TODAY’S STUDY: WIND’S CHALLENGES
Wind Power; Issues and Opportunities for Advancing Technology, Expanding Renewable Generation and Reducing Emissions
July 2011 (Electric Power Research Institute)
Wind is an abundant, broadly distributed, carbon-free energy source that may be converted into electricity with limited adverse environmen¬tal and social impacts. Deployment continues to expand rapidly despite varying growth rates for individual countries. U.S. and worldwide capacity exceeds 40 and 200 gigawatts (GW), respectively, more than 10-fold increases since 2000. Wind turbines currently account for about 1% of global electricity production, 5% in the European Union, more than double that in Portugal and Spain, and almost 20% in Denmark.
They represent the leading source of load-serving non-hydro renewable generation in the United States, as shown in Figure 1.
Wind’s growing contributions have been driven by two interconnected factors. Government actions have mandated additions of renewable generation capacity and made investments in them attractive, while ever-larger turbines incorporating increasingly sophisticated conversion and control systems have made land-based wind an economical central-station supply option in many areas. The U.S. experience highlights the influence of government support mechanisms: Capacity additions from 2000 through the present have followed a “boom and bust” cycle, track¬ing the availability of tax credits and, more recently, cash grants.
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U.S. and international agencies have advanced ambitious visions for future onshore and offshore deployment to help satisfy burgeoning elec¬tricity demand while meeting climate stabilization objectives. Targets for 2030 include 20% of U.S. and 9% of global electricity generation, which would position wind as the largest source of supply-side reductions in greenhouse gas emissions over this period. Through 2050, goals are for at least 400 GW of U.S. capacity and about 2,000 GW worldwide. The high levels of government support that have helped fuel capacity expansion are unlikely to be sustained indefinitely, challenging the wind power industry’s ability to meet these deployment targets. With natural gas prices expected to remain low, variable-output wind already faces stiff competition from combined-cycle plants. The fight for market share is likely to intensify after 2030, when new nuclear plants and fossil plants with carbon capture and storage capability are expected to be available for widespread baseload duty.
To maintain competitiveness in the face of declining subsidies, EPRI estimates that the wholesale levelized cost of energy (COE) from land-based wind must be at least 20% lower by 2030 than it is today, and cost reductions of 25% or higher are required for offshore projects. To achieve grid parity in the absence of subsidies, COE decreases of at least 25% on land and 50% offshore likely are necessary through 2050. Advanced technologies are needed for reducing capital costs and opera¬tions and maintenance (O&M) expenses and for increasing capacity factors, all of which will contribute to a lower COE at the busbar. In addition, deployment, interconnection, and integration challenges must be addressed in order to install large amounts of wind power, transmit energy from remote areas to load centers, accommodate variable genera¬tion at penetration levels above 20%, and thereby minimize wind’s delivered COE.
This paper focuses on research, development, demonstration, and deploy¬ment (RDD&D) issues consistent with expanding wind’s worldwide role in serving consumer demand at low cost in a clean, reliable, and sustainable manner. It identifies opportunities for cost-performance improvements in resource assessment, energy capture, O&M, transmis¬sion, grid operations and planning, project development, and impact mitigation. Based on EPRI’s technology assessments, this paper also highlights RDD&D priorities that individually promise the greatest sav¬ings and productivity gains and together foretell “super turbines” capable of achieving aggressive cost, performance, and deployment targets.
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Overview & Status
Modern wind turbines convert the kinetic energy of moving air into mechanical energy and then electricity. They harness wind created by the differential heating and cooling of the Earth’s surface, the passage of fronts and storms, and other phenomena. As shown in Figure 2, they employ a three-bladed rotor spinning around a horizontal axis. The rotor is connected to a drive train, generator, and control systems contained within a nacelle that sits atop a tower and rotates on a vertical axis to orient blades into the wind. Grid-connected turbines may be deployed over all scales, from individual residential and commercial units rated in the kilowatts (kW) and supplying on-site energy, to wind farms incorporating hundreds of multi-megawatt (MW) machines that generate bulk power to serve load. This white paper focuses on central-station applications.
Turbines are getting bigger all the time—hub heights and rotor di¬ameters for utility-scale, land-based units commonly are 80 m (262 ft) and greater, rated capacity has reached 3 MW, and even larger machines are under development. For offshore deployment, tur¬bines ranging from 2.5 to 7.5 MW are being sold and prototyped, respectively, and models with capacity up to 10 MW are envisioned. Control systems also are getting more complex, helping increase the efficiency of components and systems as well as the productivity of individual turbines and larger arrays. As technology has advanced and size increased, wind has become the world’s fastest growing source of utility-scale energy production. Wind projects supplied almost 71 terawatt-hours (TWh) to the U.S. grid in 2009—far and away the most important non-hydro renewable generation option (Figure 1)—and 164 TWh globally in 2007, accounting for 1.9% and 0.9% of total electricity production, respectively. (EIA, 2010a; AWEA, 2010; GWEC, 2010)
Despite challenging economic conditions in 2009, deployment totaled 10 GW in the United States and more than 38 GW globally, pushing installed capacity above 35 GW and 158 GW, respectively. Federal stimulus funding helped accelerate year-over-year growth to almost 40% in the United States, which remains the world leader in wind capacity even with far slower growth in 2010. In deploy¬ing 13.8 GW in 2009, China more than doubled its installation base and became the second largest global market. Across Europe, slower rates of deployment in Germany and Spain—now third and fourth in installed capacity worldwide, respectively—were offset by growth in Italy, France, UK, and other countries, bringing overall additions to more than 10 GW and cumulative capacity to more than 75 GW. In 2010, total offshore capacity exceeded 3 GW, with more than 75% off the UK and Denmark. Several other European countries and China also host offshore projects. (AWEA, 2010; GWEC, 2010)
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For individual regions and countries, both wind development activity and deployment potential depend on resource availability, a given site—the major determinant of capacity factors and project economics—largely is a function of both the average wind veloc¬ity at hub height and the wind speed frequency distribution. Wind resources are classified based on mean power density (in W/m2) or equivalent speed (in m/s or mph) at a specified height above the Earth’s surface, ranging from 1 (the worst) to 7 (the best). Class 3 and better resources—average wind speed of at least 6.9 m/s at 80 m above the surface—generally are required for economical grid-connected wind generation. As shown in Figures 3 and 4, open plains, mountain ranges, coastal areas, and offshore environments offer site-specific development opportunities in many regions of the United States and around the world.
Currently, wind projects benefit from high levels of government support because they generate power without requiring energy im¬ports, depleting finite fuel supplies, releasing air pollution, emitting greenhouse gases, or producing liquid and solid wastes. Renewable portfolio standard (RPS) mandates, investment and production tax credits, direct grants, and feed-in tariffs represent common support mechanisms. Climate policies that increase the cost of electricity from fossil power plants also improve the competitiveness of wind generation.
The broad distribution of resources and the widespread availability of incentives help explain why 36 U.S. states host utility-scale proj¬ects, 14 states each have more than 1 GW of capacity, 25 nations installed at least 100 MW in 2009 alone, and substantial—albeit slower—growth in deployment is expected over the near and longer terms. (AWEA, 2010; GWEC, 2010) Figure 5 displays global trends through 2035 based on the National Energy Modeling Sys¬tem (NEMS) and the World Energy Projection System Plus main-tained by the U.S. Energy Information Administration (EIA). Wind capacity is projected to triple across the next quarter-century, with the fastest growth occurring in the next five years. (EIA, 2010b)
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Deployment projections vary depending on policy, technology, load growth, and other assumptions. (See box, p. 7.) Figure 6 illustrates this concept, showing estimates of future U.S. wind generation from EIA’s NEMS and from the Regional Energy Deployment System (ReEDS) Model developed by the U.S. National Renewable Energy Laboratory (NREL). Depending on the scenario, future U.S. wind production levels range from 200 to 600 TWh in 2020 and from 200 to 1,200 TWh in 2030. These projections demonstrate the potential impact of possible federal RPS and climate policies in expanding wind generation, but the REeDS model estimates much higher levels of deployment because it employs more optimistic technology cost and performance assumptions, particularly for offshore wind. (EIA, 2009; EIA, 2010c; NREL, 2010)
In the “20% Wind Energy by 2030” study supported by the U.S. Department of Energy (DOE), REeDs analyses indicate that 300 GW of wind could supply 20% of U.S. electric demand in 2030. (DOE, 2008) NREL’s ongoing “Renewable Energy Futures” study—which considers the technical feasibility of having renew¬able generation options supply up to 80% of electricity needs in the continental United States by the middle of this century—includes REeDs scenarios in which installed wind capacity ranges from about 400 to 600 GW and accounts for 32 to 44% of U.S. needs in 2050. (NREL, in press) The International Energy Agency (IEA) envisions global wind deployment of more than 1,000 GW by 2030 and 2,000 GW by 2050, supplying 9 and 12% of the world’s electric-ity requirements, respectively. (IEA, 2009) Projections by the wind industry are even more optimistic.
For aggressive deployment targets to be achieved, wind power will need to be cost-competitive with energy from other genera¬tion options at the wholesale level, in the absence of subsidies, and in a broad range of markets. This will require continued progress in wind turbine and other technologies, a selection of which are shown in Figure 7. Already, land-based wind offers advantages as a mature, low-risk, non-emitting renewable energy technology. At current subsidy levels, central-station projects are highly competitive in many U.S. markets, even though installed costs have increased to about $1,750/kW from less than $1,500/kW several years ago. (EPRI, 1021379; EPRI, 1019772) These increases have been driven primarily by supply constraints associated with the rapid expan¬sion in deployment and by rising prices for steel, concrete, copper, and other materials. Recent growth in the number and capacity of turbine manufacturers has addressed some—but not all—supply is¬sues. Meanwhile, effects on wholesale COE have been offset in large part by deployment and production incentives, as well as increases in capacity factors attributable to technological advances and reli¬ability improvements. High O&M costs for installed capacity are a continuing concern. As components age, outage and failure rates increase for gearboxes, blades, and other components. Technologies for increasing energy capture and for reducing inspection and repair expenses, productivity losses, and replacement power costs represent high-priority RDD&D needs.
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Shallow-water offshore wind has only about a decade of commer¬cial experience. Because components must be ruggedized for and installed in difficult environments and turbines must be connected to each other and the grid with undersea cables, the distribution of costs differs from that of land-based projects (Figure 8 and 9). Offshore capital costs are roughly double, and O&M expenses are several times higher as well. On the plus side, better wind resources lead to higher capacity factors, and offshore environments offer economies of scale through larger turbines and bigger projects. Nonetheless, the average wholesale COE from shallow-water offshore wind remains substantially higher than that for land-based turbines. Technologies for greater water depths are not commercially available. Fixed support structures and floating platforms suited for these applications are RDD&D priorities.
Transmission interconnection and grid integration are critical tech¬nical issues influencing future wind deployment. Many areas with outstanding resources are remote from load centers, creating the need for new transmission capacity, system upgrades, advanced tech¬nologies, and novel planning and cost allocation strategies. Once deployed, individual turbines and projects generate output that var¬ies across all time scales and is often poorly matched with load, yet they must be integrated in an infrastructure designed to maintain reliability while instantaneously and continuously balancing produc¬tion with consumption across a wide geographic area.
Operating experience and modeling studies generally indicate that wind power may be accommodated without significant impact at penetration levels, on an energy basis, of about 10 to 20% in individual control areas. At higher generation fractions, integration challenges relating to regulation, load following, and scheduling de¬pend on the correspondence between wind output and load profile, the grid support capabilities available at turbine and project levels, the characteristics of other power plants, and additional factors. Maintaining adequate backup and high levels of reliability imposes new demands on existing fossil assets and requires additional fast-response generation capacity to supply reserve services, as well as creates the need for innovations in forecasting, situation awareness, power electronics, storage, and other technologies.
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Interconnection and integration requirements increase wind’s deliv¬ered COE, while environmental and social considerations also come into play in wind project development and operations. Utility-scale wind farms require a substantial footprint. This may create siting challenges, mitigated by the fact that turbines and other infrastruc¬ture occupy only a small portion of a project’s area and thus do not exclude other uses. Turbines deployed along ridge lines, in open landscapes, and near coastlines and other population centers may be highly visible, engendering opposition. Potential impacts on birds, bats, and additional wildlife may create constraints on siting and op¬erations. Noise, shadow flicker, radar interference, property values, and other concerns may as well. Improved knowledge and advanced mitigation strategies are needed for both onshore and offshore ap-plications.
EPRI’s analyses indicate the following:
• Wind power will continue to represent a low-cost, low-risk option for supporting RPS compliance and reducing carbon dioxide (CO2) emissions from the electric sector;
• Substantial capacity expansion will occur in the near term and over the next few decades given technological progress, abundant resources, favorable economics, growing demand, and energy and climate policy mandates; and
• Technical, political, environmental, and social issues will pose challenges to deployment as wind penetration expands.
To improve wind’s competitiveness with other generation options and grow its role in meeting domestic and global energy needs, advances will be needed in tower and foundation, blade and rotor, drive train and power electronics, O&M, transmission intercon¬nection, grid integration, siting and development, and impact mitigation technologies. To reduce costs, increase productivity, and enhance environmental performance, substantial and sustained RDD&D investment will be required.
Towers & Foundations…Blades & Rotors…Drive Trains & Power Electronics…Operations & Maintenance…Transmission Interconnection…Grid Integration…Environmental Considerations…Siting & Development…
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Implications & Conclusions
Megawatt-scale wind turbines are proven solutions for generating large amounts of carbon-free power with minimal adverse envi¬ronmental and social impacts. Consistent with wind power’s track record, the massive global resource base, and existing and antici-pated policy objectives, international and U.S. energy agencies have set extremely ambitious wind deployment goals across the next 2 decades and through 2050. The actual pace and extent of future additions of onshore and offshore wind capacity will depend less on resource characteristics than on technological progress, political and regulatory decisions, and additional drivers that together will influ¬ence the wholesale and delivered COE from future wind projects.
According to EPRI’s analyses, land-based wind resources are suf¬ficient to meet more than 50% of future U.S. electricity needs at a wholesale COE of about $80 to $90/MWh. However, wind’s competitiveness will be determined by fuel prices, climate policy and RPS mandates, and other factors influencing the economics of power generation options, while the challenges associated with integrating huge amounts of variable-output generation may pro¬duce sharp increases in delivered COE. Further, today’s incentives are not likely to be sustained indefinitely, project financing is scarce, the economics of harnessing higher-quality offshore wind resources remain daunting, and siting and permitting challenges are expected to continue. These cost, interconnection, integration, and develop¬ment issues apply in the United States and around the world.
Based on goals set by U.S. and international energy agencies and its own analyses, EPRI has established the cost-performance targets outlined in Table 1. These 2030 and 2050 targets are designed to ensure that both onshore and offshore wind power become increas¬ingly competitive in the face of declining subsidies, ultimately achieving grid parity. Meeting them will require substantial and sus¬tained technological progress to increase capacity factors and reduce capital and O&M expenses, all of which will contribute to higher productivity and a lower COE at the busbar. Advances in wind turbine components and systems are not enough, however. Wind interconnection and integration innovations are needed to reduce wind’s delivered COE by providing access to abundant resources in remote areas, delivering large amounts of power over long distances to load centers, and accommodating variable-output generation at penetration levels well above 20%. In addition, new project development and impact mitigation technologies are necessary to decrease up-front costs and risks, as well as potential environmental and social concerns.
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Table 2 characterizes the advanced turbine technologies identi¬fied by EPRI as offering the greatest potential impacts in terms of productivity gains and cost savings. For increasing energy output, offshore floating platforms and towable gravity foundations—along with taller towers for land-based applications—represent priori¬ties based on their abilities to expose rotors to higher-quality wind. Drive train technologies have the greatest potential to reduce wholesale COE: Single-stage gearboxes with medium-speed genera-tors and direct-drive configurations with low-speed PM and HTS generators are expected to provide a combination of increased pro¬ductivity, lower capital costs, and reduced O&M costs. Sweep-twist and smart blades, passive and active aerodynamic controls, and on-line monitoring and control systems are capable of both significant performance improvements and cost reductions by balancing short-term production against long-term reliability. The various gearbox, generator, blade, and O&M innovations are expected to deliver particular value by improving availability and reducing downtime later in a turbine’s lifetime when failures become more frequent. (EPRI, 1019772; EPRI, 1019773)
Also listed in Table 2 are the highest-priority transmission intercon¬nection and grid integration technologies. Energy storage systems and probabilistic operations and planning tools have the greatest potential to both facilitate high penetration levels and minimize impacts on power system reliability, other generating plants, and wind’s delivered COE. Advanced forecasting, high-voltage power electronics, and inter-area management tools offer significant inte¬gration benefits, while HTS cables and HVDC systems are critical for long-distance bulk power transmission between remote wind resource areas and load centers.
Many advanced wind energy technologies are being developed by turbine and component maufacturers. In 2011, EPRI’s focus is on tower height, low-cost HMS systems for installed wind capacity and LIDAR-based tools for increasing turbine productivity. Given accelerating offshore wind deployment in Europe and Asia and the high degree of interest in projects off the Atlantic Coast and in the Great Lakes, technologies for significantly reducing near-term capital and O&M costs are being assessed. Advances in interconnec¬tion, integration, grid operations and planning, energy storage, and impact mitigation also are being pursued. (See box, p. 36.)
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Generally, the advanced components, systems, and tools identified as RDD&D priorities in Table 2 are not mutually exclusive. In fact, “super turbines” integrating multiple advanced technologies and feeding power to an expanded, more robust, and more flexible transmission grid will be required to achieve the cost-performance targets shown in Table 1, as well as deployment goals such as 20% of U.S. generation and almost 10% of global generation by 2030 and at least 400 GW of U.S. wind capacity and about 2,000 GW worldwide by 2050. (DOE, 2008; IEA, 2009; NREL, in press) Fig¬ure 30 displays two possible “super-turbine” configurations expected to emerge within the next decade. Land-based machines likely will have a minimum capacity of 3 MW and incorporate large-diameter rotors, segmented blades with active aerodynamic controls, direct-drive PM generators, and hybrid concrete-steel towers at least 120 m high to allow economical generation in low-wind environments. Offshore, 5- to 10-MW turbines likely will employ towable gravity foundations or floating platforms with dynamic balancing sys¬tems, and smart blades will serve low-speed HTS generators at hub heights influenced more by rotor diameter than the need to avoid wind shear and turbulence above the water surface. Power electron¬ics, sensing, control, forecasting, HMS, NDE, and other innova¬tions will further improve the performance of individual turbines and entire projects.
Of course, meeting ambitious wind capacity expansion and genera¬tion objectives also is contingent on getting projects sited, permit¬ted, financed, and built and then operated with minimal adverse environmental and social impacts. Priorities include better resource characterization and site assessment methods, improved understand¬ing of wildlife interactions, advanced impact mitigation technolo¬gies, and streamlined permitting and approval frameworks.
EPRI is conducting research to help grow the role of wind energy technologies in cost-effectively meeting demand, diversifying supply portfolios, satisfying renewable energy mandates, and responding to climate change and security concerns. In addition, EPRI is collabo¬rating with utilities, government agencies, and other stakeholders to ensure that RDD&D priorities for the immediate, mid, and longer terms are being addressed.