Executive Summary

The Committee on Powering the U.S. Army of the Future considered a range of Army power and energy needs through 2035, identifying the breadth of requirements, gaps, and opportunities therein. This was a challenging task, given the tremendous diversity of needs, both in terms of the quantity of power needed and who is using it.

Given the range of technologies that will drive future power and energy (P&E) demands, the committee decided to focus the scope of the study on the power needs surrounding dismounted soldiers, existing vehicle platforms, and forward operating bases, as well as innovations under development that are expected to be in service in 2035, and technologies that could enhance the Army’s capabilities to fight as part of a multi-domain force.

The committee further scoped the study to place a heavy focus on the needs of an Armored Brigade Combat Team (ABCT) because they expend prodigious amounts of energy and the Army expects them to remain a primary, independently maneuvering unit for the foreseeable future. The ABCT provided a baseline that scaled well and allowed the committee to assess technologies across dismounted, mounted, and semi-stationary units.¹

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¹ Army aviation accounts for a considerable portion of the Army’s jet propellant 8 consumption. Due to time and expertise constraints, the committee did not focus on primary propulsion for aircraft. However, many of the recommendations in the report are applicable to aviation secondary power.

Using predictions of the Operational Logistics (OPLOG) Planner modeling tool provided by the Combined Arms Support Command (CASCOM), the committee anticipates that a typical ABCT will expend 18,800 megawatt-hours (MWh) of energy over a 12-day mission.² This equates to an average energy consumption of roughly 1,600 MWh per day and an average power level of 65 megawatts (MW). It must be noted that during mounted maneuver, power demands are significantly higher than during sustained lower-intensity operations. These energy demands will only grow for the foreseeable future as ongoing improvements in communications, electronic sensing, artificial intelligence processing to improve battlefield situational awareness, increased vehicle mobility, and more lethal weaponry threaten to overwhelm any feasible improvements in efficiency.

In finalizing its report, the committee concluded that some past power/energy studies advocating widespread use of pure battery electric ground combat vehicles recharged in the field with mobile nuclear power plants are not likely to be technically feasible in the time frame of this report. To be more specific, the committee concluded that jet propellant 8 (JP8), diesel, and biodiesel³ (a renewable fuel) should serve as the primary sources of power and energy brought to the battlefield for the foreseeable future. Their high energy density (particularly per unit volume) is unmatched by most other liquid and gaseous fuels. It is this density measure that defines how many supply trucks in convoys carrying fuel are needed, which in turn increases the risks faced by soldiers and contractors and the integrity of the supply chain with each added convoy or truck.⁴

Transportation of energy to the battlefield presents risks to soldiers and contractors. Minimizing this risk must, therefore, be considered in the development of any power and energy strategy. As shown in Figure ES.1, bulk petroleum represents 39 percent of the total volume of materials and equipment delivered to the battlefield.

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² R. Schwankhart, RAND Corporation, 2020, “Energy Consumption Requirements Overview—Armored Brigade Combat Team (ABCT) Case Study,” presentation to the study committee on April 16.

³ Although biodiesel, renewable diesel, and e-diesel refer to fuels produced by different processes, their performance properties are very similar, enabling them to be used interchangeably. As all three are environmentally friendly, a single term, “biodiesel,” is used to refer to all three such fuels throughout this report.

⁴ Although this study concludes that supply convoys will continue to be needed, there are multiple opportunities now under investigation to reduce the risk of lost lives in transport. These include active protection systems, autonomous vehicles, vehicle platooning, mine-sweeping vehicles, and helicopter and ground escorts.

Diesel is a very reasonable choice for powering military vehicles and could be preferred over JP8 in selected climates during wartime conditions. It is readily abundant in many locations, which in certain situations would enable local resupply. Diesel has a 9 percent higher volumetric energy density than JP8, making it possible to reduce the number of supply trucks dedicated to fuel by an equivalent amount. Furthermore, the technology exists today for employing closed-loop combustion controls to allow vehicles and generators to operate seamlessly between JP8 and diesel and any mixtures in between. This same technology will also improve fuel economy by adjusting injection timing for JP8 in recognition of its highly variable cetane rating.5

Given the growing need to address climate change, biodiesel (a renewable, carbon-neutral fuel) could serve as a preferred fuel source during peacetime. The same technology that enables seamless transitions from

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⁵ Note that cetane rating refers to the ease of initiating an autoignition combustion event, analogous to octane rating for gasoline.

JP8 to diesel would also enable JP8 to biodiesel transitions, albeit potentially requiring acceptability certification of the various biodiesel sources. When the United States is at peace, reduction of greenhouse gases may be a more important concern than minimizing the number of trucks in fuel convoys. In addition, biodiesel is fairly available worldwide.⁶

It must be noted that future use of multiple fuels would violate the Army’s long-standing reliance on a “single fuel policy,” which provides for a common fuel to be used across all ground vehicle platforms, generator sets, and turbine-powered aircraft. Therefore, the advantages of using multiple fuels detailed above need to be balanced against the logistic complexity challenges associated with their distribution. If such logistics prove to be excessively challenging in certain situations, then JP8 use remains the preferred method of transported energy to the battlefield, to remain compatible with aircraft needs.

The committee’s analysis has concluded that all-electric ground combat vehicles and tactical supply vehicles (i.e., fully reliant on battery energy storage versus liquid fuel) are not practical for a majority of battlefield vehicles now nor in the foreseeable future for two reasons. One is that the energy density of batteries today is roughly two orders of magnitude less than JP8 today, resulting in excessive package weight and volume to meet maneuver needs. Advances in battery energy density will undoubtedly take place, but not enough to offset that magnitude of a disadvantage. The second, and more important, reason from a practicality standpoint is that recharging such vehicles in a short period of time would require massive quantities of electric power that are not available on the battlefield.

To put this assertion in perspective, the committee’s analysis (confirmed by the Army’s internal analysis; see Figure 6.5) shows that to recharge just one heavy combat vehicle (50 to 70 tons) within 15 minutes, a power source of 14 to 29 MW would be required. Hardly practical when an ABCT may have 30 or more Abrams and a comparable number of other supporting armored ground combat vehicles.

Similarly, all-electric tactical vehicles have limited practicality on the battlefield given their recharging requirements. For example, the committee’s analysis showed that each Joint Light Tactical Vehicle would require roughly a 2.6 MW power source to recharge within 15 minutes.

Because nuclear energy dwarfs JP8 and diesel in terms of energy density, some have suggested that a mobile nuclear-based power source might meet the power demand needed to enable all-electric vehicles on

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⁶ N. Sönnichsen, “Leading Biodiesel Producers Worldwide in 2019, by Country (in Billion Liters),” Statista, https://www.statista.com/statistics/271472/biodiesel-production-inselected-countries/, accessed January 2021.

the battlefield. However, the latest design proposals indicate that such a device would weigh 40 tons, require delivery of two 20-foot ISO⁷ containers to the battlefield, and have setup and cooldown times of 3 days and 2 days, respectively. Such operational constraints are not consistent with the multi-domain operations (MDO) strategy of deploying and operating mobile forward operating bases.

As still another constraint, the prototype nuclear power plant currently being developed for expeditionary use, with 2027 production planned, would provide only 2 MW of electricity, which is a far cry from the 65 MW average consumption of one maneuvering ABCT or the 14+ MW required to recharge just one heavy ground combat vehicle in 15 minutes. Nevertheless, in a more enduring base location that requires substantial energy for sustainment operations, such a nuclear plant might be attractive as a modular capability for 24/7 power, independent of fuel logistics, for an extended period of at least 3 years.

This assessment does not mean that all-electric vehicles will not have an encouraging future in the domestic consumer, commercial, and trucking world. Rather the committee concluded that an all-electric tactical force would not be suitable for the Army to adopt through 2035. Non-tactical electric vehicles (EVs) require significantly less power or may operate over shorter ranges. They can return to the same location with a permanent connection to a high-power grid, and can be fully charged overnight. Contrast that with a multi-domain combat scenario where, in many cases, the energy must be brought to a constantly changing battlefield location and rapidly resupplied.

Of particular significance, hybrid technologies using internal combustion engines (ICEs), gas turbine engines, generators, power electronics, and battery storage can deliver many of the electrification advantages to the field without the recharging time and range constraints of EVs. Of particular importance is the improved fuel economy of up to 20 percent that hybrids provide.⁸ The Army and its supporting defense industry suppliers have already initiated much encouraging work in this area.

Hybrids also provide low noise and low thermal signatures while idling or traveling over short distances, using the energy stored in the battery with the onboard power electronics to operate when the ICE is shut down. With existing battery energy densities, they may range up to 3 to 10 miles without engine engagement, a distance that will increase as battery energy density increases over time. Lastly, it would be possible to tap into vehicle hybrid energy systems (up to

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⁷ ISO refers to International Organization for Standardization.

⁸ See Appendix K.

and including 1 MW for a heavy main battle tank) to provide power for a local microgrid, for a mobile weapon system, or to recharge dismounted soldier power packs.

The committee identified a number of fuel-efficiency opportunities that would enable the Army to further reduce the number of presently sized fuel trucks and/or convoy trips needed to bring power and energy to the field. Improvements in horizontally opposed two-stroke piston engines, a technology already pursued by the Army, are possible in the areas of fuel efficiency, power density, and heat rejection. Also encouraging are some of the four-stroke diesel technologies under development that offer lower friction, better combustion, and waste heat recovery, as part of the Department of Energy SuperTruck programs.

Further longer-term opportunities may exist in the form of free-piston engines and linear generators. A possible additional application for these emerging low fuel consumption ICE engines is applicability for relatively long-duration unmanned aerial/ground vehicles (UAVs/UGVs) where the fuel consumption (and fuel tank size) advantage overcomes the present power/weight advantage of gas turbines.

To improve self-sustainability, energy consumption needs to be minimized and its counterpart, energy efficiency, needs to be maximized throughout the complete chain from energy storage to power delivery. For example, lower rolling-resistance tracks, higher temperature–capable power electronics, batteries, motors, and more-efficient cooling systems together could enable considerable reductions in parasitic cooling and friction losses.

It must be noted that the above-mentioned opportunities would significantly reduce the amount of liquid heavy hydrocarbon fuel that would need to be transported to provide an equivalent amount of energy. As a rough quantification, Figure ES.2 is provided.

Note that a 48 percent improvement in fuel efficiency results in a 32 percent reduction in the fuel that needs to be transported to the field to provide an equivalent amount of energy. These numbers should not be considered a commitment but a vision of what may be possible and

should be pursued. Experience has shown that it may not be possible to realize all of the fuel economy opportunities on a roadmap.

The committee identified some encouraging increases in battery energy density, which will provide more capable hybrids and UAVs, as well as lighten the load of the dismounted soldier. A number of these opportunities where further investment is justified are discussed in the report. Particularly encouraging are recent developments showing that zinc-based batteries with reconfigured three-dimensional (3D) architectures, once moved to a new performance curve, bypass the safety issues associated with rechargeable Li-ion batteries while providing significant improvements in both energy and power density at the system level.

Direct energy conversion technologies being pursued by the Army continue to advance. For example, solid oxide fuel cells (SOFCs) offer promise in operations where a low noise signature over long distances is desired. Work is now proceeding on onboard JP8 reformers sized to fuel 10 kW SOFC auxiliary power units (APUs) for ground combat vehicles. The challenge, though, is significant; SOFC requires the sulfur level in the fuel to be below about 1 ppm, whereas JP8 and the ultra-low sulfur domestic diesel are allowed to have sulfur levels of 3000 ppm and 15 ppm, respectively. In addition, SOFCs operate above 700°C, so somewhat lengthy start-up times (30 minutes to a few hours) need to be factored into their deployment. Proton exchange membrane (PEM) fuel cells, which are now being used to power commercial trucks and buses, could provide fast start-up but also introduce a new challenge of providing and handling hydrogen in the battlefield.

To assess the importance of stealth operation in selected prime propulsion powertrains, the use of combat force-on-force simulation studies are recommended. SOFCs (low acoustic signature) and PEM fuel cells (low acoustic and thermal signatures) may offer certain advantages in selected applications. A key question to consider is the following: When adversaries are employing drones and enhanced sensor technologies, can a ground combat vehicle brigade with or without tracks ever truly be undetectable?

In terms of forward operating bases and tactical command posts, the committee was encouraged by and commends high-priority Army advancements now under way on new microgrid concepts, such as the Secure Tactical Advanced Mobile Power (STAMP) project using a Tactical Microgrid Standard (TMS). The objective integration of power generation, distribution, battery storage, metering, control systems, and on-board vehicle power from mobile tactical platforms into an AC/DC microgrid essentially will make JP8 and electricity more fungible, thereby enhancing “Energy-Informed Operations” capability to manage energy more effectively to meet battlefield needs.

Consistent with past studies, the committee did not find wind, hydro, large-scale solar, or waste recovery to be practical for battlefield deployment. However, as with the case of small nuclear power plants, they may have an appropriate place in semi-stationary bases located in permissive locations. In addition, although they were not a focus of this study, small flexible roll-up solar panels and small solar trailers now commercially available and can provide expeditionary personnel with a fallback battery charger or power source for laptop computers and radios.

The study noted that the demands of some future operating environments (smaller formations supported by logistical and fire support) suggest that the Army’s P&E efforts should have an increased emphasis on how to support a distributed force structure, including the dismounted soldier.

For the dismounted soldier, the committee was particularly impressed with some of the work under way to adapt thermophotovoltaic (TPV) devices, another direct energy–conversion technology, to tactical application. The soldier silent power (SSP) project utilizes a micro-combustor to convert JP8 or diesel to heat a nano-engineered infrared emitter, and tuned photovoltaic (collector) cells to convert the heat to power. This solid-state conversion technology offers the potential to significantly lighten the dismounted soldier’s load as the Army seeks to increase the self-sustainment period from 3 to 7 days. TPV technology could also be used for other Army applications. It has already been proposed for small UAV propulsion. Furthermore, it could potentially be used to power “mule vehicles” intended to lighten the dismounted soldier’s weight burden.

The Army already has such work on mule vehicles under way with their small multi-purpose equipment transport (SMET) program. Each mule has the capability of carrying up to 450 kg of equipment while providing up to 3 kW of electrical power while stationary and 1 kW while moving. Other unmanned vehicles are actively being developed with the capability to export up to 30 kW of electrical power. Extra sets of rechargeable batteries could be carried and recharged on the mule vehicle while the dismounted force was moving. This ability to replenish energy storage off of the warfighter would minimize the size of the batteries carried by each soldier as they could be swapped whenever needed with the replacement set on the mule vehicle.

Substantial opportunities have arisen to enhance the battlefield situational awareness essential for MDOs by 2035, many of which will require significantly more power. For example, 5G communications has much higher bandwidth, but requires greater power to provide the same range as 4G. Service coverage is a particular challenge that needs to take into account varied terrain and environmental conditions. Energy-efficient power conversion using advanced power electronics, improved

power-management control schemes, directional antennas, and dynamic network operation will be critical enablers for effective 5G mobile ad hoc networks (MANETs). Specific recommendations for future Army MANET studies are detailed within this report.

Use of nuclear isotope–decay devices, such as those used for space probes, may be practical for remote sensors, requiring extended lifetimes with relatively low power demands. However, their relatively low power-to-weight ratio limits them to an auxiliary role (such as battery charging) for higher power–demand applications such as the dismounted soldier or handheld weapon systems.

The committee became aware of several technologies that would generate hydrogen in the field, as an alternative to transporting it by a supply convoy. This locally produced hydrogen could then be used with PEM fuel cells, providing silent-range operation over extended ranges. One approach involves the use of electrolyzers, which are commercially available today. In this commercial application, the produced hydrogen is used as a storage mechanism today for energy produced by renewable sources.

Another approach, albeit less developed, to generating hydrogen in the field involves the use of aluminum alloys that produce hydrogen when activated and combined with water. Questions associated with this approach include what sort of apparatus would be required to generate the hydrogen, dehumidify it, compress it, and manage its flow in a given application. Despite the lower level of readiness for this technology, further work including detailed definition of a potential application and preliminary design is warranted.

Future P&E studies would benefit greatly from a series of detailed battlefield scenarios against which various P&E alternatives could be evaluated. Furthermore, given the importance of P&E on overall operational capabilities, it is strongly recommended that the scope of future warfare computer simulations (i.e., tactical exercises without troops) be expanded to include P&E considerations. These simulations should include identification of the quantity and form of energy to be transported to the battlefield, how much of this mission-required energy could be replaced with local sources, where it would be stored, any setup or takedown times, at what rate (i.e., power) that energy could be released, and how the energy needs of operating bases, vehicles, and dismounted soldiers would be replenished, including any refueling or recharging time requirements. When tabletop wargames are undertaken without computer simulation, personnel with power and energy expertise should be part of the adjudication and evaluation teams. It is worth noting that this is not a new insight, as a previous study by the Defense Science Board recommended “conducting realistic wargames

TABLE ES.1 Decision/Trade-Off Matrix

NOTE: FE = fuel efficiency; ICE = internal combustion engine; JP8 = jet propellant 8;

PEM = proton exchange membrane; UGV = unmanned ground vehicle.

and exercises that accurately reflect the threats to and capabilities of the joint logistics enterprise.”⁹

In short, the committee found many opportunities to enable a more capable Army within a very challenging and a somewhat uncertain future multi-domain environment. As in any study of multiple alternatives, there are some trade-offs. For example, if silent mobility and low thermal signatures are mandatory with an extended range, there may be a need to deploy a limited number of hydrogen PEM fuel cells, albeit with penalties in the number of convoy transport trucks. Some of these trade-offs for the major recommended technologies are summarized in the trade-off/decision matrix in Table ES.1.

Based on the technological opportunities presently being studied by the Army and those the committee identified for future study, the committee expects that this enhanced operational capability can be achieved with properly directed research and development efforts.

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⁹ Defense Science Board, 2020, “Task Force on Survivable Logistics: Executive Summary,” https://www.hsdl.org/?view&did=820550.