UltraCapacitor--Eye Candy
1 - 20 of 60 Posts
Joined
·
30 Posts
Hmmmm looks like batteries. So, what kind are they? Stats?
Joined
·
7,791 Posts
http://www.maxwell.com/ultracapacitors/products/modules/bmod0165-48-6v.asp
Total of 14 in series. Seven shown.
major
Joined
·
7,791 Posts
Joined
·
4,326 Posts
when you get a minute... I am sure LOTS of us would like to learn more about how ultra-cap (F) capacity translates into Watt-hrs for getting a handle on possible braking regen.
I don't know enough about how to compare the possible energy from regen braking, figure the ultra-cap needed, and get a price to compare to just adding more batteries....
Joined
·
7,791 Posts
Hi Dan,
I have a minute or two. They just moved my office and I'm not unpacked yet. All references still in the box. But from memory......
Energy (E) in the capacitor E = (1/2)*C*V^2, where C = farads, V = volts.
Charging the capacitor happens during regeneration. The cap voltage is low to start and increases as energy is delivered to it.
The total energy for this event is E = E(high) - E(low).
Discharging the capacitor happens during the acceleration. The cap voltage is high at the start and decreases as energy is delivered from it.
The total energy for this event is E = E(high) - E(low).
Obviously E(high)max occurs at the maximum allowable voltage for the capacitor. And E(low)min occurs at the lower voltage limit for the system. V(max) and V(min), respectfully. For normal utilization which is compatible with the propulsion system, I use V(max) = 2*V(min). This is a 2 to 1 voltage swing. Or the working voltage window.
Put a little more math to it, and you'll see that a 2 to 1 voltage window allows you to effectively use 3/4 of the total energy stored in the capacitor, calculated from the manufacturer's specified maximum voltage and farad rating. So, E(usable) = (3/8)*C*V(max)^2. Using farads and volts yields the energy in units of Joules. A Joule = watt*second. J=Ws. You have 60 s/minute and 60 minutes/hour. So a Wh=3600Ws. And 1000W/kW, so 1kWh=3,600,000J.
1 Joule seems pretty small in the energy dimensions used for EVs. One watt second is like the energy from a 1 volt battery delivering 1 amp for 1 second. Wouldn't get the old EV down the road very far
Now, how to equate the energy needed for regen. For now, assume no elevation change. Where does the energy come from in the first place? It originally came from the source which was used to accelerate the vehicle to the speed (velocity) at which it is currently traveling. At this velocity, by virtue of the mass of the vehicle, it has a certain energy in and of itself, called kinetic energy (KE). From basic physics, KE = (1/2)*M*v^2. Use units of kilograms (kg) for mass (M) and meters/second (m/s) for velocity (v) and the KE units are Joules (J). Surprise!
In a loss free universe, one would set the capacitor energy E(usable) equal to the KE of the vehicle at the desired velocity. Which isn't too bad of way to go. But, the road losses (friction and aero) always play against you. Or in other words, subtract from the KE and cannot be converted into electrical energy to be stored in the cap. And also, the propulsion system losses work against you. More energy which will not make it into the cap. So, in reality, depending on a lot of factors, you might actually be able to put 50% of that KE into the energy storage device (capacitor) during the deceleration event. You could effectively size the capacitor to this energy.
Fifty percent is just a guess. A lot of factors play into this. A big one is how quickly you decelerate. For effective regeneration, the quicker the stop, the better. Less energy is lost to the imperfect universe in friction and windage. And then there is the system efficiency. I have seen overall energy recapture rates as high as 70%, but feel you could also see very low rates, 20 or 30%.
When it comes to electrical energy storage, you pay for every Joule. So one would attempt to choose the capacitor just large enough for the energy which can be effectively delivered to it during the regen event. Ultracapacitors are particularly well suited for this because when they are sized this way, they will likely have the needed power capability. Most batteries must be oversized in terms of energy capacity in order to provide the power capability for regeneration.
I mentioned elevation change above. I live in like the flattest county in the country, so don't pay much attention to elevation. However, hilly terrain can be a big factor for regeneration. In this case, one must take into consideration the change in potential energy (PE), which is added to the change in KE for the regeneration event. Say for a downhill travel, the regen brakes are used to maintain a constant speed. Then there is no change in kinetic energy. All the regen energy comes from the change in PE. It is calculated in a similar manner as above. See your local physics text book or web site for details.
Regards,
major
Joined
·
4,326 Posts
super info, I think I can convert to metric units and come up with the total (aproximate) energy available in my minimal urban stop light example. Say.... stopping a 2500# vehicle going 45mph as the 'average' available energy.
Then sizing the ultra-cap system to twice that amount to allow for working voltage range and losses would give an aproximate cost to evaluate when considering the regen would probably extend range 20-30% for typical urban conditions; as compared to adding 20-30% more standard lead batteries for the same range.
THEN.... if the cost/benefit is acceptable, we'd have to learn a lot more about the wiring required to regulate the voltage so as not to blow up controller and motor, and how to pump energy in during braking, and use from capacitors first during accel until cap voltage drops to the floor, which would presumably be the satandard battery voltage.
Then sizing the ultra-cap system to twice that amount to allow for working voltage range and losses would give an aproximate cost to evaluate when considering the regen would probably extend range 20-30% for typical urban conditions; as compared to adding 20-30% more standard lead batteries for the same range.
THEN.... if the cost/benefit is acceptable, we'd have to learn a lot more about the wiring required to regulate the voltage so as not to blow up controller and motor, and how to pump energy in during braking, and use from capacitors first during accel until cap voltage drops to the floor, which would presumably be the satandard battery voltage.
Joined
·
7,791 Posts
Hi Dan,
Twice is too much. The losses always work against you. Even allowing for the working voltage range of 2 to 1, I'd use "equal" instead of "twice". Ultracaps ain't cheap. No need to buy and carry more than you'll need. But then again, when it comes to energy, more is better. Having more energy in the cap than you'll use allows you to move the voltage window up and therefore get more power at your current limit. It all comes down to design choices.
To get above 20% extended range from regen, you'd need a very efficient system and be making like 10 stops per mile, every mile. Your expectations may be inflated. But you can define a drive cycle and do an energy spreadsheet analysis and see if it is in the ballpark.
Using regen to extend range compared to adding batteries is a tough nut to justify. But, consider brake wear saving and the fact the regen system and caps will last the life of the vehicle whereas extra batteries will eventually need replacement, and it looks better. Also, regen is fun. It is a good feeling when you come to a regen stop and know that energy wasn't wasted warming the atmosphere.
Good luck.
major
Joined
·
1,506 Posts
Considering their largest capacitor they make of that model (165 farads at 48v) is 213,000 joules roughly, or 0.059kwh, why didn't they just use lithium ion batteries?
I assume one of those caps was in the $1000 range a piece... for a mere 0.06kwh, they'd be hard pressed to have the bus functioning by the time lithium dies, let alone make the added durability of caps worth it.
Am I missing something on the cost effectiveness table here?
A small 1 cell lithium ion battery could replace 5 of these batteries, let alone what a prismatic batt could do.
Ice engine won't last 1,000,000 battery cycles, 50 ice engines probably wouldn't
I assume this system cost 15k-20k in caps. Whereas the 0.84 KWH in this pack would cost roughly $200 in lithium form.
Maybe I'm missing something here on energy densities/price, but usually ultracaps are freaking expensive
Joined
·
7,791 Posts
What would the total cost be for a lithium battery system capable of the 160 kW required to stop the bus in 7 seconds for a couple million cycles?
Bus has been running for 4 years. This was a capacitor up-grade. It is a parallel hybrid, energy recovery/launch assist. It increases fuel economy about 25% on stop intensive routes. The ultracapacitors will last the life of the vehicle whereas any battery available will not. Total life cycle cost is better with capacitors than batteries.
Yes, energy density for batteries is much higher than ultracapacitors. But that is not the only factor when choosing the energy storage system. This application required little energy and lots of power, high efficiency and durability. Ultracaps made sense.
Regards,
major
Joined
·
7,791 Posts
The Cummins ISB should do several million accelerations, maybe with a little maintenance. What does a battery cycle have to do with it?
Joined
·
1,506 Posts
low voltage high amperage cylindrical cells would work fine as they could be charged at a 10-15C rating in bursts for a lot of short cycles.
You're correct it will last the life of the vehicle, no doubt about it. The question is if 2 lithium cell replacements is cheaper than 1 set of those, my guess is yes since you're upgrading after 4 short years.
I don't disagree with the choice per se, I'm just curious on the cost effectiveness or if the government should have been going with different engineering standards. Ultracaps are well and good but the weight addition, cost addition likely will be more damaging to overall performance per dollar than batteries or dropping the hybrid design completely. Granted I lack the political "prowess" to do much else besides cost benefit schema, clearly there is other motivations out there.
Buses are a decent application for hybrid designs because of the constant stopping/going in short distances. I don't doubt you will see the 25% figure for efficiency increase. I simply don't know if ultracaps are doing something lithium can't here. How much amperage for a 30mph stop are you REALLY dumping into these caps? I can't imagine it's over 1500A.
Anywho it's definitely an interesting project
If you're dumping 160kw for say 10-15 seconds at a time (which would be indeed a hard stop and a speedy one), you'd need something like a 20-30C charging rating for bursts... I'm sure Li-ion could supply you with something like that, might be able to extend range too with the large pack you'd want (say 4kwh of lithium)
According to my sources the 165F 48v verison is 1372 Euros, or 842 Euros in quantities over 100, so 1130USD/ea at the cheapest from this particular source.
There's always the option of having a high power DC-DC converter to transform the amperage to something useable and the voltage sky high on the recharge cycle.
Cheers
Joined
·
7,791 Posts
Hi Technologic,
It is fused at 300 amps. 680 volts max.
I looked into batteries for this application. Still am. Basically anything that is actually available, with the required support (BMS), cost more than a single set of UCs. And the up-grade was to the best UC available because the original UC (sourced from Russia 5 yrs ago) was not an option for continued development and commercialization.
If you can add a component (or system in this case) to an existing vehicle which can recover cost in the life of the vehicle and reduce fuel consumption and emissions, what's wrong with that?
Hey, give me a battery which will do the job, or a fuel cell, or a rubber band. I really don't care. I'm just trying to make this place a little easier to live in.
Regards,
major
Joined
·
1,506 Posts
The BMS for such a system would have to be custom I'd imagine, but in reality that voltage/amperage would be a simple thing to find in a fast charging lithium component...
It's an interesting project...
Joined
·
4,326 Posts
ok, I am trying to work my way thru this...
so, energy available from stopping a 2500# car going 45mph is....
KE = (1/2)mv^2
(1/2) * 2500#* 1kg/2.2# * (45miles/hr*(1609 m/mile) * (1hr/3600 sec))^2
229837 Joules available from a single typical stop,
...now, since Joule = W*s, and an average stop is maybe 10 seconds of braking, that means it would come in at about 23000 watts for 10 seconds. WOW. so at 96 volts, thats a max of about 240 amps. Obviously more than (any kind) batteries can absorb...
...right?
so the next step, to figure number of caps required requires knowing the voltage. My EV happens to be a 96v system, so I am assuming I could build a bank of ultracaps in series or parallel to provide a matching 'low voltage' of 96 v, and a max of double that, right?
my question is which voltage to use in looking at the ultra cap specs to match this all up. and then, if the 'charged' ultracaps are at 2*96 volts, how would I bleed energy back out to my system without frying controller and motor?
Joined
·
68 Posts
I think it probably has a lot to do with charge/discharge rate. The "deceleration event" would typically happen over the course of maybe 5 to 10 seconds, and the acceleration event is similarly short. While the total energy may seem small from an EV perspective, the Power is massive, because the time is very short.
You would need to grossly over size your lithium pack in order to accept the charge in the short time it is being generated by the slowing vehicle. Likewise, you need to be able to use that power very quickly. (very high discharge rate compared to the total capacity).
Ultra caps have very low internal resistance, and are able to accept and deliver charge at a much higher rate than any kind of battery.
jp
Joined
·
68 Posts
oops, didn't notice there was another page of comments. Mine was redundant.
jp
jp
Redundancy is good, for rockets, nuclear power plants
and especially for human memory retention.
Now, where did I put my glasses and what did I want to see
after I found them?
Mike
Joined
·
7,791 Posts
Yeah, I guess. Still haven't found my calculator, so trust your math. Now the 230kJ was just the kinetic energy. So you'd subtract losses from that and really need only like maybe 160 or 170kJ of usable energy storage. And the current profile for a complete decel will resemble a triangle. So, the maximum current will probably be on the order for 400 amps. Just kinda guessing. But it looks like you got the right idea going there.
This all gets into the system design. You have to keep the maximum capacitor voltage where the manufacturer has set it. And you have to keep max voltage below the limit of your motor controller. So, you either live with half voltage when the caps are discharged, or look at some type of voltage controller between the caps and the rest of the propulsion system. Such a device is often called a buck-boost DC/DC converter. Not something you can buy off the shelf. And would be on the order of power and size of the motor controller. Maybe you can steal one from a Prius. Or just design the system around a 2 to 1 voltage swing.
Regards,
major
1 - 20 of 60 Posts
Join the discussion
