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Highlights • Simulation of a microchannel H 2 O-LiBr desorber using a microporous membrane. • Sensitivity analysis of cooling capacity/desorber volume to various parameters. • Direct diffusion and boiling desorption modes are compared. • Parameters to be optimized at the design stage of the desorber are identified. • Solution inlet concentration is the most relevant operational parameter. Abstract A parametric analysis of a membrane-based microchannel desorber working with H 2 O–LiBr is presented. Mass and heat transfer equations are solved, for different membrane properties and design and operating conditions. Sensitivity of the ratio between chiller cooling capacity and desorber volume (r qV) to changes in the following parameters is evaluated: width and height of solution and hot water channels; solution concentration, temperature, pressure and mass flow rate; hot water temperature and mass flow rate; membrane porosity, pore diameter and thickness; thickness and thermal conductivity of the wall between solution and hot water and vapour pressure. The analysis includes direct diffusion and boiling modes. In both cases, during design, most important parameters to maximize r qV are solution channel height and wall thickness. Additionally, effect of channel length is remarkable during boiling. Good operation requires adequate inlet concentration and hot water temperature. In direct diffusion, vapour pressure is also a relevant parameter. [ABSTRACT FROM AUTHOR]

Highlights • A novel adiabatic membrane-based microchannel absorber is experimentally tested. • Absorber performance is evaluated under variable operating conditions. • Solution mass transfer resistance is dominant in the absorption process. • Higher absorption ratios than in previous adiabatic configurations are obtained. • The ratio between cooling power and absorber volume reaches 559 kW/m3. Abstract In the interest of reducing the size of absorption chillers, a novel adiabatic membrane-based micro-absorber prototype is experimentally studied. Water–lithium bromide solution is used as the working fluid flowing through 50 rectangular microchannels of 0.15 mm height, 3 mm width and 58 mm length. In the present study, a laminated microporous PTFE membrane of 0.45 µm pore diameter, separating the solution from the vapour, is tested. It incorporates a supporting layer of polypropylene. Different operating parameters were tested, including the inlet solution mass flow rate, temperature and concentration and the pressure potential for absorption. The measured concentration and temperature of the solution at the absorber outlet are used to evaluate the mass transfer characteristics of the micro-absorber. It is demonstrated that the process is controlled by the solution mass transfer resistance. Calculated results of the absorption rate and the absorption ratio show the advantages of the proposed design considering its compactness. The cooling power of a hypothetical chiller equipped with the tested micro-absorber of 73.7 cm3 effective volume, for the range of variables considered in this study, is 41 W. The modular configuration of the absorber allows to easily scale-up the cooling capacity. [ABSTRACT FROM AUTHOR]

In this paper a microporous membrane is used in combination with rectangular microchannels in the absorber of an absorption chiller, working in two different configurations: cooled by a water flow and adiabatically. In the non-adiabatic case, the configuration of the channels allows the heat released during absorption to be extracted using a cooling water flow. The results for solution concentration, pressure potential, absorption coefficient, absorption rate, temperatures and power exchanged/stored by the working fluids along the absorption channels are presented. The ratio between the cooling power of the chiller equipped with the simulated absorber and the absorber volume, r qV , is used to characterise the absorber compactness. A parametric analysis is also performed to evaluate the influence on the ratio r qV of the inlet solution mass flow rate, the solution inlet temperature, and the height and width of the solution channels, for both absorbers. For the base case considered in this study, both absorber configurations offer r qV higher than 1 MW m −3 . This ratio is higher than usual values found in falling film absorbers using conventional circular tubes. Moreover, the new adiabatic configuration presented has significant advantages respect to the non-adiabatic one in terms of higher r qV and fabrication simplicity. [ABSTRACT FROM AUTHOR]

The simulation of the heat and mass transfer in a H 2 O–LiBr microchannel desorber endowed with a microporous membrane is presented. The heat and mass transfer processes are modelled by means of selected correlations gathered from the open literature. The simulation provides the evolution along the channels of the parameters involved in the process: heat and mass transfer coefficients, solution concentration, temperatures of the working fluids and pressure potential. The calculated values of the desorption rate are compared to experimental data gathered from the open literature. For the case considered in this study, the maximum ratio between cooling capacity of the chiller and desorber volume is 2312 kW m −3 . This value is more than one order of magnitude higher than the ones found in conventional small-size absorption cooling chillers. [ABSTRACT FROM AUTHOR]

A microporous membrane is used in combination with rectangular microchannels in the absorber of an absorption chiller with the aim of reducing the size of this cooling technology. The simulation of the heat and mass transfer between the solution and the vapour phase in a H 2 O–LiBr absorber using porous fibres is considered. Heat and mass transfer processes are modelled by means of selected correlations and data gathered from the open literature. This new model is applied for the simulation of the absorber under typical operating conditions of absorption cooling chillers. Absorption rate, heat and mass transfer coefficients, solution concentration, temperatures of the working fluids and pressure potential along the absorption channels are calculated. For the case considered in this study, the absorber channels are of 5 cm length, offering a maximum ratio between cooling capacity of the chiller and absorber volume of 1090 kW/m 3 . This ratio is higher than twice the usual values found in falling film absorbers using conventional circular tubes. The mean absolute error between the model results and the experimental data gathered from the open literature is 8.5%, showing the capability of the model to predict the performance of membrane-based absorbers. [ABSTRACT FROM AUTHOR]