Emerging UPS technologies, applications
Several emerging UPS technologies are gaining acceptance. Some of these up-and-coming UPS types include superconductive magnetic energy storage (SMES), flywheels, super capacitors, and fuel cells.
Keith Lane, PE, RCDD/NTS, LC, LEED AP BD+C, Lane Coburn & Assocs., Bothell, WA
SMES: A SMES system involves circulating dc in the field of a magnetic coil. A solid-state switch controls the circulation of the current in the system. The large magnetic coil creates a large amount of inductance when the solid-state switch is in the open, or nonconducting, state. This high inductive force pushes current into a capacitor. Control circuits in the system preserve a predetermined voltage level across the capacitor. Under normal operating conditions, the current circulates through the switch when it is closed to keep the magnetic field charged. An internal inverter converts the dc source into ac. The capacitor can discharge the required voltage primarily during short power-quality anomalies, such as utility switching events, providing power-quality protection to critical loads in the facility. After discharging, the system can recharge within a few minutes. The amount of stored energy can be in the MW range.
Currently, superconducting magnetic energy storage systems are available with low-temperature options, which use helium for cooling. High-temperature systems that use nitrogen are still in development.
Flywheel: A flywheel is a device that couples a motor-generator (MG) with a mass that rotates to store energy for a short period of time. Typically, flywheel systems can store less than 15 sec of kinetic energy for a fully loaded distribution system. The MG draws power from the serving utility or other power source to spin the rotor within the flywheel assembly. During a power-quality anomaly or a power outage, the spinning flywheel converts kinetic energy to ac through an inverter and control system to serve critical loads.
Even if a standby generator is not designed for life safety loads, it can typically be up and running within 10 to 15 sec after a power failure. Most flywheel UPSs will supply 15 sec of full-load power and can actually supply power for up to several minutes at less than full load. In addition, most utility disturbances last for 5 sec or less, according to the U.S. Dept. of Energy’s Federal Energy Management Program (FEMP). For these reasons, a rotary or flywheel UPS can be a viable option over conventional battery backup UPS systems.
A flywheel’s rotational speed has a dramatic effect on the amount of stored energy available for critical loads. According to FEMP, doubling a flywheel’s rpm will quadruple the available stored energy. High-speed flywheel systems require a different design approach than low-speed systems. High-speed systems are typically made from carbon or carbon and fiberglass composite materials, which can withstand the higher stress associated with high-speed systems. These systems also use magnetic bearings and vacuum enclosures to reduce rotational friction and system losses.
The traditional flywheel is built from steel and is restricted to about 2,000 to 3,500 rpm. Carbon fiber flywheels can spin at 30,000 to 55,000 rpm.
Flywheels have several advantages over battery systems including:
- Flywheel reliability is typically greater than a single battery string
- A flywheel requires less maintenance than UPS systems that use batteries
- Flywheel UPS systems do not require the controlled environment that battery-based UPS systems typically require: Batteries must be kept around 77 F, and can present explosion or acid spill risks
- Flywheels require less space than battery-type UPSs
- A battery system can require hydrogen sensors, spill control, and neutralization. Eye-wash stations may also be required with battery installations. In addition, battery system should be monitored continuously to ensure top performance and life expectancy.
Flywheels can be coupled with generators to provide continuous power for both short-term and long-term outages.
Super capacitor: Ultra capacitors (also known as super capacitors) work by storing electrostatic energy within the capacitor. In a typical application, super capacitors work with other energy storage devices, such as batteries, to provide both short-term and long-term power-quality protection. Super capacitors can provide protection from short-term events, such as voltage sags. Batteries can provide long-term protection for power-quality anomalies, such as voltage brownouts and complete outages. By combining the two systems, the super capacitor can handle a majority of the power quality events, leaving the batteries to deal with only the long-term events. This reduces the duty cycle on the batteries and can greatly increase the life of the entire power quality protection system.
Fuel cells: A fuel cell provides dc voltage that can be used to power devices. Current technology allows a proton-exchange membrane fuel cell the size of a piece of luggage to power a car. Fuel cells for homes and commercial use are just starting to become available. This technology can use methanol, natural gas, and propane as the fuel source. Issues such as hydrogen infrastructure and fuel-cell life must be addressed and improved before this technology can become more widespread.
Fuel cells are now an approved standby power source for both life safety and legally required standby loads based on the 2008 edition of the National Electrical Code.
Electrical engineers or electrical distribution system designers should know about these emerging technologies. They are certainly not commonplace today, but may become more common as these systems are improved and become more cost-effective and viable.
Lane is president and CEO of Lane Coburn & Assoc. He is a member of the Consulting-Specifying Engineer editorial advisory board.
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