XT810 - Kilogram Power Wheel & Torus

"XT810 - Kilogram Power Wheel & Torus" is an extract from "DS903: The KPW (Kilogram Power Wheel)". This looks at flywheel devices used for domestic energy storage, in place of banks of electrical batteries.

What is a KPW?
The Kilogram Power Wheel is a device used to store energy at the domestic scale. Intended as part of an electricity-supply facility, it may be used instead of a bank of electrical batteries (sometimes called a "power-wall"). It is essentially a magnetically-levitated flywheel operating in a vacuum.

Energy Storage for Households
Households could generate enough energy from solar panels to supply all their needs and go "off-grid", if they could store excess power during the day and call on the stored power during the night. At present, this means storing energy in a bank of electrical batteries.

Fig. DS903-F3. Energy storage with batteries. From [4].

According to [4], the main options currently available for household energy storage are lead-acid and lithium-ion batteries. Other less common options include nickel-cadmium, nickel-metal hydride and flow batteries.

The trend is likely to be that battery assemblies ("power-walls"), similar in size to a refrigerator, will be developed to the stage where many households will go off-grid in the future. The Tesla PowerWall 1, intended for exactly this household use, offers a storage capacity of 6.4 kWh, with a continuous power draw of 5 kW [19]. It weighs a little under 100 kg.

It's worth noting that batteries store electricity as DC, while domestic supply is usually AC. Some household devices need the AC transformed to DC. Solar panels produce DC.

Summary of Energy Storage Techniques
The chart following summarizes most classes of energy storage. The vertical axis shows Specific Energy in Wh/kg (Watt-hours per kilogram, essentially the amount of energy which can be stored in a given mass). The horizontal axis shows Specific Power in W/kg (Watts per kilogram).

Fig. DS903-F9. Energy Storage Chart. From [10].

Most of the items on the chart have already been mentioned, the exception being Flywheels. Flywheels are the base topic of this article -- they are seen here as the most promising and practical technique for household energy storage, and a new technique is described below which could greatly increase their utility. This could move Flywheels up to the top right corner of the chart.

Storing Rotational Energy in Flywheels
Flywheels are an ancient method of storing energy, with perhaps their oldest use in the early potter's wheel, where the potter spins a circular table on which a pot is to be thrown. Electric potters wheels are still used today, and earlier industrial uses had them driven by belts connected to steam engines, but the earliest models seem to have spun by a treadle powered by the potter's feet.

By storing energy in a heavy rotating wheel, the potter is able to achieve a regular circular motion -- thrown clay pots are based on shapes with circular cross-sections, varied in diameter by the separation of the potter's two hands.

Fig. DS903-F10. Throwing a Clay Pot on a Potter's Wheel. From [17].

According to Wikipedia [18], a stone potter's wheel found at the Mesopotamian city of Ur in modern-day Iraq has been dated to about 3129 BC, but fragments of wheel-thrown pottery of an even earlier date have been recovered in the same area. However, southeastern Europe and China have also been claimed as possible places of origin.

There is a good explanations of Flywheel Energy Storage Systems (FESS) in [8]. Below are some extracts from there. Essentially, a FESS is just an electric motor/generator -- electrical energy can be fed in, increasing its rate of rotation, or drawn out, causing the rotor to slow.

"Flywheel energy storage systems (FESS) use electric energy input which is stored in the form of kinetic energy. Kinetic energy can be described as 'energy of motion', in this case the motion of a spinning mass, called a rotor. The rotor spins in a nearly frictionless enclosure.

When short-term backup power is required because utility power fluctuates or is lost, the inertia allows the rotor to continue spinning and the resulting kinetic energy is converted to electricity. Most modern high-speed flywheel energy storage systems consist of a massive rotating cylinder (a rim attached to a shaft) that is supported on a stator by magnetically levitated bearings. To maintain efficiency, the flywheel system is operated in a vacuum to reduce drag.

The flywheel is connected to a motor-generator that interacts with the utility grid through advanced power electronics. Some of the key advantages of flywheel energy storage are low maintenance, long life, and negligible environmental impact.

Flywheels can bridge the gap between short-term ride-through power and long-term energy storage with excellent cyclic and load following characteristics. Typically, users of high-speed flywheels must choose between two types of rims: solid steel or carbon composite. The choice of rim material will determine the system cost, weight, size, and performance. Composite rims are both lighter and stronger than steel, which means that they can achieve much higher rotational speeds"

Fig. DS903-F11. Flywheel stores Energy. From [10].

Construction of Flywheels
The amount of energy that can be stored in a flywheel is a function of the square of the rpm (Revolutions Per Minute), making higher rotational speeds desirable. Currently, high-power flywheels are used in many aerospace and UPS (uninterruptible power supply) applications and in telecommunications applications. For utility-scale storage a "flywheel farm" approach can be used to store megawatts of electricity for applications needing minutes of discharge duration.

The amount of energy that can be stored is proportional to the object's moment of inertia times the square of its angular velocity [8]. To optimize the energy-to-mass ratio, the flywheel must spin at the maximum possible speed. Rapidly rotating objects are subject to significant centrifugal forces, so while dense materials can store more energy, they are also subject to higher centrifugal force and thus may be more prone to failure at lower rotational speeds than low-density materials.

Therefore, tensile strength is more important than the density of the material. Low-speed flywheels are built with steel and rotate at rates up to 10,000 rpm. More advanced FESS achieve attractive energy density, high efficiency and low standby losses (over periods of many minutes to several hours) by employing four key features: 1) rotating mass made of fibre-glass resins or polymer materials with a high strength-to-weight ratio; 2) a mass that operates in a vacuum to minimize aerodynamic drag; 3) mass that rotates at high frequency; and 4) air or magnetic suppression bearing technology to accommodate high rotational speed.

Advanced FES systems have rotors made of high strength carbon-fibre composites, suspended by magnetic bearings, and spinning at speeds from 20,000 to over 50,000 rpm in a vacuum enclosure [7]. Such flywheels can come up to speed in a matter of minutes --reaching their energy capacity much more quickly than some other forms of storage. These FESS operate at a rotational frequency in excess of 100,000 rpm, with tip speeds in excess of 1000 m/s. FESS are best used for high power, low energy applications that require many cycles.

Fig. DS903-F12. NASA G2 Flywheel. From [7].

In summary, flywheel systems have several advantages over chemical energy storage. They have high energy density and substantial durability, which allows them to be cycled frequently with no impact to performance. They also have very fast response and ramp rates. In fact, they can go from full discharge to full charge within a few seconds or less. They are especially attractive for applications requiring frequent cycling, given that they incur limited life reduction if used extensively (i.e., they can undergo many partial and full charge-discharge cycles with trivial wear per cycle.)

Flywheels as household energy stores
The KPW (Kilogram Power Wheel)
All the technology is already present to develop small flywheel devices as household power stores, playing the same role as battery assemblies like Tesla's PowerWall. These flywheels should be much less massive than a comparable battery bank, and should be cheaper to make and easier to maintain. They should also have additional advantages in not needing rarer source materials such as lithium.

In this article, such a device is called a KPW (Kilogram Power Wheel). The target for implementation is a device about as big as a record player, with a rotor weighing about a kilogram. This is contrasted with the Tesla PW, which is the size of a refrigerator and weighs close to 100 kg.

Fig. DS903-F16. The KPW would be of similar size to a record player. From [23].

The KPW would be made up of a flywheel and the electromagnetic windings needed for it to act as a generator or motor, with the flywheel magnetically levitated (magnetic bearings) and enclosed in an evacuated capsule. This is a standard design which is virtually friction-less -- once in motion, the flywheel should continue to rotate indefinitely at the same speed until power is added or withdrawn.

Compared with most flywheels, the KPW would be on the smaller size. The energy held by a flywheel at any given time depends directly on its mass, and on the square of its speed of rotation.

We need to be able to calculate the energy held in various configurations of flywheel. A useful too here is a Web facility which uses Javascript to calculate output energy from a flywheel's mass, diameter, rate of spin in rpm (revolutions per minute) [20]. The reader can use this Flywheel Energy Calculator to make such calculations -- a snapshot of one such calculation follows.

Fig. DS903-F17. Energy stored in a 1 kg flywheel. From [20].

This base example is for a flywheel of mass 1 kg (1000 grams) and diameter 300 mm (similar to a LP record), and a spin rate of 100,000 rpm. Output is given in joules for two flywheel formats, a disc (about 617,000 joules) and a ring (about 1,234,000 joules).

These figures are for ideal disc or ring formats. The disc format is like that of an LP record, with a uniform thickness and density from centre to rim. The ring format is like that for a bicycle wheel, assumed to have all of its mass at its rim and tyre, and assumed negligible mass for its spokes and axle. Real flywheels lie between these two formats.

Here we will use kiloWatt-hours (kWh) as our comparison unit of energy. On your electricity bill, 1 kWh is usually called "1 unit". The average Australian house uses 18 kWh per day, and 6,570 kWh per year [25]. It was noted above that the Tesla PowerWall 1 offers a storage capacity of 6.4 kWh.

Another Web facility at [21] does conversion of Joules to kWh. You can do this yourself by dividing a figure in Joules by 3,600,000 (3.6 million).

So the 1 kg, 300 mm, base flywheel example above, with a spin rate of 100,000 rpm, has an energy storage of about 0.34 kWh for the ring format, and 0.17 kWh for the disc format. We would need to increase this energy storage by a factor of 3 to 20 for a useful device.

Increasing the flywheel mass by 10 times, to 10 kg, would give 10 times the output, about 3.3 or 1.65 kWh. A bank of 10 of the base units would have the same total result. This would need an engineering reconfiguration to accommodate the heavier flywheel.

Doubling the diameter of the flywheel, to 600 mm, would give four times the output. For the base example, the outputs would be 1.37 kWh (disc) and 0.68 kWh. This would involve a major re-design of the device, taking it out of the "record-player" heft.

Increasing the rotation speed of the flywheel is the easiest way to raise its energy storage. At a rate of 200,000 rpm, with the disc configuration, the storage would be 0.68 kWh -- without any change to the engineering. At 432,100 rpm, the ring-configuration flywheel would have a capacity of 6.4 kWh -- the same as the Tesla Powerwall.

So increasing the flywheel rotation rate is the obvious way to go. No re-configuration is needed, just feed in more power. The limitation is in the strength of the rotor -- with high enough centrifugal forces, the rotor will disintegrate.

Here then is the main development route to get commercial KPWs available for supply and fitting in domestic usage. Currently, carbon-fibre rotors are out-performing metal ones as far as maintaining integrity under high rotation rates is concerned. Fused-silica rotors appear to have promise here.

Fig. DS903-F18. Cover to a communications service pit. From [26].

Because of the rotor-disintegration possibility, household KPWs would normally be mounted in small service pits, as commonly used now for communications and utilities. It would be good practice to have two KPWs in separate pits at any site -- this would be convenient anyway, for maintenance and redundancy.

Two pits would also be more convenient for an external power top-up -- the contractor would bring in a buggy with its own large flywheel store, and transfer energy to each of the home flywheels in turn. This should be a rapid procedure, taking only a few minutes, unlike re-charging from mains power.

The KPT (Kilogram Power Torus)
The KPW storage device as described can be manufactured with today's technology. An improved device, the KPT or Kilogram Power Torus, is possible by applying a little R&D input to modify the KPW.

Fig. DS903-F19. Carbon-Fibre Torus vessel with encased electro magnet windings. From [24].

In this modification, the KPW rotor is replaced by a torus, kept suspended by a magnetically levitating harness. The motor/generator windings are within or around the torus, so there is no axle in the device.

In theory, a KPT rotor should be less prone to breakage under high centrifugal forces, as it floats within a harness rather than forming a solid connection with an axle. Using high-strength magnets, it should be possible to control the torus within the harness at much higher rotation speeds, and so to achieve much higher energy storage rates.

The illustration is from a description of an experimental aircraft propulsion system [24], "Basic Design of the Dual Mercury Torus Propulsion System". This notes that it contains a "Carbon-Fibre Torus Structure with Gold wire windings placed Integral to the structure. Each band of windings is a separate electro-magnet that can be turned on and off by a computer program, allowing acceleration to tremendous speeds".

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References and Links

[4]. Battery storage safety: frequently asked questions (FAQ). https://www.solaraccreditation.com.au/consumers/solar-battery-storage-faqs.html .
[7]. Flywheel energy storage. https://en.wikipedia.org/wiki/Flywheel_energy_storage .
[8]. Flywheels. http://energystorage.org/energy-storage/technologies/flywheels .
[10]. Y T Qiu. Energy Storage: Why is Energy Storage Important?. http://zebu.uoregon.edu/disted/ph162/l8.html .
[17]. Pottery Wheel Throwing. http://www.wise-adventures.ca/-new--potter-s-wheel.html .
[18]. Potter's wheel. https://en.wikipedia.org/wiki/Potter%27s_wheel .
[19]. Tesla Powerwall. https://en.wikipedia.org/wiki/Tesla_Powerwall .
[20]. Flywheel Energy Calculator. http://www.botlanta.org/converters/dale-calc/flywheel.html .
[21]. Joules to kWh conversion. http://www.rapidtables.com/convert/energy/Joule_to_kWh.htm .
[23]. Turntables. https://www.jbhifi.com.au/headphones-dj/turntables/ .
[24]. TR3 Black Manta "A51". http://tr3blackmanta.com/ .
[26]. Dig Dug Mini Digging. http://www.digdug.com.au/prepro_leadin.html .

"XT810 - Kilogram Power Wheel & Torus" is an extract from "DS903: The KPW (Kilogram Power Wheel)". This looks at flywheel devices used for domestic energy storage, in place of banks of electrical batteries.

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Version 1.0 placed on Web 2017 Nov 3.