Posts Tagged flywheel

Comparing Data Center Batteries, Flywheels, and Ultracapacitors

Posted by on August 16, 2012  |  No Comments

White Paper 65

Data centers require energy storage devices to address the risk of interruptions to the main power supply. Energy storage applications can be divided into three major functional categories:

  1. Power stability – When the power supply coming into the data center is unstable (e.g., power surges and sags), stored energy can be used as needed to balance out disturbances and assure a clean power supply to the load.

  1. Power bridging – When switching from one source of power to another (e.g., utility power to generator power), stored energy can be used (from seconds to hours) to assure consistent power.

  1. Energy management – This is the cost-optimizing strategy of charging stored energy when energy cost is low, and using stored energy when energy cost is high. This energy storage application is not discussed in this paper.

Although many varieties of energy storage technologies are available today, this paper will limit its analysis to those that are most applicable to data centers. Although some storage technologies can function across a range of applications, most are limited in their specific application because of economic considerations. The three technologies that qualify for practical use in data centers—batteries, flywheels, and ultracapacitors—are the subject of this paper (see Figure 1).

The intention of this paper is neither to provide detailed technical descriptions nor to compare in-depth TCO scenarios of energy storage alternatives. This paper attempts to simplify the analysis of energy storage alternatives by providing a relative comparison of mainstream and emerging energy storage technologies.

“Comparing Data Center Batteries, Flywheels, and Ultracapacitors” Full White Paper (Click Here To Download)

Executive Summary:

Most data center professionals choose lead-acid batteries as their preferred method of energy storage. However, alternatives to lead-acid batteries are attracting more attention as raw material and energy costs continue to increase and as governments become more vigilant regarding environmental and waste disposal issues. This paper compares several popular classes of batteries, compares batteries to both flywheels and ultracapacitors, and briefly discusses fuel cells.


  • Energy storage and energy generation defined
  • Energy storage efficiency
  • Energy storage cost
  • Factors that influence the business decision
  • Data center storage technologies
  • Additional considerations


The landscape of alternative energy storage is gaining more recognition. When selecting an energy storage solution, the first step is to determine the criticality of the data center operation; i.e., what would be the consequence of an unplanned IT equipment shutdown? A less critical operation may be able to tolerate an occasional shutdown as long as it can “ride through” the momentary power interruptions that make up the majority of power outages. A more critical operation may require a longer stored energy reserve.

As new energy storage technologies emerge, a fundamental question should be posed: What is the benefit of instituting a longer runtime (e.g., 15 minutes) as opposed to a short runtime (30 seconds)? If no benefit exists, flywheels, ultracapacitors, and smaller battery systems can represent a huge savings.

Why, then, aren’t data center professionals abandoning their batteries in droves and replacing them with flywheels, ultracapacitors, and smaller battery systems? In some cases, buyers of energy storage solutions cite issues such as cost, mechanical moving parts with lower reliability, or the inability to meet length of life goals. However, additional reflection leads to the conclusion that it is people, human beings, and not just pieces of equipment, that are ultimately responsible for the success or failure of the data center.

As computer operations become more and more critical, the majority of data centers today require longer UPS runtimes, and, as a result, batteries continue to outperform flywheels and ultracapacitors in terms of cost, reliability and availability. Despite the growth of alternative technologies, the view over the next few years is that batteries will still remain the principle resource for energy storage in the data center.

For most data center professionals, time to react and respond to a problem or emergency is perceived to be at a premium during a crisis situation. Extra time during an emergency might allow a human to correct the problem such as discovering that an auto switch was erroneously left in a manual position. In addition, since most data centers are equipped with monitoring software, when a fault occurs, an automatic data center backup copy is launched. After the backup copy, the remaining battery time is used to launch a safe server shutdown. The servers are stopped cleanly and restarted immediately when power returns. From a data center operator’s point of view, the more time to resolve an issue, the better. Since batteries currently provide people with more time to react, they are favored and take on the role as the primary energy storage mechanism in the data center.

As power generation and storage technologies combine (e.g., fuel cells combining with ultracapacitors) and manufacturers strive to introduce cost effective and cleaner hybrid solutions to the marketplace, choices for viable data center energy storage technologies will increase.

White Paper Written By:

Stephen McCluer is a Senior Manager for external codes and standards at Schneider Electric. He has 30 years of experience in the power protection industry, and is a member of NFPA, ICC, IAEI, ASHRAE, The Green Grid, BICSI, and the IEEE Standards Council. He serves on a number of committees within those organizations, is a frequent speaker at industry conferences, and authors technical papers and articles on power quality topics. He served on a task group to rewrite the requirements for information technology equipment in the 2011 National Electrical Code.

Jean-Francois Christin is Business Development Manager for APC-MGE’s Secure Power Solutions organization.  His 17 years of experience in the power systems industry includes management of technical support in APC-MGE’s South Asia and Pacific region, and management of technical communication and business development in the EMEA/LAM region.  He is member of LPQI, actively participates in international power and energy conferences, and trains subject matter experts on topics related to power quality.

Universal Networking Services’s partnership with Universal Power Group, Inc. has enabled us to build a strong distribution network of battery and related power components that meet consumer needs for accessibility, portability, security and mobility, coupled with value added offerings such as battery pack assembly and battery replacement/recycling programs.

Please feel free to contact us if you have any questions regarding this topic.