The first commercially available wind turbines were designed and manufactured by Danish engineers, who were mainly in the agricultural equipment industry. Spurred by the two ‘oil shocks’ of the 1970s, the Danish Government created policies to promote wind turbine research and development, allow access to the grid with automatic grid access and create a good investment climate for farmers and small co-ops with long-term power purchase agreements. These Danish policies collectively started the modern era of wind turbine development.
In the early 1980s, tax incentives in California created a ‘wind rush’ and thousands of sub 100 kilowatt (kW) machines were installed. These came mainly from Denmark and the United States, but it was largely the Danish turbines that were technically successful. Many of these 20-year old turbines are still in operation. The original Danish turbines were 55 kW, with 15 metre rotor diameters, fixed pitch (stall regulated) and fixed speed with induction generators and hub heights of 18 metres. During this period the turbines increased in size from approximately 50 to 250 kW.
In the mid 1990s the first megawatt machines were designed and manufactured. The impetus was from a European Union technology development incentive to produce megawatt machines. The following decade saw the industry accelerate towards developing larger, more sophisticated wind turbines and the market now offers machines in the range of 2 to 6 megawatt (MW) turbines with 80–120 metre rotor diameters. The Enercon E-112 6 MW machine has a 112 metre diameter rotor and a 120 metre hub height. In generator rating, this machine is over 50 times larger than the early 1980s turbines.
Building bigger turbines
There are many reasons (mostly economic) why wind turbines are being built with larger generating capacities.
Firstly, consider the physics. The energy produced by a turbine is proportional to the swept area of the rotor which is proportional to the square of its rotor diameter. A taller tower also puts the rotor into higher wind speeds, so it makes sense to have fewer larger and taller turbines than many small ones. Moreover, fewer turbines mean fewer foundations and expensive erection operations, less power connection cabling and (in theory) lower maintenance costs. Moreover, some control system components do not increase in price with turbine size. Aesthetics is also a consideration in choosing fewer, slower spinning larger turbines over more, faster spinning smaller models.
If this is the case, why not just scale up all the components? Unfortunately it has not been that straight forward. All wind turbine manufacturers have found that the simple fixed speed, stall regulated design so successful up to 1 MW could not be used for a variety of reasons for much larger turbines. As a result, most turbines of 1.5 MW and above now operate with variable pitch power regulation and variable speed rotors. This has resulted in more sophisticated electrical and drive train designs and more complex control systems.
Not only must technology advance to overcome the physical limitations in scaling up turbines, it must advance to challenge economic limitations. Garrad Hassan’s work on turbine design and manufacturing costs has shown that the energy capture increases with the square of the rotor diameter, while wind turbine costs increase with weight which is roughly proportional to the cube of the rotor diameter. At a certain limit, the latter relationship takes over. It is in fact this limit which is being pushed higher and higher by increased turbine efficiency and higher power to weight ratios which have resulted from advances in technology. In 1990 Vestas were responsible for a significant technological breakthrough in reducing the weight of their blades for its V39-500 kW turbine. Lighter weight blades and lighter weight nacelles are now the design objective for all manufacturers. Additionally, the performance of wind turbine rotors has become more aerodynamically efficient. Since the early 1980s the efficiency of converting wind kinetic energy into electrical energy has increased significantly and now a number of modern turbines have coefficients of performance exceeding 50 per cent.
The other driver of lower costs has been volume, creating significant economies of scale. In the early 1980s the industry was manufacturing in the order of several hundred MW per annum. During 2007 the generation capacity of wind turbines manufactured was almost 20 gigawatts (GW), roughly a 100-fold increase. At the same time, wind farm projects have grown significantly and in the same order-of-magnitude increase.
25 years of development
A major outcome of the last 25 years of technology development, volume production and wind farm size has been lower electricity costs from wind energy, and this has been one of the main drivers to the market achieving 100 GW of installed capacity with annual increases of 20-30 per cent over much of this period.
Where will wind turbine size go in the future? Turbines will continue to grow in size, particularly for offshore wind farms. There are designs in progress for 10 MW offshore wind turbines. There is no reason to believe that these are not technically and commercially viable for offshore sites. Land based wind farms will impose lower limits on turbine size, but simple terrain with good transport routes to large ports will allow the deployment of very large machines.
Finally, as the technology extends into diverse markets there will also be an increasing need for turbines of less than 1 MW, particularly where road infrastructure is poor, the availability of large cranes is limited or grid size limits installed capacity.
