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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.

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.