TECHNOLOGY FORESIGHT KERS A DEEP DIVE INTO KINETIC ENERGY RECOVERY SYSTEMS – PART 1 20 www.autotechwww.autotechreview.comreview.com AUTHORS INTRODUCTION issue of kinetic energy recovery through regenerative braking has received Vehicle manufacturers, and their suppli- increased attention. A pressing need to ers, are exploring every possible opportu- reduce battery size, increase range and nity of improving fuel economy. One such performance as well as the ability of elec- opportunity exists in recovering the tric drives to operate in regenerative mode kinetic energy that is lost due to braking. has made regenerative braking specifically ARGHYA SARDAR For a vehicle, the positive inertia kinetic relevant for electric drive vehicles. Regen- is Scientist E & Head, Transportation energy equals the negative inertia kinetic erative braking along with Anti-lock Brak- Division at TIFAC, Department of Science & Technology, Government energy over a stop-start sequence and ing System (ABS) feature enhances the of India. over a driving cycle. driving experience because of the precise In a conventional vehicle, deceleration control on the acceleration and decelera- is caused by four forces – aerodynamic tion of the vehicle. drag, rolling resistance, driveline friction, It is in this context, a review of various and friction braking. Among these, the KERS and their future potential is rele- first two are irretrievable losses. However, vant. This article attempts to explore how it is possible to recover a part of the various KERS would evolve and the type RAKESH KUMAR DEY kinetic energy that would otherwise have of vehicles they would be applied to. In is Project Associate at TIFAC, been lost in friction braking as heat. the second part of the article, scheduled Department of Science & Technology, Government of India. Kinetic energy storage devices have for our July 2015 edition, we would com- been in use since ancient times – pottery pare the technologies, and discuss the wheels and spinning wheels being some regenerative braking strategies. of the examples. Flywheels have been used with steam engines and internal combustion engines to smoothen the fluc- TYPES OF KINETIC ENERGY tuating torque produced by the reciprocat- RECOVERY SYSTEMS SURESH BABU MUTTANA ing motion of the pistons of such is Scientist C at TIFAC, Department machines. Regenerative braking is a very There are three types of kinetic energy of Science & Technology, Government of India. well-established technology in transporta- recovery systems available currently – the tion, particularly in electric locomotives. mechanical energy storage system in the Currently used in racing and sports form of a flywheel, hydraulic system and vehicles, Kinetic Energy Recovery Systems an electrical energy storage system in the (KERS) are being seriously considered for form of battery or ultra capacitor. conventional IC engine as well as hybrid Although kinetic energy recovery electric vehicles. With the electric and through regenerative braking is a well- hybrid electric vehicles starting to make established technology in case of locomo- inroads into the commercial space, the tives, there is a major difference in case of Containment disks Flywheel Rim Vacuum seal Touchdown ring Flywheel Hub Flywheel bearing Vacuum port Containment Ring Speed sensor Flywheel Housing 1 Typical Flywheel and components autotechreview June 2015 Volume 4 | Issue 6 21 TECHNOLOGY FORESIGHT KERS Flywheel LOW SPEED HIGH SPEED flywheel needs to rotate at high speed, or it should have a large diameter. Thus, RPM < 10,000 15,000-100,000 for automotive applications, high speed is the desired characteristics, so that TECHNOLOGY Mature Recent both weight and footprint can be mini- mised. Rotor materials selection, its ROTOR Steel (Vmax = 300 m/s) Composite Fibre (Vmax = 1,000 m/s) dimensioning and the structure connect- ing the rotor become important issues. BEARING Conventional (heat) Ceramic; magnetic As a technology, flywheel has evolved over the last few decades. 2 SURROUNDING Air (air resistance) Vacuum lists a brief status of the technology. Five categories of materials that are SAFETY REQUIREMENT Low High generally used for flywheel design and their properties are given in 3. In order COST Low High to reduce weight, engineers who design Formula 1 race cars have tried to use 2 Type of flywheels and their characteristics composite materials in flywheels. Fur- ther, in order to reduce friction, the fly- wheels were sealed inside a vacuum road vehicles. Whereas in case of loco- sion is used to control and transfer the chamber. R&D efforts on modern fly- motives, there are options either to feed energy to and from the driveline. The wheels focus on materials such as power (DC lines) back to the grid, or pro- transfer of vehicle kinetic energy to fly- alloys, carbon-fibre composites, ceram- vide power to other trains in the line, in wheel kinetic energy can be seen as a ics, and crystalline materials such as case of road vehicles, an on-board energy momentum exchange. When braking is single crystals of silicon. A central part storage system that can receive the recov- initiated, the vehicle is at a high speed of rotor design is the reduction of radial ered energy is essential. and the flywheel at a low speed, giving tensile stresses. Some are specifically For conventional IC engine vehicles, a certain gear ratio between them. At designed to ensure they shatter safely such storage system will have to be the end of braking, the function reverses into tiny fragments, should they spin an additional system, whereas for – the vehicle speed is low, and the fly too fast. electric and hybrid electric vehicles, wheel is at a high speed, thus changing High-speed flywheels have rotors the options are either to use the traction the ratio of speed. During the energy suspended on magnetic bearings, spin- battery, or combine it with an ultra- transfer, the ratio between vehicle speed ning at high speeds (up to 100,000 rpm) capacitor or a flywheel. and flywheel changes continuously. in a vacuum chamber. R&D efforts are The recovered kinetic energy is also being focussed on high temperature stored in the spinning wheel, to be superconductors as bearings, in order to FLYWHEEL released upon acceleration. The amount increase stored energy. of energy a flywheel stores depends on The challenge, however, is to develop In the mechanical system, the available its moment of inertia and the speed at such systems for passenger cars, where, kinetic energy is used to spin a flywheel which it rotates. The moment of inertia, unlike racing cars, the system needs to at high speeds and this motion is trans- in turn, is dependent on the mass and be affordable, and also should last for a ferred back to the wheels via a trans- its distribution. For storing more energy much longer period. In case of racing, it mission, 1. The variable drive transmis- without increasing the mass, either the may be enough for the system to last only for a couple of hours at a time. Using a similar design for consumer cars would need a system to maintain Material Allowable STRESS [σ], MPa DENSITY ρ, KG/m3 the vacuum with pumps and valves, which will add complexity and cost. GREY CAST IRON 220 7,340 Flywheels have been frequently used in high technology application, and usu- ALUMINIUM ALLOY 400 2,700 ally feature vacuum enclosures and magnetic bearing systems so as to mini- MARAGING STEEL 900 8,000 mise frictional losses. CARBON FIBRE COMPOSITES (40% epoxy) 750 1,550 Flywheel EXPERIMENTS E-GLASS FIBRE (40% epoxy) 250 1,900 Use of flywheel as the sole energy storage 3 Flywheel Materials system in the vehicle has been attempted 22 www.autotechreview.com since long. The ‘Gyrobus’ – a city bus mechanical energy for up to 30 min, 20% fuel savings and reducing car- developed by the Maschinenfabrik Oer- releasing it when required to aid in bon emissions. likon in Switzerland in the 1930s, used a acceleration. A six pound flywheel flywheel connected to an electrical made of carbon-composite, wrapped machine as the sole on-board energy around a steel hub, spins in a vac- ELECTRIC STORAGE SYSTEM source. At regular bus stops, power from uum at up to 60,000 rpm, recapturing electrified charging stations was used to somewhere around 40 hp of instanta- Regenerative braking system utilises the accelerate the flywheel. In between the neous power. It is claimed that the electric motor, providing negative torque bus stops, the kinetic energy of the fly- system has the potential to reduce to the driven wheels and converting wheel was used to produce electrical fuel consumption by up to 25 %, and kinetic energy to electrical energy for energy by the electrical machine, which in improve acceleration by 1.5 s by recharging the battery, 4. Kinetic energy turn, ran an electrical motor. Once reaching a speed of 100 km/h from can be converted back into electrical charged, Gyrobus could run six km in reg- being stationary in 5.5 s. energy, which can be stored in batteries ular traffic. There are examples of devel- :: GKN Gyrodrive, a flywheel system for reuse to propel the vehicle during the opment efforts to use flywheel-based that has been used in motorsports, is driving cycle. The existing inverter of the KERS in vehicles with IC engines. Some available for hybrid buses and other electric drivetrain can be utilised for examples are: passenger and commercial vehicles. recovering the braking energy with added :: Jaguar has tested a flywheel hybrid Go-Ahead Group plc, the UK-based regenerative control algorithms. (IC engine + flywheel) that promises operator of train and bus services, In many modern electric and hybrid a power boost of about 81 hp, and up plans to implement this technology electric vehicles, electric KERS has been to a 20 % fuel economy improve- in 500 of its 4,600 buses.