|dc.description.abstract||This thesis examines aspects of the wind resource potential (the potential for energy conversion by wind turbines) in a complex, coastal terrain setting. The main objective of this study is to use a combined modelling and field based measurement approach to assess how atmospheric and climate phenomena, over a range of spatial-temporal scales, can contribute to the quantity and quality of the wind resource in a complex, coastal terrain setting. A major theme of this study is the adoption of a holistic approach, to include the examination of atmospheric and climate phenomena that are characterized over a range of spatial-temporal scales, including those at the synoptic, mesoscale and microscale. Studies aimed at establishing linkages between aspects of the wind resource and atmospheric and climate phenomena remain sparse, in both the international and New Zealand focused published literature. However, establishing and quantifying these linkages carry important practical and economic implications including: forecasting the energy extractable by wind turbines over short time periods; allowing better informed decisions regarding turbine placement to be made; and understanding how the wind resource will be affected by 21st century climate change. These implications are particularly important given the recent and forecasted growth rate of wind energy developments both globally and within New Zealand.
The methods adopted in this research involved a field based measurement campaign combined with mesoscale numerical modelling. This research was conducted on the ridge top of Porteous Hill (approximately 400m a.s.l) (45.6895° S; 170.5829° E) situated near to the township of Waitati, north of Dunedin. The choice of field site, within a complex and coastal terrain setting, permitted examination of a number of atmospheric processes and phenomena, operating over a range of spatial-temporal scales, potentially important to the wind resource in such a geographical setting. Data from four ridge top temporary automatic weather stations were used in this research, including a sonic anemometer to analyse properties of the near surface ridge top turbulence. Measurements from these stations were made at several different heights above the surface which ranged from 2m to 30m, and across different time periods between 1 September 2011 and 31 August 2013. The Air Pollution Model (TAPM) was also used in this study; TAPM was employed to test model skill in calculating metrics for wind resource quantity and to untangle linkages between processes and phenomena that operate over different scales.
Substantial variability in the monthly mean power density (a commonly used metric for wind resource quantity) was found between months over the two year examination period. Changes in the mean power density between months were linked to anomalous Southern Hemispheric monthly mean sea level pressure fields derived from reanalysis data. In particular, the wind resource quantity in a given month examined was enhanced when high pressure centres were located anomalously north of New Zealand, and shown to be degraded when high pressure centres were located directly over New Zealand or to the southeast of New Zealand; related to the associated changes in the strength of the synoptic westerly wind over the South Island. A probabilistic analysis linking Kidson weather types to generation potential over shorter (daily) time periods yielded similar findings, with the results of this analysis subsequently used to infer how larger scale climate oscillations might also contribute to variability in the wind resource quantity.
At the sub-synoptic scale, sea breezes were often found to be of sufficient magnitude to be important for energy generation at this site, with TAPM deemed suitable for simulating a number of aspects of the circulation. Subsequent ‘idealized’ simulations carried out by TAPM revealed that the strength and direction of the ambient synoptic wind might be an important determinant of the sea breeze contribution to the wind resource. In particular, under offshore directed ambient synoptic flow of sufficient magnitude (10 ms-1), the counteraction of the sea breeze could result in an extended period of calm winds below wind turbine cut-in thresholds. However, through further model based sensitivity experiments, it was estimated that the net contribution of the sea breeze to the wind resource quantity remained positive. At the microscale, ridge top wind shear was generally found to be lower than that assumed under commonly employed tools (ie. the ‘1/7th power law’) to extrapolate wind speed with height in wind resource assessments. Variability in ridge top wind shear exhibited a complex non-linear relationship with wind speed, time of day and wind direction. The application of Monin and Obukhov similarity theory (MOST) appeared limited in this setting also; the performance of MOST for extrapolating wind speed with height was often inferior to the more simple 1/7th power law method, especially in the stable boundary layer. A number of these findings potentially have practical utility for wind resource assessments, not only at this site but also in the more general context of assessments elsewhere.||