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Meeting the Energy Needs of Future Warriors
FIGURE C-1 The capacity of a battery changes with the rate of discharge. SOURCE: Panasonic, 2003.
typically focus on specific power, but the power density can be as important or more important for portable electronics, especially when a certain form factor is required. Because power is a function of the current drawn from the system, the power is often defined as the maximum power that can be delivered by the system. Alternatively, power can be specified at the practical operating current of the source. The specific power and power density of batteries are typically measured with respect to their full, packaged system. Fuel cells and engines can be defined by the power and weight of their conversion system only or of the conversion system and fuel combination.
Specific energy and energy density are the energy per unit weight or volume of the system. The most common units for portable power devices are Wh/kg and Wh/L. The amount of energy that is generated by a system is affected by operating conditions, as discussed above, so there is often a practical range in the reported values of the specific energy and energy density of battery chemistries. For clarity, the specific energy of systems is often reported as a single value, but it should be understood that, in practice, a variation should be expected.
The weight and volume term for batteries is measured for the full packaged system, and that for fuel cells and batteries includes the conversion system, plus the fuel and fuel tank, and relevant auxiliary components. Note that a fuel cell or engine with no fuel has no energy, so to accurately estimate these terms the amount of fuel consumed over a period of time must be known. Furthermore, when determining the specific energy and energy density of a developmental power source, care should be taken to qualify exactly what is included in the weight and volume terms, as the developer will not want to be saddled in the future with unforeseen components that add to the weight and volume of the system and make it less attractive. Additional weight might also be necessary for ruggedizing systems for military use.
Classes of energy storage/conversion systems are frequently compared by plotting the log of specific power as a function of the log of specific energy—such a plot is referred to as a Ragone plot (pronounced rah-GO-knee). The technique is used to compare classes of technologies and chemistries and can also be used to show how specific power and energy interrelate for a given battery or energy converter/ fuel solution as it is discharged at different rates. Ragone plots are useful when trying to select a power source for high energy or high power. Figure C-2 shows an example of such a plot for several rechargeable batteries and an internal combustion engine. Note that the energy of a battery is inversely proportional to the power, so draining a battery quickly (at high current and, thus, high power) tends to lower the specific energy and draining it slowly (at low power) leads to higher energy densities. Care should be taken when evaluating batteries to discern whether the specific power and energy were determined at the same current, or whether they are the best-case scenario on the Ragone plot.
The efficiency, η, of an energy conversion or storage device is a function of both thermodynamics and engineering and determines the system energy and thermal signature. Systems can be described by either their thermal or electrical efficiency. The thermal efficiency, ηth, of an energy conversion device is defined as the amount of useful energy