How much energy is actually regenerated?

Hey Greg,
Are you talking about kinetic energy as in- a 3200 lb object traveling at 60mph has xx kinetic energy and the brakes generate xx kWh?
Or are you looking to compare: it took xx kWh to get up to 60mph and the brakes give us xx kWh back?

Two different things I imagine...



Here's my calculations for kinetic energy (in kWh) for a 3200lb vehicle at 60mph velocity..

KE = 1/2 (M * (V * V))
1451.49kg [3200 lb vehicle]......traveling at .....26.8224 m/s [60mph]
(1451.49*.5= 725.75) * (26.8224^2= 719.44) = 522,133 Joules

522,133 Joules = 0.145 kWh in kinetic energy

 

When I googled around for answers to this recently I noticed there was a quite a bit of ambiguity in the sparse information I could find. Sometimes the question and answer was framed as "how much does a typical driving session capture from regeneration?". I don't think that is your question.

I think you are asking how efficiently the kinetic energy from driving down the road is captured into the battery when a driver is deliberately slowing the Prius PHEV without friction brakes. Also, a related but different question is how much energy is lost to friction vs. being regenerated when you apply the brake pedal at different speeds and pressures.

I haven't seen deeply detailed numbers on this for the Prius Plugin. I have a guess based on numbers I have seen for other cars and based on reported efficiencies for the different components in the PiP involved in regenerative breaking.

The motor and inverter in the Prius are each about 90-95% efficient typically and the battery is about 96-98% efficient. That implies that regenerating into the battery is probably about 80-85% efficient. Using that regenerated power to drive the wheels back through those components again leads to an overall round trip efficiency of about 60-75%.

In any case, it seems unlikely to be less than 50% under typical conditions as long as you aren't counting any partial friction braking that might come about from stepping on the brake pedal.

I'm guessing the PiP can only absorb about 10-20 kW of energy back into the battery during regenerative braking but I haven't seen many detailed numbers. There are some test data which imply that regeneration for coasting and braking is often under 10 kW (2nd link below) vs. up to 47 kW in the Volt under the same test conditions. This is not a problem for coasting but is likely a limitation for stronger braking.

So, the main issue might not be actual regeneration efficiency but rather the regenerative power limitations due to the battery size.

For component efficiencies see around page 62:
http://info.ornl.gov/sites/publications/files/Pub26762.pdf

See the left-side table on Track Testing on page 2:
http://avt.inel.gov/pdf/phev/fact2013toyotapriusphev.pdf

 

I think Jeff is right on the money from what I have found using real world guesstimates on my PiP.... There are two places where I commonly am able to see the amount gained going down a hill using Regenerative braking and then have to turn around and go back up the same hill later. Both are right the end of a longer trip.

One is out at Boy Scout Camp. There is a mile long (basically up and down some rolling hills) gravel road going into camp. I gain .5 miles of EV according to the guesstimate on the dash braking down the hill. On the way out I use .8 miles of EV to get out to the main road.

The other is a large hill right before the driveway into the parking lot at the Rochester and Genessee Valley Railroad Museum in Rush, NY. It is about 1/2 mile long and fairly steep. Again I will generate .5 to .6 mile of EV going down that hill. Leaving the Museum I can get about 3/4 the way back up the same hill on the acquired EV. It uses about .8 mile of EV to climb the hill.

Purely anecdotal but appears to be consistent every time I make those trips.

 

I like and agree with the feedback given so far, and have two small points to add:

From the equation in the first response, you'll note that the regenerated energy is related to the square of velocity. That means that you'll see exponential decreases as you slow down. In other words, at 30mph, your recovered energy will be, at most, 25% of that available at 60mph.

The capacity of the PiP to recover energy can be large or small, depending upon the current state of charge. As an example, I live on a mountain/canyon road, so it's all steep downhill when I leave. From the time I leave my house to the time I reach the bottom, I can go from 0 EV miles indicated to about 1.9-2.0 indicated. (if I don't already have a full charge)

 

I believe your answer is in post #3.

SPH-L300 ?

 

I've seen the 30-35% number before for Prius regenerative braking efficiency but the details behind that figure were ambiguous. My guess is that to get a number that low you have to be counting a portion of friction braking that was used to absorb excess energy that the battery wasn't able to take in.

 

I don't have a link handy, but when the first gen Prius came out Toyota claimed about 30% efficiency. And in the second gen they claimed it was vastly improved to about 50%. Of course these are probably "peak" numbers, not average numbers.
That would be 50% of the kinetic energy of the moving car is converted to watt-hours that you see on the display as regen.

Mike

 

They claimed 25-55% and then said they thought it would be easy to increase that to 70% by changing the final gear ratios from their motor to the road. They were also counting drivetrain and tire resistance in their test which isn't really part of regenerative braking since those would be a drag on their car if they hadn't done any braking.

They have different motors, inverters, and batteries than the highly refined Prius components. They said their motor may have been operating at about 70% efficiency in their test speeds which were under 30 mph. They don't actually have efficiency maps for their model of the motor. The Prius motor maps shown at the 1st link I posted previously show much higher efficiency.

These were interesting numbers but not directly comparable to a Prius.

The earlier Tesla quote is closer but also not directly comparable. That quote was from a 2007 article about the early model year of the Roadster. The Prius uses permanent magnet motors which can be somewhat more efficient in some circumstances than the induction design used by Tesla.

The full article is at:
The Magic of Tesla Roadster Regenerative Braking | Blog | Tesla Motors

 

I don't believe there is much if any friction breaking as long as the HSI is in the right 75% of the regen area. I'll admit I haven't watched the Scan Gauge II friction brake value much during regen, I'll have to check it on some steep hills.

 

The flywheel is an interesting concept, but I'm sure it has its own set of limitations. One of which, I would imagine, would be transfer of power from the flywheel at a standing start. It would take careful gearing accommodations and monitoring to prevent huge losses to friction. Same for capturing all the energy as the vehicle slows... If the flywheel is spinning faster than the input, it's not providing any benefit.

 

Thanks, that answers a question I had been meaning to ask, because that is what I had observed.

So the best tactic when travelling at speed and, for example, seeing the lights turn red is to brake as hard as the regen limit (half the bar to the left?), until you get down to just enough momentum to coast to a stop?

 

I have watched amps in/out of traction battery and applied friction brake force. The friction brakes are not applied until after you lit up all the ReGen. The last (3 of 3) white regen bar on my Prius v corresponds to a range of the highest Amps going into the traction battery. The interesting thing is that there is still some regen to be gained when the third white regen bar first lights up. Each white bar corresponds to a range of ReGen, so I found that I can continue at apply more brake pedal force as I cross into the third white region bar until I see approximately 90 Amps going into the battery, then I can start to see brake force being applied unless I back off the brake.

The PiP display that corresponds to ReGen is more detailed than any others. It has those little pixels in the triangle corner areas of the display that I feel show very detailed, very small values.

I have not observed battery in/out Amp values in my wife's PiP Adv, but I would assume that as long as you can maintain brake pedal force when you hit the end of the ReGen area on the display that you would not use any friction breaking on the PiP.

I need to check for larger Amp values going into larger (than our Gen II and v) traction battery, which would tell me that we can use ReGen breaking even more in the PiP vs non plug-in Prius (more assumptions to verify).

Hope this helps.

 

Great post, Chazz! Thank you!

Would love to see the numbers from the PiP.