DRIVING FUEL CHOICES
Te reality is that the Army is not behind. It has experts in all of these fields who have been conducting research on alternative energy sources and hybrids for military vehicles for more than 20 years. In fact, the Army Rapid Capabilities and Critical Technologies Office recently awarded BAE Systems a $32 million prototype agree- ment to integrate a hybrid electric drive system onto a Bradley Fighting Vehicle. Te development program is part of the Army’s effort to increase vehicle efficiency and boost power generation to support integration of future technologies and improve mobility for combat vehicles on the battlefield.
However, the bottom line is that there is a good reason the Army hasn’t unilater- ally decided to switch to alternative fuels. Te Army has a unique set of operational requirements, and no current fuel source meeting those requirements contains as much energy, by weight, as diesel or gaso- line. Nor is there an alternative form of energy that can be carried or generated on board that, in terms of size or volume, is not at least four to 10 times that of gas and diesel. Electrification of the Army’s ground fleet (for example, by fielding integrated starter generators on tactical vehicles), is very important, but it still relies on diesel as the primary energy source.
Tis analysis focuses on energy density (the amount of energy stored in a system per unit of volume), conversion of that energy, as well as mass and volume requirements, to determine how they compare with current sources. Te full technical paper was published in the Jour- nal of Energy Resources Technology by Andrew Mansfield et al. Titled “Assess- ment of Conventional and Alternative Energy Carriers for Use in Military Vehi- cle Platforms,” the paper was published online Aug. 31.
BACKGROUND Tis study builds on past studies conducted primarily through the U.S. Army Hybrid- Electric Vehicle Experimentation and Assessment program. To understand why the Army still relies on diesel and gas, we need to understand the Army’s require- ments, operating environment and how energy is stored and transformed into vehi- cle motion.
That’s done through a process that starts with an energy carrier, which is a substance (fuel) or “material state” that stores energy that can be later converted to other forms such as mechanical work or power. We use the term “energy carrier” because it refers to energy that can exist in a variety of forms and can be converted from one form to another. Such carriers may include springs, flywheels, electri- cal batteries, pressurized air, hydrogen, petroleum, coal, wood and natural gas. A flywheel is a spinning mechanical device that is used to store rotational energy that provides continuous energy when the energy source, say torque from the engine, is intermittent.
Tus, energy carriers are the vehicle’s onboard store of energy, which is trans- formed into useful mechanical energy through a conversion device, such as combustion engines, electric motors or fuel cells. Tis process provides mechan- ical energy that can drive a vehicle transmission, which then can produce power such that:
Energy Carrier → Conversion Device → Mechanical Energy
Tere are several categories of transportation- applicable energy carriers, i.e., those that can reasonably be stored on board a vehi- cle, such as chemical and mechanical energy carriers. Te various energy carri- ers and conversion devices considered in
this analysis are illustrated in Figure 1. Tey are: hydrocarbon fuels (gas or diesel), batteries, supercapacitors, hydrogen (fuel cell) and flywheels.
Batteries and supercapacitors (electro- chemical devices similar to batteries but designed to produce very high specific power, i.e., for use in regenerative brakes on hybrid vehicles) are often compared in terms of their energy and power. Batteries have a higher energy density (the amount of energy that the system can store), compared with supercapacitors, which have a higher power density (the rate of energy that the system can release).
Tat makes supercapacitors particularly suitable for storing and releasing large amounts of power relatively quickly, whereas batteries are capable of storing large amounts of energy over long peri- ods of time.
ANALYSIS Key military requirements assume that fuels will need to be transported to the battlefield, so there is a need for high- energy dense fuels with low weight, as well as a way to generate electricity. Tat’s because a typical Soldier carries four or five electronic devices—and in the future that burden will likely double—and such devices are useless without electrical power generation. Modern lithium ion (Li-ion) batteries cannot keep up with demand even if a Soldier could carry a large supply.
Figure 2, Page 103, summarizes the performance characteristics of commer- cial energy carriers and promising future technologies, and illustrates the distinct superiority of diesel and gasoline fuels. Tey are by far the most widely used energy carriers and are used in conjunc- tion with internal combustion engines or gas turbines with varying levels of commercialization.
100 Army AL&T Magazine Fall 2020
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