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Managing and delivering power to an autonomous robot are the main challenges to their development and implementation. Robots have a range of power requirements depending on a given task and require instant high power for maximum load activities, such as moving quickly or lifting. They also need to be useable for long periods before recharging, so their power source needs to have a high energy density in as lightweight a form as possible.
Ideally, a completely autonomous robot, one that doesn’t need to return to a charging point is the ultimate goal of developers devising power strategies and systems to provide those power needs. Supercapacitors are a promising development of an established technology that could be a component in making that goal a reality.
What is a Supercapacitor?
Supercapacitors are also known as ultracapacitors or Electrochemical Double-Layer Capacitors (EDLC).
An electrochemical capacitor is similar to a battery in that it has two electrodes, an anode and cathode, an electrolyte and a conductive charge path. They have an additional component and that is the separator that electrically isolates the two electrodes. Positive ions diffuse to the negative electrode and negative ones to the positive electrode when a voltage is applied to the electrodes. An accumulation of electric charge occurs at the surface of each electrode, forming a double layer (hence the name electric double-layer capacitor).
Each double layer works the same way as a conventional capacitor but with one at each electrode. A supercapacitor is effectively two capacitors in series by design. The capacitance (that is proportional to the energy that can be stored) is directly proportional to the plate area and inversely proportional to the plate separation.
In a normal capacitor, the plate separation is the thickness of the dielectric — measured in tens of microns while in a supercapacitor that distance is only nanometers (one-thousandth of a micron). The supercapacitor uses carbon technology in its electrodes that allows for a greater surface area. The architecture of the carbon makes the effective area up to 100,000 times greater than the square area of the electrode itself.
What are the Advantages and Disadvantages of Using a Supercapacitor in a Robot?
Supercapacitors charge in seconds and can be charged through many more cycles than a battery. Because they charge quickly they can also discharge at speed delivering a power of up to 10,000 W/kg. Li-Ion batteries are in the range of 2000-3000 W/kg. While a battery can deliver its voltage consistently as it discharges a supercapacitor drops voltage as it discharges meaning it needs additional management to deliver its power smoothly. They also are inefficient at holding a charge, losing 10-20 percent of their charge per day.
A battery and a supercapacitor have advantages and disadvantages that are opposite to each other and it makes them perfect partners in a power strategy. However, supercapacitors are a relatively new technology and consequently are an expensive initial outlay compared with a battery. Volume production and new materials will close the gap as will the longevity of the supercapacitor that should have at least 3-4 times the life of a battery.
Why Use Supercapacitors in Robots?
It is the complementary characteristics of supercapacitors and lithium-ion batteries that make the former a valuable component of a robotic hybrid power system. As a supercapacitor can provide instantaneous high power and effectively store the regenerative energy when a mobile robot is braking or going downhill, combining them with batteries seems a perfect solution and this is an area of significant research activity.
In a small robot or micro swarm robots, you could use supercapacitors as the only power source charging with a renewable energy source such as solar.
Developments using graphene and carbon nanotubes are going to make the supercapacitor a cheaper and even more efficient power storage solution. Ultra-thin graphene-based stretchable supercapacitors that could be used to power soft robots and advanced wearable technologies have been developed by scientists for the first time by scientists at Nanyang Technological University in Singapore. Their research will allow them to further increase the surface area of the electrode meaning it can store more energy.
An Estonian start-up, Skeleton Technologies, run by two twenty-year-olds have identified the huge potential in graphene-based supercapacitors Oliver Ahlberg one of the founders predicts that the market size for large supercapacitors (those with a capacity over 100 farads) is expected to grow from €500m to €3.6bn over the next five years. Their largest competitors in the market are Maxwell Technologies, Ioxus, and Nesscap.
It is inevitable that supercapacitors will become an increasingly important element in power solutions for mobile robots and even electric vehicles as their costs come down, power delivery increases and it is realized that they can be produced using cheaper and more environmentally acceptable materials than batteries.
References and Further Reading
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