• Energy Research

Abstract Absorption heat pumps offer a significant energy saving opportunity because of their capability to utilize low grade heat from solar thermal collectors, combustion systems and numerous other industrial applications. Reducing the required source temperature, size, and cost can greatly enhance the market potential of these systems. Here, a new compact plate-and-frame generator design is introduced with an approximately 3 times higher desorption rate at a substantially lower mass flux compared to conventional generators at only a 10 °C wall superheat temperature. The new design utilizes a new surface structure to produce a uniformly thin solution film and to continuously interrupt the concentration and thermal boundary layers. At low wall temperatures, the desorption rate increased linearly with temperature. The desorption rate then exponentially increased due to a transition from direct diffusion desorption mode to nucleate boiling. The transition temperature was a strong function of the solution flow rate. A comparison of the desorption rate in the direct diffusion desorption mode with predictions of the laminar flow theory suggested that increasing solution flow rates results in mixing within the solution film. The high desorption rate at low mass flux enables significant reduction in the generator size and cost.

Abstract Absorption heat pumps offer a significant energy saving opportunity because of their capability to utilize low grade heat from solar thermal collectors, combustion systems and numerous other industrial applications. Reducing the required source temperature, size, and cost can greatly enhance the market potential of these systems. Here, a new compact plate-and-frame generator design is introduced with an approximately 3 times higher desorption rate at a substantially lower mass flux compared to conventional generators at only a 10 °C wall superheat temperature. The new design utilizes a new surface structure to produce a uniformly thin solution film and to continuously interrupt the concentration and thermal boundary layers. At low wall temperatures, the desorption rate increased linearly with temperature. The desorption rate then exponentially increased due to a transition from direct diffusion desorption mode to nucleate boiling. The transition temperature was a strong function of the solution flow rate. A comparison of the desorption rate in the direct diffusion desorption mode with predictions of the laminar flow theory suggested that increasing solution flow rates results in mixing within the solution film. The high desorption rate at low mass flux enables significant reduction in the generator size and cost.

Abstract While the use of energy efficient absorption heat pumps has been typically limited to the high capacity commercial and industrial applications, the use of a semi-open absorption heat pump for water heating has been demonstrated to be an energy efficient alternative for residential scale applications. A semi-open absorption system uses ambient water vapor as the refrigerant in the absorber where its heat of phase change is transferred to the process water, cooling the solution in the absorber. The solution is pumped to the desorber, where by adding heat, the water vapor is released from the solution and condensed in the condenser. The heat of phase change of water vapor is transferred to process water again in the condenser. This cycle when implemented with a membrane-based absorber in a plate and frame form of heat exchanger using ionic liquids can overcome the challenges related to the system architecture of conventional absorption heat pumps like the lower efficiency at small scale, crystallization/corrosion issues with the desiccants and the high cost of hermetically sealed components. The cycle COP for such a system was previously demonstrated by Chugh et al. for high humidity conditions. In this experimental study, design improvements were made that expand the system’s applicability to more practical and standardized test conditions. With these improvements, the performance of the system was evaluated. The results presented in this study demonstrate the improved system’s viability as a heat pump water heater conforming to standard water heater test conditions. Performance was measured at a cycle thermal COP of 1.2 with a hot water delivery water temperature of 56 °C and ambient air at 19 °C and 49% RH.

Abstract While the use of energy efficient absorption heat pumps has been typically limited to the high capacity commercial and industrial applications, the use of a semi-open absorption heat pump for water heating has been demonstrated to be an energy efficient alternative for residential scale applications. A semi-open absorption system uses ambient water vapor as the refrigerant in the absorber where its heat of phase change is transferred to the process water, cooling the solution in the absorber. The solution is pumped to the desorber, where by adding heat, the water vapor is released from the solution and condensed in the condenser. The heat of phase change of water vapor is transferred to process water again in the condenser. This cycle when implemented with a membrane-based absorber in a plate and frame form of heat exchanger using ionic liquids can overcome the challenges related to the system architecture of conventional absorption heat pumps like the lower efficiency at small scale, crystallization/corrosion issues with the desiccants and the high cost of hermetically sealed components. The cycle COP for such a system was previously demonstrated by Chugh et al. for high humidity conditions. In this experimental study, design improvements were made that expand the system’s applicability to more practical and standardized test conditions. With these improvements, the performance of the system was evaluated. The results presented in this study demonstrate the improved system’s viability as a heat pump water heater conforming to standard water heater test conditions. Performance was measured at a cycle thermal COP of 1.2 with a hot water delivery water temperature of 56 °C and ambient air at 19 °C and 49% RH.

Abstract Membrane-based liquid desiccant system is a promising technology for efficient humidity control. Also, in comparison to systems using conventional desiccants, ionic liquid (IL) desiccants enable increased system operational envelope and efficiency. In this study, a finite difference numerical model is developed for an IL-based counter and cross flow internally cooled polymer heat and mass exchanger (i.e. absorber). A super-hydrophobic membrane separates the IL desiccant and air flows while allowing moisture transfer from air to IL. The numerical model determines the outlet conditions of all three absorber fluids (water, desiccant, and air), establishing the absorber heat and mass transfer performance. The model was compared with the experimental data obtained from an IL desiccant absorber under a wide variety of water, desiccant, and air inlet conditions. The maximum discrepancy between the model predictions and experimental data for the air exit temperature, air exit relative humidity, cooling water exit temperature, and solution exit temperature are 4%, 9%, 5%, and 2%, respectively. A comprehensive parametric study is then conducted to evaluate the sensitivity of the absorber performance to different input conditions. This highly accurate model and parametric study of a membrane-based absorber can be utilized in design and performance analysis of emerging liquid desiccant dehumidification and separate sensible and latent cooling (SSLC) systems.

Abstract Membrane-based liquid desiccant system is a promising technology for efficient humidity control. Also, in comparison to systems using conventional desiccants, ionic liquid (IL) desiccants enable increased system operational envelope and efficiency. In this study, a finite difference numerical model is developed for an IL-based counter and cross flow internally cooled polymer heat and mass exchanger (i.e. absorber). A super-hydrophobic membrane separates the IL desiccant and air flows while allowing moisture transfer from air to IL. The numerical model determines the outlet conditions of all three absorber fluids (water, desiccant, and air), establishing the absorber heat and mass transfer performance. The model was compared with the experimental data obtained from an IL desiccant absorber under a wide variety of water, desiccant, and air inlet conditions. The maximum discrepancy between the model predictions and experimental data for the air exit temperature, air exit relative humidity, cooling water exit temperature, and solution exit temperature are 4%, 9%, 5%, and 2%, respectively. A comprehensive parametric study is then conducted to evaluate the sensitivity of the absorber performance to different input conditions. This highly accurate model and parametric study of a membrane-based absorber can be utilized in design and performance analysis of emerging liquid desiccant dehumidification and separate sensible and latent cooling (SSLC) systems.