Flywheel Hybrid - Electro Mechanical Battery

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1 Flywheel Hybrid - Electro Mechanical Battery Use of Flywheel as a compliment to Battery in Hybrid Vehicle Nikhilesh Reddy Kalluri Student, Department of Mechanical Engineering PES Institute of technology Bangalore south campus Bangalore, India nrkalluri@gmail.com Harish Tiwari Student, Department of Mechanical Engineering PES Institute of technology Bangalore south campus Bangalore, India harish.tiwari077@gmail.com Abstract The advancement in automotive industry has taken a higher note with the improvement in hybrid technology to drive the vehicle. Hybrid vehicles which uses battery as an energy storing device to power the vehicle has limited its use due to less travelling distance and loss of energy due to energy conversion taking place during its use. One of the overseen technology in hybrid technology is the use of flywheel to power the vehicle. Flywheel systems are attractive in hybrid vehicles due to their ability to handle power during acceleration and braking. The combination of a flywheel device with a battery source has several advantages such as peak power capacity, high energy density and reduction in number of charge and discharge cycles of battery. Keywords Flywheel, Battery, transmission, properties of flywheel, performance of flywheel hybrid vehicles. I. INTRODUCTION As the world s consumption of fossil fuels has increased, so have the concerns about the environmental effects associated with our dependence on this non-renewable resource. One result of this concern has been a growing interest in electric and hybrid electric vehicles. All vehicles act as energy storage systems by virtue of stored kinetic energy which increases with speed. It is therefore not surprising that flywheels have been considered as suitable energy storage devices for vehicles from an earlier time. Kinetic energy is stored in a similar way to the vehicle and, if energy is toggled between the two, the power needed for acceleration can be eliminated in theory, creating the effect of a zero inertia vehicle. Similarly, energy normally lost in braking can be captured and regenerated back into the vehicle. Given that the form of energy in the flywheel is mechanical kinetic, the same as that of the moving vehicle, the energy can be transmitted from one to the other and not transformed, maximizing potential for high efficiency. By comparison, any electrical means of storing energy will fundamentally require transformation of the form of energy, this needs to be done twice in a round trip from vehicle to storage and back again for energy regeneration. If the electrical storage is an electrochemical battery, then the two chemical transformations must then also be added. This is not a judgement against the use of electrical systems, since it can sometimes make good sense to use electrical means to transmit power to and from a flywheel. Pure electric vehicles typically use a large bank of electrochemical batteries to provide both power and energy storage, but experience range limitations due to poor energy storage density (compared to gasoline) and excessive recharge times (compared to gasoline refueling times). Hybrid electric vehicles address the range issue by using a small internal combustion prime power unit to drive an electric generator to meet the average power requirements. To address peak power requirements, an auxiliary energy storage and power conversion device (typically a flywheel battery or a bank of electrochemical batteries) supplements the prime power unit. In this type of hybrid system, or when used in conjunction with a bank of batteries in a pure electric vehicle, the energy storage device (e.g., flywheel battery) is used for this power averaging role. Power averaging is best accomplished by storing energy generated during vehicle braking and releasing it during acceleration. The generation of braking energy on a vehicle is accomplished through the use of electric traction motors which operate as generators as the vehicle decelerates. A motor/generator in the flywheel battery is used to convert the electrical braking energy into mechanical energy in the spinning flywheel during this period. When vehicle acceleration is needed, the flywheel battery motor/generator converts the energy stored in the flywheel back into electrical energy to power the traction motors, completing the storage and recovery cycle. Because regenerative braking recovers previously wasted energy, the fuel efficiency of the vehicle can be increased dramatically. The peak power required of the prime power unit is also reduced, which reduces both initial system costs and operating costs. While both power averaging and regenerative braking have been demonstrated with both electrochemical batteries and flywheel, there are significant benefits to using a flywheel. But it is not a complete substitute to the electrochemical battery due to the limitations of the flywheel. Whereas use of a flywheel along with electrochemical battery looks to be a viable substitute.

2 This paper includes information about using the flywheel along with a battery in hybrid vehicle, technology of flywheel, types and performance of flywheel and finally analysis of flywheel. II. RELATED WORK The notion of flywheels may conjure up images of large cast iron wheels on steam engines or the device fitted to internal combustion engines even in the minds of engineers. The other immediate issue that springs to mind is that of safety, the spinning flywheel being like a bomb that is inherently unsafe. It is these kinds of notions that have perhaps dissuaded low carbon vehicle powertrain developers from giving this technology more serious attention, an attitude that has recently changed. A simple calculation can show the amount of energy typically needed to be stored; this number being at the heart of the flywheel size and safety issue. The stored kinetic energy Ekin in any moving object is given by: Ekin = 1/2 MV^2, where M is mass in kg and V velocity in m/s. There have been attempts to see whether a flywheel or number of flywheels could be used as an alternative to a battery. In order to propel the vehicle for several miles, the amount of energy needed would be far higher than the amount calculated for regenerative energy recovery. The result would be a flywheel or flywheels of too substantial weight since the energy density of flywheels is several times lower than a battery. III. PROBLEM Addition to the extra weight, concern is the safety issue, which is discussed later but if the amount of kinetic energy stored is several times that of the vehicle, it is of far greater concern. Added to this is the issue of run down loss with flywheel charge for economic designs running down in the order of tens of minutes or hours not weeks and months. The notion of using flywheels with energy storage capacity much greater than the kinetic energy stored in a vehicle should thus be dismissed. IV. SOLUTION There is a very good case for using flywheels as a complement to batteries to overcome the shortcomings. These shortcomings include low regeneration efficiency, reduction in life and thermal issues when charging or discharging at high power levels. A flywheel can protect a battery from damaging surge demand particularly when the battery is either cold or hot. This is counterintuitive when the choice that is considered more obvious is the ultracapacitor, keeping everything electric rather than mixing mechanical with electric. However, the vehicle is a mechanical energy store so using a flywheel with a battery can make good sense. Ultra-capacitors have lower energy density, and require dedicated power electronics to match a variable voltage to the approximate constant battery voltage. At present, they present an expensive solution. The safety of flywheels is something often cited as a major concern and reason for dismissing this technology. The surface speeds may be cause for alarm approaching sonic and often above sonic for some designs. This has to be compared with speeds in the order of m/s found in automotive turbochargers and much higher speeds found in aviation jet engines. In the turbocharger example, the rotor weight is less of course and for aviation jet engines, rotors are inspected and maintained carefully. However, in the latter, turbine discs cannot be contained in the event of a rotor failure due to the need to make the casings very light and thin. For the automotive flywheel, weight of the casing is an issue but having a containment system of around the same weight as the rotor is not a problem since the mass is still relatively small in comparison with the vehicle. Indeed, there are ways to ensure that the rotor fails in a more benign manner by minimizing the size of rotor fragments in case of a full or partial rotor disintegration. Given this, the containment issue associated with these high speed rotors is not one simply to be dismissed but is also not an intractable problem. It has and can be solved by engineering solutions. A further benefit of flywheel storage is the potential for low cost. Once production has exceeded the order of 100,000 units, the cost of most artefacts tends towards base materials cost plus the cost of any special processing for the materials, particularly if energy intensive. Much, if not, all the flywheel and transmission can be made of steel, plus it is fundamentally a simple technology. A material often used for the flywheel is carbon fiber composite, which is relatively expensive, but the amounts required are small and the shapes simple. This contrasts with variable speed and voltage electrical devices needed for electrical energy regeneration, which are expensive for high power levels. The power that can be stored or extracted from the flywheel is only limited by the transmission, if the mechanical type; hence the energy storage capacity and power can be chosen independently, something not true of most other storage means. All of these fundamental benefits have led to recent resurgence of interest in flywheel energy storage for vehicles. These Flywheels when incorporated with battery to run the vehicle gives added benefits than when separately used and eliminates some of the limitations of the hybrid vehicles. V. STRESSES AND MATERIALS OF FYLWHEEL A discussion of the effect of fiber properties upon flywheel stored energy is needed to quantitatively demonstrate the benefits of composite materials. To simplify the analysis, an idealized flywheel will be considered in which all of the mass in the rim is concentrated in a thin ring of radius r. Figure Below shows Idealized flywheel rotating thin ring with wall thickness t.

3 Table 1. Comparison of different materials and two basic flywheel shapes Idealized flywheel rotating thin ring with wall thickness t. Material And Type Aluminium T7075 Titanium Density (Kg/m3) Design stress (Mpa) Peripheral velocity (m/s) Rotor mass (Kg) The kinetic energy, Ek, stored in the flywheel rotor is given by: Ek = 1/2 I ω2 (1) Where, I is the polar moment inertia of the rotor and ω is its angular velocity. The polar moment of inertial equation is given by: I = m r2 (2) Where, m is the total mass of the idealized zero thickness rim. Substituting (2) into (1) gives Ek = ½ m r2 ω2 (3) For the idealized flywheel with all mass at radius r, the hoop stress in the ring is σ = ρ r2 ω2 (4) Where, ρ is the density of the ring material. Solving for r2ω2 in (4) and substituting into (3) yields Ek = ½ m σ / ρ (5) The specific kinetic energy, defined as kinetic energy per unit mass, is maximized for maximum σ, so Emax / m = ½ σmax / ρ Showing that to maximize kinetic energy the constituent material needs to have high strength and low density. Table 1 shows the properties of different materials than can be used to make a flywheel, depending on the result obtained above the best suitable material can be chosen depending on the shape. Steel Aluminium solid T7075 Titanium solid Steel solid Glass epoxy Carbon epoxy By considering the values in the above table it is seen that carbon epoxy is the best suitable material to produce a flywheel. Even though the cost of carbon epoxy is high, the amount required is very less and reduces the weight of the flywheel compared to other materials which is an added advantage dude to the decreased load on the vehicle. VI. TRANSMISSION FOR FLYWHEEL In the case of an automobile using a flywheel as an energy storage device, a suitable means to connect the flywheel to the driveline is needed that would allow the flywheel to change its speed continuously. As mentioned in the previous sections, the flywheel exchanges its energy with the automobile by increasing or decreasing its rotational speed when the vehicle s speed is changing in the opposite direction. In other words a continuously variable transmission (CVT) is essential for the flywheel. The requirements of the CVT used in a flywheel hybrid vehicle are slightly different to that used in a conventional vehicle. The main difference is that the CVT in a flywheel hybrid vehicle should be able to transfer energy in the forward as well as the backward direction at high efficiency. Another important difference is that the ratio range, which is defined as the ratio of the maximum to the minimum speed ratio, has to be relatively large especially when using a high speed flywheel with a low depth of discharge. The other requirements of low cost, light weight and ease of control are standard. Difference between transmissions is shown below. Fig. 1. Shows difference in transmission of electric and mechanical power trains.

4 suitable for intra city bus or trolley coach application. However, beyond that, this type does not find much use. Fig. 1. Comparison of typical hybrid electric and mechanical energy storage and power delivery systems Traction continuously variable transmission transfers torque between two objects through adhesive friction. The transmission ratio is varied by changing the radius of the point of action of forces. The traction drives usually have a limited ratio range. There are predominantly two types of traction drives: the belt drives and the rolling traction drives. But this method of transmission proves to be less efficient because of losses due to friction between moving parts. By using flywheel along with battery in hybrid vehicle, Electrical transmission becomes possible which is formed by using two electrical machines in series, one functioning as a motor and the other as a generator. The mechanical energy is converted into electrical energy at one end and reconverted into mechanical energy at the other. The continuously variable transmission is achieved by controlling the torque of the machines by varying the voltage or the current of the machines. The electrical machines offer the advantage of flexibility due to the absence of any rigidly connected mechanical links. However they tend to be on the expensive side. In the case of flywheels, it is common to attach a motor generator to exchange energy in and out of the flywheel and this is commonly called an electromechanical battery or flywheel battery. B. Flywheel and internal combustion engine-based powertrain: This type is particularly useful since the conventional IC Engine vehicle does not have any means of capturing brake energy and adding a flywheel gives that option by using a complex mechanical transmission consisting of integrating and differentiating planetary gear sets. Usually non electrical transmissions will be used in this type of powertrain. In this combination, flywheel is used for initial torque and IC engine is used after a certain speed is reached in which case the consumption of fuel is reduced. This improvement in fuel economy is mainly the result of improved engine operation, and regenerative braking has limited impact. The parasitic losses of the flywheel system need to be optimized to improve fuel economy. C. Flywheel in battery electric powertrains: It is a well-known fact that the crucial element that inhibits the electric vehicle (EV) is its batteries. The limited range and cost of the electric vehicle are its limitations. These limitations can be partially offset by the combination of a high power device such as the flywheel which would load level the battery, and the battery can be designed as a high energy eleme0nt. The battery would provide low average power to the vehicle and all the high power events such as acceleration and deceleration would be handled by the flywheel. This would not only lead to increasing the range of the vehicle but also improving the battery life as high transients are taken over by the flywheel. The cooling requirements of the battery are also reduced. The flywheel is usually added on the system as a Flywheel Battery (FWB) shown in Fig. 2, with very few known exceptions. The FWB combines a motor generator with a flywheel. This is a viable substitute for the IC Engine power train but with less range. IC Engine also can be incorporated to increase range as discussed next. VII. PERFORMANCE EVALUATION The flywheel energy storage system (FESS) can be applied in an automotive powertrain in different ways depending on the powertrain type and structure as well as the performance and the vehicle requirements. Not only is the design of the powertrain but also the energy management of the system crucial to the performance. A. Pure flywheel-based powertrain: The FESS can be the sole energy source in the powertrain, in this case the flywheel is usually charged at stationary terminals during the journey. The flywheel is generally acting as a FWB which is charged and then is used to provide power to the traction motor. Since the whole of the propulsion needs to come from the flywheel, the flywheel has to be sized large enough to carry the vehicle a reasonable distance. This type of arrangement could be Fig. 2. Flywheel Battery

5 D. Flywheel in a hybrid electric powertrain: As we have seen the advantages of incorporating a flywheel with an IC engine and battery powertrains, there is a very good chance of adding a flywheel to the hybrid electric powertrain to get maximum efficiency. The flywheel with a battery increases the battery life and range of vehicle, while the flywheel takes care of the high power events as in a flywheel electric powertrain. Addition of the IC engine increases the range even more, and the pollution caused by the engine is reduced. This also increases fuel economy as in Flywheel IC Engine powertrain. Common configuration is a hybrid electric series configuration which is shown in Fig. 3. Here the IC Engine is the prime mover connected to a generator and the FWB is there in place of the usual electrochemical batteries. It can also be in parallel hybrid configuration, where it consists of an IC Engine, an electric asynchronous machine, batteries, a flywheel and a wide range continuously variable transmission as shown in Fig. 4. Fig. 3. Series Configuration Fig. 4. Garret Electromechanical transmission VIII. TECHNICAL CHALLENGE By far the greatest technical challenge facing the developer of mobile flywheel systems is the issue of safety and containment in the evening for failure. For vehicular applications, the containment has to be low cost and more importantly low weight. One method is to design the flywheel in such a way that the ultimate stress point is never reached in practice or at least the risk is extremely low. Certain developments have shown promising designs in this aspect. One example would be the rotatable liner design by UT-Austin in which a free floating, cylindrical rotatable structure was able to mitigate the flywheel burst failure mode. Another example is the system from Ricardo where failure modes can be detected by the automatic monitoring of out of balance vibration by bearing sensors, which would result in shutdown of the system before any further damage is done. IX. CONCLUSION The view that mechanical solutions will be swept away by an entirely electric future is creaking in the face of the need to find pragmatic affordable solutions. There is an urgency for manufacturers to produce low carbon vehicles whose powertrain cost is not substantially greater than the conventional ICE with mechanical transmission. Despite considerable investment and advances in battery technology worldwide, cost of this key technology is currently too high and performance inadequate on its own. Added to this is the high cost of traction drives at torque levels demanded by the customer. Flywheels can offer a low cost means of bridging the gap and making electric vehicles with good performance more affordable. Flywheels offer a low cost and efficient solution both in energy efficiency and use of recyclable benign materials. They may be a transitional technology but are more likely to remain complementary to other prime movers for a very long time. The reason for this stems from the fundamental physics that a vehicle is a kinetic energy storage system itself and it makes sense to use the flywheel to toggle energy to and from the vehicle in the mechanical form. Now that the benefit of flywheels has been demonstrated, one concludes from the comparison between the performance of flywheels and batteries that the most effective utilization of flywheels is in providing high power while providing just enough energy storage to accomplish the power assist mission effectively. Flywheels meet or exceed the power related goals (discharge power, regenerative power, specific power, power density, weight and volume) for HEV and EV. Flywheels provide high power energy storage, with emphasis on the high power. Safety is the area which requires further development; not that there is a fundamental concern but agreed methodologies and standards need to be established to ensure that rotor failures are contained to within an acceptable probability. No other prime mover or energy storage technology is completely risk free and there is no reason why flywheels cannot meet similar safety standards.

6 References [1] K. R. Pullen and A. Dhand, Mechanical and electrical flywheel hybrid technology to store energy in vehicles, city University, London, UK: published [2] R.J. Hayes, J.P. Kajs, R.C. Thompson, J.H. Beno, Design and Testing of a Flywheel Battery for a Transit Bus, paper no , SAE, Center for Electromechanics The University of Texas at Austin: 1998 [3] James G. R. Hansen David U. O Kain, An Assessment of Flywheel High Power Energy Storage Technology for Hybrid Vehicles, Prepared for Vehicle Technologies Program: OAK RIDGE NATIONAL LABORATORY Dec 2011.