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What is a Flywheel Hybrid?

How Flywheel Storage Works in a Hybrid Vehicle

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What is a Flywheel Hybrid?

Typical layout of a chain driven CVT (Continuously Variable Transmission)

© Nissan

A flywheel hybrid is a type of hybrid vehicle that uses a mechanical flywheel storage device instead of an electric battery to accumulate and then release—when needed for acceleration—regenerative braking energy.

How does a flywheel store energy?

Physics defines it as a simple mathematical equation: MV2 where M = the physical mass of the flywheel and V = the velocity of the spinning mass. In plain English, that means it is a heavy, high-speed rotating disc that builds up kinetic energy (the force that causes movement) as it spins, and further, it stores exponentially more energy the heavier it is and the faster it rotates. Think of the discus thrower in the Olympics. He winds-up, building an increasing store of force and energy as he spins, and then releases the disc and sends it flying through the air.

How does flywheel storage work in a vehicle?

The transfer of energy in both directions (captured from the driveline during coasting and braking, and released to the driveline for boost) is managed through a CVT (Continuously Variable Transmission) gear box.

Packaged inside a single housing is a shaft-mounted flywheel that is connected via a chain/gear or belt/pulley drive to a series of discs and rollers (the CVT). During braking and coasting, the flywheel spools-up (accelerates as it spins) and absorbs a storehouse of otherwise wasted energy (heat from friction brakes). During power delivery, as the vehicle begins to accelerate, the pent-up energy in the flywheel is released and it turns the shaft. The rollers within the CVT can change position across the discs and either retard or augment the torque of the spinning flywheel shaft much like a conventional step-up or step-down gear box. This “gearing” is necessary, because unlike aircraft, and to a certain extent watercraft, which travel at a relatively constant load and speed, earth-bound vehicles travel at regularly and greatly varying speeds and loads as they negotiate traffic and topography. It is this variable output velocity that allows for smooth power transmission from the flywheel to the driveline as the vehicle travels over the roadway.

Pros of flywheel systems

  • Compact weight and size -- The entire system (the CVT, the flywheel and the housing) is roughly half the weight and packaging of a battery hybrid system.
  • Twice as efficient -- Battery-electric structures lose kinetic potential during the conversion of energy from mechanical to electrical to chemical, and then back again. It’s a fundamental of the Second Law of Thermodynamics: transforming energy from one form to another introduces losses. Battery-electrics are approximately 34 percent efficient. Flywheel drives are all mechanical and suffer no conversion losses. Most of the energy loss that does occur comes from normal friction between moving parts. These systems are about 70 percent efficient.
  • Lower cost -- Smaller size and weight and reduced complexity make these arrangements about one quarter the cost of a battery-electric system.

Cons of flywheel systems

  • Less range --The power stored by these systems has much less potential vehicle range than normal battery-electric hybrid units.
  • Complex construction --To maintain high efficiency, flywheel storage units require high strength materials, nearly friction-free magnetic or “vacuum” bearings, and sometimes, multiple individual flywheels.

The Flywheel Hybrid Prognosis

When you think about the energy losses incurred by battery-electric hybrid systems, it seems plausible to reason that efficient flywheel hybrids would soon become the norm. But of course it’s not quite so black and white, and further analysis shows that a combination of battery-electric and flywheel energy storage is probably the ideal solution for hybrid vehicles. Batteries are great for storing energy over long periods of time, but aren’t well suited for the shock loads of heavy draw-down during fierce acceleration and the subsequent quick charge-up of repeated episodes of regenerative braking. Unless carefully managed, these cycles (especially with sharp swings of charge/discharge state) can severely shorten battery life.

In general, flywheel systems have energy losses due mainly to bearing friction, which makes them less efficient than a battery-based system for storing energy for long periods of time.

It makes sense that in real world driving conditions, where short periods of boost are required in cut-and-thrust traffic, a flywheel would prove very effective. On longer highway cruises, the battery-electric system could provide steady, even-loaded boost to the vehicles main drive engine. This arrangement would allow each system to do what it does best while enhancing the other. The flywheel would protect the battery from shock loads and the battery would augment the flywheel’s short burst of power. And of course, the internal combustion engine would provide hundreds of miles of vehicle range while receiving assistance from the battery system and the flywheel unit.

Battery-electric hybrids have been on the road many, many years while flywheel power units are still under development. And while major automotive manufacturers have shown interest, none have yet to produce vehicles beyond the R&D department.

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