Wind speed characteristics and implications for wind power generation: Cape regions, South Africa

Abstract

Spatio-temporal dynamics of near-surface wind speeds were examined across the Northern, Western and Eastern Cape regions of South Africa. The regions assessed were geographically subdivided into three zones: coastal, coastal hinterland and inland. Wind speed data (10 m) were evaluated at monthly, seasonal, annual and zonal resolutions, with the aim to establish wind speed attributes and trends. Data from 19 weather stations with high-resolution wind records between 1995 and 2014 were evaluated. The majority of stations (79%) recorded a decrease in mean annual wind speed over the study period. The mean rate of decrease across all stations over the 20-year period equates to -1.25%, quantifying to an annual decrease of -0.002 m/s/year (-0.06% pa). The largest seasonal decline of -0.006 m/s/year (-0.15% pa) was recorded in summer. Statistically significant declines in mean annual wind speed are somewhat more pronounced for the coastal zone (-0.003 m/s/year, -0.08% pa) than over interior regions (-0.002 m/s/year, -0.06% pa) for the study period. The largest decrease (-0.08% pa) was recorded for the coastal zone, followed by the inland zone (-0.06% pa), equating to an annual reduction in available energy of 0.18% pa and 0.09% pa, respectively. When considering all stations over the study period, the mean inter-annual variability is 3.11%. Despite such decreases in wind speed, the variance identified in this study would not have posed any risk to power generation from wind across the assessed stations, based on the period 1995 to 2014.

Significance: 

• Mean recorded wind speed decreases have been marginal under recent (last 20 years) climatic conditions and change, thus adding confidence to the justification by the South African government to procure additional wind-generated electricity capacity.
• Wind speed trends and variance measured over the research period (1995–2014) for the Cape stations in this study, would not have posed any risk to power generation from wind.
• Only four stations (28.6%) recorded statistically significant trends at the 0.05% level; of these 75% were decreasing trends.
• Coastal zones recorded a statistically larger decrease in mean wind speed compared to inland regions.
• When considering all stations over the period 1995–2014, the mean inter-annual variability is 3.11%.