Limestone quarry becomes a wind farm
by T Wizelius, Vindform, Sweden

One hundred metres from the coast at Smöjen in eastern Gotland, an abandoned limestone quarry has been turned into a wind farm. Ten turbines with a total capacity of 10 MW began delivering power to the grid at the end of August 1999.

Background

The coastal quarry, which was closed in 1962, is an ideal location for a large wind farm. The company Siral first applied for permission to install wind turbines here in 1991, when wind power development in Sweden was in its infancy. At the time, the military authorities refused for strategic reasons. However, a few years later their viewpoint changed and they allowed two Enercon E40, 500 kW wind turbines to be installed in December 1995. Later, permission was given to erect more turbines, on the condition that these would be dismantled at short notice should a military crisis arise in the area. In 1997, both the local community and regional authority agreed to allow installation of a larger wind farm and Slitevind AB, who own and operate the wind farm, asked manufacturers and the local utility GEAB to submit tenders. Last summer, Siral’s plans were finally fully realised when the two Enercon turbines were joined by eight larger Vestas turbines, comprising four 660 kW and four 1.65 MW machines.

The installation

Construction of the site infrastructure began in October 1998, once Slitevind had accepted a tender from the Danish turbine manufacturer Vestas. Access roads were built and trees felled to accommodate the foundations and allow room for the mobile cranes and for mounting the rotors. During March 1999, the foundations were excavated with the use of explosives. The holes were then lined with 90 tonnes of steel shuttering and cast with 1300 m3 of concrete. By early June the concrete had hardened, the turbines had arrived and mounting began.

The four 660 kW turbines were installed in a single day, using an ordinary 400 ton mobile crane. Heavier equipment was needed to install the huge 1.65 MW turbines and a special crane, with an 80 m high tower, was shipped from Denmark specifically for the task. Installation was carried out by an experienced crew employed solely to install Vestas turbines.

The rotor, with its three 30 m blades, was first assembled on the ground. The crane then lifted the rotor so that it hung perpendicular, while two teams used ropes to pull it into the right position. The rotor was lifted until its centre was 67 m above ground level at the height of the nacelle. Finally, the crane turned 180°, allowing the rotor to be moved carefully into place at the top of the tower.

It took 24 hours to dismantle, move and reassemble this crane in order to transport it just a few hundred metres to the next turbine location. Nonetheless, all eight new turbines were mounted within a fortnight, thanks to non-stop shift work by the 20-man crew. Electrical installations were completed during July and August and by late August all turbines were connected to the grid and the whole wind farm was on-line.

Grid connection

During the site construction phase, the local utility GEAB and other firms were preparing to connect the wind farm to the grid. The wind farm is expected to produce some 27 GWh a year, so grid connection has been a significant issue. Although negotiations with GEAB about grid connection took some time, the final technical solution was both better and cheaper than expected.

In the past, wind turbines have usually had an external transformer, where voltage from the turbine (normally 690 V) has been raised to that of the grid. However, the eight new turbines at Smöjen have internal transformers: in the 660 kW turbines this is located in the “cellar” below the door to the tower, while in the 1.65 MW turbines the transformer is mounted in the nacelle, on top of the tower. The turbines deliver power at 20 kV, through cables cut into the limestone. These are connected firstly to a common interlocking-plant and from there by a common 4.5 km long underground cable to another interlocking-plant, where the voltage is raised from 20 to 70 kV and the wind farm is connected to the grid.

The ten turbines have a total installed capacity of 9.64 MW, just below the limit allowed by the Swedish government. The four largest turbines are only allowed to run at 1.5 MW – the limit under Swedish law for small-scale power plants and also in the rules relating to grid connection and payment. If the turbines were run at full capacity GEAB could, and would, refuse to buy the power produced.

Economics

The total cost for the eight new turbines at Smöjen amounted to 80 million SEK (9.6 million US$), which includes grid connection costing 11 million SEK. Of this, some 10 million SEK has been granted by the state as an investment subsidy.

Environmental factors

At Smöjen, the combined use of different sized turbines has solved one particular environmental concern. The larger turbines are located within the quarry itself, with only four of the smaller turbines situated in the surrounding forest, thus reducing the amount of tree clearance required.

Visitors to the wind farm find it almost impossible to differentiate between the large and small turbines, which all appear to be of the same size. This is an optical illusion created by two factors: firstly, the distance between the turbines, which stand in two parallel lines, is the same and secondly, the proportions between tower height and rotor diameter are identical.

Local inhabitants have been kept well informed about the wind farm through public meetings and have been consulted at different stages throughout the project. The wind farm is generally viewed far more favourably than the previous quarrying activities at the site.

For more information contact Mats Johansson, KanEnergi Sweden AB, PO Box 41, S-532 21 Skara, Sweden. Tel: +46 511 347 873; Fax: +46 511 347 665; e-mail: mats.johansson@kanenergi.se

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to Pauline Toole, Editor, CADDET Centre for  Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.