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Energy Storage - Flywheel
Posted by Luis de Sousa on October 5, 2011 - 9:45am
This piece resulted from a challenge within the staff to write a collaborative post on emerging energy storage technologies. I left Chemistry back in high school but one technology that has long fascinated me led me to volunteer on a project: the flywheel. It seemed a good justification to study why industry hasn't developed many applications for these ancient mechanisms. The elusive quest for an answer to that question lead to a rather long writing, that justifies a post on its own. Hopefully this shall be the first of a series on energy storage.
Flywheels are very simple mechanisms. If you have a bicycle you can see how it works: lift one of its wheels from the ground and give it an impulse so that it starts spinning. If the wheel hub is in proper condition the wheel keeps on spinning for quite some time. In fact, were your bicycle in space and the wheel could spin for ever, all due to the law of energy conservation - the work employed by your hand on the wheel is stored as kinetic energy as the wheel spins. Here on Earth, the bicycle flywheel slowly grinds to a halt because air friction and hub imperfections slowly dissipate this energy.
Flywheels are very simple mechanisms. If you have a bicycle you can see how it works: lift one of its wheels from the ground and give it an impulse so that it starts spinning. If the wheel hub is in proper condition the wheel keeps on spinning for quite some time. In fact, were your bicycle in space and the wheel could spin for ever, all due to the law of energy conservation - the work employed by your hand on the wheel is stored as kinetic energy as the wheel spins. Here on Earth, the bicycle flywheel slowly grinds to a halt because air friction and hub imperfections slowly dissipate this energy.
Basic concepts
Flywheels are nothing more than discs or cylinders that spin around a fixed axis. The amount of energy a flywheel can store is proportional to its mass (m), the square of the speed at which it spins (w) and the square if its radius (r). The general equation for a solid disc is of this form:
E = 1/4 · m · r2 · w2
Flywheels have been known to Man at least since the Neolithic, when the first potter's wheels were built. At the dawn of the Industrial Revolution flywheels started to be employed widely as mechanisms to translate the work of steam engines into constant rotational motion; this solution is still employed today in modern internal combustion piston engines. At face value, a flywheel presents several advantages when compared to chemical batteries:
First commercial applications, first disappointments
This design was the first to be used in commercial applications back in the 1950s in buses that used heavy steel flywheels as their sole energy storage mechanism; these vehicles got the name of Gyrobus. They were employed in routes with low passenger loads that didn't justify full electrification. Gyrobuses could travel about 5 Km on a full charge between recharging stations. Recharging would take no more than 3 minutes, since flywheels can easily absorb high voltage electrical currents. These buses were also equipped with regenerative braking systems that recharged the flywheel.
Commercial Gyrobus services were started in Switzerland connecting Yverdon-les-Bains and Grandson in 1953, and soon after services were initiated in several routes in the city of Kinshasa in the Belgian Congo; in 1956, another service was started in Belgium, linking Ghent and Merelbeke. With time several problems came up, mostly due to the huge weight of these machines – the Gyrobuses used in Congo weighed more than 10 tonnes. They were hard to drive, damaged roads and above all, were electricity guzzlers; a tram used on the same service could easily slash electricity consumption by a third. By the end of 1960, all of the Gyrobus routes mentioned above had been scrapped.
This early commercial experiment brought to light the main disadvantages of flywheels:
Though Gyrobuses haven't returned, research continued on the application of this technology; at the beginning of the 1990s there seemed to be engineering solutions to deal with all the issues above. The first big change was the introduction of new composite materials: they immediately tackled the weight issue but also ameliorated the container, these materials disintegrate into very small particles much easier to retain. Then the bearings issue was elegantly solved with magnets, creating a magnetic field that makes the flywheel levitate, thus spinning without any contacts with other parts (this requires a small consumption of electricity). Finally, the angular momentum has been addressed with the employment of gimbals, which, while not completely solving the issue, greatly reduce its effects.
All these solutions were employed by a company named Rosen Motors in a venture project aimed to produce a car without an internal combustion engine. The concept was based on a small gas turbine coupled with a composite electric flywheel for regenerative braking. In 1997 the first test run was made with the system installed in a Saturn production car; it covered 130 Km in about 2 hours, in what as an engineering breakthrough in many aspects, earning good reactions from several auto-makers. Months later the venture capital of $24 million ran out. Without a single auto-maker wishing to pursue the project the company was forced to fold.
Until today no commercial car has ever been fitted with a flywheel based regenerative system. Nevertheless, the research conducted by Rosen Motors proved that flywheel electrical storage had reached technological maturity, with many different potential applications opening up.
Many promises, few realizations
In the rail industry, flywheels have been used more extensively, though it can't be said their usage is widespread. They have been employed to store energy in electric locomotives to guarantee motion along non- electrified sections of rail lines and also to power small locomotives; beyond a few exceptions these solutions have remained mostly experimental. Flywheel powered trams can be particularly convenient in cities for they dispense with overhead electrification. Notwithstanding this fact, today there's only one commercial case to refer, in the Stourbridge line in London. Ever since 2002, 3 different units built by Parry People Movers have been tested, with 2 of them providing regular service since 2008. This sort of tram can also be fitted with diesel engines for longer distances; since the flywheel deals with all acceleration and braking, this engine can be designed to function at optimal revolutions per minute (rpm), thus being very small and efficient. So far the apparent success of these trams hasn't triggered any serious market penetration, though trial services and demonstrations have been run in other lines; a new trial service is set to start next month on the Mid-Hants Railway, also in Britain.
More recently, interest has been growing on the employment of flywheels as static batteries by the rail side. They can be used to stabilize the electric current feed to locomotives and also to store energy locomotives feed back to electric lines when braking. In 2009 the press reported a 5.2 million $ project to implement a rail-side 2.4 MW flywheel system on the West Hempstead line in Long Island, US. In parallel, the US company Urenco Power Technology, has been developing and testing smaller rail-side flywheels in the underground lines of New York, London, Tokyo and Lyon. Satisfied with the maturity of the system, at the beginning of this year a spin-off company was launched, Kinetic Traction Systems, with the specific aim of commercializing the technology.
Another US company has been working with similar objectives: Beacon Power, but with applications for the electrical distribution grid. The company developed a large scale flywheel that spins up to 16 000 rpm, with a maximum storage capacity of 25 kWh, that can be delivered back to the grid at maximum power rate of 100 kW (over 15 minutes). These flywheels are gathered in clusters that can be used together. Here's a video from 2009 describing the system:
Contrary to what this video suggests, Beacon Power seems quite healthy today, especially after the opening, already this year, of the first commercial flywheel farm, composed by 200 units and installed at Stephentown in New York. This flywheel farm has been deployed primarily as an electricity frequency stabilizer, a perfect match to the flywheel's prompt discharge/recharge characteristics; beyond that, the farm is used to store cheap electricity available in the grid during the night. The company maintains close collaboration with US government authorities through several development programs with broader aims at grid wide stability. The state of Pennsylvania also seems interested in this technology, with capital already committed to a flywheel farm. Further applications are being envisioned, particularly the marriage of flywheel farms with wind farms in order to decentralize load balancing; in this case the system will also be coupled to a thermal generator feed by diesel or gas, that once again can be greatly optimized by the prompt aid of flywheels. If there's an application where flywheels seem bound for serious market penetration it is this one; nonetheless, considering how long intermittent technologies have been penetrating the electricity production market, these seem yet small steps.
When fun meets technology
To fully understand the flywheel state-of-the-art, a final (and longer) story needs to be told. In recent years flywheels took a boost from an unexpected source: Formula 1. In a bid to “green” the sport and provide deeper technological transfer to the auto industry, the FIA introduced new rules for the 2009 season that allowed teams to optionally fit their cars with a regenerative electric storage system with a fixed maximum capacity. This system was baptised Kinetic Energy Recovery System - KERS. These new rules seemed to put those teams opting for the KERS at an advantage, but their late introduction gave little room for the new technology to be developed and absorbed, especially concerning the extra weight applied on the cars. The 2009 season ended up dominated by one of the low budget teams that opted to not even develop a KERS.
After this débâcle no F1 team used KERS in 2010, but a left over from 2009 brought the curiosity of many: while most teams opted for chemical batteries, the Williams team had developed a flywheel, that though not successful on track, seemed quite promising for the road. With maximum spinning rates of 60 000 rpm, the Williams flywheel presented itself as such an advantageous systems that the team set up a corporate arm to commercialize it.
At the beginning of 2010 the charismatic German marque Porsche commemorated the 110th anniversary of the first hybrid car in history, developed by its late founder Ferdinand Porsche, with the public presentation of a hybrid version of the track going flavour of its flagship. The Porsche 911 GT3 R Hybrid features two electric wheels at the front axle complementing the traditional 6 cylinder engine plus a KERS – the flywheel developed by Williams. This car made its race début that year at the 24 hours of Nurburgring held on the mythical 25 km circuit. Covering 25% more distance on each fuel tank, this car lead the race from the 14th to the 23th hour, with the thermal engine giving up its ghost 45 minutes from the chequered flag. The impact of this near feat was such that Porsche presented a new flywheel fitted race car in 2011, the 918 RSR, termed by the company as a “racing lab” for the technology, though so far it hasn't taken part in any race. This flywheel system is presenting such an advantage over traditional cars in endurance racing that it's actually becoming hard for these cars to be accepted by race organizers. In its latest version, the Williams flywheel has a maximum capacity of 340 Wh, but it can produce more than 200 hp (~ 150 kW). In time, the urge to “green” the sport and reduce energy consumption will likely force endurance race and championship organizers to set specific rules for regenerative systems, once and for all opening the doors to flywheels.
As much happened in 2011 in Formula 1, with improvements on KERS regulations resulting in most teams re-adopting it. This season the system is limited to 60 kW peak output and maximum storage of 100 Wh. The next big rules revision will come in 2014 when engines will take a huge downsize from 2.6 litres to 1.6 litres; this will be matched by an increase in flywheel peak output to 120 kW. Several companies are today developing flywheels to use in Formula 1 and motorsport in general. Notable among these is the Flybrid, which is coupled to the transmission, thus avoiding electric conversions with direct kinetic energy translation. Another charismatic car maker, Jaguar, is presently studying the introduction of the Flybrid system into its production cars. In the long run Jaguar aims at completely replacing the traditional combustion engine by small turbines functioning at constant, highly efficient regimes. Here's a corporate video on the application of the Flybrid to city buses:
So, why aren't flywheels popular?
Porsche owns Volkswagen, the largest car maker in Europe, and Jaguar is part of the Tata group, the largest car maker in India - could this be the dawn of a new wide-spread technology or just a curiosity restricted to 100,000 € plus cars? Answering this question may start by comparing flywheel state-of-the-art with present chemical battery solutions. This wasn't exactly a simple task, since data varies widely from source to source on certain technologies. For what it is worth, I stuck to the numbers found at Wikipaedia. Here's a compilation of energy density (energy per unit volume) and specific energy (energy per unit mass):
These are all round numbers that intend, above all, to present the relative positioning of each technology. Clearly flywheels do not present any drastic advantage above chemical batteries in terms of density, being somewhat above Nickel Metal Hydride, getting close to Lithium batteries but far from Zinc-Air batteries. The only advantage that flywheels may have in this regard is that they don't have funny names in their components; in the long run this may mean a cost advantage to flywheels: carbon is abundant, they have much longer lifetimes (more charge cycles per capital cost) and do not present the same recycling issues. But the lack of data, since presently few systems are in commercial operation, makes an assessment of this sort hard to perform. In any event, flywheels do not seem to be the most appropriate means of pure energy storage, hence it is not to be expected their success on applications of that genre.
Things start to look entirely different regarding specific power (power per unit mass) which tells how fast the system can store and/or deliver energy:
Flywheels not only are clearly ahead of everything else, they also appear at the antipodes of those systems that are ahead in terms of energy density and specific energy. The conclusion is straightforward: for applications where energy must be made available fast and in large quantities, or likewise stored rapidly, and the overall energy capacity isn't critical, flywheels are at a clear advantage.
An illustration can be useful, coming again to transport applications. What amount of energy does a car dissipate when braking, say a vehicle weighing 1 tonne and moving at 100 Km/h? To answer this we must dig into the old high-school Physics trunk for the kinetic energy expression:
KE = ½ · m · v2
Or in English: half the mass (m) times the square of velocity (v). In SI units the mass is 1000 kg and velocity is 27.(7) m/s; doing the math our illustration results into 385 kJ, or little over 100 Wh. Meaning that a flywheel with 1 kg and occupying about half litre could store the energy needed to bring a car moving at 100 Km/h to a standstill. Depending on how hard the brakes are stepped on, this energy can be produced in just a handful of seconds. If it takes 10 seconds, average power output of such braking will be 36 kW. While an 8 kg flywheel can easily deal with such power, a Lithium-ion battery would have to be much larger, weighting some 120 kg. This means a flywheel is useful even in fully electric cars, dealing with acceleration/deceleration, whilst a chemical battery package could be dedicated exclusively to vehicle range.
An answered question
I started preparing this post in the hope of finding an answer to the lack of commercial application of flywheels as a means of electrical or kinetic energy storage. With the writing finished, I can't say I achieved such an objective. There are a few commercial applications where flywheels are starting a shy market penetration, namely on the rail industry for regenerative braking and cable-less storage and as supporting infrastructure to load balancing within the electrical grid. But precisely where they seem to be more advantageous, in road transport, commercial applications simply do not exist. Car makers have so far chosen storage technologies for their hybrid solutions that seem to go against logic, preferring specific energy to specific power; especially so when the technology has been available for 15 years. Considering that only expensive car makers are developing flywheels (Jaguar, Porsche), could this be a cost issue? There isn't satisfactory data to answer that, but the flywheel's simplicity, long lifetime, and lack of rare and/or polluting materials seems to point against it. Nevertheless, the likely success of this technology on the electrical grid and rail industry, plus the unexpected push from motorsport may change things in the years to come.
Acknowledgement
Thanks to Engineer Poet, Rembrandt, and Jonathan for the help and motivation.
Flywheels are nothing more than discs or cylinders that spin around a fixed axis. The amount of energy a flywheel can store is proportional to its mass (m), the square of the speed at which it spins (w) and the square if its radius (r). The general equation for a solid disc is of this form:
E = 1/4 · m · r2 · w2
Flywheels have been known to Man at least since the Neolithic, when the first potter's wheels were built. At the dawn of the Industrial Revolution flywheels started to be employed widely as mechanisms to translate the work of steam engines into constant rotational motion; this solution is still employed today in modern internal combustion piston engines. At face value, a flywheel presents several advantages when compared to chemical batteries:
- Efficiency – charge and discharge are made with very small losses; as an electrical storage system a flywheel can have efficiencies up to 97%;
- Fast response - flywheels can promptly store huge bursts of energy and equally rapidly return them;
- Lifetime – flywheels built in the XVIII century for the early rail industry still work today.
- Maintenance/decommission – flywheels are kept in vacuum containers, functioning with zero material wear in modern designs; they also do not pose the chemical recycling/decommission issues of conventional batteries.
First commercial applications, first disappointments
This design was the first to be used in commercial applications back in the 1950s in buses that used heavy steel flywheels as their sole energy storage mechanism; these vehicles got the name of Gyrobus. They were employed in routes with low passenger loads that didn't justify full electrification. Gyrobuses could travel about 5 Km on a full charge between recharging stations. Recharging would take no more than 3 minutes, since flywheels can easily absorb high voltage electrical currents. These buses were also equipped with regenerative braking systems that recharged the flywheel.
Commercial Gyrobus services were started in Switzerland connecting Yverdon-les-Bains and Grandson in 1953, and soon after services were initiated in several routes in the city of Kinshasa in the Belgian Congo; in 1956, another service was started in Belgium, linking Ghent and Merelbeke. With time several problems came up, mostly due to the huge weight of these machines – the Gyrobuses used in Congo weighed more than 10 tonnes. They were hard to drive, damaged roads and above all, were electricity guzzlers; a tram used on the same service could easily slash electricity consumption by a third. By the end of 1960, all of the Gyrobus routes mentioned above had been scrapped.
This early commercial experiment brought to light the main disadvantages of flywheels:
- Weight – alloy flywheels can easily weigh several tonnes; for transport applications this can be a serious issue, due to the added inertia they impose on acceleration and breaking;
- Failure – if a flywheel fails by some reason at high rotation, it disintegrates, sending shrapnel as fast as bullets in random directions; to prevent damage they be must kept within an armoured container, adding further weight to the system;
- Bearings – alloy bearings proved to wear out quite rapidly, at first reducing efficiency and later rendering the flywheel useless; Gyrobuses required constant service because of this;
- Angular momentum – the momentum stored in the flywheel will act against direction changes, which in transport can make turns a complex task.
Though Gyrobuses haven't returned, research continued on the application of this technology; at the beginning of the 1990s there seemed to be engineering solutions to deal with all the issues above. The first big change was the introduction of new composite materials: they immediately tackled the weight issue but also ameliorated the container, these materials disintegrate into very small particles much easier to retain. Then the bearings issue was elegantly solved with magnets, creating a magnetic field that makes the flywheel levitate, thus spinning without any contacts with other parts (this requires a small consumption of electricity). Finally, the angular momentum has been addressed with the employment of gimbals, which, while not completely solving the issue, greatly reduce its effects.
All these solutions were employed by a company named Rosen Motors in a venture project aimed to produce a car without an internal combustion engine. The concept was based on a small gas turbine coupled with a composite electric flywheel for regenerative braking. In 1997 the first test run was made with the system installed in a Saturn production car; it covered 130 Km in about 2 hours, in what as an engineering breakthrough in many aspects, earning good reactions from several auto-makers. Months later the venture capital of $24 million ran out. Without a single auto-maker wishing to pursue the project the company was forced to fold.
Until today no commercial car has ever been fitted with a flywheel based regenerative system. Nevertheless, the research conducted by Rosen Motors proved that flywheel electrical storage had reached technological maturity, with many different potential applications opening up.
Many promises, few realizations
In the rail industry, flywheels have been used more extensively, though it can't be said their usage is widespread. They have been employed to store energy in electric locomotives to guarantee motion along non- electrified sections of rail lines and also to power small locomotives; beyond a few exceptions these solutions have remained mostly experimental. Flywheel powered trams can be particularly convenient in cities for they dispense with overhead electrification. Notwithstanding this fact, today there's only one commercial case to refer, in the Stourbridge line in London. Ever since 2002, 3 different units built by Parry People Movers have been tested, with 2 of them providing regular service since 2008. This sort of tram can also be fitted with diesel engines for longer distances; since the flywheel deals with all acceleration and braking, this engine can be designed to function at optimal revolutions per minute (rpm), thus being very small and efficient. So far the apparent success of these trams hasn't triggered any serious market penetration, though trial services and demonstrations have been run in other lines; a new trial service is set to start next month on the Mid-Hants Railway, also in Britain.
More recently, interest has been growing on the employment of flywheels as static batteries by the rail side. They can be used to stabilize the electric current feed to locomotives and also to store energy locomotives feed back to electric lines when braking. In 2009 the press reported a 5.2 million $ project to implement a rail-side 2.4 MW flywheel system on the West Hempstead line in Long Island, US. In parallel, the US company Urenco Power Technology, has been developing and testing smaller rail-side flywheels in the underground lines of New York, London, Tokyo and Lyon. Satisfied with the maturity of the system, at the beginning of this year a spin-off company was launched, Kinetic Traction Systems, with the specific aim of commercializing the technology.
Another US company has been working with similar objectives: Beacon Power, but with applications for the electrical distribution grid. The company developed a large scale flywheel that spins up to 16 000 rpm, with a maximum storage capacity of 25 kWh, that can be delivered back to the grid at maximum power rate of 100 kW (over 15 minutes). These flywheels are gathered in clusters that can be used together. Here's a video from 2009 describing the system:
Contrary to what this video suggests, Beacon Power seems quite healthy today, especially after the opening, already this year, of the first commercial flywheel farm, composed by 200 units and installed at Stephentown in New York. This flywheel farm has been deployed primarily as an electricity frequency stabilizer, a perfect match to the flywheel's prompt discharge/recharge characteristics; beyond that, the farm is used to store cheap electricity available in the grid during the night. The company maintains close collaboration with US government authorities through several development programs with broader aims at grid wide stability. The state of Pennsylvania also seems interested in this technology, with capital already committed to a flywheel farm. Further applications are being envisioned, particularly the marriage of flywheel farms with wind farms in order to decentralize load balancing; in this case the system will also be coupled to a thermal generator feed by diesel or gas, that once again can be greatly optimized by the prompt aid of flywheels. If there's an application where flywheels seem bound for serious market penetration it is this one; nonetheless, considering how long intermittent technologies have been penetrating the electricity production market, these seem yet small steps.
When fun meets technology
To fully understand the flywheel state-of-the-art, a final (and longer) story needs to be told. In recent years flywheels took a boost from an unexpected source: Formula 1. In a bid to “green” the sport and provide deeper technological transfer to the auto industry, the FIA introduced new rules for the 2009 season that allowed teams to optionally fit their cars with a regenerative electric storage system with a fixed maximum capacity. This system was baptised Kinetic Energy Recovery System - KERS. These new rules seemed to put those teams opting for the KERS at an advantage, but their late introduction gave little room for the new technology to be developed and absorbed, especially concerning the extra weight applied on the cars. The 2009 season ended up dominated by one of the low budget teams that opted to not even develop a KERS.
After this débâcle no F1 team used KERS in 2010, but a left over from 2009 brought the curiosity of many: while most teams opted for chemical batteries, the Williams team had developed a flywheel, that though not successful on track, seemed quite promising for the road. With maximum spinning rates of 60 000 rpm, the Williams flywheel presented itself as such an advantageous systems that the team set up a corporate arm to commercialize it.
At the beginning of 2010 the charismatic German marque Porsche commemorated the 110th anniversary of the first hybrid car in history, developed by its late founder Ferdinand Porsche, with the public presentation of a hybrid version of the track going flavour of its flagship. The Porsche 911 GT3 R Hybrid features two electric wheels at the front axle complementing the traditional 6 cylinder engine plus a KERS – the flywheel developed by Williams. This car made its race début that year at the 24 hours of Nurburgring held on the mythical 25 km circuit. Covering 25% more distance on each fuel tank, this car lead the race from the 14th to the 23th hour, with the thermal engine giving up its ghost 45 minutes from the chequered flag. The impact of this near feat was such that Porsche presented a new flywheel fitted race car in 2011, the 918 RSR, termed by the company as a “racing lab” for the technology, though so far it hasn't taken part in any race. This flywheel system is presenting such an advantage over traditional cars in endurance racing that it's actually becoming hard for these cars to be accepted by race organizers. In its latest version, the Williams flywheel has a maximum capacity of 340 Wh, but it can produce more than 200 hp (~ 150 kW). In time, the urge to “green” the sport and reduce energy consumption will likely force endurance race and championship organizers to set specific rules for regenerative systems, once and for all opening the doors to flywheels.
As much happened in 2011 in Formula 1, with improvements on KERS regulations resulting in most teams re-adopting it. This season the system is limited to 60 kW peak output and maximum storage of 100 Wh. The next big rules revision will come in 2014 when engines will take a huge downsize from 2.6 litres to 1.6 litres; this will be matched by an increase in flywheel peak output to 120 kW. Several companies are today developing flywheels to use in Formula 1 and motorsport in general. Notable among these is the Flybrid, which is coupled to the transmission, thus avoiding electric conversions with direct kinetic energy translation. Another charismatic car maker, Jaguar, is presently studying the introduction of the Flybrid system into its production cars. In the long run Jaguar aims at completely replacing the traditional combustion engine by small turbines functioning at constant, highly efficient regimes. Here's a corporate video on the application of the Flybrid to city buses:
So, why aren't flywheels popular?
Porsche owns Volkswagen, the largest car maker in Europe, and Jaguar is part of the Tata group, the largest car maker in India - could this be the dawn of a new wide-spread technology or just a curiosity restricted to 100,000 € plus cars? Answering this question may start by comparing flywheel state-of-the-art with present chemical battery solutions. This wasn't exactly a simple task, since data varies widely from source to source on certain technologies. For what it is worth, I stuck to the numbers found at Wikipaedia. Here's a compilation of energy density (energy per unit volume) and specific energy (energy per unit mass):
| Energy Density (Wh/l) | Specific Energy (Wh/kg) | |
| Compressed Air | 17 | 34 |
| Supercapacitor | 35 | 20 |
| Lead Acid Battery | 40 | 20 |
| Nickel Metal Hydride | 90 | 90 |
| Lithium-Iron-Phosfate | 220 | 110 |
| Flywheel | 210 | 120 |
| Lithium-ion | 440 | 175 |
| Zinc-Air | 1600 | 470 |
These are all round numbers that intend, above all, to present the relative positioning of each technology. Clearly flywheels do not present any drastic advantage above chemical batteries in terms of density, being somewhat above Nickel Metal Hydride, getting close to Lithium batteries but far from Zinc-Air batteries. The only advantage that flywheels may have in this regard is that they don't have funny names in their components; in the long run this may mean a cost advantage to flywheels: carbon is abundant, they have much longer lifetimes (more charge cycles per capital cost) and do not present the same recycling issues. But the lack of data, since presently few systems are in commercial operation, makes an assessment of this sort hard to perform. In any event, flywheels do not seem to be the most appropriate means of pure energy storage, hence it is not to be expected their success on applications of that genre.
Things start to look entirely different regarding specific power (power per unit mass) which tells how fast the system can store and/or deliver energy:
| Specific Power (W/kg) | |
| Zinc-Air | 100 |
| Lithium-ion | 300 |
| Nickel Metal Hydride | 600 |
| Lithium-Iron-Phosfate | 3000 |
| Supercapacitor | 3500 |
| Flywheel | 5000 |
Flywheels not only are clearly ahead of everything else, they also appear at the antipodes of those systems that are ahead in terms of energy density and specific energy. The conclusion is straightforward: for applications where energy must be made available fast and in large quantities, or likewise stored rapidly, and the overall energy capacity isn't critical, flywheels are at a clear advantage.
An illustration can be useful, coming again to transport applications. What amount of energy does a car dissipate when braking, say a vehicle weighing 1 tonne and moving at 100 Km/h? To answer this we must dig into the old high-school Physics trunk for the kinetic energy expression:
KE = ½ · m · v2
Or in English: half the mass (m) times the square of velocity (v). In SI units the mass is 1000 kg and velocity is 27.(7) m/s; doing the math our illustration results into 385 kJ, or little over 100 Wh. Meaning that a flywheel with 1 kg and occupying about half litre could store the energy needed to bring a car moving at 100 Km/h to a standstill. Depending on how hard the brakes are stepped on, this energy can be produced in just a handful of seconds. If it takes 10 seconds, average power output of such braking will be 36 kW. While an 8 kg flywheel can easily deal with such power, a Lithium-ion battery would have to be much larger, weighting some 120 kg. This means a flywheel is useful even in fully electric cars, dealing with acceleration/deceleration, whilst a chemical battery package could be dedicated exclusively to vehicle range.
An answered question
I started preparing this post in the hope of finding an answer to the lack of commercial application of flywheels as a means of electrical or kinetic energy storage. With the writing finished, I can't say I achieved such an objective. There are a few commercial applications where flywheels are starting a shy market penetration, namely on the rail industry for regenerative braking and cable-less storage and as supporting infrastructure to load balancing within the electrical grid. But precisely where they seem to be more advantageous, in road transport, commercial applications simply do not exist. Car makers have so far chosen storage technologies for their hybrid solutions that seem to go against logic, preferring specific energy to specific power; especially so when the technology has been available for 15 years. Considering that only expensive car makers are developing flywheels (Jaguar, Porsche), could this be a cost issue? There isn't satisfactory data to answer that, but the flywheel's simplicity, long lifetime, and lack of rare and/or polluting materials seems to point against it. Nevertheless, the likely success of this technology on the electrical grid and rail industry, plus the unexpected push from motorsport may change things in the years to come.
Acknowledgement
Thanks to Engineer Poet, Rembrandt, and Jonathan for the help and motivation.
“To be thrown upon one's own resources, is to be cast into the very lap of fortune; for our faculties then undergo a development and display an energy of which they were previously unsusceptible.”
—Benjamin Franklin
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Great subject to write about.
My guess for the lack of commercial interest in flywheels is two things:
#1 - It doesn't seem like it would work at the small levels that we can conceptualize. Think about walking around with a boom box or power drill or some other battery-operated item. The idea of having a battery charged full of energy to power it . . . that works. Your mind can picture that working well. But trying to imagine the device working because it has a spinning wheel of some sort inside it to store the power. . . not so much. It's a matter of scale. Things that are practical at larger scales aren't always practical at smaller scales, and engineers are still humans susceptible to biases of this kind. And the younger the technology is, the more that people's perceptions affect its speed of development. There is a lot of chicken & egg issues with developing new technology. People don't think something is viable, because there is no evidence, because people don't think it's viable.
#2 - The flywheel really shines when you factor in the indirect costs of all the various power storage options, and we have a civilization-wide problem of weighing all those costs much too lightly. By this I mean the costs of mining the materials to produce the item, the maintenance costs, the feasibility of waste disposal and recycling its used parts, the environmental damage at every step of the way, etc. We tend to hire one set of people to make something work and then we hire a totally different set of people to weigh the practicalities of it.
A power drill that uses a flywheel instead of an accumulator and goes "Wmmmmm" as it is recharged in 60 seconds would be an instant hit if it were light enough and could be used for some tens of minutes. People would probably understand it better then the chemistry of an accumulator, "there is a spinning thing in it that revs upp when charging and revs down when I drill and when it gets tired I recharge it".
But to use a similarly small-scaled example (not entirely the same) as your power drill, there are existing flywheels that can be completely viable as tool power, such as the flywheel-based Bike Exercise machines. I've stashed one in the shop and haven't constructed the PTO 'Power Take Off' for it yet, or the tool table that will replace the handlebars, but I can see no reason why it won't easily convert leg-power into a steady source of kinetic energy for a great number of cutting, drilling, grinding and sanding and smaller lathe jobs, (to name a few) taking a much more balanced input from the builder than what's required in holding all the tools and pieces for the fully manual versions of those tasks. I'll also have a generator available in the drive-line, in case I want to charge batts or do some other electrical process with it..
I've found that sanding and scroll or coping saw work can become very labor-intensive, but the effort is as much in holding everything in the right positions as it is in the cutting work.
Another outlet for such a source is putting some kind of simple pedal/flywheel arrangement into a narrow slot under a kitchen counter, with the PTO up on the counter, for connecting to various cutting, grinding, mixing and blending tools.
A few million Treadle Sewing machines were a great early demonstration of the brilliance of this approach, and most didn't even have ball bearings. I've still got Three of the Tables for similar future task-ideas. (And one still with the Sewing machine, which is supposed to be tough enough for leather and other heavy stitching.)
Bob
A great and highly informative article!
The biggest impediment to the adoption of flywheel technology , at least for stationary uses, may be the lack of manufacturing scale and standardization.I have recently been inside a furniture factory where dozens of machines are equipped with powerful electric motors-ten to a hundred horsepower, typically, which are loaded and unloaded intermittently but regularly over cycles lasting a few seconds-the operator puts in a dresser top to be sanded, it takes five seconds to pass thru the machine, and then five to ten seconds to feed in another piece, for example .
If the motor were to be equipped with a flywheel, it could be smaller, and possibly cheaper to make, considering copper prices, etc, and it probably would use somewhat less electricity.I have been told that the monthly electricity bill for the plant is about eighty thousand dollars.
We old farmers have a working acquaintance with flywheels that goes back a long way.
A haybaler is a great example;most older ones are fitted with a flywheel that wieghs around fifty to one hundred kilograms.This allows the tractor engine and the drive line-all the gears, bearings, shafts, etc-to function at nearly constant speed and power levels as the hay is rammed into a compact bale by repeated blows from the "piston head" of the baler, much as the piston of an ice compresses the fuel and air charge on the compression stroke.It would require an extremely large engine and extremely heavy duty drive line to compact the hay without this flywheel.
We used to have have an old wood saw , built to cut slab wood(the outside pieces left from sawing logs into boards) into stove lengths, which had a flywheel on it. The engine, which was a ten horsepower or so one cylinder aircooled unit, would run at close to full throttle all the time,if you fed the slabs thru it steadily.
The saw (and the engine of course) would slow down somewhat while actually making the cut, but regain the lost speed while retracting the feed table( which swung in and out from the revolving but otherwise stationary blade) and advancing the slab to make the next cut.
We could saw about as fast with the ten horsepower engine on this rig as we could with one we obtained later, which had no flywheel, this one pto driven by a sixteen horsepower tractor , while using considerably less fuel.
Just about all these old saws have scrapped due to the fact that local sawmills and therefore slabs are things of the past too.Wood is too valuable to give away of sell slabs cheaply these days anyway.
If anyone runs across one in working order, PLEASE understand that it is EXTREMELY dangerous to saw ROUND wood on it-meaning anything from a tree limb to a log.These old saws worked ok with slabs because you could turn the flat surface down and push the slab back against the vertical rear part feed table, making it reasonably stable as it met the blade.
The slightest unintentional movement of a piece of round wood can cause the blade and round wood to bind or pinch with potentially disastrous results-the blade might explode, or the wood might be thrown off at bulletlike speeds.
There is still considerable demand for squarebaled hay, which fetches a modest price premium from horse owners, etc, and lots of small farmer still use the old style balers simply because they have one that still works, and because the newer, more efficient round balers are so expensive.
Most people here in the states refer to electric motors rather than electric engines-this very minor point might clear up some confusion among some less technically oriented readers, when added to the second point that an electric motor and an electric generator are essentially one and the same basic machine, and that one single machine can indeed be easily made to function both ways, as both motor and generator, if it is so designed.Undoubtedly the vast majority of regular readers understood the article without needing this minor clarification , but some of us who are less technically oriented need to have our hand held a little more tightly than others when crossing cognitive streets.
OFM, former professional holder of small uneducated hands.;-)
Thanks Mac. Great tools!
You had me thinking about an old Maine Story from Marshall Dodge, where they are waiting for the motor on the Bluebird, their fishing boat, to finally cut out.. and it keeps sputtering along..
"G__- D__ed FLYWHEEL!..." (It was cause of that flywheel that if she fired once every three, four cycles, she was 'hummin pretty good'..)
The woodworking tools story made me think of the flywheels typical on stamping presses to allow a sharp mechanical impulse load to be served by a small, continuous electrical load, and also of a 40,000ton forging press which was designed without any such significant energy storage and which was connected to the electrical grid at my company without adequate attention by the customer to the limitations of my electrical grid, despite clear and repeated warnings. You can't tell some people anything.
What I really want to know is what the TOC (total operating cost) would be over say a 25 year period compared to batteries. The thing which gets me intrigued is that it could reduce the need for peaker plants and it could mean you could treat your entire fleet of renewables as 24/7 baseload power generation. I didn't catch any forecasts for how long one of these systems could expect to last, anyone want to hazard an educated guess?
I too think the potential for flywheels associated with intermittent renewables is huge, really huge. Luckily the programmes taking place in the US shall pave the way to that.
Regarding longevity, electric bearings flywheels are something entirely apart from the machines we are used to, there's no wear of internal materials during operation. My wild guess is that what will eventually set the longevity of a modern flywheel is the external electric engine. Meaning that they might only need to be retrofitted to get back to operation.
This article has to be one of the coolest I've seen on TOD. So really now all we need to know is how much one of these units actually 'costs' to find out how ready the technology is for wider deployment.
I'm surprised you didn't mention that Volvo is working on bringing this tech to passenger cars.
Another thing I saw from the flywheel wiki page was this:
I guess magnetic bearings gets around this issue though...
Interesting topic.
Mechanical or magnetic the same forces have to be dealt with.
Wouldn't horizontally orientated North & South and tilted up equal to the latitude work anywhere?
For lowest change in angular momentum forces, you need to avoid any change in the spinning disk axis, so the disk needs to match the equator-circle-plane.
So it would be locally 'edge on' at the equator, and lying flat at the poles. (issues are now seasonal, not daily) (and always edge on, viewed from the Sun)
Of course, Gravity forces still work the way they always do, so the bearing design would get more challenging on a tilted wheel.
Yep, strange old things gyroscopes.
An interesting side effect of aligning one EW instead of NS is that as it tries to turn once a day it is possible to get one to provide energy from the earths rotation. Even above the amounts required to keep the gyro spinning. An odd little experiment but it'll never provide useful amounts of energy. Not much torque and only 1/1440 rpm.
There were some "perpetual motion" devices that tried to exploit it, but the bearing technology let them down.
EDIT - For vehicles to avoid the dreaded cornering issue it's best to mount the gyro vertically, and ideally have two counter-rotating. This setup works as a stabilizer on boats.
Yeah, flywheels on vehicles keep hitting up against this problem. For a car, you're probably best off with a vertical axis flywheel, but every time you pitch - i.e, start up or come down over a hill, turn into a sloped driveway, accelerate, brake, etc. - your car's going to pick up a roll. Likewise, when your car has a roll, like it would in a banked turn or even a flat one since the body will roll to the outside of the turn, or riding over an undulating surface, the car will pick up a pitch. Because most direction change in a car is in yaw, a vertically-mounted flywheel is preferable to horizontal, but even at that, the flywheel taking on or dumping torque would have to get transmitted to the car.
Sure, you can do the counterrotating pair, but at greater cost, complexity, and weight. Besides, the flywheels would still generate the reaction forces; they'd just get borne by the structure(s) that connect them. And yes, you can use a gimbal but you add problems of power transmission in and out of a moving apparatus and take on considerably more volume and weight.
And even sitting in a garage, unless that flywheel's axis is parallel to the Earth's, there's going to be a mechanical load caused by that 0.00069RPM rotation the entire car is undergoing.
Finally, there's the concept of "undesired sudden energy release" of the sort that might occur in an accident or even a seal failure. Whatever causes it, all that spin has to go into either motion or heat...or chemical reactions...
Modern systems like the flybrid (http://www.flybridsystems.com/Technology.html) work to diminish the gyroscope effects from angular momentum conservation by using very high rotation rates...angular momentum is proportional to rotation rate while energy storage goes like rotation rate squared, so large rotation rate gives maximum energy storage for a given angular momentum. It doesn't go away, but it doesn't seem to be a big problem in formula 1 applications.
I think the undesired sudden energy release is one of the main reason flywheels have not seen widespread application in the transportation market. Their strength is their ability to release a lot of energy fast, which can be a big problem in some cases. The flybrid simply doesn't store that much energy (400 kJ=111 Whr which is less that 1/10 of a 2010 Prius battery).
Quite a ride.
Still the merry-go-round rat race loops do have entertaining moments. If and when NASCAR starts to look beyond the ICE we will have evidence that a real sea change is occurring in the USA. I think NASCAR is first going to fuel injection from carburators in 2012 (if they don't put it off again). I think their big 'greening' (I hate the term but it truly fits here) was adding ethanol to the fuel mix a year or few back. Real innovators aren't they!
I didn't know about this. Flybrid announced a partnership with Volvo last June, fresh news that I missed.
There are different ways to deal with this.
First, just point the axis towards the north star. works everywhere, not just at the equator. this will totally get rid of the effect, independent of season. pointing it towards the sun would be a mistake, it needs to be in north-south direction, in 3D, parallel to earths axis of rotation, and everything will be fine.
secondly, use a flywheel that is longer along the axis, a cylindrical shape like the shape of an oil barrel or even longer. That is also desireable because it reduces strain on the material used to make the flywheel. It will reduce the effect. Of course when you build flywheels you calculate all this to see what the best solution is. I think with a good design the force exerted by the gyroscopic effect might be rather tiny compared to the force exerted by the enormous weight of the flywheel, which the bearings have to deal with anyway.
i say this problem with the gyroscopic effect is a non-issue. Its a minor design challenge that any big flywheel design needs to take into consideration, but every big flywheel design that works well is a testiment to the fact that this problem can be solved and is being solved in the design process.
(side note: also, it would be possible to construct a flywheel arrangement that can be used to extract energy from earths rotation, slowing down its rotation. Or the other way around, if we ever feel the need of spinning up our planet a little more for shorter days, we can do it with flywheels, investing energy into speeding up earths rotation.)
Luis,
there is at least one reason why flywheels are inapplicable at least in moving objects:
Like in a gyroscope, the moving mass has a high angular momentum, which means that the flywheel (and the object where it is mounted on) strives to keep the same orientation.
So if the flywheel was e.g. mounted on a car it would strive to run in the same direction and difficult to go around curves.
This obstacle was also mentioned in a German report about storage technologies.
If interested, check out the 911 GT3 Racing Hybrid w/electro-magnetic flywheel.
There's plenty of articles about it, but again, hardly practical at this point. It may turn out to be a good proof of concept for this application, but the diesels are showing up most everything in endurance racing anyway :-)
http://www.popularmechanics.com/cars/alternative-fuel/hybrids/porsche-91...
Hmmm..
I was just drawn off into an Earth Orbit, thinking about flywheels in space and all those fun discussions we could have about lifting costs and microwave transmission beams.
For a minute (less than 20 seconds, really), I was stuck on what you can hold STILL in order to extract the energy from.. and apply spin energy TO this flywheel, and then of course I landed upon having Two Opposing Flywheels, one with the rotor, one the stator, or perhaps a Stator sandwiched between two or more opposite/complementary-rotors, where a little bit of speed-control offers some of the attitude management for the satellite.
Fun to think about.. almost as fun as the little Flywheel Steel Rolly Car on my desk that my daughter has all-but stolen from my toy collection. (Kinda like this one.. http://www.bonanza.com/listings/Zecar-Kikkerland-Flywheel-Wind-Up-Race-Z... )
The ISS is stabilized by flywheels.
"Attitude control is maintained by either of two mechanisms; normally, a system of four control moment gyroscopes (CMGs) keeps the station oriented," -- ISS, wiki
Morning, Johkul,
I forgot about the one flywheel in my life that brought me more fun than just about any other single tech toy I ever saw-although this one never belonged to me.
It belonged to a physics teacher who had a pretty good home hobby shop and excellent hands on skills, and made it himself.
He took a very large briefcase, of the sort used by some teachers to tote around students papers , etc, and inside it he mounted a battery operated drill motor and a heavy gear salvaged from someplace to act as the flywheel, with the two connected by means of a very tiny chain and gears , and a free running clutch.
There were two switches-both concealed.He could turn one on, and rev up the gear, which would spin for a minute or two, fast enough to make the briefcase act "alive", after you turned that switch off.
The other switch was mounted in the handle in such a way that if you picked up the briefcase, it turned on the motor.
His favorite trick was to place this briefcase somewhere handy in plain view of the class and ask an unsuspecting student to bring it up front for him.I've told that one girl actually screamed and threw the briefcase and ran out of the classroom because startled her so badly to have a "possessed" briefcase come to life in her hands.Probably an exaggeration of course, just as the fish I used to catch get bigger every year.
I'm going to make a couple of these myself someday if I ever have the time and inclination on the same day.
Mac;
Not to date your age or anything, but I wonder if you and my Dad were ever in the same crowd. He spent some years in Sacramento,CA and Houston,TX in the Air Force, early 1950's, and has told me a similar tale, where the other great trick was to take the Gyro suitcase into a hotel lobby and set it down well off the edge of a table or counter, where it would hang there by a mere corner, as some bellhop would dive frantically for it, expecting the drop.
It seems possible that once this bag was seen around, others would be built soon after..
Bob, son of Frank Fiske, Jr
Back to Ya, Bob
My Dad was too young for WWII by a hair, although a couple of his brothers served, and he missed Korea, so I'm told, because my first sister and I were there at the right time as dependents. He was never in uniform, although there are current and many former members of the armed forces in my family.
The teacher who had the gyro in a briefcase was a friend of mine back in the seventies.I can't remember where he told me he first saw such a gyro toy, but it may have well been in the service, as he was a vet.I do remember that he had seen one himself before he built his.
It must have been a lot of work, considering that he probably had to modify the drill motor considerably to get it to run on a couple of dry cells.Low voltage, battery operated tools were not mass produced in thise days.Of course it is possible that he used some other type of motor, such as a starter motor salvaged from a motorcycle or something along those lines-it's been a long time, and my memory is not all it could be.
But there were a few small power tools around made to run on twelve volts, mostly used by in- the- field mechanics who generally had close easy access to 12 volt batteries -these tools used cords and alligator clips to clip directly too the battery of the machine being worked on.
As I recall it, the one Dad saw had a couple AirForce Low-volt Gyros in it, at 90 degrees to one-another.. which was how they were able to power them.. tho' it was said to be D____ed heavy, as you can imagine..
As you mentioned that Slab Cutter, I'm chewing over a flywheel rig (for some months now) that would drive my Bow-saw for some more standardized cutting of pallette-wood. I can get loads of pallettes around here for now.. and am also playing with some fun Lever and Cog setups to pry these pallettes apart with. Maybe it's overkill on the brain-work.. but for me, it's like my version of a Sunday Crossword Puzzle.
Great article, but more emphasis should be on rail transport.
Flywheels have the greatest advantage where quick charge and discharge applications are required. Batteries lose too much energy in this application due to internal resitance energy loss (goes up with square of ampheres) and lower life span in quick charge applications. Super capacitors have high cost and questionable life cycle.
The best application for flywheels is storage of electrical power for rail transport. They are used on the London Underground system to reduce peak power load and capture energy during braking of trains, IIRC. If freight rail systems were electrified, flywheels could be positioned in mountain territory to capture energy of train descending, then later provide power to get another train up grade. Same goes for high speed rail and intercity rail systems where energy normally lost in stopping at a station could be stored in a flywheel for getting same train back to 90% of its operating speed.
No application for air transport, airlines are toast in post peak oil world.
I had the impression that per KWhour of storage capacity flywheels are pretty pricy. What they are good at is providing lots of shorts bursts of power, and absorbing short bursts as well. I doubt they are economic for storing power for several hours however. Energy storage is going to have many niches, flywheels probably occupy one niche, high frequency and low loss, but pricy. Some other storage is needed for smoothing transitions such as starting/stopping thermal turbines in response to load conditions, here you need storage thats effective for roughly the timespan of spinup spindown of the turbines (including gradually raising/lower operating temperatures. Not sure if flywheels play well there. Lastly we would love to bridge day/night or even seasonal changes in demand and renewables generation.
Most larger flywheel units operate in a near vacuum so having them spinning for an hour or two is no problem, but some slight loss occurs. The issue of cost is mainly a problem of return on investment. Flywheels have a very long service life, unlike batteries that last five to six years in this type of service. So if the cost of money is high flywheels are at a disadvantage somewhat.
As application in electric power production you mention smoothing demand, but coal fired power plants like nuclear power plants keep steam turbines running at constant RPM (they must do this to keep HZ from fluctuating) and they throttle the steam in response to load. Utilities try to use gas turbines to handle moderate duration load variations like increase from noon to 5pm in the summer. These gas turbines (either simple gas fired or combined with steam cycle) can be quickly started and stopped to handle the load variation.
The overall load on the grid does not vary a large percentage from minute to minute but may over the course of several hours, so I doubt flywheels could help with power storage for the grid to any large degree. Flywheels have better application to frequent large variations in power draw in small systems like rail transit and sections of electrified rail, both high speed and freight.
I would think you would get a lot of loss from the bearing friction. For short storage of electricity an ultracapacitor makes more sense.
Except for very large flywheels no bearings are needed. The flywheel spindle or shaft ends are magnetically levitated. Some power is required for this and can be drawn from the energy stored in the flywheel. A flywheel has the capability to more easily control outflow of energy than a capacitor since the device is simply storing rotational energy that provides torque to a motor.
On the other hand a capacitor uses very high voltage to get the "ultra" in ultra capcitor and is thus more difficult to control the storage and release of the energy, or at least more complicated.
Baloney,
Magnetic levitation is a bearing because it "bears" up the weight of the shafts. try lifting a couple of tons a see how much energy it takes. Remember this energy is being used all the time you need to store the energy. You then have to convert mechanical energy to electric.
A capacitor can be charged with any voltage up to its rated value. A couple volts in the case of ultracapacitors. They get higher voltages by wiring them in series. But care must be taken to not over charge the individual capacitors. The ultra is high value not high voltage.
You can easily control its discharge by the load you put on it. a 10 cent resistor for example. much simpler once you understand ohms law.
http://en.wikipedia.org/wiki/Electric_double-layer_capacitor
http://gigaom.com/cleantech/how-ultracapacitors-work-and-why-they-fall-s...
http://www.maxwell.com/products/ultracapacitors/
The size of the capacitor is increased by using activated charcoal to
increase the surface area of the plates of the capcitor.
People keep saying that, yet I keep thinking "Zeppelin"
I knew a woodturner who got a commission for the World's Largest Myrtlewood Bowl. He laminated up a huge, maybe 10 foot, blank and after a while the screws holding it onto the lathe came loose. In celebration of the $$ deposit he had traded in his old reliable vintage 53 Chev pickup for a new one. The Bowl bowled itself through the barn door; then did in the grille, hood, cab and tailgate of his hour old truck; then down the driveway, across California Highway 1, over the beach, and floated south at about two knots.
Luckily, the dealership still had his old '53 and he looked upon the experience as retribution for being unfaithful to her and took her back. Of the lessons here, perhaps the most important one is to keep to the 'from' side of a vertical flywheel and assume nothing.
I have other stories of personally being far too near failing mechanisms with a 300 mph peripheral speed and colossal angular velocity. Exploding lead acid isn't nice either. I think I could gain affection and confidence in water/gravity. But you need water and a hill.
Nice to see an article on this. Thank you.
I remember seeing a bus, somewhere in northern europe that had a flywheel. As I recall it was direct drive rather than electric: the purpose was to use the energy to get the bus moving again, as opposed to using the ICE to do this. It always struck me as a good usage, where weight was not particularly crucial (i.e. larger vehicles) but where frequent start/stopping was occurring. I thought that garbage trucks would be a good candidate for this too.
Forgive my poor physics, but if the wheel turning in a specific direction tends to make the vehicle "want" to turn in that direction, would not two counter rotating wheels offset this? I haven't played with gyroscopes enough to test this out..
In theory yes, two flywheels rotating at the exact same speed in opposite directions would cancel out the angular momentum. The problem is that if they don't rotate at the same speed they'll send a relevant vibration to the vehicle. So far no design that I'm aware of has been able to technically achieve the required level of synchronization.
In the Williams design the two flywheels are geared together. The big problem is matching the speed of the flywheels with the speed of the vehicle. As the flywheels slow down the vehicle speeds up, and vice versa. For this they use a variable-ratio mechanical coupling that requires some fancy frictional materials.
This is rather garbled.
'Vibration' is not clairvoyant, it does not change based on the speed matching.
Vibration is purely a function of imbalances.
Two flywheels cannot cancel our momentum, which is more like a mass multiplier.
The issue with momentum, is the CHANGE of momentum, needs a force, and that CHANGE for a flywheel, includes any change in AXIS (not just RPM) - which is why mobile applications are less commercially practical.
Some have tried this approach. But if you turn counterrotating flywheels, each individually induces a torque. Sure these torques cancel out, but a lot of stress is created in the connecting axle and the flywheels theselves. Since outside of the flywheel system this torque is cancelled out, it is easy to imagine yanking the thing hard enough that it catastropically fails. The key is to keep the axis of the flywheels pointing in the same direction. A vertically oriented axis on a perfectly horizontal surface will do fine. But try to go up/down a ramp, and you will need gimballing.
I used to ride an in-line twin motorcycle with shaft drive. Revving the engine used to produce some interesting movement. Taking left and right hand bends was always slightly different. You got used to it but the effect of the rotation was quite noticeable especially if the engine revs changed in the middle of a manoeuvre..
NAOM
Does anyone have costs for these flywheels. It seems component wise that carbon fiber is a much more sustainable substance than any battery technology. Could flywheels replace batteries for storage of solar PV and wind generators. I wonder if one could set up lines of lightning rods and capture lightning strike energy and store it in such flywheels.
I purposefully avoided this issue. Flywheels themselves are very simple and cheap pieces of equipment, the costs are on the remainder paraphernalia to armor them and integrate them with the full system. Both Williams and Flybrid claim major cost reductions relative to similar systems based on chemical batteries. Porsche also claims a 50% reduction on weight (sorry couldn't find reference).
All in all costs can't be properly assessed at the moment for two reasons: first for commercial flywheels either do not exist, or operate in niches for which there isn't much comparison; and secondly because their longevity is unknown - long, but how long?
Armor is certainly needed. Long ago when funding was tight I worked in lab where my handy (practical genius) friend had built a bench centrifuge and achieved critical new capabilities. He used to set it going while he had his coffee. I stopped taking my coffee break in his room because I found sitting next to a naked few kilos rotating even at a modest 12000 rpm, too unnerving.
I would think that instead of merely heavy armouring, that one would put a bunch of FES devices effectively underground at a reasonable depth, surrounded by an absorbtive medium, be it soft earth, sand or peagravel perhaps. Safe Distances between them could be figured out, as could perhaps some level of early warning, so that any vibrational or similar indication that a Flywheel is becoming unstable or disintegrating would trigger a program that would immediately start dumping power from its immediate neighbors off to the more distant ones, so that they aren't as susceptible to multiplying the breakdown.. ??
Jok
Daresay you are right, but what sort of scale? Domestic or neighborhood? If latter, a modern District Heating system with CHP and a mini-grid would be further up my priority list for a communal energy setting. Also, anything rotating more than a few thousand revs for any length of time will need a vacuum. I think the Swedes planned a huge one in an old granite mine in a mountain a while back? Not sure I would want them in an earth tremor zone. Vertical axis and magnetic levitation seems to have been produced for small vertical axis wind turbines on the top of lamp posts, so the control gear might exist, (er ... might), at potentially low cost??? Maglev looks a daft idea for wind turbines but ... flywheels? http://www.alibaba.com/product-gs/407510794/Magnetic_Levitation_Vertical...
I used to work at this place. More power in a flywheel than you could wish for.
JET - Joint European Torus
Each flywheel generator is capable of providing 3750 megajoule (1041 kWh)of energy for the JET pulsed power systems, with a maximum of 400 megawatts power output. The integrated rotor flywheel consists of a 650 tonne/10 m diameter rim carrying the poles of the machine.
http://www.jet.efda.org/focus-on/jets-flywheels/
Hi C.R., any idea what kind of thrust bearings are used ? - j
This is clearly incorrect, and should be edited.
It is more correct to claim, as wiki does
flywheels with magnetic bearings and high vacuum can maintain 97% mechanical efficiency, and 85% round trip efficiency.
and one alternative, as a reality check, is
Modern pumped storage has about an 83% round trip efficiency for energy storage (provided the two reservoirs are close together), and the facilities have a very long life. With variable frequency controls, very fast regulation functions are now possible for pumped storage facilities, similar to the function of flywheels or batteries for voltage support and frequency regulation, but at lower cost
and this from flybrid's website
The systems are also very efficient with up to 70% of braking energy being returned to the wheels to drive the vehicle back up to speed. The devices are readily recycled and relatively inexpensive to make as they can be made entirely from conventional materials.
round trip efficiency is the important number.
"This is clearly incorrect, and should be edited."
I beg to differ. Pumped water energy storage has losses associated with parasitic turbulent flow in the out flow of the turbines. The analogous loss in flywheels that have electrical input and output of stored energy is resistive losses in electrical conductors. These resistive losses can always be reduced more by adding more copper to the conducters, but always at additional cost. So round trip efficiency is whatever the customer is willing to pay for. Since there are no serious customers, there are no serious design calculations.
Pumped storage has been used for many years by PG&E, the public utility in northern California, at a facility just east of Oroville, CA. The justification was, I think, to allow them to operate some nuclear plants elsewhere in their service area over night at full power. They might even have considered flywheels during initial proposal writing. Certainly they were aware of the flywheel storage of electricity that was then operating at Lawrence Rad. Lab. for the large particle accelerator there. That flywheel was designed and installed in order to smooth the power draw from PG&E, and PG&E engineers were very much aware of its existence. For cycling with a 24hr period, the market seems to have decided that pumped storage is better than flywheels.
For applications that have a shorter cycling period, flywheels are better, but still have competition from the very simple expedient of just letting the small expected energy saving go to waste. For example, the JETS flywheel worked very well at providing short bursts of very high power, to a fusion power research facility. But how many fusion power research facilities will there be in the post carbon world? And how much energy will be saved? And does it make sense to spend the money, if the only benefit is saving energy? In fact the recearch facility could not have received power from the grid, if it had insisted on receiving in the kind of short bursts that it needed. It would never have received regulatory approval without this kind of power shaping as part of the plan and part of the budget. But saving energy? Not really.
"This is clearly incorrect, and should be edited."
I beg to differ. Pumped water energy storage has losses associated with parasitic turbulent flow in the out flow of the turbines. The analogous loss in flywheels that have electrical input and output of stored energy is resistive losses in electrical conductors. These resistive losses can always be reduced more by adding more copper to the conducters, but always at additional cost. So round trip efficiency is whatever the customer is willing to pay for. Since there are no serious customers, there are no serious design calculations.
My point was 97% was mechanical only, NOT round trip.
A misleading claim that still needs editing.
Those round trip numbers DO include the resistive losses.
There are plenty of working, measured flywheel systems, so I'll use those numbers - should you build a system to your rather loose claims, and measure its round trip efficiency, then we can add that.
Until then real, numbers win.
No, the mechanical efficiency is essentially 100%. For a flywheel spinning in a vacuum on magnetic bearings, there are no mechanical (frictional) losses.
There's an analog to frictional losses in the form of current dissipated in controlling the magnetic bearings. The magnitude of that loss depends very much on the design of the magnetic bearings. Most designs that I've looked at are pretty dumb (IMO), using electromagnets with variable current through their coils to maintain dynamic equilibrium. Ugh! It's possible to do better.
Regardless of that, any parasitic losses are separate from round-trip efficiency losses. Parasitic losses happen steadily, independent of charging and discharging. Round-trip efficiency losses are directly associated with charging and discharging. Losses in the motor-generator can be quite small -- down to a couple of percent. A much larger toll on round-trip efficiency is in the AC - DC and DC - AC conversions in the power control unit. But if you exclude the PCU, 97% is realistic for the round-trip efficiency of the flywheel and its motor-generator.
Losses in the motor-generator can be quite small -- down to a couple of percent. A much larger toll on round-trip efficiency is in the AC - DC and DC - AC conversions in the power control unit. But if you exclude the PCU, 97% is realistic for the round-trip efficiency of the flywheel and its motor-generator.
?!? - if you exclude some part of the round trip, then you do NOT have a real Round trip number !!
Commercial Electric motors have an Efficiency curve, and they peak at ~94-95%, with average efficiencies much lower than that, away from the operational 'sweet spot'.
Even at peak, you have to square this number because it applies twice : once for Energy In (as a motor), and again for Energy Out (as a generator), so there are good reasons practical reported real round trip values are closer to 80%.
(0.94*0.94*.91 will give you ~80%)
A flywheel is not a fixed-frequency/fixed load operation, so it will spend little time at the Motor sweet spot, and yes, you DO need to include all parts of the round-trip.
Your comments about commercial motor efficiency are in the vicinity of correct for the <500HP machines which make up the vast majority of the motor market, but for very large machines such as those used in generating stations, efficiencies can be dramatically higher. 98.5% is typical at 100MVA scale these days. 99% is seen in the very largest machines. Several manufacturers are working on 99.5% efficient machines. I don't see any reason to doubt the 97% number. Incidentally, the most modern pumped storage round trip efficiencies are substantially better than you think, as well,
Incidentally, the most modern pumped storage round trip efficiencies are substantially better than you think, as well,
Great. Links to real results are always appreciated.
Your motor numbers make the same error of others, they miss the full round-trip, and also miss that a Flywheel is NOT a constant velocity machine. Electric motors efficiency drops significantly away from the single frequency sweet-spot, as too do inverter efficiencies.
The original claim was not that some fractional elements of the round trip could be 97% at some time in the future, but "as an electrical storage system a flywheel can have efficiencies up to 97%" - if you (or the OP) can post any links, showing anywhere where a flywheel was able to deliver 97% round trip electrical storage system behaviour, I would be very keen to see them.
Don't forget that there have been huge improvements in the control electronics for electrical conversion that can substantially change the 'sweet spot'. Regenerative braking is one area that has been made practical by this.
NAOM
Sure, better Electronics helps, but you still bump into physics.
Unlike hydro solutions, where you can set a fixed target rpm, and voltage, with a flywheel, the rpm by definition is widely varying.
Look at any Motor curve, and imagine a 5:1 or 10:1 demand on rpm, and look at any inverter data, and imagine what a 5:1 or 10:1 demand on voltage, does to the average round-trip efficiencies.
The 'high nineties' numbers people like to wave about, are usually only at ONE measurement point.
If it was as easy as some are claiming, real systems would already be delivering these round-trip numbers. They aren't.
The best numbers I can find published for Direct Drive MW class wind turbines, which have a similar varying speed problem (but at the other end of the scale, in rpm) is around 94%, and that will be at a single operating point, NOT over a wide speed range. (and this is only half of a round trip, which becomes 88.36% sweet spot, and it will be less than that, on load-curve average)
With wind, and regenerative braking, the energy is essentially free, so you do not sweat the small percentages too much.
(note Wind DD designers are more likely to really want the cooling advantages of lower losses, over the extra kwh)
However, with flywheels used as an electrical storage system, you are BUYING the power, so going from 95% to 90%, doubles your cost creepage.
You have to hope the cyclic power prices are large enough to still give you a positive operating margin.
Great article! I thoroughly enjoyed it.
Is somebody working on small, static home units to replace batteries for living off the grid?
best regards,
jcrocco
It would also be nice if it used the 24 hr procession to point solar cells at the sun.
Here are a couple starting points..
(They are Low-key.. I wouldn't think one would run an AC or Table Saw from them.. but they are great concepts to add to your imagination..)
http://www.youtube.com/watch?v=8aWobDU0Cm0&feature=related
(The Missing Link Between Steam-Piston Power, and Electric Motors. 1910 Patent.. maybe impractical, but FUN! and a bit out of the box.)
Bike Flywheel Generator - (And of course, the input power doesn't HAVE to be your feet..
http://scienceshareware.com/bgsp.htm
Pedal Powered Hacksaw
www.youtube.com/watch?v=cENF4WKv-o0&NR=1
Pedal to Flywheel to Generator..
http://www.youtube.com/watch?v=sQwx_m8Vghw&feature=related
Of course, as I look at that Bike flywheel I think about the homebuilt wind gennies out there, and how those guys would probably start simply gluing magnets to the Flywheel itself, inching a handwound stator in there somewhere, and not suffer the transmission losses of belts and pulleys, etc.
http://www.lowtechmagazine.com/2011/05/bike-powered-electricity-generato...
Very good site to look at....
In the old days I worked as a boiler inspector in the Panhandle in Texas. There was an old meat packing company called SunRay packing in Amarillo that had an old Corliss engine that ran the entire plant using belts and pulleys. I had the good fortune of seeing this monolith. The plant had long since stopped using the engine so it sat there as in a museum. The flywheel was approximately 25 ft in diameter and about 1.5 ft thick-don’t hold me to the dimensions it has been some years. The engine ran off of a number of A type boilers.
It is fairly common to see magnetic bearings in rotating equipment today. There are a couple of drawbacks to their use. One, they are complicated and require skilled technicians to maintain them. The next question is What happens if the power drops? The control panels are very complex to levitate the rotor.
An alternative to this is the airfoil bearing partially developed by NASA. NASA likes to beat their chest but they did only part of the research. Airfoils levitate using foils much like a wing. Loss of power does not affect them and there are no control panels. Airfoil is gaining some traction but probably will not see much use for some time.
Hi :)
I think airfoil bearings are good for especially high-speed operation, like in microturbines / turbo generators. You need to reach a high speed before the foil bearing will work, and it will not work without air!
and there we have the problem. thats why airfoil bearings cannot be used in turbomolecular pumps, these run at insane speeds so airfoil bearings would be nice. But its a vacuum pump, and airfoil bearings require air to work.
So airfoil bearings have two disadvantages: First, you cannot evacuate the container of the flywheel, or the rotating axis needs to be elongated and brought out of the vaccum chamber, requiring something like a ferrofluid barrier at the axis. Second, you need to stay above a quite high velocity. I think this combination just kills them for the flywheel-application.
Atlas Copco is using air foil bearing in some of their compressors. These bearings are not just for hard drive applications
And then there's the electronic flywheel: http://en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage
That's really interesting. I wonder if it would be possible to make a superconductor loop around the whole country and use it for both energy storage and transportation. Kind of like a giant energy reservoir that a lot of different things could feed into and draw from.
Re Beacon Power - looks very good.
http://www.beaconpower.com/products/smart-energy-25.asp
Can someone please check my maths: While there is not a spec sheet as such, seems that stated energy stored is 25kWh.
Mass is 200 lbs (from the video - "200 pounds of carbon fibre")= 90.7kg. Rotation speed is up to 16,000 rpm. Outer radius (deduced from 'twice the speed of sound on the rim) = 0.4m, and inner (by eyeometry) = 0.2m. Best I can get for this spec is 12.7 M Joules stored = 3537 Watt Hours = 3.5 kWh stored, not 25 kWh. Do they mean 25kWh for ten units, perhaps?
Ah!
http://www.reuters.com/article/2011/06/21/idUS107216+21-Jun-2011+GNW2011...
"...the
Company's first flywheel energy storage plant in Stephentown, New York, has achieved its full 20-megawatt (MW) capacity. The plant, which is the largest advanced energy storage facility now operating in North America, utilizes 200 high-speed Beacon flywheels.."
= 100kW per unit - as the discharge rate. OK
and Beacon's own site says they are clustered (10) to give the required capacity. So the 'Smart Energy 25 flywheel' will store 25kWh, provided its done up in bunches of ten. Fair enough.
The article is completely wrong where it suggests the use of flywheels in Formula 1.
NONE and I mean NONE of the cars on the grid in Formula 1 is using a flywheel today.
NO Formula 1 car has EVER used a flywheel.
All the cars are using an electric converter/engine coupled with LI-ION battery packs.
Williams, which bought its flywheel tecnology from an external firm, have never been able to bring a working flywheel to their cars and Now they are using the battery pack plus the engine/converter.
I'm guessing you have slightly better information on the design of the KERS formula 1 used in 2009 and reintroduced in 2011 than formula1.com gives on this page. They said KERS can be electrical , mechanical(flywheel), or even hydraulic, but at least in my cursory exam they didn't specify which type/s are being or have been used.
There was no such a thing like "reintroduction" for KERS in 2011. Simply in 2010 there was a gentleman agreement between all the teams Formula 1 to avoid the use of this device which proved to be costly, heavy and unreliable.
The agreement expired in 2011, therefore teams were free to decide whether to use it or not.
Rules in Formula 1 do not state in which form the energy must be stored, they simply limit the amount of energy supplied by KERS that can be used for each lap. In this freedom electric motoconverter emerged as winning design while it was impossibile for Williams to bring to circuits even a prototype of their flywheel. Right now they are using a motoconverter with LI-ION batteries and I strongly believe they will be using this design even beyond 2014 since it has weight and weight distribution advantages.
I have seen a flywheel farm a decade ago :)
in the small town of Garching, north of Munich, we have some supercomputers, a linear accelerator, some space telescopes are controlled from there, ESO Headquarters are there, and we have a nuclear research reactor. Lots of high-tech research going on there.
A few years back we also had a nuclear fusion experiment, the Wendelstein 7-AS. That was before superconducting magnets, so the stellerator worked with huge copper coils. To power it and heat up the plasma inside up to 100 million degrees centigrade, much energy was needed. In fact, the experiment over a few seconds required 10 times the amount of energy that Munich requires, which is a city of about 1.3 million people. So the grid was totally incapable of delivering that kind of energy.
The solution back then was a flywheel farm, with massive concrete flywheels weighing hundreds of tons each. They would accelerate over 3 days, and then deliver all that energy within 20 to 30 seconds, enabling scientists to fire up their nuclear fusion experiment. So whenever the scientists needed the amount of energy that is normally used to power about 13 million people living in a city, they could get it using the flywheel farm. The Experiment couldnt run for more than a few seconds anyway, because the heatup of the massive copper coils for the extremely large and powerful magnetic fields was very rapid. And then the experiment needed to cool down for hours or even days anyway, so the flywheel farm was just perfect and did flatten that extremely spiky consumption curve.
So i know first hand that flywheel farms can work, and they can be tailored for specific needs, in this special case it was about slowly storing lots of energy, and releasing it very rapidly, going from maximum RPM close to standstill in half a minute. Powering an experiment that needed practically no energy almost all of the time, and then for a few seconds has an energy demand similar to that of New York City. It used small electric motors to speed up, and really huge generators that were only connected to the flywheels during energy takeout.
p.s.: I have no idea what happened to the flywheel farm after the fusion experiment was over, maybe it was decomissioned, maybe it was put to a different use. I now intend to find out, each year they have one day where you can access almost everything, even the most restricted areas like the nuclear research reactor and the supercomputers. And that will be in two weeks, if i find out more about that good old flywheel farm ill update here.
I think there is a flywheel energy storage system for a wind farm in the Azores. Hopefully the author of this article can find some more information about this as he is from Portugal?
I worked on piece of mining equipment (diesel-electric) that used a flywheel to even out the load on the diesel (1800hp) motor over a digging cycle of 30-60 seconds. The flywheel could store about 10-30kWh and was 8 feet in diameter and turned at 1200 rpm max. It was a bit of a pain to deal with and as far as I know it is the only set up in existence. Also a bit unnerving having 7 tons of rotating flywheel close by. Friction losses not an issue over 1 minute and it took about 45 minutes to spin down by friction losses on its own when things went wrong so you could not use to store energy for a day. No fancy vacuum or magnetic bearings just white metal and behind a metal grill.
From what I understand, flywheel energy storage is great for timescales of minutes - not so good for hours or days. That and super capacitors (which are being looked at for mining trucks at the moment) seem to be competitors in the same field.
Flywheel helicopters for timber operations seem to make sense. You power them up, fly out a tree, repeat.
Would the flywheels be light enough?
Another approach to high power / short term energy storage is a variable displacement hydraulic pump with an accumulator. For vehicle use it's got many of the same desirable characteristics of a flywheel without the inertia issues. Eaton has been developing a system for several years now:
http://www.eaton.com/Eaton/ProductsServices/ProductsbyCategory/HybridPow...
Several years ago Chevy built a hydraulic regenerative demonstrator using a Silverado pickup, but for some reason they decided to go with a battery pack for regenerative braking instead.
For stationary use, compressed air storage presents interesting opportunities. If air is compressed to 120 PSIG it heats up to about 500 F. A windmill connected to an air compressor could provide space heat and energy storage without batteries. This would probably work best if the windmill used a driveshaft down to ground level. As an added bonus, when room temperature air is expanded through an air motor from 120 PSI to atmospheric, it cools off to about -180 F. This could be used to provide cooling to a refrigerator / freezer. Since space heating requirements usually go up in windy weather, a system like this could provide a simple mechanical triple play for home energy storage.
I recall an assignment from my university studies (mechanical engineering) where we were investigating different concepts for accumulating braking energy in a city bus, that stops and starts frequently. Flywheel, hydraulic accumulator and other concepts were compared. The winner, 100% efficiency and by far the cheapest, was to build a small hill at every bus stop. The "hill" doesn't have to be that high, a foot or two actually saves quite a bit of energy over a year. Here in my town city buses stop every few minutes throughout the day, a bus wheighs maybe 15-20 tonnes. 50 cm elevation reduces the need for braking, and the potential energy is re-used as the bus starts to roll down again. Do the math yourselves.
/Mats Lindqvist, Sweden
Mats,
That is a great example of some "out-of-the-vehicle" thinking!
A 0.5m rise for a 15ton bus is 75kJ or 21Wh. Doesn't seem like much, but that represents the mechanical output of 8mL of diesel at each stop.
If the bus does 20 stops per hour, and runs 16 hrs a day, that is 2.7L of diesel/day, or 1000L/yr.
Only thing is, it misses out on energy recovery at traffic lights, which for a city bus is probably as many stops again.
Still, I am impressed with the mechanical simplicity of the plan. As a civil engineer I should have thought of this myself - it is not very often we get to trump the mech. eng's with energy solutions!
Sounds like a great solution for a dedicated busway system - do you know if it has ever been implemented anywhere?
I haven't seen this for buses, but apparently (see more comments below) it is used for underground/subways. But electrified railways etc already have the possibility to brake electrically and feed energy back to the train cable. So the gain ought to be better for diesel powered buses. As you calculated, a saving of 1000L of diesel fuel the first year, for one bus stop should be a good investment. Reduction of noise, pollution, reduced brake-maintenance are other benefits. Here in my town, Lund in souther Sweden, there are a few bus-only streets and lanes, in these cases this should be a viable solution.
The "other benefits" - brakes, noise, diesel engine not blowing (as much) black smoke, etc are probably at least of equal value to the fuel saving.
A group in Japan was building a prototype of a rail system that was based on this concept - similar to roller coasters;
But it doesn't seem this project has gone anywhere.
Which is too bad, as the roller coaster principle has over 100years of real world experience behind it, and it would make transit riding fun!
Back to the buses, if you had a dedicated urban busway, with stops every 1km, and made the "hill" 2m high (partly by depressing the lane between stops, then you would have 300kJ of potential energy recovery at each stop. This is enough, by itself, to get a (frictionless) bus moving at 22km/h
Yair...Paul. One of the early open cut mines in Qld was using Euclid belly dumps for hauling coal. As I understand it the haul road had been set up specificaly to suit the V12 Detroit two stroke and Allison auto transmission.
The trucks only stopped at the shovel. Once loaded they proceeded at WOT to the process plant and a series of ever steepening grades dropped the tranny back through the gears so that as they came to the dump grid they were in low gear and a seperate pedal kept the Jimmy percolating at a pre-set speed...probably fifteen to eighteen hundred... while they dumped. The brakes were never used in normal operation.
Once dumped and doors shut it was on the loud pedal again and they dropped off down a series of grades to get the Jimmy boogieing once they hit the flat.
It always seemed like a slick system to me but bottom dumps are out of fashion so I guess it didn't work.
Cheers.
A lot of stations on underground/subway railways are on a little 'hill' too for the same reasons. Save on brakes when coming into the station and save on acceleration power when leaving the station.
My utility, NorthWestern Energy, is adding a flywheel system to its new natural-gas fired turbine plant just west of me in Anaconda:
http://www.marketwatch.com/story/beacon-power-and-northwestern-energy-hi...
I'm still waiting for them to install wind turbines on the arsenic-polluted land in the same area, which used to be the home of a major copper smelting operation. It has a good wind resource that plows down out of the surrounding mountains.
The ideal flywheel would not be a wheel at all. It would be an underground vacuum sealed
circular wheel. If very wide it could reach very high velocities without excessive force.
Energy:
E=1/2MV^2
Force on rotating object:
F=MV^2/R
The higher the radius the less units of force per units of stored enery
Imagine a mile wide utility scale storage ring containing a wheel many feet thick, and weighing
hundreds of tons.
To clarify it would not be a typical wheel, it would lack spokes.
mercurius,
I know you mean well, but alas this is an example where any of us can be easily fooled by the math.
The equation you show above for centripetal force appears correct, however, it fails to explicitly show that velocity (V) is a function of radius (R).
Another way for expressing that force is F= m * R * w^2
See: http://en.wikipedia.org/wiki/Centripetal_force
When seen in that way, it becomes more apparent that centripetal force increases as radius increases.
Thus the mile-wide disc would be a disaster.
Don't let this dissuade you from thinking up of new ideas.