Batteries & Flywheels: What is your best energy storage option?

April 25, 2019 10:32 am || || Categorized in:


A mission critical facility’s greatest liability to maintaining uptime is power quality. But why is power quality so important to a Data Center’s operation? Because the quality of power to the IT equipment directly impacts the equipment performance, uptime, and potential damage to equipment. The key to power quality is choosing the right uninterruptible power supply (UPS) for the facility’s application. This post will focus on two different UPS technologies: battery and flywheel. The operational principle of a flywheel is a mechanical energy storage device that utilizes rotational momentum inertia to store and deliver back energy. Conversely, a battery is a chemical energy storage device that delivers and recharges by execution and reversal of a chemical reaction. Currently, the battery UPS is the most common energy storage technology with the most common battery type being lead-acid [1]. In this post, we will examine the benefits and shortfalls of each technology to identify their best application scenarios. Categories to be considered are the installed size, energy storage capabilities, lifespan, operating conditions, efficiency, and carbon footprint.

Focusing on installed size, the flywheel offers a more compact design than batteries for applications larger than >50kW for equivalent output energy [2].  According to Vycon, a leading flywheel manufacturer, their technology can save their customers 50 to 75% of the typical UPS footprint. The footprint size of a UPS can be critical when building space is limited and usable floor space must be maximized for IT equipment.

Operating conditions are an important consideration to the HVAC load and environmental conditions of the UPS. Batteries require a temperature controlled operating location and a storage environment of approximately 77 degF to maximize lifespan. In comparison, a mechanical storage device can withstand a larger range of operational conditions; up to 100 degF. The environmental operating conditions required by the UPS have an impact on the installation location and the usage of the conditioned environment floorspace in the facility. If the UPS can be installed in a less controlled environment, then a larger percentage of the conditioned space is available for the IT load. Additionally, it gives more flexibility to expand the UPS without increasing the HVAC needs if greater redundancy or load capability is needed.

Another critical component when evaluating the equipment selection of a UPS is the total energy storage. The total energy storage is directly related to the amount of time a facility can operate on UPS supplied power, this metric is referred to as runtime. The runtime available in a UPS has a large impact on how quickly the backup power for the facility needs to respond in the event of a power fault. A shorter runtime with UPS power can lead to more generator starts for short power disruptions and longer run time for generators during extended outages. In this performance category, the battery UPS is the unrivaled leader with scalability to store from a few minutes to an hour of load delivery on UPS power [2]. In contrast, the flywheel has energy storage to full load for only approximately 30 seconds for large loads, even with multiple units in parallel [3]. In addition to more generator starts, a flywheel’s shorter runtime translates to shorter response time to issues encountered on generator startup and transitioning load such as poor power quality, synchronizing frequency, and a generator failure to run.

A metric to evaluate the maintenance requirements for an energy storage device is the lifespan of a unit. The VRLA is the most common battery in the data center application and has a manufacturer rating for replacement on a 3 to 5 year interval [4]. A wet cell battery has a longer lifespan but has more maintenance and safety requirements due to the need to refill liquid inside the battery cells. In comparison, the flywheel has a nearly unlimited lifespan with regular bearing maintenance. Due to the mechanical energy storage method, the flywheel can have a nearly unlimited number of cycles in its lifespan, and only regular maintenance on wearing components such as bearings. However, some magnetic levitation designs do not require bearings, for an even reduced maintenance requirement. In a lifecycle comparison example, a 20-year data center lifespan, a VRLA UPS may have a complete replacement or battery replacements a minimum of four times, while the flywheel will only require bearing maintenance replacements as needed by design type.


Energy efficiency of the supporting equipment such as UPS, line losses, and HVAC is becoming increasingly important to data center design. According to a Schneider Electric white paper, 6% of data center power is consumed by UPS operational energy of inverter and standby loss/trickle charge loss. This can be a significant factor in the data center operational cost and total cost of ownership [5]. If we assume the inverter operational costs are equivalent between a battery and a flywheel; differences will focus on the continuous energy requirement of maintaining a charge. Depending on the design of the flywheel, it requires a range of 0.2-2% of kW rating to maintain spin. In comparison, a battery requires a 0.2% of kW rating to maintain charge [2]. This difference in operational energy consumption is one of the major considerations of a UPS selection and can have a large impact on the overall power consumption and operational costs of the data center.

Finally, we will consider the overall carbon footprint impact on the lifecycle of a battery and a flywheel UPS based on the size, replacement interval, and energy efficiency of the operation. Using an example by Schneider Electric of a 1000kW UPS capacity with a 20-year lifecycle, the cumulative carbon footprint of a flywheel is over 1000 tons CO2, while a battery lifecycle footprint is over 350 tons CO2. This large difference in carbon footprint is due to the higher flywheel standby loss to keep the spin-charge full addressed in the last segment. Note that this evaluation was based on a 1% standby charge loss for flywheel and 0.2% for battery [6]. This evaluation was conducted for one configuration and will change based on application; however, it is an important consideration for the power consumption and the lifecycle environmental impact of the device chosen.

Whichever UPS device is chosen, commissioning services are key to delivering the full value of a UPS to the mission critical facility. Commissioning services provide installation testing before startup, functional testing and system-wide testing; all before any critical equipment is installed. The benefits of regimented commissioning process are that each component is tested before startup to verify correct installation to avoid damaged components due to improper installation. Next, the component is tested to verify is operating as designed and all outputs are within specification of downstream equipment. This testing ensures that no critical equipment will be damaged by improperly tuned and programmed UPS equipment. Additionally, the commissioning process can save time in startup because the problems with installation and startup will have been tested and resolved before the critical equipment arrives and setting the facility up for successful operation and maximum uptime.

In summary, there are benefits and drawbacks for both the battery and flywheel technologies used for data center UPS installations. Due to the longer runtime, lower carbon footprint, and lower energy consumption, I believe that battery UPS’ will continue to be the preferred method of power quality assurance for an IT data center.

Learn more about our data center services here, and contact Niall Killeen, our Global Director of Building Commissioning to discuss your next project: 

About the Author:

Schnieder, AnnAnn Schneider brings 2.5 years experience as a Project Engineer and Project Manager of construction projects in the Oil & Gas industry. Her engineering experience includes design reviews, operational support for existing facilities, and hydraulic analysis of systems. Before the Oil & Gas Industry, Ann worked as an Intern for an Electric Utility.