Single-pressure absorption heat pumps enable refrigeration using solely thermal input, and are ideal candidates for distributed and passive cooling applications, such as vaccine refrigeration in developing countries. Liquid circulation in single-pressure systems has conventionally been achieved with a gas-fired bubble-pump generator (BPG) - a vertical tube in which intense localized heat from a flame desorbs refrigerant. The buoyancy of rising refrigerant bubbles lifts solution to an upper reservoir, which feeds flow through other system components. While this method of refrigerant separation and fluid circulation is mechanically simple, it necessitates a source of high-grade fuel, and thus cannot be employed in waste-heat recovery applications or many remote settings. In this investigation, a novel full-length coupling-fluid heated BPG design is investigated. In this implementation, heating fluid is circulated through an annular jacket around the bubble-pump tube. Because the heat transfer area is much larger than in conventional spot-heated BPGs, relatively low temperature thermal sources can be employed. Results are presented from an experimental study of a 7.8 mm internal diameter BPG with water-steam working fluid over a wide range of operating conditions. This BPG can operate with thermal input as low as 11 K above the fluid saturation temperature. This investigation demonstrates that coupling-fluid heated BPGs are a promising alternative to conventional gas-fired implementations, and can enable passively-driven refrigeration using widely available low-grade thermal energy sources.
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