To avoid coming into contact with the support towers, longer blades have to be cantilevered away and as the blade length increases beyond, say 50m, it will probably be necessary to use cfrp or a glass/carbon hybrid and the cost will be the deciding factor. An average wind speed is 8m/s and any changes are an important factor in controlling the cost. A decrease of about 1m/s results in a cost increase of 33%, whilst an increase of 1m/s provides a reduction of 25%. The power generated is a function of the cube of the wind speed.
Wind generators come in a range of sizes with rotors as small as 0.5m diameter to as large as 90m diameter. The smaller sizes produce a DC supply, whilst the larger industrial and utility designs produce an AC supply.
Vestas, the world’s largest wind turbine manufacturer, markets a range of turbines from 660 kW-2.0 MW and have a model with 47m diameter blades, generating a nominal output of 660kW at 1650 rmp at an optimum wind speed of about 15m/s.
In Denmark, Vestas have a joint venture with a 40% share of Gamesa Eolica s.a. in Spain, who manufactures carbon fiber wind turbine blades. Another front runner is Danish manufacturers NEG Micon A/S, who acquired the Danish company Wind World in 1997 and Nedwind of the Nether lands in 1998.
Wind for the next millennium is discussed and Gallet and Hamlyn describe a new approach in filament winding for large complex parts.
Sound level, both mechanical from gearbox, or generator and aerodynamic from swish, when a blade passes the tower has to be taken into account when situated near dwellings.
The Aeromax Corp, has developed stealth acoustic carbon fiber blades, molding the cfrp to slim aerodynamic profiles. The safety aspect is enhanced by using cfrp, which keeps the rotational inertia low even at high speeds. The possibility of lightening strike will have to bo accommodated and the longer blades may have to be made on site due to difficulties in transporting by road. It is interesting that the factory of Aerolaminates on the Isle of Wight, UK is situated next to a navigable river, permitting transportation by sea.
There is, however, a possible market for small wind machines to supply individual homes and outlying farms when cfrp becomes a feasible proposition.
Tidal turbine blades: Marine current turbines derive their power from the tides, which unlike the wind flow, is in two directions only. A major difference is that since water is some 800 times denser than air, the thrust on the blades is so much greater and the blades tend to be shorter and sturdier. The turbines are designed to operate in a current of 4.5 knots (2.3m/s) but a 1MW turbine operating at full power would have to withstand some 900 tons/s of water passing through its blades, which is indeed onerous.
Composites are an ideal choice for the blades and Marine Current Turbines Ltd, are operating a twin axial flow rotor 11m diameter trial unit driving a generator via a gearbox with a coefficient of performance of 40-45% and an energy capture some 27% better than expected. To achieve these results, the blades are stiffened using carbon fiber in the main spar, surrounded by a grp envelope made in two halves and bonded together to give the blade the requisite hydrodynamic shape, with the grp envelope supported by carbon fiber reinforced ribs attached to the main spar. Although this is early days, the system does appear to have great promise.