Question L1-C5

The question is about the various possibilities of energy storage for a mobile robot that has to be energetically autonomous. The question is: "A company has developed a nanoprobing mobile robot (sliding on a plate using ultrasound actuators) operating under vacuum. Now they want to remove the powering cables that disturb the robot’s movements and integrate an energy storage system inside their robots. The devices do not consume much and need to be fully operational the maximum amount of time. They can go to a loading station, but the loading operation should be as short as possible so the ratio working-loading is maximized. Which energy storage would you suggest, being simple to implement and well adapted to this situation?" Four possible answers are proposed:

Answer A. Li-po accumulators. 
This answer is wrong. Li-Po accumulators are slower to charge than capacitors (possibly 1000x longer than any capacitor tech). As this application requires a quick charge, this is a wrong answer, even if the Li-Po accumulators. In fact this is not critical as the robot can come back to recharge.
In the explanation of the student we would like that the student mentions that the Li-Po accumulators are slow to charge. 

Answer B. Capacitors.
This answer is correct. Capacitors are by far the simpler energy storage to implement. They are good for low consumption applications, and no other option will charge as nearly as fast as a capacitor.
In the explanation of the student we would like that the student mentions that capacitors are fast to charge and therefore well adapted for this application. 

Answer C. Ni-Cd accumulators.
This answer is wrong. Ni-Cd accumulators are slower to charge than capacitors. Ni-Cd accumulators are electrochemical devices where the charging chemical reaction is not, yet, fast.
In the explanation of the student we would like that the student mentions that the Ni-Cd accumulators are slow to charge. 

Answer D. Fuel cells". 
This answer is wrong. Fuel cells under vacuum conditions would required a sophisticate mechanism for refuelling. Therefore they are not simple to implement, even if refuelling could be fast.
In the explanation of the student we would like that the student mentions that the Fuel cells can be charged fast but the refuelling would be complicated.

