Matthew L. Lazarewicz
Richard L. Hockney
Beacon Power Corporation, Wilmington, MA, USA
The information era has spawned the demand for reliable electrical power. Beacon Power Corporation is using its flywheel energy storage technology to create new products to meet the rapidly expanding demand for reliable, distributed electrical power. While there are many promising applications for flywheel energy storage systems, Beacon believes that the communications markets (internet, telephone, wireless, and cable television) are the most attractive entry.
Compact high-energy flywheels are a recent development. Beacon has used new materials such as high-strength fibre, efficient electric drives, and high performance magnetic bearings to create a new generation of flywheel products. The flywheels are fabricated from high-strength fibre composites, such as graphite, which allow the flywheels to rotate at high speeds and store large amounts of energy relative to similar size and weight machines made from metals.
In a typical, grid-connected application, electronics convert the AC utility power into DC power from which the flywheel is charged, and then the flywheel energy storage unit maintains power when the utility power fails.
A 0.5 kWh to 1 kWh system is sufficient to accommodate the power quality and transition to generator markets where high power for short periods of time are delivered. The telecommunication market, on the other hand, requires relatively low power (0.5 kW to 2 kW) for four to eight hours. Energy storage systems of 2, 4 or 8 kWh, or even higher, are required.
Currently this market uses lead acid batteries to provide the energy storage. Generators, fuel cells, and new technology batteries are also being tried as alternatives to lead acid batteries. Flywheels, however, have several inherent advantages over the other technologies:
- High reliability and quiet operation
- Long life (10-20 years)
- Low (or no) maintenance
- Does not use hazardous materials
- Underground installation
- Can free up remote terminal volume by displacing batteries
- Easily monitored and controlled on-line
- Instant load response (charge and discharge)
- Fast recharge (approximately 1/2 hour for 2 kWh wheel)
- Low sensitivity to ambient temperature
- High cyclic life (>1000 deep discharge cycles).
A flywheel energy storage system consists of a rotating cylinder (rim) in a container, electronics for control and monitoring and a motor/generator. The motor/generator transforms electrical energy into stored kinetic energy by spinning the composite rim. The flywheel is able to spin for extended periods with great efficiency, because friction and drag are virtually eliminated by employing magnetic bearings and a vacuum in the container.
When the product is called upon to produce electricity, the spinning flywheel drives the motor/generator, and its bidirectional inverter converts the kinetic energy into electrical energy. This conversion continues, and the flywheel slows, until it is “discharged”.
The battery replacement system connects directly to the 48 V battery bus and gets its charging power from the existing rectifiers. The AC input system gets its charging power directly from the utility line, reducing the current requirements for the rectifiers. The fast recharge system is designed to take advantage of the extra power capacity of the utility interface and quickly return the flywheel to full charge. The high power system produces the higher voltage and power needed to service emerging powering strategies.
The 2 kWh flywheel is designed for low cost, high reliability, and inexpensive underground installation. It is roughly 1.2 m tall and 0.81 m in diameter with a weight of approximately 272 kg.
Energy is stored in the composite rim which spins at a top speed of 22 500 r/min and spins down over a three to one speed range in order to give up its 2 kWh of energy. Energy is converted from electrical to mechanical and back via the permanent magnet motor generator mounted on the shaft with the flywheel.
The entire rotor weight is suspended on magnetic bearings for high reliability, low loss operation. All of the components of the flywheel module are contained within a vacuum enclosure with a self-contained vacuum maintenance system which is required to maintain the ambient pressure level low enough so that the drag losses are acceptable.
The energy storage rim is made from a combination of carbon fibre and glass fibre composite materials with epoxy matrix and stores a total of 2700 Wh at its design speed of 22 500 r/min.
The five-axis active/passive magnetic bearings are designed to operate for 20 years under all environmental conditions and duty cycles without degradation of operation. The bearings are biased using permanent magnets for low loss operation. Mechanical bearings are employed as coupling to the radial dampers, and also serve as backup for the magnetic bearings in the event of a failure or a seismic occurrence which exceeds the capability of the magnetic bearings.
The electric machine which performs the bidirectional electromechanical energy conversion is a permanent magnet brush-less design. The iron-less stator is conduction cooled. The rotor uses neodymium boron iron magnets in a two pole-pair configuration which achieves more than 96 per cent efficiency when averaged over a complete charge/discharge cycle.
Figure 1. Typical back-up time, power and energy requirements by application. There is an increasing demand from the telecommunications industry for reliable electric power
Beacon’s proprietary vacuum maintenance system is completely self-contained inside the vacuum housing, and has no moving parts. This system is projected to last 20 years when employed in the mechanically-sealed housing.
All of Beacon Power’s flywheel systems have the capability for remote control and monitoring of system performance. Presently, this remote access uses either a telephone modem for remote terminals, or Telnet for internet cable installations. The monitoring interface displays any faults occurring in the system, and a list of system diagnostics.
The controller will either automatically shut down the system to protect it and the user equipment from any serious faults, or try to restart.
The control interface screen allows the user to access internal operating parameters which can facilitate system diagnostics. The user can also attempt to restart a faulted unit. In addition, this interface allows the user to change selected configuration variables, including the ability to spin down the unit for servicing.
The flywheel energy storage system (FESS) is powered by a simple switch that allows AC power to start the flywheel in motion. System startup is then monitored locally with the operator interface screen.
Beacon considers flywheel safety to be of paramount importance. The system is designed to prevent a high energy, short duration event. The rotor must be designed for an extremely low probability of failure, but if failure were to occur, the system must be designed to maintain full control of the debris to protect people and property. The design should also slow the rate of energy release to minimize external forces.
In order to maintain the safety aspects, the composite rim on the flywheel is designed not to burst. Burst stresses are at approximately 25 per cent of allowable limits at operating speed (400 per cent margin). Less than one per million of the rims are expected to fail at these margins. The current rotor is limited by the metal hub which attaches the composite rim to the shaft with a generous stress margin of 50 per cent.
Because of the difference in margins, the hub is always expected to fail before the rim. In addition to this, the rim has strength capability to contain possible hub failures and/or fragments at design speed.
The control system is designed to prevent the possibility of an overspeed using three independent detection modes – each mode would individually prevent the overspeed.
Both full system and component testing have demonstrated that the most catastrophic hub failure results in mechanical damage, but does not result in a rotor burst.
Flywheels are not new, but recent technological advancements in composite materials, power electronics, vacuum technology, internet communications and efficient magnetic bearings have enabled this new generation of flywheels to be created. The technology comes at the right time to support the distributed powering needs of the telecommunication industry.