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From Child's Toy to the ISS: Flywheels Hold the Power
As dangerous as a power outage in a home, power plant, or a hospital can become, it's nothing compared to the crisis humans may face if they lose power on a spacecraft. Take the International Space Station (ISS), for instance. It relies on electricity for all its myriad functions. The ISS uses electricity to power its lights, run its life support systems, and energize its computers, as well as several expensive and scientifically important experiments. Electricity even splits water molecules to create the air the ISS astronauts breathe. Electrochemical batteries provide power for the station when it enters the Earth's shadow each orbit, but NASA is seeking an improved method of storing electricity on spacecraft.

Credit: Texas A and M University
Right: Alan Palazzolo and his team at the Center for Space Power in College Station, Texas, have been working to increase the reliability of the magnetic suspension of the flywheel energy storage system for use in spacecraft. This high-speed test rig is used for developing a monitoring and diagnostic system to detect and correct anomalous conditions and to adaptively optimize the flywheel magnetic suspension for improved flywheel efficiency.

Principal Investigator Alan Palazzolo of the Center for Space Power (CSP), a NASA commercial space center at Texas A&M University, College Station, Texas, has been working on various parts of a flywheel energy storage system, or kinetic battery, for many years. He and his colleagues are taking aspects of the current terrestrial technology and fine-tuning them for space applications. Their work has already yielded several patents for CSP and its research partners.

Currently, the ISS is powered by several solar arrays, which contain multiple solar, or photovoltaic, cells. More arrays will be added as construction of the space station continues. These photovoltaic cells, which produce electric current from sunlight, are pointed toward the Sun to catch as much light as possible. While the station is in light, the cells power the craft directly and charge a bank of rechargeable nickel-hydrogen batteries that then provide power while the station is in shadow. Because it is in low Earth orbit, the station can be in shadow for as long as 36 minutes of its 92-minute orbit. Thus, these batteries constantly discharge and recharge.

Unfortunately, electrochemical batteries, such as nickel-hydrogen rechargeables, can be discharged and recharged only a limited number of times. Replacing worn-out batteries by space shuttle is expensive -- in cost of the actual batteries, in lost cargo space on the space shuttle, and in installation time.

A flywheel energy storage system would be more efficient, weigh less, and have a longer life than electrochemical batteries under the same conditions. Flywheel batteries could save NASA tens of millions of dollars a year in equipment and energy costs, as they could be used in satellites as well as the space station, and they would yield significant benefits for terrestrial applications as well.

Batteries work on a simple premise: Energy can be neither created nor destroyed, just transferred from one type to another. In simplified physics, batteries transfer potential, or stored, energy into kinetic energy, or movement. An electrochemical battery stores potential energy through the chemical reaction of its components. The reaction creates free electrons that, when connected to a circuit, will move through the circuit and drive a load (a motor or lightbulb, for example). This reaction will continue until the chemical reactants are depleted.

For rechargeable electrochemical batteries such as those used on the ISS, reversing the current through the circuit reverses the chemical reaction. Electrons are returned to a higher energy state by rebuilding the chemical composition. This restores the chemical reactants to their original composition and transfers energy (from sunlight, in the case of the ISS) to potential energy once again.

Flywheel batteries also work by transferring energy. First, kinetic energy spins up a flywheel. What's a flywheel? Think of a child's toy top, advises Fred Best, director of the Center for Space Power. "If you imagine a child's toy top spinning, that's fundamentally what a flywheel is, only a flywheel spins at much higher rates of speed and is much more massive than a child's toy. And the reason that the flywheel is useful is that the spinning aspect -- that high rotational speed -- is a way to store energy." In a flywheel battery, external electrical energy (through a motor) is the kinetic energy that powers the motor that spins up the flywheel, transferring the electrical energy to rotational kinetic energy. As the flywheel is discharged and spun down, the stored rotational energy is transferred back into electrical energy by the motor -- now reversed to work as a generator -- and creates electricity to supply power where it is needed.

While flywheel systems do the same job as rechargeable electrochemical batteries, new developments have made them vastly superior in several ways. Best explains, "Recent advances -- and by recent I mean over the past 10 years -- in both the materials that you can make flywheels out of and the way to control flywheels, have allowed us to start spinning the flywheels up to very much higher energies than was possible in the past. What this means is that the energy we talk about being stored in the flywheel -- its kinetic energy -- begins to surpass, on a mass basis, the energy stored in an electrochemical battery."

Researchers are finding that they can store much more energy per unit mass in a flywheel system than in electrochemical batteries. They have also determined that flywheels can be discharged at a higher percentage, meaning that more of the stored energy is available. As much as 80 percent more energy can be recovered from flywheels than from electrochemical batteries, given the same conditions. This increase in what is called depth of discharge offers several advantages. The flywheel systems could weigh less than electrochemical batteries, a benefit on spacecraft, where weight is a limiting factor. They could also survive more cycles of charging and discharging, with less wear and tear on the system and a longer life span. An electrochemical battery lasts between four and five years on the ISS, whereas a flywheel would last as long as 15 to 20 years.

While flywheel technology is now in use in several terrestrial applications -- such as providing backup power for hospitals and serving as a power bridge (filling the gap between power outage and generator startup) in manufacturing plants -- it is still a young technology. NASA is trying to capture some of the potential of this promising energy storage system by sponsoring innovative research, and the payoff may not be for the space program alone.

Flywheel research at NASA is based at the Aerospace Flywheel Technology program at Glenn Research Center in Cleveland, Ohio. Project Manager Ray Beach heads the program and is responsible for coordinating research from many different NASA projects, with the eventual goal of building a demonstration unit. This unit, which may be ready within the next five years, will be used to determine whether flywheels are a viable replacement for the electrochemical batteries on the ISS. Like the batteries, the flywheel system will be charged by current from the photovoltaic cells of the solar arrays. The current will spin up the flywheel through a motor, which will then be turned into a generator, and the flywheel will be spun down to transfer the energy back to the generator to create electricity. Palazzolo and his researchers at CSP are among the main contributors to the work being done at Glenn. When Palazzolo started this research, the technology was not capable of the high speeds and split-second controls necessary for use in space. His solution was to develop better magnetic bearings. Best explains, "When you're trying to rotate something at 60,000 rpm, if you had mechanically contacting bearings, the friction from that would be prohibitive in terms of the energy that would be consumed. Things would just melt. Palazzolo's magnetic bearings suspend the rotor in a vacuum, and that's what actually allows us to have these things spinning at 60,000 rpm." This top rotational speed of 60,000 rpm is faster than any other flywheel system, and it can hold between three and four times more energy per unit weight than any other flywheel system that has been measured.

An additional challenge has been controlling the rotating shaft of the flywheel. Palazzolo is working on a feedback system that monitors the rotor and can make minute and rapid corrections to keep it true. Palazzolo describes how it works: "If [the shaft] deviates from the target [optimal location], then the feedback control takes that error, sends a correctional signal to the electromagnetic bearings that support the shaft, and the electromagnets then pull the shaft back toward the target.

"In a system like a flywheel, that's spinning at a thousand revolutions per second," Palazzolo adds, "the response time of the control system has got to be on the order of fractions of one millisecond." Any deviation of the flywheel for more than a split second could have potentially damaging results, especially at full speed. In addition to shifting the flywheel back into balance, the control system can shut it down to prevent damage to the spacecraft. Development of these controllers resulted in a patent for Palazzolo and other researchers at Texas A&M University.

Another aspect of Palazzolo's research for Glenn is the potential dual use of the flywheel as a battery and as a momentum wheel to assist with attitude control. "Because flywheels rotate, they can affect the spacecraft that they're on," says Best. "The Hubble Space Telescope has momentum wheels on it, and they act to allow the telescope to orient itself in space, to point in a given direction." Without momentum wheels, spacecraft would have to use thrusters to "steer" themselves. Palazzolo is working on the possibility of creating a unit with both energy storage and attitude control capabilities.

While much of Palazzolo's work is with the Glenn project, the focus of CSP is leaning more toward the commercial application of their research. CSP is one of several commercial space centers within the Space Product Development Division of the Office of Biological and Physical Research at NASA. One way flywheels could be extremely beneficial to the commercial space industry is as an energy source for satellites, particularly communications satellites. Most satellites are geostationary, orbiting 22,300 miles above Earth and remaining fixed over one point on the equator. While perfect for television, these satellites are awkward for voice communication, because the lag time is too great to conveniently carry on conversations. Although satellites that have low orbits would be perfect for communications (because there would be virtually no lag time in receiving signals), their 90-minute orbit puts batteries through so many charge-discharge cycles that, like those on the space station, they wear out in four to five years. This makes the satellites not very commercially viable. Flywheel batteries could open up a whole new market for low-orbiting satellites. In addition to working with the commercial space industry, Palazzolo and his team have worked with several ground-based commercial partners, assisting them in developing better systems. One of CSP's most promising ground-based projects is with the Federal Rail Association and the Center for Electromechanics (CEM) at the University of Texas, Austin. Together, these partners are developing applications for flywheels in trains.

geostationary and low Earth orbit
Credit: Dave Dooling and Jacky Edwards

Right: The efficiency of flywheel batteries may make placing satellites in low Earth orbit economically feasible. Currently, most satellites are in geostationary orbit, where they spend less time in shadow and so extend the life of their electrochemical batteries. Flywheel batteries would enable satellites to fly closer to Earth, where they could be used for communications without lag time.

"The CEM intends to reuse energy from braking," says Palazzolo. "With one of these flywheels in a subway train, each time it comes to a stop, instead of losing all the energy to heat in the brakes, it is stored in the flywheel. Then, when the train leaves the station, it uses that same energy and really improves the efficiency," explains Palazzolo. "There has been considerable effort and investment for applying this regenerative braking scheme to automobiles. Hopefully this will have a big payback in cleaner air and less dependence on foreign oil."