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Background |
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Most current personal cooling systems
consist of clothing with a phase change material (PCM) incorporated into
it. The garment must first be placed inside a freezer until the PCM
solidifies and then it may be worn until the PCM has melted. These garments
do not provide sustained cooling and are not useful for long duration
missions. To provide cooling for extended mission durations a cooling cycle
must be used. The type of cooling system depends on where the energy to
drive the cycle will come from. Vapor compression cycles provide a simple
cycle but require large amounts of mechanical shaft work. This mechanical
energy must come from an electric motor or an internal combustion engine.
Large batteries or a fuel cell would be required to drive the electric
motor. The internal combustion engine would use a liquid fuel, thus
reducing system mass and volume but it would also be loud. An absorption
cycle is a more complex cycle but the main energy input would be heat from
burning a liquid fuel. The absorption cycle would still require small
amounts of electrical energy for pumps but the required batteries would be
much less than those required for an equivalent vapor compression system. |
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Motivation |
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These novel cooling systems are expected to be used as
a personal cooling system for hazardous duty suits, and for automobile
comfort cooling systems. When used in hazardous duty suits they are
expected to reduce heat-related stresses, increasing productivity and
allowable mission duration, reduce fatigue, and lead to a safer working
environment. They could also have an immense impact in the medical field
for patients suffering from diseases such as multiple sclerosis, whose
mobility is impaired due to their sensitivity to temperature changes. Other
applications include the transportation of biological tissue and organs. |
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Past Work |
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A wearable cooling system,
powered by a small-scale engine was developed and tested at elevated ambient
conditions. The cooling system was a vapor compression system assembled in
a backpack configuration. The overall cooling system, including the
wearable evaporator, had a total mass of 5.31 kg (11.7 lb) and measured
0.318 × 0.273 × 0.152 m (12.5 × 10.75 × 6 inches). Testing was conducted in
a controlled environment to determine system performance over a wide range
of expected ambient temperatures (37.7-47.5°C), evaporator refrigerant
temperatures (22.2-26.1°C), and engine speeds (10,500-13,300 RPM). Heat
removal rates of up to 300 W, which is the cooling rate established in the
literature as being required for maintaining comfort at an activity level
comparable to calisthenics or moderate exercise, were demonstrated at a
nominal ambient temperature of 43.3°C (110°F). |
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Ongoing Work / Future Directions |
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Future work will develop a
portable system based on an absorption cooling system. The system will be
designed to take advantage of advanced manufacturing techniques such as
precision photo chemical etching, and diffusion bonding to reduce the size
and weight of the system as well as the fluid inventory of the system.
Initial testing of a miniaturized ammonia-water absorption cooling system
has demonstrated the feasibility of this concept. These preliminary results
and ongoing experiments will guide the development of future prototypes and
application-specific optimization. These mass-producible, modular and
portable cooling systems are expected to offer revolutionary means of
cooling at the small scales under environmentally challenging conditions.
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Sponsors |
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