Are electric flywheel buses in Portland's near future? In stop-and-start bus travel, most of the energy used to get the bus moving gets wasted two blocks later when it brakes. Flywheels capture that energy and use it to get the bus moving again. —Steve R.

For decades, flywheel energy storage has been one of those just-over-the-horizon technologies that make the cover of Popular Mechanics every couple of years, but somehow are never quite ready for prime time.

In principle, storing energy in a flywheel is simple: You use the energy to spin a wheel obscenely fast, and when you want said energy back, you use the wheel to drive (usually) a generator.

If you seal that wheel in a vacuum and support it with magnetic bearings, flywheels can hold their energy for a long time—and, unlike batteries, they never lose their ability to hold a charge.

However, flywheels' energy capacity is limited by their mass and speed. The most powerful flywheels currently in use can store in the neighborhood of 150 kilowatt hours of energy. They spin at 16,000 rpm, create 130,000 G's of centrifugal force at the perimeter, and weigh 3,000 pounds.

It would take four of these—12,000 pounds—to hold the 600 kWh carried by modern all-electric buses. And that's before the gimbal-style mounting system you'd need to keep the bus from doing crazy stuff (balancing on two wheels, etc.) because it's carrying a 6-ton gyroscope.

Significantly more modest flywheels have been deployed on buses, however. In 2015, some buses in the United Kingdom were fitted with 0.4 kWh flywheels—just enough to accelerate the bus from 0 to 30 mph once, to be recharged on the next braking cycle.

The problem is, you don't need a flywheel for regenerative braking—most hybrid cars have it, as does the all-electric MAX. For its part, TriMet is skipping flywheels and rolling out a pilot program featuring those modern all-electric buses mentioned above, powered by "100 percent wind energy."

Meanwhile, I can't find any news about that British flywheel program after the 2015 rollout, and there's no mention of the system on the company's website. But, y'know, it'll catch on—just a couple more years!