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Control for Advanced Energy Systems

Advanced Controls Enable Airborne Wind Power Generation

Complex control challenges that a few years ago seemed impossible, have now become manageable thanks to improvements in processing power, FPGA technology and software control, making innovative wind power generation possible.

BRIAN MACCLEERY, NATIONAL INSTRUMENTS

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Just a few thousand feet above our heads blows a vast untapped resource that could supply civilization with ample quantities of cheap, clean and abundant energy. No leaps of science are required to tap into it and no unsolvable technical hurdles prevent it from becoming a practical and reliable source of electricity. This resource has a terrific “energy payback ratio,” meaning the amount of energy required to build and maintain it is much less than the total energy produced during the system’s lifespan. Additionally, it has an impressive “capacity factor,” as its average power production is higher and more stable than other renewables. This could someday tap into the highest “power density” source of renewable energy on the planet. Finally, it has great “land use intensity,” meaning the amount of power produced per acre of real estate is excellent. It could be installed profitably almost anywhere in North America, even close to large population centers, and produce terawatts of clean energy, which is orders of magnitude less expensive than that from fossil fuels. Ultimately, you could extract enough power to supply the entire world’s energy demand without negatively impacting the climate. So why haven’t you heard of it before?

The answer lies with the initial complexity, cost and Moore’s Law. Just ten years ago, the necessary processing, instrumentation, sensors and control software were prohibitively expensive and complex to design. Today, exponential increases in computing power, advanced instrumentation technology and high level software development tools, tightly integrated with COTS reconfigurable embedded systems, make it possible for the pioneers of this new industry to build prototypes by the dozen and shorten time between designs. 

The world needs a clean energy hero as we all have a stake in the outcome, and the urgency is palpable. If necessity is the mother of invention, this child may break the “cost barrier” and reach the tipping point where natural market forces take over and drive adoption at hyperbolic rates. Please welcome to the stage a new contender that may have what it takes to carve out a healthy slice of the $6 trillion dollar clean energy technology (ET) market—airborne wind.

This is not your father’s wind turbine. It has no blades. It has wings. No massive steel tower. It has a tether cable. There’s no human pilot aboard this experimental aircraft. Even Chuck Yeager could not survive the G forces. 

How Does It Work? 

Airborne wind borrows many established technologies from the conventional wind energy industry. The main feature that differentiates airborne turbines is the way they extract energy from the wind. Instead of a large steel tower structure, a tether cable anchors the system to the ground. Rather than rotating blades, specially designed airfoils sweep a path across the sky. 

This ability to sweep through a larger cross section of the wind is one of its fundamental attractions—facilitating a modestly sized airfoil to extract large amounts of energy from the stronger, more consistent wind higher up. Like the tip of a conventional turbine blade, the airfoil ?ies crosswind in a circle or figure 8 pattern at many times the speed of the wind, as shown in Figure 1. With a wingspan comparable in length to a wind turbine blade, an airborne turbine can sweep a larger region of the sky to harness nearly ten times more energy.

Figure 1

Mechanically, airborne turbines benefit from being cushioned in a pillow of air during flight rather than rigidly connected to the ground. However, the G-force loads caused by their fast moving patterns can put significant stress on airfoil structures and tether lines.

Flying one to two thousand feet above our heads, airborne wind is on track to become a cost-effective, practical and utility-scale-ready segment of the wind industry within the decade. Bringing utility-scale airborne wind to market at those altitudes doesn’t require any breakthroughs—just solid engineering work, R&D investment and the support and guidance of the experienced aerospace and wind engineering communities. 

At least thirty startups and research groups around the world are busy at work to make airborne wind a reality. Over the years, their prototypes have proven the basic principles of airborne wind and grown into the tens of kilowatts. The next step for the industry leaders is to prove their systems can scale up to megawatt production levels and perform reliably during long-term continuous operation in the field. If airborne wind makes it off the ground, it just might change the world.

“Would airborne wind have been cost-effective ten years ago?” comments Joby Energy Business Development Director, Archan Padmanabhan. “No. It’s definitely advances in technology that make it cost-effective today—from inexpensive aircraft materials, to low cost GPS sensors, autonomous flight software and the increasing power of embedded computing. The biggest technical hurdle that has been overcome is in the area of control systems. Thanks to the aerospace industry, flight control systems have become a lot more robust than any time before. Commercial airlines today are primarily flown on autopilot and people trust those systems—no one expects airplanes to come crashing down. The aircraft industry has a lot to offer and we are learning from it.” 

Harnessing high altitude wind is a bold vision that brings with it a wide range of technical and logistical challenges—from finding tether lines that are strong and light enough to gaining Federal Aviation Administration (FAA) approval and airspace clearance. Even at two thousand feet altitudes, FAA permitting questions need to be resolved. At least for now, making high altitude (tropospheric) wind commercially viable is likely to remain elusive. 

Similar to the early days of the ground-based wind industry, researchers in airborne wind are testing numerous design options to determine what works best. Even with computer models, there are no substitutes for physical prototypes, which help convince skeptics and attract investors. 

From Prototype to Production with CompactRIO

Windlift, a North Carolina-based airborne wind startup, is developing mobile airborne wind turbines that have attracted interest from the U.S. military because their high power density makes them a future replacement for diesel generators and the vulnerable fuel convoys that must supply them. Windlift uses National Instruments LabView graphical programming language and NI CompactRIO ruggedized embedded instrumentation systems for control and dynamic monitoring, as shown in the interface for their 12 kW prototype system in Figure 2.

Figure 2

Windlift chose to use National Instruments LabView software and CompactRIO hardware for several reasons including their FPGA processing performance and flexibility. Ultimately, they chose it because “National Instruments illustrated a clear development pathway with the CompactRIO from prototype to production with the same hardware and software.” 

For the controller, Windlift uses the embedded computer that runs LabView Real-Time and LabView FPGA. Within the system, the microprocessor and FPGA work hand in hand to share tasks. Seven I/O modules are included in the system, each providing the front-end conditioning for the type of signals needed to interface, including digital I/O, RTD temperature sensors, load cells and CAN fieldbus communication. These modules interface directly with the FPGA, which can perform high-speed signal processing and/or pass the signals through to the real-time microprocessor.

Windlift’s system uses a three-line airfoil as the prime mover for their reel in/reel out system. A single tether attaches to the leading edge of the wing and carries most of the load. Two steering tethers attach to the trailing edge and are used to steer the wing and control its angle of attack. To steer, the system sends commands to two servo motors based on inputs from a human pilot on the ground using a “fly by wire” interface. “The direction we’re heading is to make that flight control fully automated,” explains control design engineer Matt Bennett. 

Pilot in the Loop

The prototype consists of two linear slides driven by a servo motor for steering, and an AC induction motor/generator, similar to what might be used in a hybrid bus. Each servo has its own internal controller, which receives position commands. For the tension control, the system adjusts the torque settings on the generator.

CompactRIO uses proximity sensors to measure the rotation of the drum and calculate how much tether remains on the drum; the pilot only needs to steer the kite, while CompactRIO manages all of the power generation functions, including detecting the end of tether and retracting the wing. Windlift plans to add inertial measurement sensors and telemetry systems to automate steering of the wing and enable unattended operation.

With two prototype systems that are working proofs of concept, Windlift estimates the next system will be production-ready. They are developing a 12 kW system and hope to start on the design in Q2 of 2011. To put this in perspective, an equivalent conventional wind tower would measure 65’ tall with a 75,000 lb reinforced foundation. Windlift expects to offer a more affordable option compared to a conventional turbine in this size class, even with the cost of their built-in energy storage system.

Ground-based generator systems, like those being developed by Windlift in the U.S., KITEnergy in Italy and SwissKitePower in Switzerland, produce power when the airfoil pulls a tether line. The torque and velocity of the tether cable produces electricity by spinning a generator which is attached to a rotating winch drum. As illustrated in Figure 3, there are two distinct modes of operation—the traction phase and the recovery phase. In the traction phase, the airfoil slowly pulls the tether line and electricity is produced until the maximum tether length or altitude is reached. Then the recovery phase begins; the airfoil is flown back while the tether cable is winched in. Recovery actually uses a small amount of power as the generator becomes a motor drive to retract the cable. Then the process repeats. 

Figure 3

For steering, the airfoil wirelessly transmits GPS coordinates and roll, pitch and yaw information from an inertial measurement unit (IMU) in the air to a kite steering unit (KSU) on the ground. KITEnergy uses the National Instruments PXI platform as the ground control unit, as shown in Figure 4, which acquires and processes the sensor signals and executes advanced control algorithms to command the winch motor/generator and steer the kite. “Theoretical, numerical and experimental results so far indicate that KITEnergy technology could provide large quantities of renewable energy, available practically everywhere, at lower cost than fossil energy,” comments Mario Milanese, KITEnergy founder. Shown in Figure 5, SwissKitePower also uses LabView in the networked computers for their airborne wind systems in addition to a ground-based generator approach.

Figure 4

Figure 5

Other companies, such as Joby Energy and Makani Power, are pursuing airborne generator designs. In this case, a number of small, propeller driven generators located on the aircraft are used for power generation and power is sent down the tether cable to the ground. Airborne generator systems are typically more like an aircraft and less like a kite—featuring an onboard computerized autopilot system and flight control surfaces to control roll, pitch and yaw like elevators and ailerons. A great deal of engineering effort at airborne wind companies is focused on perfecting these flight control systems and making them robust to withstand any sort of problem from gusting winds to actuator and sensor failures. The Makani Power system is being designed so it can even disconnect from the tether and land autonomously if needed. 

Not surprisingly, for such a budding child in the wind industry, the dust has yet to settle on which design choices prove to be the most practical and cost-effective. Airborne wind has a way to go before becoming a mature technology, but one thing is for sure—it’s an exciting time. Each new prototype that takes flight helps to convince skeptics and investors alike that “above ground wind power” isn’t such a crazy idea. Consider lending your talents to help airborne wind get off the ground.  

Airborne Wind Energy Consortium
[aweconsortium.org].

 

Joby Energy
Santa Cruz, CA.
(831) 426-3733.
[www.jobyenergy.com].

 

KITEnergy
Torino, Italy.
+39 011 225 8261.
[www.kitenergy.net].

 

Makani Power
Alameda, CA.
(510) 629-4316.
[www.makanipower.com].

 

National Instruments
Austin, TX.
(512) 794-0100.
[www.ni.com].

 

SwissKitePower
[swisskitepower.ch].

 

Windlift
Durham, NC.
[www.windlift.com].