GENERATION Sharing Eskom’s experiences in wind energy by Ian Smit, Dr. Louis van Heerden and Riaan Smit, Eskom This article shares some of Eskom’s experiences over the past few years in wind energy at the Klipheuwel wind energy demonstration facility (KWEDF), just north of Cape Town, the only large wind turbine facility currently in sub-Saharan Africa. The research and demonstration facility was established by Eskom’s corporate division. The division’s research and demonstration portfolio in the renewable field focuses on bulk renewable energy systems including solar, biomass, and wave and wind energy. The KWEDF facility consists of the Danish and does not influence the radio and power, duty cycle and cost characteristics Vestas V47 660 kW and the V66 1,75 MW satellite communication sites in the vicinity. of each individual approach will be used wind turbines and a French Jeumont J48 Audible noise measurements before and to populate a future technology matrix that 750 kW wind turbine. The three turbines after commissioning confirmed that rural ultimately will contain a comprehensive suite are all different in design and operational background levels were not increased by the of appropriate renewable energy technology characteristics to enable comparison of wind turbines. Ongoing research is focusing options (wind being the first). the different technologies employed in the on the monitoring of avian interaction (not Tower dynamic characteristics and vibration industry under similar conditions. The three a single confirmed bird strike to date) and condition monitoring baseline measurements wind turbines were commissioned between localized pollution effects. Further resource were completed. The wind farm is connected August 2002 and February 2003. To date analysis is continuing throughout South to a healthy 11 kV distribution network more than 15GWh has been generated by Africa. and the impact from the wind farm on the facility which is connected to the grid at Research the grid or vice versa is negligible, with the Klipheuwel 11 kV/66 kV substation. The research goal defined in this project monitoring of the quality of grid supply Environmental and network impact supports both the national view and Eskom’s continuing. The production analysis is used The site selection was determined through a strategic intent with respect to renewable to determine impact on future marketing of full environmental impact assessment, the energy technology. The main deliverables of green energy. this project were to quantify the performance first for a South African wind farm. Important Resource analysis aspects considered included proximity to of the generating plant installed, identify issues Cape Town for research and demonstration that impact on the performance of the plant, Power in the wind suggest processes or remedies that could be purposes, grid availability and direct impact An important characteristic of wind energy applied to address the identified issues and on the environment. Normal agricultural is that the power output of a wind turbine to assess the techno-economic feasibility activities are continuing on site. (or power in the wind) is proportional to the of various options aimed at optimizing wind The research to date showed that generation under South African conditions, third power of the wind speed, while a direct electromagnetic interference is negligible and for Eskom’s purposes.The individual relationship to the air density (containing temperature, humidity and ambient air pressure parameters) also exists. Therefore, the precision requirements of wind speed statistics for energy assessments are higher than for most other purposes. The basic equation is : Power in the wind (Watt) = ½ AV3 where = air density (kg/m3) A = rotor swept area (m2) V = wind speed (m/s) A modern horizontal type wind turbine typically has a mechanical conversion efficiency of around 40%. Betz’s law states that the wind behind a turbine cannot be stationary, so the maximum useable power in the wind is only about 60% of that calculated above. Seasonality Another noteworthy characteristic of the wind is the seasonal and year-to-year Fig. 1: “On top of the world” – the V66 blade is longer than the wing of a Boeing 747-400 at 33,5 m. variations of the wind conditions. An accurate energize - April 2008 - Page 38 GENERATION determination of wind climatologies must take account of these variations and therefore, if possible, several years of wind data should be used in the analysis. A well known study of climatic variability in northern Europe (Larsen et al., 1988), showed that during the seventies the mean power in the wind was close to the mean for the period 1873 to 1982 (a full 109 years!). The study also showed, however, that variations of up to 30% can be expected from one decade to another. This information is crucial in long-term viability of a possible site with an expected operational life in excess of 20 years. Hence, the application of wind measurements to wind power calculations demands a long time series of high-quality wind data. In the Western Cape, weather is predominately affected by the cold front systems in winter and associated NW winds, and by the good old “Cape Doctor” in the summer months, the latter providing good production statistics as will be seen in the next section [1]. Roughness The wind speed measured at a meteorological station is determined mainly by two factors: the overall weather systems, which usually have an extent of several hundred kilometres, and the nearby topography within a few tens of kilometres from the station. The terrain classification comprises four roughness classes, each class corresponding to a typical terrain. The KWEDF is typically classified as a class II type terrain. Height variation Another very important concept is the effect of height variation or speed-up effect due to a hill-top etc. These effects of height variations in the terrain on the wind profile can most clearly be demonstrated by the well known results from the international field experiments at the Askervein hill (a perfect “koppie” in the middle of nowhere) on the Isle of South Uist in the Hebrides (Taylor and Teunissen, 1987; Salmon et al, 1987). Their results showed: • The speed-up at the crest is 80% as compared with the undisturbed upstream mean wind speed. • The negative speed-up (speed-down) in the front and lee of the hill is 20 to 40% as compared with the undisturbed upstream mean wind speed. It is evident from the discussion that hills exert a profound influence on the flow, and that this has to be taken into account as carefully as possible. But one should be aware that all the height changes in the terrain influence the flow: a 5% height increase can have a 5% impact on the mean wind speed - possibly at hub height - resulting in a 15% increase of the available power or even worse the reverse just in front of the hill! It is often difficult, impossible in complicated terrain, to apply simple formulas. For this reason it is necessary to determine the wind resource at specific locations and then in most cases to use a numerical model (computational fluid dynamics) for the calculations as found typically in WAsP [2]. Density - temperature, humidity and ambient air pressure We know from the above power equation that the air density plays a major role in the amount of lift (work capability) available in air or the wind. The power curve of a wind turbine is usually referred to at standard density of 1,225 kg/m3 that corresponds to conditions of dry air at standard sea level pressure of 1013,25 mbar and a temperature of 15°C. A power curve applied to a site where the average air density is different from the standard value is commonly assumed to be proportional to the ratio of the site air density to the standard value. The most important energize - April 2008 - Page 39 GENERATION parameters when determining density are temperature, ambient air Sea breezes pressure and humidity. The higher the temperature the less dense the When the air moves from sea to land areas or vice versa, two effects air! The higher the humidity the less dense the air! Air consists mainly of are of major importance for wind resource climatology, namely: the oxygen, nitrogen and carbon dioxide (and of course all our pollutants change of surface roughness and thermal surface properties. Well such as dust and soot and SOx and NOx). Water vapour displaces air away from the coast, the wind climate is either of the maritime or inland molecules and thus reduces the lift capabilities (H2O is lighter than O2 type, but in between it is a mixture of both. The width of the coastal zone and N2 etc). The climate in the Western Cape typically constitutes hot, dry summers with cold, wet winters. The same wind speed (strength) varies with climate and topography. A common phenomenon in many can cause a 10% variation in production between summer and winter coastal areas is the occurrence of a land/sea breeze. The sea breeze [3]. Typical air densities for these two conditions are: is a local wind blowing from sea to land, caused by the temperature difference when the sea is colder than the adjacent land. Therefore, it Cape Town summer: 1,14 kg/m3 usually blows on relatively calm, sunny, summer days. The land breeze Cape Town winter: 1,27 kg/m3 is the oppositely directed, usually weaker, night-time wind. Owing to the generally low wind speeds associated with the land/sea breeze it adds little to the wind energy potential of coastal areas [1], [4]. Production The production to date has proven that an average wind resource is available at the KWEDF, with best production during the “windy” summer months.