Reinventing the Wheel for Greater Energy Efficiency
With fuel prices edging ever higher, engineers are scrambling to find new ways to improve the energy efficiency of vehicles. We’ve already seen a few attempts on the market. Hybrid automobiles — vehicles that combine electrical and internal-combustion powertrains — are becoming commonplace. Pure electric vehicles, such as those in development by Tesla Motors, are expected to be seen on the road in the next few years.
Unfortunately, the weakest link of all-electric powertrains is the energy storage medium: batteries.
Even with the best technology available, batteries are expensive and bulky and have a low energy density. The result is more-expensive vehicles with lower range between fill ups. Since batteries can’t be topped off in a few minutes like fuel tanks can, the ability to easily extend range also suffers.
Innovators are taking a look at some of the oldest-known technologies as alternatives to batteries. In a sense, they are reinventing the wheel — the flywheel, that is.
The flywheel idea is an old one that dates back to the potter’s wheel: A heavy wheel was used to store rotational energy so that the potter could form symmetrical shapes out of clay.
Unlike batteries, which are a chemical form of energy storage, flywheels store energy mechanically in a rotating device. Flywheels can store more energy than batteries in a package a fraction of the size.
Flywheels were used as a form of energy storage in Swiss “gyrobus” electric buses of the 1950s. Developed for use on bus routes where overhead power lines weren’t available, gyrobuses charged their mechanical “batteries” at passenger stops or terminals.
At these stops, the buses would connect to an electric power source and a motor would spin the flywheel at speeds up to 3,000 revolutions per minute. Upon leaving the stop, the motor would act as a generator instead, using the stored spin energy to deliver electrical power to motors driving the wheels. On a full charge, gyrobuses could travel 3.7 miles at speeds of up to 37 miles per hour.
Unfortunately, flywheel energy storage has had its shortcomings. Energy losses from friction limited the utility of the concept. Older flywheel designs used mechanical bearings and conventional materials, which curtailed the speed at which the wheels could be spun. Since the flywheel housings themselves weren’t vacuum sealed, there were also significant friction losses from spinning against atmospheric gases. However, new technologies are addressing these problems, and there is a resurgence of interest in the concept.
Formula One racing teams, for example, have been using a flywheel-based Kinetic Energy Recovery System (KERS) since 2009. KERS uses a flywheel to recover deceleration energy lost through braking. When the F1 race car exits a sharp turn into a straightaway, the energy, stored in a flywheel, can be released for higher acceleration. The driver has a special boost button he can engage whenever he wants an extra burst of power.
The original 2009 design, named Flybrid, was made of a steel/carbon fiber composite rotating at 60,000 rpm inside a vacuum chamber. The unit is good for 80 horsepower over a period of 6.67 seconds per lap and can decrease the time spent on a lap by up to four-tenths of a second. Four-tenths of a second is make or break in the highly competitive sport. Furthermore, since there is no energy conversion loss as in battery-based recovery systems (which must convert kinetic energy to electrical and then back again), the system is inherently more efficient.
Flybrid and Torotrak PLC, which is the licenser behind much of the technology in the F1 KERS system, is partnering the technology with major automakers. Volvo, for example, is looking at taking flywheel energy storage beyond the race circuit and into everyday automobiles.
Thanks to advanced materials technology, the flywheel Volvo is testing weighs a mere 13 pounds and has a diameter of only 7.8 inches. It also spins in a vacuum to minimize friction losses.
Volvo believes it can be produced at a much higher volume than the battery-based energy recovery technology used in today’s crop of hybrid vehicles. Knowing what we do about resource constraints (whether real or politically caused) affecting the mass production of rare earth permanent magnet motors and lithium-ion batteries, this is a big deal.
According to Volvo, flywheel technology has the potential to deliver double-digit gains in fuel economy and makes a four-cylinder car feel like a six-cylinder.
You may have played with friction-motor toy cars as a child. I had one with a flywheel you would wind up with a zip cord. Once set down on the floor, it would zip to the opposite end of the room. These days, the concept is expanding into more practical uses. Volvo plans to demo a flywheel-enhanced vehicle this fall, and in a few years, your next vehicle might feature the technology.