Of course, BMW are being very coy about the exact technical details of what’s under the bonnet of their X5 as they don’t want their competitors to find out exactly where they are in their research. Having said that though, I think we can still come up with a reasonably accurate educated guess as to what’s under the skin of the new X5 by performing some simple sums from the figures that BMW have
1) Power output
According to the article, the electric motor provides an additional torque of about 878lbft at 1,000rpm. So maximum power of the motor must be at least
167bhp, or 124kW (878*1000/5252).
2) Energy storage
Assuming that maximum power can be maintained for only 7 seconds, this means that the capacitor in the X5 contains a maximum charge of no more than 240Wh (0.24kWh) – approximately one eighth that of the Prius battery maximum storage amount.
3) Weight of ultracapacitor
The best ultracapacitors today have an energy density of around 6Wh/kg (vs ~65Wh/kg for the Prius battery). This means that the capacitor in the X5 must weigh a minimum of 40kg, not including housings and ancillaries.
4) Cost of capacitor
This is really difficult to quantify, but assuming that the ultracapacitor used in the X5 is made up of a bank of commercially available individual 2.5 volt, 5,000 Farad capacitors (see below for an example of what these look like), they would need approximately 55 of these to provide 240Wh of storage. I’ve worked this out on the principle that 240Wh is equivalent to 864,000 Joules, or 276,480 Farads at 2.5V full charge – somebody please write in if I’ve made an error somewhere!
These individual capacitors currently cost something like $150 each, so we can guess-timate that to make a bank this big yourself would cost at least $8,250 – and that’s before all the control circuitry and traction motors are factored in! However, Maxwell technologies reckon that by next year, if ultracapacitors are manufactured in volume, the cost will drop to around $0.01 per Farad, or $50 per 5000F unit capacitor ($11,500/kWhr). At this price the X5 pack would cost a more reasonable $2,750 (compared to the market value of around $1,200 for the current 1.8kWhr Prius battery).
What capacitors could mean for the Prius
The whole circuitry of the Prius is geared to being kind to the battery. If it wasn’t, the NiMH battery would be ruined very quickly. So, rapid charging or discharging is avoided. Complete depletion of the battery is also avoided as much as possible. Finally, every effort is maintained to ensure the battery only ever undergoes the shallowest possible depths of charging and depletion – meaning that for much of the batteries life it remains close to a fixed optimum percentage of battery charge. Thus, in everyday driving, the battery may only go up or down by maybe 20% of total charge, allowing many more charge/discharge cycles than the 1,000 limit if it were fully discharged each time.
On the other hand, ultracapacitors don’t need to be looked after anything like as much as traditional batteries. They can be charged from 0 to 100% charge in seconds, allowing huge power inputs which in turn means that a much higher proportion of braking events and hence braking energy can be directed to a capacitor compared to a battery. What’s more, this can me done millions of times over before any loss in efficiency is observed.
This means that for most applications for most of the time, you can get away with using a much smaller
ultracapacitor than you can with a battery. For example, the Prius may use for the majority of the time a charge/discharge depth of only 20%, or about 0.36kWhr. This is equivalent to a 60kg bank of todays ultracapacitors which despite the lower overall total capacity could do much the same job as the battery in the Prius, most of the time
As discharge is also extremely quick, a capacitor could easily be added to provide, say, an extra 200bhp for a 10 second burst of acceleration
. This amounts to around 410Wh in energy, or 68kg in weight - easily worth the weight disadvantage. By way of comparison, the maximum depletion rate (and hence power output) of the Prius battery is currently 15bhp for a maximum of 10 seconds.
So todays ultracapacitors could
have a place in todays hybrids if they weren’t so expensive! But my opinion is that if there’s any area of technology that gets cheap very quickly, it’s electrical components. Also, I’ve heard rumblings on the internet that a new wave of ultracapacitors may be on the way, borrowing ideas from the latest lithium polymer and organic electrolyte research, with energy densities approaching 100Wh/kg. If these things came to be, you could recharge an electric car in under one minute for a range of at least 150 miles.