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New Materials Boost Supercapacitors
New developments in supercapacitors have opened the door for energy-saving, environmentally-friendly product designs.
By Bill Campbell, Chief Executive Officer, Nanotecture Ltd, Southampton, UK
One product sector which has advanced rapidly in recent years is that of Supercapacitors (also known as Ultracapacitors), driven by the development of new materials and lower-cost manufacturing. Supercapacitors exhibit a number of characteristics that place them well above conventional capacitors and either in combination with or as replacements for batteries in certain applications. Measured against other capacitors, supercapacitors generally have high energy density, while retaining high power density, which distinguishes them from batteries.
While batteries generally have much higher energy density, supercapacitors measure lifetimes in 100,000s of cycles compared to batteries, which are normally specified in 1,000s of cycles. This dramatic difference in cycling lifetimes provide system design teams with greater choices, so that they can specify energy delivery and storage systems that are more closely optimized for the particular application.
An example of an innovative supercapacitor development is the product under development at Nanotecture. This product is termed an "asymmetric" or "hybrid" device since the two electrode plates are made from dissimilar materials. This is in strong contrast to conventional supercapacitors where the two electrodes are made of the same material.
Asymmetric supercapacitors are generally recognized as having superior energy density to their symmetrical counterparts. The distinguishing feature of the Nanotecture product is that one electrode is formed using a nano-porous metal oxide developed using the company's proprietary Liquid Chrystal Templating technology. This approach uses the same wetting agents as are commonly found in soaps and shampoos to create a controlled scaffolding structure, around which the metal oxide is deposited. The agent is then washed out to leave particles with nano-sized porosity, just like a honeycomb. The high surface area material compares favorably with nanoparticles in performance but with much easier handling and lower production cost. By constructing one electrode with this material, the supercapacitor achieves both high energy density and high power density.
A significant trend in the development of next generation electricity generation is the increasing deployment of localized back-up power systems and in particular "Bridge Power".
Bridge Power is used as short-term power which is necessary to "bridge" from one long term power supply to another. An example of the use of Bridge Power is in the event of a main power failure when the standby power generator may not be immediately available or, indeed if the generator itself suffers a failure. Bridge Power is typically implemented with battery or, increasingly, with supercapacitor banks which are deployed locally to support temporary power to mission-critical equipment. This can either be as a standalone uninterruptible power supply (UPS) or integrated into specific equipment.
The combination of both bridge and long-term power generation is necessary because the cost of long-term bridge power is high when compared to a standby generator. Critical installations are increasingly using a "cascading" architecture where a number of power sources are utilized for back-up power. In addition to traditional Diesel generators and batteries, more sophisticated systems are now using fuel cells, supercapacitors and flywheels. With the growth in demand from telecommunications and data systems which are highly sensitive to power outages, this trend towards fast-response, short-term bridging will continue.
The use of supercapacitors in Bridge Power applications is driven by a number of key factors. Since the supercapacitor is used as a high speed bridge, its power density profile is ideally suited to support high power periods of between 30 to 100 seconds. A battery is typically sized to deliver power over longer periods.
Supercapacitors are able to hold charge voltage over long periods without loss of capacity whereas batteries typically lose their charge when held in standby. Most importantly, supercapacitors provide long lifetime and low maintenance costs when compared to lead-acid batteries.
Since many back-up systems are in remote locations, as in support of cell phone base stations, reliability and longevity are critical. The optimization of the system according to the response profile also results in smaller systems, since the batteries no longer have to be overspecified to deal with high power demands.
Supercapacitors, such as those from Nanotecture, exhibit excellent low-temperature performance, which is often essential in such applications.
Based on this evidence, it would appear that supercapacitors are an obvious choice, yet while supercapacitors have been available and evaluated for some time, they have not yet made a major breakthrough in this application. Back-up power system manufacturers have highlighted two major barriers to entry, both of which are directly addressed by Nanotecture's products.
The most significant barrier to adoption is cost. Supercapacitors typically cost more than lead-acid batteries. While back-up power manufacturers will accept some level of premium in order to gain the advantages of supercapacitors, the balance has been in favor of lead-acid.
Nanotecture is able to address the cost question at two levels. First, our device is based on a water-based electrolyte which enables a low-cost manufacturing process. By comparison, the current generation of capacitors use an acid-based electrolyte which requires a costly drying process to remove moisture from the electrodes prior to insertion in the acid. Second, and most important, is the unique hybrid electrochemistry which combines an asymmetric electrode configuration with our patented nanoporous material — a technological breakthrough which means that the supercapacitor can not only deliver a high discharge rate, but also stores significantly more energy per unit volume than competing products. The benefit for back-up power manufacturers is that, at the system level, fewer supercapacitor cells are needed to meet system specifications, and fewer cells mean lower cost.
A further example of more cost-effective and efficient storage and delivery of electrical energy is in the use of supercapacitors in engine starting of commercial vehicles as an alternative to lead-acid batteries. Much of the motivation for truck manufacturers is the need to respond to Government anti-idle legislation and to reduce the lifetime costs of the engine starting system. With high rates of discharge and lifetimes which remove the need for replacement during the lifetime of the truck, supercapacitors are a preferred alternative and are now under consideration by truck manufacturers worldwide. The cost arguments which apply to Bridge Power are also a key consideration in this application.
Today's changing landscape in the business of energy generation and storage, capture, and delivery has created new opportunities for energy system designers. Advances in new technologies and materials will play their part in supporting this trend, and will aid in the development of renewable energy systems and in electrification of more traditionally fossil-fueled transportation which brings with it the added advantages of a cleaner environment.
These issues will not be fully addressed by one technology or by single-storage chemistry, but rather by a range of approaches that can be combined to deliver the best solution for the specific application — the best, not a compromise. A wider range of options presents the design community with the opportunity to optimize functions depending needed performance.
Supercapacitors represent one more piece of the technology puzzle, and can best be used in combination with batteries and potentially with fuel cells in applications ranging from bridging power and truck starting to regenerative braking on electric vehicles, buses and trains. These applications present new challenges for electrical energy storage and delivery, highlighting the need for new and innovative technologies.
Contact: Nanotecture Ltd., Epsilon House, Enterprise Road, Southampton Science Park, Southampton SO16 7NS, UK
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