Desiccant technology is a category of HVAC equipment used for dehumidification. A desiccant material is a material that attracts and holds large amounts of water vapor. Desiccant materials are used in complete desiccant cooling units, air pretreatment devices, and HVAC system enhancements. The instructional module has been developed to introduce engineering students to desiccant technology and the use of desiccant systems. In the typical engineering curriculum, a number of courses could contain topics related to desiccant systems. Thermodynamics, heat transfer, HVAC, thermal systems design, and alternate energy systems courses are appropriate for desiccant related topics. The instructional module contains lecture material and review questions and exercises relating to desiccant systems and their uses.

In warm and humid climates, a primary source of building energy consumption is dehumidification of conditioned air supplied to the building spaces. The proposed system utilizes a selective membrane to remove water vapor from ambient air as opposed to a vapor compression cycle or a desiccant. This work provides an analysis of the membrane dehumidification system with a focus on the energy performance of the system. A system performance goal was set at the beginning for a given inlet and outlet ambient air condition and a total cooling load of one ton. The target COP of the combined sensible and latent cooling is 3.58 with a target value for only the latent system of 3.34. Two different simulations were developed including an initial simulation which uses a basic mass transfer model and a simpler condenser model. The initial model was used to develop the system, analyze operating parameters and provide initial performance results. The initial simulations indicate that the system requires two optimizations to meet the target performance: condenser pressure optimization and the use of multiple membrane segments operating at different pressures. The latent only COP including the optimizations was a maximum of 4.23. A second model was then developed which uses a more detailed mass transfer model and a more detailed condenser model based on the operating conditions. This simulation yielded a maximum latent only COP of 4.37 including the optimizations. The work also analyzes two different combined systems capable of providing both sensible and latent cooling. The first utilizes a conventional vapor compression cycle for sensible cooling and has a maximum COP of 3.93. The second uses multiple evaporative coolers in between multiple membrane dehumidification steps and was found to have a maximum COP of 3.73. Second law analysis of the systems was also conducted and found that the greatest reduction in latent system exergy loss can be obtained by improving the selectivity of the membrane. Apart from improving the membrane selectivity, the results show the greatest improvement can be found in improving the operation of the gas compression devices.

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Yes / Models’ accuracy and reliability are important factors for designers of the humidification-dehumidification (HDH) desalination systems. A model used for designing the system must consider all important parameters in order to maintain high accuracy over the wide range of fluctuating conditions. The empirical models for HDH systems which are mostly available in literature are simple and easy to develop but also have limited predictive accuracy for extreme conditions because of consideration of only a few of many influential parameters. Usage of these models may lead to an expensive redesign at latter stages in development of the real system. Therefore, the aim of this paper is to propose the mechanistic model of the HDH desalination process with an improved prediction accuracy as an alternative to conventional models. This model is developed by coupling the heat and mass transfer equations at the water–air interface into enthalpy equations. Performances of the proposed model and an empirical model from literature are compared against experimental data obtained from the HDH system, which is also designed in this work. Results show the proposed model has relatively low mean square error (0.4) hence more accurate than the empirical model with mean square error of 7. It was also found that, the recovery ratio attained by the system increases substantially with an increase of the feed water temperature, but decreases with an increase of water-to-air flow ratio. Freshwater productivity increases with an increasing packing's specific area while doubling of dehumidifiers’ surface area improves the recovery ratio by 16%.

A numerical model for isothermal and non-isothermal flat-plate liquid-desiccant dehumidifiers and regenerators was developed and implemented. The two-dimensional model takes into account the desiccant, water and air flow streams. A parametric analysis was performed to evaluate the influence of some of the most important operational parameters on mass transfer performance, such as flow configuration, water mass flow rate and inlet temperature, and desiccant mass flow rate. The results indicate that the water temperature and mass flow rate have a strong effect on the performance of the dehumidifier and regenerator, with the isothermal wall case acting as an upper limiting case. Increasing the desiccant mass flow rate improves the water transfer performance, but the improvement is asymptotic with mass flow rate. An experimental rig with a single channel prototype was also built and tests were run for 18 different cases, with varying water mass flow rate, desiccant mass flow rate and flow configuration. The results show trends similar to those observed in the numerical results. However, the discrepancies between the numerical and experimental results are larger than the estimated experimental uncertainty at a 95% confidence level. There is some indication that poor desiccant wetting of the channel walls was partially responsible for the discrepancies. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2007-12-31 22:12:39.184

abstract: The horizontal desalination units belonging to the humidification-dehumidification family purify water using air as a carrier gas. The temperature required for separation can vary from ambient to 99 °C so waste heat, fuel combustion, or solar collectors can drive the process. A unit in which air flows horizontally affords several advantages over similar vertical “Dewvaporation” towers (as an example), including ease of construction and potentially increased efficiency. The objective was to build and test horizontal units and identify areas of potential efficiency improvements. The desalination units consisted of: 1.) A series of aligned, corrugated, polypropylene sheets covered on the outside with absorbent, water-wettable cloth. 2.) A basin that caught saline water flowing downward from the absorbent cloth. 3.) Ten pumps to cycle the basin water back onto the cloth. 4.) An air blower on the front of the unit that drove air horizontally across the cloth, increasing the humidity of the air. 5.) A steam generator on the back of the unit producing steam that mixed with the incoming air to increase the temperature and humidity. 6) A steam box that caused the air to mix with the steam and return to flow inside the corrugations in the plastic sheets, creating a countercurrent heat exchanger as the exiting air transferred its heat to the incoming air and causing purified water to condense from the cooling, oversaturated air. The tested unit produced distillate at a rate of 0.87 gallons per hour with 13 parts per million total dissolved solids and an energy reuse factor of 2.5. Recommendations include the implementation of a continuous longitudinal pump design, a modification of the basin to accommodate top and bottom unit center dividers, increase in insulation coverage, and optimization of air flow rate. / Dissertation/Thesis / Masters Thesis Chemical Engineering 2015

This thesis is focused on understanding the correct method to simulate atomistic models to calculate coefficient of diffusion of water through the membrane. It also aims to fix the method previously used in molecular modelling in which the simulation results did not match the experimental results. These membranes will be used in air dehumidification systems. The four types of membranes namely, polyurethane, polyurethane with silica nano particles, polyurethane with silica nano particles and amine surface modifier, and polyurethane with silica nano particles and aniline surface modifier. These membranes were also simulated to understand the effects of temperatures and pressure using molecular dynamics. The software packages used are MAPS 4.3, Avogadro, EMC, OVITO, and LAMMPS. MAPS, Avogadro and EMC were used to model the membrane at an atomistic level while LAMMPS is used to simulate the model generated. OVITO is used to analyze the simulation visually. The movement of water vapor molecules were tracked through the membrane in the simulation and diffusion coefficient was calculated using Mean square displacement equation. To create a realistic model, silica was dispersed in the Polyurethane matrix, simulated under standard atmospheric conditions. These results will help in further optimizing the membrane for air dehumidification. This will be an energy efficient, and environment friendly way of dehumidification compared to the traditional heat pump type.

A simplified model is developed to predict the heat and mass transfer between air and fallingilm liquid desiccant during dehumidification in a parallel-plate absorber. Compared to the second-order partial differential equations that describe fluid motion, first-order, non-coupled, ordinary differential equations are used to estimate the heat and mass transferred and explicit equations are derived from conservation principles to determine the exiting conditions of the absorber for different flow arrangements. The model uses a control volume approach that accounts for the change in desiccant film thickness and property values. The model agreed with a more complicated parallel flow model in literature. Using existing experimental data for a counterflow arrangement the model was validated over the range of input variables at the level of 8% for varying inlet desiccant flow rates and 10% for varying inlet air mass flow rates when an experimentally determined mass transfer coefficient was used in the model.

This research proposes a computer evaluation model that assists architects and designers in producing buildings with low energy consumption. The model is based on computer-designer interaction. Here, the designer suggests a range of design alternatives, and, in turn, the computer evaluation model generates a matrix of design solutions and performs various environmental simulations. The performances of the various design solutions then analyzed by a statistical analysis package that derives relationships. These relationships explain the impact that the different building components have on energy consumption. The relationships are represented in the form of statistical relations and interactive data charts. The evaluation model was tested and used to support new ventilation strategies for the Beliveau House in Blacksburg, Virginia. The designer of this house implemented strategies for integrating solar radiation, thermal mass, thermal insulation, and air ventilation to conserve energy. A field study and computer simulation were conducted to monitor the actual performance of the house and to validate the evaluation model results. Based on the evaluation model results, this research suggests new direct and indirect ventilation control strategies to reduce cooling energy and to improve comfort. The research also suggests general design guidelines to improve the energy performance of buildings and to enhance thermal comfort. These design guidelines are based on a holistic view of integrating the building components that has significant impact on buildings thermal performance. / Ph. D.