Thermodynamic Analysis of Irreversible Desiccant Systems

Abstract

A new general thermodynamic mapping of desiccant systems' performance is conducted to estimate the potentiality and determine the proper application field of the technology. This targets certain room conditions and given outdoor temperature and humidity prior to the selection of the specific desiccant material and technical details of the system configuration. This allows the choice of the operative state of the system to be independent from the limitations of the specific design and working fluid. An expression of the entropy balance suitable for describing the operability of a desiccant system at steady state is obtained by applying a control volume approach, defining sensible and latent effectiveness parameters, and assuming ideal gas behaviour of the air-vapour mixture. This formulation, together with mass and energy balances, is used to conduct a general screening of the system performance. The theoretical advantage and limitation of desiccant dehumidification air conditioning, maximum efficiency for given conditions constraints, least irreversible configuration for a given operative target, and characteristics of the system for a target efficiency can be obtained from this thermodynamic mapping. Once the thermo-physical properties and the thermodynamic equilibrium relationship of the liquid desiccant mixture or solid coating material are known, this method can be applied to a specific technical case to select the most appropriate working medium and guide the specific system design to achieve the target performance.Entropy 2018, 20, 595 2 of 13 (i.e., the temperature of the air entering a conditioned room can be relatively high). Low exergetic level heat (60-90 • C) (Tu et al. [3]), rather than electricity, can be used as the main energy source, enabling the use of renewable sources (Jani et al. [4]) and waste heat recovery (Ge et al. [5]). A higher system reliability (Giampieri et al. [6]) can be ascribed to the absence of a compressor. Desiccant-cooling systems employ a sorptive material that undergoes a cycle of thermodynamic transformations by sequentially facing the process air and the regeneration stream, thus realising the necessary heat and mass exchanges. These processes are driven by the characteristic vapour pressure at the desiccant-air interface, which is related to the equilibrium isotherm of the specific desiccant material: a univocal relation between the moisture content, pressure and temperature. During the sorption of moisture, the related heat of sorption is released and transmitted through the material itself, which decreases its sorption capacity. Therefore, heat and mass transfer are coupled processes influenced by the contactor configuration, the thermo-physical properties of the desiccant material, the fluid dynamics of the air streams and the contingent operating conditions. Furthermore, a large variety of desiccant mixtures and coatings are available and more new materials are under development. For an updated literature review about absorptive or adsorptive materials and desiccant ...

Design of quaternary ammonium type-ionic liquids as desiccants for an air-conditioning system

“…Kanakubo et al suggested that the dehumidification capability of the ILs was dependent on not the cationic part but the anionic part of the IL [ 36 ]. Since many types of ILs have a moisture absorbent property, ILs are proposed as a desiccant source for the liquid desiccant type air-conditioning system [ [37] , [38] , [39] , [40] , [41] , [42] , [43] , [44] , [45] , [46] ]. Luo et al attempted to use of several ILs as desiccants for an air conditioner [ 37 , 38 ].…”

“…Luo et al attempted to use of several ILs as desiccants for an air conditioner [ 37 , 38 ]. Zegenhagen [ 41 ], Verela [ 42 ], and Giannetti [ 43 , 44 ] also demonstrated the use of ILs as desiccant materials for an air-conditioning system. We recently reported that tributyl(methyl)phosphonium dimethylphosphate ([P 4441 ][DMPO 4 ]) displayed an excellent dehumidification capability; the [P 4441 ][DMPO 4 ] aqueous solution (77%(w/w)) could be used as a desiccant for a liquid desiccant air conditioner [ 46 ].…”

Dicationic-Type Quaternary Ammonium Salts as Candidates of Desiccants for an Air-Conditioning System

“…Mu and co-workers proposed that water molecules in the air are captured by [C 2mim ]­[OAc] through the formation of four hydrogen bonds with an ion pairtwo hydrogen bonds with two oxygen atoms of the acetate anion and two more bonds with hydrogen atoms at the 4- and 5-positions of the [C 2mim ] cation . Due to the hygroscopic nature of ILs, − their feasibility as desiccants for air conditioners has been studied by many groups, including us. − Luo et al identified several imidazolium-based ILs as potential desiccants for air conditioners, , while Feycon reported that choline carboxylate ILs, cholin (2-hydroxy- N , N , N -trimethylethan-1-aminium) alkylcarboxylates ([Ch]­[RCO 2 ]), acted as an efficient desiccant . Aparicio et al proposed a water capture mechanism of [Ch]­[Lac] based on their DFT simulation study .…”

Energy and Exergy Analysis of an Absorption and Mechanical System for a Dehumidification Unit in a Gelatin Factory

“…Another matter is related to the refrigeration industry, regarding the use of cooling fluids [ 5 ]. A liquid desiccant absorption system is one option for a vapor compression cooling system [ 6 , 7 , 8 ]. A Previous investigation [ 9 ] has shown that it is possible to achieve a reduction in overall energy consumption, although it has high initial system costs [ 9 ].…”