“The Wind Bloweth where it Listeth”: The Past, Present, and Future of the US Wind Energy Industry (Part 3)
Depending on who you talk to about wind energy and how it powers the electric grid, there’s bound to be… well, some spin. Much of the discussion centers the reliability of wind energy in terms of how it meets base load compared to the zero fuel cost and extremely low carbon emissions, as well as concerns about whether or not the wind industry can compete with other forms of energy fuels. With this new three-part series, we hope to provide a quality overview of the wind energy industry – its early development, how it operates now, and where the industry is heading.
In the third and final installment of “The Wind Bloweth where It Listeth,” we’re going to look at the challenges the wind energy industry faces for future development, as well as to what heights the fickle winds may carry us.
Currently in the US, there are 60 gigawatts (GW) of installed wind energy capacity mostly in the familiar form of towering land-based wind turbines. Though wind energy appears at present to be the most affordable renewable energy source (especially in light of the of the wind production tax credit (PTC)), it does have three characteristics dogging its development.
- Wind farms are located far from urban centers and require expensive transmission lines.
- Wind energy by its nature is intermittent, making it (at present) unreliable for covering base electrical load.
- Both land and sea-based wind turbines rarely catch enough wind to meet their generation capacity.
The answer to all three challenges lies in technical innovations that are not just idle pie-in-the-sky daydreams, but currently down-to-earth hardware.
Getting Wired Up
While wind turbines are popularly seen as sources of green energy, the transmission lines that deliver their generated power to the rest of the grid are despised and often fought against tooth and nail because people generally don’t want them in their back yards. Apart from the NIMBY sentiment, transmission lines are expensive construction projects costing billions of dollars. These costs are often passed onto the consumer as part of their utility bill’s distribution charges. According to the American Tradition Institute’s 2012 report, The Hidden Costs of Wind Electricity, wind energy projects require big new power lines: “Because its best locations are remote from major cities, wind requires new long-distance transmission lines which were rarely necessary before, and would not be necessary today, except to support wind.”
While this is an important point, it ignores the larger fact that growing energy demand is already driving the construction of new transmission lines to move conventionally generated energy from one part of the country to the other. These include the Susquehanna-Roseland Electric Reliability Project, the Gateway West Transmission Line Project, and the Illinois Rivers Transmission Project among many others which would not be necessary today, except to support increased demand and system reliability. In all likelihood, new transmission lines that serve wind farms will be needed to route power from one part of the national grid to the other.
As mentioned in the previous installment of this series, wind farms can be idled by transmission line problems. In Texas, from December 2008 to December 2009, between 500 MW and 2000 MW of wind energy was curtailed daily, totaling 16% for the year. Such curtailments were caused by transmission congestion when demand outstrips the transmission line’s physical capacity to handle the amount of current wanted. Most power lines today use aluminum conductor steel reinforced cable (ACSR). These are aluminum conductors wrapped around a steel cable that carries the load of the cable. When the cable gets hot because of high voltage, it begins to resist conducting electricity. So more current is required to send the same amount of electricity through. This makes the cable get hotter and resist more. Left unchecked, the cable will stretch and sag from the towers until it breaks and causes a blackout.
Aluminum Conductor Composite Reinforced (ACCR) and Aluminum Conductor Composite Core (ACCC) are made of strong, lightweight carbon composite materials that can endure heat and the strain of supporting the aluminum conductor wrapped around it. (The milling of the aluminum conducting cables into trapezoidal shapes that fit tightly together creating a higher capacity conductor.) Already in service for several years, composite-based cables transmit more power efficiently even at high temperatures without thermal stretching and sagging that would otherwise destroy ACSR cables.
Another option is High voltage DC power (HVDC). HVDC can transmit bulk power over long distances (even under water) with less line loss. AC systems need to be in sync in order to transfer power between them. Distance can cause two AC system to fall out of sync. HVDC can transfer power between AC systems that are asynchronous and this reinforces grid stability. For off-shore wind farms, HVDC would be the solution for successful power transfers.
Where It Listeth
Even with improvements to transmission technology, the chief problem with the wind is its intermittent nature. Sometimes it blows constantly, sometimes not. Sometimes it blows a lot, sometimes a little. The thing about running an electrical grid is that, in order to supply demand reliably, you need to be able to produce a constant stream of energy for a set period of time; say 1 megawatt for one hour. Wind energy output can be irritatingly variable; say 800 kilowatts for 10 minutes, then 1.2 MW for 15 minutes, then down to 750 KW for 20 minutes, etc. This is caused by many factors, including wind speed, turbine blade length, and the turbine drive train.
Over the course of 20 years, wind turbine blades have grown from 75 feet to over 150 feet in length making full rotor spans some 300 feet across. Just recently, British Petroleum (BP) and Shell announced they were investing $25 million to help build wind turbines with lightweight carbon fiber blades that were 300 feet long. While monster-sized turbines like these are scaled for the 10 MW towers that sit off-shore in the ocean, the principle is still the same: bigger blades mean more surface area to catch more wind. But just because you can catch more wind doesn’t mean the turbine will put out enough power.
For decades, wind turbines were designed so that the propeller shaft was connected to a gearbox. The gearbox transferred the spinning motion of the blades into the correct speed and energy to turn a generator and make electricity. As the need for greater output led to bigger blades, it also led to re-thinking about how to build more efficient electrical generators. Now, both Siemens and General Electric are building direct drive generators. These are connected directly to the propeller shaft and use rely on permanent magnets used to generate electricity. As a consequence, the turbine weighs less and has fewer moving parts. Couple this strategy with innovations in generator stator designs that work at lower speeds, and wind turbines are able to produce higher output and work more efficiently with less wind.
A long term goal of the wind energy industry is to be able to store energy it gathers into large capacity batteries. While that technology is still years away, battery storage is being used now as part of a control strategy. Since “shakey” wind speeds make wind energy output “spikey,” in order to smooth out a wind farm’s electrical output, banks of batteries are being used to momentarily store energy before it is transmitted to the grid. This also provides cushioning to the grid so that other generators, such as natural gas, can come up to capacity to shoulder the load as wind energy slackens. The strategy has caught on to the point now that new wind turbines come with battery storage already built-in to them.
The Sky’s the Limit
Wind turbines and wind farms are said to have a certain “name-plate capacity”, that being the upper limit of the amount of electricity they can generate. Unfortunately, both land and sea-base wind turbines rarely operate near their generation capacity and instances where they exceed 40% are rare. On February 9, 2013, about 28 percent of the system load in ERCOT (Texas) was covered by 9,481 MW of wind power. That works out to nearly 85% of the installed capacity of 10,400 MW for an 8 to 12 hour period.
But if land and sea-based wind turbines can only reliably achieve 20-40% of their capacity, isn’t there a better way?
To overcome wind energy’s intermittent nature, wind turbines need to be placed where they can catch the wind 24/7/365. There’s only one place where the winds are constantly moving, and that’s 350 to 1000 feet up in the sky.
Admittedly, such a recommendation sounds like it belongs to science fiction, but a number of companies have recently committed to developing airborne wind turbines. The concept is very simple: mount one or a dozen wind turbines on a balloon (or blimp) or kite (or wing), tether it with a cable that also transfers the power to the grid, and send it up into the sky. There is the one little snag. As Robert Creighton, founder of WindLift, observed, “Of course, generating electricity with a kite has some challenges you don’t have with a standard wind turbine. Turbines can’t crash, for example.”
Not to mention wind energy “No-fly zones”. In fact, the FAA is seeking public input on developing a policy to help integrate “airborne wind energy systems (AWES)” into the National Airspace System.
Right now, there’s lots of companies launching kites, wings, and blimps to catch the wind. Makani Power (and its recent investor, Google) successfully completed a full autonomous flight of its 30 KW system on May 13, 2013, the kite flying a circular pattern between 800 and 2,000 feet. On March 27, 2012, Altaeros Energies tested a 35-foot scale prototype of the Altaeros Airborne Wind Turbine (AWT) at the Loring Commerce Center in Limestone, Maine. The AWT climbed up 350 feet high, produced power at altitude, and landed in an automated cycle. The AWT uses a helium-filled, inflatable shell and lifted a Southwest wind power Skystream 3.7 (400kWh) residential-sized wind turbine aloft where it generated twice the power normally captured from a 30 foot mast.
How much more energy can be harvested from the wind? “Geophysical Limits to Global Wind Power,” a study on the upper atmosphere published in Nature Climate Change, found that high-altitude wind power could extract more than 1,800 terawatts (TW). That’s about 100 times greater than the world’s current power demand.
With more innovation and more investment in the wind energy industry, it sure looks like the sky’s the limit.