Power pptx

Power H Wenzl, Clausthal University of Technology, Clausthal-Zellerfeld, Germany & 2009 Elsevier B.V. All rights reserved. Definitions and Terms In rapid-response situations, the ability of supercapacitors and batteries to provide power (W) over a short period of time is more important than their ability to provide en- ergy (Wh) over a long period of time. Familiar examples are the starting of internal combustion engines, the pro- tection of computers and telecommunications equipment against brief interruptions of mains power supply, the starting of electrical motors with high inrush currents, and the triggering of fuses. Ragone diagram is a method of comparing the specific power and specific energy of dif- ferent power sources in these circumstances. Power is the rate at which energy is used. From a practical point of view, ‘high power’ corresponds to the discharging of a battery in a few minutes or the dis- charging of a supercapacitor in a few seconds. For certain types of industrial Ni–Cd batteries, the discharge rate may even be as brief as one second. This article focuses on applications of electrochemical power sources that have been designed or are used for such high power discharges. There are many complications when trying to deter- mine the power capability of electrochemical systems. For example, instead of a constant power discharge, a constant current discharge is sometimes used, in which case the average power must be calculated. In addition, in certain applications, it is also necessary to ascertain how well a system can reabsorb power, e.g., during regenera- tive braking in electric vehicles, or during sudden gusts of a wind turbine. Furthermore, for fuel cells and redox flow cells, the concepts of specific capacity, specific energy, and energy density are completely inappropriate, because they depend on the size of storage tanks and not on the size of the electrochemical conversion units. Notwithstanding these difficulties, the po w er of a fuel cell can be calculated as the product of the current, the voltage per cell, and the number of cells in series. Em- pirically, it has often been found that the ability of a fuel cell to prov ide pow er beyond the maximum rated pow er, ev en for a very short period of time, is very limited. In c ontrast, batteries and supercapacitors may actually exceed their maximum r ated pow er considerably f or a few seconds. Power of Batteries For applications that have a very high power requirement for a short period of time, batteries are often used. This is because of their low internal resistance, which minimizes the generation of heat. Figure 1 shows the equivalent circuit diagram. Figure 2 shows the internal resistance of a 6.5 Ah Ni– MH module, having a nominal voltage of 7.2 V, measured 1 ms and 1 s after the start of discharge. It can be seen that the internal resistance is only a weak function of dis- charge current, implying low polarization of the cells. The internal resistance R i (O) is calculated as R i ¼ ðV OC À V ðt ÞÞ I where V OCV the open-circuit voltage (OCV) prior to discharge (V), V(t) the voltage (V) at the time of meas- urement (t), and I the discharge current (A). This internal resistance (R i ) is the sum of the internal ohmic resistance and the resistance related to electrodes overpotential. The small increase of internal resistance after 1 s in Figure 2 is possibly the result of depletion of the con- centration of hydrogen at the electrode/electrolyte interface. As in all electrical systems, the maximum power is delivered when the internal resistance and the external resistance are equal. However, this point is at 50% of the cell voltage and therefore outside the voltage range that is specified by the manufacturer. For determining the power capability of a cell, an application-specific power or current curve followed by a rest period is often used, for instance, the European Council for Automotive R&D (EUCAR) power assists cycle for hybrid vehicles. The power requirement is fulfilled as long as the voltage during the test does not fall below a set voltage limit. Figure 3 shows the power that can be delivered (and taken up) during short, 10 s pulses at different states of charge (SoCs). The ability to a bsorb power is important for hybrid vehicle applications, and for stabilizing the R i E 0 Load Figure 1 Equivalent circuit diagram of a battery; E 0 is the open- circuit voltage of the cell at rest and R i is the lumped internal resistance (combining all effects that lead to a change in voltage during current flow). 559 . power and specific energy of dif- ferent power sources in these circumstances. Power is the rate at which energy is used. From a practical point of view, ‘high power corresponds to the discharging. on applications of electrochemical power sources that have been designed or are used for such high power discharges. There are many complications when trying to deter- mine the power capability of electrochemical. determining the power capability of a cell, an application-specific power or current curve followed by a rest period is often used, for instance, the European Council for Automotive R&D (EUCAR) power