This article discusses different features of underwater kites and its advantages in the turbine industry. The underwater kite moves fastest when it slaloms through the current in this way, much like a water skier. Electricity generated by the mounted turbine generator is transmitted along the tether to a moored, floating buoy, and then onto the power grid. This concept, now known as the Tethered Undersea Kite (TUSK), was first conceived by Magnus Landberg, a researcher in Sweden, in 2007. Underwater kites look to be feasible to build using commercial available technology. According to economic analyses conducted by other research teams, TUSK systems may be able to produce electricity at about half the current cost for fixed hydrokinetic turbines, and a bit below the cost of the power produced by offshore wind turbines. Those researchers attribute the lower costs to improved power-to-weight ratios derived from replacing the inner blades and support tower of a traditional turbine with a lightweight, low-cost tether.
A boat cuts across a placid lake, towing a water skier. As long as the skier points his skis in the same direction as the boat, their speeds match. But the skier swerves and starts to trace graceful curves behind the boat. Even though the skier and the boat are still connected by a rope, the skier's speed relative to the water increases, sometimes by as much as 50 percent.
Objects at the ends of ropes or tethers can pick up surprising amounts of speed. A trapeze artist at the bottom of her swing can reach a speed of 20 mph while the balls on the end of an Argentinian bola swung by a gaucho circle at over 50 mph. Perhaps the most deceptive are kites. Miles Loyd, an engineer at Lawrence Livermore National Laboratory, showed as early as 1980 that a winged kite moving through the wind can travel at a velocity equal to the wind velocity times two-thirds of the wing lift-to-drag ratio. For a conservative lift-to-drag ratio of 9, the wing accelerates to six times the wind velocity.
A kite tracing arcs in a stiff breeze on a very long line can reach speeds close to one hundred miles per hour.
With those kinds of speeds comes the potential for harvesting power from the wind, or an ocean current.
For decades, researchers have been pursuing the concept of tapping hydrokinetic energy from ocean or tidal currents to generate electric power. Still, ocean-current (or hydrokinetic) energy conversion remains a largely unfulfilled promise. Virtually all of the existing generation units use fixed turbines mounted on the seafloor. These stationary generation units depend on the currents to spin their blades and crank their generators.
The amount of generated power is proportional to the cube of the water velocity flowing through the turbines; increase this velocity by 26 percent and the power doubles. The velocity—and the generated power—can be increased even more dramatically if the turbine, instead of standing still, can be put in motion so it can actively move through the current.
Tethered wings that slalom across water currents can drive turbines.
Mount the axial-flow turbine onto a wing-shaped kite, attach the kite to the end of a tether, and “fly” the kite underwater, tracing figure-eights across the flow of the current. The kite moves fastest when it slaloms through the current in this way, much like a water-skier. (In fact, high-performance hydrofoil water skis, which lift the skier off of the water to reduce drag, also use an underwater wing.) Electricity generated by the mounted turbine-generator is transmitted along the tether to a moored, floating buoy, and then onto the power grid.
This concept, now known as the Tethered Undersea Kite, or TUSK for short, was first conceived by Magnus Landberg, a researcher in Sweden, back in 2007. I have been part of a group researching the same idea at Worcester Polytechnic Institute for more than three years now, and we believe it has the potential to generate vast amounts of inexpensive, renewable power.
Applying Loyd's calculation for the maximum velocity of a crosswind kite, a turbine flying on a rigid-wing, underwater kite at six times the current velocity could generate 216 times as much power than a stationary turbine in the same current.
Power output is, of course, restricted by inefficiencies and the theoretical Betz limit for turbines, and this is not violated for power kites.
The kite moves fastest when it slaloms through the current in this way, much like a water-skier.
I was first inspired to work on kite power after reading a June 1979 Mechanical Engineering article by Jitendra Goela, a researcher in India at the time, who has also consulted with our WPI research team. In the mid-2000s, when I discovered Goela's article, research teams around the globe were starting to design kitebased power systems, collectively known as Airborne Wind Energy, or AWE. Such systems use large fabric kites or rigid wings (with onboard wind turbines) that fly at the end of a flexible 300 m tether. At that altitude, kites can take advantage of faster, steadier winds than conventional wind turbines can reach, which means that more energy can be extracted from the wind with AWE systems.
AWE systems have faced some challenges. When the wind dies, an airborne kite will crash into the ground unless carefully controlled. It is likely that such systems will have to fly over unpopulated areas. The 300 m tethers attached to the kites are heavy and degrade the kite's flight performance.
About four years ago my research team began wondering whether the experience we had gained with AWE systems could be translated into the study of a power-generating system using “underwater kites.” Like an airborne kite moving in a breeze, the wing in a TUSK system is able to travel at high speeds solely due to the ocean current flowing past the moored buoy and wing. No on-board propellers are required to generate thrust. In short, underwater wings act as current speed enhancement devices. As a result, they can be used in low-speed currents where conventional hydrokinetic turbines do not generate enough power to be economical.
And underwater kites may prove to be more practical than kites in air. Instead of crashing in a slack tidal current, for instance, an underwater kite will safely hang beneath the waves on its tether until the tide picks up again. The highest ocean current speeds are actually found near the ocean surface, so that TUSK tethers can be much shorter and weigh less.
Underwater kites look to be feasible to build using commercial available technology. Kites with wingspans of up to 15 m and areas of up to 35 m2 that weigh about 10 metric tons can be constructed using carbon fiber or glass-reinforced plastics commonly used in boat hulls. Tethers with lengths of up to 100 m can be made from high-modulus polyethylene fibers, which have already been proven as AWE tethers and have sufficient maximum strength to withstand the large tether tensions (more than 1 million Newtons) predicted for underwater kites. Buoy and mooring requirements should be comparable to those for floating, offshore wind turbines. Practical control systems needed to autonomously manipulate the kite elevators, ailerons, and rudders can be designed.
We were quickly struck by the extraordinary potential of the idea. Ocean currents represent a remarkably concentrated source of energy—a current of 1-to-2 m/s has an available power density of about 1 kW/m2, or about 800 times what is available in air moving at the same speed. A TUSK system with wing area of 30 m2 operating in that current will produce about 300 kW of power. A school of 100,000 such wings operating in the world's ocean currents would generate 30 GW of power, enough for about a quarter of U.S. households. And by the way, a similar number of wind turbines, about 250,000, now spin in wind farms around the world.
Operating in marine environments is always a challenge, and the wings must be compatible with both passing ships and ocean life. But a design that allows for the wings to be reeled in and pulled out of the water for servicing should make it easier to perform maintenance on a TUSK system than on a fixed hydrokinetic turbine.
According to economic analyses conducted by other research teams, TUSK systems may be able to produce electricity at about half the current cost for fixed hydrokinetic turbines, and a bit below the cost of the power produced by off-shore wind turbines. Those researchers attribute the lower costs to improved power-to-weight ratios derived from replacing the inner blades and support tower of a traditional turbine with a lightweight, low-cost tether. In essence, the wing sweeping a circuitous path through the water acts like the tip region of a traditional turbine blade, where most of the power is produced.
Proof of Promise
A team of WPI students and faculty members, including myself and Michael Demetriou, began working to help see if TUSK could fulfill its early promise. An early model we created to solve ordinary differential equations for kite-tether dynamics showed encouraging power output estimates. That model incorporated the important physics that determine system performance, from hydrodynamic lift and drag to buoyancy forces and moments and cavitation effects on the blade tips of the lightweight axial-flow turbines. Solving the resultant differential equations in Matlab software provided outputs for such factors as instantaneous kite velocities, accelerations, and orientation in addition to system power output. The software also models control systems needed to create the desired slalom motion of the wing.
We also designed, built, and tested a small-scale TUSK kite and tether. The kite, fabricated with a 3-D printer, has a wingspan of 40 cm, weighs about one-half kilogram, and uses NACA 0005 airfoils for its wing. A scaled threebladed turbine and micro-generator mounted at the front of the kite extracts energy. To simplify our early tests, a rigid, hollow carbon fiber rod is used for the tether instead of HMPE fibers. A single rudder adjusts the kite's yaw orientation to control its slalom motion. The kite can be manually controlled via a joystick, or with an autonomous control system still under development.
Operating in marine environments is always a challenge, and the wings must be compatible with both passing ships and ocean life.
To save money, we had the clever idea of turning the swimming pool in WPI's Sports and Recreation Center into a water tow tank. We attached the tether and kite to a wheeled cart that we rolled along the pool deck to simulate an ocean current. This allowed us to system check our data acquisition instrumentation during brief slalom runs.
We next tested the model in a 6-meter-wide, two-meter-deep water flume at the Alden Research Laboratory in Holden, Mass., where we exposed it up to 1 m/s simulated currents. Proof-of-concept tests of the scale model were conducted in spring 2016, during which repeatable slalom motions were achieved, with kite trajectories and velocities measured. Future tests in the flume at Alden will further refine the scale model to better estimate the potential power output of full-scale TUSK systems.
The scale-model tests are supported by computational fluid dynamics simulations that we have developed in collaboration with Gretar Tryggvason at the University of Notre Dame. Those simulations use a numerical grid that moves with the kite wing, and partial differential equations (the Navier-Stokes equations) are solved at every grid point to determine the flow velocities and pressures near (and on) the wing as it moves.
No onboard propellers are required to generate thrust... (and) they can be used in low-speed currents where conventional hydrokinetic turbines do not generate enough power to be economical.
Simulating the kite in this way allows us to better predict kite lift and drag, tether tension forces, kite trajectories, and power output than we could by using the ordinary differential-equation-based models we created earlier. Flow visualization studies allow us to picture the flow over the kite to better understand how it interacts with the current, and we can compare the results from the CFD simulations to those from our scale-model tests to help us better design new underwater kites.
We are not the only ones looking into the feasibility of underwater kites. An European company, Minesto UK Ltd, founded in 2007 by Anders Jansson, has developed an advanced system it calls Deep Green. That system uses an underwater wing with upturned tips, a kitemounted turbine, and a tether that runs to the ocean floor. Minesto has conducted long-term sea trials of quarter-scale kites, which have been underway off the coast of Northern Ireland since 2010. At the same time, researchers at the University of Strathclyde in Glasgow, working with Minesto, have considered potential risks during operation and maintenance of Deep Green. Minesto's next step is to implement a 500kW power plant off the coast of Wales in 2017.
A Canadian company, HydroRun Technologies Ltd., started developing its Freestream Glider technology for use in river currents back in 2012. Its underwater wing actually resembles a glider, complete with rudders and elevators on tail surfaces behind the wing. The glider is tethered to a generator on a buoy on the river surface. After successfully testing a 40 kW pilot plant on the Fraser River in British Columbia, HydroRun suspended operations in 2015 due to some changes in the economics of the energy industry in Canada.
A Dutch company, SeaQurrent, has developed the TidalKite which consists of several wings tethered to the seabed. Forces generated by the high velocity wings drive pressurized fluid through a hydraulic motor to generate power. A scaled prototype of the TidalKite has recently been tested at the Marin Research Institute in the Netherlands.
Only a few companies and schools have worked on underwater kites, a relatively new concept compared to airborne wind energy, which has benefited from the work of researchers at dozens of institutions worldwide. We expect more researchers to begin working on this emerging technology in the coming years. What will likely drive this future interest is the realization that tethered undersea kites have the potential to provide cheap electricity without producing pollution.
Ongoing work has begun to fulfill the promise of underwater kites, but more development is needed in the next decade to fully realize this potential. Improved methods for controlling underwater kites for optimal power production are needed, materials that will extend system life must be developed, and more accurate simulations of system performance will aid future design improvements. Prototypes are undergoing sea trials, and power plants that will electrify the grid are on the horizon. At the same time, research and development also continues at universities to further refine and optimize the technology.
At WPI we have started to study one such refinement where the wing and turbine are suspended under a tethered, floating hull. A similar idea has been patented by HydroRun. This winged hull should be easier to control than a fully submerged kite, since it only needs to move along the surface. This set-up also moves the tether between the hull and buoy out of the water and into the air, where it experiences far less drag. As a result, we expect that the hull and turbine will slalom at higher speeds, much like our water skier. So far, we have designed and built a test apparatus for this concept, and completed a first test in our water flume.
Studies like this, we believe, will continue to advance this exciting new technology.