The figure depicts a passive radiative cooler designed for use in outer space. The design of this device conjoins radiative and conductive thermal-isolation features, which, in further conjunction with a favorable spacecraft attitude and on-orbit thermal environment, can be utilized to cool specimens of high-temperature superconducting materials to operating temperatures. Once installed on a spacecraft or even on the lunar surface, the passive radiative cooler will perform the cooling function that would otherwise be performed by a more expendables-hungry cryogenic system. This device, which has the added advantage of no moving parts, can operate in low orbit around the Earth in the space-shuttle cargo bay. Small and adaptable to many spacecraft and mounting configurations, this device can be used to demonstrate applications that involve superconductivity. Commercially, this device can advance the art by providing a simplified alternative for satellites equipped with infrared (IR) detectors or apparatuses that exploit superconductivity.
The current art in cooling sensors or specimens of superconducting or other materials in outer space involves the use of cryogenic cooling systems; the operation of such systems is more complex and certainly more costly in expendables (cryogenic fluids) than is the use of the passive radiative cooling capability of outer space itself. Though radiative coolers other than the present one have been used before in outer space, those devices can operate only on interplanetary spacecraft, on spacecraft in high orbit around the Earth, or on spacecraft in low orbit around the Earth under tailored illumination conditions (i.e., Sun-synchronous orbits). The thermal-radiation environment of a high orbit around the Earth differs markedly from that of a low orbit around the Earth: the proximity of the Earth gives rise to undesirable heating by IR radiation from the Earth plus reflected solar radiation reflected from the Earth (albedo) from a large portion of the field of view. The present passive radiative cooler provides shielding against the radiation from the Sun and Earth and is of a size and simplicity that make it suitable for operation in the space-shuttle cargo bay, which is also a source of IR heating.
The three main components of the passive radiative cooler are a mounting base, a conical shade, and a conical radiator/sample tray. The mounting base includes a low-thermal-conductance structure for holding the conical shade. The conical radiator is suspended within the conical shade by tensioned nonmetallic cords, which provide (1) a high degree of isolation against thermal conduction and (2) protection against vibration for the sample tray and the radiator. Both the outside of the conical radiator and the inside of the conical shade are fabricated with a low-IR-emittance finish for a high degree of radiative isolation. The inside of the conical radiator is given a high-IR-emittance finish to promote high radiative transfer of heat to space.
An item that one seeks to cool (e.g., a specimen of a high-temperature superconductor) is affixed to the sample tray, and the passive radiative cooler is mounted on a spacecraft structure that faces away from the Sun or a planet. As the spacecraft orbits in a specified attitude, the sample tray and sample are cooled radiatively. The cone angle of the radiator is chosen to afford adequate radiative heat rejection while enabling the cone to shield the sample from viewing other bodies (the Earth, the Sun, or nearby objects) that could adversely affect heat balance of the sample.
A relatively high degree of thermal isolation can be achieved. For example, in a test of a prototype of the passive radiative cooler, a sample temperature of 116 K was achieved in the presence of a mounting surface at a temperature of 240 K.
The passive radiative cooler is expected to function within a remarkable temperature range. The basic passive-radiative-cooler design is flexible and scalable; for example, if the device is to be mounted in a location with few nearby obstructions, it could be beneficial to design for a more open cone. The passive radiative cooler can be mounted on a spacecraft for cooling samples of high-temperature superconductors or other materials, detectors, or sensors, provided the environment and spacecraft attitude meet the specified criteria. Finally, again assuming that sufficient isolation from the surface can be achieved, the passive radiative cooler can even be used on the lunar surface to cool sensors.
This work was done by Steven L. Rickman, Ross G. Iacomini, David S. McCann, Robert G. Brown, and Yuan-Chyau Chang of Johnson Space Center and by Jeffrey A. Clayhold, Ching-Wu (Paul) Chu, Allen W. Linnen, Jr., and Yuyi Xue of the Texas Center for Superconductivity, University of Houston.