DESIGN AND FABRICATION OF HYBRID COMPOSITE FLYWHEEL ROTOR Jung D. - PDF document

18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS DESIGN AND FABRICATION OF HYBRID COMPOSITE FLYWHEEL ROTOR Jung D. Kwon 1 , Seong J. Kim 1 *, K. Hayat 1 , Sung K. Ha 1 , Sang C. Han 2 1 Department of Mechanical Engineering, Hanyang University 1271, Sa 3-dong, Sangnok-gu, Ansan, Kyeonggi-do, 426-791, Korea 2 Korea Electric Power Research Institute 103-16 Munji-dong Yusong-gu Daejon, 305-380 Korea * Corresponding author (sjongkim@gmail.com) Keywords : composite flywheel rotor, inter hybrid, intra hybrid, multi-rim, interference, press- fit, in-situ cure 1 General Introduction The performance of a flywheel rotor made of composite material is always deteriorated by tensile An advanced composite flywheel has recently been stress in the radial direction, which is generated by developed for various energy storage applications both the curing process and the centrifugal force including electric utilities, frequency regulations, during rotation. The press-fit of multiple inter-hybrid hybrid or electric vehicles and spacecraft [1-5]. The rims (“inter-hybrid” means each rim is made of a composite flywheel rotor has characteristics of single composite material, either CFRP or GFRP, distinctively high energy density, long life and but the material might differ from rim to rim) with lightweight. Recent efforts in development of the interferences has been known as an effective way of flywheel have been devoted to material hybridized reducing the radial tensile stress. However, it is and press-fitted multi-rim rotor to further increase relatively expensive and difficult in the viewpoint of the performance and insuring the safety of the manufacturing. On the other hand, a flywheel rotor flywheel [6]. The pre-stresses developed by the consisting of a single intra-hybrid rim (a rim that is interference between multi-rims reduce the net made of comingled composites with different stresses in the rotating rotor, and yield a higher carbon-glass ratio) is less costly and relatively easy rotating speed and higher energy storage capacity [7- to manufacture, but the radial tensile stress cannot be 9]. Filament winding with the fiber tension is a effectively reduced. typical process of manufacturing the composite rotor, followed by the stages of heat buildup, curing, and 2.2 Stress analysis of a rotor cooling. The curing process causes tremendous A stress analysis to calculate the deformations and amount of tensile stresses primarily due to the the strength ratios of the multi-rim rotor with anisotropic thermal expansion of individual plies, interferences as shown in Figure 3 can be found in which deteriorate the performance of the rotor since many previous publications [10-12]. The governing the inertial forces also generate the radial tensile equation for a rotor of rotational speed is written as stresses during the rotation [10].     An advanced composite flywheel rotor consisting of d      2  (1) r r r 0 intra and inter hybrid rims was designed to optimally dr r increase the energy capacity, and was manufactured using filament winding with in-situ curing. In this Where  r and   are radial and circumferential stress, investigation, a comprehensive study was conducted respectively,  denotes a density, and  is the with the intent to implement composites in high rotational angular velocity. Assuming the plane performance flywheel applications. stress state, the stress-strain relationship can be written as         Q Q   T  2 Design of the advanced flywheel rotor     11 12 (2)            Q Q T       2.1 Inter hybrid rotor and Intra hybrid rotor r 21 22 r r

Where  and Q are the strain and the stiffness matrix and inter-hybridization technology, we named it in cylindrical coordinates. In the axisymmetric case, Advanced Hybrid Rotor. To evaluate the stresses within such an Advanced Hybrid Rotor, three the strains are defined by the radial direction sources of stresses were considered. Fig. 2 is the u : displacement r schematic representation of those sources, which are      u r  the thermal residual stresses developed in each intra-   r (3) ε        u r     hybrid rim after curing process, the stresses r r developed after press-fitting of two intra-hybrid rims In case of Intra hybrid rotor, Between rims, the due to the existence of interference, and the stresses following compatibility conditions should be met: generated by the centrifugal effect in high-speed rotation. The superposition of stresses from the  ( j  1)   ( ) j aforementioned three sources is the stress state of r r ( j =1, 2, 3, … , N -1) (4) i o the Advanced Hybrid Rotor. ( j  1)  ( ) j u u r r i o 2.4 Design of hybrid rotor In case of Inter hybrid rotor, Between rims, the The design of the Advanced Hybrid Rotor was following compatibility conditions should be met: optimized with respect to internal stress distribution and manufacturing cost. The predetermined  specifications of the rotor are listed as following:  ( j 1)   ( ) j r r ( j =1, 2, 3, … , N -1) (5) i o usable energy 35kWh, maximum rotating speed  ( ) j  ( j 1)   ( ) j u u 15,000 rpm, inner diameter 539.5mm, length r r o i 1340mm, maximum hoop-directional strain of the Three strength ratios are considered: R r =  r (the inner surface during rotation 0.8%. Three different designs satisfying those given conditions and radial stress)/Y, R  =   (the circumferential stress)/X possessing the same value of I p /I t (the ratio between polar moment of inertia I p and transverse moment of     k for 0  inertia I t ), mass, and specific energy density (SED), k X  k   (6) R were compared. Those designs were the inter-hybrid  k    rotor which consists of 4 separately wound rims, the k for 0  k  X  intra-hybrid rotor which consists of 4 rims wound at k the same time, and the Advanced Hybrid Rotor. To avoid excessive radial compressive stress applied to the inner intra-hybrid rim during the press-fitting of 2.3 Advanced hybrid rotor the Advanced Hybrid Rotor, the interference between two rims was calculated such that the radial In order to reduce the stress along radial direction compressive stress was limited to below 6 MPa. The induced by the centrifugal effect during high-speed properties of the hybrid material used for each rim in rotation, the inter-hybrid rotor is the best option three different designs are listed in Table 2, which because the interferences between adjacent rims can the result of optimization is listed in Table 1. effectively decrease the radial stress. However, Comparing the radial stress within each rotor during fabricating all the rims separately is costly and time- rotation, it was discovered that the following consuming. In this study, a new method was devised inequality held: Inter < Advanced < Intra, while in by winding every two rims together to form an intra- terms of manufacturing cost, the reverse order held: hybrid rim, which means the original 4-rim rotor Inter > Advanced > Intra. Fig. 4 shows the radial and now consists of two intra-hybrid rims, and the hoop strength ratio along the radial direction of the interference was given between two intra-hybrid rotor corresponding to 0 rpm and 15,000 rpm. It can rims. Although the reduction of radial stress in the be seen from Fig. 4(a) that for the inter-hybrid rotor comprising 2 intra-hybrid rims is not as much design, the hoop strength ratio is lower than the as that in the rotor comprising 4 inter-hybrid rims, radial one in most region along the radial direction the manufacturing process was simplified when the rotor is at rest; when the rotor is rotating at considerably. Since the rotor incorporated both intra-