Coupling-fluid heated bubble pump generators: Experiments and model development

Single-pressure absorption heat pumps operate using solely thermal input and are ideal candidates for distributed cooling applications. Liquid circulation in such systems has conventionally been achieved with spot-heated bubble-pump generators, vertical tubes in which intense localized heat desorbs refrigerant. Rising refrigerant bubbles lift solutions to an upper reservoir, which feeds other system components. While this method of refrigerant separation and fluid circulation is simple, it necessitates a source of high-grade energy and thus cannot be employed in many settings. In this investigation, a full-length coupling-fluid heated bubble-pump generator 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 bubble-pump generators, relatively low-temperature thermal sources can be employed. Results are presented from an experimental study of a 7.8-mm-internal-diameter bubble-pump generator with water-steam working fluid over a range of operating conditions. This bubble-pump generator can operate with thermal input as low as 11 K above the fluid saturation temperature. A mechanistic fluid flow and heat transfer model is developed and validated. This investigation demonstrates that coupling-fluid heated bubble-pump generators are a promising alternative to conventional spot-heated implementations and can enable refrigeration using low-grade thermal energy.