WhiteBlue wrote:
The harvesting will occur in very short bursts of typically less than 2 s and that should be ok for the A123 prismatic cells to be used in 33C mode.
I have given you the written reference to that.
Beyond that I can only say that we are not in the same position as the teams who probably have access to customized battery systems with more advanced capabilities.
No WB, you have given no reference to any source, that it is possible to charge these accus with 33C.
The references in the file I posted and in other publications, all refer to the discharge rate.
You make the assumption, that the process is 1:1 reversable, which it is not.
At the same time, you choose to ignore the other figures given in the same paper, because they don´t support your theory. But this is fine with me, don´t worry.
I´m sure you have made up your mind, and won´t listen anyway, nevertheless for any other reader here, I will try to get some things straight, and waybe at the end WB will be a happy person, because he is 70% right.
After all, it´s Christmas, so this is my present to him:
From the limited freely published data about the A123 cells, we can get the following informations.
The cells have a nominal voltage of 3.3V@0.2C load.
If the load increases, the voltage will drop, due to the internal resistance of the cells.
The operational temperature limit is ~60°C, higher temperature will weaken the cell and the will loose capacity and cycle life.
The cells are able to whitstand burst of 33C discharge rate (some sources claim up to 60C).
On the A123 website is a max. charge current from 12C published which will result in ~90% in 5 min. charge/capacity ( I will come to that later)
The max. charging voltage per cell is 3.7V, higher voltage will result in destruction of the cell. (this is important, as we will see)
WB has assumed a cell voltage of 2.75V for his calculations, accoding to the informations we have, this is the cell voltage @ a 16C discharge, but we are looking for the double. Published data is availible up to 20.5C discharge rates.
The quoted cell voltage is 2.61V (not 2.75). But we looking for a even higher rate
30-33C. From the data given some pages ago, I have compiled the following table.
To the best of my knowledge, and fom the data availible, I have extapolated the values up to 33C, but I´m happy fo people to come foward with better data and prove me wrong, no problem with that.
For "my" calculations I will use a value of 2,41V per cell at 33C load.
Following WB´s choosen voltage level of ~305V we will need to add cells to our battery to have this voltage under load.
We need to increase our cell count to 127 cells and will have ~306V, we will keep our 3 cells in parallel to account fo the current needed.
Now we have 127x3=381 cells.
At 0,40kg pro cell our battery will weight in at ~152 kg.
This is for the cells only, it does not include the housing and not cooling.
And we will need a lot of cooling, as we will see.
So, now we have a battery powerful enough, to supply the requiered energy to make ~333kW power for 12s - no problem (306V*11A*3(cells parallel)*33(C)=333kW
(we just need two 18,5mm thick cables to get the current to our MGU´s, but that´s all cool), but this was not our challenge.
As we maybe only can spend 120kW in drive mode anyway (depending on the rules).
How we get the energy back into the battery?
Can we charge with 33C (that´s 333A) and why does A123 says max 12C charge?
Now if we look at the table and the highlighted yellow field, after I have converted the temp. values into °C, we see the answer. At ~12C we each the max. permitted temperature of ~60°C. As A123 probably does not expect the customers to put the batter into the fridge while charging, this is the "safety" limit.
Now, I see WB´s face light up, he will say, no problem, we water cool the battery anyway, so is he right?
Allmost, one small problem, we can´t have a higher voltage then 3.7V per cell (~468V for our battery, I will use 3.68V for my calcs), otherwise our cells will
fail.
Every battery has a lower internal resistance when it is empty, for a given voltage, this defines our charging current. As the charge increase (and thereby the voltage of the battery), the difference between the cell voltage (which is around 2.5V when empty) and or chaging voltage decreases and therefore our charging current. To keep the charging current, we would need to increase the voltage. But as in the case of this battery type the max. Voltage is limited to 3.7V our current will decrease untill it is allmost zero, when the battery is fully charged.
That´s why A123 says quickcarge 90% of capacity after 5min. It is not possible to quickcharge 100% without exceeding the max. cell voltage.
O.K. long story short, I have tried to back engineer the charging current, and that´s in the last (green) colum.
In average, we will, theoretical, be able to charge with ~213A if we provide a voltage of ~470V. (470V*213A*3 = ~300kW), and thereby tranfer ~300kW, which over our average 12s would make ~3.6MJ
Various sources quote the cycle efficiency of these type of battery with 90% in high load mode, such as an HEV. 300kW/333kW=0.9=90%.
Therfore I think, my estimates are not totally "on the mooon", but I´m happy to correct some parts, if somebody can provide better data.
The last line in the table (red) and the last colum (green) are values I have extrapolated, the other data a based on published test data.
So, everybody can make up his own mind, if he want´s to take them or not.
Now where does all that leaves us? Was WB at the end right?
Perhaps, allmost. It this theoretical a possibility? Yes !
Would any team in it´s right mind trying to attempt a system like this?
I don´t think so, but feel fee to make your own judgement.
Some other "fact´s?" based on published data:
If we take the battery efficiency as 90%, that means we turn 33,3kW into heat, that does not include the MGU´s or inverters, nor the cables and connectors.
To provide some perspective the heating system of a average single family house in Western Europe has ~18-20kW
(but we use it perhaps more then 7,68h per day, which is the equvivalent to 19.2s/minute)
So some good cooling is requiered.
At an price of 4,25$/Wh, such a battery would cost 3,3V*127*11Ah*3*4,25$/Wh=~58800$
(this does not include any labour, the housing or cooling system, just the cell price)
At a load of 16C the lifecylce is quoted with 280 cycles, as we use around the double, we may want to cut this in half 140 cycles = laps.
Which brings us in the ballpark of the quoted 2 races per battery, which is claimed from the 2009KERS
In a roundabout way, this would make 58800$*20races/2*2cars= ~1.2 Mill$/year per team. In reality the costs will be most likely higher.
This calculation was just for "giggles"
If we take our battery weigth, and add some weight for the 3MGU´s and inverters, we may say the car will be 185kg heavier, as the same car with no KERS.
If we take the 0.3s/10kg rule, the Renault F1 engineer has mentioned.
We have made our car ~2,55sec a lap slower, compare to a car with the same engine but without KERS.
If we take our 3.6MJ energy budget and spend it on a 75s lap, which means 63s where we not brake, and let´s say we have only 40s where we are not traction limited, we can average the energy over 40s into 90kW more power, then a non KERS car.
Assuming a 640kg KERS car and a 450kW I4turbo engine that´s 450kW+90kW/640kg=~0,85kW/kg
a non KERS car, if we did not have the min. weight limit would have 450kW/640kg-185kg = ~0,99kW/kg
both would have the same fuel (weight goes on top) and the same tires. Chances are that the ligther car would be able to use the softer tire for longer etc.
Some other calcs to put the "it´s no problem" statement into perspective:
in reality, it would be better to have 3 accu packs and inverter, one at each MGU.
But I´m not sure you can package 2 accu packs into the front nose.
If we use 3 systems, the current comes down to 363A, and you don´t need to run the one main accu with 3 cellblocks in parallel, which has advantges in terms of cross current risks etc. It would make the cables thinner, but you need more.
363A it´s still pretty decent, and contact resistance etc. can quickly add up to some serious loses.
Some other thoughts:
470V is a quite high voltage, think accident open wires etc., some serious isolation is needed.
Think racing in the rain, or trying to extinguish a burning car, after an accident.
50mA current flow through the body/heart can/will kill a person
I think segedunum made a good and IMHO valid comment.
If the attempt to harvest this amount of energy, and flywheel system will become a very interesting proposal.
IMHO it would make sense to try to charge the flywheel mechanical, direct from the engine or the gearbox.
Something like a double clutch, which under braking disconnects the engine, and connects via a planet drive the flywheel.
Then use the energy to drive one or two MGU´s on the front wheels, as the rear will be most likely be traction limited anyway even with only the IC alone, to help
acceleration out of the corners.
As a mechanical diven flywheel, will have some "upfront weigth" which is not so much dependent fom the amount of energy/power (all the gears), it is not really feasable fom a weigth perspective, with the Micky Mouse KERS we have now.
But it becomes more and more competetive as higher the energy. I can perfectly understand Patrick Heads perspective and agenda.
Let´s see how it pans out. But with much higher power and energyy levels, a all electric/battery solution becomes less and less competetive, unless we make a quantum leap in accu technology. - IMHO
Merry Christmas
some KERS MGU infos (not sure if 100% correct, maybe superceeded by now)
30% duty cycle on a 75s lap is 22,5s
