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This paper describes the performance benefits and current technology progress of an active heat pump loop (HPL) thermal control bus for spacecraft and planetary thermal control applications. Having initiated this research more than 14 years ago, this paper also briefly highlights the technical developments and obstacles overcome during this 14-year development. This paper discusses the unique features of the HPL approach that make it an attractive design choice for future manned thermal control applications: the use of an heat pump to reject heat to space at a temperature above the heat acquisition temperature, the use of non-toxic thermally stable working fluids, and the use of high-performance lubrication-free (gravity independent) refrigeration compressors. The HPL approach has the performance benefits of a traditional two-phase pumped loop thermal bus coupled with the simplicity of a single-phase pumped loop.

This paper will describe a patent-pending approach of using a vapor compression system to also provide a forced two-phase indirect heat transfer loop. This system can be configured with water boiler peak cooling thermal control hardware to avoid the high ambient temperatures associated with supersonic low altitude flight. Due to the very short duration of this high ambient condition, water boiler transient cooling techniques have potential. The water boiler can also be used for ground based cooling when flight-line ground cooling carts are unavailable. The use of a PID controller to accurately control the cold plate temperatures when used with a solenoid activated by-pass circuit will be described.

This paper investigates the use of several zero-ozone depleting potential (zero-ODP) HFC refrigerants, including HFC-134a, HFC-227ca, HFC-227ea, HFC-236ea, HFC-236cb, HFC-236fa, HFC-245cb, and HFC-254cb, for centrifugal chiller applications. We took into account the thermodynamic properties of the refrigerant and aerodynamic characteristics of the impeller compression process in this evaluation.. For a given operating temperature lift, there are significant differences in the pressure ratio required by each refrigerant and this variation in pressure ratio directly affects compressor size, efficiency, and performance. A comparison of the HFC refrigerant candidates with CFC-114 shows that HFC-236ea, HFC-227ca and HFC-227ea are viable alternatives for centrifugal water chillers. HFC-236ea has properties closest to CFC-114, and will result in comparible performance, but will require a slightly larger impeller and a purge system.

This paper will describe the screening and development of absorbents for HFC-134a in the chemical/mechanical heat pump. The absorbents must have low volatility, low melting point, high solubility for HFC-134a vapor, high heat of mixing with HFC-134a, suitable vapor pressure/temperature concentration characteristics when mixed with HFC-134a, low toxicity, low flammability, and thermal stability. A screening procedure was used to select approximately 15 absorbents for experimental evaluation. Measurement of the key physical and thermodynamic properties of the absorbent/HFC-134a mixtures, such as vapor pressure/temperature/concentration properties, materials compatibility, and thermal stability, is described. From these measurements, activity coefficients, enthalpy of mixing, and entropy of mixing of the liquid solution were determined.

We have successfully used techniques found in computational chemistry to identify a series of compounds for use as nontoxic heat transfer fluids for habitat two-phase thermal control systems. Our approach allows us to screen compounds, based on their molecular structure, where no thermodynamic data exists. Once compounds that have the desired properties are identified, their properties can be measured. The key problem in developing the most efficient and versatile two-phase pumped fluid system is the selection of the appropriate working fluid. Although many fluids are good candidates in terms of their thermodynamic properties, they cannot be used inside pressurized, habitat volumes due to toxicity, flammability, and thermal stability concerns.