Optimal design of press-fitted filament wound composite flywheel rotors

doi:10.1016/j.compstruct.2004.10.012

Composite Structures

Volume 72, Issue 1, January 2006, Pages 47-57

https://doi.org/10.1016/j.compstruct.2004.10.012 Get rights and content

Abstract

This paper explores the design of press-fitted, cylindrical, filament wound composite flywheel rotor rims using two-dimensional plane stress anisotropic elasticity solutions and an optimization method based on simulated annealing. A method of accounting for residual stresses in press-fitted composite rims is proposed and demonstrated. The starting point for the designs is an open core rotator motor/generator with a pre-determined, fixed pair of concentric rotor rings, upon which several carbon/epoxy rings with different structural properties are press-fitted. For certain assumed sequences of carbon fiber types versus radius, optimal designs are determined with the objective of maximizing specific energy of the rotor subject to constraints defined by expected operating speeds and temperatures as well as material strengths. The optimal solutions suggest that specific energies of 40–50 W · h/kg and tip speeds of 800–900 m/s are feasible with 5–8 press-fitted carbon/epoxy rings.

Introduction

Composite flywheels present an attractive alternative to chemical batteries for energy storage due to their comparable specific energy (energy per unit mass) and superior specific power (power per unit mass), charge/discharge behavior, efficiency, temperature range, and cycle life [1], [2], [3], [4]. The basic components that may be found in a flywheel energy storage system are: (i) the motor/generator, which stores and extracts energy by changing the angular speed of the rotating mass; (ii) the rotor, which includes all rotating parts of the system such as, for example, portions of the bearings and motor/generator, the shaft, a hub, and a rim—i.e., extra material added at the outermost radius of the machine primarily to increase the polar moment of inertia; (iii) the stator, which contains stationary parts of the motor/generator and bearings; (iv) the housing, which enables operation of the rotor in a vacuum to minimize losses and captures the rotor or rotor fragments in the event of a malfunction; and (v) the electronic control system. In the context of flywheel energy storage, a high performance rotor can be defined as one with a specific energy greater than ∼25 W · h per unit mass of the rotor [2]. High strength fiber reinforced polymer composites are strong candidates for portions of high performance flywheel rotors because of their high strength per unit mass density and their benign failure mode in comparison to metallic flywheel rotors [2]. Flywheel energy storage systems are being considered for space applications such as satellites and space stations and terrestrial applications such as uninterruptable power supplies. In space applications, a high premium is placed on cycle life, as the cost of replacing an energy storage unit is prohibitive. This paper concerns the design of a high performance composite flywheel rotor for energy storage in a small satellite.

Section snippets

Previous research on the design of composite flywheel rotors

Literally hundreds of publications describing a tremendous variety of composite flywheel rotor designs have appeared in the literature since the advent of new, affordable, high performance fibers such as carbon and aramid in the early 1970s. The most recent key references on composite rotor design are the book on flywheel energy storage systems by Genta [2] and the book chapter on composite rotor designs by Portnov [5]—both appearing in the 1980s. It could be argued that the subsequent

Objectives

The objectives of the present investigation are as follows:

•
Optimize a cylindrical, multi-ring, press-fitted, hoop-wound rim of an OCR flywheel rotor to maximize the specific energy of the rotor for a predetermined, fixed motor/generator design;
•
Assess the reliability of the “simulated annealing” method for determining an optimum set of dimensions of a particular set of rings in such a rotor rim.
•
Demonstrate the significance of properly accounting for process-induced thermal stresses in

Analysis

The OCR motor/generator contains two rings that were designed previously for optimal electromagnetic performance [14] and is therefore considered fixed in the present optimization problem (Fig. 1). The innermost ring consists of 24 arc-shaped samarium cobalt (SmCo) magnets with inside and outside radii of 7.396 and 8.218 mm, respectively, while the outermost ring is laminated amorphous iron with inside and outside radii of 8.218 and 9.273 mm, respectively. The motor/generator is designed to

Optimization problems

Four baseline operating conditions for the entire rotor were selected to encompass the anticipated extremes of speed and temperature (Table 5). The listed changes in operating temperature in Table 5, ΔT, are relative to 20 °C, whereas changes in temperature used in the stress calculations are relative to the presumed stress free temperature, T_sf, of each ring.

The three design problems investigated are as follows:

1. For the four operating conditions of Table 5, find the dimensions of a fixed

Problem 1—evaluation of simulated annealing

In this problem, the optimization routine was run eleven times so that the ability of the SA algorithm to repeatedly find an optimum solution could be evaluated. Five c/ep rings were used in the rim: T700, T1000a, T1000b, M46a, and M46b from inside to outside. Specific energies for each run as well as group statistics are shown in Table 7. Much, if not all, of the small variation among runs can be attributed to inaccuracy associated with the random nature of a search-based optimization method

Conclusions and recommendations

Based on the results generated in this investigation, some general conclusions and recommendations on the optimal design of OCR flywheel rotors with press-fitted filament wound rims can be made. Minimizing residual stresses in rings can simplify a rotor design by reducing the number of rings required to maximize the specific energy. The deleterious effects of residual stresses are well known, but the quantitative effect of these stresses and an appropriate method of analyzing these stresses in

Acknowledgment

This research was funded by NASA Grant NAG3-2598 and monitored by Dr. Barbara Kenny of NASA Glenn Research Center. Numerous colleagues at Penn State provided useful technical assistance with various aspects of this investigation: Dr. Ryan P. Emerson and Mr. John M. Noland of the Engineering Science and Mechanics Department and Prof. Heath Hofmann, Mr. Yiming Liu, and Mr. Wensen Wang of the Electrical Engineering Department.

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Optimal design of press-fitted filament wound composite flywheel rotors

Abstract

Introduction

Section snippets

Previous research on the design of composite flywheel rotors

Objectives

Analysis

Optimization problems

Problem 1—evaluation of simulated annealing

Conclusions and recommendations

Acknowledgment

Composites

Mathl. Comput. Model.

Flywheels

Sci. Am.

Kinetic energy storage: theory and practice of advanced flywheel systems

Composite flywheels

Optimum design of thick-walled composite rings for an energy storage system

J. Compos. Mater.

Optimum design of multi-ring composite flywheel rotor using a modified generalized plane strain assumption

Int. J. Mech. Sci.

A high-efficiency electromechanical battery

Proc IEEE