Flywheels For Short-Term Power

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andylaurence
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Flywheels For Short-Term Power

Postby andylaurence » Tue May 12, 2009 10:36 am

I'm still at the beginnings of my electric Mini conversion (too many projects, not enough time) and I'm still pondering power sources. Power density is the holy grail for me to get the performance I'm after. Lead acid batteries would triple the weight of the car to get enough power, Lithium would cost a fortune and still weigh a goodly amount or need regular replacement. Given this, I've been thinking about batteries for long term storage (A-B) and using contra-rotating flywheels for short term power. Bosch do an F1-spec system that produces 80bhp and weighs very little. Being destined for F1, two things can be deduced; it'll be well made and cost a fortune! Has anyone researched this kind of setup and/or found a flywheel storage system for sale? I'd build my own but the thought of 100000rpm spindles of my own making is terrifying! Is the concept as sound as I think it is?

Cheers,
Andy
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Deker
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Flywheel storage

Postby Deker » Sat May 16, 2009 2:31 am

Hi Andy.

Only this week I heard about the flywheel kinetic storage system for F1.
This has been researched for many years, the bit I heard about was for buses, to give an initial acceleration boost from a standing start.

The flywheel/s are brought up to speed from braking systems, those years the main worry was of the flywheel/s bursting.

As for the F1 system, it is designed to give a only a few seconds boost.

If your vehicle is not used in a start/stop cycle there is probably no gain.
I would go as far to say in gentle braking there would be no gain.
By reason of, the flywheel/s could not get up to speed.

I would think, that with an E vehicle you'd be better off with Re-gen braking - it's a whole lot lighter.

Just done a quickie calculation, at around 60 MPH, to get a flywheel up to 100,000 RPM you would need roughly an 83 to 1 overdrive gearing.
Assuming a tyre diameter of 18 inches.

On the other hand, if your Mini is going at 200 MPH, then to get your flywheel/s up to 100,000 RPM you would only need a 26:1 overdrive !!!

Which is similar to what wind power generators have, yes they do have gearbox failures, and the anti wind gen lobby jump on this by saying wind generated electricity is costly.
There are other reasons by way of financial loan repayment structures, over heat generated electricity.

Some smart character will no doubtedly correct me on the exact overdrive gearing on wind generators, which does vary according to the diameter of the blades.

The point is - torque loadings on the geared components is ginormous, not really for the home constructror.

Best of luck.

Deker.

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Jeremy
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Postby Jeremy » Sat May 16, 2009 3:58 pm

Years ago flywheel powered buses were used for a while. The flywheel was spun up at bus stops by external power, allowing the bus to travel in between stops by drawing power solely from the flywheel.

Here's a Wiki entry on them: http://en.wikipedia.org/wiki/Gyrobus

Jeremy

andylaurence
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Postby andylaurence » Sat May 16, 2009 9:26 pm

I suppose I should've explained more accurately. I wasn't thinking of a direct mechanical connection between the driven wheels and the flywheel. Instead, I was thinking of using the flywheel as a high power density, low energy density electrical energy storage device. Essentially, this means that peak electrical current can be met with the flywheel, yet the longer term energy requirements can be met with the batteries. In short, this means that the peak power of the batteries could be 10KW, yet a flywheel capable of 50KW for a few seconds could meet instantaneous demands. My thinking is that it'll reduce weight needed and battery costs for a vehicle that only needs a range of 10 miles, yet requires high power.

Cheers,
Andy
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Jeremy
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Postby Jeremy » Sun May 17, 2009 11:48 am

But batteries are able to deliver high peak loads, so where's the problem?

The peak to mean power ratio for a small car is around 6:1, but this is based on an internal combustion engine, that has peak torque at a high rpm. For an electric vehicle, one can take advantage of the high torque at low rpm characteristic to reduce this ratio by a fair bit, perhaps to around 3:1, and still have good performance.

If you want a one hour endurance, this means you need batteries capable of about 3C peak. This is well within the capability of all battery technologies that are currently available, even cheap lead acid cells can deliver around three or four times this without much bother. Some of the better lithium cells are good for around 15C or even 20C or more, although they get pricy as peak discharge rating goes up.

Factoring the added losses of powering up the flywheel (around 15% to 20% energy loss) and taking power out of the flywheel (another 15% to 20% loss), plus the weight of the thing, together with the weight of the flywheel energising motor/generator, and it seems a less than desirable alternative to just using reasonably good batteries.

As an example of battery peak power, I have an old Red Top race car battery sat by my desk. It's a sealed lead acid, rated at 16Ah, that will deliver well over 600 amps peak. That's around 37C............

Jeremy

andylaurence
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Postby andylaurence » Sun May 17, 2009 12:39 pm

So, taking an example. I've found some 30C lithium cells. They're 3.7v and 2.2Ah, the max. current is stated as 66A. Each cell weighs 54g. By my reckoning, if I wanted to achieve 100KW, I'd need 410 cells, which would weigh 23KG and store around 3KWh in total. I'd need to shell out over £5000 on these cells. Then there's the BMS, both in cost and weight. Added to that, there's the lifespan of lithium, which I would expect to be significantly reduced with regular charge/discharge cycles. A flywheel is able to be "charged" and "discharged" almost infinitely in comparison. High discharge rates kill batteries, yet flywheels are designed purely for that. The batteries can backfill the shortfall.

Perhaps I should be in the garage, working on the car, rather than typing away here on the forum!

Cheers,
Andy
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Jeremy
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Postby Jeremy » Sun May 17, 2009 1:56 pm

I think there's some confusion here between power, battery capacity and endurance (although I may be wrong).

For a small car, the mean power requirement is going to be quite modest, perhaps 5kW or so for commuting use. The peak power might be two or three times this figure for reasonable acceleration and hill climbing ability.

Let's assume that your ten mile commute is at an average speed of 30mph. this will need an average power of 5kW for 10 miles over a 20 minute period, so you need at least 1665 watt hours battery capacity, ignoring losses and Peukert factor. Let's assume a 60% factor for losses and Peukert, so the battery capacity needed is about 2800Wh, which is very modest for a car.

Let's pick a battery voltage for this example, of 48V. To obtain 2800Wh from a 48V battery pack needs about 58Ah of battery capacity. The current drawn from this 48V battery at the mean power level of 5kW will be about 104 amps, or about 1.8C. The peak current drawn at three times this power level will be about 5.4C. Even the very cheapest lithium cells will easily deliver 5C, so there is no need to think of going to anything like 30C cells.

Using your example cells, then each cell has a capacity of about 8Wh, but has a near unity Peukert factor, so the capacity needed for your ten mile range (at 30mph average speed) will reduce to around 2000Wh. You will therefore need about 250 cells to get your required 10 mile range/20 minute endurance, and the cells will be discharging at an average rate of around 2 to 3C. You will have a peak power capability from this 30C pack of at least TEN times the mean power, so there will be masses of power in hand from the batteries for acceleration (more than the motor or controller will take, by a fair factor, I expect).

Is this a bit clearer?

Jeremy

andylaurence
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Postby andylaurence » Sun May 17, 2009 3:38 pm

Yes, I understand that I could pootle along for 10 miles with just a few kW of power. My project is not for a commuter vehicle. I'm aiming for a lot of power as a bit of fun for motorsport. I'm looking for a range of a few miles but to be able to utilise a lot of power over that period. I'm planning to use a front-wheel-drive setup initially with a 4WD solution in the future.

At the moment, I'm getting the weight down as much as possible. I've removed all the engine, 'box, exhaust, fuel tank and associated items. I'm also replacing panels with lighter weight items (a fibreglass boot and front, for starters) and removing unnecessary items like the rear seats, carpets and sound deadening. Once I've refurbished the bodyshell and the suspension/braking systems, I'll be working on the drivetrain.

I saw your post about multiple motors and have been thinking about how I can mount them in my car. I have been leaning towards mounting them on the hub, driving a spur gear, although there's the possibility of mounting the motors in the engine bay, attached to the drive shafts. Some of these motors produce upto around 8hp, weighing less than 1.5kg and costing under $100US. If I could use six 6kW motors on each front wheel, then I'd have almost 100hp, which would make the car spritey to say the least! The motors weigh less than 20kg, which is about the same as a tank of fuel.

To power that, it's a lot of lithium. 30C or not, it's still a lot of power to be drawing, hence why I'd thought about flywheels for energy storage, simply because the instantaneous demands for that amount of power may just be too much. I just want to be sure there isn't a really great solution staring me in the face. I do agree that it's not so important for a commuter vehicle though as the number of cells required for adequate range ensure there's enough peak power.

Cheers,
Andy
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Jeremy
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Postby Jeremy » Sun May 17, 2009 4:32 pm

I understand the desire to use the flywheels to boost peak power, but can't see how it would be of benefit when the ability to deliver instantaneous power isn't down to the batteries, it'll be limited by other parts of the system way before the batteries present a problem.

I think that what you're looking for is lots of torque, rather than lots of power. Because we're used to thinking in terms of ICEs with their inability to generate torque at low rpm, we seem to equate power with acceleration. In fact, what's needed for maximum acceleration is just enough torque to keep the wheels as close to possible to losing grip. This takes very little power at low speeds, in fact at the instant of launch from a standstill the power is actually zero.

Power is needed to overcome the forces acting to hold the vehicle back at speed, primarily aerodynamic drag, hill climbing and rolling resistance. Even then, it doesn't take much power to make a small car do 60mph on the level, 20hp will probably be more than enough.

You can "reverse engineer" the power that any car needs to travel at a particular speed if you know the fuel consumption at that speed. 56mph is a good figure to get data, because it's used in the official tests. If a car does 50mpg at a steady 56mph, then as a good four stroke engine needs about 240g of fuel per kWh, it's possible to work out the power that the engine is delivering. 56mph at 50mpg gives a mass of fuel used in one hour of about 3.66kg. Divide this by the BSFC of 240g/kWh gives a power of just over 15hp.

Aerodynamic drag is the major resistance factor at speed, and power required to overcome it is proportional to the cube of the speed difference. Increasing the speed from 56mph to, say 70mph, will increase the power required from about 15hp to about 30hp.

You can factor these figures for different numbers easily enough. For example, if your car only does 40mpg at 56mph, rather than 50mpg, then the power would be about 19hp, rather than 15hp and the power needed at 70mph would be around 37hp rather than 30hp.

Hill climbing requires extra power too, depending on the weight of the vehicle and it's speed. For example, a 1000kg car travelling at 60mph up a 5% hill needs about 18hp of additional power over that required to do 60mph on the level.

Jeremy

andylaurence
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Postby andylaurence » Mon May 18, 2009 12:17 pm

Jeremy wrote:I understand the desire to use the flywheels to boost peak power, but can't see how it would be of benefit when the ability to deliver instantaneous power isn't down to the batteries, it'll be limited by other parts of the system way before the batteries present a problem.


My thinking is that the batteries are a limiting factor. Even at 40mph, whilst only 10hp may be needed to maintain speed, a much larger amount will be needed to overcome the grip of the tyres. That will be limited by the batteries, unless a large number of them are used.

Jeremy wrote:I think that what you're looking for is lots of torque, rather than lots of power. Because we're used to thinking in terms of ICEs with their inability to generate torque at low rpm, we seem to equate power with acceleration. In fact, what's needed for maximum acceleration is just enough torque to keep the wheels as close to possible to losing grip. This takes very little power at low speeds, in fact at the instant of launch from a standstill the power is actually zero.


You need both. They are directly linked by definition. Power is equal to the torque and the speed. As such, higher torque for a given road speed equals more power. Yes, that means it takes less power at lower speeds, however, it also means large amounts of power at higher speeds.

Jeremy wrote:Power is needed to overcome the forces acting to hold the vehicle back at speed, primarily aerodynamic drag, hill climbing and rolling resistance. Even then, it doesn't take much power to make a small car do 60mph on the level, 20hp will probably be more than enough.

You can "reverse engineer" the power that any car needs to travel at a particular speed if you know the fuel consumption at that speed. 56mph is a good figure to get data, because it's used in the official tests. If a car does 50mpg at a steady 56mph, then as a good four stroke engine needs about 240g of fuel per kWh, it's possible to work out the power that the engine is delivering. 56mph at 50mpg gives a mass of fuel used in one hour of about 3.66kg. Divide this by the BSFC of 240g/kWh gives a power of just over 15hp.

Aerodynamic drag is the major resistance factor at speed, and power required to overcome it is proportional to the cube of the speed difference. Increasing the speed from 56mph to, say 70mph, will increase the power required from about 15hp to about 30hp.

You can factor these figures for different numbers easily enough. For example, if your car only does 40mpg at 56mph, rather than 50mpg, then the power would be about 19hp, rather than 15hp and the power needed at 70mph would be around 37hp rather than 30hp.

Hill climbing requires extra power too, depending on the weight of the vehicle and it's speed. For example, a 1000kg car travelling at 60mph up a 5% hill needs about 18hp of additional power over that required to do 60mph on the level.

Jeremy


That to me says I need lots of power to make a fast EV and that's what I'm planning (comparatively). Perhaps I'll revisit this when I have got to the point where the drivetrain needs to be powered.

Cheers,
Andy
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