of 315 kW and a hub height as well as a rotor diameter of 30 m (ZELENÝ & FOKEN, 1995). This location offers a nearly flat fetch during wind from west to north as well as steep slopes for southerly winds. Hence, the footprint model of SCHMID (1994) was used in connection with a digital terrain model to develop new definitions of the so-called "macro-roughness" (ZELENÝ & PRETEL, 1986) for the description of the relief roughness. A computer program was created to combine both models and calculate the macro-roughness values automatically.

The stability dependence of the measured turbulence parameters was analyzed and in most cases considerable deviations from the models for plain areas were observed. Subsequently, the wind parameters of a stability range, in which they showed no dependence upon stability in the latter analysis, were summarized to means of 30° sectors and inspected for their dependence on wind direction and thus on the relief. The normalized turbulence characteristics and the slope of the wind vector rose significantly and monotonously with growing slope of the fetch, whereas the drag coef-ficient seemed to decrease slightly. The turbulence intensity and the standard deviation of the wind direction displayed a very similar course and varied from sector to sector without indicating a general trend. This led to a highly significant linear correlation between macro-roughness, the normalized turbulence characteristics and the wind vector slope, which also held in somewhat weaker form for the drag coefficient. The turbulence intensity could not be described by the used macro-roughness definitions, for there was no clear difference between steep and flat fetches perceptible. The examination of the influence of the wind on the power performance of the WECS revealed a distinct effect of the turbulence intensity respectively the standard deviation of the wind direction and the wind vector slope on the deviations of the measured power from the expected power curve. This relationship was described within the framework of a non-linear regression analysis by means of turbulence intensity and the slope of the wind vector, so that the portion of the deviations due to these parameters could be calculated to 55 % at least. It became evident, that a high turbulence intensity, as it is typical for mountainous areas, caused considerable power deficits in comparison to the power curve above certain wind speeds. For this WECS it could also be proven, that a sloping fetch leads to a reduction of the power performance due to the slope of the wind vector. Other turbulence parameters did not show influences on the power performance. From this, consequences for the future exploitation of wind energy in mountainous areas were formulated concerning both questions of siting as well as producers of wind power plants.

last modified 2003-12-12