Electrical Engineering

Technical summary: Supercapacitors

Rapid invention and evolution in the state-of-the-art have made it difficult to stay versed in the ever-growing variety of components, devices, and technologies that could be used in the design of a product.

These developments often promise the possibility of new capabilities, improved performance, or decreased cost. To help readers stay informed, Design Concepts will be authoring a series of articles to provide a high-level overview of various technologies, what their benefits might be, and how they might be utilized. The intent is not to provide a detailed engineering discussion, but a general understanding of the topic area with enough insight to determine if it might warrant further investigation.

What is a supercapacitor?

From the micro to the nano and the mega to the giga, the development of technologies that are either diminishingly small or increasingly large are major trends. Supercapacitors, also known as ultracapacitors, fall into this category. In this review, we will explore what makes this quickly evolving type of capacitor “super,” and how its unique characteristics might be utilized.

Electrical engineers are all familiar with the capacitor as one of the fundamental elements in circuit design. It stores an electric charge with its most basic defining specification, capacitance, being the amount of charge stored per volt applied. By controlling the rate and manner by which they are charged and discharged, capacitors can be used in a variety of applications including simple timing circuits or as part of electronic filters, such as in a radio receiver, to select signals of interest.

Because they store energy when charged, capacitors are also employed in the design of power supplies. Supercapacitor development has focused on expanding the energy storage capabilities of the basic capacitor and is creating new opportunities in a variety of power related applications.

History and evolution

Development of the supercapacitor began in the early 1960s by the Standard Oil Company of Ohio as a spin-off from work being done on fuel cells. Through major changes to the internal construction, the groundwork for today’s electric double layer capacitor or EDLC (the more scientific name for a supercapacitor) was laid. The new technology increased electrode surface area tremendously and decreased the spacing between electrodes, yielding dramatic improvements in device capacitance.

Increased capacitance meant that a much greater charge was stored at a given voltage. Capacitance is measured in Farads (F) and standard capacitors in the range of 100 microfarads (.0001F) are common. In the late 1970s, Panasonic began offering the Gold Capacitor™, a new EDLC with a capacitance around 0.5F. While able to store 5,000 times more charge than its older counterpart, these early supercapacitors suffered from high internal resistance that limited how fast the charge could be extracted. For this reason, their use was limited to low power applications such as providing backup energy storage for electronic memory chips. Panasonic continues to sell a more refined version of the Gold Capacitor™ for this type of purpose today.

Over the past 30 years, companies like Maxwell Technologies have worked to extend supercapacitor capabilities not only by increasing their capacitance, but also decreasing the internal resistance. Their latest BOOSTCAP™ family exemplifies the current state-of-the-art, with capacitance as high as 3,400F and an internal resistance less than 1/10,000th of the early predecessors. The tiny internal resistance means that the capacitors can handle very large charge and discharge currents, even exceeding the capabilities of batteries.

Storage mechanism

Capacitors store energy in a manner different from batteries. In batteries, energy is stored and released electrochemically through reactions between materials within a cell. Thus, charge/discharge cycles cause physical and chemical changes within the battery and eventually cause it to wear out. EDLCs store energy as an electrostatic charge between its electrodes, no chemical reaction takes place and no changes to the internal structure occur. This allows supercapacitors to perform through millions of charge/discharge cycles.

Supercapacitors are finding application in electric automobiles, rapid transit rail, and load leveling for renewable energy generation.

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