Li-Zhi Zhang

Energy performance of independent air dehumidification systems with energy recovery measures

Abstract Independent air dehumidification provides an attractive alternative to traditional coupled air dehumidification with reduced energy use, better humidity control and indoor air quality. According to this concept, latent load is treated by an independent system and the sensible load is treated by chilled-ceiling panels. In this work, four independent air dehumidification systems with energy recovery strategies are proposed. They are as follows: system 1, mechanical dehumidification with heat pump; system 2, mechanical dehumidification with sensible heat exchanger; system 3, mechanical dehumidification with membrane-based total heat exchanger; and system 4: a heat pump incorporating an active desiccant wheel and evaporative cooler. They are compared with a mechanical dehumidification system with no heat recovery. Hour-by-hour energy analysis is performed on the systems proposed. The results show that the system of mechanical dehumidification with membrane total heat recovery (system 3) consumes the least primary energy. However, since, the systems employ energy recovery measures, the energy savings for the four systems are in the same order, around 4.40×106 kJ per person.

Fabrication of a lithium chloride solution based composite supported liquid membrane and its moisture permeation analysis

Abstract A novel composite supported liquid membrane has been prepared for ventilation air moisture recovery. The membrane is composed of three layers: two hydrophobic protective layers and a sandwiched hydrophilic support layer in which LiCl solution is immobilized to facilitate water vapor transfer. A test is conducted to measure the moisture permeation rate through the composite membrane. Various resistances in the cell and in the composite membrane are clarified. Linear equilibrium relations between humidity, temperature, and LiCl concentration in the liquid solution layer are obtained to aid in the model set-up. It has been found that the mean moisture permeation rate through the composite membrane is around 1.14×10−4 kgm−2 s−1, almost two times higher than that through a solid hydrophilic cellulose acetate membrane with comparative thickness. Further, the supported liquid layer only accounts for 12% of the total moisture transfer resistance in the cell, indicating that there is much potential for further performance improvement.

Selective permeation of moisture and VOCs through polymer membranes used in total heat exchangers for indoor air ventilation.

UNLABELLED Fresh air ventilation is central to indoor environmental control. Total heat exchangers can be key equipment for energy conservation in ventilation. Membranes have been used for total heat exchangers for more than a decade. Much effort has been spent to achieve water vapor permeability of various membranes; however, relatively little attention has been paid to the selectivity of moisture compared with volatile organic compounds (VOCs) through such membranes. In this investigation, the most commonly used membranes, both hydrophilic and hydrophobic ones, are tested for their permeability for moisture and five VOCs (acetic acid, formaldehyde, acetaldehyde, toluene, and ethane). The selectivity of moisture vs. VOCs in these membranes is then evaluated. With a solution-diffusion model, the solubility and diffusivity of moisture and VOCs in these membranes are calculated. The resulting data could provide some reference for future material selection. PRACTICAL IMPLICATIONS Total heat exchangers are important equipment for fresh air ventilation with energy conservation. However, their implications for indoor air quality in terms of volatile organic compound permeation have not been known. The data in this article help us to clarify the impacts on indoor VOC levels of membrane-based heat exchangers. Guidelines for material selection can be obtained for future use total heat exchangers for building ventilation.

Effects of substrate parameters on the emissions of volatile organic compounds from wet coating materials

Solvent-based interior coating materials have long been recognized as a major source of volatile organic compounds (VOCs) in the indoor environment. In the emission process, substrate acts as a secondary source. The sink effects are studied with a detailed mass transfer model considering convective mass transfer in air streams, the VOCs diffusions in painting film, and the sorption and diffusions of VOCs in substrate. The model is proposed and validated by the emission profiles of a water-based emulsion paint in a standard field and laboratory emission cell. The focus is on the role the substrate plays in the emission process. The effects of the substrate parameters, such as the substrate diffusivity and sorption characteristics, on the emission profiles are investigated. This is helpful in exposure control through both selecting healthy materials and proper ventilations.

Transport Phenomena in a Cross-Flow Hollow Fibre Membrane Bundle Used for Liquid Desiccant Air Dehumidification

The transport phenomena in a hollow fibre membrane bundle for liquid desiccant air dehumidification were investigated. In the bundle, the liquid desiccant solution flows inside the fibres and the process air flows across the fibre bundle; where air is dehumidified by moisture permeation through the membrane. This study investigated the fluid flow and conjugate heat and mass transfer in the cross-flow hollow fibre membrane bundle by considering the interactions between the neighbouring fibres. Two regularly packed arrangements: in-line and staggered, were considered. Due to the periodicity of the fluid flow and heat and mass transfer across the bundle, two representative periodic unit cells, which simultaneously include 2 to 3 neighbouring fibres, were selected as the calculation domains. The equations governing the fluid flow and heat and mass transfer in the two cross-flow streams (solution and air) in the membranes, were solved together with the heat and mass diffusion equations. The friction factor and the Nusselt and Sherwood numbers on both the air and the solution sides were then calculated and experimentally validated. The results were compared to those available data calculated from the free surface model.

Self-cleaning of Surfaces: the Role of Surface Wettability and Dust Types

The self-cleaning property is usually connected to superhydrophobic surfaces (SHSs) where the dust particles can be easily removed by the rolling motion of droplets. It seems that superhydrophobicity (its durability is questionable nowadays) is a necessity. However here, it is disclosed that self-cleaning can also be realized on an ordinary surface by droplet impinging. The effects of surface wettability and the types of dust particles are considered. The self-cleaning is realized by two steps: (1) the pickup of particles by the water-air interface of an impinging droplet, (2) the release of the impinging droplets from the surface. It can be observed that only the trailing edges of the droplets can pick up particles when the droplets recoil from the inclined surfaces. The hydrophilic surface can also achieve self-cleaning under some conditions. This interesting finding may be helpful for the successful implementation of self-cleaning with common surfaces.

DEVELOPMENT OF FRACTAL ULTRA-HYDROPHOBIC COATING FILMS TO PREVENT WATER VAPOR DEWING AND TO DELAY FROSTING

Superhydrophobic films fabricated on copper and aluminum surfaces have potential applications to solve water condensation and frosting problems on chilled ceiling system. The rough surfaces of copper foils obtained by solution immersion method exhibit the existence of fractal structures. The hydrophobicity of copper surfaces is enhanced with fractal structures. The relationship between contact angles (CAs) and the fractal dimensions (FDs) for surface roughness of Cu samples with different etching time is investigated. Moisture condensation and frosting experiments on the two kinds of surfaces are conducted in natural environment under different chilling temperatures. During condensation, micro water condensate droplets drift down the surface like dust floating in the air. Several larger condensate droplets about 1–2 mm appear on the substrates after 3 h condensation. This continuous jumping motion of the condensate will be beneficial in delaying frosting. The results demonstrate that dense nanostructures on copper surfaces are superior to loose lattice-like microstructures on aluminum surfaces for preventing the formation of large droplets condensate and in delaying the icing. The large water droplets of 2–3 mm in diameter that would form on a common metal foil are sharply decreased to dozens of microns and small droplets are formed on a modified surface, which will then drift down like a fog.

Durable superhydrophobic surfaces made by intensely connecting a bipolar top layer to the substrate with a middle connecting layer

This study reported a simple fabrication method for a durable superhydrophobic surface. The superhydrophobic top layer of the durable superhydrophobic surface was connected intensely to the substrate through a middle connecting layer. Glycidoxypropyltrimethoxysilane (KH-560) after hydrolysis was used to obtain a hydrophilic middle connecting layer. It could be adhered to the hydrophilic substrate by covalent bonds. Ring-open reaction with octadecylamine let the KH-560 middle layer form a net-like structure. The net-like sturcture would then encompass and station the silica particles that were used to form the coarse micro structures, intensely to increase the durability. The top hydrophobic layer with nano-structures was formed on the KH-560 middle layer. It was obtained by a bipolar nano-silica solution modified by hexamethyldisilazane (HMDS). This layer was connected to the middle layer intensely by the polar Si hydroxy groups, while the non-polar methyl groups on the surface, accompanied by the micro and nano structures, made the surface rather hydrophobic. The covalently interfacial interactions between the substrate and the middle layer, and between the middle layer and the top layer, strengthened the durability of the superhydrophobic surface. The abrasion test results showed that the superhydrophobic surface could bear 180 abrasion cycles on 1200 CW sandpaper under 2 kPa applied pressure.

Investigation of Membrane-Based Total Heat Exchangers with Different Structures and Materials

Membrane-based total heat exchangers are devices to recover both sensible heat and latent heat from the exhaust air. The performances of exchangers assembled with different structures and membranes vary dramatically. To investigate performances, five modules are fabricated for comparison. A test rig is built to measure the performance of these total heat exchangers. The heat and moisture transfer in the cores are studied simultaneously. These cores can be divided into two categories: with different structures and with different membranes. For the first category, parallel-plates, plate-fins and cross-corrugated structures are used. For the second category, three kinds of membranes, i.e. one-step hand-made CA membrane, hydrophobic-hydrophilic composite membrane and machine-made CA membrane are used. The heat and mass transfer coefficients, sensible cooling and latent effectiveness are obtained through experimental measurements. The experimental results show that the cross-corrugated ducts can enhance heat and mass transfer effectively. And the one-step hand-made CA membrane has the lowest resistance in heat and moisture transfer.

Moisture transport through asymmetric porous membranes with finger-like holes for indoor humidity control: A lattice Boltzmann simulation approach

Asymmetric porous membranes with finger-like holes are one sort of promising membrane materials for indoor humidity control. Moisture transport in these materials is the key factor influencing humidity control performance. To overcome the difficulties in modelling meso-scale mass transfer in these materials, a lattice Boltzmann simulation (LBM) methodology has been proposed to model the pore-scale gas flow and mass transfer in the asymmetric membranes with finger-like holes. A typical membrane is classified into three sub-layers: a porous support layer, a layer with finger holes, and a denser skin layer. Simulated annealing technique was used in our study to reconstruct the calculating domain. Then fluid flow and mass transfer in the membrane were predicted with LBM, and permeability and effective diffusivity were evaluated. The existence of finger holes in the matrix could dramatically enhance the overall mass transfer in the membranes. Besides, the inhomogeneity in membrane structures would make the macro-scale lumped parameter prediction of membrane performance questionable. Our findings show differences in comparison to previous macro-scale multi-layer analysis. The dominant resistance is in the skin layer. There should be a change in emphasis for membrane optimization with a focus on the skin layer rather than the porous layer.