Backup Power Products Keep Data Centers Alive and Cool

June 28, 2006
Active Power has launched two back-up power products based on its TACAS technology., The products, CoolAir DC and CoolAir UPS, are designed to supplant battery-based energy sources in small- to medium-sized data centers.

Active Power has launched two back-up power products based on its thermal-and-compressed air energy storage (TACAS) technology. The products, CoolAir DC and CoolAir UPS, are designed to supplant battery-based energy sources in small- to medium-sized data centers. Besides providing backup power, these products also produce cool (59ºF) air as a byproduct of power generation. That air is used to cool the data center during the main power outage. Meanwhile, the company is also leveraging the low maintenance of TACAS to make the CoolAir products available through a usage agreement that eliminates large upfront capital expenditures.

Capable of generating up to 100 kVA of back up power with runtimes as high as 45 minutes, the CoolAir products are best suited for smaller facilities (under 4000 square feet) that currently have no generator for extended power backup. CoolAir DC is essentially a replacement for the conventional battery cabinet associated with data center UPSs. Using the TACAS technology, CoolAir generates up to 80 kW of dc output at an adjustable 360 V to 480 V, which can then be used to power a three-phase UPS.

With TACAS, thermal energy and compressed air are used to spin a small expansion turbine that generates ac power, which is then rectified by IGBT-based power converters. (For a more detailed description of how TACAS works, see the references below.)

TACAS energy storage depends on both air temperature and air pressure. Consequently, CoolAir DC runtime is partially a function of the number of air storage tanks in the system and the product’s modular construction allows different numbers of tanks to be connected depending on the target runtime.

CoolAir UPS includes the CoolAir DC product, which is housed in its own cabinet, and adds a three-phase, double conversion (480-V) UPS in a separate cabinet. The UPS is offered in three different models for varying power requirements. The lowest-power UPS delivers 20 kVA to 40 kVA; a medium power unit, 50 kVA to 80 kVA; and the highest power unit, 100 kVA.

The CoolAir DC product consists of a base unit that provides power generation and power conversion (for dc output) and one or more air tank cabinets. The base unit measures 78 in. x 31.5 in. x 32.5 in., as does a tank cabinet containing 9 cylinders. The CoolAir DC datasheet specifies a scaled down version of the base unit plus air tank that measures 78 in. x 48.5 in. x 32 in. However, this unit, which only holds three cylinders of compressed air, provides just 1.5 min of runtime at full load. Three additional tank cabinets (for a total of 30 cylinders) must be connected together to obtain the full 15-minute runtime at full load.

These mechanical dimensions can be interpreted another way. The base unit on the CoolAir DC occupies less than 7 sq. ft. of space, while the air tank cabinets will occupy a total footprint that varies from 4 sq. ft. for a 1.5 minute configuration to 25 sq. ft. for a 15 minute configuration. While this sounds like a significant amount of real estate within a data center, the entire CoolAir solution is designed to operate in ambient environments of 0ºC to 40ºC. Consequently, the CoolAir units can be placed outside of premium air conditioned environments. Another option is to locate the air tanks outside the data center and provide an air feed to the CoolAir DC unit.

The three versions of the UPS unit measure 71 in. x 47 in. x 32 in. (100 kVA); 71 in. x 31.5 in. x 31.5 in. (50 kVA to80 kVA), and 71 in. x 27 in. x 31.5 in. (20 kVA to 40 kVA).

Unlike battery-backed sources, the CoolAir products promise minimal maintenance and replacement requirements over a 20-year product life. And whereas a conventional UPS generates heat, CoolAir DC produces cool air in the course of generating backup power. This cooling can be used to maintain operation of data center equipment during backup power delivery, rather than shutting down loads to keep equipment within thermal limits.

In addition to exploiting its TACAS technology to offer backup power and cooling, the company is also exploiting the low-maintenance quality of the CoolAir products to offer a different business model for purchasing backup power. Instead of making a large upfront investment in UPS equipment and batteries, customers can lease the CoolAir equipment through a service plan that provides a fixed, predictable annual price that includes extended warranty and maintenance coverage.

For the customer, this usage or leasing model eliminates the unpredictability associated with irregular battery replacement in battery-backed UPSs. Active Power recently announced that Georgia State University signed such a usage plan agreement for CoolAir UPS, which is being installed in the university’s Network Operations Center.

Active Power plans to introduce new members of the CoolAir product family in the future. The company will offer a CoolAir product for the international market. In addition, a future version of CoolAir will allow units to be paralleled for higher power output.


1. “Leveraging Thermal and Compressed Air Energy Storage,” Power Electronics Technology, October 2004, pages 95-96, available online

2. “Battery-Free Energy-Storage System” Power Electronics Technology, July 2004, pages 10-11, available online at

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