The objective of this project was to develop a more efficient, lightweight, and air-transportable heat pump using helium, a gas that does not deplete ozone or contribute to global warming, instead of existing fluorocarbon-based refrigerants, such as R-22, for Air Force bare base heat pump units. The successful implementation of a Non-Ozone Depleting Mobile Heat Pump (NODMHP) will bring the Air Force and the Department of Defense (DoD) into further compliance with the 1989 Clean Air Act.
A NODMHP was developed by the Air Force Research Laboratory (AFRL), Air Base Technology Branch that uses single-phase gaseous refrigerants, such as helium, to provide efficient, yet environmentally-benign, heating and cooling refrigerants. Unlike available hydrofluorocarbon alternatives, NODMHP does not contribute to global warming. The objective of this effort was to build and test a 1-ton single-phase pulse-tube prototype for heat cycle efficiency and capacity and then compare the results to current Air Force R-22 systems and commercial-off-the-shelf alternatives. The single-phase pulse tube demonstrated exceptional potential since the only work that must be added to the system occurs at the compressor, which generates thermodynamic heating by compressing the working fluid into a pressure wave. As the helium working fluid rejects heat and passes through the regenerative cooler, it expands and cools, thus absorbing heat. As the working fluid propagates through the system, it once again compresses under its own momentum and reverberates a pulse wave back through the system. During the entire process, the helium returns as much as 80 to 90 percent of its initial compression back to the compressor. Test results revealed that the prototype unit is a good adiabatic heat provider for efficient space heating of deployable shelters and met the unit size reduction goals. However, the pulse tube unit was not capable of meeting the cooling efficiency requirements set in the Air Force operational requirement document. It has been determined that the cooling limitations of pulse tube technology can not be overcome. AFRL/MLQC has terminated further work in the pulse tube heat pump technology and has transitioned to carbon dioxide cycle technology. The recent carbon dioxide cycle research technology has shown highly promising results.
In addition to its environmental benefits, the helium working fluid is inexpensive, easy to use, and has a significantly longer service life, making NODMHP a good candidate for both DoD and commercial industry applications. AFRL/MLQC determined this technology provides a good heating capability for extreme cold location applications.
In order to achieve the goal of eliminating the military's dependence on heavily regulated ozone-depleting chemicals, it will be necessary to pursue the development of a different technology. Currently, research is being conducted on the use of carbon dioxide in place of fluorocarbons.