A flywheel (a mass rotating around an axis) is a simple machine. Originally these machines were made of steel; however, advancements have been made through the primary use of carbon fibers. These higher strength fibers allow the flywheel to be considerably smaller in dimension and lighter in weight. In addition, features have been integrated into current flywheel systems to ensure proper safety operation for not only the flywheel but also maintenance staff who work on the flywheels.

Figure 2. Satellite dishes outside WBRZ.

A flywheel collects energy in the form of kinetic energy. The energy stored is similar to the stored energy that powers a rubber band car. As you twist and tighten the rubber band by pulling the car backward, energy is stored in the rubber band. Once the car is released, the potential energy stored in the rubber band is transferred into usable kinetic energy making the car go forward. The flywheel spins around an axis, the motor-generator rotor, by the use of magnets in a vacuum. This energy is then stored in the motor-generator rotor. If normal power in a building is interrupted, the energy that is stored in the rotor is released and used as the power source. Once this begins, the flywheel generator motor begins converting all of the stored rotational kinetic energy into electrical energy, according to a publication entitled Flywheel Basics from Active Power. This generator produces the electricity needed to handle the emergency power loads in the building.

Figure 3. Pentadyne's flywheel unit.

Today, flywheels are being used throughout the United States in different applications. Most current installations are in the governmental sector for telecommunications and data storage centers. There are also uses of flywheels in medical facilities and 24- hour news networks, including all three national television networks.

Flywheels are important in telecommunication, data, medical, and news operations because of the large amount of computer and processing equipment located in each application. Non-computer equipment and other loads have a constant power requirement and would be plugged into a building receptacle like a lamp, vacuum, or television; computers, copiers, and medical monitoring devices produce fluctuations in power requirements. These fluctuations in power are seen as spikes and dips needed to run the specified piece of equipment. Flywheels clean up power very efficiently and eliminate the fluctuations that computer equipment creates. The flywheel levels off all spikes and peaks extending the life of the equipment, says Beacon Power’s publication Flywheels and Frequency Regulation.

For instance, Fort McPherson, an Army base in Georgia, replaced 572 lead-acid calcium cell batteries with two flywheel uninterruptible power supply (UPS) systems (see figure 1). These batteries were located in a 2,400 square foot space and required considerable maintenance.

Luke Wyland, energy conservation program manager at Fort McPherson, said the flywheels are primarily used for backup power in the telecommunication and data systems for the whole base. Since the data system is very large for the base, the UPS system incurs large amounts of oscillating power requirements (spikes and dips) because of the computer loads when normal grid power is lost. Wyland and Fort McPherson installed flywheel UPS because of the way that flywheels handle harmonics and because they conserve energy compared to lead-acid batteries.

Wyland also found that maintenance on the lead-acid batteries was very labor intensive. Just maintaining the batteries each year cost Fort McPherson a total of $128,000. Converting the power backup system from batteries to flywheels drastically decreased the amount of maintenance required for Fort McPherson’s energy backup system. Maintaining the flywheels requires that a trained factory technician from Active Power change the oil and vacuum pump each year, change the bearings every two and half years (a factory recommendation), and change out the steel wheels that float inside the vacuum every four years.

The total cost of maintaining both flywheels was $125,000 for a four-year period for an annual cost of $31,250, representing a savings of $96,750 on annual maintenance budget. In addition, maintenance labor decreases as employees at Fort McPherson perform only general maintenance to the room, which frees up the 94 days a year from battery maintenance. The flywheel also decreased the mechanical and electrical requirements for the HVAC system because of the decrease in conditioned floor space.

A final energy savings occurred in the recycling of hazardous materials. When the flywheels were installed at Fort McPherson, all 572 batteries were sent off to the Department of Energy recycling facilities in Minnesota. These batteries contained 118,000 pounds of regulated waste that would have been hazardous to the environment if not disposed of properly. All the lead and plates inside the battery were recycled and the facility was even able to recycle all of the fluid and plate containers. The electrolytes were also extracted and used to make lawn fertilizer that is sold in stores throughout the United States.

The newsroom at WBRZ-TV, the ABC television affiliate, in Baton Rouge, LA, is another flywheel application. The news operation is active 24 hours every day of the year. Television and radio stations are required by law to continue broadcasting if power is lost in the power grid.

When Hurricane Katrina hit the coast of Louisiana in August 2005, WBRZ-TV stayed on the air to alert the public of the hurricane’s danger and the damage it could cause.

Jean-Philippe Poirrier, a Pentadyne Power representative, said the power systems of WBRZ (see figure 2) were not disturbed and were fully operational throughout the encounter with Hurricane Katrina. This was accomplished through the use of a flywheel, the VSSdc model from the Pentadyne Corporation shown in figure 3.

The Pentadyne VSSdc unit at WBRZ is rated at 160 kilowatt for 13 seconds. This flywheel is utilized as backup power for the equipment in the WBRZ-TV 24 hour news center and corporate data center. In other parts of the building, WBRZ used another UPS system rated at 50 kilovolt-ampere (kVA).

Figure 4. Active Power also has success in television markets. This unit is installed at an Echostar facility

Television news stations switch to their emergency power system prior to a storm so power will not be interrupted. Any interruption in power would prevent the station from broadcasting important weather information until a generator was fully on line. In addition, the sensitivity of the equipment and its restart time make it imperative that power transitions seamlessly. WBRZ-TV switched to the flywheel UPS system before the hurricane made landfall. Throughout the storm, WBRZ-TV relied on the flywheel and generator system as its primary source of power.

Hurricane Katrina hit the coast of Louisiana as a Category 4 hurricane. WBRZ was responsible for informing their viewers of the progression of the storm. According to Poirrier there was never a loss of power to the WBRZ television studios throughout the hurricane. Other news stations in the area reportedly went down during the hurricane.

Clyde Pierce, the director of Engineering and Operations at WBRZ-TV, said, “I am very pleased with the flawless performance and reliability that the Pentadyne VSSdc demonstrated throughout Hurricane Katrina,” in a press release on Business Wire on October 19, 2005.

WBRZ had encountered problems with their emergency battery backup system during the few years prior to switching over to a flywheel system. The battery system that WBRZ initially utilized required too much maintenance and downtime compared to the flywheel. The switch to a flywheel was also advantageous because of the overall dimensions of the flywheel. WBRZ did not have space to allow the UPS to expand beyond the current space occupied by the batteries. The significantly smaller footprint of a flywheel filled this criterion and provided the station with additional storage floor space as shown in figure 4.

Enlarge this picture Figure 5. Graphic cost comparison between flywheel and battery systems.

Currently, WBRZ uses one flywheel for backup power. After the engineers of the building saw how the flywheels performed during the hurricane, they gained more confidence in the flywheel system. Eventually, they decided to switch to a total flywheel system. As the battery strings begin failing in the system, the engineers plan on removing those battery sets and installing other Pentadyne systems until the whole station relies on multiple flywheels for backup power.

In addition, almost every major television network in the United States uses a flywheel combination to keep their stations up and running.

According to Active Power, most of those stations are using the CleanSource technology. Specifically, ABC News switched to an initial CleanSource flywheel technology in 1998 when Active Power initially began its development. They use two Active Power CleanSource DC systems to provide power protection for their equipment ensuring that there is an uninterrupted source of power whether it be a simple disturbance from the utility or a major event that results in complete power loss.

An important factor in the decision aspect of the design is the cost of the equipment to be installed. Costs to be evaluated include, but are not limited to, hardware and installation, removal/replacement, annual costs for system upkeep, and mechanical/electrical requirements. These costs were compared in the table and figure 5.

Furthermore, the lifecycle cost comparison is easily seen in figure 6. The flywheel has a steady cost over the life of the equipment compared to the lead-acid battery. Initially the lead-acid battery is lower in cost, but after 20 years, the cost is much larger. Inflation has not been considered.

Enlarge this picture Figure 6. Cost savings increase with time.

Flywheels are drawing more attention today because of their focus on “green” or environmentally friendly technology. The use of flywheels for power generation is a promising technology. Innovations in use and efficiency will continue to bring flywheels to the forefront of power-system design.

Enlarge this picture The table compares the costs of battery and flywheel systems.

About the Author's: Craig Wanklyn recently graduated with a Master’s Degree in Architectural Engineering from Kansas State University. Craig is now employed as a Project Engineer at M-E Engineers in Wheat Ridge, Colorado.

Julia Keen, P.E., is an Assistant Professor of Architectural Engineering and Construction Science at Kansas State University.