By comparison, any electrical means of storing energy will ..... numerous publications on this subject and the most comprehensive is Dailey and Nece (1960) ...
Mechanical and electrical flywheel hybrid technology to store energy in vehicles K. R. Pullen and A. Dhand, City University London, UK Abstract: This chapter deals with flywheels and their applications as energy storage devices in automotive powertrains. A brief introduction about flywheels is given which is followed by the historical development of flywheel usage in automobiles. The important characteristics of the flywheel are discussed including the safety aspect. This is followed by an explanation of the types of transmission needed to connect the flywheel to the vehicle’s driveline. Various types of utilisation strategies of the flywheel in the vehicle are discussed using specific examples. Finally the current challenges to flywheel technology, conclusions and future trends are presented. Key words: flywheel, transmission, properties of flywheel, performance of flywheel hybrid vehicles.
15.1 Introduction 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 (Genta, 1985). 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, maximising 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, denoted a battery, then the two chemical transformations must then also be added. This is not a judgement against the use of electrical systems per se since it can sometimes make good sense to use electrical means to transmit power to and from a flywheel. However, good round trip efficiency of flywheels and transmission is often cited as a substantial benefit as compared to low efficiencies in the order of 30–40% when regeneration is done via electric motors, power electronics and batteries (Pullen and Ellis, 2008). The real challenge, however, is that as the vehicle speeds up, the flywheel must slow
down such that the sum of squares of the vehicle and flywheel speeds is constant in the ideal system. There is no simple, non-friction drive gearbox able to create this relationship known to the authors so it must be done in different ways as described later in the chapter. 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 a 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 MV2 [15.1] where M is mass in kg and V velocity in m/s. The kinetic energy stored in a one tonne vehicle at 70 mph (Vveh = 31.3 m/s) is 489 kJ. If the flywheel is assumed to be a thin-walled hollow cylinder of steel with a feasible mean speed of the steel of 300 m/s, the mass of the flywheel needed to store the same energy as this vehicle is only 10.9 kg. This approximate calculation assumes a stress of 720 MPa and the dimensions of the cylinder for a 10 mm wall thickness of 0.2m diameter 0.2m length. This can further be illustrated by the analogy of moving a liquid volume, analogous with an amount of energy, between two tanks (Fig. 15.1). The long tank is the vehicle, the short one the flywheel, denoted as a surge power unit (SPU). The length of the tanks represents the mass in the energy equation and the radius represents the velocity. The energy is proportional to velocity squared hence the radial section tank. Kinetic energy flows to and fro between the vehicle and the SPU (grey tint in Fig. 15.1). Taking into account casings and bearings, for the simple steel cylinder example, allowance for the shaft, bearings and casings would mean a flywheel package of less than 30 kg. The high speed flywheel even of this type can therefore store the energy of the vehicle in a unit of the order to 3% of the mass of the vehicle. Of course, better, lighter designs than steel cylinders are available. There has been a notion that only one means of energy storage should be used on a vehicle and 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. Of additional 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. However, there is a very good case for using flywheels as a complement to batteries to overcome the shortcomings of the latter. 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. Ultracapacitors 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
15.1 Flow of liquid between tanks energy analogy (Pullen and Ellis, 2008). The square law enables only a few tens of kilos of rotor to store most of the kinetic energy from the host vehicle.
Mechanical and electrical flywheel hybrid technology 479
be compared with speeds in the order of 300–500 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 minimising 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, what is often framed ‘benefit for bucks’. 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 fibre 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. However, getting it to work successfully is not as easy as might first be thought. It has taken a number of innovations both in flywheel design and in transmissions to get to a point where commercial exploitation of flywheel based vehicle powertrains is becoming reality. A brief history is now given as background to the more current developments described in the later sections of this chapter.
15.2 The development of flywheel technology To date, the best historical account of flywheel technology inclusive of automotive applications can be found in Genta (1985). Probably one of the earliest applications of flywheels in vehicles worth mentioning is the Oerlikon Gyrobus which operated in the 1950s in several cities of Europe and Africa before it was phased out due to several technical challenges (Anon., 1955). In England in the 1960s Clerk presented designs of transmissions to couple energy storage flywheels to commercial and passenger vehicles (Clerk, 1964). These were relatively complex. In 1969 Rabenhorst presented the concept of the so called superflywheel, which used high uniaxial tensile strength material and could store energy of 30 Wh/lb (14 Wh/kg) which was deemed significant at that time (Rabenhorst, 1969). In the 1970s the oil crises as well as other factors triggered a huge interest in flywheels with the US government funding numerous projects to evaluate flywheel-based vehicle designs. Unfortunately, interest in flywheel energy technology fell as oil prices stabilised towards the end of the 1970s and the research into flywheels reduced as programmes petered out by the mid 1980s. There was a revival in the 1990s with stricter emissions legislation coming into force worldwide. Some notable examples were electromechanical batteries developed by Lawrence Livermore Laboratory in the US (Post, 1996) and the University Eindhoven in the Netherlands (Thoolen, 1993). With the advent of production hybrid vehicles in the late 1990s, the interest in electric and hybrid vehicles became stronger. Since then, interest in vehicle flywheels has strengthened with the development of high speed motor generators made possible with advent of high frequency power electronics and high strength permanent magnets. A major step for the flywheels was the declaration by the Federation
Internationale L’Automobile (FIA) about the intention to employ recover and reuse the kinetic energy in Formula 1 (F1) race cars in 2006. The first season in which the F1 race cars implemented kinetic energy recovery system (KERS) was 2009. Some of the KERSs developed were flywheel based systems but, with relatively restricted power and energy limits coupled with packaging issues, the rules favoured battery based systems. Given large budgets, expensive batteries could be used and expended without concern, something not realistic for energy recovery systems for road vehicles. For reasons of lower cost and higher efficiency potential, a number of OEMs have started getting involved in developing flywheels for vehicular applications and the authors believe that flywheel-based powertrains might be seen in production vehicles in the near future.
15.3 Types and properties of flywheels A flywheel is inherently a very simple device, essentially a disc or drum supported on bearings with an output shaft. An electrical motor-generator (M/G) may be integrated into the flywheel for electrical transmission but even here the complexity is little more than that of a standard electrical machine. However, there are some differences associated with the need for relatively high peripheral speeds that create particular engineering challenges. These take practical vehicle energy storage flywheels beyond routine design methods and are listed and treated one by one. These include: rotor design and materials for maximum ratio of stored energy to weight aerodynamic loss and vacuum system low loss bearings containment in the event of rotor failure gyroscopic moment potentially affecting vehicle dynamics.
15.3.1 Rotor design and materials for maximum ratio of stored energy to weight The purpose of this section is to give a simple treatise on rotor design with rudimentary analysis and also discuss the issues with all metallic and composite designs. Clearly, the lower the mass of the flywheel rotor the better since any added weight to the vehicle increases the power needed for acceleration and rolling resistance loss. The higher the speed that the flywheel can operate for a given material density, the more energy will be stored and the lighter will be its weight for a given energy requirement.
Maximum speed is governed by the maximum design stress in the rotor and this stress s is given by: s = k rV2 [15.2] where r is the material density, V is the peripheral speed and k is a constant depending on the rotor shape. For a thin-walled cylinder, the value of k is unity and for other shapes k can be calculated. For a solid cylinder without a central hole, the value of k is 0.413 and can be determined by consideration of elasticity theory known as the Lamé formulae (Benham and Warnock, 1976). For a thick-walled cylinder with the radius of the central hole half of the outer radius, k is 0.869 and with the smallest central hole, k = 0.825 which is double that without the hole. The lowest values of k can be obtained for tapered discs as typically found in high speed axial turbomachines. Designs of this type are discussed later. Given knowledge of material maximum allowable stresses and densities, calculations can be done to establish the relative merits of different materials. Table 15.1 presents a list of candidate materials that could be considered for flywheels for two shapes, a thin cylinder and a solid cylinder without central hole. The latter is not possible for composite materials since in filament form the have high strength in one direction only. The values of design strength include account of reductions that are necessary for practical designs in the experience of the authors. For metallic materials, there must be allowance for adequate fatigue life and methods for this are well understood. Composites are more resistant to fatigue but this must also be considered. There can be considerable variation in properties obtained and the strength of the composite is often controlled not by the fibre strength but the quality of the matrix material. Also given in Table 15.1 is the weight of the rotor and its peripheral speed that would be required in order to store the figure of 489 kJ, the approximate energy requirement derived in Section 15.1. What is immediately apparent is that most metals compared with each other have similar performance yet the composites, even with conservative strengths for reasons explained, are considerably lighter. However, a composite flywheel (a cylinder) must have a metallic shaft and discs which must be connected to the composite. Given that composites typically have a lower Young’s modulus than metals when highly stressed, the radial displacement is high and it can be difficult to attach the composite part to the metal shaft via metal discs. Composite discs are more difficult to make due to the orthotropic properties of composites so this material is usually wound into a cylinder. For these reasons, most composite rotors typically include
a considerable mass of metallic rotor parts and the ratio between weight of a practical composite-metal rotor and an all metallic rotor is reduced from around 1:8 in Table 15.1 to between ¼ to ½ of this depending on the design. For applications which are less cost sensitive, composites certainly hold the advantage but if cost is the major issue, steel can be highly attractive as long as the energy storage requirement is not too high. It should also be noted that the energy stored depends on peripheral velocity and therefore a higher speed flywheel of lower diameter does not store any more energy than a larger diameter lower speed one as long as the peripheral speed, rotor shape and mass are the same. A higher speed flywheel will have a longer but lower diameter rotor like a drum with a low speed flywheel being more like a disc. The choice of speed will therefore affect the package shape not the flywheel rotor mass neglecting detail design effects. What is affected is the transmission, particularly if mechanical where a higher speed flywheel will require a greater speed reduction to get down to the order of a few thousand rpm range of gearbox input speeds or lower further down the drive train. Table 15.1 Comparison of different materials and two basic flywheel shapes 3
Material and type
Density (kg/m )
Design stress (MPa)
Peripheral velocity (m/s)
Rotor mass (kg)
Aluminium cylinder T7075
2800
350
354
7.8
Titanium cylinder
4430
520
374
7.0
Steel cylinder
7800
720
304
10.6
Aluminium solid T7075
2800
350
550
6.5
Titanium solid
4430
520
582
5.8
Steel solid
7800
720
473
8.7
Glass Epoxy cylinder
2150
1100
715
1.9
Carbon-epoxy cylinder
1670
1500
938
1.1
15.3.2 Aerodynamic loss and vacuum system Given the high peripheral speeds of flywheels, the aerodynamic friction loss would be excessive unless the pressure inside the casing is reduced. Losses occur due to the effects of skin friction but also pumping by the discs as air near the rotor is pumped radially outwards. Not only would the loss of energy be excessive but the rotor might overheat if the rotor ran in under atmospheric conditions. Near complete removal of the air may be
seen as a simple and attractive option by creating a very high vacuum but even in a hermetically sealed design with no shaft seal, preventing leakage can still present a major challenge and many of the materials particularly composite matrix materials, generate gas due to outgassing. Another issue is the lubricant for the bearings which will vaporise if the pressure is too low unless the bearings are kept outside the vacuum environment. If a shaft seal is required for mechanical shaft output then sealing a very high vacuum is a challenge. One solution to the leakage issue is to have vacuum pump on board the vehicle to run continuously or intermittently albeit with additional cost and weight. A better starting point is to evaluate what level of windage loss can be tolerated without excessive loss of energy relative to the energy savings or other benefits the flywheel can provide. If the pressure is not too low then the losses can be calculated based on equations for continuum flow. There are numerous publications on this subject and the most comprehensive is Dailey and Nece (1960) which covers windage in enclosed chambers. If a very high vacuum is required then continuum flow gives way to molecular flow and a different set of equations are needed (Genta, 1985). The transition from continuum to molecular flow is gradual so this regime is denoted Knudsen flow. Windage losses are proportional to approximately the 5th power of diameter and the cube of the speed. However, these losses are proportional to pressure to the power of 3/4. A higher peripheral speed composite flywheel will hence need to operate at a higher vacuum than a metallic one for the same energy storage.
15.3.3 Low-loss bearings The choice of bearings for automotive flywheels is relatively limited to the rolling element type given the need for low loss, low cost and high capacity. Although the weight of the rotor and out of balance loads are relatively small, the bearings must support loads due to acceleration shocks and those imposed due to gyroscopic effects. A magnetic bearing might be highly suitable for a stationary application but would have to be substantially oversized to cope with these additional loads. One solution is to put the flywheel in a gimbal such that the flywheel and casing is free to precess without imparting gyroscopic moments on the rotor and bearings. As well as the added complexity and space with this solution there is an added problem of containing the flywheel and casing in the event of rotor failure when momentum is imparted into the casing. Rolling element bearings offer low loss moments and when sized to
give adequate fatigue life for normal running, can accept shock or short term loads many times the steady design load. Angular contact bearings are typically chosen and, depending on the cost considerations, can be all steel or ceramic balls of different levels of precision. Correlations for losses are available from most manufacturers and contain a fixed loss moment plus a loss moment element that is dependent upon speed. It can be noted that rolling element bearing technology has moved on considerably over the last few decades in terms of increased life and lower loss moments. The choice of lubricant depends on the vacuum pressure required with more limitations occurring when the vacuum is high. For such situations, silicon-based oils are needed albeit with inferior lubricating properties.
15.3.4 Safety in the event of rotor failure Safety is of paramount importance and the approach to this issue and solutions are explained in Hansen and O’Kain (2011). The choice can briefly be stated as: design with such a high safety factor that failure will not occur ensure failure is incremental and contains fewer fragments than the entire rotor use instrumentation to detect and prevent failure incorporate containment tailored to the flywheel design. It is the authors’ opinion that for an automotive flywheel, the only option is one which guarantees containment under all foreseeable circumstances without reliance on instrumentation or other controls. Since the need for low cost means that materials cannot be controlled or inspected nor can the life of condition of the flywheel including instrumentation be controlled. Not only must the flywheel be contained but the rate of angular deceleration of the fragments not be too great lest a very high torque is transmitted which could cause the casing itself to break free or excessive torque be applied to the vehicle. Metallic flywheels tend to break into three large pieces if a crack grows to a critical size causing burst. Here, the entire energy of the flywheel is released in three large chunks that require a heavy casing to contain it. A solid flywheel without a central hole has the added problem of a triaxial stress at the centre, which promotes fracture over yielding. A solution to this is to use a laminated sheet type structure, laminated axially such that only a small fraction of the rotor can burst at any one instant; the probability of two or more failing at the same time being negligible. A flywheel of this type is shown in Fig. 15.2.
Composite flywheels have been promoted as safer on the basis that the material tends to turn into dust upon failure hence the containment vessel experiences a pressure of rotating mass more like a liquid rather than chunks. Other types of failure mode have been seen and what occurs is complex and requires extensive research in order to understand it. One of the challenges is how to create a realistic burst condition since composite flywheels tend to have such high safety factors and bursting with massive overspeed would be unrepresentative of a likely failure condition in service. The methodology used for vehicle crash assessment can be adopted for flywheel burst containment evaluation based on dynamic finite element analysis simulation validated against results of highly instrumented experiments. There is a need for standards to be developed along these lines to create a clear framework for the design of automotive energy storage flywheels as the technology becomes established.
15.3.5 Potential effects of gyroscopic moment on vehicle dynamics The issue of gyroscopic torque from flywheels on a vehicle is treated thoroughly in the paper by McDonald (1980). A gyroscopic torque will
15.2 A laminated steel flywheel designed at City University.
result if the axis of the flywheel is rotated and it acts perpendicular to the rotor axis. The magnitude of the torque is the product of the flywheel rotor moment of inertia, the flywheel angular velocity and the angular velocity of the flywheel axis. The vehicle will experience rotating in the yaw axis from cornering, roll axis also when cornering (at entry and exit) and pitch when riding through a road dip or crest of a hill. Yaw angular velocity is likely to be the greatest in the case of vehicle spin and effects on the flywheel can be eliminated by making the axis vertical. Sometimes this is less convenient particularly for mechanical transmission where an in line or transverse axis is preferred. If the axis is transverse, the gyroscopic torque can be used to counter roll moment and hence enhance road holding by virtue of transverse reducing weight transfer. However, the main conclusion from McDonald (1980) was that the gyroscopic torques generated were generally insignificant in comparison to other moments on the vehicle. It is an issue that should be considered but is not of great concern. Of more concern is the ability of the bearings to take the resulting loads to support the gyroscopic torque as mentioned previously.
15.4 Transmissions for flywheels 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. The different types of transmissions considered for a flywheel energy storage system (FESS) are described in the next section
15.4.1 Hydrostatic transmissions The hydrostatic transmission transmits energy using hydraulic fluid. The usual transmission has two hydraulic devices; one working as a variable displacement pump and the other working as a motor connected by hydraulic lines. The pump converts mechanical energy into pressure and the motor reconverts the pressure energy to mechanical energy. By varying the displacement of the pump a continuous ratio from zero to the maximum value can be obtained thereby forming an infinitely variable transmission (IVT). As a result the hydrostatic transmission does not require any starting clutch. The torque direction is reversed by the pump acting as the motor and the motor acting as the pump. The hydrostatic transmissions are simple to construct and offer flexibility; however, they are bulky and noisy, so they tend to be used more in heavy vehicles than in passenger cars. Figure 15.3 shows a typical hydrostatic transmission (Beachley and Frank, 1979).
15.4.2 T raction continuously variable transmission Traction CVT 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. Belt drives The transmission takes place over belts which are placed between two pulleys. By varying the diameter of the pulleys the ratio change is achieved continuously.
15.3 Hydrostatic transmission (Beachley and Frank, 1979).
The belts are usually made of rubber or steel. Rubber v-belt CVTs have been used for many years. Van Doorne of Holland developed a v-belt made up of steel blocks linked by steel bands which was a significant improvement over the rubber belt drives (Walton, 1959). It is a compression belt, where the driver pushes the driven pulley. Figure 15.4 shows the schematic of this type of CVT. A similar type of belt was developed by Battelle Columbus Laboratories (Swain, 1980). Another popular CVT developed by Kumm Industries is the flat rubber belt design shown in Fig. 15.5. In this case the belt is positioned radially by drive elements located by guideways in the side of the two pulleys and the ratio change is achieved by changing the radial position of the belt along the guideways (Kumm, 1980).
Rolling traction drives There are a number of designs of this type where the transmission takes place between two elements, which are loaded against each other, and are separated by a lubricant film. The speed ratio is varied by changing the point of action of forces or the effective pitch radius. A common design is the toroidal design in which the power is transmitted from the input discs to the output discs via rollers. The ratio change is achieved by varying the inclination of the rollers. This type of transmission has been developed in the past by Perbury of England (Stubbs, 1980) and is currently developed
by Torotrak (Brockbank and Greenwood, 2009). Figure 15.6 shows the Torotrak design. Another rolling traction drive is the Milner CVT (Akehurst, 2001) which uses three or more spherical planetary rollers to transfer power
15.4 V-belt CVT (Beachley and Frank, 1979).
15.5 Kumm belt design (Kumm, 1980).
15.6 Torotrak design (Brockbank and Greenwood, 2009).
from an inner race set to a carrier assembly. The ratio change is achieved by changing the axial displacement between the outer race halves. Figure 15.7 shows the Milner CVT.
15.4.3 Electrical transmission The electrical transmission 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
15.7 Milner CVT (Akehurst, 2001).
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 (EMB) (Post, 1996) or flywheel battery (FWB) (Hayes et al., 1999).
15.4.4 Mechanical transmission The simplest way of achieving a pure mechanical continuously variable transmission is to use a slipping clutch in series with a stepped gear box. The gearbox has discrete ratios and at each step the clutch can be slipped to achieve the desired ratio. If a gearbox with a large number of ratios is used the power lost due to the slipping of the clutch can be reduced. Having a large number of gears would be expensive but Read developed a methodology of using a gearbox with a few steps plus a smaller gearbox to create the effect of a large number of gears (Read, 2010). Beachley suggested such a design as one of the ways to connect the flywheel to the driveline for their flywheel hybrid (Beachley et al., 1981). More recently a digital hydraulic system has been developed by Artemis Intelligent Power Ltd (Artemis, 2012) and has been applied to flywheel transmissions. The other way of achieving a CVT through purely mechanical means would be by using the planetary gear set (PGS) as a two degrees of freedom device. In the conventional applications it is common to hold one of the elements of the planetary gear set stationary while the other two act as an input or output. However, in case of being used as a CVT all three branches of the PGS would be free to rotate. Since the PGS is a speed coupling device, by determining the speeds of any two branches the third one can be controlled. Diego-Ayala used a simple PGS with a brake to design a flywheel based powertrain (Diego-Ayala et al., 2008).
15.4.5 Power split principle For the power split (PS) principle the previously mentioned CVTs will be called variators or the continuously variable unit (CVU) and the complete transmission will be referred to as CVT. Usually the variators suffer from poor efficiency and the power split principle is used to overcome this effect. It is used effectively in the Toyota Prius. A general PS-CVT is shown in Fig. 15.8. As the name suggests, the input power is split between the variator and the highly efficient direct mechanical path which is added up at the output. The PS-CVT can be input coupled which means that the torque is split at the input or it can be output coupled where the speed is divided at the input. The PS-CVT usually has a higher efficiency than a variator only CVT; however, its ratio range is limited due to the fact that the power carried by the variator is less than the input power. However, in case the power carried by any of the branches of the PS-CVT is higher than the input power, it is called power recirculation. Power recirculation can be positive or negative
depending on whether the power going through the variator is in the direction of input power or opposite to it. The case of power recirculation provides an interesting trade-off, whereby the ratio range can be increased at the expense of efficiency. White explains the power split principle in greater detail (White, 1967). Figure 15.9 shows the different schemes of the PS-CVT principle. A drawback of power recirculation is that the bearings carrying the branches are overloaded and have to be designed to take the high power. However, another way of achieving a higher ratio range while avoiding power recirculation is by using multi regime CVT. Such kinds of CVTs have been discussed in literature (Mantriota, 2001). The basic idea is to use the variator multiple times by the use of clutches and gears.
15.8 General PS-CVT (Beachley and Frank, 1979).
15.9 Different PS-CVT configurations (Martinez-Gonzalez, 2010). GV and GM are fixed gears and V is a variator.
The PS-CVT can be constructed using different types of variators explained in previous subsections. One of the common designs used in the 1970s and early 1980s was the hydromechanical design. A number of flywheel based powertrains conceived during that period use this type of design (Frank and Beachley, 1975). Another design that was developed to be used in a flywheel vehicle and patented in 1980 was the electromechanical CVT using a planetary gearset and two electric machines (Rowlett, 1980). As well as these, a number of designs have been developed using the traction rolling and belt variators in combination with planetary gearsets.
15.5 Performance evaluation of flywheel hybrid vehicles The 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.
15.5.1 Pure flywheel-based powertrain The FESS can be the sole energy source in the powertrain such as in the Oerlikon Gyrobus (Anon., 1955). 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 suitable for intra city bus or trolley coach application. However, beyond that, this type does not find much use. Figure 15.10 shows the Gyrobus.
15.5.2 F lywheel and internal combustion engine-based powertrain This type is particularly useful since the conventional ICE vehicle does not have any means of capturing brake energy and adding a flywheel gives that option. The first application of this type was developed in England (Clerk, 1964) for bus and car application using a complex mechanical transmission consisting of integrating and differentiating planetary gear sets. The University of Wisconsin (Frank and Beachley, 1975) developed a similar powertrain and more were developed by University of Eindhoven (Kok, 1999) and Imperial College London (Diego-Ayala et al., 2008). U sually non electrical transmissions would be used in such a powertrain.
There are various configurations and operating modes possible. The flywheel can be connected in a parallel configuration or in a series configuration. At least one study explores the use of a flywheel with two CVTs in a series arrangement (Loscutoff, 1976). However, the common principle is the parallel configuration. One such system from Eindhoven University is illustrated in Fig. 15.11. In this type of configuration the flywheel is used in combination with the ICE up to a vehicle speed of 100 kph, and beyond that the ICE only is used. In the low speed hybrid mode (from 10 kph to 55 kph) the ICE is run at its most efficient operating point and is used to charge the flywheel up to its operational limit. As soon as the desired flywheel speed is reached, the ICE is shut down and the flywheel is used to drive the vehicle, thus the ICE operates in an on-off mode. From 55 kph to 100 kph the ICE is always running and the flywheel assists the vehicle. Beyond that the flywheel is disconnected and ICE powers the vehicle. According to the results shown by Kok (1999), the 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 optimised to improve fuel economy. The improved prototype of the flywheel hybrid driveline shows fuel savings up to 35 % in urban traffic. Another configuration is the one developed at Imperial College London. Fig. 15.12 shows the schematic. Here the flywheel with a mechanical transmission is an add-on to the conventional ICE powertrain. The primary purpose of the flywheel is to capture the regenerative braking energy which otherwise would be lost. The flywheel is charged during braking and the energy is released
15.10 Gyrobus (Lawson, 1978).
15.11 Eindhoven concept (Kok, 1999).
in the subsequent acceleration. The ICE is shutdown when the flywheel is operating. According to Diego-Ayala et al. (2008), the improvement in fuel economy using such a system is more to do with the engine operating less frequently rather than the engine operating at its most efficient range. It has been found that improvements in fuel economy and reductions in emissions up to 22% and 33% for a car and bus, respectively were obtained as compared to conventional vehicles. This approach differs to the earlier mentioned one. Here the flywheel and the CVT are not intertwined with the ICE operation and operate independently, whereas in the earlier one the flywheel and the ICE operations are coupled together. Since 2009 F1 has been employing limited KERS in races. Most KERSs are electrical in nature but the Flybrid-Torotrak system employs a mechanical flywheel based KERS. This system is shown in Fig. 15.13. Projects are underway in various organisations to make this system implementable in road vehicles (Birch, 2011).
One such application is the project headed by Ricardo plc in the UK called ‘Flybus’. The application involves a hermetically sealed flywheel coupled to a Torotrak CVT. The use of a hermetically sealed flywheel provides an opportunity to avoid the vacuum pump which is usually required to reduce windage losses, though it is difficult to predict if periodic service would be required to maintain the vacuum lost due to leakage. Another innovation in this work is that the flywheel is coupled to the transmission via a unique magnetic coupling thereby avoiding a moving seal. The CVT can transfer over 60 kW in a package that is less than 10 kg in weight. The vehicle also features an electrical power take off for recharging the batteries (Fuller et al., 2011).
Flybrid Automotive Ltd in the UK has developed a small high speed flywheel for various applications including F1 and road cars. The flywheel is very light and spins at maximum speed of 60,000 rpm. It is made up of steel, aluminium and a small quantity of carbon fibre (Cross and Hilton, 2008). The flywheel has been used in various projects to develop road applications. Volvo is developing one such application to use the Flybrid flywheel with a Torotrak CVT. The flywheel is also being used by Jaguar in their development of hybrid vehicles (Birch, 2011). Williams Hybrid Power (www.williamshybridpower.com) has developed a fully integrated electromechanical flywheel. The flywheel rotor is made up entirely of composite material and has over speed protection built into
15.12 Imperial College concept (a) brake-only hybrid version and (b) CVT-brake hybrid version (Diego-Ayala et al., 2008).
it. It has been used in F1, in the Porsche 911 GT3 R hybrid race car where it drives the rear axle and in the Audi R18 e-tron Quattro race car. It is also being developed for commercial passenger bus application in the UK (www. willamshybridpower.com).
15.5.3 F lywheel 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 element. 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 FWB with very few known exceptions. The FWB combines a motor generator (MG) with a flywheel in three topologies (Hayes et al., 1999). 1. Fully integrated: in this version the flywheel and the MG operate as one unit and are fully contained in a single containment to give a highly compact system. There are no mechanical connections coming out of the FWB and it has only electrical connections as the output. However, since the machine is contained in a vacuum, heating is a problem. 2. Partially integrated: in this version the rotor of the MG is contained in the containment but the stator is outside. This partially offsets the heating problem from the stator but rotor cooling has the same limitations as the fully integrated one.
15.13 Torotrak design (Brockbank and Greenwood, 2009).
3. N on-integrated: in this design the complete MG is outside the containment and it requires rotating seals like in a mechanical connection. It is larger in size but the machine cooling is easier to engineer. FWBs have been developed by a number of organisations including Eindhoven University (Thoolen, 1993), University of Texas at Austin (Hayes et al., 1999) and Lawrence Livermore Laboratories (Post, 1996). Magnetic bearings are commonly used in FWBs. Figure 15.14 shows a typical FWB. Research has been carried out for a number of years on using FWBs in EVs as a load levelling device. In recent research Lundin (2011) used a FWB with double stator windings placed between the battery and the traction motor of an EV. The high voltage side is connected to the traction motor and the low voltage to the batteries. This allows the FWB to charge or discharge both the traction motor and the batteries at two different power and voltage levels. Results show a significant decrease in partial charge/discharge cycles, maximum current and battery resistive losses with the flywheel as compared to one without the flywheel. The flywheel can be connected mechanically to the EV driveline, which gives the advantage of low cost, simplicity and avoids energy conversion. Some limited government funded research was carried out in the 1970s, notably in the US after the 1973 oil crises sparked an interest in EVs. However the
EVs at that time were greatly inferior to the conventional vehicles and even with the flywheel were not deemed competitive to ICE conventional vehicles (Agarwal, 1982). However with the current technical situation in flywheels
15.14 Flywheel battery (Hayes et al., 1999).
and with the availability of production EVs today, there is research being conducted at City University London in this field. Figure 15.15 shows the EV with flywheel using electromechanical transmission built by Garrett.
15.5.4 F lywheel in a hybrid electric powertrain Another common configuration is a hybrid electric series configuration which is shown in Fig. 15.16. Here the ICE is the prime mover connected to a generator and the FWB is there in place of the usual electrochemical batteries. Such an arrangement has been shown in the applications of the FWB developed by University Eindhoven, UT- Austin and University of Alberta (Wang et al., 2009). Similar to the battery based series HEV powertrain, this arrangement has been primarily implemented in public transportation such as transit buses. However, there are a few exceptions. The Chrysler Patriot sports car using a gas turbine engine alternator and a FWB developed in the
early 1990s is such a system (Shortlidge, 1996). An example of the parallel hybrid powertrain was developed at ETH Zurich and called the ETH-Hybrid III vehicle (Dietrich et al., 1999). Figure 15.17 shows the schematic of such a powertrain. It consists of an ICE, an electric asynchronous machine, batteries, a flywheel and a wide range CVT. Due
15.15 Garrett electromechanical transmission (Lawson, 1978).
to the presence of various elements, a large number of operating modes are possible and in essence it combines the battery load levelling function and the strategy similar to the Eindhoven University concept described before. However, it is fairly complex in construction and control.
15.5.5 Energy management Since there are various configurations for a possible flywheel hybrid vehicle no one control strategy fits all. One of the common strategies is to keep
15.16 Series configuration (Hayes et al., 1999).
15.17 ETH-III powertrain (Dietrich et al., 1999).
the total kinetic energy of the flywheel and the vehicle constant. This has been applied in various cases. It is a simple strategy though not optimal and suffers from the so called history dependence problem. As an example, after
a light deceleration where the flywheel has not recuperated enough energy, the control strategy would demand additional energy from the prime mover to make up for the loss ignoring the other demands of the vehicle. Gilbert suggested that an anticipatory control strategy would avoid this problem (Gilbert et al., 1972). Martinez-Gonzalez (2010) showed with an example that a ‘looking ahead’ strategy using predictive journey estimation has the potential to improve fuel economy in a flywheel hybrid vehicle.
15.6 Technical challenges in flywheel development The greatest technical challenge to the implementation of flywheels in road vehicles seems to be certainty of containment in the event of a failure. As stated earlier there is no fundamental issue but rather the need to mitigate against unexpected scenarios. Exactly the same type of issue applies to more or less all electric vehicles which can catch fire as voltages and battery energy storage systems are increased. It is a case of managing change at a time when vehicle health and safety is treated very differently to the period of the advent of the motor vehicle. Engineers do have at their disposal sophisticated tools for simulation and prediction and, in comparison to a battery, a flywheel is a relatively simple device even with composite materials. 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 (Strubhar et al., 2003). 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 (Hansen and O’Kain, 2011).
15.7 Conclusions and future trends 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. Interest and development of flywheels for automotive use has increased dramatically with a number of companies offering products in limited production. Investment has been substantial and has been kick started by the funding available for motor sport applications such as KERS in F1 and Le Mans. Now that the benefit of flywheels has been demonstrated, it is clear that this technology is becoming established and implementation in mass produced vehicles in only a matter of time. Key to this step is the development of low cost solutions in the order of a few $100s manufacturing cost and this will likely mean flywheels with less composite content, all steel or steel with partial composite. 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.
15.8 References Agarwal, P.D. (1982) Energy utilization of electric and hybrid vehicles and their impact on US national energy consumption, International Journal of Vehicle Design, 3(4) pp. 436–449. Akehurst, S. (2001) Performance investigations of a novel rolling traction CVT, Transmission and Driveline Systems Symposium, Detroit, MI, USA. Anon. (1955) The Oerlikon Electrogyro: its development and application for omnibus service. Automobile Engineer, December. Artemis Intelligent Power Ltd, http://www.artemisip.com/ (accessed 26 January 2012). Beachley, N. and Frank, A. (1979) Continuously variable transmissions: theory and practice, Lawrence Livermore Laboratory Report, UCRL-15037. Beachley, N.H. et. al. (1981) Continuously-variable transmission designs for flywheel-hybrid automobiles, Proceedings International Symposium Gearing Power Transmissions, Tokyo, pp. 393–398.
Benham, P. P. and Warnock, F. V. (1976) Mechanics of Solids and Structures, Pitman International, London, UK. Birch, S. (2011) Volvo spins up flywheel technology research, Automotive Engineering International Online, June. Brockbank, C. and Greenwood, C. (2009) Fuel economy benefits of a flywheel and CVT based mechanical hybrid for city bus and commercial vehicle applications, SAE paper 2009-01-2868. Clerk, R. (1964) The utilization of flywheel energy, SAE paper 640047. Cross, D. and Hilton, J. (2008) High speed flywheel based hybrid systems for low carbon vehicles, Hybrid & Eco Friendly Vehicles Conference, Coventry, UK. Dailey, J. W. and Nece, R. E. (1960) Chamber dimension effects on induced flow and frictional resistance of enclosed rotating disks, Journal of Basic Engineering, March, 82(1), 217–230. Diego-Ayala, U., Martinez-Gonzalez, P., McGlashan, N. and Pullen, K. R. (2008) The mechanical hybrid vehicle: an investigation of a flywheel-based vehicular regenerative energy capture system. Proceedings of the Institution of Mechanical Engineers Part D Journal of Automobile Engineering, 222(D11), 2087–2101. Dietrich, P. et al. (1999) Results of the ETH – Hybrid III – vehicle project and outlook, SAE paper 1999-01-0920. Frank, A. and Beachley, N. (1975) Improved fuel economy in automobiles by use of a flywheel energy management system, Flywheel Technology Symposium Berkeley, CA. USA. Fuller, J. et. al. (2011) Hardware development of FLYBUS – Flywheel Based Mechanical Hybrid Systems for bus and commercial vehicles, 2nd VDI Conference Transmission in Commercial Vehicles Friedrichshafen, Germany. Genta, G. (1985) Kinetic Energy Storage: theory and practice of advanced flywheel systems, Butterworths. Gilbert, R. et al. (1972) Flywheel drive systems study, Final report, contract no. 68-040048, Lockheed Missiles and Space Company, Inc. Hansen, R.G. and O’Kain D.U. (2011) An assessment of flywheel high power energy storage technology for hybrid vehicles, Oak Ridge National Laboratory Report ORNL/ TM-2010/280, December. Hayes, R. et al. (1999) Design and testing of a flywheel battery for a transit bus, SAE Paper 1999-01-1159. Kok, D. (1999) Design optimisation of a flywheel hybrid vehicle, PhD Thesis, Technical University Eindhoven. Kumm, E.L. (1980) Design study of flat belt CVT for electric vehicles, report prepared for US Department of Energy, Electric and Hybrid Vehicle Division Office of Transportation Programs. Lawson, L.J. (1978) Applications of kinetic energy storage to transportation systems, High Speed Ground Transportation Journal, 12(3), 1–27. Loscutoff, W. (1976) Flywheel/heat engine power for an energy-economic personal vehicle, Prepared for the Energy Research and Development Administration under Contract E(45-1): 1830. Lundin, J. (2011) Flywheel in an all-electric propulsion system, Licentiate Thesis,
Uppsala University. Mantriota, G. (2001) Power split continuously variable transmission systems with high efficiency, Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 215, January 357–358. Martinez-Gonzalez, P. (2010) A study on the integration of a high-speed flywheel as an energy storage device in hybrid vehicles, PhD Thesis, Imperial College London. McDonald, A.T. (1980) Simplified gyrodynamics of road vehicles with high-energy flywheels, Flywheel Technology Symposium, US Department of Energy, pp. 240–258, October. Post, R.F. (1996) A new look at an old idea: the electromechanical battery, Science & Technology Review, April, pp. 12–19. Pullen, K. R. and Ellis, C. H. W. (2008) The Vehicle as Kinetic Energy System, ATZ Auto Technology, 8 October. Rabenhorst, D.W. (1969) Primary energy storage and the superflywheel APL/JHU Report TG-1081. Read, M. (2010) Flywheel energy storage systems for rail, PhD Thesis, Imperial College London. Rowlett, B.H. (1980) Flywheel drive system having a split electromechanical transmission, US Patent 4233858. Shortlidge, C. (1996) Control system for a 373 kW, intercooled, two-spool gas turbine engine powering a hybrid electric world sports car class vehicle, ASME Turbo Asia Conference, Jakarta, Indonesia. Strubhar, J. et al. (2003) Lightweight containment for high-energy rotating machines, IEEE Transactions on Magnetics, 39(1), pp. 378–383 January. Stubbs, P.W.R. (1980) The development of a perbury traction transmission for motor car applications. ASME paper no.80-C2/DET-59, August. Swain, J.C. (1980) Design study of steel v-belt CVT for electric vehicles, Report prepared for US Department of Energy, Electric and Hybrid Vehicle Division Office of Transportation Programs. Thoolen, F.J.M. (1993) Development of an advanced high speed flywheel energy storage system, PhD Thesis, Technical University Eindhoven. Wang, C. et al. (2009) Application of flywheel system in series hybrid transit bus, IEEE Vehicle Power and Propulsion Conference, Dearbon, MI, USA. Walton, H. (1959) Belt drive, Popular Science, December. White, G. (1967) Properties of differential transmissions, The Engineer, 224, 105–11.
