Review of Desiccant in the Drying and Air-Conditioning Application

Desiccant is a hygroscopic substance generally used in the dryer and air-conditioning system as a drying agent. The function of desiccant is to remove moisture from the air to reduce the humidity of the surrounding air been conditioned. This paper presents several works on the performance of desiccant material in the drying and air-conditioning application. It puts focus on the various advantages and disadvantages of the use of desiccant as a drying agent. There are some advantages of using desiccant include consistent drying and low energy usage. However, there are several disadvantages of using desiccants which are low capacity for moisture absorption and pressure drop in solid desiccant. Solar drying applications have some advantages such as being comparatively cheaper than other methods and less risk of spoiling the product. On the contrary, drying applications have disadvantages include being lower in comparison to the original foodstuff and drying foods eventually leads to shrinkage. The advantages of using desiccant in air-conditioning applications offer dehumidified fresh air to keep the building's temperature in a comfortable range and enhances water recovery efficiency. There are disadvantages such as desiccant will substantially impact the system's performance and desiccant should be cooled after completely dried.

Evaporative Cooling Integrated with Solid Desiccant Systems: A Review

Evaporative cooling technology (ECT) has been deemed as an alternative to the conventional vapor-compression air conditioning system for dry climates in recent years due to its simple structure and low operating cost. Generally speaking, the ECT includes two types of different technologies, direct evaporative cooling (DEC) and indirect evaporative cooling (IEC). Both technologies can theoretically reduce the air temperature to the wet-bulb temperature of outdoor air. The major difference between these two technologies is that DEC will introduce extra moisture to the supply air while IEC will not. The enhanced IEC, Maisotsenko-cycle (M-cyle) IEC, can even bring down the air temperature to the dew point temperature. The ECT integrated with solid desiccant systems, i.e., solid desiccant-assisted evaporative cooling technologies (SDECT), could make the technology applicable to a wider range of weather conditions, e.g., weather with high humidity. In this paper, the recent development of various evaporative cooling technologies (ECT), solid desiccant material and the integration of these two technologies, the SDECT, were thoroughly reviewed with respect to their configuration, optimization and desiccant unit improvement. Furthermore, modeling techniques for simulating SDECT with their pros and cons were also reviewed. Potential opportunities and research recommendations were indicated, which include improving the structure and material of M-cycle IEC, developing novel desiccant material and optimizing configuration, water consumption rate and operation strategy of SDECT system. This review paper indicated that the SDECT system could be a potential replacement for the conventional vapor-compressed cooling system and could be applied in hot and humid environments with proper arrangements.

Experimental Investigation of Desiccant Dehumidification Cooling System for Climatic Conditions of Multan (Pakistan)

In this study, experimental apparatus of desiccant dehumidification was developed at lab-scale, using silica gel as a desiccant material. Experimental data were obtained at various ambient air conditions, while focusing the climatic conditions of Multan (Pakistan). A steady-state analysis approach for the desiccant dehumidification process was used, and thereby the slope of desiccant dehumidification line on psychrometric chart (ϕ*) was determined. It has been found that ϕ* = 0.22 in case of silica gel which is lower than the hydrophilic polymeric sorbent, i.e., ϕ* = 0.31. The study proposed two kinds of systems, i.e., (i) standalone desiccant air-conditioning (DAC) and (ii) Maisotsenko-cycle-assisted desiccant air-conditioning (M-DAC) systems. In addition, two kinds of desiccant material (i.e., silica gel and hydrophilic polymeric sorbent) were investigated from the thermodynamic point of view for both system types, using the experimental data and associated results. The study aimed to determine the optimum air-conditioning (AC) system type, as well as adsorbent material for building AC application. In this regard, perspectives of dehumidification capacity, cooling capacity, and thermal coefficient of performance (COP) are taken into consideration. According to the results, hydrophilic polymeric sorbent gave a higher performance, as compared to silica gel. In case of both systems, the performance was improved with the addition of Maisotsenko cycle evaporative cooling unit. The maximum thermal COP was achieved by using a polymer-based M-DAC system, i.e., 0.47 at 70 °C regeneration temperature.

Adsorption Isotherm and Kinetics of Water Vapor Adsorption Using Novel Super-Porous Hydrogel Composites

Deliquescent salts have high water vapor adsorption capacity, but they dissolve in water by forming crystalline hydrates. That restricts their use in different water vapor adsorption applications. However, this limitation can be overcome by incorporating deliquescent salts within a polymer matrix which will keep the salt solution in place. Furthermore, if the polymer matrix used is also capable of adsorbing water vapor, it will further improve the overall performance of desiccant system. Therefore, in this work, we are proposing the synthesis and use of a highly effective new solid polymer desiccant material, i.e. superporous hydrogel (SPHs) of poly(sodium acrylate-co-acrylic acid (P(SA-co-AA)), and subsequently its composite with deliquescent salt, i.e. calcium chloride (CaCl2), to adsorb water vapors from humid air without the dissolution of the salt in the adsorbed water. Parental PAA-SPHs matrix alone exhibited an adsorption capacity of 1.02 gw/gads which increased to 3.35 gw/gads after incorporating CaCl2 salt in the polymer matrix. Both materials exhibited type-III adsorption isotherm and the experimental isotherm data fitted to the Guggenheim, Anderson and Boer (GAB) isotherm model. However, the adsorption kinetics followed linear driving force model which suggested that this extremely high adsorption capacity was due to the diffusion of water molecules into the interconnected pores of SPHs via capillary channels followed by the attachment of adsorbed water molecules to the CaCl2 salt present in the polymer matrix. Furthermore, the adsorbents were used successively for six cycles of adsorption with a very little loss in adsorption capacity. Therefore, the proposed polymer desiccant material overcomes the problem of dissolution of deliquescent salts and opens the doors for a new class of highly effective solid desiccant material.

Synthesis and experimental investigation of V-CaCl2 composite desiccant for performance enhancement in evaporative cooling applications

This study investigates novel vermiculite–calcium chloride composite solid desiccant material for desiccant-assisted evaporative cooling applications. In addition, comparative performance analyses between direct and desiccant-assisted evaporative cooling were performed. According to the study results, temperature drop around 10 ℃ with an average cooling rate of 700 W and coefficient of performance in the range of 5–6 were obtained for both processes. The specific cooling capacity was found 3.5 kJ/g for desiccant assisted and 2.6 kJ/g for direct evaporative cooling process. It was concluded that at low air mass flow rates, performances of both processes are similar whilst at higher air flow rates, desiccant-assisted evaporative cooling becomes more efficient in terms of specific cooling capacity and coefficient of performance. The experiments also showed that vermiculite–calcium chloride provides effective moisture sorption rates above 40% air relative humidity levels. Therefore, this desiccant material could be suitable for desiccant-assisted evaporative cooling applications.

Theoretical analysis of heat and mass transfer processes in an evaporative cooling system with zeolite desiccant powered by solar energy

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Concerns about energy sources depletion and environmental pollution issues have been raised and is a top priority of the global community. Refrigeration machines have been received the major of attention because of their energy consumption and pollution. Different from traditional cooling strategies, desiccant cooling technology (DCT) has been emergent as a promising alternative giving the fact that the economic-ecological air conditioning system is not restricted to hot and dry climates only. In the current experimental study, an advancement solar assisted desiccant cooling system (SADCS) is presented. The advantage is to use only a fraction of the energy of typical compressor-based cooling systems. The advancements have taken place into the dehumidification, evaporator, and regeneration sections. The role of appropriate choosing of the desiccant material type on the adsorption process has been presented, and a Faujasite (FAU) 13X zeolite is utilized in the dehumidification stage. A novel monolayer coating method has conducted. The new coating method has insured no external water vapor condensation ruins the desiccant material during relative humidity working range (20-97) %. Moreover, the new coating method allows to increase the performance of the adsorption and desorption processes, respectively. By giving the coming air stream the ability to pass through and surround the zeolite beads, accessibility to adsorb and desorb water vapor molecules is easier as more rooms are available. That clearly has been stated as 6450 g of zeolite holds 684 g of water in full saturation status within 37 minutes and regenerates by less than 120 [degree]C air temperature within 66 minutes. In the evaporative cooling section, an effective small compact evaporative cooler (CEC) system dealing only with product flow is introduced. The new CEC is utilized direct and indirect evaporation of water mechanisms combined in cross channels to cool air. Dropping air temperature by (5-7) [degree]C has recorded without desiccant stage. Supplying the required regeneration energy from a green energy source was essential in this study. For this purpose, an innovative flat plate double-mesh air solar collector has designed and carried out. In the new collector, a double copper mesh frames were fixing within a double Plexiglas covers flat plate collector. Experimental results show a good consistent with the mathematical model. With an average 0.71 solar fraction and 80 [degree]C exit air temperature, the collector presents 0.73 as thermal efficiency. As the improvements of the desiccant air-conditioning system turns out globally recognized by the progress in different research outcomes, designs, setting up and evaluation methods, it is expected that the system will be one of the most important alternative systems for the maintenance of human's environment comfort and air quality when considering the reduced dependence on conventional energy usage. Present work and results provide a reference data sets related to real adsorption dehumidification process and show that the advancement SADCS has a great potential in the future of the evaporative cooler systems.