Material selection




Flywheels are made from many different materials; the application determines the choice of material. Small flywheels made of lead are found in children's toys.citation needed Cast iron flywheels are used in old steam engines. Flywheels used in car engines are made of cast or nodular iron, steel or aluminum. Flywheels made from high-strength steel or composites have been proposed for use in vehicle energy storage and braking systems.

The efficiency of a flywheel is determined by the maximum amount of energy it can store per unit weight. As the flywheel's rotational speed or angular velocity is increased, the stored energy increases; however, the stresses also increase. If the hoop stress surpass the tensile strength of the material, the flywheel will break apart. Thus, the tensile strength limits the amount of energy that a flywheel can store.

In this context, using lead for a flywheel in a child's toy is not efficient; however, the flywheel velocity never approaches its burst velocity because the limit in this case is the pulling-power of the child. In other applications, such as an automobile, the flywheel operates at a specified angular velocity and is constrained by the space it must fit in, so the goal is to maximize the stored energy per unit volume. The material selection therefore depends on the application.

The table below contains calculated values for materials and comments on their viability for flywheel applications. CFRP stands for carbon-fiber-reinforced polymer, and GFRP stands for glass-fiber reinforced polymer.

Material Specific tensile strength Comments
Ceramics 200–2000 (compression only) Brittle and weak in tension, therefore eliminate
Composites: CFRP 200–500 The best performance—a good choice
Composites: GFRP 100–400 Almost as good as CFRP and cheaper
Beryllium 300 The best metal, but expensive, difficult to work with, and toxic to machine
High strength steel 100–200 Cheaper than Mg and Ti alloys
High strength Al alloys 100–200 Cheaper than Mg and Ti alloys
High strength Mg alloys 100–200 About equal performance to steel and Al-alloys
Ti alloys 100–200 About equal performance to steel and Al-alloys
Lead alloys 3 Very low
Cast Iron 8–10 Very low

The table below shows calculated values for mass, radius, and angular velocity for storing 250 J. The carbon-fiber flywheel is by far the most efficient; however, it also has the largest radius. In applications (like in an automobile) where the volume is constrained, a carbon-fiber flywheel might not be the best option.

Material Energy storage (J) Mass (kg) Radius (m) Angular velocity (rpm) Efficiency (J/kg) Energy density (kWh/kg)
Cast Iron 250 0.0166 1.039 1465 15060 0.0084
Aluminum Alloy 250 0.0033 1.528 2406 75760 0.0421
Maraging steel 250 0.0044 1.444 2218 56820 0.0316
Composite: CFRP (40% epoxy) 250 0.001 1.964 3382 250000 0.1389
Composite: GFRP (40% epoxy) 250 0.0038 1.491 2323 65790 0.0365

Comments

Post a Comment

Popular posts from this blog

History

Image
The principle of the flywheel is found in the Neolithic spindle and the potter's wheel, as well as circular sharpening stones in antiquity. The mechanical flywheel, used to smooth out the delivery of power from a driving device to a driven machine and, essentially, to allow lifting water from far greater depths (up to 200 metres (660 ft)), was first employed by Ibn Bassal (fl. 1038–1075), of Al-Andalus. The use of the flywheel as a general mechanical device to equalize the speed of rotation is, according to the American medievalist Lynn White, recorded in the De diversibus artibus (On various arts) of the German artisan Theophilus Presbyter (ca. 1070–1125) who records applying the device in several of his machines. In the Industrial Revolution, James Watt contributed to the development of the flywheel in the steam engine, and his contemporary James Pickard used a flywheel combined with a crank to transform reciprocating motion into rotary motion.
Read more

Flywheel

Image
A flywheel is a mechanical device specifically designed to use the conservation of angular momentum so as to efficiently store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, if we assume the flywheel's moment of inertia to be constant (i.e., a flywheel with fixed mass and second moment of area revolving about some fixed axis) then the stored (rotational) energy is directly associated with the square of its rotational speed. Since a flywheel serves to store mechanical energy for later use, it is natural to consider it as a kinetic energy analogue of an electrical inductor. Once suitably abstracted, this shared principle of energy storage is described in the generalized concept of an accumulator. As with other types of accumulators, a flywheel inherently smoothes sufficiently small deviations in the power output of a system, thereby effectively playing the role of a low-pass filt...
Read more

Table of energy storage traits

Image
Flywheel purpose, type Geometric shape factor (k) (unitless – varies with shape) Mass (kg) Diameter (cm) Angular velocity (rpm) Energy stored (MJ) Energy stored (kWh) Energy density (kWh/kg) Small battery 0.5 100 60 20,000 9.8 2.7 0.027 Regenerative braking in trains 0.5 3000 50 8,000 33.0 9.1 0.003 Electric power backup 0.5 600 50 30,000 92.0 26.0 0.043 For comparison, the energy density of petrol (gasoline) is 44.4 MJ/kg or 12.3 kWh/kg. High-energy materials edit For a given flywheel design, the kinetic energy is proportional to the ratio of the hoop stress to the material density and to the mass: E k ∝ σ t ρ m {\displaystyle E_{k}\varpropto {\frac {\sigma _{t}}{\rho }}m} σ t ρ {\displaystyle {\frac {\sigma _{t}}{\rho }}} could be called the specific tensile strength. The flywheel material with the highest specific tensile strength will yield the highest energy storage per unit mass. This is one re...
Read more