- Flywheels typically supply full-load energy for much shorter periods of time.
- Flywheels require less space and weigh less.
- Flywheel air-conditioning and ventilation requirements are less restrictive.
- Flywheels consume slightly more power.
- Flywheels require less maintenance.
- Flywheels have a potentially longer service life.
- Flywheels do not require storage battery spill prevention and recycling at the end of service life.
- Flywheels are not limited by the number of discharge cycles they can supply.
- Flywheels are limited in the frequency of discharge cycles they can supply.
Flywheels vs. traditional storage batteries: Key considerations
The UPS reserve energy source must support the UPS output load, while UPS input power is unavailable or substandard. This situation normally occurs after the electrical utility has failed and before the standby power system is online. As you determine whether flywheels are appropriate for a project, the amount of time that the reserve energy must supply the UPS output is key. For comparable installed cost, a flywheel will provide about 15 seconds of reserve energy at full UPS output load, while a storage battery will provide at least 10 minutes. Given 15 seconds of flywheel reserve energy, the UPS capacity must be limited to what one standby generator can supply. In 15 seconds, the standby power system must complete the following tasks:
- It must recognize the utility loss.
- It must wait for any utility automatic transfer switch or re-closer to try to restore utility power.
- If the utility power is not restored, it must crank the generator.
- It must transfer the UPS to the generator when its voltage is adequate.
With less than 15 seconds of reserve energy, there is not enough time to reliably parallel two or more standby generators. You can arrange switching to provide generator redundancy, but at the end of the day the UPS system capacity is limited by the capacity of a single generator. A 3,000-kW, standby-rated generator (the largest capacity readily available in the U.S.) limits the maximum UPS system capacity to about 2,200 kW. I view this limit as the major consideration.
How flywheel backup power differs from other methods
Space requirements. Assuming 2 feet of side clearance, 2 feet of rear clearance and 3.5 feet of front clearance, a flywheel for a 675 kW UPS module requires about 121 square feet of floor area and weighs about 9,400 pounds. A vented wet cell storage battery on two-tier racks for the same UPS module requires about 350 square feet of floor area, or almost three times the area required by the flywheel, and weighs 33,000 pounds (over three times that of the flywheel). A valve-regulated lead acid (VRLA) storage battery in cabinets for the same UPS module will require about 250 square feet of floor area (twice that required by the flywheel) and weigh 35,000 pounds (almost four times that of the flywheel).
Ventilation. Flywheels require the same wide operating temperature range as do UPS equipment (32 degrees Fahrenheit to 104 degrees Fahrenheit), while storage batteries should be maintained at 77 degrees Fahrenheit for rated performance. Storage batteries also require ventilation to prevent hydrogen accumulation, and hydrogen detectors to alarm on accumulation. Therefore, flywheels can be installed in the same room as UPS equipment, while storage batteries should be installed in separate battery rooms.
Energy consumption. Flywheels consume more energy than storage batteries. For a flywheel supporting a 675 kW UPS module, one manufacturer advertises 1.5 kW losses (0.2% of module output), while another advertises 6.6 kW losses (1% of module output) when the flywheel is fully charged and online. In my experience, the float charging of a storage battery supporting the same 675 kW UPS module requires about 0.3 kW (0.04% of module output). Assuming 95% UPS rectifier efficiency, a site power usage effectiveness (PUE) of 1.7 and 10 cents per kWh cost of purchased electricity, the flywheel described above would cost $1,900 to $9,900 more per year to operate than a storage battery.
Maintenance requirements and service life. A reliable storage battery requires more maintenance than a flywheel. At minimum, vented wet cells need maintenance twice a year – a 6-month checkup and a more in-depth maintenance check six months later. In addition, they require interim quarterly maintenance when batteries are new and more frequent electrolyte replacement. VRLA cells also need maintenance every six months. One flywheel manufacturer recommends annual maintenance and a change of bearings every three years, while another recommends annual maintenance and a capacitor change every six years.
Modern flywheel manufacturers advertise a 20-year service life, which is based on their own forecasts. But none of the products in use today have been in service for more than 11 years. I expect that they will reach the 20-year service life that corresponds to today's UPS products. In my experience, storage batteries have a shorter service life than their warranties imply; vented wet cells rarely exceed a 14-year service life, despite a 20- year warranty, while low-cost VRLA cells rarely exceed a four-year service, despite a 10-year warranty. It is reasonable to assume that the storage battery must be replaced during a UPS system's service life, while a flywheel will not.
Hazardous materials. All commonly used storage batteries contain an acidic electrolyte and lead, hazardous substances that are governmentally regulated. Spill prevention is required on most sites with storage batteries, and recycling of storage batteries is mandatory. Flywheels do not contain large amounts of hazardous materials and are not governmentally regulated.
Discharge cycle limits. There are limits on the number of discharge cycles that a storage battery can supply during its service life. One manufacturer's standard vented wet cell is rated for 2,700 discharges of 30 seconds or less, with a premium cost model rated for 10,500 such discharges. Discharge data on VRLA cells is less available, although one manufacturer states that its product is rated for 100 discharges of 15 minutes. Flywheel manufacturers state no limit on the number of discharges they can supply during service life. As a practical limit, I assume that the limit is something less than 20 per hour, since one manufacturer states that its product requires 2.5 minutes to recharge after a discharge. These discharge cycle limits become relevant only where the utility supply is very unreliable.
Let's assume a desired 12-year service life for a vented wet cell that is rated for 2,700 discharges; this is an average of 18 discharges per month, or one discharge every 38 hours. If the utility is less reliable than the calculated average, expect that the battery service life will be less than desired. If you assumed the premium-cost battery and a 14-year service life, then the average would be 63 discharges per month, or one discharge every 12 hours.
However, there are limits on the number of discharge cycles that a flywheel can supply during a short time period. A typical flywheel requires about 2.5 minutes of recharge time after a discharge. Like a storage battery, flywheel recharge is usually accomplished when the UPS is supplied by the utility so as to minimize the standby power requirement. If another discharge is needed before recharge is completed, the flywheel may not be adequate. Discharge frequency should be considered. Let's assume that you have a site with a UPS storage battery and the utility supply is less reliable than advertised, but most of the utility outages are less than 15 seconds. Your storage battery's service life is being rapidly reduced. Under either scenario, the standby power plant will get a lot of exercise and burn a lot of fuel, driving up operating costs and potentially exceeding air quality permit limits. Assuming that the standby power plant operates for two hours after every start, 18 battery discharges per month becomes 430 hours per year of standby power plant run time. I would not recommend that a client consider a site with a utility supply that is this unreliable.
I will now partially contradict the previous sentence. Let's assume that you have a site with a UPS storage battery and the utility supply is less reliable than advertised, but most of the utility outages are less than 15 seconds. Your storage battery's service life reduces rapidly. You could remedy this problem by installing a flywheel in parallel with the storage battery. The flywheel controls can be arranged so that flywheel will supply the UPS until its stored energy is exhausted (15 seconds), at which time the storage batteries supply the UPS. You could also delay the start of the standby power plant to coincide with when the flywheel is exhausted. This remedy can enable you to prolong the storage battery life and reduce the number of standby power plant operations.
In this article, we've covered the key differences between flywheels and traditional storage batteries. Now you can make an educated decision about whether flywheels for data center backup power are an option for your data center.
ABOUT THE AUTHOR: Christopher Johnston is a Vice President and Chief Engineer for Syska's National Critical Facilities Team, the head of Syska's Critical Facilities' Technical Leadership Council and a member of Syska's Green Critical Facilities Committee. He leads research and development efforts to address current and impending technical issues in critical and hypercritical facilities, and specializes in the planning, design, construction, testing and commissioning of critical 7-24 facilities. Johnston is also regular presenter at conferences such as Data Center Dynamics and the Uptime Institute.
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This was first published in February 2009