The military places greater demands on its equipment — electronic and otherwise — than any other market segment. It is also true that it places its greatest thermal demands on the men and women who serve in the armed forces. In cold environments, body armor acts as thermally protective clothing to keep a soldier warm, but in hot climates where removing body armor to cool the human is not practical or advisable, another solution is required.
Humans have been wearing armor for thousands of years. In earliest times, men wore animal skins and over time, this evolved into the wearing of metal plates, which could protect against more energetic projectiles, but only at the expense of increasing their thickness and weight.
In the 1960s, engineers developed a reliable bullet-resistant armor that a person could wear comfortably. Unlike traditional armor, this soft body armor was not made out of pieces of metal; it was formed from advanced woven fibers that can be sewn into vests and other soft clothing (see photo).
Modern body armor is divided into two main categories: hard body armor and soft body armor. Hard body armor, made out of thick ceramic or metal plates, functions basically the same way as the iron suits worn by medieval knights — it is hard enough that a bullet or other weapon is deflected. That is, the armor material pushes out on the bullet with the same force (or nearly the same force) with which the bullet pushes in, so the armor is not penetrated.
Typically, hard body armor offers more protection than soft body armor, but it is much more cumbersome. Police officers and military personnel may wear this sort of protection when there is high risk of attack, but for everyday use, they generally wear soft body armor, flexible protection that one wears like an ordinary shirt or jacket.
Thermal Management of the Human Body
The human body is designed to hold its core temperature between 98°F and 100°F (with some additional variability amongst individuals) when the ambient temperature is between approximately 68°F and 130°F. There are many assumptions that depend on the relative importance of the various mechanisms the body has for removing heat; for in - stance, dry or wet skin, calm or rapidly moving air, etc.
The body exchanges heat with its surroundings through four primary mechanisms: radiation, conduction, convection, and evaporation of perspiration. These processes can act in concert or alone. The entire process is regulated by the brain (hypothalamus) and, depending on the external conditions, one mechanism or another may dominate.
The human body requires significant heat pumping to make an individual feel cooler while dropping the skin temperature a few degrees. Temperature drops greater than 10°C can be harmful to the body and small surface temperature drops will be compensated for by the active thermal management system of the human body itself.
While the body armor that is available today is a significant improvement over that used by the knights of the Middle Ages, it has one drawback in common. If armor is designed to keep things from getting in, then how do things get out, and in particular, how does the heat generated by the human body get removed from the protective armor? The primary reason for doing this is that if the armor is uncomfortable, it will not be used. In fact, recent feedback from police officers shows that 50% of officers do not wear protective vests due to overheating.
For ambient temperatures (20- 25°C) below human body temperature, where convection is excluded, heat loss will be dominated by radiation losses. For ambient temperatures above the human body temperature, perspiration will dominate.
Body armor as it exists today provides an extra layer of thermal resistance and hence, reduces the cooling by conduction and interferes with cooling by convection or enhanced cooling by perspiration assisted by convection. Enhancing the existing passive thermal management system is unlikely to provide the desired benefit, resulting in the need for an alternative solution such as integrating an active thermal management system into the armor.
Active Systems for Thermal Management
If adding cooling while not increasing the bulk of the body armor is desired, active thermal management must be added. Given that passive heat removal is only a linear function over distance of the temperature, work must be put into the system to obtain a greater rate of cooling and a smaller device.
Active thermal management devices such as thermoelectric coolers (TECs) have been employed in several new applications to provide this additional heat pumping and temperature stabilization capability. Thermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump that transfers heat from one side of the device to the other side against the temperature gradient (from cold to hot), with consumption of electrical energy. Such an instrument is also called a Peltier device, Peltier diode, Peltier heat pump, solid-state refrigerator, or thermoelectric cooler.
TECs today are manufactured using two methods. Bulk TECs are manufactured using pressed sintered powders, and thin-film TECs are manufactured using semiconductor manufacturing processes. Each method has benefits, but the most obvious difference is that the thin-film TECs are 10 to 20 times smaller in each linear dimension and therefore add almost no weight.
One of the drawbacks to using TECs is that, in doing work to move the heat in an almost discontinuous manner, they will add heat to the system. This heat will be added downstream and needs to be accounted for when determining how much heat must be pumped.
TECs have had a reputation for poor efficiency, but that is because they are usually operated at or near their operating limits. The human body, as mentioned above, requires only a small temperature difference (ΔT) while requiring large amounts of heat pumped. In this operating environment, a TEC can operate with efficiencies greater than one in which the coefficient of performance is defined as the heat pumped divided by the electrical energy used to move that heat.
How and Where to Use Active Thermal Management
The last question to consider when employing active thermal management in this manner is where to locate it within the armor.
Studies have shown that placing cool water or cold packs on the skin in areas of high blood flow reduces the temperature of the blood, which then recirculates throughout the body, lowering the overall body temperature. Also, to keep the temperature drop across the TEC to a minimum, the device should be placed at a location where the blood flow is close to the surface. Therefore, this approach suggests that the wrists, neck, or armpits — where blood vessels are close to the skin — offer ideal locations for placement of this type of thermal management. Cooling anything other than the blood (such as layers of fat or muscle) will mean additional work for the TEC and hence, will require additional heat to be added downstream for rejection into the environment.
Embedding thermoelectrics into body armor at strategic locations of high blood flow can keep the temperature of the human body at the ideal core temperature, thus making armor more comfortable to use and reducing the chances of heat stroke without putting soldiers at risk.
This article was written by Dr. Paul A. Magill, Vice President of Marketing and Business Development at Nextreme Thermal Solutions, Durham, NC.
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