Suorva – arctic wind turbine in northern Sweden
by J Norling, Vattenfall AB, Sweden

The wind conditions in parts of northern Finland and Sweden make these areas attractive for wind turbine installations. However, little is known about the problems associated with building and operating wind turbines in prevailing arctic conditions. Now, a Swedish utility company has built its first arctic wind turbine with the aim of gaining knowledge and experience in this area.

The arctic equipped wind turbine at Suorva

Introduction

Northern Scandinavia boasts several arctic wind power plants – in Finland, Norway and Sweden – but question-marks over public acceptance, environmental impact, acoustics, power performance, icing, availability, operation and maintenance costs etc mean that the future of this technology is uncertain. The Suorva project was started with the aim of tackling these uncertainties.

The project

In October 1998, the Vattenfall company built a wind turbine equipped for arctic conditions at a Swedish site called Suorva, 100 km north of the Arctic Circle near the Stora Lule River. Suorva is 110 km from the nearest village, Porjus. The existing local 10 kV grid and road are only 300 m from the turbine site.

The project was conceived in 1992 when a local group explored the possibility of building a wind power plant to exploit the area’s favourable wind conditions. In 1995, a 36 m meteorological measurement mast was built on a ridge between two hydro-power dams. The positive results led to discussions with Vattenfall regarding the building of a wind turbine and, in late June 1998, the necessary permission and support from the authorities was obtained.

The project is being financed jointly by Vattenfall and by the Swedish State Energy authority, Energimyndigheten. The project budget is SEK 6 million (where SEK is the Swedish krona), with 30% of this provided by Energimyndigheten. Vattenfall is responsible for overall management of the project using both its own experts and those from external organisations.

In early August 1998, the company signed a contract with Bonus Energy A/S to deliver an arctic-equipped 600 kW Mk IV wind turbine to Suorva. Work started on 18 August, with a proposed erection date of 8 October. Despite the risk of long delays in building due to high wind speeds and snow, the turbine was completed on schedule. On 13 October 1998, the wind turbine delivered electricity to the grid for the first time. An evaluation programme, running from 1999 to 2001, covers:

  • operation and maintenance;
  • public acceptance;
  • environmental impact;
  • power performance;
  • ice – loads/stresses;
  • acoustics.
The site

Mountain peaks of 1,400–1,500 m above sea level lie within 5–6 km of the Suorva site, to the south-west and north-east. The base of the turbine is

470 m above sea level on a small ridge in the valley. North-west of the site is Sweden’s largest constructed water reservoir which is used for hydro-power. Results from the first year’s measurements of wind conditions showed that the annual mean wind speed at a height of 35 m was about 7.5 m/s, with the highest observed wind speed being

32 m/s. Channelling effects in wind distribution in the direction of the valley were observed. Icing conditions, defined as occasions with a relative humidity above 95% and a temperature below 0¡C, were experienced 2–4% of the time.

Wind speed is measured on a mountain 7–8 km south-east of the turbine by a 50 m mast located 900 m above sea level. As an example of the very high wind speeds, measurements on the mountain made in December 1997 showed one-minute average wind speeds exceeding 50 m/s during a five-hour period before the anemometer broke down.

The people that live and visit the area are reindeer herders, service personnel for two hydro-power plants and tourists. Their opinion about wind power in this area is being assessed by interviews. During the first year, the media were positive about the Suorva project.

Power plant data

The turbine is a three-bladed Bonus Mk IV installed with arctic equipment. The rotor is a 44 m diameter upwind model with 1,520 m2 of swept area. Rotor speed is 27 rpm or 18 rpm, depending on which generator is used. Output is 600 kW from an asynchronous generator at 690 V. This is transmitted by a 20 m cable to an 800 kVA transformer (690 V to 12 kV).

The transformer housing is coloured grey to minimise its visual impact on the landscape. The housing is anchored to the rocks in each corner with steel wires to avoid the risk of being moved by strong winds.

Erecting the wind turbine

The three 19 m blades have “JE-system” heating that uses a carbon-fibre surface on the outer part of the blades and on the leading edge. Heating is regulated by a control system triggered by signals from an ice-detector and thermistors between the carbon fibre and the original glass-fibre surface. If the ice-detector shows ice and the wind speed is below 11 m/s, the heating is 9 kW on each blade; above 11 m/s, the heating is 15 kW.

The arctic equipment also includes a heated wind vane and anemometer and a heated gearbox and control system. The lubricants are of the low-temperature type with low viscosity.

To control the site there are two telephone channels – one for the meteorological mast system and the other for the data acquisition system, control system, energy measurement system and the separate fault alarm system.

Measured parameters are wind speed and direction, temperature, output power, flap and edgewise bending on the blades, torque on the main shaft, and parameters from the anti-ice system; extended meteorological measurements are taken within a 10 km radius of the turbine.

Experience gained

During 1999, the Suorva wind turbine’s energy production was exceeded by only two other larger turbines in Sweden; it produced 1,806 MWh. However, this may have been a temporary peak due to an unusually long period of high-pressure weather conditions in southern Sweden, which pushed low-pressure fronts northwards.

During the start-up phase, some problems occurred in the blade-heating system, which have now been solved. Other problems encountered were low temperatures and moisture in the foot of the tower, and snow penetrating into the transformer room. High-power-effect problems have been solved by turning the blades 1.5¡ and assembling extra 10 mm triangular stall-strips on the front side of the blades to reduce output power.

The availability was 95% during 1999. The system is constructed to cope with temperatures down to –20¡C. During 1999, there were 9–10 days below this temperature that led to the turbine being shut down. The lowest temperature so far has been –37¡C. The energy demand for de-icing has, so far, been less than 3% of the generated energy.

For more information contact Jan Norling, Vattenfall AB. Tel: +46 26 835 21; Fax: +46 26 836 70; e-mail: jan.norling@utveckling.vattenfall.se; or the CADDET Swedish National Team in Stockholm.

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