mission needs, one must consider source and sink characteristics along with mission requirements and duty cycles.
Any two sources may be combined into a hybrid to satisfy soldier system needs. As described previously, the combination of rechargeable batteries and fuel cells might be used to meet periodic high current demand and high energy needs in combination. The combination of an air-breathing generator and rechargeable batteries might be better for a mission that requires a soldier to be immersed in water.
The Army has defined several mission scenarios, and each must be validated so that appropriate systems can be selected confidently. Before any system is selected, it is important that the combination of energy source, energy sink, and soldier mission requirements and duty cycles be considered jointly.
Comparing power source performance metrics under identical load conditions and operational scenarios allows for the best assessment of energy alternatives. This requires knowledge of the power demand and time data for every piece of equipment for every soldier for a statistically significant number of missions. The OFW-ATD Program includes a modeling effort that predicts power demand by positing mission scenarios, estimating duty cycles, and using power sink specification data for all components of the Land Warrior soldier system. In addition, the OFW Program plans to monitor the power demand of specific components during actual or simulated missions. The information gathered will be used to validate the models and provide realistic boundary conditions for total energy, average power, peak power, and duty cycles for various missions. The validated models should lead to more effective planning and designs. For example, the optimal suite of energy storage and energy conversion devices, fuel quantities, etc., could be determined for each mission.
Commercial developers of power systems simulate the power use of their systems so they can rapidly optimize hybrid system choices. These modeling efforts require detailed descriptions of the power supply characteristics of the candidate components and are considered to be key proprietary parts of their in-house design processes. The dynamic response characteristics of components—that is of the available battery types, capacitors, and fuel cells—as a function of current flow rate and ambient conditions must be known to obtain accurate results.
The Army will need a similar modeling approach in order to make appropriate system design choices. The type of duty cycle encountered in real field applications is key to acquiring an optimized design and must be determined through experimentation. The Army should invest in such a modeling capability, which would be essential for effective power management at the system power input level. This capability could be combined with other power management capabilities focused on system power output and power demands.
Researchers at the University of South Carolina have developed modeling software known as the Virtual Test Bed (VTB), with the goal of optimizing the usage of charge storage devices for specialized applications. They can input the parameters for general battery systems and then study how the battery will perform under specified loads. Software can also be used to model hybrid power sources, where a battery is used for low power and an electrochemical capacitor is used for pulsed power. With this approach, the South Carolina team successfully improved power utilization for a device that utilizes a hybrid system (Dougal et al., 2002). The results are largely nonintuitive, and extensive modeling was needed to identify the optimum power source.
It is clear to the committee that high-fidelity modeling will be needed to optimize the Army soldier system. For models to be useful, the power and energy usage of a mission must be specified. However, until the OFW system has been put in place and usage data gathered, the power and energy inputs to these models are not available. The mission requirements are especially critical when pulsed power is needed, in which case a battery + capacitor system might be useful; also, the duty cycle of the pulsed power must be known. For instance, a minute-long pulse can be delivered efficiently with a battery, but a capacitor + battery hybrid power source might be better for a device that senses at low power and then transmits data using a millisecond pulse. Because engineers of military equipment are often looking to adapt new technology, they resist modeling efforts until they have their system design completed. Although the modeling effort can be time consuming, it should not be delayed.
One challenge with the modeling approach is to develop code with general rules that can be rapidly adapted by systems engineers to help guide their choices toward components that may save power in the overall system. Also, the models need to be able to take into account the behavior of real systems—that is, systems that fade in performance over time or have a range of performance values.
In summary, modeling has the potential to save time and money in the development of efficient portable electronic systems if accurate system inputs can be supplied. The modeling can complement experimental data as it narrows down the parameters of optimization. Any power solution ultimately needs to be verified with experimental data, but modeling can expedite selection of the power source. Ideally, the military should develop and acquire new equipment based on recommendations and considerations gained from power sources modeling, so that the lifetime of the equipment can be maximized.