Reinventing the wheel is considered such an unnecessary act that the phrase itself connotes pointless effort. But it’s a good thing James Watt, the pioneering 18th-century Scottish engineer, was willing to tinker around with that ancient technology.

By using a wheel to convert the up-and-down thrusts of steam-powered pistons into a continuous rotational motion, Watt invented the modern flywheel. That and other improvements he made to the steam engine helped spawn the Industrial Revolution (and earned him immortality as the namesake of the watt).

Now, it seems, the time has come to reinvent the flywheel.

Today’s engineers are repurposing Watt’s device into a promising alternative to batteries and other high-tech means of storing energy. Among their aims: using flywheels to make renewable energy more useful, and turning trains and buses into hybrid vehicles.

In its simplest form, a flywheel is a wheel, disk or cylinder that spins around a stationary axis. It works on the same principle as the potter’s wheel, which has been used to mold clay vessels since the dawn of civilization.

Flywheels can store enegery efficiently. (Louie Psihoyos/GETTY IMAGES/SCIENCE FACTION JEW)

A flywheel’s momentum, which depends on how heavy the wheel is and how fast it’s spinning, can contain an impressive amount of energy. And if something acts as a brake on the wheel, this energy can be released — in the form of electricity, for example — with impressive suddenness.

Finding new, improved ways to store energy is crucial partly because of the growth of renewable energy sources such as wind and solar power. The former is available only when the wind is blowing. The latter, only when the sun shines. Unless some of that energy can be stored for later use, renewables will never be able to power the world’s needs around the clock.

Better energy storage methods could also improve fuel efficiency and curtail costs at conventional power plants, reduce their carbon emissions and prevent blackouts.

Even though conventional plants can often increase or decrease energy production as needed — by throwing more coal in the furnace, for example — they can’t adjust their output instantaneously.

Energy demand, however, often comes in sudden surges. A large factory powers up its equipment all at once. A rush of commuters arriving home almost simultaneously turn on all their air conditioners and TVs. A baseball stadium’s lights blink on. These and other daily occurrences cause sharp and not entirely predictable spikes in demand for electricity.

Consequently, power plants routinely generate more electricity at a given moment than their customers are expected to consume. The excess goes to waste. But that’s more acceptable than being overwhelmed by an unexpected spike in demand, which can rapidly lead to a blackout.

If a standby supply of stored energy is available for rapid deployment, plant operators can use it as a buffer against demand surges. They could then get away with less overproduction, minimizing waste and emissions. Starting in 2009, financial incentives from the federal government and some states have encouraged companies to seek ways to store energy and release it to the grid quickly when demand spikes.

In January, Massachusetts-based Beacon Power Corp. began serving the New York state electrical grid with the nation’s largest flywheel plant. When fully operational, the Stephentown, N.Y.-plant’s array of 200 flywheels, each weighing more than a ton and rotating up to 16,000 times per minute, will be able to provide 20 megawatts of power. That’s enough energy to meet about 10 percent of the state’s daily needs, the company says.

Having that extra juice around could help prevent blackouts such as the one that plunged New York and parts of other states and Ontario into darkness in August 2003.

Beacon already has a smaller flywheel plant operating in Massachusetts, and last month it announced a deal to build a plant in Montana.

Technological improvements since Watt’s day have made flywheels about 90 percent efficient at storing energy, meaning that all but about 10 percent of the energy put into them can be retrieved later. Some batteries, by comparison, can rapidly lose a tenth or more of their charge, with further losses as time goes on.

The flywheel’s efficiency is a product of the fact that once a body is in motion, it tends to remain in motion. As long as a flywheel maintains its rotational speed, it retains its stored energy. To eliminate air resistance that could slow down the spin, engineers have enclosed flywheels in vacuum chambers. To prevent friction, they’ve dispensed with physical bearings, instead using “magnetic bearings” that levitate the spinning wheel and permanently suspend it in place.

Other improvements in materials and manufacturing have increased flywheels’ durability and capacity.

Previously made from steel, flywheels are now mostly constructed from lighter carbon-composite materials. Steel’s density gave a flywheel a lot of momentum at a given rotational speed. But the outer edge of a spinning flywheel is under tremendous centrifugal force. As strong as steel is, it has a tendency to explosively disintegrate under such a strain. A flywheel made of composites, however, can spin many times faster than a steel counterpart without coming apart. That extra speed more than makes up for its relative lack of heft.

The power grid isn’t the only place where flywheels can be useful. Because they’re able to produce powerful bursts of electricity, flywheels have various industrial applications, from powering lasers to testing circuit breakers.

Vehicle makers see the technology as promising, too. Some Formula One race cars have been equipped with flywheels that help boost their acceleration. But past efforts to use flywheels in commercial vehicles never got far off the starting line.

In today’s hybrid cars, large batteries capture and store the energy generated when the driver brakes. But batteries are too heavy and inefficient to be optimal for that purpose in larger vehicles such as trains and buses. Now that composite materials can make flywheels lighter, flywheels could replace batteries in hybrid mass-transit vehicles.

With all the stopping and starting that commuter trains and buses do, that could give fuel efficiency a real boost. Researchers at the University of Texas at Austin estimate that a hybrid locomotive with an onboard flywheel could use 15 percent less fuel than a standard train along the nation’s busy Northeast Corridor.

The train has long been an iconic symbol of the Industrial Revolution. With the help of the flywheel, it’s poised to come full circle.

Harder is the general manager of Health and Science at U.S. News & World Report.