hybrid system. Here there would be no significant difference in discharge times, though the voltage dip during high power pulses would be smaller for the hybrid.

Table 3-1 illustrates the differences between single and hybrid sources for several performance categories. In addition, effort should be devoted to understanding critical factors such as self-discharge and temperature effects. It is well known that capacitors generally have higher self-discharge rates than batteries. Thus, for a battery + capacitor hybrid, the capacitor will drain energy from the battery, shortening mission life. Similarly, Zn/air cells do not perform well at low temperatures. Thus, for a hybrid power source involving Zn/air as one of the components, low temperatures might necessitate a larger Zn/air cell in order to achieve the same performance as at room temperature.

SYSTEM CONFIGURATION CHOICES

The choice of system elements must consider the specific characteristics of both the energy source and energy sink elements of the system. The wide range of energy sinks includes standard computer and display hardware using powers of milliwatts to watts; laser target designators demanding 100 W or more; soldier cooling hardware demanding tens of watts continuously; and exoskeletal devices that demand very large amounts of power and energy.

The proposed sources of energy include primary batteries (with high energy densities and modest internal ohmic resistance); rechargeable batteries (with lower energy density and lower ohmic resistance); fuel cells (with low energy output but high energy density related to their use of high-energy fuels); and engine-driven generators (with high outputs and high energy densities but problematic noise and heat signatures).

Hybrid systems have the ability to improve system power usage by limiting the voltage drops that are imposed on the power supplies for higher current demand duty cycles. This ability reduces the heat generated by ohmic resistance in the power supply. Additionally, such hybrid combinations can avoid the transitions into low effectiveness operation of one of the power supply components. See Figure 3-7 for a comparison of data for the Zn/air battery curve as one example.

These power sources might be used at different times or in combination to best satisfy soldier needs. To determine the most appropriate combination for satisfying a soldier’s

FIGURE 3-7 Performance of hybrid as compared with performance of single components in power load cyclic profile of 9 min, 12 W, and 1 min, 40 W. Hybrid’s gain is approximately 4 hours. SOURCE: Graham and Feldman, 2003.



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