The third issue relates to battery comparisons. Performance specifications for batteries are given for specific cell sizes and discharge rates. The specific energy data quoted in this study are valid for the discharge rates under consideration. However, there will be a packaging penalty (weight and volume) for battery packs. For example, a lithium ion laptop computer battery may be in a square configuration 2.5-cm thick, but in reality there are eight cylindrical 18650-Li ion cells inside this package. The performance specifications of the 18650-cell should be discounted to account for this. A good rule of thumb is to deduct 15 percent from the cell performance figures.

Fueled systems, which are in various stages of development, can be used to replace batteries or supplement them as part of a hybrid system. For systems supplying more than about 1 kWh, fueled systems offer a significant mass advantage over batteries. Figure 2-1, taken from the 1997 report, illustrates this point. It can be seen that the battery mass is directly proportional to mission energy requirement. In contrast, for fueled systems, mass comprises the fuel (including fuel tank) mass, which is a function of the mission energy requirement, and the energy converter mass, which is a function not of the mission energy requirement but of the mission power requirement. The y intercept in the figure is the dry mass of the energy converter and the slope is the product of the energy content of the fuel and the system energy conversion efficiency. These issues are explained in detail in the earlier report (NRC, 1997).

ANALYSIS OF ALTERNATIVES

The known performance of state-of-the-art lithium/ manganese dioxide and lithium/carbon monofluoride (Li/ MnO2 and Li/(CF)x) primary batteries and lithium ion (Li ion) rechargeable battery technologies was compared with that of promising energy conversion technologies. The interpretation of data for new technologies was intentionally conservative, with every effort made to use performance data obtained from completely packaged systems. In some cases, projections were made from subsystem data if system data were unavailable. Assumptions and references used to make these projections are documented in Appendix D. Low-TRL concepts (e.g., lithium/air, carbon/air) were not compared if too many assumptions were needed to predict system-level performance. Similarly, low-performance technologies—that is, those that were known not to exceed lithium battery performance—were not included in the analyses.

Plots of total system mass including fuel versus 24- and 72-hr mission durations were developed for alternatives in each power regime (Figures 2-2 to 2-5). The corresponding numerical data are included in Tables 2-3 and 2-4. The technologies were rated on their ability to provide both average and peak powers for a given regime. The battery mass needed to produce the equivalent amount of energy was calculated from cell data. Because the typical 15 percent mass penalty for packaging these cells into battery packs was not included, battery performance is slightly overestimated in these charts

FIGURE 2-1 Graph showing the crossover points for battery and fuel cell power systems as functions of available energy and system mass. The assumed system power level is 5 W. PEMFC is proton exchange membrane fuel cell. SOURCE: NRC, 1997.



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