Miniature, lightweight, low-power, low-vibration Joule-Thomson Rankine-Cycle refrigerators have been proposed for cooling portable scientific instruments. These refrigerators would be made largely from silicon wafers by micromachining techniques like those used to fabricate integrated circuits. The compressors in these refrigerators would be microperistaltic pumps, in which voltages applied in spatial and temporal sequences to multiple electrodes positioned along channels would give rise to waves of electrostatic attraction that would cause membranes to pinch the channels closed at intervals in peristalsislike waves. [A fuller description of microperistaltic pumps was presented in "Microscopic Heat Exchangers, Valves, Pumps, and Flowmeters" (NPO-19093), NASA Tech Briefs, Vol. 22, No. 7 (July 1998), page 66.]
A single- or multiple-stage refrigerator according to this concept could be made from two fused wafers. The figure schematically illustrates a single-stage refrigerator, in which a microperistaltic pump would compress the working fluid (a mixture of gases as described below) from a lower pressure of 5 psi (34 kPa) to a higher pressure of 25 psi (170 kPa). The compressed fluid would flow along a microchannel, where it would be partly cooled by transfer of heat into the surrounding wafer material. Continuing along its flow path, the compressed fluid would be cooled further and condensed in the first of two microchannels in a highly thermally conductive counter-flow heat exchanger within the wafer. After leaving the heat exchanger, the fluid would flow along a microchannel to an expansion nozzle in a cold pad that is thermally well insulated except for contact with the object to be cooled.
Upon expansion in the nozzle, the fluid would evaporate, drawing latent heat of vaporization from the cold pad. The vapor would flow into the second microchannel in the heat exchanger, where it would absorb heat from the compressed fluid in the first microchannel. Upon emerging from the heat exchanger, the fluid would return to the lower-pressure port of the microperistaltic pump, completing the cycle.
The Joule-Thomson-cooling capabilities of a number of gas mixtures have been studied to assess their utility as working fluids for a refrigeration cycle between an exhaust temperature of 200 K and a refrigeration temperature of 70 K. One suitable fluid was found that consisted of nitrogen and five hydrocarbons. With a mass flow rate of 0.001 mole/second and a heat-exchange efficiency of 0.98, the refrigerator could handle a maximum heat load of 0.3 W while maintaining a temperature of 71 K.
For most terrestrial applications, it would be more practical to exhaust heat at a higher temperature, giving rise to the need for two stages of refrigeration to reach a low temperature of 70 K. For example, the first stage could exhaust heat at 313 K and provide cooling at 190 K, while the second stage would be like the single-stage refrigerator described above, with its exhaust heat removed by the cold pad of the first stage. The composition of a suitable working fluid for the first stage consisted of carbon tetrafluoride and miscellaneous hydrocarbons. With a mass flow rate of 0.001 mole/second and a heat-exchange efficiency of 0.98, the first stage could handle a heat load of 2 W while maintaining a temperature of 190 K.
This work was done by Frank T. Hartley and Jack A. Jones of Caltech for NASA's Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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