Article

Recent trends in liquid desiccant materials and cooling systems: Application, performance and regeneration characteristics

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Air conditioners (ACs) use an enormous amount of energy as they are widely used to provide people with favorable indoor conditions. In recent years, liquid desiccants have received considerable attention as they can reduce the energy consumption of ACs in particular. This paper examines the different liquid desiccant materials, properties, operating conditions, desiccant dehumidifiers, performance improvements with different regeneration configurations. The paper also highlights the effect of the liquid desiccant system on indoor air quality and, more specifically, on volatile organic carbon, particulate matter, and the removal of bacteria. Besides, key performance parameters are introduced in order to understand the performance of liquid desiccant better. The operation of different materials or mixtures of liquid desiccants can be effectively compared with these parameters. Finally, various methods of liquid desiccant regeneration, including the most advanced method of photovoltaic/electrodialysis regeneration, are elucidated. The review could provide a suitable guidance for design and operation of a liquid desiccant-based building air conditioning system.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Optimizations on systems, components and materials in liquid desiccant air dehumidification Gao et al. [11] 2021 Integration and design of desiccant air-conditioning systems for overall performance improvements Gurubalan et al. [13] 2021 Designation and configuration of dehumidifiers and regenerators in LDAS Salikandi et al. [32] 2021 ...
... Multifarious factors should be taken into consideration for the selection of liquid desiccant. For the sake of long-term and large-scale application of LDAS, liquid desiccant is expected to possess the characteristics including non-toxic, non-flammable, nonvolatile, odorless, stable, non-corrosive, economically efficient, outstanding moisture absorption ability, easy regeneration, good heat transfer performance, etc [7,32]. ...
... It is convinced by researchers that biological pollution is an essential factor for sick building syndrome, while liquid desiccant plays positive roles in filtering, killing or inactivating biological pollution to large extent [9]. Typical liquid desiccants, e.g., LiCl-H 2 O and LiBr-H 2 O, were claimed as biocides to kinds of microorganism (e.g., streptococcus, staphylococcus, anthrax spores, SARS virus), attributed to the biodegrading phenomenon of genetic material in these liquid desiccants [24,32]. Skinner et al. [214] experimentally demonstrated the inhibition on the replication of several DNA and RNA viruses in LiCl-H 2 O. Except for salt solutions, TEG solution was also tested and the results exhibited positive evidence bactericidal effect [215]. ...
Article
Liquid desiccant air-conditioning system (LDAS) becomes an attractive option for reducing the energy consumption of conventional air-conditioning systems. Despite lots of published papers on LDAS in various aspects, there is not yet a comprehensive and up-to-date review on the properties of liquid desiccants, while the selection of liquid desiccant plays essential role in the overall performance of LDAS. In this paper, a state-of-the-art review on the properties in regard of energy and environmental performance is delivered for present and potential liquid desiccants, including vapor–liquid equilibrium, specific heat capacity, safety concerns. The current situations and future concerns of liquid desiccant investigation can be obtained, while different kinds of liquid desiccant candidates can be compared and evaluated comprehensively. Existing liquid desiccant of halide salt faces severe drawback of corrosiveness in long-term use. Compared with existing liquid desiccants, the candidates of weak acid salt, ionic liquid and deep eutectic solvent behave low toxicity and friendly corrosiveness, whereas their weak moisture absorption ability, high cost or high viscosity is the bottleneck for further applications. Therefore, the mixture of them can be regarded as a promising candidate in LDAS applications, but the fundamental properties are urged to be measured. The work in this paper provides momentous reference and guidance for the exploration of new liquid desiccant as well as the evaluation of future prospect of LDAS.
... One notable example can be the integration of MD into the liquid desiccant air-conditioning (LDAC) process (Duong et al. 2017a(Duong et al. , 2017bDuong et al. 2018;Lefers et al. 2018;Zhou et al. 2020aZhou et al. , 2020b. LDAC is a potential game changer in advancing the air-conditioning industry to become greener and more energy-efficient (Gurubalan et al. 2019;Chen et al. 2020;Salikandi et al. 2021). Currently, most conventional air-conditioning systems are based on the vapour compression process, whereby the air is first dehumidified by deep cooling to dew point temperature to condense moisture and then reheated to achieve a desired temperature. ...
... lithium chloride (LiCl) solution). The hygroscopic nature of the liquid desiccant solution drives the moisture removal without the need for deep cooling and reheating the air; therefore, the energy consumption of the LDAC systems is markedly reduced compared to that of the conventional vapour-compression based air-conditioners (Gurubalan et al. 2019;Chen et al. 2020;Salikandi et al. 2021). ...
... Most current LDAC systems rely on thermal evaporation for the regeneration of liquid desiccant solutions (Cheng and Zhang 2013;Duong et al. 2019aDuong et al. , 2019b. This regeneration method involves heating the diluted liquid desiccant solution to a high temperature prior to spraying it in counter-current flow with a hot air stream in a packed bed media (Lowenstein 2008;Cheng and Zhang 2013;Salikandi et al. 2021). The direct contact between the hot liquid desiccant solution and the air stream inevitably leads to the carry-over of desiccant droplets in the air stream, which is regarded as a vexing technical issue of the thermal evaporation regeneration method (Duong et al. 2019a(Duong et al. , 2019bGurubalan et al. 2019;Chen et al. 2020;Salikandi et al. 2021). ...
Article
Full-text available
Membrane distillation (MD) has been increasingly explored for treatment of various hyper saline waters, including lithium chloride (LiCl) solutions used in liquid desiccant air-conditioning (LDAC) systems. In this study, the regeneration of liquid desiccant LiCl solution by a pilot direct contact membrane distillation (DCMD) process is assessed using computer simulation. Unlike previous experimental investigations, the simulation allows to incorporate both temperature and concentration polarisation effects in the analysis of heat and mass transfer through the membrane, thus enabling the systematic assessment of the pilot DCMD regeneration of the LiCl solution. The simulation results demonstrate distinctive profiles of water flux, thermal efficiency, and LiCl concentration along the membrane under cocurrent and counter-current flow modes, and the pilot DCMD process under counter-current flow is superior to that under cocurrent flow regarding the process thermal efficiency and LiCl concentration enrichment. Moreover, for the pilot DCMD regeneration of LiCl solution under the counter-current flow, the feed inlet temperature, LiCl concentration, and especially the membrane leaf length exert profound impacts on the process performance: the process water flux halves from 12 to 6 L/(m2·h) whilst thermal efficiency decreases by 20% from 0.46 to 0.37 when the membrane leaf length increases from 0.5 to 1.5 m.
... Hygroscopic salts reduce the vapour pressure of the aqueous solution with increasing concentration, shifting the equilibrium in favour of absorption of water from the air into the liquid phase. The most commonly used salts for air dehumidification are lithium chloride (LiCl), magnesium chloride (MgCl2) or potassium acetate (KAC) [53][54][55]. Glycols such as triethylene glycol [54,55] and ionic liquids [56,57] are also used as hygroscopic sorbents. Compared to glycols, salts have the advantage that they have no vapour pressure and therefore remain in the aqueous phase when heated. ...
... The most commonly used salts for air dehumidification are lithium chloride (LiCl), magnesium chloride (MgCl2) or potassium acetate (KAC) [53][54][55]. Glycols such as triethylene glycol [54,55] and ionic liquids [56,57] are also used as hygroscopic sorbents. Compared to glycols, salts have the advantage that they have no vapour pressure and therefore remain in the aqueous phase when heated. ...
Article
Full-text available
Water scarcity is a growing global and systematic problem in regions with low groundwater availability. Atmospheric water generation (AWG) technologies are an innovative solution to the water shortage problem, as atmospheric water vapor is a readily available resource even in arid regions, with the drawback of high energy consumption. In this paper, the viability of AWG technologies on an energy and economic level is investigated by thermodynamic modeling of three main active AWG systems consisting of cooling condensation, adsorption and absorption processes. A location analysis model is developed to evaluate the performance based on representative weather data of temperature, pressure and relative humidity over a period of one year to account for seasonal shifts and daily variations in climatic conditions. The specific energy consumption kWh/kg, water production trend and total specific cost are calculated for each technology. Water production by seawater desalination at the nearest coastline and transportation to the site by tanker truck, as well as bottled water prices, are used as benchmarks to assess economic viability. The results show that active AWG systems can only be an economically viable alternative if the water consumption site is relatively far from the coast or other water-rich regions and low electricity costs are available (distance >600 km, electricity price <0.10 US$/kWh). Compared to bottled water, all AWG technologies are in a competitive price range. Absorption systems have an energy efficiency advantage over conventional cooling condensation and adsorption systems (cooling condensation: average 0.42 kWh/kg; absorption: average 0.38 kWh/kg; adsorption: average 1.16 kWh/kg), but require a higher degree of process and plant design development. However, because of the high fluctuation in water production, atmospheric water generation technologies should be considered as a complementary supply to conventional water sources.
... For use in LDAC, liquid desiccants must be highly hygroscopic, nontoxic, and relatively low in cost. To reduce the energy required for pumping, they should exhibit favorable transport characteristics such as low surface tension and low viscosity [11][12][13][14][15]. Salts of weak organic acids meet these requirements. ...
... Among these, potassium acetate (CH 3 COOK) is a promising candidate [11,12,[16][17][18]. CH 3 COOK has a similar dehumidification potential to chloride salts, at a fraction of the cost [14]. ...
Article
Full-text available
For commercial buildings, liquid desiccant air conditioners (LDACs) could provide up to 80% energy savings compared to high-efficiency vapor compression AC, but commonly utilized liquid desiccants are highly corrosive. This precludes the use of metallic components, necessitating specialized plastics and thereby driving up cost, weight, and limiting operational temperature and pressure ranges. Less corrosive alternatives are sought. Here, potassium acetate solutions are investigated as less-corrosive alternatives to the chloride salt solutions that are typically used in LDAC systems. Corrosion evaluations for a Cu alloy (C12200) and two Al alloys (Al3003 and Al1100) in both potassium acetate and chloride salt solutions are presented. We show that yearly corrosion rates are lower in potassium acetate solutions by up to three orders of magnitude. Active corrosion behavior is largely absent in potassium acetate solutions but is present in chloride salt solutions. Furthermore, solid corrosion products are observed in chloride salt solutions. Thus, we conclude that potassium acetate is a promising candidate as a less corrosive alternative liquid desiccant for LDAC systems with metallic components.
... The difference is that solid adsorption dehumidification uses the solid desiccant carried by medium with a relatively large specific surface area. Liquid desiccant dehumidification, also known as solution dehumidification, absorbs moisture in the air driven by the vapor pressure difference between the air and the surface of the liquid desiccant [112,113]. One representative type of the liquid desiccant dehumidifiers mainly includes the dehumidifier, the regenerator, cooling and heating coils, the heat exchangers, and other equipment, with the schematic diagram shown in Fig. 14. ...
... The internally cooled dehumidifiers would allow the dehumidifying capacity to be maintained at the expense of increased cost. Salikandi et al. comprehensively reviewed various liquid desiccant materials and their operating temperatures and concentrations [112]. The widely used liquid desiccants mainly include lithium chloride, lithium bromide, calcium chloride, and glycol-based substance [122,123]. ...
Article
The moisture from residual droplets during construction and infiltration of wet air during operation can cause metal corrosion of main cables on suspension bridges. Main cable dehumidification, emerged as a prospective corrosion protection approach, which provides an internal environment of relative humidity below 60%, would significantly mitigate the corrosion. This paper presents a state-of-the-art listicle review of existing dehumidification systems implemented on 47 suspension bridges (e.g., the Akashi Kaikyo Bridge, the Runyang Yangtze River Bridge). Dehumidification methods adopted in suspension bridges, including solid desiccant dehumidification, condensing dehumidification and integrated dehumidification, are elaborated. Based on that, crucial limitations and adaptations of various systems for main cable applications are discussed. Then, technical suggestions concerning the moisture load matching, sensor configurations, full-bridge sealings, dehumidification equipment placement and system division, are given to realize system sustainability and energy efficiency. Finally, this paper examined one potential development of a multi-stage dehumidification system that uses condensing dehumidification and solid desiccant dehumidification, which facilitates cascade utilization of the waste energy and enhances the system efficiency.
... Desiccant solution properties are influential factors on regeneration performance. Thus, hygroscopic, thermophysical, physiochemical, rheological and electrochemical properties of a desiccant solution influence the overall performance of the LDAC system [5][6][7]. ...
... The crystallization drawback of these solutions in low operating temperature impelled researchers to search for alternative solutions working fluids [16]. TEG and some organic solvents tested in air conditioning systems are less corrosive to metallic equipment and less toxic, yet, possess limitations like high volatility, high viscosity, flammability and respiratory irritation [5,17]. Compared with some organic desiccants, ILs are generally non-combustible, have good thermal and chemical stability, and non-volatility under normal conditions. ...
Article
The energy-saving propensity of liquid desiccant air dehumidification technique compared with its counterparts, mitigating the limitations of conventional desiccant solutions and the need for energy optimization in indoor space cooling, has invigorated the exploration of alternative desiccant solutions. This study investigates different binary deep eutectic solvents as a potential alternative working fluids in liquid desiccant air conditioning systems. Hydrogen bond acceptors (Choline chloride, N, N-Diethylethanolammonium chloride, 1-Butyl-3-methylimidazolium chloride, ZnCl2) and hydrogen bond donor (Ethylene glycol) were synthesized in different ratios to form binary deep eutectic solvents. The physiochemical, thermophysical and rheological properties of the investigated solutions were characterized and evaluated. DES A1 (mixture of choline chloride and ethylene glycol in ratio 1:2) and DES D1 (1-Butyl-3-methylimidazolium chloride and ethylene glycol in ratio 1:1) were found best among investigated binary DES solutions. The dehumidification and thermal regeneration potential of these promising solutions were further investigated in a humidity chamber and a drying oven, respectively. At 65 % relative humidity, 30 °C air temperature, and 3.77 × 10⁻⁴ m² moisture-desiccant contact area, the estimated dehumidification mass flux of DES A1 and DES D1 are 4.61 × 10⁻² and 3 × 10⁻² g/m²∙s, respectively. Empirical correlations obtained for deep eutectic solvents moisture absorption in this study provide an error limit between ±2.6 % to ±3.5 %. These solutions are found promising as alternative solutions for dehumidification and thermal regeneration.
... Consequently, LDACs using LiCl-based desiccants require special corrosion-resistant pipes, thereby increasing production costs and preventing widespread adoption of LDACs. 18 Moreover, lithium is limited in supply, largely due to its use in Li-ion batteries that power a wide range of modern electronic devices. 19 Thus, it is desirable to find alternative desiccant materials that can replace Li salts. ...
Article
Due to their many attractive physicochemical properties, ionic liquids (ILs) have received extensive attention with numerous applications proposed in various fields of science and technology. Despite this, the molecular origins of many of their properties, such as the moisture absorption capability, are still not well understood. For insight into this, we systematically synthesized 24 types of ILs by the combination of the dimethyl phosphate anion with various types of alkyl group-substituted cyclic cations—imidazolium, pyrazolium, 1,2,3-triazolium, and 1,2,4-triazolium cations—and performed a detailed analysis of the dehumidification properties of these ILs and their aqueous solutions. It was found that these IL systems have a high dehumidification capability (DC). Among the monocationic ILs, the best performance was obtained with 1-cyclohexylmethyl-4-methyl-1,2,4-triazolium dimethyl phosphate, whose DC (per mol) value is 14 times higher than that of popular solid desiccants like CaCl2 and silica gel. Dicationic ILs, such as 1,1′-(propane-1,3-diyl)bis(4-methyl-1,2,4-triazolium) bis(dimethyl phosphate), showed an even better moisture absorption, with a DC (per mol) value about 20 times higher than that of CaCl2. Small- and wide-angle X-ray scattering measurements of eight types of 1,2,4-triazolium dimethyl phosphate ILs were performed and revealed that the majority of these ILs form nanostructures. Such nanostructures, which vary with the identity of the IL and the water content, fall into three main categories: bicontinuous microemulsions, hexagonal cylinders, and micelle-like structures. Water in the solutions exists primarily in polar regions in the nanostructures; these spaces function as water pockets at relatively low water concentrations. Since the structure and stability of the aggregated forms of the ILs are mainly governed by the interactions of nonpolar groups, the alkyl side chains of the cations play an important role in the DC and temperature-dependent equilibrium water vapor pressure of the IL solutions. Our experimental findings and molecular dynamics simulation results shed light on the moisture absorption mechanism of the IL aqueous solutions from a molecular perspective.
... Combining these desiccants can improve dehumidification performance and substantially reduce costs and energy usage [86]. Both Salikandi, M. et al. and Al-Farayedhi [87,88] have examined the performance of numerous two-solvent combinations. In the majority of applications, the mixed solvent must possess characteristics such as a high boiling point elevation, a high latent heat of condensation and dilution, a low-vapor pressure, a low crystallization point, simple handling at low temperatures, a low viscosity, a high density, and a low price. ...
Article
Full-text available
Desiccant agents (DAs) have drawn much interest from researchers and businesses because they offer a potential method for lowering environmental impact, increasing energy efficiency, and controlling humidity. As a result, they provide a greener option to conventional air conditioning systems. This review thoroughly analyzes current issues, obstacles, and future advancements in liquid desiccant agents (LDAs) for drying, air conditioning, and dehumidification applications. The importance of LDAs in lowering energy use and greenhouse gas emissions is highlighted, emphasizing their potential for environmentally friendly humidity control. The current review examines key parameters such as novel materials, enhancing desiccant qualities, integration with technologies, and long-term durability while examining recent developments in LDAs and investigating their applications in diverse industries. The main conclusions from the evaluated publications in this review are also highlighted, including developments in LDAs, new applications, and developing research fields. Overall, this review advances knowledge of LDAs and their potential to shift humidity control systems toward sustainability and energy efficiency.
... The most commonly used liquid desiccants include Lithium Chloride, Lithium Bromide, and Calcium Chloride. Salikandi et al. [12] conducted a review and comparison of various liquid desiccants based on several criteria. Their conclusion was that among the different options, Calcium Chloride and Lithium Chloride are the most suitable liquid desiccants. ...
... LiBr is approximately 20% more Sustainability 2022, 14, 15917 3 of 25 costly than LiCl, but it possesses the same regeneration and dehumidification capabilities. As the most readily available desiccant, CaCl 2 solution has the lowest cost but can be unstable depending on the solution's concentration and the air conditions at the inlet [17]. Moreover, the dehumidifier in the LDDS system was equipped with an external cold source for internal cooling, consequently lowering the solution temperature and enhancing overall dehumidification capacity. ...
Article
Full-text available
A liquid desiccant dehumidification cooling system is a promising, energy-saving, high-efficiency, environmentally friendly technology that maintains thermal comfort effectively indoors by utilizing renewable energy sources or waste heat to enhance system efficiency. In this research, a small-scale (6 kW cooling capacity) hybrid liquid desiccant air-conditioning system (HLDAC) is proposed to evaluate the dehumidification performance of a non-corrosive potassium formate (KCOOH) solution. For this, four input parameters, namely, inlet air flow rate, inlet desiccant temperature, inlet desiccant concentration, and inlet specific air humidity, were selected. Moreover, the different combinations of experiments were designed by employing response surface methodology (RSM) to evaluate the dehumidification performance parameters, namely, dehumidifier latent heat load, coefficient of performance of hybrid system, and moisture removal rate (MRR). Further, a comparative performance analysis between the hybrid system and a standalone vapor compression system (VCS) unit was carried out. The result showed a remarkable increase in coefficient of performance, which was observed at about 28.48% over the standalone VCS unit. Furthermore, the economic assessment of the proposed hybrid system is presented in this paper. Finally, from the economic analysis, it was concluded that the hybrid system had a payback time of 2.65 years compared to the VCS unit.
... Potassium formate (KCOOH) solution is considered as one of the ionic solution, which is widely used as the antifreeze of heat source tower [1][2][3][4] and liquid desiccant [5][6][7][8]. KCOOH can reduce freezing point and improve moisture absorption compared with traditional lithium chloride and lithium bromide [9][10][11][12], which has broad application prospects due to its advantages of high solubility, low cost and no pollution. As we know, KCOOH solution will become invalid because it absorbs water during operation as the antifreeze or liquid desiccant, the original performance of which needs to be restored by means of solution regeneration, however, the existing methods of solution regeneration are consuming a lot of energy when dealing with high concentration solution. ...
Preprint
Low energy consumption is the regeneration goal of high concentration potassium formate (KCOOH) solution, which can reduce freezing point while improve moisture absorption when being used as the antifreeze or liquid desiccant. This paper proposed a mechanical vapor compression (MVC) regeneration system, which can greatly reduce energy consumption when dealing with high concentration solution. The developed models of MVC regeneration system were in good agreement with the experiments, whose relative deviation between the model and experimental values is from −13.49 to 10.18 %, to predict and analyze the regeneration performance of KCOOH solution. Experimental results shows that the MVC regeneration system had still a high COP (12.22-16.30) , when dealing with the high concentration solution. Performance prediction displays that the optimal heat transfer temperature difference was from 7 to 11°C. Research of this paper provides the technical support for the design optimization and engineering application of MVC regener-ation system.
... An alternative is sodium and potassium format and acetate, but their humidification capacity is limited (Qu et al., 2017). Therefore, the interest in ionic liquids as desiccants has increased over the past decade (Salikandi et al., 2021). To analyze the potential application of aqueous solutions of ionic liquids, it is necessary to know the vapor pressure of these systems. ...
Article
Full-text available
Desiccants are an important part of dehumidifier systems, contributing to absorbing the humidity of the air. The vapor pressure is one of the most important properties in the dehumidification process. For this reason, the application of the Cisternas–Lam model can overcome difficulties such as the lack of experimental vapor pressure measurements. In this work, this model is applied to aqueous solution of several desiccants such as CH3COONa, CH3COOK, NaClO4, and KClO4, including ionic liquids such as [EMIM][OMs], [EMIM][OAc], [EMIM][OTf], [E3MPy][OMs], [EMIM]Cl, [BMIM]Br, [BMIM]Cl, [BMIM]I, [EMIM][ES], [EMIM][MS], [BMIM][DBP], [EMIM][DEP], [MMIM][DMP], [EMIM][DMP], [EEMI][DEP], [EMIM][Tf2N], [EMIM][DCA], [DMIM][BF4], and [MMIM][MS], estimating their K value in binary aqueous systems. In addition, the prediction capacity of the model is tested in interpolation and extrapolation examples and shows good results. Finally, this model is used to predict the vapor pressures of aqueous ternary systems that contain ionic liquids. The results showed that the Cisternas-Lam model is appropriate to correlate and predict vapor pressure in the systems studied.
... Honeycomb pads gave better results than aspen pads and found that honeycomb pads were more applicable for hot and dry climates. Salikandi et al. (2021) reviewed liquid desiccant dehumidification where dehumidifier system with different desiccants was studied. Also, this work focused on indoor air quality measurement and its effect on the occupants. ...
Article
Full-text available
In the present performance parameters is determined for the varied operating conditions using mathematical modelling. Outlet humidity ratio, Dry bulb temperature (DBT), cooling efficiency and cooling effect have been predicted by varying the air velocities, inlet DBT, inlet relative humidity (RH) and pad thickness for three different wettability of Celdek packing. Predicted results revealed that cooling effect, saturation efficiency, ΔDBT and humidity ratio are found to be increasing with the increase in pad thickness and wettability of the material. An increase in the inlet air flow rate and RH resulted in a decrease in ΔDBT, humidity ratio and cooling efficiency. The maximum performance of ΔDBT, ΔRH, saturation efficiency and cooling effects have been observed for Celdek 7090 and are equal to 6°C, 55%, 90% and 7000 Watts for the thickness of 0.3 m, wettability of 630 m2/m3. Wettability of 630 m2/m3 & thickness 0.3 m gave maximum performance.
... Various liquid desiccant regeneration methods are described in this research, including the most advanced form of photovoltaic/electrodialysis regeneration [21]. The most popular liquid desiccant (LD) cooling system is the packed bed. ...
Article
Full-text available
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.
... In cooling systems, a desiccant is used to absorb the extra heat and humidity of the inlet air. Several liquid desiccants that have been widely used are calcium chloride (CaCl 2 ), LiCl (lithium chloride), LiBr (lithium bromide), and triethylene glycol, since they have low vapour pressures and have shown good performance [110]. The possibility of using bittern as a liquid desiccant was investigated to further process the salt byproducts and exploit the hygroscopic properties of bittern for greenhouse cooling [6,29]. ...
Article
Full-text available
In addition to high-purity salts, salt farms and industries generate a substantial amount of highly concentrated brine (bittern) byproducts. The presence of high salinity in this waste stream may lead to adverse environmental impacts. Environmental concerns and strict regulations on its disposal have driven the development of innovative practices for bittern management to achieve sustainable resource use and recovery. Meanwhile, commercial materials (Li, Rb, and Cs) that may be contained in the bittern can be seen as attractive value-added resources. This review article comparatively discusses the available technologies for bittern recovery, their advantages and disadvantages in terms of their technical aspects, their estimated energy and/or technological costs, the recovery efficiency of the targeted products, and the possibility of the utilization of bittern to aim for both minimal and zero liquid discharge targets. Of the bittern recovery technologies evaluated, electrodialysis offers efficient and highly selective separation of ionic compounds, while evaporation and precipitation are the most efficient methods of obtaining solid salt products (MgCl2, NaCl, KCl, etc.). In a minimal liquid discharge system, electrodialysis can be coupled with a thermal process or precipitation to achieve a less concentrated bittern effluent for safe disposal. However, in a zero liquid discharge process, more complex recovery technologies are applied, before the utilization of liquid bittern as a CO2 absorbent, cooling agent, or coagulant. Finally, we highlight areas of future research regarding technological developments that aim to enhance the effectiveness at larger scales and to improve the sustainability of bittern recovery and utilization.
... Air conditioning is an essential component of the HVAC sector, and research has revealed that air conditioning systems in buildings alone consume about 50% of the total energy. 1 This percentage is much more for nations with tropical or subtropical climates. In Gulf countries alone, 60% of the total electrical energy use has been attributed to comfort conditioning systems. 2 Moreover, over the years, there has been an unprecedented growth in the market for air conditioning systems, as shown in Fig. 1. ...
Article
Traditional air conditioning systems run on the vapor compression cycle, which utilizes electricity generated from fossil fuels, the reserves of which are fast depleting. Moreover, the refrigerants used in such systems have exacerbated ozone layer depletion and climate change. Liquid desiccant air conditioning (LDAC) systems appear as a favorable option in light of these drawbacks. This paper has developed the numerical model of an LDAC system using a dehumidifier that is internally cooled and has a finned coil. The study of this particular model has been limited in the past. The model has been validated against a reference study available in the literature. Moreover, the commonly used desiccant materials tend to be corrosive and detrimental to the air quality. Therefore, the feasibility of an ionic liquid (65% [Emim]OAc) and an organic salt solution (73% HCOOK) as desiccant materials has also been investigated in this paper. With the proposed scheme for room air conditioning, the air temperature and humidity levels within the range of comfort (21 ∘ C, 53% R.H.) could be supplied to the conditioned space. Performance analysis revealed that the eco-friendly desiccants could achieve dehumidifier and regeneration effectiveness similar to that of the traditionally used corrosive salt solution of calcium chloride (CaCl 2 ). Furthermore, the estimated dehumidifier efficiency of about 76% is found to be higher than that available in the previous studies. Economic analysis provides a comparison of total cost between the conventional and the novel desiccant air conditioning systems and also reveals HCOOK to be a more economical choice as desiccant material.
... The cooling capacity of the honeycomb and the aspen pads are found to be 2.33 and 1.64 kW, respectively. Salikandi et al. [24] reviewed different desiccants, operating conditions, and packing configurations. They highlighted indoor air quality and regeneration techniques to regain the desiccant concentration back. ...
Article
Evaporative cooling is one of the easiest methods of achieving thermal comfort inside defined space, especially in arid regions. The current work adopts a novel approach in the design and fabrication of a humidifier unit that works on the centrifugal effect. The system consists of a cylindrical mesh that is filled with packing material driven by a motor. The rotating packing cylinder is soaked by the water dripping by gravity. The vortex effect caused by the centrifugal action on the packing allows dispersing the fine water particles in the air. Airblown inside the chamber gets humidified due to the evaporation of water. Output parameters such as evaporation rate (ER), cooling efficiency (HE), coefficient of performance (COP), cooling effect, and specific cooling effect (SCC) are determined for various combinations of air velocities and cylinder speed of rotation. Experiments are carried for both static and dynamic packing conditions. It is found that dynamic packing with 150 rpm and air velocity of 8 m/s gave a superior performance in terms of change in DBT, change in RH, humidification efficiency, and evaporation rate. The system gave a maximum COP of 5.85, an evaporation rate of 0.51 g/s, and a humidification efficiency of 79.23%. Transient analysis indicated that required thermal conditions could be attained in the dynamic case earlier than the static conditions saving both energy and valuable time. Overall, the performance of dynamic packing is superior compared to the stationary packing in terms of higher ΔRH, ΔDBT, humidification efficiency, and evaporation rate.
... The introduction of ILs as desiccant provided alternative low melting point and non-corrosive working fluid to surmount the constraints posed by common liquid desiccants (Halide salt solutions). 31,58,59 Watanabe et al. 31 reported that dimethyl ester of phosphate acid, methyl phosphate {P 4441 }{DMPO 4 } has the best dehumidification capacity of about 10 944%/mol. The dehumidification capacity is a crucial parameter in dehumidifier selection because it indicates the maximum moisture removing strength of any moisture removal equipment. ...
Article
Liquid desiccant air dehumidification has gained substantial attention recently due to its attractive energy‐saving capability, high moisture retention, and low regeneration temperature. However, there are still unresolved limitations in liquid desiccant air conditioning systems (LDACs). Among on‐going studies are the search for greener desiccant solvents, high‐performance membrane, and regeneration techniques hybridization. This review discusses up‐to‐date development of the performance influential components of LDACs, such as desiccant properties, regeneration techniques, membranes, energy sources, and hybrid system configurations. The corrosive nature of conventional halide salt solutions, non‐biodegradability and high viscosity property of most ionic liquids necessitate the search for alternative solvents in LDACs. Deep eutectic solvents (DES) properties, such as non‐corrosive, hygroscopic, biodegradable, and low viscosity idealize promising alternative desiccant solutions. Therefore, DES may be usefully explored and further investigated in LDACs to establish the degree of their capacities in replacing conventional desiccants. Non‐thermal regeneration techniques and nanoparticle enhanced membranes were also found to improve the overall energy performance of the LDAC system. Non‐thermal regeneration techniques can operate below 40°C and reduce energy utilization between 10% and 50% in indoor space cooling. The coefficient of performance (COP) of this regeneration category is capable of being as high as 6, which is an indication of its promising energy‐saving propensity. Highlights • Review on liquid desiccant materials and the potential of deep eutectic solvents as bio‐desiccants for air dehumidification. • Nanoparticles enhanced ion exchange membrane tendency to eliminate membrane fouling and improve regeneration performance. • Non‐thermal liquid desiccant regeneration techniques eliminate re‐cooling energies requirement in LDACs.
Article
Air-conditioning systems condense air water vapor to achieve dehumidification. This process entails excessive cooling followed by reheating, resulting in high energy usage. As a more energy-efficient alternative, liquid desiccant-based dehumidification eliminates the need for excess cooling by dehumidifying through an absorption process. Advanced internally cooled liquid-desiccant dehumidifiers enhance overall air conditioning efficiency by incorporating internal cooling mechanisms. These devices are three-stream heat and mass exchangers, involving air, liquid desiccant, and cooling water. Simulating them involves discretization and solving comprehensive partial differential mass and energy balance equations to capture the interconnected heat and mass transfer phenomena. The models require detailed dehumidifier information, are computationally expensive, and pose challenges in convergence. These requirements make them unsuitable for integration into Building Energy Simulation software or inclusion within a comprehensive air-conditioning system model. This study aims to generate models with less computational demands during simulation using artificial neural networks. A comprehensive finite difference model was used to generate training data for developing the neural network. Various network configurations were explored to assess their impact on prediction precision. The trained network, integrated into an independent code, underwent evaluation for prediction accuracy using a distinct dataset from the training stage, proving accurate to simulate the internally cooled dehumidifier, reaching R-values up to 0.96 for the 5 predicted outlet variables. This is the initial step towards incorporating neural network models into specialized Building Energy Simulation software, as the foundation for conducting system-level, transient, and long-term simulations.
Article
The escalating growth in the traditional air-conditioning industry has led to an increased demand for energy. However, this industry has the drawbacks of high energy consumption and is non-environmentally friendly. Solar cooling technologies emerge as a pivotal solution to overcome these challenges, presenting an ideal alternative for energy and environmental considerations. However, the main drawbacks of solar cooling technologies are their initial high cost and comparatively lower performance, which remain obstacles to their broader implementation. Despite recent advancements, these technologies still struggle to compete with traditional cooling methods due to cost, performance, and maintenance challenges. This review focuses on categorizing solar cooling systems and provides a detailed examination of each type, with a specific emphasis on the most significant ones. Additionally, this work includes a historical overview of the utilization of solar energy in various cooling and refrigeration processes, covering aspects of design and application. Specifically, the article provides an extensive review of various solar-powered cooling techniques, including photovoltaic, photovoltaic-thermal (PVT), absorption, desorption, and mechanical systems, and their potential to enhance energy efficiency. It also covers the contributions of optimization and machine learning in improving the efficiency of solar cooling technologies. The current study conducts a broad survey of diverse cooling systems utilizing solar energy for either full or partial operation. Recent studies encourage for multi-functional hybrid solar cooling technologies that offer innovative solutions with low energy consumption, high performance, and cost-effectiveness. These systems are well-suited for both industrial and domestic applications. The findings of this study align with previous research, affirming that solar absorption systems are the most prevalent among various solar cooling systems. The efficacy of solar cooling is contingent on several factors, with paramount importance placed on weather conditions, sunlight intensity, and the size of the space intended for cooling. The optimal utilization of solar energy collectors and advancements in flat plate technologies have significantly improved the performance of closed adsorption and absorption cooling systems, as well as enhanced the effectiveness of their components. Additionally, the study concluded that solar cooling systems are integrated solutions, commencing with a thoughtful selection of either photovoltaic panels or solar collectors, followed by energy conservation storage, and concluding with a suitably matched cooling unit. Finally, the review emphasizes the need for continued research and development in solar cooling technologies, with a focus on optimizing performance, enhancing efficiency, and expanding their application in various settings. Future studies should focus on enhancing the coefficient of performance (COP) and cooling capacity through heat and mass recovery, multi-stage and multi-bed configurations, and refining absorption/adsorption structures and operating conditions.
Article
Due to rising environmental concerns and decreasing fossil fuels, renewable energy-driven liquid desiccant packed bed dehumidifiers are fast developing as a viable alternative for air dehumidification. In this article, the effect due to the material of packing as well as the packing density of an individual packing on dehumidifier performance is numerically investigated by coupled heat and mass transfer analysis through two sets of case studies. A parametric study using less corrosive potassium formate (KCOOH) solution is conducted for the set of design variables at the inlet viz. air flow rate m˙ a ( ) , desiccant flow rate m˙ s ( ) , desiccant concentration (ξ), inlet air specific humidity (ωa), desiccant temperature (Ts) and ambient temperature (Ta). The numerical model is validated with literature and in-house experiments. As the packing density increases, moisture removal rate (MRR) and pressure drop (PD) non-linearly vary with air velocity due to material properties. Further, numerical analysis shows an overall improvement in dehumidification performance in terms of MRR, moisture effectiveness (εabs), latent heat ratio (LHR) and PD when packing density of Mellapak 250Y is varied in the range 100–700 m2/ m3. Performance of the dehumidifier increases with ωa, ξ, m ˙ s and m ˙ a but decreases with Ts. An improvement of 52 % in MRR is found with higher ξ which is further improved by 97 % at higher packing density. PD increases by 99 % with m ˙ a and packing density but decreases under higher humid conditions. An optimal packing density and m ˙ a with higher ξ and m ˙ s (based on limitations in the pump capacity) guarantee maximum dehumidifier performance. Introduction
Article
Liquid desiccant air-conditioning (LDAC) system is a promising alternative to the traditional vapor compression system. The most energy-consuming part of LDAC system is desiccant regeneration. An urgent issue that needs to be addressed is the inefficiency of traditional regeneration method. Solar heat localization is a novel and highefficiency strategy for seawater desalination. However, the wide application is confined due to the low-efficient of water condensation and harvesting. In this work, the regeneration performance of heat localization method was investigated and an electric heating sheet was adopted to circumvent the effect of solar reflection. Compared with traditional heating method, the regeneration performance can be significantly improved by heat localization method. The regeneration efficiency of desiccant with 40 % concentration was significantly improved by 6 times (about 23.15 %) through heat localization method. Heat localization method allows immediate concentration of high-concentration desiccant with a low-temperature rise of regenerated desiccant. The low-temperature rise is beneficial for LDAC system due to less cooling energy is needed. This work is expected to extend the application of solar heat localization and provide some new information for liquid desiccant regeneration.
Article
A saturated vapor pressure model with a wide range of applications (covering traditional salt solutions and emerging ionic liquid solutions) and simple frame form is the foundation of the research in the field of solution gas-liquid equilibrium. The existing typical thermodynamic models usually improve the prediction accuracy at the cost of complex forms (such as increasing the number of adjustable parameters), resulting in highly nonlinear functions, which are not convenient for practical calculation. Based on the theory of statistical thermodynamics and considering the dipole interaction of ion-water molecules, this paper proposes a saturated vapor pressure explicit model with temperature and ion concentration for salt and ionic liquid multi-component liquid desiccant, in which only three adjustable parameters are introduced to characterize each ion. Compared with the experimental data and several typical thermodynamic models, the results show that the prediction accuracy of the model is comparable to those of typical models. However, its advantage over these typical models is its simpler framework and significant computational reduction. Through the application of some examples, it is found that the new model can have the potential of direct extension, which is manifested: from one temperature range to another temperature range, from ions to compounds, from single-component to multi-component, and from the water system to other solvent systems. This demonstrates that the three adjustable parameters introduced for ions in the model have clear physical meanings, which can significantly reduce the dependence on experimental data. The proposed model can quickly calculate the saturated vapor pressure of the solution containing inorganic salts, organic acid salts, and ionic liquids, and can guide the optimization of dehumidifying materials.
Article
Vapor Compression Systems (VCS) are the most common air conditioning technology. VCS cool the air to its dew point temperature (overcooling) to remove water vapor in the air through condensation and then reheats the air back to the comfort temperature for direct use. The VCS process is inefficient due to overcooling and reheating. Liquid Desiccant Dehumidification (LDD) is a potentially energy-efficient air conditioning. LDD removes water vapor in the process air using liquid desiccant’s high-water affinity. It hybrids with sensible cooling to control temperature and humidity separately. The LD in the LDD becomes weak after dehumidification. The LDD needs additional heating to regenerate the weak Liquid Desiccant (LD) to a high concentration for dehumidification. Earlier versions of the LDD systems use highly concentrated liquid desiccant (large water removal capability) to dehumidify the air by only dealing with latent load. It leads to highly elevated temperatures above 60°C of heat sources (combustion or electric resistance-based heating) for regeneration. The energy needed for the elevated temperature heat resource significantly reduces or demolishes the benefit of LDD systems. In the recent two decades, researchers have investigated a new configured LDD system that couples an LDD with a heat pump at both dehumidification and regeneration sides for better efficiency. The heat pump provides cooling (from the evaporator) for both dehumidification and sensible cooling and simultaneous heating from the condenser for regeneration. The highly integrated system (HP-LDD) with improved efficiency enables the LDD to operate at lower concentrations and temperatures in dehumidification and regeneration. This paper depicts the working principle behind HP-LDD and its heating and cooling requirements. It reviews the comparison between the HP-LDD systems and the conventional LDD systems regarding system configurations, component design, energy efficiency, and dehumidification performance characteristics. The main findings from the review include the preferred use of packed bed over membrane-based dehumidifiers, the use of internally cooled dehumidifiers enabled by the HP cooling capacity, the high dispersion of HP operation conditions, and the dependence of dehumidification performance on various dehumidifiers. An outlook for future research on HP-LDD strategies is presented based on the reviewed works and their limitations.
Article
Liquid desiccant air-conditioning (LDAC) systems are one of the most promising alternatives to conventional air conditioning systems due to the high energy efficiency. The most energy-consuming part of LDAS system is desiccant regeneration, while traditional regeneration methods often fail to balance the energy grade and energy efficiency. Inspired by solar heat localization for seawater desalination, the regeneration performance of this novel method has been investigated. Here, we fabricated a wood-based regenerator by drilling holes and spaying Chinese ink, which features high sunlight absorption and strong water transportation ability. Owing to the high hydraulic conductivity of drilled holes and low thermal conductivity of wood substrate, liquid desiccant can be regenerated efficiently with a low-temperature rise. Even for 40 wt% CaCl2 solution at 30 °C, the regeneration rate and efficiency can reach 0.64 kg·m⁻²·h⁻¹ and 43.2 %. Comparisons between solar interfacial regeneration method and other traditional regeneration methods were also conducted. Results show that the solar interfacial method can significantly improve the regeneration rate at the same temperature. Given the low manufacturing cost and operating cost, high regeneration rate and efficiency, superior salt-rejecting property, and low-temperature rise, the wood regenerators based on solar interfacial regeneration show great potential in LDAC systems.
Article
Liquid desiccant air conditioning (LDAC) has been proposed for greenhouse cooling. LDAC achieves lower temperatures than conventional evaporative cooling while also allowing closed air recirculation which has advantages over open ventilation. However, liquid desiccant regeneration is challenging and energy intensive. A novel multi-stage nanofiltration (NF) system is proposed as a regeneration method to enable both greenhouse cooling and irrigation in a closed water cycle. This concept study shows that, during winter and summer respectively, a 9-stage NF regenerator recovers 4.8 and 6.6 L of water per day per m² of greenhouse floor area. Using commercial NF membranes with burst pressures of ∼40 bar, a 9-stage regenerator using CaCl2 could achieve a yearly COP of ∼12.4 and monthly greenhouse temperatures below 32°C, almost independent of location. This improves greatly on LDAC using open evaporative regenerators, which only provides a COP of 0.3-0.5. Future research allowing NF membranes to operate with burst pressures of 55 bar could achieve the same COP by using just a 4-stage regenerator, making the system less expensive and more compact. Perspectives on the proposed design and implementation are discussed. This study shows the potential of the multi-stage NF regenerator to cool greenhouses in hot climates where it is difficult to produce crops during summer.
Article
Liquid desiccant air conditioning system (LDAS) was considered a promising air conditioning system due to the advantages of low-grade heat utilization, effective humidity control and environment-friendly. The performance of LDAS significantly depends on the heat and mass transfer components, namely dehumidifier and regenerator, while the regeneration process of traditional regenerator is energy-inefficient. Inspired by the high-efficiency solar interfacial evaporation which can produce drinking water from seawater, this method was introduced in this work for liquid desiccant regeneration to overcome the inefficiency of the regeneration process. Wood was prepared as the regenerator by alkali-assisted method and surface carbonization. The regeneration performance of solar interfacial regeneration was experimentally investigated and compared with that of packed-bed tower regeneration and membrane distillation regeneration. Results show solar interfacial regeneration is very competitive due to the high regeneration rate and efficiency, which can reach 0.57 kg m⁻² h⁻¹ and 42.5% for 40 wt% LiCl solution under 1-sun irradiation. Moreover, the low temperature rises of concentrated solution, cheap construction costs and operating costs make LDAS more energy-saving. This work is expected to supply a novel method for liquid desiccant regeneration.
Article
In this work, the ED regeneration experiments were conducted using KCOOH (potassium formate) liquid desiccant, which is attracting attention due to its non-corrosiveness and low toxicity. Varying solution concentration, solution temperature, and current density, effect of the operating conditions on overall transport number, overall hydration number, overall osmotic coefficient, and area-specific resistance (ASR) was analyzed. In particular, effect of the solution temperature on the ASR, which was frequently ignored in previous research, was evaluated. As the solution temperature increased from 21 ℃ to 30 ℃, the ASR decreased 21% and 17% at different concentration conditions. Additionally, the overall transport number of the KCOOH were compared with experimental data of different liquid desiccants in other research. The result showed that at low solution vapor pressure condition, KCOOH recorded the largest overall transport number so that it has a potential to be an energy-efficient working fluid for ED regeneration system. Furthermore, the analysis results and the regression equations can be utilized in developing a system modelling of the liquid desiccant regeneration using ED with KCOOH desiccant in future research. *Keywords: Electrodialysis, Potassium formate, Liquid desiccant regeneration, Overall mass transport model, Transport number, Hydration number
Article
Liquid desiccant air-conditioning (LDAC) system is being considered as a promising alternative to vapor compression refrigeration system owing to its effective temperature and humidity independent control and great energy-saving potential. Regeneration of liquid desiccant is an important desalination process in the LDAC system, which aims at re-concentrating the diluted desiccant solution to renew its dehumidification capacity for system continuous operation. This review provides a comprehensive overview of the developments in liquid desiccant regeneration to date, focusing on its application for LDAC systems. The operation principle and evaluation index of liquid desiccant cooling system and regeneration process are firstly presented. Then, attention has been paid to various developments in liquid desiccant materials and regenerators, which play a pivotal role in the liquid desiccant regeneration process. Furthermore, hybrid liquid desiccant regeneration systems with various driven energy sources and the enhancement technologies for improving the regeneration performance are summarized and reviewed. Finally, future needs and recommendations for liquid desiccant regeneration technologies are discussed. This review may help to identify the research gaps and explore promising approach for future study to further enhance the efficiency of liquid desiccant regeneration for the LDAC system.
Article
In this paper, an experimental device for falling film dehumidification of circular tube is established. The effects of desiccant inlet concentration(30%-40%), velocity(0.01–0.06 m/s), temperature(26–36°C) and air inlet velocity(0.7–1.2 m/s), temperature(26–36°C) and moisture content (15–25 g/kg) on heat and mass transfer dynamic process and the internal detailed temperature, velocity were investigated by experiments and CFD model. The results show that the CFD simulation is in good agreement with the experimental results, with an average error of 4%. Then, the heat and mass transfer and desiccant flow process inside the dehumidifier are analyzed.
Article
The review describes the development and advancement of thermoelectric based dehumidifier systems. Thermoelectric cooling in thermoelectric dehumidifiers has various advantages over conventional techniques, such as lightweight, compact size, noise-free (no moving part), and eco-friendly operation. The review initially describes the basic principle and different techniques used for dehumidification. Then, thermoelectric dehumidifiers are introduced and their governing parameters are discussed. Further, thermoelectric dehumidifier-based prototypes are reviewed and discussed over their design and performance. The patents and commercialized products developed on thermoelectric dehumidifiers were also briefly discussed and compared. Lastly, this review states the challenges for the more efficient and large-scale operation of thermoelectric-based dehumidifiers and simplifies ways for further improvement. Thus, the study illustrates the current status of thermoelectric dehumidifiers and provides futuristic scopes for their development.
Article
An energy conservative air-conditioning system is critical to promoting energy conservation and emission reduction. One promising alternative is liquid desiccant cooling system (LDCS), which can be driven by low-grade heat and has superior dehumidification performance. However, energy waste from traditional regeneration process restricts its application from widespread use. To overcome this disadvantage, a wood-based heat localization regenerator is presented: it localizes the thermal energy on the regeneration surface avoiding the unnecessary heating of bulk solution. The real performance of this new regenerator has been investigated theoretically and experimentally in this paper. The regeneration process of desiccant solution has been analyzed and a regeneration performance prediction model has been developed. Some performance indexes have been derived from experimental data. A regeneration performance comparison has been conducted between the heat localization method and the traditional heating method. The results show that the new method improves regeneration thermal efficiency by above 50% and the maximum value can achieve 43.6%. Besides higher performance, the reduction of energy consumption, initial cost and complexity has achieved with the heat localization regenerator. The overall improvement makes LDCS such a competitive alternative to the current air-conditioning system.
Article
Liquid desiccant dehumidification (LDD) system is an emerging technology with energy-saving benefit in HVAC applications. It removes moisture from the inlet air stream and handles the latent load without overcooling and reheating. The most energy-intensive process of the LDD system, the desiccant regeneration process, can be driven by renewable solar thermal energy with a temperature lower than 100 °C. However, the integration of solar heat needs to consider the inconsistent availability of solar radiation and regeneration heat demand. In this study, a pinch-based Cascade Analysis (CA) approach is used to optimally size the solar thermal collectors and thermal energy storage (TES) water tanks, which are the main components of the solar thermal system. From the analysis, the overall system efficiency, minimum area of solar thermal collectors and total TES volume are 78.8%, 59.83 m² and 7.10 m³, respectively, with an average daily regeneration heat demand of 213.48 kWh. The overall system installation and operating cost is approximately 14219.98 USD or 948.00 USD annually over 15 years. This work serves as a preliminary study to provide an overview of the implementation of solar thermal systems for decision-makers who intend to implement solar-based LDD systems in HVAC or drying applications.
Article
The liquid desiccant solution can able to dehumidify the air by absorbing the water vapour content present in the air. The current work aims to test the dehumidification performance of liquid desiccant system with aqueous CaCl2 and ABS (Acrylonitrile butadiene styrene) packing in a humid climatic condition of India i.e., Kozhikode. The experimental setup is made with packed type dehumidifier. The dehumidification performance such as specific moisture removal, moisture removal rate and dehumidification effectiveness are studied with the effect of variation of solution flow rate, air flow rate, outdoor humidity ratio and solution temperature. The increase in solution flow rate and outdoor humidity ratio positively affect the dehumidification performance whereas the increase in solution temperature negatively affects the dehumidification performance. The increase in air flow rate increases the moisture removal rate but decreases the specific moisture removal and the effectiveness. The dehumidification performance shows a promising results, particularly performs better in higher outdoor humidity conditions. The dehumidification performance can be further improved by adding the cooling system.
Article
Full-text available
The main objective of this study is to investigate the indoor air quality enhancement performance of two different liquid desiccant and evaporative cooling-assisted air conditioning systems, such as the variable air volume (VAV) system with the desiccant-enhanced evaporative (DEVap) cooler, and the liquid desiccant system with an indirect and direct evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS), compared with the conventional VAV system. The transient simulations of concentration variations of carbon dioxide (CO2), coarse particles, and fine particles (PM10 and PM2.5) in a model office space served by each system were performed using validated system models that were found in the literature. Based on the hourly thermal loads of the model space predicted by the TRNSYS 18 program, each air conditioning system was operated virtually using a commercial equation solver program (EES). The results indicated that the LD-IDECOAS provided the lowest annual indoor CO2 concentration among all the systems considered in this research, while the VAV system with DEVap cooler exceeded the threshold concentration (i.e., 1000 ppm) during the cooling season (i.e., July, August, and September). For the indoor particulate contaminant concentrations, both liquid desiccant and evaporative cooling-assisted air conditioning systems indicated lower indoor PM2.5 and PM10 concentrations compared with the reference system. The LD-IDECOAS and the VAV with a DEVap cooler demonstrated 33.3% and 23.5% lower annual accumulated indoor PM10 concentrations than the reference system, respectively. Similarly, the annual accumulated indoor PM2.5 concentration was reduced by 16% using the LD-IDECOAS and 17.1% using the VAV with DEVap cooler.
Article
Full-text available
Suitable control of the humidity can contribute to electric energy savings. However, the present dehumidification system has many weak points. The liquid desiccant air-conditioning system has recently gained growing interest from the stand point of reducing energy consumption during dehumidification. In order to find the appropriate ionic liquids (ILs) as a desiccant for the liquid desiccant air-conditioner system, we conducted a systematic evaluation of the humidification capability of 16 types of ILs. Among the tested ILs, tributyl(methyl)phosphonium dimethyl phosphate ([P4441][DMPO4]) exhibited the best dehumidification capacity and had a less corrosive effect on four types of metals as possible piping materials. It should be noted that this [P4441][DMPO4] has a very stable nature and produced no odor while conducting the experiment and storing for over 1 year at room temperature under ambient conditions. Furthermore, it was revealed that a 77% (w/w) aqueous solution of [P4441][DMPO4] worked as an efficient desiccant liquid for the liquid desiccant air-conditioner system. © 2019 Institute of Process Engineering, Chinese Academy of Sciences
Article
Full-text available
Humidity control of indoor space using the conventional air conditioning system is energy intensive. The liquid desiccant dehumidifier, which operates on low grade energy sources, is one of the energy efficient alternatives for humidity control. Membrane dehumidifiers avoid the desiccant carryover and hence are preferred over the packed bed dehumidifiers. However, their performance is lower due to the additional resistance in the membrane. Internal cooling is one way to improve the performance of the membrane dehumidifier and the present study experimentally investigates such a dehumidifier. The operating parameters considered are specific humidity, mass flow rate, temperature and of inlet air. The performances of the adiabatic and internally cooled dehumidifiers are presented in terms of moisture removal rate and latent effectiveness. It is found that these are higher by 60 and 50%, respectively, for the internally cooled dehumidifier.
Article
Full-text available
The traditional vapor compression cooling system (VCS) has been criticized for its heavy reliance on electricity consumption and limited control ability of humidity. Compared with the VCS, the liquid desiccant cooling system (LDCS) is more efficient by handling the sensible and latent load separately. The present study investigated the regeneration and corrosion characteristics of an anodized aluminum plate regenerator for the first time. Comparative tests were conducted to present the regeneration and corrosion characteristics of the normal and anodized regenerator. The influence of internal heating on the regeneration performance was also identified. The results showed that the corrosion rate was reduced from 0.0005218 mm/year for normal aluminum to 0.000011 m/year for anodized one under the same operating conditions. However, pitting corrosion was observed at operating conditions with high solution temperature, as the anodized layer of anodized aluminum was damaged due to the high temperature. Compared with the normal aluminum regenerator, the anodized one had an average enhancement of 24% and 23.7% in terms of regeneration rate and effectiveness, respectively. It was attributed to the increment of surface energy from 26.4 mN/m for normal aluminum to 47.6 mN/m for anodized plate. Besides, it was found that, compared with adiabatic one, the internally-heated regenerator improved the regeneration rate by 6.0%~38% and the regeneration effectiveness by 6.3%~32% depending on the operating conditions. The advantage of present study is that it can guide the design of anodized aluminum regenerator for LDCS.
Article
Full-text available
Desiccant cooling systems have been considered as an efficient method of controlling moisture content in supply air. They do not use any ozone-depleting coolants and consume less energy as compared with the vapour compression systems. This communication provides an extensive review of liquid desiccant systems (LDSs). All the components of an LDS such as dehumidifier, regenerator, packing material and liquid desiccant properties along with its energy storage capabilities have been discussed in detail. In addition, hybrid of LDSs with sensible cooling technologies has been studied. Various types of mathematical models to predict the outlet parameters of the desiccant system and current issues in liquid desiccants have been reviewed in detail. Moreover, solid and other advanced desiccants have also been discussed briefly. Finally, a summary of some successful case studies and economic evaluation of desiccant systems have been given.
Article
Full-text available
In this study, hollow fiber membrane contactors have been applied for gas dehumidification using tri-ethylene glycol as absorbent. The effects of gas and liquid flow rates, inlet concentration of tri-ethylene glycol and flow direction on dehumidification efficiency have been investigated. The results reveal that membrane contactors can remove water vapor very effectively using tri-ethylene glycol, and very low water dew point as low as −50 °C can be achieved. It also shows that liquid phase is not the limiting resistance in this system, as a result of very high solubility of water in tri-ethylene glycol.
Article
Full-text available
Air handling processors with structure packing are widely utilized in the air-conditioning system, in which coupled heat and mass transfer occurs between air and water/desiccant. As an additional benefit, the processor could remove particles at the same time. An experimental setup for a cross-flow packed processor was established to investigate the filtration efficiency, in which air-water and air-desiccant were chosen as flow media. Effects of air and water/desiccant parameters on the filtration performance were experimentally analyzed. The results show that for typical air-to-liquid flow rate ratio (≈1), PM5 arrestance efficiency of packed processor with water is 40%∼50% while that with LiBr solution is 20%∼30%. The filtration efficiency with LiBr solution is higher than that with water when splash effect of liquid is omitted.
Article
The liquid desiccant cooling system (LDCS) is a promising alternative for the conventional vapor compression system due to its high energy efficiency. To enhance the mass transfer between the regeneration air and liquid desiccant in the regenerator, the paper firstly introduced a kind of surfactant called polyvinyl pyrrolidone (PVP-K30) which was added into the LiCl solution for better desiccant regeneration performance. The falling film characteristics and regeneration performance were investigated and compared with and without surfactant. The results indicated that the contact angle of desiccant solution on plate decreased from 58.5° to 28.0° by adding surfactant with 0.4% of concentration. The average wetting area increased from 0.174 m² to 0.209 m² with a relative increment of 20.1% with the addition of surfactant. Correspondingly, the film thickness had a reduction of 0.103 mm from 0.696 mm to 0.593 mm averagely. The regeneration rate had an average enhancement of 26.3% under the same working conditions, resulting from the decrement of solution contact angle with the addition of the surfactant. Finally, correlations to predict the mass transfer coefficient were proposed with and without surfactant and the mean absolute relative deviation between the results of correlations and the experiments were kept within 8%. The results and findings of present paper could be applied to guide the design of compact regenerator for higher regeneration performance.
Article
The potassium formate (KCOOH) solution is regarded as a promising alternative for the widely used salt solution in liquid desiccant dehumidification. However, few studies have been conducted to investigate the regeneration characteristics of KCOOH solution. In the present study, it firstly measured the vapor pressure of KCOOH solution with different temperature and concentration. Then the corrosion behavior of stainless steel 316L in both KCOOH and LiCl solution was studied with electrochemical test. Finally, experiments were carried out to study and compare the regeneration performance of the two liquid desiccants. Results showed that the vapor pressure of KCOOH solution with the concentration of 64.3% to 73.3% was similar to that of LiCl solution with the concentration of 33% to 38% when the temperature was from 45 °C to 65 °C. Both the regeneration rate and mass transfer coefficient were almost the same for KCOOH solution and LiCl solution in the same experimental conditions. However, the application of KCOOH solution could reduce the investment of liquid desiccant by up to 931 USD/100 kg, and dramatically alleviate the corrosion as well. In addition, a correlation to predict the mass transfer coefficient was developed and validated with the mean absolute relative deviation of only 3.92%.
Article
Air conditioning (AC) systems demand a significant portion of the total energy consumed by the building sector. Conventional AC based on vapor compression refrigeration (VCR) is neither energy efficient nor environment-friendly due to its method of humidity control and use of refrigerants with global warming potential respectively. Liquid desiccant air conditioning system (LDAS) is a promising alternative to VCR. This review provides a comprehensive overview of the developments in LDAS so far. It explains the principle of operation and classification in detail. The various developments in dehumidifier, regenerator, desiccant material, and mathematical modelling are discussed. The various types of performance parameters, and the design criteria and effect of operating parameters are also detailed. Finally, the climate feasibility, performance control strategies and indoor air quality are explained. This communication will be useful to identify the research gaps to explore new pathways for future research to further improve the efficiency of LDAS.
Article
Membrane-based liquid desiccant air dehumidification (MLDAD) is a separation process driven by the vapor pressure difference. Different from the conventional liquid desiccant air dehumidification, MLDAD uses semi-permeable membranes to separate the processing air and desiccant liquid so that only water vapor molecules in the air side can transfer through the membrane and be absorbed by the solution. In this process, the vapor is removed from the air stream, and the carryover of solution droplets in the air is also able to be eliminated. This review addresses the characteristics of liquid desiccants and membranes, the design of MLDAD modules and systems, the performance assessment and comparison of the dehumidification and regeneration module, as well as the system-level energy analysis. State-of-the-art research results are presented, and the future needs for this promising technology are discussed.
Article
Compared with the extensive attention drawn by salt solutions, the promising potassium formate (KCOOH) solution which has smaller causticity and lower price receives little attention in the field of liquid desiccant cooling system (LDCS). Consequently, the present paper studied both the thermal properties and mass transfer performance of KCOOH solution for its practical engineering application. The vapor pressure and density of KCOOH solution under different concentrations and temperatures were measured and then fitted in the form of polynomials with the mean absolute relative deviation (MARD) of 0.06% and 0.551% respectively. Referring to the data in the existing literature, the heat capacity of KCOOH solution was also fitted with the MARD of 0.15%. Mathematical models of dehumidification/regeneration with the film shrinkage on the plate dehumidifier/regenerator were developed and validated by the experimental data. Results showed that the wetting ratio predicted by the model matched well with the measured one with the relative difference of 3.4%. Besides, the dehumidification/regeneration performance obtained by the model in terms of absolute moisture change was also in line with the experimental ones with the MARD of 6.05%/4.2%. In summary, the correlations of KCOOH solution thermal properties, mathematical model, and experimental results were valuable for the engineering application of KCOOH solution in LDCS.
Article
Heating and cooling systems around the globe are the largest energy consumer and with the ever-increasing population and development the need for heating and cooling systems is increasing. Control systems in solar assisted desiccant cooling are especially important as they allow the solar fraction to be increased when paired with thermal storage. Low cost efficient thermal storage methods are also important in increasing the efficiency of solar assisted desiccant cooling. Transient System Simulation tool (TRNSYS) simulation is presently used for estimation of energy use in building. It is targeted to include solar desiccant based evaporative cooling using MATLAB so that it can be readily used for control. Building Energy Performance Scenarios - BEPS Tools (Energy Plus, TRNSYS, ESP – r, Mathcad) do not provide sub-models for proper control mechanism. So MATLAB co-simulator could be targeted in order to control a TRNSYS simulation. The use of TRNSYS-MATLAB co-simulator as a relevant tool is discussed along with its applications for the system. A comparison of TRNSYS and MATLAB to other building energy performance simulators is then provided to evaluate the performance of solar desiccant cooling for hot and humid region.
Article
Liquid desiccant cooling systems are considered a promising technology for accurate humidity control and high energy efficiency. The dehumidifier and the regenerator are the two main components in the system, and their performance directly determines the system performance. This paper is a comprehensive review of the empirical correlations for the determination of mass transfer coefficient and moisture effectiveness in both adiabatic and internal cooling/heating dehumidifier/regenerators, and it further discusses approaches to enhance their mass transfer performance. These methods include structural improvements, such as structural modification, ultrasound atomisation and membrane-based modules, and modification of liquid desiccants, such as the addition of surfactants and nanoparticles. Finally, a brief summary and some suggestions for future work are outlined and addressed.
Article
Salinity gradient between two solutions can be transformed to an electrical power by reverse electrodialysis (RED) process which comprises of alternatively cation exchange membrane (CEM), dilute solution, anion exchange membrane (AEM) and concentrate solution. Although spacers are generally equipped between the membranes to keep their intermediate distance, the presence of them also have a significant effect on RED stack resistance, often called as spacer shadow effect. In this study, the spacer shadow effects on both membrane and solution compartment resistances are investigated by using 16 spacers having different geometries with 56-84 % porosity and 0.100-0.564 mm thickness. In addition, we search and propose the suitable spacer geometric parameter for estimating these spacer shadow effects. The results indicate that the spacer shadow effect on membrane has a good correlation with a parameter consisting of area fraction and diameter of spacer filaments, whereas, a parameter consisting of both area and volume fractions of the spacer agreed well with those on solution compartment in a wide range of spacer geometric condition. These results are useful for not only estimating the spacer shadow effect on both membrane and solution resistances for enhancing the subsequent RED power output performance, but also designing a suitable spacer geometry specified for RED.
Article
This study first fabricated a new mixed liquid desiccant solution via the addition of hydroxyethyl urea to lithium chloride solution to reduce its causticity in a metal-based regenerator. The formula of the new solution—25% LiCl, 39% hydroxyethyl urea, and 36% water—was determined according to the vapor pressure measured with the static method. Accordingly, its basic thermal properties (e.g., density, viscosity, and conductivity) were measured and compared with those of a 35% LiCl solution. The corrosion characteristics of the mixed solution and the LiCl solution were analyzed with an electrochemical testing method. The regeneration performances of the solutions were experimentally studied, and the results show that the new solution significantly reduced causticity due to the addition of hydroxyethyl urea and a reduction in LiCl concentration. The regeneration effectiveness showed an average relative increase of 14.1% because of the larger wetting ratio and greater fluctuation of falling film. The wetting ratio increased from 81.5% to 87.8%, and the standard deviation of the film thickness increased from 25.441 μm to 31.672 μm with more rigorous fluctuations. Finally, an empirical equation for regeneration effectiveness was developed with an average absolute relative deviation of 4.01%, which provides a useful guide for the design of regenerators.
Article
Hollow fibres have been studied as the substitute for metallic materials due to the advantages such as light weight, corrosion resistant and low cost in heat and mass transfer applications. A novel polymer hollow fibre liquid desiccant dehumidification system, in which a cross-flow hollow fibre module (fibre inside diameter = 1.4 mm) serves as the dehumidifier, is presented in this paper. This novel hollow fibre integrated liquid desiccant dehumidification system can be used in an air conditioning system to provide a comfortable indoor environment for hot and humid area. Compared with other conventional liquid desiccant dehumidifier, the polymer hollow fibre has a very small diameter which leads to significantly increased surface area. Moreover, the porous feature of the hollow fibre module can help to eliminate any liquid desiccant droplets carryover into the process air. As a less corrosive and more environmental friendly working fluid, aqueous potassium formate (KCOOH) solution has been selected. The dehumidification performance of the proposed system were analysed experimentally under the conditions of incoming air temperature in the range of 30–45 °C. The variations of dehumidification sensible and latent effectiveness, moisture removal rates were studied by varying the incoming air velocity from 0.65 m/s to 4.5 m/s. With the various values of incoming air relative humidity in the range of 55–75% and the solution concentrations between 36% and 62%, the experimental obtained latent effectiveness are in the range of 0.25–0.43 and the sensible effectiveness are in the range of 0.31–0.52, which is in a satisfactory agreement with the empirical correlation of effectiveness-NTU in the literature.
Article
Liquid desiccant dehumidification system has gained much progress recently for its considerable energy saving potential without liquid water condensation. Within the system, regeneration is of great importance since diluted desiccant solution after dehumidification needs to be re-concentrated. The operational characteristics of a membrane-based flat-plate heat and mass exchanger used for liquid desiccant regeneration are investigated in this study. The liquid desiccant and air are in a cross-flow arrangement, and separated by semi-permeable membranes to avoid carry-over problem. The regeneration performance is examined by numerical simulation and experimental test. Solution side effectiveness, temperature decrease rate (TDR) and moisture flux rate (MFR) are applied to evaluate heat and mass transfer in the regenerator. Effects of main operating parameters are assessed, which include dimensionless parameters (i.e. number of heat transfer units NTU and solution to air mass flow rate ratio m∗), solution inlet properties (i.e. temperature Tsol,in and concentration Csol,in) and air inlet conditions (i.e. temperature Tair,in and humidity ratio Wair,in). It is found that m∗ and NTU are two of the most important parameters and their effects on the regeneration performance are interacted with each other. There is hardly benefit to the performance improvement by increasing NTU at low m∗ or increasing m∗ at low NTU. Even though the regeneration performance can be improved by increasing m∗ and NTU, its improvement gradient is limited when m∗ and NTU exceed 2 and 4 respectively. It is also found that increasing solution inlet temperature is an effective approach to enhance the regeneration performance, while air inlet temperature and humidity ratio have negligible effects on it.
Article
In an effort to minimise electricity consumption and greenhouse gases emissions, the heating, ventilation and air-conditioning sector has focused its attention on developing alternative solutions to electrically-driven vapour-compression cooling. Liquid desiccant air-conditioning systems represent an energy-efficient and more environmentally friendly alternative technology for dehumidification and cooling, particularly in those cases with high latent loads to maintain indoor air quality and comfort conditions. This technology is considered particularly efficient in hot and humid climates. As a matter of fact, the choice of the desiccant solution influences the overall performance of the system. The current paper reviews the working principle of liquid desiccant systems, focusing on the thermodynamic properties of the desiccant solutions and describes an evaluation of the reference thermodynamic properties of different desiccant solutions to identify which thermodynamic, physical, transport property influences the liquid desiccant process and to what extent. The comparison of these thermodynamic properties for the commonly used desiccants is conducted to estimate which fluid could perform most favourably in the system. The economic factors and the effect of different applications and climatic conditions on the system performance are also described. The paper is intended to be the first step in the evaluation of alternative desiccant fluids able to overcome the problems related to the use of the common desiccant solutions, such as crystallization and corrosion to metals. Ionic liquids seem a promising alternative working fluid in liquid desiccant air-conditioning systems and their characteristics and cost are discussed.
Article
The large electricity consumption due to the extensive use of vapour compression machines in the air conditioning and industrial sectors dramatically increases the emissions of greenhouse gases. Besides, the synthetic halocarbon refrigerants used in said machines are responsible for ozone depletion. These environmental concerns give an opportunity to solar thermally-driven liquid sorption systems, whose energy consumption primarily lies on the regeneration heat of liquid sorbents. This paper surveys both theoretical and experimental studies on solar thermal regeneration methods of the hygroscopic solutions used in liquid sorption systems. A brief review of some conventional and alternative liquid desiccants utilised in said systems is firstly presented. Furthermore, information about several configurations of regenerators and their performance is covered in detail, putting special emphasis on solar collector/regenerators, which use directly solar energy to reconcentrate the weak desiccant solution. A comparative assessment of relevant thermophysical properties of some hygroscopic liquids is realised in order to determine their suitability for use in sorption systems. The results show that the aqueous solutions of potassium formate and ionic liquids are very promising desiccants due to their low vapour pressures, specific heat capacities and dynamic viscosities, as well as their non-corrosiveness. A performance comparison between typical regeneration units and solar collector/regenerators is also carried out. It could be shown that solar collector/regenerators have a high potential for enhancing the water desorption rate from a diluted desiccant solution and, consequently, the capacity of the air dehumidification process within the absorber.
Article
The paper describes the development of a hybrid solution that improves air dehumidification. It comprises the integration of a composite desiccant and a nano-woven membrane for air dehumidification. The solution compliments any building HVAC project where the removing moisture from the air via an energy-efficient means is a concern. Longer sustainable performance of the desiccant is achieved as the non-regenerative membrane assists in partial air dehumidification. Accordingly, the hybrid system requires a lower regenerating temperature while producing air of very low humidity. In sum, the proposed hybrid solution involves the composite desiccant and membrane to work hand-in-hand in order to achieve enhanced moisture removal efficiency and improved energy efficiency by up to 40% compared to the best grade commercial silica-gel desiccant.
Article
The desiccant-enhanced dew point evaporative cooling system is proposed as a potential alternative for air conditioning. It enables independent control of the air humidity and temperature so that latent and sensible heat loads are judiciously decoupled. In this paper, we carried out a thermodynamic analysis on a hybrid membrane liquid desiccant dehumidification and dew point evaporative cooling system. A cross-flow membrane liquid desiccant dehumidifier and a counter-flow dew point evaporative cooler were developed and investigated. Experiments were conducted to demonstrate the ability of the dehumidifier and cooler to achieve sufficient moisture removal and temperature reduction. The supply air could be regulated to the suggested thermal comfort zone of 20.0–28.0 °C temperature with humidity ratio of below 12.0 g/kg. A detailed mathematical model was formulated by considering the momentum, continuity, energy and species equations. The model was able to predict well the experimental data with a maximum discrepancy of ±5.0%. Key findings that emerged from this study include: (1) the proposed system was able to deliver the product air at 18.3 °C temperature and 10.9 g/kg humidity under nominal conditions; (2) the channel heights of the dehumidifier and cooler should be maintained at 2.0 and 3.0 mm, with their channel lengths at 0.2 and 0.6 m, respectively, to achieve thermal comfort conditions; and (3) a solution flow ratio of 2.7 with the membrane diffusion resistance not larger than 20 s/m achieved sufficient humidity reduction of more than 9.0 g/kg.
Article
Operating parameters of a membrane-based parallel-plate liquid desiccant dehumidification system are investigated in this paper. The liquid desiccant and air are in a cross-flow arrangement, and separated by semi-permeable membranes to avoid carry-over problem. A numerical model is developed to simulate the system performance, and validated by experimental and analytical results. Impacts of main operating parameters on the system performance (i.e. sensible, latent and total effectiveness) are evaluated, which include dimensionless parameters (i.e. solution to air mass flow rate ratio m^* and number of heat transfer units NTU), solution properties (i.e. concentration C_sol and temperature T_sol) and inlet air conditions (i.e. temperature T_(air,in) and relative humidity 〖RH〗_(air,in)). It is found that m^* and NTU are two of the most important parameters influencing the system effectiveness. Even though the system performance can be improved by m^*and NTU, its increasing gradient is limited when m^*and NTU exceed 1 and 4 respectively. Decreasing solution temperature does not make a great improvement to the system performance, however, increasing solution concentration is a good approach to enhance the latent effectiveness without influencing the sensible effectiveness. The system shows the broad adaptability in various weather conditions, and has the ability to provide relative stable state supply air.
Article
This paper presents the performance investigation of a solid desiccant dehumidifier integrated with Maisotsenko Cycle (M-Cycle) based cross flow heat and mass exchanger (MC-DAC). The experimental test rig consisting of a silica gel based desiccant wheel and a heat recovery wheel is coupled with M-Cycle indirect evaporative cooler. The effect of wide range of inlet air parameters such as ambient temperature, humidity ratio, and regeneration temperature on the performance of the integrated system was analyzed and compared with the conventional desiccant air conditioning (DAC) system. Set of experiments were carried out for both systems at constant process as well as return air mass flow rates under different operating conditions. MC-DAC system was observed to be around 60-65% more efficient than the other system in terms of COPth providing same supply air conditions at low regeneration temperatures.
Article
A novel transport model is developed for a space conditioned by a cooled liquid desiccant dehumidification membrane ceiling (LDMC-C) and displacement ventilation DV in which the space is divided into three zones: an occupied cool with fresh air zone; an upper recirculation zone; and a ceiling adjacent boundary layer zone where the air is drawn at the exhaust grill. The adjacent air boundary layer at the ceiling membrane predicted the latent and sensible heat transfer to the desiccant solution. The boundary model was validated experimentally in a climatic chamber. The developed model predicted of the LDMC-C/DV system operational parameters such as the supply flow rate and temperature and the liquid desiccant concentration, flow rate, and inlet temperature that meet thermal comfort and air quality requirements. Extensive simulations were performed on a typical office load of 75 W/m² to generate design charts of the system for three DV supply air temperatures of 18, 20, and 22 °C to identify the appropriate DV flow rate and inlet desiccant (CaCl2) solution temperature that provide comfort and air quality and ensures that condensation is unlikely to occur. The LDMC sensible and latent load removal from the space load is directly read off the charts.
Article
The main purpose of this research was to empirically investigate the removal of volatile organic compounds (VOCs), airborne particles, and microorganism contaminants from air conditioned by a liquid-desiccant (LD) dehumidifier. The dehumidifier was used in an existing liquid-desiccant evaporative cooling–assisted 100% outdoor air system (LD-IDECOAS) using both direct and indirect evaporative coolers. The ventilation efficiency of the LD-IDECOAS was also measured via a tracer-gas test with carbon dioxide. The concentrations of two representative VOCs, toluene and formaldehyde, the number of particles with various sizes, and the colony-forming units of microorganisms such as bacteria and fungi in the process air were measured before and after liquid-desiccant dehumidification. Then, the measurement results were compared with those acquired by bypassing the liquid-desiccant unit in order to verify the impact of the liquid desiccant on air-contaminant removal. The experimental results showed that the removal efficiency of the LD unit was 36% for toluene, and 29.6% for formaldehyde. The particle-removal efficiencies measured as part of this research were 48.2%, 64.1%, and 84.9% for the particle sizes of 2.5–5 μm, 5–10 μm, and 10–25 μm, respectively. The bacteria-removal efficiency was 77.5% and 81.3% for the sampling process air of 200 and 500 L, respectively, while the fungi removal efficiency was 38.8% and 44.4% for 200 and 500 L of sampling process air, respectively. In addition, the LD-IDECOAS provided an air change rate approximately 3.1 times higher than that of the VAV system when both systems delivered the supply air to the test chamber at the same rate.
Article
In a global era, continuous increment in energy requisition with its associated cost and relevant climate problems is causing accentuation in exploring more efficient ways to provide air conditioning in enclosed space without degradation of the environment. In the hot and humid area, major part of conventionally produced electrical energy is consumed by air conditioning. Also in the rapidly growing world scarcity of clean water is dilemma as equal as greenhouse and ozone layer depletion. Thus, there is emergence of dehumidified air for human comfort conditions with having potable clean water same time. In summer with largely available solar radiation, use of hybrid solar system is viable option to overcome problems of dehumidification of liquid desiccant as well as of potable water. Up till now the conventional dehumidification with desiccant solution has been improved or replaced by less costly new systems. This paper provides an extensive literature review on development of liquid desiccant regeneration using solar energy. The paper also includes the recent findings of hybrid solar system in which either two sources of heat is used for regeneration of liquid desiccant or solar energy is used for regeneration of liquid desiccant along with other application.
Article
A membrane-based liquid desiccant dehumidification system is one of high energy efficient dehumidification approaches, which allows heat and moisture transfers between air stream and desiccant solution without carryover problem. The system performance is investigated experimentally with calcium chloride, and the impacts of main operating parameters on dehumidification effectiveness (i.e. sensible, latent and total effectiveness) are evaluated, which include dimensionless parameters (i.e. solution to air mass flow rate ratio and number of heat transfer units ) and solution properties (i.e. concentration and inlet temperature ). The sensible, latent and total effectiveness reach the maximum values of 0.49, 0.55, and 0.53 respectively at = 3.5 and = 12, and these effectiveness are not limited by and when > 2 and > 10. Both the latent and total effectiveness increase with , while almost no variation is observed in the sensible effectiveness. All effectiveness can be improved by decreasing . The experimental data provide a full map of main design parameters for the membrane-based liquid desiccant air conditioning technology.
Article
Liquid desiccant dehumidification has received much attention in recent years due to its effectiveness in humidity control and great potential in energy saving of buildings. At the same time, indoor air quality (IAQ) problems associated with liquid desiccant dehumidification also aroused people's attention since IAQ has a great significance for human health and life. This paper focuses on the impact of liquid desiccant dehumidification on IAQ and reviews recent achievements and progress in this respect. The recent researches in this field are reviewed on: (1) The removal capability of liquid desiccant solution for volatile organic compounds. (2) The remove, filter, kill or deactivation effect of liquid desiccant solution on bacteria and virus. (3) The capture function of liquid desiccant dehumidification system for particulate matter. (4) The carryover of liquid desiccant.
Article
A membrane-based liquid desiccant dehumidification cooling system is studied in this paper for energy efficient air conditioning with independent temperature and humidity controls. The system mainly consists of a dehumidifier, a regenerator, an evaporative cooler and an air-to-air heat exchanger. Its feasibility in the hot and humid region is assessed with calcium chloride solution, and the influences of operating variables on the dehumidifier, regenerator, evaporative cooler and overall system performances are investigated through experimental work. The experimental results indicate that the inlet air condition greatly affects the dehumidification and regeneration performances. The system regeneration temperature should be controlled appropriately for a high energy efficiency based on the operative solution concentration ratio. It is worth noting that the solution concentration ratio plays a considerable role in the system performance. The higher the solution concentration ratio, the better the dehumidification performance. However simultaneously more thermal input power is required for the solution regeneration, and a crystallization risk in the normal operating temperature range should be noted as well. The system mass balance between the dehumidifier and regenerator is crucial for the system steady operation. Under the investigated steady operating condition, the supply air temperature of 20.4°C and system COP of 0.70 are achieved at a solution concentration ratio of 36%.
Article
Membrane-based liquid desiccant dehumidification has attracted increasing interests with elimination of solution droplets carryover problem. In this study, a membrane-based hybrid liquid desiccant dehumidification cooling system is developed, which is mainly composed of a dehumidifier, a regenerator and an evaporative cooler. The system is capable to remove latent load by the liquid desiccant dehumidification unit and simultaneously to handle sensible load with an evaporative cooling unit. This paper presents a performance evaluation study of the hybrid system with calcium chloride as liquid desiccant based on experimental data. Series of tests are conducted to identify influences of operating variables and conditions (i.e. desiccant solution concentration ratio, regeneration temperature, inlet air condition, etc.) on the system performance. The experimental results indicate that the system is viable for dehumidification cooling purpose. Furthermore, it is noteworthy that mass balance between the dehumidifier and regenerator should be achieved for system steady operation. Thermal COPth of 0.70 and electrical COPel of 2.62 are achieved respectively under steady operating condition at CaCl2 concentration ratio of 36%.
Article
One of the major obstacles to improving solar thermal cooling technologies is the high operating temperature requirements of most solar thermal cooling systems. This paper reviews recent advances that could reduce the required heat source temperatures for solar desiccant cooling to the range of 50°C–60°C. These approaches include (i) isothermal dehumidification (e.g. two-stage dehumidification or internal cooled dehumidification) and (ii) pre-cooling of the entry air with ambient heat sinks (e.g. indirect evaporative cooling or geothermal exchange). These techniques can potentially leads to a more thermodynamically efficient solution for utilising recovered heat from flat plate photovoltaic thermal (PV/T) collectors for desiccant regeneration.