So, i’m sitting here having a beer and watching Rays Baseball on Mothers Day and the thought occurred to me that…
Couldn’t the wheels be fitted with generators and charge the batteries as they spin?
I know they have to still be charged with a plug-in eventually, but still.
Wouldn’t it be prudent to harness all that spinning, as you basically use energy to move forward and as you let off the pedal the residual energy could be recouped and not wasted.
Also there is the braking energy that could be reserved.
Yep, it’s already done, and it’s called “regenerative braking”. It’s actually even simpler than your concept of adding additional generators. Turns out, that a generator and an electric motor are basically the same thing working in opposite directions. If you spin the output shaft of a motor, you generate current. If you apply current to a generator, its input shaft will spin. So in a typical hybrid or electric car, hitting the brakes will send some power “backwards”, through the drive shaft, to the motor which then acts like a generator.
Regenerative braking is one of the major reasons that hybrids and electrics are so efficient – otherwise, whenever you brake, all of the car’s kinetic energy is turned into heat and lost forever.
Many, if not all, electric cars and hybrids do recapture energy during the braking process. And you don’t even need to add a generator to do it; the electric motor itself becomes a generator. See the Wikipedia entry for regenerative brake.
But it only works to recapture energy that would otherwise be lost. If you had a car with electric motors driving the rear wheels, for example, and you mounted generators on the front wheels, you wouldn’t gain anything. Compared to free-rolling front wheels, the motors would have to work harder to turn the wheels with generators attached. In theory, the motors would consume as much power as the generators would produce. In the real world, nothing is 100% efficient, and you never get back as much energy as you put in.
I have a somewhat related question: In the process of regenerative braking, in which the motor acts as a generator, thus converting mechanical energy into electrical energy rather than vice versa, as would the motor that acts as a motor (during the nonbraking phases I assume), does the current generated by the generator recharge the battery?
Yep, as has been said, existing hybrids and electric vehicles do this. And also has been said, it’s typically the electric drive motor that flips over to generator mode when coasting or braking, no need for separate generators at the wheels.
As an example, we have a couple of hybrid Ford Escapes at work. When I drive those, braking is initially almost all regeneration (the load of the electric drive motor-turned-generator dumping power into the battery slows the car) until almost at a stop, then the normal brakes engage.
The current gets saved somehow, not necessarily in the battery. It could be stored in capacitors. If you had lots of electrical items turned on (headlights, radio, etc.), some of the generated current could go to powering those things. If the car is a hybrid, that takes some of the load off the engine and it burns a little bit less fuel, so some of the saved energy is in the gas tank.
And this is because regenerative braking just damps the motion: If that were all that was stopping you, it would take an infinite time to reach a complete stop. Of course, in the real world, there’s also friction of various sorts, but it’s still not a bad idea to help it along.
Capacitor technology is nowhere near good enough to capture even a fraction of the energy generated in braking. And why would you want to use them? You have a perfectly good energy storage device in the battery pack.
The braking energy, yes. As other have said it’s already done. It also seems like you’re suggesting trying to harness just the ‘spinning’ when you’re traveling down the road. That won’t work. There is no free lunch.
When you hit the pedal on a regenerative brake it engages the the generator(s) to the wheels. The wheels cause the generator to spin and that spinning creates electricity that is used to charge the batteries.
Now, it’s not just the spinning mass of the generator that causes the car to slow down. The magnetic fields in the generator are opposing each other so imagine that, just like taking two magnets and pushing them together at their north poles,there is a resistance. That’s the essence of electrical generation. There is energy being “pushed” into opposing magnetic fields. That energy has to go somewhere so it ends up “pushing” electrons and causing power flow into the batteries.
The reason you can’t just spin the generators while cruising is that you need that opposing magnetism to generate power and that opposing magnetism has a significant braking effect.
Capacitors can be charged (and discharged) a lot more rapidly than batteries. Because of this, the amount of energy re-captured by regenerative braking is only a small fraction of the total amount available.
Some back of the envelope calculations: the fastest the Leaf can charge is ~ 20 kWh in 30 min = 40 kW. (And Nissan warns that frequent fast charging will damage the battery). If you’re driving it at 60 mph and slow to a stop briskly in 5 seconds, the brakes are absorbing 108 kW. With a sufficiently large bank of capacitors, you could recover 2-3x more braking energy than you can with batteries.
I agree that it’s not practical with current technology – the best capacitors still have energy density lower than batteries, so you’d need a huge and expensive capacitor bank.
Capacitors are also capable of an essentially infinite number of charge-discharge cycles. Batteries are limited in how many times they can be charged and discharged. Capacitors don’t have anywhere close to the energy density of batteries, but would be more convenient for storing energy from regenerative braking. I don’t know if any electric cars actually use capacitors, I suspect the low energy density and cost make it not effective yet.
Some do, but not as the primary sustaining source of energy. And, in fact, research into very large capacitors have automobiles in mind as an application.
The problem with using capacitors is that they can’t really drive a fixed voltage. The nice property of batteries is that if you want a constant voltage source, you’ve got one. You’re not going to get that with a capacitor.
But what they can do is provide a nice “boost” if your fuel cell isn’t ready to provide a large amount of charge up front and can provide a nice charge storage solution, as already mentioned in the thread.
So, the current application of capacitors for cars is to provide that near instant energy you need for starting up/accelerating and also for energy storage. Regular fuel cells would provide the more constant energy source you’d use for 90% of the driving, though. So, capacitors are used as buffering mechanisms for sudden changes in electrical demand.
Hmm, after a few more back-of-the-envelope calculations, using capacitors for short-term auxiliary power storage doesn’t seem quite so infeasible. As in my example above, the Leaf driving at 60 mph has about 5 MJ of energy that could be re-captured. Wiki references a a company that makes the latest and greatest capacitors, and they have a 0.1 MJ/kg energy density. In that case, you’d need a 50 kg bank of capacitors. That’s a nontrivial amount of weight, but might possibly be worth the trade-off. In fact, the company who sells that capacitor is looking at this very application…