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I Soldier Silent Power Challenges As discussed in Chapter 5 âDismounted Soldier and Light UAV/UGVs,â thermophotovoltaic power sources utilizing JP8 present a major opportunity to reduce the weight burden of the dismounted soldier. Nevertheless, some technical challenges remain to be addressed before introduction onto the battlefield. DEVELOPMENTS FOR TRL 6 Major development efforts are required to develop SSP to TRL 6. The cost of the program will be $1-2M and take 12-24 months for a feasibility demonstration in laboratory conditions, and an additional $2-4M and 12-24 months to deliver a fully functional and integrated prototype capable of operation under realistic conditions (a field experimentation). The areas of required development are outlined below. INFRARED PV CELLS Several companies are capable of producing high-performance indium gallium arsenide (InGaAs) on indium phosphide (InP) PV cells. Several fabrication-runs will be required to optimize performance in terms of open circuit voltage, fill factor, and quantum efficiency (ratio of incident photons to electrons at the terminals). The production runs will provide the cells necessary for system development and testing. PHOTONIC CRYSTAL INTEGRATION Photonic crystal integration has been demonstrated experimentally by the Armyâs Institute for Soldier Nanotechnology at MIT in small planar systems (W. R. Chan, V. Stelmakh, M. Ghebrebrhan, M. Soljacic, J. D. Joannopoulos, I. Celanovic, âEnabling efficient heat-to-electricity generation at the mesoscale,â Energy Environ. Sci., 2017, vol. 10, no. 6, pp. 1367-1371). Two minor challenges remaining for integration of photonic crystal emitters into larger cylindrical systems. The crystal emitter needs to be packaged in a vacuum (to prevent degradation by reaction with air) and the photonic crystal needs to be bonded to the micro-combustor: 1. Mesodyne has developed a process for fabricating photonic crystals on flexible substrates that can be wrapped around and brazed to a cylindrical micro-combustor. A small amount of additional work is required to perfect the process. 2. The vacuum package is an IR-transparent (quartz or sapphire) tube that encapsulates the micro- combustor and photonic crystal, which optimizes the thermal band used by the PV to produce electricity. One end of the tube is sealed, and the other end is hermetically bonded to a metal tube for subsequent bonding to the micro-combustor. There are no COTS products that match these requirements, although there are numerous companies that offer similar products and could custom PREPUBLICATION COPY â SUBJECT TO FURTHER EDITORIAL CORRECTION I-1
fabricate the vacuum package. Additional development may be required to ensure long-term stability of the vacuum with the micro-combustor at operating temperature. JP-8 MICRO-COMBUSTOR Numerous designs for JP-8 combustors at larger scales exist, and several companies have developed burners in the approximate power range required for this project. The challenge is to integrate them with the novel TPV system, or alternatively modifying the existing micro-combustor to be compatible with JP-8. JP-8 combustion itself is not the primary challenge, rather it is vaporizing the fuel because it has a narrow temperature range between boiling and decomposition. Solutions exist, but the challenge is developing something that fits within the size and weight requirements of Soldier power to meet the future 7-day mission requirement. SYSTEM INTEGRATION AND MINIATURIZATION FOR TRL 7-8 This task covers the engineering work to transition from a lab prototype to a self-contained demonstration unit as follows: â PV cell array packaging and coolingâdesign a lightweight heat sink and PV cell mount that keeps the cells cool and aligned with the emitter, while consuming minimal power with a small cooling fan. â Control systemâengineer a robust system to supply fuel and air to the microcombustor. â Power electronicsâinterface between the PV cell array and the Soldierâs battery or power manager. â Miniaturizationâengineer all components to be compatible with a Soldier wearable solution. The microcombustor and heatsink will be the most challenging because the former needs to be a monolithic unit and the later can be bulky. â Housingâdesign a robust and ergonomic housing for the system. Filled cavity photonic crystal (optional) FIGURE I.1 Filled Cavity Photonic Crystal. SOURCE: Chan, W., I. Celanovic, and J. Joannopoulos. 2020. Filled Cavity Photonic Crystal. White paper presented to committee. Silent lightweight battlefield power source: Scalable from Soldier wearable power to platform power. Massachusetts Institute of Technology Institute for Soldier Nanotechnologies. PREPUBLICATION COPY â SUBJECT TO FURTHER EDITORIAL CORRECTION I-2
In order to further improve efficiency and power density, the Armyâs Institute for Soldier Nanotechnologies at MIT has developed a next-generation photonic crystal capable of omnidirectional emission approaching the blackbody limit for in-band wavelengths (see Figure I.1). This is accomplished by filling the cavities with a dielectric material and decoupling the physical and optical dimensions. Small samples have been fabricated by MIT, but a scalable fabrication method compatible with standard semiconductor processes needs to be developed. The numbers presented here are all based on the mature photonic crystal design from the Armyâs Institute for Soldier Nanotechnologies at MIT. Maturing and adopting the filled cavity photonic crystal could improve the already very impressive bottom-line performance by an estimated 50 percent more. DEVELOPMENTS FOR TRL 9 These efforts focus on manufacturability and production scale-up, ruggedization, etc. The cost of the program will be $5-10M and take 2-4 years for the first fieldable production run. Photonic Crystal Fabrication Scaleup This effort aims to mass produce the photonic crystal. A company, Mesodyne, has already eliminated a key bottleneck in the fabrication process, i.e., removing wafer size limitations. Additional required development efforts include transitioning from small batches of small diameter wafers to large batches of large wafers, making the tantalum wafers compatible with non-academic cleanrooms. For example, by mounting the thin tantalum substrate on silicon wafers, and streamlining the process to eliminate the need for manual real time adjustments to process parameters. PV Cell Scaleup This effort aims to mass produce the InGaAs PV cells. The primary barrier is the high cost of the indium phosphate (InP) substrate. At scale (10000 wafers/month) the majority of the cost of the finished device is the substrate. Epitaxial liftoff, already developed for III-V solar PV cells, allows for multiple reuses of the same substrate, reducing cost by up to 70 percent. Additionally, the liftoff process will produce flexible cells, allowing for improved coupling to the cylindrical emitter. Integration, Miniaturization, and Ruggedization At this stage, the product can be designed for minimal weight and volume rather than ability to be built and modified rapidly. Additionally, the product will need to be ruggedized against drops and rough handling as well as dirt and water. Design for Manufacturing This effort covers manufacturing scaleup of the rest of the system. We do not anticipate major hurdles because every component has an analog that is already manufactured at scale. PREPUBLICATION COPY â SUBJECT TO FURTHER EDITORIAL CORRECTION I-3
Testing and Qualification A significant amount of testing will be required to ensure the product meets performance, safety, and environmental standards. Additionally, user testing will ensure the product is easy to use in a battlefield environment. PREPUBLICATION COPY â SUBJECT TO FURTHER EDITORIAL CORRECTION I-4