A Dual-scale Simulation Of Performance Of A Desiccant Wheel For Air Dehumidification For Total Heat Recovery

Humidity and temperature are the key parameters that needed to be controled in indoor hot and humid environment. Humidity control has a great significance for human health and life. Recovering the heat and humidity from indoor exhaust air is an important measure to reduce energy consumption in the air conditioning system. Desiccant wheel is an important method for air dehumidification and total heat recovery. Their operating process occurs on the solid-gas surface, and there is no damp surface. Thus, this process is safe and healthy, and desiccant wheel is widely used in the civilian and industrial air conditioning system. Performance of these wheels is influenced by many factors. However, these factors are in dual scale, thus presenting a dual-scale model describing the desiccant wheel is to be solved.This article is focused on the performance of honeycomb-type solid desiccant wheels(beds) for air dehumidification for total heat recovery,from the viewpoint of a “dynamic system operation” rather than a “raw material”. The main works are summarized as following:(1) The performance of honeycomb-type desiccant beds(wheels) for air dehumidification with various solid desiccant wall materials is compared, from the viewpoint of a “dynamic system operation” rather than a “raw material”. A mathematical model is proposed and validated to predict the cyclic behaviors of the cycling beds or wheels. The influences of regeneration air temperature, process air temperature, and humidity on the coefficient of performance(COP), specific dehumidification power(SDP) and dehumidification efficiency(εd) are predicted with various desiccant wall materials. Totally ten most commonly used desiccant materials are considered, with different adsorption and thermophysical properties. It is found that of the 10 materials, the silica gel 3A and silica gel RD perform better than other desiccants for air dehumidification under typical working conditions and driven by low-grade waste heat.(2) Desiccant wheels have been widely used for humidity treatment: air dehumidification and total heat recovery. Since the operating conditions are different, heat and mass transfer behaviors in the wheels are quite different. The performance of honeycomb-type desiccant wheels for total heat recovery with various solid desiccant wall materials is compared, from the viewpoint of a “dynamic system operation” rather than a “raw material”. A one-dimensional, transient heat and mass transfer model is proposed to predict the cyclic behaviors of the desiccant wheels. Effects of the working conditions, the rotary speed, the inlet velocities of process air(fresh air) and exhaust air on the performance of the wheels are investigated and compared with various desiccant wall materials. Totally ten most commonly used desiccant materials are considered, with different adsorption and thermophysical properties. It is found that of the 10 materials, the zeolites 5A and 13 X perform better than other desiccants for total heat recovery under the high rotary speed under typical working conditions.(3) Amorphous mesoporous adsorption materials such as silica gel B represent one important class of adsorption materials used in air dehumidification and cooling applications. Molecular dynamics simulation is an efficient tool to help to design novel materials. In this paper, a novel approach for the build-up of molecular models of amorphous mesoporous materials(silica gel B) is presented. Based on the molecular model set up, Grand Canonical Monte Carlo(GCMC) combined with molecular dynamics(MD) simulation methods are used to investigate the water molecules dynamic behaviors(adsorption and diffusion) in silica gel B. The radial distribution function is used to investigate the water molecules adsorption sites and adsorption mechanism. The relations between the microstructure and macroscopic performance are analyzed.(4) Performance of desiccant wheels is influenced by many factors. These factors are in dual-scale. Previous studies only involved the macro-scale heat and mass transfer in the wheels and the system performance, by neglecting the micro-scale properties of wheel materials. In this paper, a dual-scale modeling approach is proposed for a desiccant wheel with a novel organic-inorganic hybrid adsorbent material which combines high adsorption capability with good mechanical durability. The proposed dual-scale model included a micro-scale molecular dynamics sub-model for adsorbent material, a macro-scale sub-model for heat and mass transfer in matrix channels and system performance evaluation. The two sub-models are linked together through information exchange to form the dual-scale model. Through modeling, the effects of the micro physical-chemical properties of materials and macro structure of wheels, as well as the operating parameters on system performance are investigated. With the dual-scale model as a design tool, material compositions are optimized. The moisture adsorption capacity of the material is two times higher than that of silica gel B at high relative humidities. Consequently, the sensible and latent effectiveness are improved by 12% and 30% respectively.