12/01/2013 | Drew Devitt
America is blessed with long coastlines and relatively deep waters. But ironically, this has been a disadvantage to the U.S. offshore wind industry. Compared to more than 1,000 turbines that are already operating in the relatively shallow waters around the British Isles, and the significant offshore wind turbine generating capacity in many other European countries, only one offshore turbine is operational in the U.S. today. This is in no small part because shallow U.S. coastal waters are relatively close to the shoreline, which is problematic because it means offshore installations, limited by technical hurdles, must be closer to people, migratory bird patterns, and within state jurisdictions.
As for any new industry, it is a good idea to get a big picture view of the sector’s particular circumstances and objectives. First, it is a given that renewable wind generation should be relatively close to demand, yet not in someone’s backyard. Several technical trends are symbiotically conspiring to avoid “not in my back yard” (NIMBY) issues and dramatically change the offshore wind model developed in Europe. These trends include the development of floating wind turbines as opposed to seafloor-supported designs, the use of deep ocean water near U.S. coastlines as an effective head for energy storage, and the use of direct current (DC) deepwater cables in energy transmission. This article shows how these technologies could work together in the context of America’s natural resources and political landscape.
The Case for Offshore Wind Turbines
When used for offshore wind power production, floating structures have the potential to reach a much larger and significantly more energetic wind resource than seafloor-mounted turbines. At the same time, they increase social acceptance because they allow turbines to be installed far away from people.
Sandy Butterfield and his colleagues at the National Renewable Energy Laboratory (NREL) have published papers confirming the huge potential advantages of floating wind turbines, noting in 2010: “The NREL has estimated the offshore wind resource to be greater than the 1000 GW of the continental United States. The wind blows faster and more uniformly at sea than on land. A faster steadier wind means less wear on turbine components and more electricity generated per turbine. The wind increases rapidly with distance from the coast, so excellent wind sites exist within reasonable distances from major urban load centers reducing the onshore concern of long distance power transmission.”
To emphasize Butterfield’s point regarding transmission, the best winds within the continental U.S. are class 3 and 4 winds in the Great Plains and Mountain States—but typically 1,500 miles from major load centers. Comparatively, areas just 30 miles offshore from major metropolitan hubs see class 6 winds. This is significant because an estimated 70% of U.S. electricity demand is close to its coastlines and the Great Lakes.
It is also important to add that the energy in wind increases as a cube function of its velocity, so wind of 6 meters per second (m/s) has more than double the energy of wind at 4 m/s. Also, wind velocity near the ocean surface is higher than on land, as thermal boundary layers created by the sun heating the land are eliminated farther from shore. About 20 miles out to sea, wind currents aloft sink and reattach to the ocean surface, becoming trade winds. This reduces the need to elevate the turbine into the air and improves its capacity factor.
Types of Wind Turbines
Wind turbines can have either a horizontal or a vertical axis of rotation. Another important point of differentiation is that wind turbines employ two basic principles to capture energy from moving air: aerodynamic turbines use low-pressure lift (like an airplane wing), while impulse turbines use drag (like a water wheel). The differentiating factor is that the blade tip speed of aerodynamic turbines is a multiple of the wind speed, but an impulse turbine will not spin faster than the wind. Aerodynamic turbines can be more efficient than impulse turbines (Figure 1).
1. Turbine efficiency by type. This chart describes the efficiency of different types of utility-scale wind turbines. The chart’s vertical axis represents the turbine’s efficiency as a percentage of the total energy in the wind. The horizontal axis represents the relationship between wind speed and turbine tip speed. Aerodynamic turbine types have tip speeds of four to seven times the wind speed, and impulse turbines have tip speeds on the order of the wind speed. Aerodynamic turbines are favored because they have roughly twice the efficiency of impulse-type turbines, but impulse turbines have historically been used whenever cost, reliability, or capacity factor is more important than efficiency. Source: Jean Lucmenet
An anemometer, a device for measuring wind speed, is an example of an impulse-type device with a vertical axis of rotation—though vertical axis wind turbines (VAWTs) may employ aerodynamic or impulse design. Conventional horizontal axis wind turbines (HAWTs) used widely at utility scale are an example of aerodynamic turbines with tip speeds today reaching 100 m/s (360 kilometers or 225 miles per hour). The old four-bladed Dutch wind-powered mills and water pumps that used cloth-covered, wooden-framed blades as well as the iconic Wild West American multiblade wind turbines are almost impulse type systems when considered in the context of today’s modern aerodynamic HAWTs.
A lot of engineering and technical development has gone into modern HAWTs in order to drive their efficiencies to 45% at the high end. The theoretical maximum efficiency is limited by Betz’s Law to 59%. A wind turbine cannot be 100% efficient, as this would imply that the air exiting the turbine would have zero velocity and so would prevent other air from flowing through the turbine.
Efficiency factors can be misleading, though, in that they presume a certain wind speed, which is usually not noted. For instance, a HAWT may have an efficiency of 45% for a wind speed of 14 m/s, but it would not even spin—meaning it would have zero efficiency—with a 5 m/s wind. HAWTs are a logical optimization of the wind turbine specifications. The energy in wind is a cubed function of its velocity, so optimizing wind turbine efficiencies for high wind speed results in large megawatt ratings. This works well for the sales team when selling a turbine based on its megawatt rating.
What should be considered instead are capacity factors. Capacity factors are based on a power curve for the particular wind turbine as well as wind speed data from the proposed site where the turbine will be installed. Capacity factors for land-based wind turbines are typically claimed to be 25% to 35%. Comparatively, gas or steam turbine capacity factors approach 100%.
To maximize the capacity factors for wind energy, the focus of the offshore wind industry should change from the megawatt rating of a turbine to useful load matching, with more interest given to turbines optimized for higher capacity factors in average wind speeds. In the current paradigm, HAWTs have the highest efficiencies in the higher wind speed ranges, and this results in high megawatt ratings for the turbines but low capacity factors, meaning that the turbine will generate its rated capacity only a small fraction of the time. This causes “spiky power,” that is, much of a turbine’s power is made over a relatively short period of time (Figure 2). For this reason, wind turbine electricity must be associated with storage in order to be considered as a baseload power source.
2. Time versus wind energy. This chart, showing the distribution of wind speeds with respect to time, plots two years’ worth of wind speed data from a buoy at the mouth of the Delaware Bay. Total hours that the wind blew at a particular speed are shown as a bar chart. To show the energy that is contributed at each of the wind speeds, the power in the wind (a cubed function of its velocity) is multiplied by the time that the wind blew at that speed. Notice that the maximum energy was at 11 meters per second but that the wind blew at this speed only 4% of the time. Half of the total energy for the year occurred on the high-speed side of the energy peak during 15% of the total hours. Source: American Offshore Energy
In comparison, VAWTs in an impulse configuration have a relatively high efficiency in lower wind speeds because they have higher blade areas as a percentage of swept area. This could be thought of as the “barn door method” of collecting energy from the wind: Although not as efficient at higher wind speeds, impulse type VAWTs will make power most of the time the wind is blowing—making them more suitable to power companies and mitigating the need for time shifting or storing wind energy. However, utility-scale energy storage would still be beneficial to any electrical grid if it can be done cost effectively.
Ocean Energy Storage
Much has been invested in trying to develop energy storage technologies as a way to more evenly distribute renewable power across time. So far, only compressed-air storage and pumped-hydro storage have the capacity to practically “time shift” utility-scale energy, but both require specific geological features that are generally not found close to major load enters. If the electrical generation occurs over deep ocean water, though, energy storage becomes much more convenient.
In another significant advantage for floating wind turbine technology, the pressure of the deep ocean waters under the turbines can be used for utility-scale energy storage. A number of different engineering approaches to this have been explored using air or water. Colorado-based Bright Energy Storage, for instance, has a plan to pump air into huge bags in deep water, while Dr. Alexander Slocum, a mechanical engineering professor at the Massachusetts Institute of Technology, suggests using excess wind power to pump water from hollow concrete spheres (made from fly ash) that are ballasted by their own weight to the seafloor. Doubling as an anchor for floating wind turbines, a 25-meter-diameter sphere could store up to 10 MWh of power, depending on depth.
Pairing deepwater wind generation with energy storage could make wind energy the most flexible of all energy sources. A 1-MW wind turbine that is able to produce 10 MWh over 24 hours could sell all 10 MWh during the hottest hour of the next day at peak prices. Wind farm operators could even begin bidding in the frequency regulation market, where the price per kilowatt-hour is five to 10 times the price that can be negotiated in power purchase agreements. This would both improve returns for investors and provide an environmental bonus because turbines that now provide frequency regulation, being smaller and more flexible, have fewer pollution controls. It is good for everyone when clean flexible power is worth more.
Addressing Transmission Concerns
But to accomplish frequency regulation from the deep ocean, transmission capabilities are obviously required. Several developments have been made here, too. Trans-Elect Development Co. has proposed the Atlantic Wind Connection (AWC), a 6,000-MW transmission backbone running from Virginia to northern New Jersey, some 30 to 50 miles out in the Atlantic Ocean. That $5 billion plan has attracted more than $500 million in investments from companies including Google, Good Energies, and Marubeni. Trans-Elect has received a “Determination of No Competitive Interest” from the Bureau of Ocean Energy Management and is proceeding without an associated auction. Plans are in effect to have the first phase—a $1.8 billion, 150-mile-long project from Delaware Bay to Atlantic City—operational by 2016.
Notably, Trans-Elect, which was the nation’s first independent transmission company, is betting on high-voltage direct current (HVDC) cables, which it anticipates will have cost and technical advantages over alternating current (AC) transmission. Today, almost all commercially available wind turbines generate asynchronous AC current that is converted to DC, and then the DC is inverted back to three-phase AC at 60 Hz. However, a number of capital costs, efficiency losses, cooling systems, power quality problems, and maintenance headaches must be borne with this method. Wind turbines designed to generate DC current may still need a transformer to step up the voltage but would avoid even having to sync with the rest of the grid, making them simpler to implement by reducing the balance of plant—which is especially important at sea.
Though technically challenging, obtaining permits for undersea cables may be easier than for land-based lines. Also, transmission cables could be brought ashore to existing grid connections at retired power plants, many of which are located on riverbanks or at coastlines near major load centers. These could serve as perfect locations for injection of high-current frequency regulation and reactive power services to keep the grid running efficiently.
Floating VAWTs Fit the Bill
Only one offshore turbine is operational in the U.S., even though the country has the world’s second-highest onshore wind turbine capacity (after China). The UK has more than one-half of the 3 GW total worldwide installed offshore wind generation, and it has ambitious plans for even more offshore wind farms. However, as Bloomberg New Energy Finance notes, because the UK has a limited supply chain for offshore wind turbines, about 80% of what the country spends on wind technology still goes to foreign contractors or turbine suppliers. Jim Lanard, president of the U.S. Offshore Wind Development Coalition, is quick to point out that if the U.S. is to gain and keep legislative support for the offshore wind industry, it will have to generate domestic jobs.
Increased use of floating VAWTs could do just that. First, the capital-intensive supply chain needed to manufacture large roller bearings, gears, forgings, and castings would not be required. Steel fabrications and fiberglass components with relatively low capital equipment needs are all that would be required, so a supply chain based on these components would scale up much more quickly. Old shipbuilding sites and cargo transfer ports could make for good candidates for wind turbine manufacturing sites. The labor skill sets could also be filled quickly and practically deployed in many more coastal locations.
Floating VAWTs could also eliminate the need for purpose-built ships that are required to assemble seafloor-mounted HAWTs. This is important because, having never installed a foundation-based offshore wind turbine, the U.S. lacks a fleet of the jack-up ships that are necessary. And, unlike in Europe, the U.S. cannot hire foreign-flagged ships to work in U.S. territorial waters because it would violate the Jones Act, a federal statute that regulates maritime commerce in U.S. waters. The U.S. already has a ready fleet of ships that are capable of towing floating turbines out to mooring fields, though.
By eliminating a seafloor foundation, the cost structure of supply-chain issues and the costs to assemble and service turbines at sea can also be dramatically improved. As noted previously, the further away from NIMBY issues and state jurisdictions, the better the wind resource becomes, but the ability to tow a turbine back to the factory in a single day mitigates risk, reducing both insurance and banking costs for projects. Ocean transportation and sighting combined with low turbine speed could enable scalability to a huge size.
A Better Choice than HAWTs
However, most “floater” programs in development in the U.S. today are designed to employ HAWTs, which have a lot of developmental inertia based on current onshore designs, their supply chains, and government funding programs. U.S. research consortium DeepCwind, which is led by the University of Maine’s Habib Dagher, this year launched the first and only offshore wind turbine off the coast of Maine—a concrete-composite floating platform HAWT prototype that is one-eighth the size of a “VolturnUS” design envisioned for commercial installation.
One problem is that it is difficult to make conventional HAWTs float. They are cantilevered structures, reaching high off their base support with large masses and forces acting at the top. It is a fundamentally unstable structure in the context of floatation, but HAWTs are the mainstay of the wind energy industry. Almost all utility-scale wind turbines employ three blades connected to a horizontal spindle, which is mounted on top of a pole. There is no debate that this design can be the most efficient at capturing energy from wind, but a big picture, smart grid, objective look should consider all of the issues and constraints involved, not just the turbine’s maximum efficiency.
Other structures are possible. Lightweight structures can be achieved by using tension and compression design principles rather than the bending of a cantilevered structure. Examples of such structures would include bicycle wheels, suspension bridges, and sailboat masts (Figure 3).
3. Inspired design. One example of a floating vertical axis wind turbine (VAWT) is a design based on the rigid mainsail used on America’s Cup boats. The 60-ton VAWT design adds three more sail plans and a masthead ring for 1,800 square meters of projected area. The bearings and generator are located near sea level for easy on-site service and to provide a stable low center of gravity. Courtesy: American Offshore Energy
A 200-foot tall VAWT installed 30 miles offshore would not be visible or audible from land, dramatically reducing legal challenges that can delay and increase costs of wind projects. Having a high degree of solidity and a low rotation speed avoids harm to birds and provides excellent horizontal radar reflection for maritime visibility with little vertical reflections. Additionally, the VAWT has no gearbox or oil reservoir, and all the components on the turbine are waterproof and rustproof. In the event of an occasional hurricane or rogue storm, the turbines could be easily reefed or sunk by remote control, allowing them to ride out the storm safely beneath the ocean surface. When the storm has passed turbines may be again raised via remote control and recommissioned with little effort.
Floating VAWTs enable a host of advantages that dramatically improve the return on investment, the reliability of the energy stream, and the ability to usefully site the turbine. Because VAWTs would have a completely different supply chain than conventional HAWTs, their potential to generate jobs may be increased. And, if paired with HVDC power transmission and deep ocean energy storage, floating VAWTs could give the country’s new offshore sector fair winds in which to hoist its sails. ■
— Drew Devitt is founder of American Offshore Energy and a former president of the American Society of Precision Engineering.
Sunday, December 1, 2013
Drew Devitt – CTO
American Offshore Energy
4009-G Market Street Aston, PA 19014 USA