Project Size and Water Depth Effect on the Costs

By building at sea the foundation and costs of connection to the net are great. The foundation costs can be decreased by building large plants: the 1.5 MW power plant foundation is very reasonable compared to three 0.5 MW plant foundations. It is estimated that in the Tunö Knob case the 1.5 MW unit size and the newest foundation technology should decrease the costs from 6.39 c/kWh to 4.20 c/kWh. It is possible that wind power plant size will increase even further to several megawatts. This will in particular improve the economy of offshore parks. In this case the foundation costs in relationship to produced energy will decrease even further.

It will also be advisable to build large plants also from the point of view of ice loads.

When the 600 kW size is changed up to the megawatt class the ice loads will only increase a little compared to the growth of the wind turbine. The ice loads rise in ratio to the ice faced area (in the 600 kW plant the tower base diameter is 3 - 3.5 m and in the 1.5 MW class 4 m). The wind forces are in ratio to the height and diameter of the rotor (a 600 kW plant height is 50 m and rotor diameter 40 m: a 1.5 MW plant height is 60–

80 m and diameter 57–66 m). The megawatt class power plant base load is significantly bigger than that of a half megawatt power plant. The ice load effect decreases in the total load when the power station size increases.

In the case of large wind parks all fixed costs will be divided by the greater energy amount produced. Also the electricity net costs favour big project sizes. Then the sea cable costs will be divided between several units. For example in the case of the Kokkola wind park the net joining costs of 10 plants are estimated to be 185 000 / plant, in the case of 20 plants only 100 000 / plant and with 30 plants 75 000 / plant.

When the distance of wind turbines is further from the coast, the costs of joining to the electricity net also grow. For example in Denmark 7.5 MW offshore park costs are calculated to be 4.37 c/kWh when the distance is 5 km and 5.89 c/kWh when the distance is 30 km.

When the wind park size grows the cost decreases per produced power. For example 200 MW offshore calculated costs vary from 3.70–3.89 c/kWh when the distance to the continent is 5–30 km (Morthorst et al. 1997: 204).

Figure 28 shows how a foundation in deep water increases the costs. The water depth effects on the foundation costs are estimated to be 12–34 % when changing from a 5 m to 11 m depth (Lemming 1997: 41).

Figure 28. Three base type foundation costs at Horns Rev location (Lemming 1997:

41).

The technical potential of offshore wind power is on the Finnish Perämeri in shallow water areas. Between Vaasa and Tornio there is a potential of over 40 TWh / a. In this case a 7 m/s medium wind speed is demanded, a depth of water under 10 m and thickness of moving ice under 40 cm. The demands can be fulfilled in this area and the area could be filled with wind power plants (nearly 2000 km, over 11 000 power stations). In practice suitable areas are much more limited when we take into considera-tion the limitaconsidera-tions of using the area (among others navigaconsidera-tion, nature protecconsidera-tion and defence forces).

The offshore foundations are remarkably more demanding than the foundations being built on land. When planning the construct we need to take into consideration all the environmental effects on the construct. In the building material and surface treatment we should pay attention to the corrosion of the steel and the effects of freezing water on a concrete interstice and to the erosion of the bottom. The roads to the wind power plant should be useable both in reasonable surf and in winter time during the period when the ice is frozen. With the design of the foundation the ice loads can be decreased significantly. The design should pay regard to both local high ice loads and from the total ice loads accumulated over a wide area, both static and dynamic. Moving ice under 40 cm thick, which will crush against the ice cone, will affect loads under 1 MN.

The transportation of the foundation and wind turbine to the site increases costs because the portable and lifted mass is big. The foundation weighs hundreds of tons as ready-made concrete and thousands of tons filled with mass. The wind turbine weight is about 200 tons. The tower height is 50–80 m.

The cost of a one megawatt class foundation is 0.25–0.42 million including the assembly of foundation and preparation of sea bottom. For the 10 plant sea wind park preliminary cost estimates show that offshore wind power is still clearly more expensive than building wind power on land, 5.38–5.89 c/kWh compared to 4.37 c/kWh. By building big offshore parks the costs per produced kWh drop. For example in Denmark offshore parks will be built with over 100 turbines. The estimated production cost is about 5.05 c/kWh (on land about 3.36 c/kWh). At this moment 1.5 MW power plants are for sale and especially for offshore purposes 2–3 MW units are available. By development of foundation technology and by building large units at sea it is possible to

reach the same production costs as wind power plants built on land especially when the best places on the coast are already built on (Holttinen et al 1998: 110–113).