Reliability Assessment of an Energy Storing Flywheel - Report Example

Student Name: Tutor: Title: Reliability Assessment of an Energy Storing Flywheel Date: ©2016 Table of Contents Introduction 2 Average Failure Rate Data 3 Reliability Block Diagram (RBD) 4 Recommendations 5 Conclusion 5 Advantages of RBD [Ele12] 5 Disadvantages of RBD [Ele12] 6 References 7 Introduction A fly- wheel is a simple form of mechanical (kinetic) energy storage [Fed03]. Commonly referred to as a flywheel battery, it consists of a fly- wheel, a motor generator and control electronics for connection to a larger electric system. The energy is stored by causing a disk or rotor to spin on its axis. It works on the principle of sourcing electrical energy, storing it in the form of rotational kinetic energy, and delivering it to a load (output) at the time and in the form required [Heb02]. Flywheels are typically sized to provide about 15 minutes of full load power. In today’s modern buildings however, a huge portion of power disturbances last only for 5 seconds. Uninterrupted Power Supply Systems (UPS) are integrated with fuel- fired generators and build up to full power in about 10 seconds. The Direct Current (DC) flywheel system could be used as an alternative as it provides 15 seconds of full load power. Flywheels are generally classified as low speed (rpm’s in the 000’s) or high speed (rpm’s in the 0,000’s). An increase in speed significantly increases the amount of energy stored (a double increase in speed quadruples the energy stored) as evidenced by the equation below. The stored energy in a flywheel can be illustrated in the equation below [Rib01]; where E is the energy stored in the flywheel (joules) w is the rotational velocity of the flywheel (m/s) I is the moment of inertia r is the radius of the sinning disk (m) m is mass of the spinning disk (g) h is the height/ length of the rotor (m) Energy is transferred to the flywheel when it accelerates (operating as a motor) and discharges when it decelerates (the electric machine regenerates through the drive). Some models use hollow cylinders for the rotors to increase the moment of inertia at the outer radius of the flywheel, increasing the amount of energy stored. Flywheel systems, however, are subject to various failure as a result of their composition and operation. These failure modes include; High speed systems – These are subject to heavy friction losses, necessitating different design approach and the use of specialized materials to mitigate this [Fed03]. Mechanical failure – Due to the high rotation, there is need to provide a containment vessel as a safety precaution in case the flywheel fails mechanically [Rib01]. Tensile strength of rotor – The speed of rotation of the disk depends on its strength, with stronger disks allowing for greater speeds Average Failure Rate Data The average failure rate (FPMH1) for a mechanical component is given by[Rel14] ; Where T is the interval of maintenance for the renewal of the item. R(t) is the Weibull reliability function η is the characteristic life β is the Weibull shape parameter The typical average shape factor values and characteristic life of the various components is as below [Bar10]. A maintenance interval of 4 years is assumed, as it is the recommended time at which the flywheel bearing assembly should be replaced, thus [Wag16]. Table 1: Average Failure Rate Data for Flywheel System Component Shape factor Characteristic life (hours) Weibull reliability function Average failure rate (FPMH) Ball bearing 1.3 40,000 22.71 Motor (DC) 1.2 50,000 18.07 Vacuum seal 1.4 60,000 12.92 Bolted flanges 3 300,000 0.05 Flywheel/ Shaft 2 50,000 12.91 Reliability Block Diagram (RBD) The process of performing analyses on the availability and reliability of large and complex systems by showing network relationships using block diagrams can be achieved by use of Reliability Block Diagrams. The logical interaction of failures within a system whose components are required to sustain operation is defined by the RBD structure [ITE07]. Figure 1: Typical flow of Energy in a Flywheel System[Heb02] The power fed into the system is converted by the components of the input electronics into electrical energy at signals compatible with the motor component, which spins at variable speeds. The signal from the input electronics determine the speed of rotation of the motor, dictating the amount of power to be stored. The motor spins the flywheel, storing energy in mechanical form (energy stored is directly proportional to rotational speed). The flywheel energy is dissipated as it slows down, and is converted to electrical energy by the generator. The output electronics then configure the energy into the appropriate signal for the output appliance. Figure 2: Reliability Block Diagram for the Flywheel System The flywheel system is represented by blocks in series as a failure in any of the components results in a failure of the whole system. Furthermore, there is no alternative path available the process could follow to ensure the system remains operational, that is, there is no redundancy. With respect to the average failure rates calculated above, the ball bearings (top and angular contact) are most likely to cause system failure, caused by friction above permissible limits. The bolted flanges are the least likely to fail. Recommendations The main challenge facing this system is the lack of redundancy, where the failure of one component means a shutdown of system operations. To improve reliability, the system may be altered by; Installing another flywheel, with accompanying motor and generator units to be served by the same input and output electronics. This will provide an alternative path the system may follow to maintain operation should one flywheel, motor or generator fails. However, the system is still prone to failure should either the input or output electronic components fail. Figure 3: Alternative RBD for Flywheel System Conclusion The RBD reliability system employed shows the flywheel system to have a poor configuration, such that failure to any of the components results in failure of the system as a whole. The RBD analysis shows that there is need to alter the configuration of the components, introducing redundant paths, ensuring smooth running of the system despite failure. Advantages of RBD [Ele12] 1. RBD offers more flexibility when modelling the system, thus statistical analysis is simple and easy. 2. The precision of the data obtained in RBD is more than that of FMEA2. 3. Through techniques such as ‘missing plot technique’, RBD allows for analysis to be carried out even when some values regarding the components are missing 4. More information is collected when using RBD Disadvantages of RBD [Ele12] 1. Challenges arise when modelling a large number of treatments which increases the block sizes. 2. Maintaining homogeneity becomes difficult when modelling large block sizes References Fed03: , (Federal Energy Management Program, 2003), Heb02: , (Hebner, et al., 2002), Rib01: , (Ribeiro, et al., 2001), Rel14: , (Reliability Analytics Corporation, 2014), Bar10: , (Barringer & Associates, Inc., 2010), Wag16: , (Wagner Power Systems, 2016), ITE07: , (ITEM Software, Inc., 2007), Heb02: , (Hebner, et al., 2002), Ele12: , (Elementary Statistics and Computer Application, 2012), Read More