Lead acid batteries are the tried and (sometimes) true energy storage option for large uninterruptible power supplies (UPS), but many hospitals are choosing an alternative to less-than-reliable lead acid battery strings: flywheels.
A flywheel is a spinning mass, typically made of lightweight carbon fiber or heavy steel. When the utility power drops or sags, the residual spinning inertia of the flywheel produces DC electricity like a motor generator. Think of flywheel operations like your car when you take your foot off the accelerator and you coast for a short distance. The flywheel spins with a small amount of electricity, comparable in operation to a battery's float charge. When the electricity that spins the flywheel stops, the mass keeps spinning and its momentum is converted into electricity, which can provide a ride-through until the outage is over or the load can be supported by emergency generators.
It is important to understand that this flywheel ride-through is not infinite. A single carbon fiber flywheel can produce about 200 kW of electricity for roughly 12 seconds; longer run times can be achieved with a smaller load or paralleled flywheels. This short ride-through is ideal in hospital applications where generator reliability is extremely high. The flywheel is also ideal for hospital settings due to the dynamic load profile of most radiology and imaging equipment, where power draw fluctuates dramatically and can easily be handled by the flywheel.
Benefits over batteries
Compared to lead acid batteries, the benefits of flywheel energy storage are numerous. First, flywheels eliminate the fatal flaw of batteries, as every time you use the battery you shorten its life. The use of a flywheel completely eliminates battery replacements, which typically are needed every two to four years. With routine maintenance, a flywheel should achieve an operating life of 20 years.
Flywheels also have a smaller physical footprint than batteries and weigh less, enabling them to be utilized in spaces not typically thought of for electrical equipment-such as second floors or unreinforced raised flooring. And with wider environmental operating parameters, no special ventilation or emissions controls are needed with the flywheel.
Eliminating toxic lead acid from hospital operations is attractive for facilities with green initiatives. Eliminating battery replacements over a 20-year period equates to more than 300 batteries that never need to be produced and 20,000 pounds of lead that never needs to be mined.
The most beneficial attribute of flywheels is their reliability. UPS battery strings are like the old holiday lights with which dad used to fight. If one goes out, they all go out. If one battery in the string is dead, the entire string will fail and the critical load will be dropped. Battery failure is the number one cause of dropped loads. Flywheel manufacturers claim 10 to 20 times higher mean time between failure than batteries. That ultra-high reliability is why flywheels are a presence in data centers, hospitals, defense facilities, and telecom applications. High-availability systems should be protected with high-availability energy storage equipment.
Evaluation criteria for flywheels
There are many designs for flywheel energy storage, and there are critical questions that should be asked for each unique situation (see sidebar). Is the flywheel light and fast or slow and heavy? Is the flywheel magnetically levitated or supported by ball bearings? Each of these design differences impacts the operations and maintenance of the flywheel.
Another parameter to consider is the flywheel's standby losses of the float charge. Energy consumption is required to spin the mass. Depending on the manufacturer, the energy needed to power the flywheels could range from a few hundred watts to a few thousand watts.
Another key evaluation point when comparing flywheels is cycle time/recharge time. The flywheel needs to reach a high enough speed to produce enough DC electricity to support the load. After a discharge, how soon can the flywheel discharge again? Voltage sags typically come in waves, so it's important to know if the flywheel takes seconds or minutes to get back up to speed.
The last point for evaluation doesn't even relate to flywheels, but to the UPS instead. Some flywheels come integrated inside a UPS while others are UPS vendor neutral. It is necessary to understand the UPS topology used with the flywheel, as the UPS itself is also a critical component of protecting availability.
What's the catch?
The initial purchase price of a flywheel is higher than the cost of lead acid batteries. That initial investment can be difficult to swallow, but it's essential to remember the 20-year life of the system. By the time the first battery replacement comes along in three years, the flywheel will have paid for itself. When factoring in all the expected life, cost components, capital price, maintenance energy costs, and battery replacement cycles, a flywheel can save more than $150,000 versus a comparable battery string. It's important to analyze long-term, total cost of ownership.
Another concern about a flywheel is its unfamiliarity, as some facility or IT managers are hesitant of utilizing new technology. Batteries have been around since the first UPS was manufactured, and a comfort level comes along with that familiarity. But just because the battery is sitting on the rack or in the cabinet doesn't mean it will work when the time comes. Being able to check the status monitor and know the flywheel is spinning, and that it will be there when needed, should restore comfort and confidence in your energy storage systems.
For those managers who prefer the comfort of batteries but want the reliability of a flywheel system, there is always the strategy of using flywheels in conjunction with batteries. Many facilities use the flywheel to absorb the frequent, small and short voltage sags that occur every day while saving the batteries for those longer outages that exceed the flywheels' capabilities. This approach extends battery life close to design claims by eliminating battery cycling for short discharges.
A great alternative
IT and critical systems are everywhere in hospitals and their availability is essential in delivering positive patient outcomes. The use of a UPS with a flywheel for energy storage can provide maximum availability at a lower total cost of ownership and with a more environmentally friendly profile. And that's no spin. HBI
Critical questions to ask when considering flywheels
Does the cost make sense? How often are you replacing UPS batteries?
Would you benefit from using a flywheel in conjunction with batteries?
What is the UPS design topology attached to the flywheel?
Will flywheel implementation help the hospital with its green initiatives?
What is the flywheel's standby power draw?
What maintenance is required? How often is maintenance performed?
What is the flywheel bearing type and how does that impact maintenance?
What is the recharge/cycle time? How many discharges can I get in five minutes?
What installation is required? What is the weight of the unit?
