In Central Europe an increasing number of used wind turbines are becoming available at attractive prices following repowering projects. In Europe it is difficult to reinstall these turbines because they require a lot of space and this is the main bottleneck for further expansion of wind energy. A solution could be to export these wind turbines to developing countries so that the energy produced could substitute for diesel fuel. This would decrease the dependence of these countries on more and more expensive fuel imports. Lahmeyer International has undertaken a feasibility study for a wind–diesel system, and a summary is presented in this article.
The feasibility study was elaborated on behalf of GTZ (Deutsche Gesellschaft für Technische Zusammenarbeit GmbH) and DNE (Direction Nationale d’Energie) of Mali. GTZ and DNE have conducted a wind measurement campaign of more than two years. It analysed the technical and economic conditions for introducing wind energy in the remote town of Gao, northern Mali, at the southern rim of the Sahara Desert. Electricity for the town is currently supplied by an isolated grid and fed by a 4 MW diesel generator station. The main goal of the project was to install a wind park of three wind turbines, of 300 kW each, in order to save fuel, which has to be transported over a distance of more than 2,500 km. One part of the investigation was an analysis of the technical and economical feasibility of used wind energy converters (WECs) from the repowering market in Central Europe.
A wind–diesel system consists of wind turbines, a diesel power station and an isolated grid. In contrast to grid-parallel wind farms, there is much interaction of diesel and wind power generation. In some systems storage facilities such as batteries or thermal dump loads are installed. The main objective of introducing wind energy into a diesel system is to minimise the electricity generation cost by fuel savings. The second objective is to reduce fossil fuel emissions through substitution by renewable energies. The usual size of the system is between 1 kW and 10 MW. Favourable wind conditions need to be identified within the area supplied by the grid. On the institutional side the following points have to be considered:
Gao is a small city of about 40,000 inhabitants in northern Mali. The electricity grid covers an area of 10 x 15 km, and shows a peak load of about 1.8 MW. Power is generated by an existing diesel power station of 4 MW. The diesel fuel has to be transported from Lome in Togo over two borders and a distance of 2,500 km by truck and a ferry boat over the river Niger. This leads to high specific electricity generation costs for the diesel power station and also to high transport costs for the wind turbine equipment and the crane. GTZ has made wind measurements since 2001. The average wind speed is moderate (5.1 m/s at 40 m height).
Figure 1: Medium Voltage grid in Gao
The wind farm was designed for three turbines, with a rated power of 300 KW each, feeding into an existing 15kV medium voltage distribution line at 10 km distance from the diesel power station.
Diesel power station (installed capacity 4 MW,
peak load 1.8 MW, demand 9 GWh/a)
Electricity grid - Medium voltage: 15 kV,
distance 10 km (diesel station to wind park)
Wind energy converter (wind turbine 3 x 300 kW,
production 1 GWh/a, avoided CO2 emissions 880t/a
The grid connection in wind–diesel systems has to be evaluated in more detail than in stronger interconnected high voltage systems. The dynamic effects of the wind power integration on the electrical parameters of the small fragile isolated grid (such as frequency, reactive power and flicker) can become critical. Modern variable speed generators show a better suitability concerning grid stability than constant speed asynchronous generators of used turbines and the power of these variable speed generators can be fed into existing distribution lines. For the constant speed asynchronous generators a separate feed in-line up to the bus bar at the diesel power station is recommended.
The wind park will supply about 15 % of the annual electricity demand. A load simulation indicated specific fuel demand increases, because the diesel generator sets are operated more at partial load.
If the electricity demand is lower than the wind energy offer, the wind turbines have to be pitched down partially. Therefore, only pitch-controlled wind turbines can be integrated into power control systems. Stall turbines are not suitable in this situation.
A comparison of the investment cost between new variable speed WECs and used constant speed WECs with pitch control show that the lower purchase price of a used WEC is partly offset by additional grid integration costs and a lower period of useful life. High transport and infrastructure costs reduce the economic advantage of the used WEC.
The specific investment costs were estimated to be about 1,900 €/kW, which is almost double that for commercial wind farms in interconnected grids in Central Europe. This is due to the small size of the wind farm and the specific high transport and infrastructure costs.
Because of the moderate average wind speed this results in wind power generation costs of about 0.2 €/kWh. The high transportation costs also affect the diesel power generation prices. Figure 3 shows a good economic performance of the wind farm if the petrol price is above US$ 35 per barrel. The African Development Bank has already earmarked a soft loan for the project on a public–private partnership basis. GTZ is currently in negotiation for the equity financing.
Power generation prices
In the course of a project feasibility study elaborated on behalf of GTZ (Deutsche Gesellschaft für Technische Zusammenarbeit GmbH) and DNE (Direction Nationale d’Energie) of Mali a wind–diesel system was analysed. A comparison between variable speed and constant speed WECs on the dynamic effects of grid stability in small isolated grids has shown the better suitability of the variable speed type. Power control of wind–diesel systems requires pitch-controlled WECs. The low specific investment cost of used constant speed WECs in comparison to new variable speed WECs is partly offset by higher grid connection costs. The cost structure of small wind farms in developing countries is characterised by the high proportion of infrastructure and transport costs.
However, because of site-specific high diesel power generation costs, even wind–diesel systems with moderate wind potential are economically viable. The techno-economic concept developed could be transferred to many other isolated power systems in the Sahel region.
Cattle market of Touareg Nomads near Gao
This article appeared in the volume 1, Nr. 4 / 2005 issue of Windtech International and is republished with permission of the authors and publisher.