Valuing Reversible Energy Storage

Abstract

The development of new materials that provide the capability of high-performance energy storage combined with flexibility of fabrication opens up the possibility of a wide range of technological applications. On page [1326][1] of this issue, El-Kady et al. ([ 1 ][2]) describe thin and highly flexible electrochemical capacitors (ECs) that were created by means of a very simple and innovative process. Unlike the usual approaches of making thin graphene electrodes that start with a particulate and use roll-coating, screen printing, or ink-jet printing ([ 2 ][3]), their process involves focusing a low-power laser onto a thin graphene oxide deposit to convert it into graphene. The incorporation of graphene in electrodes created with mechanical processes tends to be in agglomerates that provide little performance advantage over traditional particulate-activated carbon electrodes. El-Kady et al. 's approach also contrasts with plasma-assisted chemical vapor deposition processes that have been used to grow vertically oriented graphene nanosheet electrodes ([ 3 ][4]). Although graphene structures grown by such methods are well-formed and offer performance advantages over traditional activated carbon materials, they require complicated vacuum process equipment, plus the graphene growth rate is very slow ([ 4 ][5]). The somewhat simple EC electrode fabrication process reported by El-Kady et al. therefore appears to circumvent many of the difficulties encountered with traditional processes. Whenever a new energy storage technology is reported, almost inevitably the first question asked and the first data cited focus on its “watt-hours per kilogram” (Wh/kg) value. This measure refers to specific energy, a metric that dates historically to the days when heavy batteries provided almost the only means available to store electrochemical energy. Despite, and almost in defiance of, the emergence of newer energy storage technologies, however, specific energy continues to be referenced without further consideration as the most important characteristic of any new energy storage technology, the gold standard of its worth or value ([ 5 ][6]). This is all wrong—specific energy is only one of many metrics by which the value of storage technology can be measured. Indeed, it may be one of the least important when it comes to assessing its value for use in today's newest and most innovative applications ([ 6 ][7]). ![Figure][8] Power dressing. Design concept for self-powering “smart” garments, outfitted with piezoelectric patches to harvest energy from body movements and flexible electrochemical capacitors to store the energy. Power to camouflage uniforms is one possible use. CREDIT: ISTOCKPHOTO.COM; ADAPTED FROM KRISTY JOST/A. J. DREXEL NANOTECHNOLOGY INSTITUTE/DREXEL UNIVERSITY ![Figure][8] Usually the second question asked is the cost, the most popular metric being “cost per unit of energy” ($/kWh). The table lists the three types of capacitors and two different battery technologies, using dollars per kilowatt-hour as the cost metric ([ 7 ][9]). The value for electrostatic capacitors (metalized-film capacitors) is $2.5 million per kWh. Electrolytic capacitors cost $1 million per kWh. Curiously despite such extremely high costs, both technologies are found in almost every piece of electronics available today. Much lower in cost, at a mere $20,000 per kWh, are ECs ([ 8 ][10]). But the stark comparison to lithium-ion batteries at $1000 or lead acid batteries at $150 per kWh suggests that $/kWh is not actually a very important metric in some decision-making. Cost is but one of the many ways by which storage technology can be measured, and again it may be one of the least important when it comes to assessing the value of an energy storage technology for use in applications. Reversibility, essentially the efficiency of a round-trip cycle that first stores then later uses the stored energy, is also an important metric for energy storage technology in many present and emerging applications. Unlike money that may be deposited in an honest savings bank that later is totally returned and often with interest, energy deposited in any storage device has associated losses that prevents the full return. Then the question is what fraction of the deposited energy will eventually be returned. This strongly depends on the storage media as well as the rate at which energy is stored, the storage time, and the rate at which energy is extracted ([ 9 ][11]). Unlike batteries that typically have higher losses during charge than during discharge, ECs can be totally charged and discharged very quickly with high efficiency. Energy reversibility is often a most important factor in establishing the value of a storage technology for many of today's energy conservation applications. Cycle life goes hand-in-hand in importance with energy reversibility. Some energy conservation applications, for example, regenerative energy capture during the stopping of a hybrid city transit bus, may require more than 1 million charge/discharge cycles during their operational life. A storage system can be replaced several times or “supersized” to reduce the depth of discharge in each cycle and increase cycle life, practices commonly used for battery technologies. However, both approaches mean that the storage system will have higher cost. ECs, by contrast, rely on physical rather than chemical storage and do not suffer from limitations of cycle life. They effectively can be “right-sized” at the start and last the entire life of a given application. In short, cycle life can impart great value to an energy storage technology. Storage system shape is another factor that may have high value in some applications. Energy density advantages generally can be best achieved with shapes approaching a cube, whereas power density advantages can be best achieved with thin, large-area designs. A given energy...