While some alternative energy systems are suitable as primary power sources — batteries and fuel cells, for instance — others are more suited to being interim or supplementary power providers. Ultracapacitors are increasingly seen as a short-term source of electrical energy, as they can store and release energy at a high rate — certainly faster than equivalently-sized batteries.
But alternatives based on kinetic rather than electrical energy also can store and release energy relatively rapidly. One of the most straightforward methods of energy storage is the flywheel: set it spinning and it retains its kinetic energy, only losing some energy to friction and air resistance. Connect it to a motor/generator, and you can spin it up with electricity — ideally with cheap energy such as the regenerative braking of an electric vehicle. Switching the motor to act as a generator harvests that energy at any time. The flywheel is simple, and has the advantage of long life.
A number of electric flywheel systems, designed to compensate for supply fluctuations in power generation set-ups, have operated for decades without incident. Some flywheel systems‘ energy density (in terms of kWh/kg) is as good as an electric battery, with the potential for higher peak power output, with longer cycles of operation.
Friction is critical: bearings are one issue, and many applications have used magnetic bearings (or conventional bearings with magnetic load-reduction) successfully. Another issue is the resistance of the air (or fluid) around the flywheel itself. Most automotive applications use an evacuated chamber, although this makes mechanical connection difficult – a contactless magnetic drive is preferable.
A fully-mechanical drive was the basis of some of the KERS (Kinetic Energy Recovery System) devices used in motor racing to give a boost while overtaking. One of the most advanced instances of the flywheel battery is the electrically-connected GKN Hybrid Power flywheel, or ‘Gyrodrive’, originally developed by Williams Grand Prix Engineering and used in Audi’s successful R18 hybrid Le Mans car. This uses a magnet-loaded carbon-fibre flywheel which is spun up electrically during braking, releasing the energy to an electric motor as needed. GKN Hybrid Power worked with Alexander Dennis Limited (ADL) to develop a hybrid bus using the system to assist in frequent stop-start cycles. Fourteen Gyrodrive-equipped ADL Enviro 400s with 36,000rpm flywheels were operated by Oxford Bus Company (pictured above) on its BROOKESbus operation. At the time, the operator said: “The exhaust emissions are as good as the electric hybrids in the fleet… but the add-on cost is much lower”, and claimed “fuel use is reduced by up to 25%”. But GKN ceased production of the Gyrodrive in October 2016. According to a news story at the time, “The introduction of Euro VI and the relatively low price of diesel are [cited] as factors in the decision”, although a spokesman this year said that the firm’s “focus shifted from diesel vehicles to full electrification”.
The hydraulic accumulator has some of the same properties as a flywheel: it can absorb and release energy quickly, with relatively few moving parts and a long life. In general, the accumulator consists of a pressure vessel containing both hydraulic fluid (which is incompressible), and a sealed bladder of compressible gas (usually nitrogen) to act as a spring. The hydraulic fluid is pumped into the vessel using the energy from braking, for instance, to be released as needed to drive a hydrostatic motor.
In 2003 US company Stewart & Stevenson demonstrated a military truck with a diesel/hydraulic hybrid system. It was developed by an Australian firm called Permo-Drive and US components manufacturer Dana. The vehicle was said to cut fuel consumption by 35%.
The same technology was set to be rolled out more widely to US refuse collection vehicles, but Dana’s financial problems essentially ended the project.
Meanwhile, staying across the Atlantic, postal carrier Fed-Ex worked with the US Environmental Protection Agency (EPA) to build a prototype hydraulic hybrid vehicle (HHV). Based on a 13-tonne International chassis, its 6.0-litre diesel drove a hydraulic pump which fed pressurised oil to a diff-mounted hydrostatic drive, as well as high-pressure accumulators. Wheel-driven pumps provided regenerative braking, and the hydraulic system was also used to start the engine. The system was claimed to reduce CO2 emissions by 40%. It never reached production.
Chinese researchers are currently working on HHVs that use wheel-mounted hydraulic pumps/motors, which improve the efficiency of the system and allow the IC engine to operate near its most economical range.
In 1980 Volvo demonstrated a bus which combined a flywheel with some hydraulic elements: this used a relatively small (100kW) diesel engine coupled to a hydrostatic drive and a flywheel, which could absorb up to 170kW under retardation. The system was said to be smooth, economical and needing little maintenance — with much less brake wear than a conventional bus — but the initial cost was high.
BOX: THE OERLIKON GYROBUS
While most applications of flywheel batteries have been to provide extra boost while accelerating, there has been a serious attempt to use them for primary motive power. The Oerlikon Gyrobus, which entered service in Switzerland in 1953, used a 1.5-tonne steel flywheel, mounted in the middle of the chassis on a vertical axis, connected to an electric motor-generator which drove the bus at up to 55km/h. The flywheel was ‘charged’ by connecting the motor-generator to overhead three-phase 380V mains using a roof-mounted collector, taking advantage of cheap hydroelectric power: four minutes of charging at 150kW would give the bus a range of about 5-6km of stop-start operation.
In operation, the 1.6m diameter flywheel would spin at 2,100 to 3,000rpm; remarkably, it was mounted in a sealed chamber filled with hydrogen at 0.7 bar to reduce air resistance, and would take ten hours to spin down to a halt while idling. In practice, it was ‘trickle-charged’ to keep it spinning overnight.
Reportedly, drivers had to get used to the peculiar handling resulting from the gyroscopic effect of the system, although the ride was said to be exceptionally smooth. The Gyrobus was used in Belgium and the Congo (as was; now the Republic of Zaire) as well as in Switzerland — 18 buses entered service, and performed acceptably, but the last ones were replaced by diesel buses in 1960.