Myths vs Facts
Supercapacitors and ultracapacitors are one and the same. The difference in the nomenclature can be attributed to the Europeans and the Americans. Europeans call the same device supercapacitor while the Americans know it as ultracapacitors...
- Dr P B Karandikar,
Dr N R Kulkarni,
Supercapacitors or ultracapacitors, have garnered a lot of interest as an emerging environment friendly device with its unique characteristics, like high power density and cyclability, that is currently unmatched by other storage devices. Similar to the conventional capacitor, the supercapacitor stores energy by charge separation.These characteristics are already being exploited in variousapplications ranging frommobile charging, toys, sensors,large scale transport systems like subway trains and buses, energy storage at intermittent generators and smart grid applications. The enthusiasts of this technology believe that it can garner a large part of the global $85billion battery market. But due to various factors this seems to be far from the reality.Here are some common myths about this technology, along with the facts to set the record straight.
Supercapacitors, ultracapacitors and electrochemical double layer capacitors are different devices.
Supercapacitors and ultracapacitors are one and the same. The difference in the nomenclature can be attributed to the Europeans and the Americans. Europeans call the same device supercapacitor while the Americans know it as ultracapacitors. Electrochemical Double Layer Capacitors (EDLCs) are the most common type of supercapacitors which store charge at the positive and negative electrodes by the separation of charges at the electrode-electrolyte interface. Each electrode–electrolyte interface represents a capacitor and the complete cellcan be considered as two capacitors connected in series thereby making the double layer capacitor. Contrary to what most people believe, there are other types of supercapacitors that use surface chemistry like pseudocapacitance for charge storage. They are technically known as pseudocapacitors. Another variant is the hybrid capacitor which stores charges similar to the pseudocapacitor on one electrode and like the EDLC on the other electrode. The figure below shows the classification of supercapacitors depending on the mechanism of charge storage.
Figure 1: Types of Supercapacitors...
It can replace batteries.
Replacing batteries with supercapacitors entirely is just not possible except in very few applications where for short bursts of current is the main requirement. This is due to the fact that supercapacitors do not have the high energy density which is the hallmark of batteries. A typical supercapacitor has an energy density in the range of 1-5Wh/kg while the same of the battery is range of 8-400Wh/kg. This means that when energy is required for extended periods of time, supercapacitors will not help. Thus in the near future the chances of the supercapacitor replacing the battery are remote. However, battery-supercapacitor with electronic controller combine power pack is likely to find use in many low voltage applications.
It can be used like a plug and play energy storage device.
Due to their fast charging-discharging times and high currents, the supercapacitors requires very specialized power electronic interfaces for proper functioning in an application. Unfortunately, there is a scarcity of such modules and these interfaces have to be application specific. In other words, it is not possible to use the same power electronic interface for two applications which require the same energy storage ratings with different loads.Another issue is that power electronic devices work at high frequencies but manufacturers do not provide the necessary data like bode plots and frequency response in their datasheets. Thus, it becomes difficult for any application engineer to incorporate a supercapacitor in their circuit.
It has infinite life.
Having infinite life is an ideal condition. In general supercapacitor lifetime is dependent on three things: electrolyte life, voltage rerating and temperature dissipation. The fluid in an electrolyte can evaporate and cause the supercapacitor to fail under extreme operating conditions. This device is also highly vulnerable to excess heat. Moreover, using a supercapacitor at close to its maximum voltage will cause it to fail more quickly than using it at a lower voltage. Typically, a manufacturer provides data regarding the change in capacitance and internal resistance to be expected after 1000 cycles.
If this value is less than what is required after the 1000 cycles, then the conditions. This device is also highly vulnerable to excess heat. Moreover, using a supercapacitor at close to its maximum voltage will cause it to fail more quickly than using it at a lower voltage. Typically, a manufacturer provides data regarding the change in capacitance and internal resistance to be expected after 1000 cycles.
If this value is less than what is required after the 1000 cycles, then the supercapacitor might need to be replaced. If proper attention is given to the rating and characteristics of the supercapacitor while choosing it for an application, the supercapacitor will easily outlive the system in which it is being used. The figure 2, illustrates the life in hours as a function of temperature and voltage...
Figure 2: Supercapacitor life as a function of temperature and voltage...
Commercially available materials can be used in manufacturing of supercapacitors.
The activated carbon that is used as supercapacitor electrodes have to be of very high purity with less than 1% ash content. Attaining this level of purity is costly and there are very few carbon manufacturers who have the requisite technology.
Other materials which are used as the active electrode material like carbon nano tubes, carbon aerogel and graphene have still not managed to crossover successfully from the laboratory to the industry.
Proper optimizing of the electrode-electrolyte pairing, suitable method for coating the current collector with active material so that the penetration of electrolyte is maximum and reducing the equivalent series resistance are some of the issues that need to be addressed for the proper scaling up of these technologies to make them suitable for mass production. Debate is still ongoing about the pros and cons on stacked type configuration vis-a-vis rolled configuration for the cell design. In the former, the electrodes and separatorsare stacked like in a battery and the cell has a cubical shape. In the latter case, the electrodes and separator are wound and the cell has a cylindrical shape similar to the conventional capacitor. Another major deterrent is the non-availability of the necessary grade of separators, electrolytes and current collector. The materials for electrode, current collector and separator materials needs more research. As these components need to be of very high quality, an OEM finds it very difficult and costly to procure.
This technology has potential to generate its own market.
Though partly true, this is not really the case on the ground. There is still a lot of reluctance in industry to embrace this technology. Even though the automobile industry is the largest perceived market and a few automobile manufacturers are claiming to use the supercapacitor in some of their models, we haven’t yet seen them accept this technology in their entire range. Moreover, as most of the supercapacitor manufacturers in possession of path breaking technology are small players in the energy storage market, they are not able to use existing channels of distribution and contact with customers.
This keeps them from completely exploiting the energy needs of the consumer. In many countries, apart from manufacturing of this device, developing market for this is also a challenge.
Figure 5: Supercapacitor Market Shares by Application...
In conclusion, the myths surrounding the supercapacitor are many and it is high time that they are dispelled so that the supercapacitor can be given its proper place among the various energy storage devices. At present, the low energy density, the requirement of voltage balancing circuit and power electronic interface are still a major problem for the widespread use of supercapacitors.
Another area which is a major hindrance to its acceptance is the cost. This is in spite of the fact that in the 1980s, a 470F 2.3V supercapacitor cost $2.00/F while in 2012 the same had come down to $0.03/F.
It is expected that supercapacitor demand will grow in value by about 30% overall by 2020 and remain in the hundreds of millions of dollars and not billions, as was earlier stated by various experts.
At present, the United States is the market leader for supercapacitors with more than 40% share in the market.
Europe and countries like China and Japan are also active players in this market. India is still taking baby steps and there is an urgent need to adopt this technology into the mainstream.
The present push by the government towards electric mobility and the commitment to bring in 5 to 7 million electric vehicles on the Indian roads by 2020, will certainly boost this sector.
Dr P B Karandikar
Army Institute of Technology
Dr N R Kulkarni
Vice Principal and Head of Electrical Department
P.E.S’s Modern College of Engineering
P.E.S’s Modern College of Engineering
If you want to share thoughts or feedback then please leave a comment below.