2010

Nick Meeker and Brian Walker

RIT

2/24/2010

Flywheel Technology for Energy Storage

Abstract

Energy storage is increasingly becoming a vital target in research as the potential gains are

enormous. Numerous energy storage technologies exist from the common battery, to compressed air

systems, all the way to a proposed hydrogen infrastructure. They all have their advantages and

disadvantages, but none seem to meet all our requirements; efficiencies must be high and costs low,

resources must be plentiful or renewable, emissions must be miniscule to none. Such harsh

requirements call for rigorous engineering and innovation. One might wonder, maybe our answer isn’t

looking forward at a new cutting edge technology, but rather looking back at a classic mechanical marvel

that has lasted centuries.

The flywheel has been around for hundreds of years in various forms from spindles to CVT

transmissions. New advances in materials, bearings, seals, and more help the flywheel boast near

immediate response times, long life under constant cycling, and efficiencies reported at 85% and

higher.1 These attributes make flywheel for energy storage competitive with traditional technologies.

Some drawbacks that still may need to be overcome to meet our strict energy storage requirements

include initial costs, design complexities, transmission requirements, and overall system size and weight.

With that said, this technology is currently being put to use for several stationary electricity back-up

systems and could one day find their way in a hybrid vehicle. 2

Energy Storage

There are countless ways that we can currently produce energy. We have coal power plants,

heat engines running off various fuels, fuel cells and batteries; the list goes on. In addition there are

countless ways we use them. Undoubtedly, there are various time patterns associated with how we

produce and gather energy and how we use them. These patterns at some point will vary and

somewhere in the process there is a need for energy storage. For example, solar energy for power

generation is most abundant at midday when the sun is at the strongest. This may meet peak loads

during the summer, but in the winter months the largest energy demand is at night. This is one

prominent example in which engineers today are still struggling to obtain a solution.

Through the decades many technologies have come and gone and several have remained

common. Energy storage can take form in chemical, biological, electrical, mechanical and even thermal

technologies. Some of these could be hydrogen and bio-fuels which are two of the largest prospects for

future technologies, capacitors in everyday electronics, and even a simple spring. Energy storage is

actually a very general term and each technology serves its own purpose from the long term energy

storage associated with hydrogen to nearly instant charges and discharges that can be seen in a

capacitor.

1 THE ARTEMIS PROJECT Source 2

2 Reinventing the Wheel Source 8

History Flywheel

For years, engineers and designers have focused on batteries for long term energy storage

which can only last for finite charge/discharge cycles. Only recently have people begun to think about

hydrogen as a large storage medium. In all this, one can only wonder if we have overlooked an old

technology proven through time. The flywheel is an innovative mechanical device that has existed for

hundreds of years. It has not always been used for energy storage; rather it holds other meanings in

history. Some argue that the earliest forms of flywheels date back to the 15th century when Egyptians

used them to twist strips of leather for shipbuilding while others argue that the spindle-whorl used to

make pottery was the first true form of a flywheel. With advancements in composite materials, more

modern flywheels could be seen in the 19th century with the invention of the steam engine. In these

times, flywheels were steel discs with or without rim and said to be a conventional ‘off the shelf’ design.

Eventually, inventers and designers developed interest in what they called ‘super flywheels’ which

meant they were higher in energy density and less destructive upon failure. 3

Advantages There are many advantages to using flywheel technology rather than the alternatives.

Flywheels have very high power and energy densities. They have efficiency that is about 85%, which is

considered to be very high. Contrary to traditional technologies, flywheels are extremely clean and

environmentally friendly. They have a wide range of shapes and sizes, ranging from kilograms to

hundreds of tons, which make them viable for a long list of applications. The concept of a flywheel is

very simple; they just convert electrical energy into mechanical energy, and then back to electrical

energy. They can also be connected to a shaft to output mechanical work rather than electricity. This

makes a flywheel seem like a “mechanical battery.” The outputting of mechanical work can be useful,

but it is more common for the energy to be converted to electricity.

Disadvantages There are some disadvantages to using flywheels for energy storage. First of all, they have

complex designs. They require materials that can withstand the amount of angular momentum that the

flywheel itself possesses. Flywheels also need complicated and heavy equipment to be able to function

properly. This makes for high initial costs and can also lower the efficiency and energy density of the

system

3 Kinetic Energy Storage, G. Genta, pages 1-26 Source 5

Technology

The most core concept to understand regarding flywheels has to do with the conversion of

energy. Energy is stored in kinetic energy, for however long it may be. This is very unique from most

devices in which energy is to be used as kinetic energy. To accomplish this, a disc spinning inside a casing

which provides structural support among other things, spins at higher angular velocities as more energy

is stored. As energy is removed the angular velocity is consequently decreased. To do this and create a

complete system that is useful for energy storage, several components are required including the

flywheel itself, the casing, bearings and seal systems, power transmissions, and vacuum and system

controls. Optimists claim these systems can reach efficiencies approaching 99%, however there are real

world cases that report even 85% efficient. Figure 1 shows the cross-section of a typical flywheel

system.4

Figure 1: Cross-section of typical flywheel system

Though nearly useless as a single unit, the flywheel is undoubtedly the heart of the system. Also

commonly called rotors, flywheels can vary in shape, size, and material. One way of characterizing its

shape depends on its geometry, and therefore moment of inertia. This is this commonly referred to as

the ‘shape factor’, K, which is a dimensionless quantity. The material of the fly wheel can depend on

endless design parameters, but one that holds the largest effect is the stress that the flywheel needs to

withstand (though this can also be accounted for with geometry). This component is what supplies the

system with actually energy storage in the form on kinetic energy which can easily be calculated with

Equation 1 and Equation 2 below.

4 Review of Flywheel Energy Storage System Source 10

For most flywheel systems, some type of housing is required to keep a vacuum, eliminating drag

on the flywheel, and to provide a shield for contact and also from possible failure. In applications where

speeds can reach as high as 50,000 rpm, drag can contribute significant losses. In this case the housing is

required to supply the enclosure for a vacuum. Though rare, some applications do not require housings.

In an unmanned space vehicle for example, the protection of debris in the case of a failure is

unnecessary, and if start-up occurs beyond earth’s atmosphere, even the use of a vacuum is redundant.

The aerodynamic drag acting on the flywheel can be expressed as shown in Equation 3 where ρ

is the density of air, is the angular velocity, r is the radius, and Cm is a dimensionless coefficient relying

on Reynolds number, Knudsen number, and the Mach number (relation of the radius, angular velocity

and speed of sound). Drag can also come from the friction from bearings and rotation. This can be

significantly reduced through the implementation of magnetic bearings which suspend the rotor nearly

eliminating friction entirely. These high costs however, make hybrid bearings more feasible which use

magnets to support the rotor and high-temperature superconductors to stabilize it.

Equation 3: Rotational Drag

In several of these examples of flywheel systems where a vacuum is required, there must be

some sort of connection from the inside of the chamber to the outside to obtain any power input or

output. Frequently, this is done by connecting the rotating shaft to a transmission system. This requires

a robust seal to maintain vacuum pressure while subjected to forces imparted by the rotating flywheel.

The high rotational speeds of the flywheel can be adjusted by the time is gets to the shaft through gear

ratios. However, even with speed reductions, significant heat production can occur adding another

requirement to the seal. This seemingly minuscule component of the overall system can actual have a

large impact on the system performance and can even lead to failure.

It is important to understand the difference between the many types of flywheels. The most

significant distinction is between applications of flywheels to maintain an even angular speed and

flywheels for energy accumulation. R.C. Clerk states that a flywheel accumulator is one that rotates at

least ten times during the charge and discharge cycle.5 By this definition, a flywheel of a typical car

engine is not an energy accumulator while a simple spindle whorl is. There seems to be a gray line

between such technologies. However, the vital concept to grasp is that the flywheel is a technology with

a myriad of uses, but not all are used for energy storage.

There are various ways to quantify a given flywheel system. Energy density which is very

commonly used, is the ratio between the energy able to be stored by the complete system to the mass.

As discussed earlier, a flywheel is actually a complete system with several components, so surely the

entire mass of the system should be used in this calculation. However this is not always the case as in

design work this quantity is often misleadingly expressed as the energy stored at burst speed by the

mass of the flywheel alone because of the ease in calculation from theory shown below in Equation 4.

Here the energy density at burst speed depends solely on the fly wheel design which is described by the

shape factor, K, and the material properties ultimate strength, , and density, .

5 Clerk R.C., “The utilization of Flywheel Energy”. SAE Transactions. Vol. 72, 1965, pp.508-543

Equation 4: Energy density

For easy comparison, energy density can be categorized in low, medium and high categories as

shown below. Interestingly enough, if considering the earth spinning about its axis as a flywheel, and

ignoring its non-homogeneous shape, it would fall under the medium-energy density category at about

12 Wh/kg. 6

Low-energy density: ≤10 Wh/kg (36 kJ/kg)

Medium-energy density: 10 to 25 Wh/kg (36 to 90 kJ/kg)

High-energy density: ≥ 25 Wh/kg ( kJ/kg)

Costs In general, flywheels cost more than batteries in initial costs, but require less maintenance and

will run for much longer. It can be difficult to analyze the cost of a technology that can cover such a wide

scope of applications and types. Therefore, this cost analysis will compare a flywheel system to a

comparable set of batteries. We will use a system with 250kW of uninterrupted power supply backed up

by a generator that can reach full power in 10 seconds. The unsupplied period can be supplied with

batteries or a flywheel. Also, assume the batteries can supply power for 10 minutes. From the

Department of Energy Federal Technology Alert, flywheel purchase costs can range from $100/kW to

$300/kW while installation can be simple run from $20/kW to $40/kW. Operation and maintenance

costs are also claimed to be low and infrequent. They range from bearing replacement around $10/kW

or vacuum pump replacement for $5/kW. For comparison to batteries, the figure below is based on a

low-rpm flywheel with a life of 20 years and a VRLA battery with a life of 4 years. After a life cycle

analysis, again done by the Federal Energy Management Program, the present value of life-cycle costs

was $248,129 for the batteries and $105,572 for the flywheel which results in 60% savings. 7

Figure 2: Life-Cycle Costs of Flywheel and battery

6 Kinetic Energy Storage, G. Genta, pages 27-30 Source 5

7 DOE Federal Technology Alert Source 3

Case Analysis Flywheels are becoming a valid business venture in today’s energy market. Beacon Power,

located in Stephentown, New York, has been applying the use of flywheels to the local power grid. They

are using flywheel technology to convert electrical power into the mechanical motion of the flywheel to

store the energy.

This stored energy is being used to “help in power-grid regulation, quickly balancing the second-

by-second discord between electrical supply and demand.” This means that when the supply and

demand of electrical power do not agree, the flywheel either stores the excess energy or supplies the

needed energy to meet the demand. This transfer of energy is done almost instantaneously with the

flywheel technology, which makes it perfect for this application.

With the emergence of technologies such as solar and wind power, the regulation of the power

grid is going to become even more of a challenge. Beacon Power has realized the need for

advancements in this technology and has implemented a new material for the flywheels. They are using

a material similar to that of which a golf club is made. These systems have put forth an efficiency of

about 85%, which is known to be very high.

Beacon Power is doing their part to make for a greener world. Using this flywheel technology,

rather than traditional grid-regulating techniques, greatly reduces the amount of conventional pollution

and carbon dioxide emissions. 8

Figure 3: Beacon Power Smart Energy 25Flyhweel Figure 4: Beacon Power Flywheel Storage Array

8 Advancing the Flywheel for Energy Storage and Grid Regulation – Source 1

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