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818 ELECTROCHEMICAL STORAGE FOR PHOTOVOLTAICS<br />

Volumetric energy density<br />

[Wh/kg]<br />

300<br />

200<br />

100<br />

NiMH<br />

NiCd<br />

Lead<br />

acid<br />

RAM<br />

NiNiCl<br />

Li- ion<br />

Li-polymer<br />

SuperCaps<br />

50 100 150<br />

Gravimetric energy density<br />

[Wh/kg]<br />

Figure 18.7 Practical specific volumetric energy density (Wh/l) versus specific gravimetric energy<br />

density (Wh/kg) for various secondary battery technologies (data from Table 18.4)<br />

The specific energy densities of batteries are an important parameter to characterise<br />

the different battery types. For logistic reasons on supplying the batteries to the systems,<br />

gravimetric and volumetric energy densities are a relevant cost factor also for autonomous<br />

power supply systems. Figure 18.7 shows an overview of the energy densities of different<br />

secondary battery technologies based on an analysis of commercially available products.<br />

The figure does not show the theoretical limits of the different technologies.<br />

Table 18.4 gives an overview of the most important properties of several secondary<br />

electrochemical accumulators. A more detailed description is given on the NiCd<br />

(Section 18.4.2), the nickel-metal hydride (NiMH) (Section 18.4.3), the RAM (Section<br />

18.4.4) and the Lithium batteries (Section 18.4.5) as well as on the double-layer capacitors<br />

(Section 18.4.6). Lead acid batteries are treated in greater detail in Section 18.4.7. All<br />

other battery types have no relevance in the field of autonomous power supply systems.<br />

Details on all primary and secondary batteries can be found in [2].<br />

18.4.2 NiCd Batteries<br />

NiCd batteries have been available as a commercial product for many decades and are<br />

well-proven products. They have very good properties concerning lifetime and lifetime<br />

cycles. They are widely used in heavy-duty applications and in very cold climates.<br />

Standard NiCd-battery designs can be operated easily at temperatures of −20 ◦ Cand<br />

specially designed cells can be operated even up to −50 ◦ C. Nevertheless, NiCd batteries

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