Running a flywheel energy storage system in parallel with VRLA batteries can lengthen their service life and improve reliability
Every discharge-recharge cycle reduces the life of a battery. In fact, the higher the percentage of total available energy the battery delivers during a discharge, the greater the reduction in its life. Many batteries used in UPS applications tolerate a relatively low number of discharges before they must be replaced. Furthermore, UPS battery warranties are often tied to the number and duration of discharges as well as the length of time the batteries have been in service.
Battery manufacturers know that the frequent discharge of batteries in UPS systems causes them to reach end-of-life long before what would be expected in float service. As a result, they consider the use of batteries in UPS systems as limited cycle service. This is the primary reason why some battery manufacturers require the use of battery cycling monitors to validate warranty coverage in UPS applications.
Battery discharges occur for every power disturbance that exceeds the input voltage or frequency limits of the UPS, as well as for most large step-load changes on the UPS output. Thus, the battery's useful life and warranty life can be depleted in a very short period. When flywheel energy storage is used in parallel with the battery, the flywheel provides energy to the UPS during step loads and short-term power disturbances, which greatly increases the useful life of the battery. When used in parallel with UPS batteries, flywheels can isolate them from 96% of all power events at an “average” location in the United States, according to the results of the EPRI Distribution Power Quality Project that was conducted from 1993 to 1995.
From a reliability perspective, it's important to know that the predominant failure mode of valve-regulated lead-acid (VRLA) batteries is open circuit, which means that a single-cell failure causes loss of all battery back-up capability. Adding a flywheel energy storage system (FESS) in parallel with batteries provides a redundant UPS energy source that gives sufficient backup time to ride through most power disturbances or to transfer to standby generators.
How battery isolation works. The flywheel is usually applied to the DC bus of a UPS and in parallel with the lead-acid battery string. You can set the flywheel's voltage thresholds for charge and discharge to accommodate a wide range of DC bus voltage settings so they'll always be above the open-circuit voltage of the chemical battery. This flexibility ensures that the flywheel will properly integrate with the batteries in a wide range of UPS configurations.
Short step loads occur frequently in many commercial and industrial applications, often with no indication that the batteries are being cycled. Such a step load is shown in Fig. 1 on page 34. In this case, the UPS batteries alone provide ride-through.
Fig. 2 at right shows this same 150kW step load on the same UPS but with the FESS in parallel with the UPS batteries. Once the load is applied, both the battery and FESS respond to the demand and provide energy to the UPS. However, the FESS continues to provide energy as the battery current drops back to zero. Note that the DC link voltage only drops to a minimum of 521VDC. Because this voltage level exceeds the open-circuit voltage of the battery, the battery doesn't experience a chemical discharge. The energy delivered only comes from the static charge on the plates. The FESS continues to provide power at a constant voltage while delivering the current to meet the demands of the UPS.
Input power interruptions. Because most power outages last less than a few seconds, the ability of an FESS to shield the battery from short discharges can significantly improve battery life. Fig. 3 at right and Fig. 4 on page 38 illustrate how flywheels accomplish this. Fig. 3 shows a power outage with the same 150kW load applied to the same 300kVA UPS with the same string of batteries, but without a flywheel. Note that the battery supplies all of the current necessary to support the load and the current continues to rise during the short duration of this plot. In addition, the DC bus voltage falls to the 460VDC level. When the utility feed is restored, it will be several seconds before the UPS transfers from batteries to utility power. A built-in “settling time” ensures that utility power is stable, and then the UPS must resynchronize and gradually “walk onto” the utility power. Therefore, the battery experiences a significant discharge on every utility disturbance, no matter how short.
The plot in Fig. 4 shows an identical outage on the same UPS and battery, but with the flywheel in parallel with the battery. Once again, the flywheel system and the battery immediately start providing energy to support the DC bus. However, the flywheel continues to increase its output to take the load while the static charge on the plates dissipates. The minimum voltage of the DC bus during the event is 517VDC, which is still above the open circuit voltage of the battery, so the battery doesn't discharge. The current flow out of the battery dissipates the capacitive charge that makes up the difference between the open circuit voltage and the float voltage. This charge must be partially “neutralized” to allow the battery to follow the float voltage down from 540VDC to 517VDC. The flywheel supplies energy to the UPS at a constant voltage while varying the current to meet the demands of the UPS and its critical load, controlling the DC voltage to within 1% of a preset and programmable value determined from the specifications of the UPS.
Field test data. Field data are being collected to support the earlier claims of battery life extension, and early adopters of FESSs are reporting the predicted results of battery isolation.
Fairview Hospital in the Cleveland area reports that batteries installed on June 1, 1999, continued to have impedance measurements close to those observed at an installation as illustrated by using a data storing, battery multimeter during a semi-annual battery preventive maintenance inspection performed in 2003, shown in Fig. 5 on page 38. The hospital's 160kVA UPS system consists of a single 480VDC string of 12VDC batteries and a flywheel. The system supports a mission-critical information technology operation located within the hospital.
Impedance values increase with age. The hospital replaces cells that vary more than 20% from the string average. New cell impedance values typically measure at or near 3,500 micro-ohms, with VRLA batteries typically reaching end-of-life values of 8,000 micro-ohms to 9,000 micro-ohms in five years or less.
As expected, paralleling a flywheel with the battery has been shown to prevent the battery condition from deteriorating with age. The typical impedance values for cells that are subjected to discharge and recharge cycles would be significantly higher than those observed. These results validate the battery life extension benefit of using a flywheel in conjunction with a battery.
In addition, Fig. 5 shows the different impedance values reported at the Cleveland site. The manufacturer's specification sheet for the 12VDC batteries in use at this site lists a nominal impedance value of 2,500 micro-ohms for new cells. However, this particular installation had observed values at or just below 3,500 micro-ohms when new. Making a conservative assumption that the initial value is 2,500 micro-ohms, and projecting a linear increase of impedance to the 8,000 micro-ohm, five-year-expected-useful-life-span, the data indicates the benefits of flywheel energy storage isolation.
With this assumption, a three-year-old battery used in typical UPS applications would normally be expected to have an impedance measurement of 5,500 micro-ohms. However, this set of batteries measured an average of 3,579 micro-ohms, which is just slightly above the observed new installation values. The isolation of the batteries from discharge cycles appears to have significantly reduced aging. Due to the true nonlinear nature of battery impedance values, it's difficult to extrapolate the final battery end-of-life with the flywheel in parallel. However, the trend to date indicates that the batteries will perform well past the expected useful life of five years.
Flywheel technology is now a proven, highly reliable, and environmentally responsible replacement or supplement to lead acid batteries. The simplicity of use and installation makes a flywheel system a viable part of any power quality system, and it requires significantly less maintenance and service than batteries during its 20-plus years of useful life. In addition, since a flywheel system incorporates internal monitoring and communications capabilities, it can give notice of any condition that may require further scrutiny, unlike a battery system that requires an additional external battery monitoring system to provide similar information. A flywheel installed in parallel with batteries will extend their useful life well beyond normal ranges and continue to do so for the next set of replacement batteries.
Richey is product manager with Active Power in Austin, Texas.