Using a $600 SuperCap (120 kJ) supplementing battery to capture more regen efficiently

So doing some calculations, 120,000 Joules is about 0.033 kWhr. Which translates to 48 kW for 2.5 seconds, or 24 kW for 5 seconds, and so on. The Prius battery captures regen most efficiently at about a 12 kW rate or nearly 60 Amps. It supplements braking by friction once it gets it the region of 24 kW or so. Most drivers including hypermilers could get a bump in the amount and round-tip efficiency of the regen energy captured if even a 118 kJ supercap could be integrated into the hybrid system.

While there are a number of possible ways to implement this, the main point I'm making here is that a larger more expensive SuperCap is not needed to improve efficiency significantly in a larger number of the frequently occurring driving maneuvers.

To put this in perspective a 120,000 Joules SuperCap (excluding the HV battery) at 90% efficiency would have the energy capacity to
- accelerate the Prius up from 0 to 28 mph
- decelerate the Prius from 60 to 54 mph
- decelerate the Prius from 40 to 29 mph

One can start to conjure up many benefits of using such a modest SuperCap supplementing the HV battery. The Super-capacitor starts to become the primary energy buffer with the benefit of taking the primary strain of the large short-term swings in energy capture and demand, the HV battery would then become the excess storage/supply element of the system handling the power and current excesses beyond the SuperCap energy storage. As the round trip efficiency of storing electrical energy in a capacitor is much higher than a NiMh battery, this would translate to real significant improvements for drivers that have a high proportion of non-steady state driving. It allows even more aggressive stop and go drivers to significantly improve their city mileage.

A 120,000 Joule SuperCap would would be enough to handle the excess current flow (beyond what the HV battery can handle efficiently) that gets generated when braking at the deceleration rate of average traffic approaching a stop.

So the point of all of this is a large SuperCap is not needed. Each 120 kJ addition to the SuperCap capacity, would produce significanlty smaller and smaller gains. If even only 60 kJ of the capacitor storage could be utilized due to technical challenges or cost, it would still provide enough energy to accelerate a 1450 kg Prius to 20 mph, or efficiently capture a fast 20 to 0 mph deceleration. The high currents would be handled by creating the capacitance by a larger number of parallel capacitance paths, which would built into the design of the SuperCap. HV battery life would likely also be significantly extended.

While I haven't done any full calculations. I suspect the city mpg value of the Prius could approach the 70+ mpg mark in the EPA and even near 70 mpg when tested hard with car magazine journalist with their quick acceleration and momentum killing rapid deceleration habits.

 

Pricing Source: Supercapacitor Information – Battery University
I figure this is volume commercial pricing from 2011. I suspect, pricing and specs, have probably continued to improve somewhat since then.

To add to my previous post, while flywheel recovery has potential to be more efficient overall, I think in a hybrid like the Prius having all the HSD infrastructure already in place, the supercap or ultracap, would appear to be the easiest next logical step. A 120 kJ ultracap weighs about 7kg and can be built to a geometry to allow easy fitting.

There are possibly ways to have the unavoidable varying voltage of the capacitor (as its charge level varies) to not go through inverter conversion losses on both the charging and discharging side by having the HV battery control circuitry provide the delta voltage/current. So on the acceleration side, for example, as the capacitor charge/voltage depletes, the HV battery/inverter circuitry output could gradually take up the load. Software could control the optimum amount of charge in the capacitor depending on speed (i.e. at 60 mph, capacitor charge should be low, to allow for an impending deceleration)
Edit: I think the guys in this link are doing what I'm talking about. http://web.mit.edu/scolton/www/everpres.pdf

 

That's exactly what the analysis is based on. The battery round trip efficiency is at best 50% when all conditions are optimum, 60 amp charge rate, battery temp 70 deg F, optimum SOC, optimum inverter temp, etc. Anecdotal evidence from posts on this site, see users gaining 20 to 40 % regen recovery when no friction braking is used. One of the benefits of Ultracaps is much higher braking rates while much more efficiently capturing the regen. The only drivers that would not benefit much from the efficiency of Ultracaps would be hypermilers on a closed course who allow the car to completely glide until it stopped........or.....drivers that predominantly drive on highways at nearly steady speeds

For me personally it would be nice to approach a stop sign knowing the regen energy caputured could be almost independent of the deceleration speed. Then almost any driver could get hypermiler like mpg returns in city driving involving a lot of stop and go driving. So for an average Prius Gen III driver who gets 32 mpg city driving (Consumer Reports), a modest Ultracap should easily boost that to double the mpg. If 8,000 miles per year is city driving that would save $500 per year at $4/gal by real world aggressive drivers.

If the modest 120 kJ Ultracap was a $2000 Toyota option ($1000 for cap and $1000 for associated hardware), it would easily have a net positive ROI by many measures (fuel payback time, used car residual value, "unlimited" charge/discharge cycles, extending battery life)

 

Poor Power density as opposed to Energy density (of the HV battery) is the reason why traditional hybrid electric vehicles can't achieve far better mpg in the city than they already achieve. Theoretically, at 100% regen recovery a Prius would get near 100 mpg (maybe 80 mpg in real life....A/C, low engine load, etc.) at below 30 mph regardless of the amount and intensity of the stop and go. Of course engine management would have to be tweaked to take full advantage of this.
33 w-hrs round trip via ultra cap at 90% efficiency will give back 30 w-hrs to the road (with typical rapid deceleration). 33 w-hrs round trip via battery will give back maybe 4 or 5 w-hrs to the road with typical rapid deceleration (with 50% of braking energy from friction). Google the "Citroen Hybrid Air" technology which is expected to give 80 to 90 mpg, to see why it's not so much the quantity of the regen but the regen efficiency that bumps up those city mpg numbers so much. I understand the Citroen regen roundtrip efficiency is near the high 60s low 70s %.