Fly wheel UPS

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The traditional solution to power reliability issues is a lead-acid battery uninterruptible power supply (UPS).3 However, these systems have proven unreliable for a variety of reasons. Although the batteries are typically warranted for 20 years pro rata, they actually must be replaced every four to five years because the more they are used, the faster they wear out. Leaks and spills are common, creating a risk to workers’ safety and potential damage to the environment. Battery UPS systems also require a large amount of floor space to house a dedicated air conditioning system, eyewash stations and a separate battery room. Installation and maintenance are expensive due to the requisite spill containment systems, special floor coating and the myriad of tests that must be conducted. Even when maintenance is performed appropriately, there is no way of knowing if an aging battery will survive a full discharge.

The Unreliability of Batteries
Contemporary UPSs provide both power conditioning and a source of temporary electricity for periods when the principal electrical source has been compromised or lost. It is common to include a short-term energy source in or as a part of the UPS system. This source of energy is generally assembled as an in-series string of lead-acid batteries.3 Unfortunately, these batteries have many undesirable traits, most notably including:
▲ The more they are used, the faster they wear out. A typical lead-acid battery is exhausted after 250 complete discharges. Even though they are typically warranted for 20 years pro rata, they are routinely changed out about every four to five years.
▲ Lead-acid batteries can be unreliable. Seismic events, post-seal problems, corrosion and case swelling due to excessive sulphation can all cause leaks, and regardless of maintenance or repeated load testing, there is no way of knowing if an aging battery will survive a full load discharge.
▲ A significant amount of maintenance is required for lead-acid batteries to come close to the predicted mean time between failure (MTBF). Unfortunately, this stored-energy failure rate (1/MTBF) can be significant because most available UPSs use 10 or more 12-volt batteries in series, causing string failure rate to be multiples of the individual battery failure rate.
▲ Installation of battery systems can be expensive. They require spill containment systems, hydrogen monitoring, epoxy acid resistant floor coating, eyewashes and a completely separate battery room. In addition, there are dozens, if not hundreds, of costly copper connections and DC cable runs, as well as separate air conditioning with high air exchange to meet explosion codes.
▲ Battery systems require a large amount of floor space. For example, a 240 kW (300 KVA) array requires 120 square feet of space due to the need for a separate battery room, eyewash station, spill dike and neutralizing supply room.
▲ UPS battery strings must be kept at or near 77 degrees Fahrenheit to balance performance

and life expectancy. For every 10 degrees above 77 degrees, this is cut in half. For example, at 87
degrees, a 10-year battery quickly becomes a 5-year battery, and at 97 degrees it becomes a 2.5-year battery. For temperatures below 77 degrees, life expectancy is extended, but expected performance — measured by protection time — drops below what is necessary to complete the mission.

▲ Spill containment and neutralization for lead-acid batteries are major issues, as are explosive hydrogen generation, potential catastrophic burn down , and safety issues from extensive, exposed high-voltage terminations and DC power, which cannot be shut off in certain short circuit conditions.
There is no question that the benefits of having stored energy on hand in an attempt to ride out a power outage have been proven. However, the most popular solution — batteries — is also one with many problems. In fact, a major user of UPS systems analyzed all the service actions on his UPS configuration over an extended period and found that
the battery string alone caused over 90 percent of these forced actions.

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