Figure 1. Evaporator or Condenser sheets
PNNL researchers are developing a miniature heat pump for a range of microclimate control applications, including man-portable cooling and distributed space conditioning. The heat pump is based on the well-known lithium bromide/water absorption cycle, but the miniaturization technology is based on the enhanced heat and mass transfer made possible by fluid flow in microchannels. Using many microchannels in parallel, heat pumps can be deployed with radical reductions in size and weight compared to conventional systems. For the lithium bromide/water-absorption cycle system, the heat pump size can be reduced to 1/60th of the size needed for a comparable conventional heat pump.
Most of the components of the heat pump are embodied in a metal sheet that is machined with microchannels and other features needed for a particular function. Individual or combinations of sheets are used for absorption, desorption, condensation, evaporation, and heat exchange. These sheets are stacked together to form an integrated system. The addition of an external solution pump completes the structure of the heat pump. Additional equipment is necessary for heat actuation, fuel storage and rejection of heat to air, if required.
Many workers must wear protective clothing that significantly reduces heat transfer from the body, decreasing worker effectiveness. These workers include both military and civilian personnel who need to wear personal body armor, fire-resistant clothing, or suits to protect against exposure to hazardous chemicals, biological agents, and radioactive materials.
Until now, cooling systems that are truly manportable have not been technologically feasible. Currently, cooling systems for environmental suits used in areas such as hazardous waste situations and military applications are large, heavy, inefficient, and effective for a limited amount of time.
For a 350 watt cooling load and system operation of 8 hours, our complete cooling system is expected to weigh about 11 pounds and be about 0.6 cubic feet in volume. Most of this size and weight is taken up by the equipment required to transfer heat to ambient air and for combustor fuel.
In addition, operation of the system is independent of orientation, which allows the individual wearing the suit to perform tasks with greater efficiency than in current methods. This also enables use in ships or other conveyances where a level orientation is not always possible.
All of the individual micro components of an absorption cycle heat pump have been demonstrated. A prototype, integrated system is currently being developed. Methods of mass production of man-portable cooling systems are being developed and evaluated.
The technology is protected by United States Patent 5,611,214 ("Microcomponent Sheet Architecture," issued March 18, 1997). Foreign patent applications based on this case and additional United States patent applications are pending.
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