## The wheels on the EV go round and round...

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Rigel
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### The wheels on the EV go round and round...

So I was speaking with a co-worker today who asked why we can't just put alternators on the wheels of EVs to get free energy back, and I spent 30 minutes trying to convince him there's no such thing as free energy before giving up. But then I got curious about the math.

A Nissan Leaf gets ~100MPGe. Traveling at 55mph, it will expend the equivalent of .55 gal / hr.

Gasoline has a density of 0.737 g/cm3 and an energy density of 45 kJ/g.

1 gallon is 3785 cm3
0.55 gal of gasoline is 2081.75cm3, or 1534g (1.534kg), meaning 69041 kJ (69MJ)
So the Nissan Leaf is expending 69MJ/hr just to keep moving at highway speeds.

Grabbing a random alternator from Napa, 160A at 12V, or 1,920 W (I'm assuming the speed at which each wheel is rotating would be sufficient to drive it at its max rating - this is a big assumption!)
Over one hour that would generate 6.9MJ (1,920 W *3,600 s =6,912,000 J)

So, expending 69MJ, and returning 6.9MJ with a single alternator, decreasing energy expenditure by 10%. Meaning that one on each wheel would decrease energy expenditure by 40% - that can't be right.

Do EVs already do this, and that's part of how they're so efficient? Did I get the math wrong? Or are some of my assumptions incorrect?
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Zarathustra
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Are you taking into account that turning the alternator would require an *extra* expenditure of energy equal to or more than the amount it generates? Here's why: just because the wheels are turning doesn't mean you can siphon off some of that spin for free, without affecting the rate of spin. Turning the alternator involves not only friction of its moving parts, but also the force you are countering (I assume magnetic?) in order to generate a current. So you'd have to increase the energy expenditure to maintain the same speed AND power the alternator, and since no machine is perfectly efficient, you'd have to increase this expenditure more than the amount you'd get from the alternator.

Regenerative brakes work by recapturing the energy in the wheels' spin, converting that to electricity. The reason that works is the reason why your idea wouldn't: it slows the car down. But in this case, since you want to slow the car down anyway--that's the purpose of brakes--you are getting "free" energy. But it's not really free. You paid for it by accelerating up to and maintaining that speed prior to braking. It's just recapturing energy that we usually throw away as heat.
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Rigel
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Zarathustra wrote:Are you taking into account that turning the alternator would require an *extra* expenditure of energy equal to or more than the amount it generates?
All my math is above.

Anyway, I couldn't find anything about how to compute the actual energy output based on inputs. Google is an engine designed to sell you things, so all it showed me were either auto parts stores, or websites talking about very specific models.

Even googling turbines just talks about commercial varieties; the sites not talking about specific models will, instead, say "Most commercial wind turbines have a power output..."

What I need is the principle behind it so I can accurately calculate the forces going in and the current coming out.
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Zarathustra
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Obviously Iâ€™m not addressing your math. You asked if your assumptions were wrong and I pointed out where they were wrong. Hooking up an alternator to each wheel would be no different from adding regenerative brakes to each wheel. The energy isnâ€™t free. It comes at the cost of slowing the car down. Yes, you can extract energy from spin, but that necessarily decreases the spin (or angular momentum), which would then require more energy to maintain, meaning that you havenâ€™t decreased your expenditure by extracting this energy from the spin and adding it back, rather you have increased your expenditure to create that energy while simultaneously maintaining speed under this additional load.

The math youâ€™ve provided doesnâ€™t change any of this; these are just consequences of the laws of physics.
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Thanks Z!
Your brake analogy clearly let me understand what you were saying.
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Rigel
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I'm literally asking for help with the math though, so it's not obvious at all.

My gut reaction is to say this can't possibly be useful, for exactly the reasons you outlined. But then I decided to plug the numbers in and got a surprising result; my next gut reaction is to say there's something wrong with my math, so I'm trying to find out what.

I think I found what I need to work this out by plugging in a Faraday disc, but it's been a while since I've read a physics textbook so I'm taking longer to get through the text than I expected.
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Zarathustra
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I thought it was obvious I wasnâ€™t addressing the math because I literally did not address the math.

Anyway, Iâ€™m a little confused about what youâ€™re looking for. Are you trying to say that the math seems to show that alternators would decrease the energy expenditure? Even though you know thatâ€™s impossible?

I think all you need is the efficiency of the alternator, which Wikipedia says is 70%-80%. You are converting kinetic energy into electricity, so to maximize the alternatorâ€™s production at 6.9 MJ, youâ€™d need to supply 8.63MJ of kinetic energy to overcome the drop in efficiency. Therefore, to maintain your speed, youâ€™d have to replace that 8.63MJ of kinetic energyâ€”which you removed from the wheelâ€™s spin with the alternatorâ€”with an even greater amount from the carâ€™s battery to overcome the efficiency rating of the EV to convert electricity into kinetic energy (Wikipedia says 85%-90%), which means youâ€™d draw 9.58MJ from the batteries to generate 6.9MJ of electricity with the alternator. And thatâ€™s not even counting the electricity you need to power the car at that speed without any alternators decreasing the kinetic energy.

So if the alternator is returning 6.9MJ, the cost is 9.58MJ - 6.9MJ = 2.68MJ. Thatâ€™s how much energy youâ€™d lose (per alternator) in this scenario. There are no net savings. But that can be pointed out without going through the math.
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Since another thread is discussing Chat GPT, I thought these would be a good questions to ask it. So here are its answers:

Q: Why are four alternators not used instead of regenerative brakes on an electric vehicle?

A: Four alternators could be used to generate electricity on an electric vehicle, but they would not be as efficient as regenerative brakes for several reasons.

Firstly, an alternator generates electricity by converting mechanical energy from the engine into electrical energy, while regenerative brakes convert the kinetic energy of the vehicle into electrical energy. Since regenerative braking is based on the principle of energy conservation, it is more efficient than generating electricity from the engine, which requires the use of fuel to create the mechanical energy in the first place.

Secondly, using four alternators would add significant weight and complexity to the vehicle, which would reduce its overall efficiency and performance. Electric vehicles are designed to be lightweight and efficient, and adding unnecessary components can compromise their performance.

Thirdly, regenerative braking is a more integrated and seamless solution for recovering energy from braking. It does not require any additional equipment or controls, as the electric motor that propels the vehicle is already integrated with the regenerative braking system.

Overall, while four alternators could generate electricity on an electric vehicle, they would not be as efficient, lightweight, or integrated as regenerative brakes. Regenerative braking is a well-established technology that has been optimized for electric vehicles and provides a highly effective and efficient means of recovering energy from braking.

Q: How much more efficient is regenerative braking than an alternator?

A: Regenerative braking is generally much more efficient than using an alternator to generate electricity because it recovers energy that would otherwise be lost as heat during traditional friction braking. While the exact efficiency gains will depend on the specific system and driving conditions, regenerative braking can typically recover 50-70% of the energy that would be lost during braking.

In contrast, an alternator generates electricity by converting mechanical energy from the engine into electrical energy, typically at an efficiency of around 50-60%. This means that for every unit of energy used to turn the alternator (such as from burning fuel in the engine), only about half of that energy is actually converted into electrical energy, while the other half is lost as waste heat.

Furthermore, using an alternator to generate electricity on an electric vehicle would also require using energy from the battery to power the alternator, further reducing the overall efficiency of the system. In contrast, regenerative braking on an electric vehicle directly charges the battery, making it a more efficient and integrated system.

Overall, regenerative braking is a highly efficient way to recover energy during braking, and it is a key feature of many modern electric and hybrid vehicles.
Rigel
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Zarathustra wrote:You are converting kinetic energy into electricity,
This is the part that was tripping me up.

I was assuming that a Faraday disc would affect the magnetic field and electric current, but not the kinetic force of the spinning disc. This is incorrect.

Even a frictionless flywheel that could, in theory, spin forever, would lose rotational velocity in direct proportion to the current draw. So the simple assumption that to get a current out you would have to put at least that much current (+extra for inefficiency) in is correct.
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### The wheels on the EV go round and round...

I had to look up regenerative braking. After reading it, I get why alternators would be redundant, less efficient, and add weight for no reason. Always learning
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