Technical Definition Ultracapacitors
Post on 19-Dec-2015
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Ultracapacitors: The New Era of Energy Storage
As our global population expands, so does our need for energy. Although efficient and renewable energy resources are an imperative focus, an equally key effort is to optimally store energy. For decades, batteries have been the forefront technology for storing energy. However, with an increasing need to store and release energy rapidly to meet demands, new developments have led to the ultracapacitor. The ultracapacitor, also known as an electrostatic capacitor (EC), allows for portable energy storage without several of the downsides of a battery. Basic design principles of the capacitor, comparisons with lithium-‐ion batteries, and applications of the ultracapacitor will be overviewed in this document.
What is an Ultracapacitor? An ultracapacitor is an energy storage device that stores electrical charge in two parallel electrodes. The primary basis for storing charge in the capacitor is due to a phenomenon known as the electric double layer. In order to understand the principles of the electric double layer, it is first vital to understand the design of the capacitor. The ultracapacitor is constructed in a ‘sandwich’ design, which is shown in detail in Figure 1.
The Ultracapacitor ‘Sandwich’ 1: The Bread (Carbon Electrodes)-‐ The carbon electrode is the electrical conductor of the capacitor. The electrode connects the capacitor with the rest of the circuit to allow transfer of electrical charge. Carbon is chosen as the electrode material to allow for high surface area within the conductor. The high surface area permits more charges to be stored within the electrodes. 2: The Dressing (Electrolyte)-‐ The electrolyte is a mixture of both positive and negative ionic charges that is dissolved in a solution such as water. The liquid electrolyte
1 1 3 2 2
Figure 1: The Ultracapacitor ‘Sandwich’ 1
contacts both the carbon electrodes in the capacitor. 3: The Meat (Separator)-‐ The separator is an insulator that divides the two carbon electrodes from one another. The separator is often made of some porous material such as paper, plastic etc. Although the separator prevents the electrodes from contacting, its porousness allows for the transfer of ionic charges between the electrodes.
The Electric Double Layer One of the basic principles of electricity and magnetism is that opposites attract. When an external voltage is applied to the capacitor, positively charged ions in the liquid electrolyte are attracted to the negatively charged electrode and vice versa. The two zones where the opposite ions attract in Figure 2 are known collectively as the electric double layer. This creates an electrically neutral gap between the two electrodes. This gap creates an electric potential, which is synonymous to an electric charge. The longer the voltage is applied to the capacitor, the larger the charge that builds up.
Figure 2: The Electric Double Layer 2
Ultracapacitor vs. Battery -‐ What is the difference? Batteries have been the dominant energy storage technology for as long as we have been alive. As one may have noticed with a device such as an iPhone, batteries can provide complications. The lithium-‐ion battery in the iPhone can take hours to fully charge, and it is depleted rather quickly when charged. Also, the longer a user keeps their iPhone, the shorter the charge state becomes for the battery-‐ a correlation that
is due to cycle time fatigue. Ultracapacitors are capable of providing alleviations to these common problems. When analyzing the effectiveness of ultracapacitors, it is key to understand the differences between capacitors and batteries. Here are some crucial contrasts between the two: Table 1: Batteries vs. Ultracapacitors Characteristic Lithium-‐Ion Battery Ultracapacitor Charge Time ~3-‐5 minutes ~1 second Discharge Time ~3-‐5 minutes ~1 second Cycle Life <5,000 @ 1C rate >500,000 Specific Energy (Wh/kg) 70-‐100 5 Specific Power (kW/kg) .5-‐1 5-‐10 Cycle Efficiency 50%-‐90% 75%-‐95% Cost/Wh $1-‐2 $10-‐20 Charging Method: Batteries store energy via chemical reactions. During these reactions, the battery also undergoes physical altercations between the charged and uncharged states. As the battery continues to supply charge, chemicals are used up in the reactions. When all the chemicals are used, the battery is dead. Conversely, capacitors store energy physically and they experience no key transformation in physical states amongst charges. ` Charge/Discharge Time: The energy storage and release time for an ultracapacitor takes about a few seconds or less. During this short discharge time, ultracapacitors are capable of releasing great amounts of power. Conversely, a lithium-‐ion battery can take several minutes or even several hours to fully charge. Cycle Life: Although there have been countless technological developments in batteries, the chemical changes that occur in the cell constrain the total life of the battery. Some batteries proclaim to have life cycles of around 10,000 charge-‐recharge periods, but this quantity is miniscule compared to the capacitor, whose life cycle is measured in millions of cycles. Energy Density: As of right now, batteries have a much higher energy density than capacitors. Energy density is the amount of electrical energy that is able to be stored per unit volume. This is an extremely important measurement when dealing with portable devices; higher energy densities lead to smaller and lightweight charging mechanisms. There are several other slight differences between the two that also influence functionality and performances of the devices. For quantifiable differences between the two, see Table 1.
Where are Ultracapacitors Used? Ultracapacitors are already used in a wide variety of different applications that span unique functions throughout the globe. These capacitors are only compatible with DC (direct current) power applications, thus limiting the ultracapacitor’s functionality. Ultracapacitors are typically preferred over batteries and other energy storage devices in three different scenarios:
Ø A sizeable quantity of power is required for a comparatively short time
Ø A high amount of charge/discharge cycles is needed
Ø A lengthier lifetime is essential
Figure 3 displays several applications where ultracapacitors are an essential component.
Challenges for the Future After reading this far, you may be wondering: “Why aren’t ultracapacitors used everywhere?” The answer to your question: Cost. Despite a premiere level of performance, quality, and safety, ultracapacitors are just too expensive to compete on the free market with lithium-‐ion batteries. Also, as previously mentioned, the relatively low energy density of ultracapacitors remains an improving point for researchers and scientists. In some instances these capacitors are too large to be an adequate choice for energy storage. Therefore, as technology advances, it will be imperative to lower cost and increase energy density of the ultracapacitor without surrendering the redeeming qualities that differentiate it from traditional batteries. Hopefully, in the near future, the stomach-‐turning words “my battery is dead” will be nothing more than an obsolete saying.
Power Tools Tools such as electric screwdrivers or nail guns require a high pulse of power in a short instance of time. Ultracapacitors are chosen as the energy storage method because of this short power constraint and for short charge/recharge times
Forklifts Similar to power tools, forklifts require extensive power in a short amount of time. Ultracapcitors are the utilized technology because they can meet power demands and are allowed to be volumetrically large-‐ which permits greater energy and power
Light Rails and Trams Trams in larger cities often run constantly, therefore needing a technology with a high cycle time. Capactitors are part of the electrical systems in trams to meet the longevity of the application. Cities also have strict pollution rules, and capacitors are an excellent earth-‐friendly storage device.
Figure 3: Applications of Ultracapacitors
References
1-‐ http://www.explainthatstuff.com/how-‐supercapacitors-‐work.html 2-‐ http://www.intechopen.com/books/dynamic-‐modelling/dynamic-‐
modelling-‐and-‐control-‐design-‐of-‐advanced-‐energy-‐storage-‐for-‐power-‐system-‐applications
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