Hello,

We are a group of electrical engineering students, for our project we have to design a supercapacitor system for a car.

The source of energy is a fuelcell which fluctuates between 14V and 24V. The supercaps will be charged by this fuellcell. Our question is:
What is the best configuration for charging these supercaps and how can we discharge these supercaps.

The supercaps are 350F and they have a max voltage of 2.7 Volt. We think the best solution is charging in series.

Futhermore there is a dc dc converter between the fuelcell and the motor, which boost the 14V-24V to a fixed voltage of 37Volt.
Our motor needs 37 Volts and a minimum of 200 Watts.

Thanks in advance.

 

First things first. I'm not exactly sure what you mean by a bi-directional DC-DC converter. All of the topologies I'm familiar with are uni-directional. Let's put that aside for a moment.

Your basic requirement for the motor is 200W @37 VDC. This implies a current draw of 5.41 Amperes. As I have repeated too many times to count, "the power out of a DC-DC converter is always less than, and sometimes much less than the power in". If we use an efficiency of 80% for our initial back of the envelope calculation you can see we require 250 watts of input power. Assume we have a single supercap at 2.7 volts. 250 Watts is a current of 92.59 Amperes (Don't put your lips on it!). This rate of discharge will reduce the capacitor voltage to almost nothing in short order. Now let's put say four of them in series for 10.8 volts. Now 250 Watts will require 23.15 Amperes which is still a large enough current to exponentially discharge a capacitor in an awful hurry

Because you are using a boost converter, as the supercap voltage drops, the converter is going to suck more current out of the caps, to maintain the output power level. Now try as you might, you cannot get away from the supercapacitor's EXPONENTIAL discharge characteristic.

What discharge characteristic do you want for your storage element? You want a flat discharge characteristic, such as the one provided by a battery with a high energy density.

I would only proceed with your original plan if you can get an A for trying. If you have to produce a modicum of success to get a passing grade then you need to drop the capacitor idea. It will lead you down a rabbit hole.

The buck converter to charge a four layer capacitor stack is less challenging assuming you can charge at a relatively slow rate and that you take precautions to prevent uneven and overcharging.

https://en.wikipedia.org/wiki/Buck_converter
https://en.wikipedia.org/wiki/Boost_converter