The use of ammonia-based working fluids for absorption prevails in a wide range of applications due to the low freezing temperature of the refrigerant and the absence of crystallization as well as the lack of problems under vacuum conditions. This paper presents a comprehensive overview on the use of ammonia-based absorption chillers and heat pumps. The thermodynamic and physical properties of pure ammonia and binary and ternary ammonia mixtures are presented in correlation formulas. Developments and applications in subfreezing refrigeration, heating/domestic hot water, renewable energy utilization, waste heat recovery, thermal energy storage and miniaturization of absorption systems are presented and summarized. In subfreezing refrigeration, the evaporation temperatures for single-stage absorption lie mainly between −30 °C and −5 °C, and they can reach as low as −70 °C in advanced absorption systems. Air-source and ground-source absorption heat pumps are suggested for heating/domestic hot water applications in cold regions. For renewable energy uses, ammonia-based solar absorption applications with various working fluids are quite popular, whereas geothermal and biomass energy systems are less studied. In thermal energy storage, ammonia-based working fluids are not advantageous for storage capacity or cycle efficiency, but they prevail for subfreezing energy storage. Additionally, ammonia-based fluids are also attractive options for the miniaturization of absorption systems due to the absence of crystallization.

An overview of ammonia-based absorption chillers and heat pumps

This two-part paper presents an overview of evaporation heat transfer mechanisms, a review of the experimental and prediction methods and a creation of a consolidated multi-lab database of 3601 data points and provides a detailed comparison of all the prediction methods to this broad database and finally proposes new prediction methods for the local heat transfer coefficient and the frictional pressure gradient of flow boiling within plate heat exchangers. Specifically, in Part 1, a description of the complex geometry of plate heat exchangers and an introduction to their major applications are described, followed by an extensive literature survey of experimental studies and associated prediction methods. While many prediction methods are found to work in the literature, the results of this study show that these methods have only been compared to their original data, but have not been vetted against a large database covering many fluids, plate designs and test conditions.

Flow boiling and frictional pressure gradients in plate heat exchangers. Part 1: Review and experimental database

The use of membrane contactor technology is well-known in the process industry and can be employed in many important fields; such as separation and absorption processes, membrane distillation, pervaporation, biotechnology, food industries etc. In recent years, research has been carried out regarding the use of membrane contactors in the components of absorption refrigeration systems. The use of membrane contactors makes it realizable to design compact components with improved heat and mass transfer. Heat and mass transfer performance of the components is significantly enhanced due to the higher area to volume ratio available. Membrane based absorber and desorber allow the reduction in size of the absorption refrigeration systems to a great extent and thus absorption refrigeration technology can be used in transport and small scale applications. In this paper, the applications of membrane contactors in absorption refrigeration systems are reviewed. The application of membrane contactors in the components of absorption refrigeration systems, the configurations of refrigeration cycles that employ membrane contactors and the characteristics of the membrane contactors used in absorption refrigeration systems are all reviewed in detail. Information is collected on the choice of working fluid mixture to be used in absorption refrigeration systems that use membrane based components and the compatibility of working fluid mixtures with the membrane contactor material is discussed. The significance and limitations of using membrane contactors in absorption refrigeration systems is included in this paper.

A review of membrane contactors applied in absorption refrigeration systems

The absorber is a major component of absorption cycle systems, and its performance directly impacts the overall size and energy supplies of these devices. Absorption cooling and heating cycles have different absorber design requirements: in absorption cooling systems, the absorber works close to ambient temperature, therefore, the mass transfer is the most important phenomenon in order to reduce the generator size; on the other hand, in heat transformer absorption systems, is important to recover the heat delivered by exothermic reactions produced in the absorber. In this paper a review of the main experimental results of different absorber designs reported in absorption heat pump cycles is presented.

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Cesar Oronel

Papers

Heat and mass transfer in a bubble plate absorber with NH3/LiNO3 and NH3/(LiNO3 + H2O) mixtures