Tech Briefs

Thermal and Compressed-Air Storage System Provides Alternative to UPS Batteries

Three mature energy-storage technologies are combined in a new system to replace lead-acid batteries.

Virtually all businesses and industries are vulnerable to electric power disturbances such as outages, sags, swells, and harmonics. These problems are less of an issue for data centers, protected behind their walls of Uninterruptible Power Supply (UPS) systems. But the typical battery-backed UPS is too fragile for use in less protected environments. UPS batteries must be maintained in a narrow temperature range and fail prematurely when subjected to a steady diet of step loads and motor drives. About six years ago, flywheel-based UPS products became commercially available. These devices store energy as rotational inertia, and are rugged enough to survive on the factory floor. However, flywheels have relatively short ride-through energy and are best-suited for use in locations with backup generators.

A completely different energy storage solution combines the strengths of several established technologies while mitigating limitations: Thermal and Compressed-Air Storage (TACAS). The new TACAS technology has the ruggedness associated with flywheels, and the longer runtimes associated with battery systems. Initial tests demonstrated backup times of up to 15 minutes at 85kW (100kVA) and as long as several hours at lower power levels. The basic TACAS system concept is that heated compressed air spins an expansion turbine/alternator to generate electricity.

Image Although the overall concept is novel, TACAS draws upon many mature technologies. In fact, the first working prototypes were built primarily from components available from established commercial suppliers. TACAS begins with a self-contained Thermal Storage Unit (TSU). The TSU uses conventional electrical heating elements to keep its core (a well-insulated block of steel) at operating temperature. The TSU has internal air passages and functions as a heat exchanger. Compressed air is stored at 4,500 pounds per square inch (PSI) in conventional gas cylinders or pressure vessels. This pressure is routine for Self-Contained Breathing Apparatus (SCBA) compressors and fill stations used by fire departments and diving operations.

Gas cylinders rated up to 6,000 PSI are widely available throughout the world. Likewise, the valves to regulate and direct the flow of compressed air are widely used in process-control applications. The expansion turbine is a remarkably simple device: a single turbine wheel attached to the shaft of an alternator. Its low inertia enables the turbine/alternator assembly to reach full operating speed (70,000 rpm) in approximately one second.

Image During normal UPS operation, the compressed-air/thermal storage/turbine system is in standby mode. To meet the short-duration energy needs of the UPS (for step loads and brief input power fluctuations), TACAS includes a continuous duty flywheel energy storage device. This all-new flywheel has no vacuum pump, no magnetic levitation, and no high-tech composite materials. The flywheel provides up to four seconds of bridging energy. Figure 1 shows a block diagram of TACAS. When normal input power is restored to the UPS, the TACAS system recharges itself. The flywheel regains full speed in a few seconds. The TSU heaters switch on, and begin restoring the TSU to full temperature. Likewise, the air compressor begins recharging the air cylinders. The total time required to regain full readiness is proportional to the discharge time.

The table shows how the new combined system compares to lead-acid batteries and flywheels. Each energy storage technology in TACAS brings a different set of strengths to the system, compensating for the limitations of the other technologies. The flywheel provides instant dynamic response and excellent durability in heavy-cycling service. The thermal and compressed-air storage together provide the longer runtimes that flywheels lack. The fast recharge times of the flywheel and the TSU help compensate for the slower recharge time of the air tanks. All three technologies in TACAS are environmentally benign and capable of providing 20 years of service with normal maintenance.

Image All three energy storage technologies are mature and well-proven. The only novelty is bringing them together into a commercially viable product. Figure 2 shows how a typical system would be configured. The TACAS turbine outlet air stream consists of clean air that is considerably cooler than typical ambient room temperature. Generally, the exhaust air will be at 55° F — the same as the under-floor air in a raised-floor data center.

Although the TACAS airflow is fairly modest — about 700 cubic feet per minute per TACAS unit — the cool air would be helpful during the first two or three minutes of discharge, while the computer-room air conditioning system was rebooting and coming back online. Furthermore, the compressed-air cylinders will get extremely cold during discharge. Active Power design engineers are studying ways of ducting room air through the tank cabinets to take advantage of the temperature differential.

Prototype systems have successfully met their performance goals, and the development team expects to make test systems available to select customers within a few weeks. Depending on the results of the field trials, commercial production could begin as early as Q3-05. Initial systems will be rated between 30kW and 85kW, for use with UPS systems up to 100kVA. Initial customers are likely to have small data centers and industrial applications that do not or cannot have backup generators on site. These customers have been disappointed by the premature failures associated with low-cost battery systems, but require longer backup times than flywheels provide. As higher power ratings are introduced, TACAS could become an attractive choice for a wide variety of UPS customers in other industries.

This article was written by John Sears, Product Marketing Manager at Active Power. For further information, visit Active Power at www.activepower.com.