Robots are typically powered by batteries. However, there is increasing attention towards providing them with hybrid fuel cell battery combination. Let’s take a look at plusses and minuses of each and how a hybrid system can work.

Batteries:

Advantages:

• They can deliver higher bursts of power than fuel cells. This is important for tasks such as quick moves, jumping, running.
• Batteries are simple systems, they require far less maintenance than fuel cells.
• Batteries have better energy efficiency, upto 90%, because everything is sealed and contained where chemical reactions are more straightforward with less heat and without byproducts such as water in fuel cells.
• They enable more precise actuation.
• They are more compact, modular and easily integrated into torso, easily removed and replaced.
• They are most widely used and well understood.
• Their charging is easier.

Disadvantages:

• They have lower energy density than fuel cells, which is the main limit on runtime. In other words, they last shorter duration than fuel cells. Long term autonomy is not possible with current technology. Needs to return to charging base frequently.
• Charging time is longer, even though continuous improvements are being made, it is still much longer than simply filling the fuel tank of a fuel cell.

Fuel Cells:

Advantages:

• They have higher energy density, which means they can store and deliver higher amount of energy per unit weight or volume.
• They can produce continuous power for a long time as long as they have their fuel supply, which is typically hydrogen.
• Can be refueled very rapidly, just by swapping fuel tanks.

Disadvantages:

• They cannot deliver high energy bursts as batteries.
• Their energy efficiency is lower, often around 40%-60%. This is because of the nature of chemical reactions such as combining hydrogen oxygen, producing water and heat being less efficient than the ones in a battery. More of the reaction in a fuel cell is converted into heat, which reduces efficiency.
• Fuel cells are more complex. They need pumps, hydrogen storage tanks, air intake system, humidifers, leak detection systems. They require careful treatment of hydrogen as it is highly flammable. They are more sensitive to contamination and require more maintenance than batteries.
• Fuel tanks can be bulky and it is harder to fit them inside a humanoid body. They are inherently more suited towards larger systems such as vehicles.
• They need time to warm up, they cannot start instantly.

Because of these characteristics, fuel cells fit better to heavier applications such as vehicles and backup power, while batteries are almost exclusively used in electronics and robots. As can be inferred from above, where each have their own strengths, making a hybrid system would indeed provide us with robots that can run longer time without sacrificing strength.

Hybrid Systems:

Robots are highly mobile systems which need compactness and lightweight for all components. This is especially true for humanoid robots, which not only have to balance itself while advancing but also must be able to perform all range of motions a human can. In addition, a humanoid robot needs to perform motions precisely with adequate strength. In addition, a reasonably long run time is required for a humanoid robot to be useful. Because of these reasons, a hybrid battery – fuel cell system for a humanoid robot can prove to be effective. Indeed, most advanced humanoid designs now started to employ hybrid systems.

With hybrid systems, we can now have longer total runtime, as the baseline continuous power is undertaken by the fuel cell, which relieves significant amount of burden from battery. The power provided by the fuel cell is used for a baseline continuous power, which is necessary for ongoing operation of sensors and central cpu, and for more specific power intensive tasks that require beyond baseline power, the battery and or supercapacitors kick in. Similarly for long outdoor field operations fuel cell is the main source, while for precise indoor tasks, it is the battery.

For a hybrid system to work, the power management system which we introduced in an earlier post here, must also be able to include the power cell among the tasks we had mentioned in that post. Please take the time to review that post to have a better picture at this point because the tasks mentioned there also apply here. So the additional fuel cell related tasks include: When to use battery and when to use the fuel cell, when to charge the battery from fuel cell, handling of rapid load changes by coordinating the load drawn from battery and cell, managing of active cooling strategies against heating. Additional internal sensors are needed for this, to monitor voltages, load and temperature changes, There must be certain safety cutoffs and redundancies in case of to handle small faults and ensure reliability and stability. To coordinate and manage all this effectively we need complex control algorithms which must handle all this in dynamically changing load requirements in real time.

By: A. Tuter

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