Flywheels: Improving energy efficiency

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2009

This viable energy storage system rotates in a vacuum using magnetic bearings to minimize friction losses. 

KEY CONCEPTS
• A flywheel is a viable energy storage system that stores rotational energy as a rotor spins on its axis.
• A flywheel can undergo many more discharge cycles over its lifetime than a battery without losing its effectiveness.
• Incorporation of a flywheel in mobile gantry cranes used in ports has led to significant fuel savings up to 35% and significant reductions in particulates, NO_x and carbon monoxide levels.

Energy savings and emission reduction remain as two strong drivers in the development of new lubricant technologies. One other approach that is being evaluated is to harness the energy lost during the operation of mobile machines such as automobiles.

In a recent TLT article, technology was discussed that captured the energy lost when Class 8 refuse trucks brake during operation (1). A hybrid hydraulic drive was developed to recover the kinetic energy generated during the braking process. This energy was then transmitted hydraulically into a high-pressure accumulator. Energy is tapped from the accumulator to drive the truck through the hydraulic system. In this example, up to 70% of the kinetic energy obtained during braking can be recovered and reused.

Another mode to capture and reuse energy is the flywheel. This mechanical device stores rotational energy as a rotor spins on its axis. The flywheel has been in existence for thousands of years with the earliest known application being the Potter’s Wheel.

Dr. Mark Flynn, research engineer at the Center of Electromechanics at the University of Texas at Austin, says, “The flywheel can be considered to be a viable energy storage system that contains three main parts: the electric motor, flywheel and magnetic bearings. This device rotates in a vacuum using magnetic bearings to minimize friction losses.”

In effect, a flywheel acts in a similar fashion to a battery. Flynn says, “A flywheel has a number of advantages compared to a battery. The rotational speed of the former tells the user to what degree the device is charged. For a battery, the only way to find out its charge is to discharge it. Flywheels also can execute frequent charge and discharge cycles without any harm. In many of these cases, flywheels undergo deep discharge cycles that drop their energy levels from 100% down to 20%.”

Flynn estimates that a flywheel can do 2,500 deep discharge cycles per day for 20 years and not lose its effectiveness. In contrast, batteries only deep cycle for a few thousand times during their lifetime depending upon battery chemistry. To ensure battery life can be maintained, Flynn points out that a user needs to buy 95% more battery energy capacity to run a specific application than warranted in order to ensure that battery life is not significantly shortened due to too great a discharge depth.

MOBILE GANTRY CRANES
Flynn and his coworkers have developed flywheel energy storage systems for three applications. Initial work was done on a lightweight, hybrid electric transit bus designed for use in an urban environment. The flywheel was used to supplement the primary power source. This enabled the designers to substitute a natural gas for a diesel engine with the objective of improving fuel economy.

Flynn says, “We found that utilizing the brake energy captured by the flywheel enabled the acceleration rate of the bus to double to 45 miles per hour while reducing fuel savings by 25% in road testing. The speed of the flywheel was increased through the testing to 31,000 rpm without any difficulty.”

Durability of the flywheel was tested up to 112,000 cycles without any problems. Testing was done over a wide range of speeds ranging from 27,000 to 36,000 rpm. It is not until a speed of 42,000 rpm—achieved during a laboratory-based overspeed test—that the flywheel started to show some performance problems. At that high speed, the balance of the flywheel began to change as predicted because the internal rings had started to physically separate. This wheel was further spun to 47,500 rpm without failure.

A second application was to incorporate a flywheel system into a train locomotive power system. Flynn says, “The objective of this work was to defer the cost of electrifying the railroad track in the short-term. Trains are large consumers of fuel so any changes that could be made to improve efficiency would be welcomed.”

Use of the flywheel enabled the train to be equipped with a lighter-weight turbine engine that can be run at a constant speed. The kinetic energy of the train is moved to the flywheel while the train is moving downhill. Alternatively, the flywheel can be charged from the turbine engine in anticipation of an uphill climb or an acceleration event.

The most successful application for the flywheel technology has been in the operation of mobile gantry cranes, which are used to move shipping containers in ports. These containers weigh as much as 40 metric tons each. Figure 2 shows a mobile gantry crane in an operation at a major port.


Figure 2. Flywheels have been used to capture the braking energy generated when mobile gantry cranes move containers to the ground in ports. This leads to significant fuel savings and reduced emissions. (Courtesy of the Center of Electromechanics at the University of Texas at Austin)

Flynn says, “Most of the power needed by the mobile gantry cranes is in lifting the containers. But in returning the containers to the ground, the descent must be done carefully to avoid an accident. The downward movement of the container is similar to an automobile moving downhill. Braking needs to be done to slow down the container, and this process leads to the dissipation of heat.”

Inclusion of a flywheel in the process enables the energy lost as heat to be retained for use in hoisting the next container. Flynn adds, “We have seen significant fuel savings up to 35%, as use of the flywheel leads to a reduced use of diesel fuel and a smaller engine. Emissions levels have also declined significantly with particulates reduced by two-thirds, No_x by 25% and carbon monoxide by between 20% and 25%.” 

Mobile gantry crane engine sizes can be reduced by one-third to one-half when used with the flywheel system. The only problem seen by Flynn is the propensity of operators to leave the cranes idling.

A new application that Flynn will be evaluating is the use of the flywheel in municipal subway systems. He explains, “Upon entering a station, a subway train will brake and lose energy as heat in the process. Our objective is to capture the braking energy in the flywheel and use it to accelerate a second train as it is leaving the station. The flywheel may emerge as a way to improve energy efficiency by keeping the electric voltage of the subway system nice and steady.”

Further information on the use of flywheels in the mobile gantry crane application can be found in a recent article (2). Flynn also has expressed the desire to move away from magnetic bearings and use highspeed, long-life mechanical bearings. He adds, “Magnetic bearings can only withstand impact forces up to 3Gs and are expensive.”

Flynn’s challenge is how to effectively lubricate the bearing without contaminating the flywheel while operating in a vacuum. The bearing is two inches in diameter and needs to rotate at 36,000 rpm.

Flynn would welcome any ideas for lubricating this bearing. He can be contacted at m.flynn@cem.utexas.edu. 

REFERENCES
1. Canter, N. (2008), “Hybrid Hydraulic Drive Reaps Fuel Savings,” TLT, 64 (8), pp. 14–15.
2. Flynn, M., McMullen, P. and Solis, O. (2008), “Saving Energy Using Flywheels,” IEEE Industry Applications Magazine, 14 (6), pp. 69–76.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.