As a result, typical wind turbine nameplate capacity ratings have increased dramatically since the 1980s (from roughly 75 kW to 1.5 MW and larger), while the cost of wind energy has substantially declined. Onshore wind energy technology is already being manufactured and deployed on a wind farm commercial basis. Nonetheless, additional R&D advances are anticipated, and are expected to further reduce the cost of wind energy while enhancing system and component performance and reliability.
Offshore wind farm technology is still developing, with greater opportunities for additional advancement. This section summarizes the historical development and current technology status of large grid connected on- and offshore wind turbines, discusses international wind energy technology standards, and reviews power conversion and related grid connection issues; a later section describes opportunities for further technical advances.
Basic design principles
Generating electricity from the wind requires that the kinetic energy of moving air be converted to mechanical and then electrical energy, thus the engineering challenge for the wind power industry is to design cost-effective wind turbines and wind farm plants to perform this conversion. The amount of kinetic energy in the wind that is theoretically available for extraction increases with the cube of wind speed.
However, a wind turbine only captures a portion of that available energy. Specifically, modern large wind turbines typically employ rotors that start extracting energy from the wind at speeds of roughly 3 to 4 m/s (cut-in speed). The Lanchester-Betz limit provides a theoretical upper limit (59.3%) on the amount of energy that can be extracted.
A wind turbine increases power production with wind speed until it reaches its rated power level, often corresponding to a wind speed of 11 to 15 m/s. At still-higher wind speeds, control systems limit power output to prevent overloading the wind turbine, either through stall control, pitching the blades, or a combination of both. Most wind turbines then stop producing energy at wind speeds of approximately 20 to 25 m/s (cut-out speed) to limit loads on the rotor and prevent damage to the turbine’s structural components.
Wind turbine design has centred on maximizing energy capture over the range of wind speeds experienced by wind turbines, while seeking to minimize the cost of wind energy. Increased generator capacity leads to greater energy capture when the turbine is operating at rated power (Region III). Larger rotor diameters for a given generator capacity, meanwhile, as well as aerodynamic design improvements, yield greater energy capture at lower wind speeds (Region II), reducing the wind speed at which rated power is achieved.
Variable speed operation allows energy extraction at peak efficiency over a wider range of wind speeds (Region II). Finally, because the average wind speed at a given location varies with the height above ground level, taller towers typically lead to increased energy capture.
To minimize cost, wind turbine design is also motivated by a desire to reduce materials usage while continuing to increase turbine size, increase component and system reliability, and improve wind power plant operations. A system-level design and analysis approach is necessary to optimize wind turbine technology, power plant installation and O&M procedures for individual wind turbines and entire wind farm plants.
Moreover, optimizing wind turbine and wind farm design for specific site conditions has become common as wind turbines, wind power plants and the wind energy market have all increased in size; site-specific conditions that can impact turbine and plant design include geographic and temporal variations in wind speed, site topography and access, interactions among individual wind turbines due to wake effects, and integration into the larger electricity system. Wind turbine and power plant design also impacts and is impacted by noise, visual, environmental and public acceptance issues.
Onshore wind energy technology
In the 1970s and 1980s, a variety of onshore wind turbine configurations were investigated, including both horizontal and vertical axis designs. Gradually, the horizontal axis design came to dominate, although configurations varied, in particular the number of blades and whether those blades were oriented upwind or downwind of the tower.
After a period of further consolidation, wind turbine designs largely centred (with some notable exceptions) around the three-blade, upwind rotor; locating the turbine blades upwind of the tower prevents the tower from blocking wind flow onto the blades and producing extra aerodynamic noise and loading, while three-bladed machines typically have lower noise emissions than two-bladed machines. The three blades are attached to a hub and main shaft, from which power is transferred (sometimes through a gearbox, depending on design) to a generator.
The main shaft and main bearings, gearbox, generator and control system are contained within a housing called the nacelle. In wind turbines without a gearbox, the rotor is mounted directly on the generator shaft.
In the 1980s, larger machines were rated at around 100 kW and primarily relied on aerodynamic blade stall to control power production from the fixed blades. These wind turbines generally operated at one or two rotational speeds. As turbine size increased over time, development went from stall control to full-span pitch control in which turbine output is controlled by pitching (i.e., rotating) the blades along their long axis.
In addition, a reduction in the cost of power electronics allowed variable speed wind turbine operation. Initially, variable speeds were used to smooth out the torque fluctuations in the drive train caused by wind turbulence and to allow more efficient operation in variable and gusty winds. More recently, almost all electric system operators require the continued operation of large wind farm plants during electrical faults, together with being able to provide reactive power: these requirements have accelerated the adoption of variable-speed operation with power electronic conversion.
Modern wind turbines typically operate at variable speeds using full-span blade pitch control. Blades are commonly constructed with composite materials, and towers are usually tubular steel structures that taper from the base to the nacelle at the top.
Over the past 30 years, average wind turbine size has grown significantly, with the largest fraction of onshore wind turbines installed globally in 2009 having a rated capacity of 1.5 to 2.5 MW; the average size of wind turbines installed in 2009 was 1.6 MW. As of 2010, wind turbines used onshore typically stand on 50- to 100-m towers, with rotors that are often 50 to 100 m in diameter; commercial machines with rotor diameters and tower heights in excess of 125 m are operating, and even larger machines are under development.
Modern wind turbines operate with rotational speeds ranging from 12 to 20 revolutions per minute (RPM), which compares to the faster and potentially more visually disruptive speeds exceeding 60 RPM common of the smaller wind turbines installed during the 1980s.
Onshore wind turbines are typically grouped together into wind power plants, sometimes also called wind farm projects or wind farms. These wind power plants are often 5 to 300 MW in size, though smaller and larger wind farm plants do exist.
The main reason for the continual increase in turbine size to this point has been to minimize the levelized generation cost of wind energy by: increasing electricity production (taller towers provide access to a higher-quality wind resource, and larger rotors allow a greater exploitation of those winds as well as more cost-effective exploitation of lower-quality wind resource sites); reducing investment costs per unit of capacity (installation of a fewer number of larger wind turbines can, to a point, reduce overall investment costs); and reducing O&M costs (larger turbines can reduce maintenance costs per unit of capacity).
For onshore wind turbines, however, additional growth in wind turbine size may ultimately be limited by not only engineering and materials usage constraints, but also by the logistical constraints (or cost of resolving those constraints) of transporting the very large blades, tower, and nacelle components by road, as well as the cost of and difficulty in obtaining large cranes to lift the components into place. These same constraints are not as binding for offshore wind turbines, so future turbine scaling to the sizes are more likely to be driven by offshore wind turbine design considerations.
As a result of these and other developments, onshore wind energy technology is already being commercially manufactured and deployed on a large scale. Moreover, modern wind turbines have nearly reached the theoretical maximum of aerodynamic efficiency, with the coefficient of performance rising from 0.44 in the 1980s to about 0.50 by the mid 2000s.
The value of 0.50 is near the practical limit dictated by the drag of aerofoils and compares with the Lanchester-Betz theoretical limit of 0.593. The design requirement for wind turbines is normally 20 years with 4,000 to 7,000 hours of operation (at and below rated power) each year depending on the characteristics of the local wind resource. Given the challenges of reliably meeting this design requirement, O&M teams work to maintain high plant availability despite component failure rates that have, in some instances, been higher than expected. Though wind turbines are reportedly under-performing in some contexts, data collected through 2008 show that modern onshore wind turbines in mature markets can achieve an availability of 97% or more.
These results demonstrate that the technology has reached sufficient commercial maturity to allow large-scale manufacturing and deployment. Nonetheless, additional advances to improve reliability, increase electricity production and reduce costs are anticipated. Additionally, most of the historical technology advances have occurred in developed countries.
Increasingly, however, developing countries are investigating the use of wind energy, and opportunities for technology transfer in wind turbine design, component manufacturing and wind power plant siting exist. Extreme environmental conditions, such as icing or typhoons, may be more prominent in some of these markets, providing impetus for continuing research. Other aspects unique to less-developed countries, such as minimal transportation infrastructure, could also influence wind turbine designs if and as these markets grow.
Offshore wind energy technology
The first offshore wind power plant was built in 1991 in Denmark, consisting of eleven 450 kW wind turbines. Offshore wind energy technology is less mature than onshore, and has higher investment and O&M costs. By the end of 2009, just 1.3% of global installed wind power capacity was installed offshore, totalling 2,100 MW.
The primary motivation to develop offshore wind energy is to provide access to additional wind resources in areas where onshore wind energy development is constrained by limited technical potential and/or by planning and siting conflicts with other land uses. Other motivations for developing offshore wind energy include: the higher-quality wind resources located at sea (e.g., higher average wind speeds and lower shear near hub height; wind shear refers to the general increase in wind speed with height); the ability to use even larger wind turbines due to avoidance of certain land-based transportation constraints and the potential to thereby gain additional economies of scale; the ability to build larger power plants than onshore, gaining plant-level economies of scale; and a potential reduction in the need for new, long-distance, land-based transmission infrastructure to access distant onshore wind energy. These factors, combined with a significant offshore wind resource potential, have created considerable interest in offshore wind energy technology in the EU and, increasingly, in other regions, despite the typically higher costs relative to onshore wind energy.
Offshore wind turbines are typically larger than onshore, with nameplate capacity ratings of 2 to 5 MW being common for offshore wind power plants built from 2007 to 2009, and even larger turbines are under development. Offshore wind farm plants installed from 2007 to 2009 were typically 20 to 120 MW in size, with a clear trend towards larger wind turbines and wind power plants over time.
Water depths for most offshore wind turbines installed through 2005 were less than 10 m, but from 2006 to 2009, water depths from 10 to more than 20 m were common. Distance to shore has most often been below 20 km, but average distance has increased over time.
As experience is gained, water depths are expected to increase further and more exposed locations with higher winds will be utilized. These trends will impact the wind resource characteristics faced by offshore wind power plants, as well as support structure design and the cost of offshore wind energy. A continued transition towards larger wind turbines (5 to 10 MW, or even larger) and wind power plants is also anticipated as a way of reducing the cost of offshore wind energy through turbine- and plant-level economies of scale.
To date, offshore wind turbine technology has been very similar to onshore designs, with some modifications and with special foundations. The mono-pile foundation is the most common, though concrete gravity-based foundations have also been used with some frequency; a variety of other foundation designs (including floating designs) are being considered and in some instances used, especially as water depths increase.
In addition to differences in foundations, modification to offshore wind turbines (relative to onshore) include structural upgrades to the tower to address wave loading; air conditioned and pressurized nacelles and other controls to prevent the effects of corrosive sea air from degrading turbine equipment; and personnel access platforms to facilitate maintenance.
Additional design changes for marine navigational safety (e.g., warning lights, fog signals) and to minimize expensive servicing (e.g., more extensive condition monitoring, onboard service cranes) are common. Wind turbine tip speed could be chosen to be greater than for onshore wind turbines because concerns about noise are reduced for offshore wind farm plants—higher tip speeds can sometimes lead to lower torque and lighter drive train components for the same power output.
In addition, tower heights are sometimes lower than used for onshore wind power plants due to reduced wind shear offshore relative to onshore. Lower power plant availabilities and higher O&M costs have been common for offshore wind energy relative to onshore wind both because of the comparatively less mature state of offshore wind energy technology and because of the inherently greater logistical challenges of maintaining and servicing offshore wind turbines.
Wind energy technology specifically tailored for offshore applications will become more prevalent as the offshore market expands, and it is expected that larger wind turbines in the 5 to 10 MW range may come to dominate this market segment.
International wind energy technology standards
Wind turbines in the 1970s and 1980s were designed using simplified design models, which in some cases led to machine failures and in other cases resulted in design conservatism. The need to address both of these issues, combined with advances in computer processing power, motivated designers to improve their calculations during the 1990s.
Improved design and testing methods have been codified in International Electrotechnical Commission (IEC) standards, and the rules and procedures for Conformity Testing and Certification of Wind Turbines relies upon these standards. Certification agencies rely on accredited design and testing bodies to provide traceable documentation of the execution of rules and specifications outlined in the standards in order to certify turbines, components or entire wind power plants. The certification system assures that a wind turbine design or wind turbines installed in a given location meet common guidelines relating to safety, reliability, performance and testing.
Insurance companies, financing institutions and power plant owners normally require some form of certification for plants to proceed, and the IEC standards therefore provide a common basis for certification to reduce uncertainty and increase the quality of wind turbine products available in the market.
In emerging markets, the lack of highly qualified testing laboratories and certification bodies limits the opportunities for manufacturers to obtain certification according to IEC standards and may lead to lower-quality products. As markets mature and design margins are compressed to reduce costs, reliance on internationally recognized standards is likely to become even more widespread to assure consistent performance, safety and reliability of wind turbines.
Power conversion and related grid connection issues
From an electric system reliability perspective, an important part of the wind turbine is the electrical conversion system. For large grid-connected turbines, electrical conversion systems come in three broad forms. Fixed-speed induction generators were popular in earlier years for both stall-regulated and pitch-controlled turbines; in these arrangements, wind turbines were net consumers of reactive power that had to be supplied by the electric system.
For modern wind turbines, these designs have now been largely replaced with variable-speed machines. Two arrangements are common, doubly-fed induction generators and synchronous generators with a full power electronic converter, both of which are almost always coupled with pitch-controlled rotors. These variablespeed designs essentially decouple the rotating masses of the turbine from the electric system, thereby offering a number of power quality advantages over earlier turbine designs.
For example, these wind turbines can provide real and reactive power as well as some fault ride-through capability, which are increasingly being required by electric system operators. These designs differ from the synchronous generators found in most large-scale fossil fuel-powered plants, however, in that they result in no intrinsic inertial response capability, that is, they do not increase (decrease) power output in synchronism with system power imbalances. This lack of inertial response is an important consideration for electric system planners because less overall inertia in the electric system makes the maintenance of stable system operation more challenging.
Wind turbine manufacturers have recognized this lack of intrinsic inertial response as a possible long-term impediment to wind energy and are actively pursuing a variety of solutions; for example, additional turbine controls can be added to provide inertial response.