TECHNOLOGY AREA(S): Materials OBJECTIVE: To design, develop, and demonstrate the feasibility of an efficient and lightweight multi-fuel burner for soldier-borne battery re-charging. DESCRIPTION: Power systems relying on rechargeable battery operations limit Soldier and platform mobility by the low specific energy density of rechargeable batteries (145-160 W h/kg). A typical squad leader can carry more than 14 pounds of batteries for a 72-hour mission for an average of 12 Watts with some squad members requiring more than double that amount. Hybridizing hydrocarbon or alcohol-based power sources with rechargeable batteries can provide lighter and longer running power systems than battery-only systems. For example, a 1 kg (dry), 10% efficient power source using JP-8 to produce 25 Watts for 72 hours can achieve a specific energy of 700 W h/kg, thus reducing the weight compared to a battery by 5 times or extending the mission duration by the same amount. Energy conversion technologies with minimal friction losses are necessary to convert the heat to electricity in an efficient manner at a scale that would enable a <1 kg (dry) power source weight capable of producing 10-30 Watt (electrical). Current and past programs have largely focused on improving only the thermal-to-electric energy conversion efficiency via thermoelectric or thermophotovoltaic energy conversion using non optimized propane or butane fuel mixtures commonly found in recreational camping stoves and target specific energy densities of 700-1000 W h/kg for a 72 h mission using temperatures that range from 773K to 1273K. For example, an ongoing propane fueled thermophotovoltaic (TPV) power source project described by Fraas et al. [1] and a review by Bitnar et al. [2] which surveyed efforts of TPV power sources demonstrated using single fuels suggests that a thermal-to-electric conversion efficiency of 15% may be feasible. Overall system efficiencies of 10% were by both Fraas et al. and Bitnar et al. A multi-fuel capability enabling the use of gaseous mixtures and liquids with varying thermodynamic properties and purity as well as optimization of the thermal efficiency can enable a power source capable of operating from logistics sources or from sources procured locally. Previous efforts on logistics fueled or multi-fuel capable burners have focused on large scale heating applications such as portable cooking stoves, water heating [3], or Stirling engine applications all > 1 kW thermal power aimed at heating surfaces much lower than combustion temperatures. Recent breakthroughs in micro- and meso-scale combustion technology, advanced atomization and vaporization techniques for heavy hydrocarbons, and advanced heat exchanger technologies for heat recuperation have the potential to achieve the lightweight, thermally efficient burners required to reduce the weight Soldiers carry for power by 5 times when integrated with efficient thermal-to-electric energy conversion technology. This program will advance the key enabling technologies and integration solutions to realize a multi-fuel burner capable of heating surfaces in the range of 450K to 1500K with a thermal efficiency >80% and overall system efficiency >10% in extremely small form factors to produce 10-30 Watt (electrical) with a dry system weight <1 kg for a specific energy >600 W h/kg. The effort shall look to study designs which increase the life of burner materials including catalysts if appropriate, increase efficiency of heat recuperation, improve overall thermal efficiency, minimize soot and coke deposits, and minimize parasitic losses. Fuels of interest include: JP-8, JP-5, diesel, gasoline, higher alcohols (C2+) and biofuels, as well as methanol and propane. The system should be capable of operating on low sulfur (<15 ppm sulfur) fuels at a minimum with the objective of being able to operate on fuels with up to 3000 ppm (wt.) sulfur. PHASE I: The investigation shall explore combustion concepts, designs and materials of a multi-fuel burner and select the associated thermal-to-electric energy conversion method (e.g. thermoelectric or thermophotovoltaic) for a 10-30 Watt (electrical) battery re-charger capable of achieving >600 W h/kg (including fuel) for 72 hours. The system efficiency and weight assessment including all balance-of-plant components should be completed to determine the predicted specific energy. Identification of key burner components including catalysts (if appropriate), vaporization mechanism, ignition mechanism, and heat recuperation also should be completed. A tradeoff analysis between the burner size, weight, efficiency, and the range of usable fuels should be completed. The range of fuels should include: JP-8, DF-2, and propane at a minimum and additionally light liquids, such as gasoline and higher alcohols (C2+), and heavy liquids, such as other kerosene-based fuels as an objective. The system should be capable of operating on low sulfur (<15 ppm sulfur) fuels at a minimum with the objective of being able to operate on fuels with up to 3000 ppm (wt.) sulfur. Unless the submitting organization has demonstrated thermal-to-electric energy converters (e.g. thermoelectric or thermophotovoltaics) for similar applications, partnering with an appropriate organization is desirable. End of Phase I requires demonstration of critical burner component concepts to determine the feasibility to achieve the targets of the burner. PHASE II: Fabricate prototype multi-fuel burners based on the phase I design and integrate into thermal-to-electric converters for evaluation. Verification of design targets such as the range of fuels from Phase I, efficiency and weight (consistent with >600 W h/kg Phase I target), and cost validation. Characterization and evaluation of operational parameters should include ignition power and start up time, tolerance to impurities, and operational lifetime. End of phase 2 requires demonstration of a TRL 4 level multi-fueled thermal-to-electric converter prototype for Army/ARL evaluation as a potential battery re-charger. PHASE III: Integrate the multi-fuel burner and thermal-to-electric energy converter with all necessary balance-of-plant components to fabricate a standalone prototype battery re-charger. Demonstrate system performance targets such as system efficiency re-charging a Soldier-worn battery, system weight including balance-of-plant, ability to switch between different fuel types including from liquid to gaseous fuels, system lifetime, and cost. Demonstrate a TRL 5 level Soldier portable multi-fuel battery re-charger porotype for Army/ARL evaluation. Develop partnerships with Army Project/Program Offices to enable opportunities for fielding to support future forward area Soldier and tactical platforms. Potential commercial applications for a fuel flexible portable power could include supporting emergency / disaster relief operations and operations in nations lacking a robust power infrastructure. Potential commercial applications for fuel flexible burner could include portable heaters for water or combustion systems in cold climates. REFERENCES: 1: L. Fraas, L. M., J. Avery, H. She, L. Ferguson, and F. Dogan, "Lightweight Fuel-Fired Thermophotovoltaic Power Supply," EU PVSEC 2015, Hamburg, Germany, 20152: B. Bitnar, W. Durisch, and R. Holzner, "Thermophotovoltaics on the move to applications," Applied Energy, Vol. 105, pp. 430-438, 20133: C. Welles, " Development of an Advanced Flameless Combustion Heat Source Utilizing Heavy Fuels," Natick Technical Report Natick/TR-10-018, 2010 KEYWORDS: Multi-fuel Burner, Fuel Flexible Combustion, Thermoelectric, Thermophotovoltaic (TPV), Micro-combustion, Meso-scale Combustion, Combustion Catalyst
