Cambridge, MA--Cable TV companies and telephone companies typically provide the backup power needed to maintain service for their customers with valve-regulated lead-acid (VRLA) batteries. Designed to function at 77F, the batteries experience a reduction of 50% in usable life for every 15F rise in operating temperature. Batteries present environmental problems and require on-site monitoring and regular maintenance to function properly. Despite their technical maturity and relatively low acquisition cost, VRLA batteries can prove short-lived and expensive to maintain in many regions of the U.S.
To provide an alternate means of providing an uninterruptible power source for telephone and cable companies, SatCon Technology Corp. has used its experience in building momentum wheels for satellites to produce the model 20C1000 flywheel-based energy storage system. Supported by a hub and a frictionless magnetic bearing, each flywheel's fiberglass/epoxy rotor spins in a housing maintained at a low ambient pressure by an ion pump.
To support the flywheel, SatCon's engineers designed a low-cost hybrid magnetic bearing system intended to operate for 20 years. Bill Stanton, SatCon's vice president, describes the development of the magnetic bearing system as the key to meeting the project's cost requirements. "Basically we went to a hybrid bearing approach--hybrid meaning a combination of passive and active bearings--that minimized the necessary controls and the number of active axes. We actually reduced the number of active axes to one," says Stanton.
In the SatCon system, passive (permanent) magnets form the radial bearing system, and also provide axial support of the flywheel. An active (electromagnetic) bearing at the top of the flywheel functions as the system's only magnetic actuator. The magnetic bearings operate as elements in a position control loop that uses the single active magnetic axis as a force generator.
Data from a position sensor drives a loop compensation system, which calculates the force required from the actuator to move the rotor to the desired position. A power amplifier produces the necessary current in the actuator. A proprietary damping system developed by SatCon's engineers damps the spring/mass combination formed by the flywheel and passive bearings to prevent it from oscillating. Hybrid mechanical bearings serve as touchdown bearings for the radial magnetic bearings in case a failure or seismic load defeats the magnetic bearings.
A PM brushless motor/generator draws power from an electrical bus to spin up the flywheel rotor to its steady state speed of 30,000 rpm. As long as the load served by the system receives line power, the flywheel draws a nominal load from the bus to keep the flywheel's speed constant.
When bus voltage drops (i.e. there's a power outage or brownout), the synchronous PM motor/generator, which uses neodymium boron iron magnets in a three-pole pair configuration, switches into its generator mode. It then draws stored kinetic energy from the rotor, converting it to electricity and delivering it to the bus. After complete discharge, the flywheel can recharge in five hours .
Power and control electronics convert electricity drawn from the dc bus to ac to drive the flywheel's motor. The interface to the flywheel uses a bi-directional PWM variable-frequency inverter that converts power to the three-phase variable-frequency ac needed to excite the brushless motor. Commutation sensors determine the rotor's position relative to the stator.
Measuring 25 inches high by 18 inches in diameter, the assembled system weighs 260 lbs. Nominal output voltage can be 36 or 48 Vdc, with continuous output power capability being 1 kW for two hours. The flywheel's 100-lb energy storage rim stores 2,600 W-hr at its steady-state speed of 30,000 rpm. It's attached to the system's driveshaft via a lightweight hub and can operate at temperatures to 185F without any affect on performance or cycle life.
SatCon has received purchase orders from several firms for its 20C1000 Cable Telecommunications Flywheel Energy Storage System. It will undergo demonstration and evaluation tests at those customers' sites this year.
Additional details…Contact Craig A. Driscoll, Market Analyst, SatCon Technology Corp., 161 First St., Cambridge, MA 02142-1221, (617) 661-0540.