ABSTRACT
Ragone representations for the characterization of energy storage are used in two different contexts. The first one aims to compare different technologies in Ragone charts, and will only briefly be discussed in the next section; you will find much on that topic in the popular engineering literature. The second one, which will be discussed in the remainder of this chapter, is to characterize examples for idealized energy storage devices by their energy-power relations.
In order to identify an energy storage device technology for a given application, one has to know the order of magnitude of the power and of the energy that the energy storage device should be able to deliver for its task. For this purpose one identifies the associated application area in the energy-power plane. Knowing which region is covered by which energy storage technology allows to pick the one which fits best to the application. For instance, a battery for an electrical vehicle has to deliver at least an energy that is sufficient for the minimum required traveling distance. And a minimum power is required as well, that is related to the needs associated with acceleration and speed. Take a look at Figure 6.1. Batteries are in the region with high energy density, but unfortunately have comparatively low power density. This is because the energy is stored electro-chemically in the battery volume, where it needs some time to get the energy out, e.g., by chemistry and ion migration, which puts some limits on the power. On the other hand, capacitors provide high power, but unfortunately have limited energy. Again the reason should be obvious: the capacitor charge is stored on the electrode metal surface and
can be quickly discharged. It is thus clear that if electrical energy is to be delivered fast, capacitors should be used, while for longer lasting energy needs, batteries are appropriate. Since the discharge time for an energy E at a power P is E/P , the lines which have a slope equal to one in the log-log plot belong to constant characteristic discharge times, as indicated in the figure.
