Kinetic energy storage fulfills a vital role for diverse applications

Data centers, hospitals, and other applications can benefit from the reliability, scalability, low cost of ownership, and other advantages kinetic energy storage systems offer.


Learning Objectives:

  1. Understand the importance of power protection.
  2. Learn how batteries introduce uncertainty in backup power protection.
  3. Identify how kinetic energy storage systems (flywheels) increase power availability and reduce energy consumption.

Facility managers and electrical engineers have many choices for mitigating the high cost and equipment damage that power problems cause. Choosing a solution that makes the best sense—in terms of reliability, scalability, maintenance, and return on investment—is key. Facilitating improvements in recycling, energy conservation, and heat management are other important objectives.

Backup choices

Careful planning and design go into the integration of the optimum power infrastructure. Many power systems are taken into account, such as power distribution, power switchgear, automatic transfer switches (ATS), power management software, and power backup systems, including uninterruptible power supply (UPS) systems, and engine generators. Choosing the optimum equipment can make the difference in achieving continuous power availability and high levels of system uptime. Reliability is always the first priority, but achieving maximum levels of energy efficiency is also a “must-have” to reduce carbon footprint and operational overhead expense.

Vendors of UPS and engine-gensets understand the demands of today’s facilities and design their systems to provide the best reliability and functionality. The major providers of three-phase UPSs offer very efficient systems—up to 97% at full load even in double conversion mode—to mitigate energy losses. Many double-conversion UPS designs also feature digital signal processors and field programmable gate arrays to manage output voltage for coordination with downstream power distribution units (PDUs) and static transfer switches. Still, with all this technology, the lead-acid batteries used for the UPS’s energy storage component are unpredictable.

Figure 1: This graph shows the typical battery voltage reaction to demand or load during an input power failure. When the battery is called upon to provide current, the voltage drops rapidly and then returns to a nominal voltage as the discharge continues. The instantaneous drop in voltage and the rapid return to nominal or steady state is called the coup de fouet. The battery must be sized to accommodate the instantaneous voltage drop so it does not reach the UPS preset cut-off voltage. Courtesy: VYCONEnergy storage

Batteries have long been the mainstay energy storage partner for UPS and for good reason. They are ubiquitous, have a low initial purchase cost, and are readily available. However, their reliability is always in question and they have a limited life. Statistics show that 70% of the service calls made on a failed UPS resulted from a battery problem. In 40% of cases involving a power loss due to a critical load, a failed battery system was the cause. Strong evidence shows that the battery is the highest failure point in a traditional UPS.

The service life of valve-regulated sealed lead-acid (VRLA) batteries depends on many factors, including ambient temperature, proper maintenance, and usage frequency (cycling), as well as the quality of connections and terminals. According to Eaton’s Blackout Tracker, there were more than 3,000 power outages in 2013. Testing generators monthly puts an additional 24 cycles on the battery (transfer to and from the generator cycles the battery). This means that UPS batteries are cycled an average of 56 times per year, excluding cycles due to load changes and internal faults within the facility. Most of these disturbances are short dips in voltage and may not be noticed by the user, but the battery notices them as shown in Figure 1.

Kinetic energy storage systems—also known as flywheels—have been around since the Bronze Age and are a proven method of storing energy. Why? Because they are rotating mechanical devices as opposed to batteries that are chemically based. A kinetic energy system’s rotating assembly is operated and maintained within a vacuum. Some systems use mechanical bearings; however, replacing mechanical bearings with magnetic levitation reduces maintenance cost and hours of bearing replacement downtime.

To initiate and maintain a flywheel, electricity from the grid is needed, but if the power cuts out, it uses the stored kinetic energy to generate electricity. Compared to lead-acid, battery-based UPS systems that necessitate expensive air conditioning, require large real estate, and are maintenance intensive, flywheels give mission critical facilities a green and predictable backup energy source that helps keep critical systems up and running.

Moreover, for those who just can’t part with their batteries, the flywheel system can be the first line of defense against power disturbances, prolonging the life of the batteries. A hybrid dc kinetic energy storage system consisting of flywheels and batteries combines the best of both worlds for some applications. In this configuration, the flywheel is the primary energy storage for the UPS covering nearly 100% of all outages. With a properly configured and operational power system including a backup generator, the battery will see a discharge only during system testing. Eliminating battery discharges and recharges (cycles), along with eliminating the coup de fouet, or whiplash, will increase battery life. In applications that do not have a backup generator, the flywheel provides protection from short-term outages ­(that shorten battery life) before doing a soft hand-off or walk-in to the battery. This eliminates whiplash and further increases battery life by protecting the battery at its weakest point in the discharge cycle.

Figure 2: EasyStreet’s SAS 70 Type II audited data center utilizes paralleled VYCON VDC-XE units (two shown). The flywheels are installed in a back-to-back configuration with the UPS. Courtesy: EasyStreetData centers

Rapid advances in virtual, cloud, and mobile computing have driven computer networks to be operational 24x7, no matter what. And as we move closer to an Internet of Things (IoT) infrastructure—linking physical and virtual objects—downtime is just not tolerable. According  to a recent study on the Cost of Data Center Outages by the Ponemon Institue, the cost of downtime is $627,418 per incident.  If a data center is unable to process e-commerce transactions, serve up important data, or provide fast networked communications instantly, customers are unhappy, which results in a less-than-optimal bottom line. Some of the largest and most energy-efficient data centers in the world are deploying kinetic energy storage systems as part of their backup strategy (see Figure 2). While it’s true that flywheels do provide shorter run times than batteries, flywheels have an exceptional track record of reliably transferring to on-site generators when prolonged power backup is required. In reality, if the power does not resume in a few minutes, chances are it will be much longer before power is restored. EPRI states that 80% of all utility power disturbances last less than two seconds, and 98% last less than 10 seconds.  

In this real-world scenario, the kinetic energy system’s automatic transfer switch (ATS) has ample time to determine if the outage is more than a transient occurrence, and to start the generator and safely manage the hand-off.

“The driver for utilizing kinetic energy storage is to reduce the life-cycle cost and maintenance requirements when installing large banks of batteries. In addition, the space savings by using flywheels and fewer batteries mean lower construction costs. This allows for optimum space utilization,” said Karl Smith, head of critical environments for SunGard Availability Services located in Wayne, Pa.

Hospitals and imaging centers

Hospitals and imaging centers are also reaping the benefits of replacing batteries with flywheels. While a hospital’s data center obviously must be protected, its imaging equipment is very susceptible to any kind of power anomaly. Brownouts, surges, and outages can have devastating effects on magnetic resonance imaging (MRI) equipment—especially on its refrigeration system. Without clean electrical power supplied to the MRI vacuum pump, a cryogen vent breach can occur, requiring an expensive and time-consuming re-commissioning of the MRI magnet. When this occurs, patient MRI appointments have to be rescheduled, causing time and revenue losses. Space is another important consideration in health care facilities. Few facilities have the room for electrical gear, let alone the space for a battery bank. Pairing UPSs with kinetic energy storage systems is the perfect balance between high power availability and compact size.

Return on investment

In a typical data center, health care facility, or manufacturing facility, flywheels free up 50% to 75% of space that would be taken up by an equivalent power-rated battery bank. This lowers construction costs and allows space to be used more productively. Also, unlike batteries, flywheels do not require a temperature-controlled environment, as they can operate in temperatures from 0 to 40 C.

Flywheels can be added to UPS systems for additional capacity or redundancy, to grow as energy requirements increase. Reliability, scalability, sustainability, low cost of ownership, and reduced carbon footprint are just a few of the important benefits provided by today’s kinetic energy storage systems.

Frank DeLattre is president of VYCON Inc. He has more than 20 years of experience in power quality and related industries in the global energy marketplace.

Anonymous , 05/15/14 10:11 AM:

The best UPS system I ever used was a 75KWH motor generator/flywheel with battery back up 30 years ago. The battery/inverter system provided 480V 3 phase to the motor so I was able to locate it a few hundred feet distant in an electrical/mechanical area for easy access and ventilation. After 12 years of reliable service the system was replaced with a larger, all electronic UPS (not my choice). Reliability, while good, was not the same and scheduling service was more difficult because there is no coverage for short drops when servicing the batteries and inverter. With the MG unit battery failures would show up during power blips as the inverter fired up when the drop lasted a quarter second but the load would not drop due to the stored energy in the flywheel. It was a darn good system.
Jozef , NY, United States, 05/16/14 05:49 AM:

My friends
Somebody was speaking about Emergency Generator with fly wheel of 75 kWatts diesel engine and period of 30 years. Let talk about history. I as the Senior Electrical Engineer have designed myself many the Data Centers as 1968 and it was data center used for IBM 360 Mainframe engines. We used the Parson diesel engine 400 kW and Stamford generator 400V, 3phases, with Fly Wheel unit as set for emergency support for eliminate the voltage drop. In the time I have not access to UPS and the voltage supported was by 0,5 second only by Fly wheel. In this time the ATS has been used to switch to second feeder of source supply. Latter we recover better situation and static UPS was discovered by somebody (problems with power thyristors had been set). Today we have no problem to provide the new engine with 30 seconds fly wheel working time. The problem is only with the cost of the unit. When you calculate construction cost of Fly wheel UPS is my expensive. In my opinion. the Manufacturers shall lower their cost. Or nobody will be interested with buy the Fly wheel UPS. I have money today. And I am not interested in money saving after 5 years. The technology is 3 times changing in period of 5 years. And what is good today, tomorrow will be too old for maintenance.
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