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The first energy storage flywheel for uninterruptible power supply (UPS) applications, such as large computers, was introduced about a decade ago. With these systems, energy is stored kinetically. The American company Power Thru (previously Pentadyne) has enhanced the principle using carbon fibre for the rotating group, resulting in higher speed and far more effective yield.
Flywheel systems store energy kinetically rather than chemically. Instead of dozens of 45 kg containers of lead plates submerged in sulphuric acid, flywheels use the inertia of a spinning mass to store and regenerate power.
Still sold today, the first type of energy storage flywheel for UPS applications uses a large, heavy steel disk weighing about 360 kg. This is spun at a rate of about 8,000 rpm. The trick with storing energy kinetically is that if you double the mass, you will also double the energy storage (see Equation 1). Double the rotation speed and you will quadruple energy storage. That is why it makes sense to use a flywheel that is smaller and lighter – for better energy density/smaller footprint and weight – but make it stronger and spin it faster. This is what Pentadyne (now Power Thru) brought to market a few years ago with it flywheel module (Fig. 1).
Faster, stronger, lighter, smaller
Of course, something a lot stronger than steel is needed at a much faster spin rate – 52,000 rpm. Hence the carbon fibre composite cylinder manufactured by Crompton in the UK which is used in Pentadyne’s system (Fig. 2).
Since the mass is relatively light (under 23 kg), the rotating group can be fully levitated in a 5-point magnetic field.
This eliminates wear, maintenance, heat build-up and energy inefficient drag of the mechanical bearings of legacy flywheel systems. The rotating group spins faster to maximize energy density.
Patented, bearing-free and pump-free operation
Heavy steel flywheels have mechanical bearings that must be replaced every 2- 3 years at a cost of thousands of dollars and the better part of a day’s worth of downtime. Not so with the Power Thru system. Its small spinning mass is completely magnetically levitated: no bearings. This not only eliminates a mechanical point of failure, but also the friction, heat generation/cooling needs, energy waste and maintenance/downtime issues related to bearings.
Flywheels spin in a vacuum to minimize aerodynamic drag. Other systems use a constantly operating mechanical pump to maintain a rough vacuum. Instead, Pentadyne’s awardwinning systems use a patented molecular vacuum sleeve on the flywheel shaft. This eliminates a mechanical point of failure, energy consumption and maintenance/downtime issues related to a continuously operating mechanical pump.
Exceptional energy efficiency and minimal heat generation
Flywheels have many advantages over lead-acid UPS batteries, but energy efficiency has not been one of them, until now. Thanks to the key technology aspects noted above, the standby energy consumption of a Pentadyne flywheel is a mere 300 watts, the same as the “float charge” consumption of a battery bank with similar capacity. According to the company, competitor flywheel systems use 2,500 watts. That energy waste alone costs about $3,000 or more in higher electric bills per year per cabinet. Then there is the heat generation that the cooling system will need to offset. Battery sets generate lots of heat and competing flywheels vent more than 8,500 Btu/hr. Ridding a facility of all that extra heat further impacts utility bills and cooling systems. In contrast, the innovative system generates less than 1,000 Btu/hr.
Every composite flywheel is inspected, spin balanced and overspeed tested in accordance with NEMA guidelines. A patented dual-wall containment ensures that the 50-mm thick steel inner housing retains the rotating group in the unlikely event a flywheel were to separate during operation. In repeated forced destructive tests, the carbon fibre of the flywheel cylinder expands and transfers torque to the inner housing. The inner housing is then allowed to rotate inside the outer housing. The cooling fluid in the outer housing surrounds the inner housing and acts as a dynamic brake. The stored energy is released in a controlled manner as the flywheel/ inner housing comes to a rest. The outer housing holds the entire safety system and acts as a reservoir for the cooling fluid.
The Power Thru system has many more advantages than those mentioned here. The company has sold and shipped more than 500 flywheel energy storage systems that ensure power quality and continuity for UPS-backed mission-critical operations at industrial, commercial, medical and military facilities in the US and abroad.
Using a bearing-free, fully magnetically levitated carbon-fibre spinning mass, the flywheels are – according to the manufacturer – the world's most energy efficient, using 90% less power than other commercial flywheels.