On 16/10/17 22:15, 'Dudley'transitconsult@...
[TramsDownUnder] wrote:
> Compare them with supercapacitors - these are designed for very quick discharge and

> recharge. They are believed to have a long life time - SFAIK none of the original

> supercaps have had to be replaced because they have worn out, more likely replaced for

> test and examination.


But they have MUCH lower 'energy density' than the best lithium
batteries, a figure of one tenth is given.

Upper end is 15 Wh/kg
Lead Acid battery 40 Wh/kg
Lithium battery 265 Wh/kg
Petrol 12300 Wh/kg (But a petrol engine is only 30% efficient), so say
3700 Wh/kg usable.

The capacitors are improving in energy density as materials science
comes up with new materials, but the similar gains are being made on the
battery front too.

> For buses, it seems that design is going the battery route - perhaps the familiarity of

> the car battery makes these seem better than the "new-fangled" supercap.

No - it's simply the energy density issue. You need so much MORE of them
to equal the chemical energy stored in a battery. For a given capacity
the capacitor bank has to be 10 times heavier.

Hence hybrid systems that have both - you have the capacitor system for
a quick boost (and discharge rates for a 'sprint') and batteries for the
long haul, that charge and discharge at much lower rates.

>

> With trams, I would suggest that supercaps is the way to go.

>

> Query, does anyone know if they develop much heat in use?

At high currents the 'I squared R' losses in the connecting wiring start
to become significant. While the capacitor may not get hot, it's wiring
will at high currents.

> For info, I came across this, which may or may not help:

> http://energyandcarbon.com/lithium-ion-batteries-versus-supercapacitor/

>

> Any comments, please?

>

They completely gloss over the energy density issue.