Several weeks ago, I packed up all of my worldly possessions and moved them nearly 750 miles from Southeast Michigan to central North Carolina. Part of this process involved driving a seventeen-foot U-Haul truck, fully loaded and towing my car on a trailer, through the mountains. I found this process to be highly unsatisfying, but also thought-provoking.
For perspective, U-Haul lists this vehicle as being more than eight thousand pounds empty. Fully loaded, it weighs in at just about fourteen thousand pounds. In addition, the trailer is more than two thousand pounds empty, with a more than three thousand pound car riding on top of it. The effect is a fourteen thousand pound vehicle with a five thousand pound sea anchor hanging off the back. Driving a vehicle this size through the mountains is enough to make anyone develop a multiple-personality disorder. Going uphill, I would simply lay the accelerator flat against the floor, then listen to the engine roar even while the speedometer steadily dropped beneath the sheer mass of the vehicle behind it. Going downhill, I’d stand on the brakes and try not to think about the feeling of the massive, dead weight of the trailer cramming itself up into the small of my back.
Each time I rode the brakes down a hill and then listened to a huge, gas-guzzling engine wheezing as it struggled to heave itself back up the hill, I found myself thinking about regenerative braking. Even though it would not be the kind of thing that is even included on a vehicle of that type, such a thing felt like it would be exactly what I needed. I spent several hours thinking about the efficiency of regenerative braking systems.
Regenerative braking is a technology that has been around for many years, but has only recently been included in cars. It works by taking an electric motor attached to the wheels and, rather than using it to drive the vehicle forward, instead using it to operate as a generator, producing electricity from the force of the wheels turning. In the process, the vehicle is slowed down. In this way, some of the energy used to accelerate the vehicle from a stop is recaptured as it slows back down, and can then be reused the next time it accelerates.
As an added bonus, regenerative braking relies on electromagnetic forces, rather than on friction. As a result, the parts don’t wear as the vehicle slows down. It reduces the wear and tear on brake pads, discs, and drums. Because sometimes more braking power is needed, vehicles with regenerative brakes almost always include standard friction brakes, as well. Used properly, regenerative braking will make the standard brake system of a vehicle last much longer.
This is a technology that has been used in trains, particularly passenger trains, subways and streetcars, for decades. Being on a large grid of electrical lines or rails, trains are able to simply dump the energy they recover back onto the grid, rather than having to store it on board. With the advent of gas electric hybrid cars which need to carry a battery array with them, it suddenly became practical to recapture energy during braking, because the car had somewhere to put it.
Once I finished my move, it took me a couple of weeks to get an internet connection hooked up again. However, once I had one available, I set about trying to discover how efficient regenerative braking systems really were. To my great distress, I’m still looking. Try as I might, I can’t find any reliable source of information on the subject.
I’ve located several resources with information about the efficiency of regenerative braking on trains. However, because trains use low voltage DC current, they suffer from large transmission losses. As a result, the efficiency of a regenerative braking system is dependent on the distance on the grid between the train and the switching station. The various estimates I found listed efficiencies ranging from as low as 5% to as high as 50%. I was not able to find any reliable data on the practical efficiency of such a system, independent of the circumstances of the grid it was sitting on.
The information on regenerative braking in cars is even less helpful. Claims range all over the map. I haven’t been able to find any reliable source of information. If one is willing to take the word of random people on forums, regenerative braking is anywhere from 99% efficient to actually worse than friction braking. A little bit of simple math will show either of those claims to be well beyond the pale, but this leaves a lot of questions about more reasonable numbers in between.
Companies that make hybrid cars spend a lot of time talking about the overall efficiency of their vehicles, but I wasn’t particularly interested in the gas mileage a Prius can achieve. Rather, I wanted to know how much of the available kinetic energy its regenerative brakes could recapture and make useful again. For this specific piece of information, I haven’t been able to find any kind of authoritative answer. In a twist of slightly worrisome irony, one of the better items I did find was an old homework set from a class at M.I.T. which lists the efficiency at 64%. Then again, it was written back when gas was $1.75 a gallon, so I don’t know that it really reflects the more recent innovations in the technology.
I’m hopeful that a member of the greater geek community can point me in the right direction. If anybody can direct me to a reliable source on this matter, I’d be very happy to hear about it.
-posted by Mark