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Abstract
Fossil fuels are an important energy source in transportation. Since fossil fuels are
non-renewable resources which will exhaust sometime, the need for energy-efficient vehicles
with energy recovery systems has become a necessity to optimize the utilization of fossil
fuels in transportation. Currently, great interest is being devoted to research sustainable
transportation, combining highly efficient vehicles with various forms of energy storage and
recovery. This thesis investigates a mechanical flywheel energy recovery system for hybrid
vehicle applications, which has become the subject of extensive research as an alternative to
electrochemical batteries for on-board energy storage. The objectives of the research are to
design a prototype of a low-speed flywheel system and integrate the hardware of the control
systems to demonstrate the automotive brake energy recovery process. As a part of a large
project conducted at the Ohio State University, the prototype is used to further investigate
control algorithms to improve the efficiency of the flywheel-based energy recovery system.
Introduction
For many years, fossil fuels are consumed as a primary energy source in transportation and industry. The increasing demand of fuel to meet these needs inevitably causes a rising of oil price. This issue is increasing awareness about the energy crisis of the non-renewable fossil fuel resource. To moderate the potential crisis, the need for sustainable development has become a necessity, especially in the area of the transportation.
The significant amount of rejected energy, which refers to energy that is lost in conversion
processes, is observed in the transportation sector. The total energy consumed was around
27.5 Quad Btu of which ~20.6 Quad Btu was rejected energy [3]. This huge inefficiency in
energy conversion in the transportation sector has called for developing technical solutions
towards sustainable transportation, normally combining highly efficient vehicles with
alternative energy resources.
Regenerative Braking
A Regenerative Brake is a Mechanism that reduces vehicle speed by converting some of its kinetic energy into some other kind of useful form of energy – electric current, compressed air.The energy captured is then stored for future use or fed back into a power system for use by other vehicles. For example, electrical regenerative brakes in electric railway vehicles feed the generated electricity back into the supply system . In battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of twin layer capacitors for later use. Other forms of energy storage which may be used include compressed air and flywheels. Regenerative braking practices the principal of an electric motor acting as a generator . It reuses kinetic energy by using its electric motor to regenerate electricity. Regenerative braking does not dissipate the electric energy as heat and is thus more energy efficient thandynamic braking
KERS
A type of Regenerative braking is called KERS. KERS is an automotive system for recovering a moving vehicle's kinetic energy under braking. The recovered energy is stored in a reservoir (for example a flywheel or a battery or super capacitor) for later use under acceleration [4-6]. Electrical systems use a motor-generator incorporated in the car’s transmission which converts mechanical energy into electrical energy and vice versa. Once the energy has been harnessed, it is stored in a battery and released when required [7]. Types of storage devices There are different type of devices and forms in which the Kinetic Energy lost while braking can be stored. They are: 1) Mechanical KERS 2) Electric KERS 3) Hydraulic KERS 4) Hydro-electric KERS (HESS)
Mechanical storage system
The mechanical hybrid utilizes a rotating mass (or flywheel) as the energy storage device and a variable drive transmission to control and transfer the energy to and from the driveline [4]. The transfer of vehicle kinetic energy to flywheel kinetic energy can be seen as a momentum exchange [4]. Energy is drawn from the vehicle and supplied to the flywheel. In doing this, the speed of the vehicle reduces, (effectively this is braking), whilst the speed of the flywheel increases [5]. At the start of braking the vehicle has a high speed and the flywheel a low speed, giving a certain gear ratio between them [6]. At the end of braking the vehicle has a low speed, and the flywheel a high speed, so the ratio of speeds has changed [6]. Examination of the energy transfer shows that the ratio between vehicle speed and flywheel speed necessarily changes continuously during the energy transfer event [5] [6]. Flywheel based mechanical hybrid systems are not new – systems have previously been developed by the Technical University of Eindhoven and Leyland Trucks amongst others and indeed it is possible to ride on a Flywheel powered tram from Stourbridge junction in England – and a flywheel based system is not without areas requiring focus (most notably the failure modes of the rotating mass and the method of control and transmission of the energy to and from the flywheel.) [7][8]. It has been put into use for Formula 1 cars since the year 2009
Electrical storage system
The well documented electrical hybrid systems utilize chemical batteries as the storage medium and electric motor / generator systems as the energy transfer and control media [13]. KERS components for battery storage systems are: Electric Propulsion Motor /Generator, Power Electronics – Inverter, and the Quad Flywheel Storage [13] [14] [15]. Electric Propulsion Motor and Generator in one are also known as a MGU – Motor Generator Unit [15] [16]. Capacitors are fundamental electrical circuit elements that store electrical energy in the order of microfarads and assist in filtering [16] [17] [18] [19]. The main function of a capacitor is to charge or discharge electricity [19] [20] [21]. Super-capacitors have special features such as long life, rapid charging, low internal resistance, high power density, and simple charging method as compared to capacitors and batteries [22][23].
C. Hydraulic storage system
Regenerative braking in vehicles using a variable displacement hydraulic pump/motor together with a hydropneumatic accumulator has attracted considerable interest during the last 20–25 years. Such a system is particularly suitable for application in city buses [24] [25] [26] [27] [28] [29]. Despite the significant gains in the efficient use of energy that can be brought about by hydro-pneumatic regenerative braking, its use has not attained great popularity [28] [29] [30]. The added cost, which may represent 10–15% of the total for the vehicle, is undoubtedly a deterrent [31] [32] [33].
D. Hydro-electric storage system
Hydraulic accumulator has the characteristics of higher power density and is well suited for frequent acceleration and deceleration under city traffic conditions [32-34]. It can provide high power for accelerations and can recover more efficiently power during regenerative braking [33] in comparison with electric counterparts [35]. However, the relatively lower energy density brings the packaging limit for the increasing accumulator size [35]. For example, Uzunoglu et al. [39], Rodatz et al. [40], Thounthong et al. [41] developed hybrid energy system such as battery/UC, fuel cell/UC, etc. These studies focused on the hybrid energy systems in electric forms, and publications devoted to hydraulic/electric system are relatively scarce, Ricardo [38] proposed a combined regenerative-dissipative brake system for a city bus. The regenerative component consists of a fixed displacement hydraulic pump/ motor and a hydropneumatic accumulator. Bozic [42] introduces hydraulicelectric synergy into hybrid transmission using the freepiston engine technology. A hydraulic accumulator/battery hybrid energy system, called hydraulic/electric synergy system (HESS), is designed to overcome the drawbacks of existing single energy storage sources used in heavy hybrid vehicles, which adopts hybrid energy structure to combine high power hydraulic accumulator and high specific energy battery together [28]. Figure 1 shows operating modes transition of a hybrid with Hydro-Electric Synergy System.
Analysis of Kinetic Energy Recovery
To move a vehicle on the road with a variable speed profile, the variation in kinetic energy of the vehicle should overcome the energy losses due to the aerodynamic friction, the
rolling resistance and the energy dissipation in the brakes. Unfortunately, the energy losses to
the aerodynamic friction and the tire rolling friction are non-recoverable. However, one
possible method to improve fuel economy is to a part of the vehicle kinetic energy that would
otherwise be dissipated to the brakes during deceleration, store it and utilize it to accelerate
the vehicle later. This process of recovering the energy dissipated to the brakes is defined as
“regenerative braking”. Hybrid vehicles can recover braking energy to increase vehicle
efficiency.
There are several methods developed to adopt regenerative braking for vehicle use.
One method involves using an electric generator to convert the braking energy into electrical
energy and store it in a chemical battery. The stored energy is then released to accelerate the
vehicle when needed, by using the electric machine as a motor. Alternatively, energy can also
be stored mechanically using a rotating disc (flywheel) with appropriate inertia, as well as a
power transmission system. In order to store energy, the flywheel speed should be increasing
while the vehicle speed is decreasing. Vice versa, the flywheel must decelerate while the
vehicle speed is increasing to release the energy.
Though a regenerative braking system plays a role in the vehicle‟s braking, it cannot
completely replace friction brakes for the safety reasons. In cases of potential energy storage
system failure or desired deceleration beyond the capability of the regenerative system,
friction brakes must still be capable of providing sufficient stopping capability
Overview of Hybrid Electric Vehicles (HEVs)
HEVs provide an option of sustainable transportation to improve fuel efficiency and
moderate the fuel energy crisis. HEVs use electric machines to convert the braking energy
into electrical energy and store it in a chemical battery. The electric machine behaves as a
generator and charges the battery by drawing regenerative braking energy. When acceleration
is commanded by the HEV, the electric machine behaves as a motor and provides torque at
the expense of stored energy.
Currently, HEVs are the most common hybrid vehicles that car companies produce
in the market. The first widely available electric hybrid vehicle was the Toyota Prius released
in Japan in 1997. Initially hybrid vehicles were considered unnecessary because of the low
cost of fuels, but the continuously increasing oil price in the world forced more and more
automakers to pay attention to hybrid vehicles after 2000. Statistics showed that worldwide
sales of hybrid vehicles produced by Toyota, the market leader, have topped the 3-million
mark, with more than 3.03 million units sold worldwide as of 28 February 2011 since sales
began in 1997 [5]. Figure 3 below indicates the obviously increasing trend of the Toyota
hybrid sales.
Alternative Energy Storage Systems (AESS) for Hybrid Vehicles
Besides the rapid development of the HEVs, great interest is being devoted today to
study the devices to store energy in the other forms. The alternative energy storage systems
(AESS) may offer interesting opportunities for the feasibility of low-cost hybrid powertrains.
Figure 4 shows the specific power and specific energy for some short-term energy storage
systems. Flywheels usually have higher specific power than batteries. In addition, flywheel
systems have the advantage of relatively little performance degradation over time regardless
of the depth of discharge in terms of energy storage capacity
The principle of flywheel hybrid vehicles is to store energy in kinetic form. In some
applications, flywheel is sealed inside a vacuum container to reduce the friction losses due to
the presence of air. The amount of the kinetic energy stored in a flywheel is represented as
the formula:
? = 1 2
?2 (1.1)
where? is the stored kinetic energy, ? is the mass moment of inertia and ? is its angular
speed.
The concept of a flywheel energy storage system is relatively simple. A
Continuously Variable Transmission (CVT) plays an important role to transfer the energy
between flywheel and vehicle. When a flywheel hybrid vehicle is braking, kinetic energy of
the vehicle is stored in the flywheel by changing the CVT ratio to force higher flywheel
speeds in a way that matches the decreasing vehicle velocity. This way, the flywheel works
as a brake to reduce the vehicle speed while storing kinetic energy to increase its own
rotational speed. When the vehicle is commanded to accelerate, the stored energy will return
to the vehicle by changing back the CVT ratio to force a lower flywheel speed. Figure 5
below demonstrates a simple flow of energy between flywheel and vehicle.
FIGURE 5
where FW is flywheel, CVT is continuously variable transmission, T is transmission, E is
engine and V is vehicle drivetrain.
Flywheel can be fabricated with different materials based on the maximum rotational
speed and other design constraints. High speed flywheels for speeds above 30000rpm, are
usually composed of high strength carbon fiber. A large mass is not desired for high speed
flywheels because extra mass means more energy will be needed to accelerate the vehicle. On
the other hand, low speed flywheels with speed values below 20000rpm, are generally made
of steel or other metals for low cost.
Flywheel systems have been adopted for many areas. For the 2009 racing season, the
Federation Internationale de l‟Automobile (FIA) has authorized hybrid drivetrains for
Formula 1 racing with the clear objective of developing hybrid technology for the use in
motorsport. With a focus on safety, the FIA has specified a limit on both the power rating of
8
the flywheel system at 60kW and the quantity of energy transfer per lap at 400kJ, which
provides significant benefits when applied to the racing cars.