Life as a Physicist

Hybrid cars… Hybrid accelerators November 3, 2009

By now I think most people know how the Prius and other hybrid cards operate. Most cars’ breaks are just like a bycycle break: a clamp that generates a large amount of friction and slows the car down. This is a terrible waste of energy: the car’s motion is converted into heat and damage (to the brake pads) and can never be reclaimed. Think of it as wasted gas, excess pollution, etc.

Hybrids are much more clever. They attach an electric motor/generator directly to the wheel and when you want to break then use the wheel’s motion to run the generator. This requires work – which slows down the car. Instead of the energy being lost, however, it is poured into a battery. The energy can then be reused to get the car started again. Huge savings in gas! This is also why hybrids tend to amazing at city driving, but not long distance driving (where this doesn’t help much because you aren’t stop/start).

Before we got sophisticated with generators and batteries we did something much more mechanical. At least for public transportation: the gyrobus:

Instead of a battery, however, a giant flywheel was used to store the energy. These things were built back in the 1950’s.

Guess what… the same technology has been used for particle accelerators – specifically the Bevatron!

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These are 65 ton flywheels, and there are two of them. Here is an abstract from a paper that describes the control system that ran these puppies:

The Bevatron/Bevalac main guide field power supply stores 680 MJ in the flywheel-shaft systems of two independent motor-generator sets. During the normal acceleration cycle of various heavy-ion beams, the energies of the rotating shafts are converted to energy stored in the main magnet guide field. At the end of the acceleration cycle, the magnet energy is inverted back to the shafts. Generally, this takes place from 10 to 15 times per minute. The rapid switching of ions, energy, and beam lines at the Bevalac has required various control techniques for fast switching between all operational Bevalac fields within 1 min. The power supply control systems and operating parameters are described.

The principle is same as with the hybrid car, or the gyrobus, but all the sizes and power are extreme (as usual for the field of particle physics). Imagine spinning up and down those flywheels at a rate of once every 10 seconds or so! Of course, that system would never have fit in a car!

While I don’t know the answer to this, I suspect that flywheels are still one of the best ways to store energy that has to be quickly extracted over the timescale of seconds. Batteries probably can’t do it without costing a huge amount, and capacitors probably have a much lower energy density – though they are ideal for other stored energy applications that require much faster discharge times!