Hacker News Reader

Comment by Retric | Hacker News Reader

Regenerative breaking systems is one of the obvious places for supercapacitors. They are not quite at the energy density or cost level where every car uses them, but with some minor improvements they will become standard. The advantage is of course better mileage and increased battery lifespan.

PS: There are even advantages with IC engines: http://articles.sae.org/11845/

View on HN

The density/weight needs to be improved a lot for it to be effective compared to existing battery technologies though:

Mazda supercapacitor:

0.007kWh @ 6kg - 857kg/kWh

Prius with nickel-metal hydride batteries:

1.3kWh @ 42kg - 32kg/kWh

Tesla with lithium-ion batteries:

85kWh @ 544 kg - 6.4kg/kWh

Breaking systems only really need to contain enough energy to stop a car. So, up to 1/2 * 2,000kg * (40m/s)^2 = 1.6e6 J which is a lot of energy, though less than 3.6e+6 J. Going down a mountain is an important edge case, but frankly not something that you really need to use super caps for as these systems still have mechanical breaks for backup, and the lower peaks is more acceptable for battery's.

In an electric car you would then use that energy to speed up again, thus allowing for 10s if not hundreds of charge cycles on a single trip.

Note: 2000kg ~= 4,400lb, 40m/s ~= 90mph.

You forgot to divide by two.

It's actually 1.6 MJ, not 3.2 MJ.

Just edited before seeing your note. But, thanks I always forget about that 1/2.

Just to be pedantic, it is "brakes" for slowing things down, and "breaks" for ruining or damaging something.

Are these figures right ? Gasoline lists on wikipedia [1] as 46.4 MJ/kg which Google gets as 77g/kWh.

[1] https://en.wikipedia.org/wiki/Energy_density

Those figures are right, the best batteries today are still like one percent as good as gasoline at storing energy. Although, you can only get about 20 percent of the energy out of gas while you can get about 90 percent out of a battery.

Right, and ultimately, hydrogen is one of the most (if not THE most) efficient energy storage methods. However, as far a physical storage, actually putting it in a tank, it is not as efficient. Plus, burning it is not as efficient as electricity either.

Electricity and batteries are the way to go. They are around 90% efficient today and will only get better with better cooling methods and recapturing heat loss/preventing heat.

I didn't think about regenerative breaking and of course it looks like a good place. But in the end of the cannot this be absorbed by the batteries with a good configuration? I mean, adding N supercapacitors that can absorb X power each would be proportional as configuring it as M batteries (where M >> N) charging that can absorb Y power each where Y = X * N / M => Y << X, which seems doable. So you'd either need enough capacitors to use them effectively (adding cost/complexity) or enough batteries, right?

Is that the reason why Tesla can do regenerative braking except for strong stops? Because the batteries can absorb part but not all of the sudden energy?

I imagine the strong stop problem is more about actually stopping the car fast enough. Easier to do with friction than induction.

That will depend on your speed. High speeds favor inductive braking.

Anyway, if I were to guess, I'd say it's because ABS tech is very mature, and inductive breaking is not, thus safety favoring friction.

Battery's don't really like very short partial charge cycles, and the electronics to track these energy spikes get's difficult. Storing ~e6 J in ~5 seconds takes a lot of battery's. Also of note, because battery systems can only take X energy the rest of the system is only disguised to supply X energy. Most hybrids use battery's and simply kick in normal breaks if you do more than a slow stop.

But, with super cap's a lot of this goes away. They can store more energy so you can add larger alternators and recover more energy. Further, they have a higher % efficiency for charge / discharge cycles.

So, you have 3 tiers of rate vs capacity?

* Capacitor - high charge / discharge rate (power), low capacity (total energy). (and high risk of electrocution and spot welding)

* Battery - medium power, medium capacity.

* Small ICE - low power, high capacity.

Reply on news.ycombinator.com