Bulk wind power generation
By Hermann F.W. Oelsner, Darling IPP (Pty) Ltd

Wind power is arguably the most advanced and commercially available of all renewable technologies. In recent years it has been the world’s largest growing energy source, with an average annual growth rate in sales of wind turbines of 40% over the past six years.

Our research has shown that for South African conditions wind energy is the most promising and presently most economic of all renewable technologies for bulk energy electricity generation.

Global status of wind power
By the end of 2001, more than 23,000 MW of electricity generating wind turbines were operating in almost fifty countries around the world.

The most successful markets for wind energy in recent years have been in Europe, particularly Denmark, Germany and Spain. There has also been an upsurge in the use of the technology in the United States, as well as in many developing countries, including India, China and South America.

In recognition of its environmental advantages, many countries have supported wind energy developments with government-backed incentives. The aim of these has been to stimulate the market, reduce costs and compensate for the unfair advantage currently held by conventional fuels.

A number of scientific assessments have shown that the world’s wind resources are well spread throughout six continents. The total available wind resource in the world today that is technically recoverable is 53,000 TWh per year – about four times bigger than the world’s entire electricity consumption in 1998. It is clear that the world’s wind resources are unlikely ever to be a limiting factor in the utilisation of wind power for electricity production.

South African status of wind power
South Africa is blessed with abundant wind, sun and waves. Strong steady winds blow especially along the coastline.
A dominant topographic feature is the so-called Great Escarpment, which divides the interior plateau region from the coastal areas. These coastal areas range from sandy flats to hilly terrain to steep mountain ranges higher than 2,500 m. This, together with the temperature difference between ocean and land, results in an excellent wind regime, in particular on the West Coast with the cold Atlantic.

The existing situation is that there are no large wind turbines installed yet. But there are more than 300,000 windmills installed, mainly for borehole water pumping on farms.

Following Europe’s example a realistic target would be to have approximately 4,000 MW of wind turbine generating capacity installed by the year 2020. This would amount to about 10% of presently installed total generation capacity.

Darling national demonstration Wind farm project
In 1996 the Oelsner Group identified a site on Moedmag Hill near Darling in the Western Cape as an area well suited to the siting of a wind farm.

Seventeen months of wind measurements were taken using two monitoring systems in parallel. Results were correlated with 10 years of historical wind data from Cape Town Airport, Koeberg Nuclear Power Station and Namaqua Sands at Saldhana. The result is an excellent wind regime of an average wind speed in excess of 7.5 m/s at 50-metre hub height, depending on the location on the hill.

The capacity factor is 34 %. For a 5.2 MW capacity installation a conservatively estimated output of 13.5 GWh per year is predicted.

The pattern of the electricity generation will match the demand patterns of the area by being higher in the late afternoon when local demand starts to peak. There is also a good match of supply with demand on an annual basis since the wind is stronger in the summer months when an influx of tourists comes to the coastal towns nearby.

Connecting wind power to the national grid
Eskom is responsible for the safe and economic operation of the national grid and has obligations to maintain satisfactory quality and supply to the users of the system. Eskom is not necessarily the purchaser of the electricity generated by the wind turbines.

From the point of view of the electricity network, the most important factors are:

• the rating, which is the maximum continuous output in kilowatts
• whether the turbines operate at fixed (single or two speeds) or variable speed
• whether the turbine is stall-regulated or pitch-regulated.

Wind turbines are usually located in rural areas, where the electrical connection to the nearest electricity substation can be weak and where the local demand for electricity may be much less than the wind generation capacity. One way of defining the “strength” of the electricity network is by the fault level, which is a measure of the current that will flow when there is a fault on a network. At a low fault level site, the impact of the wind turbines can be great enough to disturb other local consumers. For this reason it is sometimes necessary to reinforce the network, or to connect the wind turbines to a higher voltage or stronger part of the network further away. This will increase the costs. 

Higher voltage systems such as the 400 kV or 275 kV transmission systems have high fault levels. In general, the lower the voltage, the weaker the system. For most of South Africa’s rural areas, the distribution voltages are 66 kV and 11 kV. The 11 kV system is the most extensive, but in rural areas is unlikely to support more than three to five megawatts of generation.

In the case of the Darling wind farm, the weak feeder line from Darling to Yzerfontein is now supplemented by an additional 66 kV line, allowing easy and stable access for the planned 13 MW wind power plant.

The effects of wind turbines on electricity systems
Factors, which need to be considered, are listed below. Their relative importance will depend on the local elec-tricity network, and the choice of wind turbine technology.

Network voltage range
Electricity networks were designed to transfer power from large generators on higher voltage systems to customers distributed on lower voltage systems. If power is transported in the other direction, the voltage experienced by consumers nearby on the network could go outside the statutory limits. To ensure there are no problems for consumers, Eskom needs to know the proposed maximum output and the power factor. For larger projects, additional power-factor correction capacitors or static VAr compensators may be worthwhile. Some variable-speed turbines provide control of their power factor, and may be more acceptable in this respect.

Voltage fluctuations
When they are running, the output power of wind turbines varies second by second, depending on the strength and turbulence of the wind. The effect of the tower as the blades rotate past it also introduces a periodic disturbance in the power output, which is greater at higher wind speeds. These power fluctuations are among the factors causing voltage variations on the local electricity network, termed flicker.

Flicker is only likely to be a problem for small groups or single turbines, especially large machines connected at lower voltages. Stall regulated wind turbines tend to produce less disturbances than pitch-regulated turbines, depending on the exact design.

Any concerns that wind turbines with induction generators (as is most common) will also cause disturbances when starting, is largely a problem of the past, as “soft-start” units are fitted to most designs. However, the voltage step that will occur when a wind turbine shuts down from full output, perhaps due to high winds, must also be considered. As the number of turbines per installation increases, the probability of short term fluctuations decreases. 

Harmonics
Variable speed wind turbines can cause harmonic voltages to appear on the network, which can cause equipment to malfunction or overheat. Fitting filters at additional costs can reduce the problem. Fixed-speed wind turbines will generate harmonics for the short periods when their soft-start units are in use, but this is generally not significant.

Thermal ratings
Overhead lines, cables and transformers are designed to transport power up to a certain maximum rating. This sets an upper limit on the power that may be “exported” from a wind farm without expensive system reinforcement.

Fault contribution
The fault current, which will flow from wind turbines to a fault on a network, generally will be very much less than will flow from within the network itself, and is not usually a constraint.

Voltage unbalance
This is a problem caused by the network, rather than by the wind turbines. There could be locations where the voltages on each of the three phases are unequal by more than the regulated amount. This can cause wind turbines to shut down to protect against generator overheating, thus reducing income to the wind turbine owner.

Benefits of wind power electricity generation
Embedded generation
All wind farms and most other renewables projects are embedded generators, i.e. that they feed into the electricity supply system at the level of the lower voltage distribution network. Embedded generation can bring a number of advantages, both economic and strategic, over centralised generation (where the feed is feed is at the level of the high voltage transmission network), although the extent of these advantages depends on where the embedded generator is located in the network.

Pay-back period
Most favourable use of energy is in the fact that the energy used to manufacture and erect wind turbines will be recovered after 2 to 3 months of operation, the so-called pay-back period.

Job creation
Worldwide experience demonstrates that wind energy has a very high job creation effect, creating 10 times more jobs than nuclear and four times more jobs than coal fired power plants. Compared with large central power stations, the construction of many small power generation plants results in repetition of work processes for design, production, planning, building permission, marketing, selling, service, maintenance and operation control. Certain components for example, like rotor blades, are manufactured in very labour intensive processes to reach the required high quality standards.

Reduction in wind turbine prices
The economics of wind energy are already very strong, despite the youth of the industry. The downward trend in costs is predicted to continue. As the world market in wind turbines continues to boom, wind turbine prices will continue to fall.

Foreign investment and export potential
There is an outstanding opportunity for South Africa to establish a new exciting industry and join the rapidly expanding global wind energy market, creating employment through the export of wind energy goods and services. In Eskom’s recent EIA report on its wind turbine test installation near Cape Town, a conservative figure of 1,000 MW roll-out for the next decade is quoted. This is equal to a capital investment of R10 billion at today’s prices.

Clean development mechanism
Due to the high fossil fuel content of its power generation, South Africa will be very attractive as a partner for European countries to buy carbon credits under Article 12 (dealing with the Clean Development Mechanism) of the Koyoto Protocol. The value of these credits is estimated to be able to cover up to 20 % of total project capital costs.

Environmental benefits 
Avoidance of greenhouse gases contributes to a reduction in global warming. Avoidance of pollution and savings of natural resources from a 10 MW Darling Wind Farm (annual production of 27 GWh) are as follows:

 

Wind energy uses land resources sparingly, because 99% of the land can still be used for farming and grazing as usual. The ecological impact can easily be minimised during construction and the restoration of surrounding landscapes has become a routine task. The scrap value covers the cost of decommissioning, dismantling and restoring the site.

The final question: What happens when the wind does not blow?
Wind farms are not replacing generation capacity, but are generators of electric energy. The wind blows in general during standard and peak electricity demand periods.

At times when there is no wind, back-up supply from central power stations must be supplied, most of the time from base load generation in off-peak periods.

With its advantage of 99% availability, generated electricity can be predicted on a 24-hour basis from weather forecasts.