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. Utilization factors vary between 10% and 35% (winter/ summer). The day-to-day operations and maintenance aspects are successfully managed by Eskom’s Peaking Generation division. Most of the initial start-up problems were resolved and wind turbine availability has increased on average to around 95%, which is comparable to international performance levels. Refer to the figures below for month on month production statistics for three years at the KWEDF (each turbine shown separately).

Fig. 2: Month on month capacity factor comparison, Vestus V47. Proposed 100 MW facility on the West Coast

Eskom announced the construction of a 100 MW facility on the west coast in 2007. EIA and commercial activities are well underway. The construction will start late in 2008 and the plant will be operational by 2010.

Capacity: 100 MW

Technology: Horizontal type turbines with pitch control, gearbox reduction drives and induction generators (As is the current industry standard).

Unit Size: 1,5 to 2,5 MW each approximately 50 units)

Location: West Coast just north of the Olifants river mouth close to the town of Koekenaap.

Network Integration: 132 kV Line to Eskom’s Koekenaap or Juno substation.

Fig. 3: Month on month capacity factor comparison, Vestus V66. Wind Resource: A = 6,2 m/s; k = 2,1 annually. (Weibull distribution parameters)

Fuel Costs: R 0 per annum

Plant Availability: 90%

EUF / CF: 26%

Production: 228 GWh per annum

Actual wind measurements at the proposed site applied to typical wind turbine performance has indicated an energy utilisation factor of 26% as shown above in the base case. However climatic variation may impact this production figure by as much as 30% on a year-on- year basis (both negative and positive). This is based on European experience over the last 100 years. Our experiences in wind also indicate large variations in wind resource. This changes the possibilities of the proposed project to 16% utilisation (5 m/s average annually) Fig. 4: Month on month capacity factor comparison, Jeumont J48. and a 36% utilisation (7 m/s average annually).

energize - April 2008 - Page 40 GENERATION

Environment

The proposed west coast site consists of ocean facing dunes along the coastline (roughly 20 km wide) North of the Olifants river mouth. The area is very sparsely populated with the nearest towns Koekenaap and Lutzville (20 km away). Fig. 7 refers to the topographical map. Land use is mainly goat/sheep farming in the dunes (very few and far between) with most of the land belonging to diamond mining groups along the coast itself. Their activities are mainly focused on the shoreline and ocean itself. North of Brand se Baai, mining activity takes place on the inland dunes as well – access to the general public is forbidden in these areas. Rocky outcrops in the ocean on the shoreline are inhabited by seals. The vegetation on land is mainly small shrubbery typical of the Sandveld in this region with no natural fresh water occurring other than the Olifants River.

Temperatures can vary significantly from the coast to inland. Mild teens with lots of fog (high humidity) in the mornings are experienced at the coast with temperatures 40ºC and above typical at Vredendal 30 km inland in summer. Reasonably good access roads exist capable of transporting mining equipment and for that matter turbine towers. Refer to the red (tarred) and orange (gravel) roads in Fig. 7. Additional road infrastructure required for turbine transport will be minimal.

Fig. 5: Actual West Coast wind measurement - Transhex wind climatology

Fig. 6: TransHex typical expected daily production (Summer) (V80 2 MW Turbine - 26% CF).

energize - April 2008 - Page 41 GENERATION

experience at Klipheuwel and international best practices.

The international industry mainly utilises maintenance contracts from the OEM for their turbine maintenance. This consists of scheduled maintenance intervals with oil and filter change outs lasting a day or two per turbine. Day to day operation requires very little input from the owner as the turbines are self dispatching. A dedicated remote monitoring system is included per turbine usually linked up via a broad band link to the facility (DSL/GPRS/3G/HSDPA). Between scheduled maintenance intervals preventative intervention is minimal and inspections are limited to twice monthly per turbine. Two years worth of start up spares and full maintenance contracts are usually included in the original purchase of wind turbines. Eskom’s experience has indeed Fig. 7: Map of the proposed West Coast site. been that all the work can be done internally backed up by the OEM support services with a once off annual visit from the OEM to ensure conformity to latest industry trends, modifications etc.

Conclusion

The results of the work done in this project provided immediate economic and technical benefit, as well as valuable strategic information to assist in achieving its long-term obligations regarding renewable energy application.

Wind resource in South Africa can be considered moderate when compared to northern European conditions. Its application leaves some challenges in the current South African electricity market at somewhere Fig. 8: Dunes facing the Atlantic ocean at Transhex with the Olifants river mouth in the background. between 50 c/kWh to 60 c/kWh. (This is without

Construction, operation and maintenance and network integration that is usually left to the owner/developer. Industry standard No fatalities have been recorded in the for turbine erection and commissioning is wind industry during operation of plant world typically 1 tower per week from start to finish. wide. Since the early 1970s the wind energy This was also the experience at Klipheuwel. industry has experienced only 14 worker A 100 MW facility consisting of 50 turbines fatalities, directly or indirectly during wind farm will thus take roughly a year to construct and construction. All of these deaths could have commission. been prevented if today’s safe work practices had been adopted. Reference from the Electricity generation from wind is still a European Wind Energy Association (EWEA). relatively new field of applied technology and significant advances are still being made. The Wind turbines are typically purchased on a mainstream (industry standard) wind turbine is turnkey basis from the OEMs internationally currently rated at around 2 MW. Application and require very little input from the of wind energy in costs roughly €1000 / kW or owner/developer during construction and $1200 / kW (installed) (Fig. 10). commissioning. This was also the case at Klipheuwel. The erection process requires a The operation of the proposed 100 MW small dedicated OEM team, large hydraulic facility requires six permanent personnel crane and operator and some rigging staff located close to the facility consisting of to assemble the modular components that four maintenance crew. Vredendal would are shipped to site in crates and containers. be ideal for the establishment of an office The only requirement before hand is the (not required on site). This figure was verified establishment of the road infrastructure by the generation division based, on its own Fig. 9: Lifting of a rotor at Klipheuwell.

energize - April 2008 - Page 42 GENERATION

Fig. 10: Report U.S. wind-turbine transaction prices over time. any subsidisation for a full commercial facility based on sound economic principles). Continued research in wind energy in South Africa must be supported at all levels, to ensure the optimal use of this renewable energy source. The current program is striving to highlight the key factors for funding, viability and resource analysis and future impact in the local South African context.

Fig. 11: Eskom’s first generator without a fence.

Public interest in the wind energy facility and renewable energy in general is on the increase. Site visits and demonstrations, various presentations and posters on site for visitors are still successfully used to disseminate information. Public awareness has increased significantly since the official opening of the facility, and visible commitment to renewable energy has been extremely well received with more than 3500 visitors officially visiting the site in guided tours to date. The worlds wind installed base is close to 94 GW (end 2007).

Acknowledgement

The authors would like to thank Eskom, Corporate Services, ERID and Research for their support of this project.

This article was presented at the SAIEE Generation Technology conference, March 2008, and is republished with permission.

References

[1] “South African Wind Atlas”; R. Ragoonanthun, D vd Westhuyzen and I Smit, Eskom Enterprizes, TSI, 2001. [2] WAsP is developed and distributed by the Wind Energy Department at Risø National Laboratory, Roskilde, Denmark, http://www.risoe.dk/ [3] “Evaluation of Wind Energy Applications in the South African Environment” Eskom Research Report, RES/TE/04/23193, 1994. [4] “Danish Wind Energy Manufacturers Association Web Page”; http://www.windpower.dk/ Contact Ian Smit, Eskom, Tel 021 980-3633, [email protected] v

energize - April 2008 - Page 43