Liquid Desiccant Dehumidification System Research Articles

A Review of Internal Cooling Strategies in Liquid Desiccant Dehumidification and Cooling Systems

Air conditioning systems have become an integral part of society, and their utilization is increasing day by day. Most air conditioning systems use the vapor compression refrigeration (VCR) principle, which is a 190-year-old technology and has undergone significant changes to improve its performance and efficiency. However, the major drawback of VCR is its high energy consumption and use of highly polluting refrigerants. In this 21st century, energy conservation and environmental protection are the need of the hour. To mitigate the challenges, the search for alternative cooling systems that are eco-friendly and have less energy consumption is gaining wide attention. Liquid desiccant systems are gaining popularity due to less energy consumption and being eco-friendly. Therefore, this study attempts to review the recent developments in liquid desiccant technology by focusing on internal cooling strategies. As the internal cooling improves the overall performance of the liquid desiccant by controlling the liquid temperatures. Therefore, this review highlights the various methods, configurations and mathematical simulations in internal cooling, which are less explored in the available literature.

Optimizing the amount of solution in packed liquid desiccant systems for sustained dehumidification

Liquid desiccant dehumidification technology, driven by renewable energy, presents an energy-efficient solution to dehumidification. However, a large solution amount is always configured to extend the dehumidification process and address the unavailability of renewable energy. This is due to a lack of design guidance regarding the optimal desiccant solution amount. This study aims to address this gap by offering guidelines, with a particular focus on the dehumidification duration. Initially, a benchmark for the solution amount is defined for convenient quantification in further study. Then, a theoretical analysis of the dehumidification degradation process is conducted. Results reveal that dehumidification duration is primarily constrained by the temperature rise of the desiccant solution, rather than the solution amount. Consequently, a natural cooling unit, instead of mechanical cooling, is introduced to remove the temperature rise. Following this guideline, multiplying the benchmark for the solution amount can significantly extend the dehumidification duration from minutes to hours, while increasing it by tens of times can extend further to days. Furthermore, the solution amount can be reduced by over 90 % with the design guideline compared to the traditional method. This study paves a way to significantly reduce the solution amount and relevant costs for liquid desiccant dehumidification systems.

Application of biomass-based wood shaving packing in a liquid desiccant dehumidification system – a source of sustainable energy

ABSTRACT The present work focuses on the construction of a counter flow dehumidification test rig where calcium chloride liquid is used as the desiccant and is made to spray on the novel organic packing made of wood shaving with different densities and porosities. The mass flow rate of the air is varied and the performance parameters such as moisture removal rate (MRR), dehumidification effectiveness, coefficient of performance (COP), sensible heat factor (SHR) and mass transfer coefficient have been investigated. Results demonstrated an increase in the relative humidity, mass transfer coefficient and drop in the COP, MRR and dehumidification efficiency with the increase in the flow airflow rate. On increasing the CaCl2 concentration, dehumidification performance parameters such as MRR, COP and mass transfer coefficient are found to be increased. Out of the three different densities tested, 500 kg/m3 shows best performance with MRR, ΔRH and dehumidification effectiveness being 9.7%, 18.7% and 4.9% lower than the Celdek pad when measured at 5 m/s of air velocity for 30% desiccant concentration. Optimisation of the input parameters has been done and the results revealed that air velocity should be maintained within the range of 4–5 m/s to minimise energy consumption and optimum performance.

Solar driven liquid desiccant dehumidification system: Measurements and annual system simulations

Liquid desiccant air conditioning (LDAC) systems are used for dehumidification of air to low dew point temperatures. In the presented study a TRNSYS model of a LDAC unit is developed and validated with laboratory measurements. This model is coupled with a solar thermal system that provides heating water for the regeneration process. With this model dynamic annual system simulations can be carried out to simulate the dehumidification of a building. The solar fraction is evaluated for various infiltration rates of the building and solar collector areas with an internally cooled as well as an adiabatic absorption process.The simulation results reveal that high solar fractions of 70% to 81% can be achieved when dimensioning the solar thermal system to cover the regeneration heat demand on a good summer day. This is due to the good correlation of solar irradiance and dehumidification load. Without internal cooling of the absorption process (adiabatic conditions), the solar fraction is reduced by about 10percent points for the same size and operating conditions.

Study on the heat transfer characteristics of CO2 and liquid desiccant CaCl2 aqueous solution in the gas cooler and evaporator

The heat exchangers are essential components in the CO2 heat pump-driven liquid desiccant dehumidification system since the heat transfer performance of heat exchangers directly affects the energy efficiency of the system. However, the heat transfer characteristics of CO2 and liquid desiccant in the heat exchanger have yet to be fully revealed, resulting in a lack of appropriate strategies to improve the heat transfer performance. To address this issue, numerical gas cooler and evaporator models are developed and verified. The effects of parameters such as the weight concentration of the CaCl2 aqueous solution, the CO2 inlet pressure, mass flow rates, and the length of the heat exchanger on the heat transfer performance are investigated respectively under various operating conditions. Results show that the heat exchange load of the gas cooler can be significantly impacted by various inlet parameters of both fluids. In particular, the increase in the weight concentration from 36 to 54 wt.% deteriorates the heat exchange load by over 15 %. Nevertheless, the heat exchange load of the evaporator is hardly influenced by the weight concentration. This is attributed to a great gap in the two fluids’ heat capacity rates, causing the evaporator to be in the maximum heat transfer state. The sign of the maximum heat transfer state of the heat exchanger is whether the thermal effectiveness is close to 1, which is related to the heat transfer area. Therefore, modifying the heat exchanger length could avoid the thermal effectiveness approaching 1. Results indicate that the appropriate length of the evaporator is 3–4 m under the designed operating conditions.

Study on the optimal solution recirculation ratio of liquid desiccant dehumidification system based on matching rate evaluation

The recirculation of dehumidification and regeneration solutions has shown promising moisture removal rates and energy-saving effects in liquid desiccant dehumidification systems. The different combinations of dehumidification solution recirculation ratio and regeneration solution recirculation ratio under varying operating conditions can have different impacts on the system. Nevertheless, the system performance under full operating conditions has not been comprehensively studied. To address this research gap, this study proposes a liquid desiccant dehumidification system with a solution recirculation structure, defines a new evaluation index — matching rate, and conducts a parameterized study and optimization of the steady-state performance of the liquid desiccant dehumidification system with a solution recirculation structure within the range of solution recirculation ratios under all operating conditions, by integrating the system cooling capacity and energy effectiveness ratio. The performance of the system is studied by varying seven key parameters: dehumidification solution recirculation ratio, regeneration solution recirculation ratio, total solution mass flow rate, dehumidification solution inlet temperature, regeneration solution inlet temperature, and ambient air temperature-humidity ratio. The results show that under design conditions, the solution recirculation ratio within the full operating range can process air to a range of 17.8℃～18.5℃ and 12.5 g/kg～17.4 g/kg. The maximum system cooling capacity, energy effectiveness ratio and matching rate values are 21.5 kW, 0.21 and 0.42 respectively. The recommended (dehumidification solution recirculation ratio, regeneration solution recirculation ratio) schemes for high, medium and low demand of system cooling capacity are (0.7～0.9, 0～0.5), (0.4～0.6, 0～0.7) and (0～0.3, 0.4～0.7) respectively, based on the requirements of system cooling capacity.

Membrane-based packed-sheet liquid desiccant dehumidification system

Humidity control plays a vital role in both buildings and greenhouses. The thermally-driven liquid desiccant dehumidification systems are getting an increasing traction because of their high moisture removal capacity and the possibility of their integration with waste-heat or renewable energy sources. In this work, a novel design concept for liquid desiccant dehumidification systems is proposed to overcome the practical challenges of the conventional systems, such as solution carryover, crystallization, and corrosion. The presented system utilizes a compact “packed-sheet” sorption bed that houses spherical membrane-based micro-absorbers with LiBr as a liquid desiccant. A bench-scale system is designed and tested under the typical dehumidification working conditions. The experimental results showed that the proposed design has up to two-fold higher moisture removal rate per volume (MRR = 75 g/s-m3) than a conventional LiBr liquid desiccant dehumidification system (MRR = 35 g/s-m3). In addition, a one-dimensional coupled heat and mass transfer mathematical model is developed to simulate the transient behavior of the proposed system. An optimization study, using the validated model, suggested that the performance of proposed design can be maximized to realize moisture removal rates of up to 135 g/s-m3 (270% higher than the conventional liquid desiccant systems) with a coefficient of performance of 0.25.

Performance characteristics of a compact self-circulating liquid desiccant air dehumidification system coupled with a heat pump: A comprehensive parametric study

Integration of a heat pump with a liquid desiccant system for air dehumidification has received growing attention in recent years in view of its more compact size and lower electricity consumption. This paper presents a compact combined heat pump and self-circulating liquid desiccant air dehumidification system targeted at small cooling load applications. The compactness lies in the integration of the dehumidifier and evaporator as well as the integration of the regenerator and condenser. Besides, the design of solution self-circulation (SSC) is intended for energy savings and the auxiliaries are employed to help achieve energy matching between the heat pump and the air dehumidification system. First, a comparative study is performed to identify the more energy efficient one of the two strategies for energy matching. Based on this, a thermodynamic model-based parametric study is conducted to investigate the effects of various influencing factors on the energy matching relationship and performance of the proposed system, as well as the energy-saving potential relative to the basic system without SSC. The results show that there exists an optimum solution self-circulation ratio (SCR) to achieve the maximum COP under a given condition. Basically, the proposed system with a proper SCR could yield a significant energy-saving effect regardless of the performance variations under studied operation conditions. It is found that a reduction of 29.68 % in energy consumption and an improvement of 42.20 % in COP are achieved under the ambient condition of 30 ℃ temperature and 20 g/kg humidity ratio. Furthermore, a correlation analysis is used to reveal the association strength and direction between any two main operation and performance parameters. Finally, the heat recovery is proved to effectively reduce the cooling coils load. This paper provides an in-depth understanding of the energy matching relationship and demonstrates the energy-saving performance benefiting from SSC for future practical applications of the proposed system.

Study on the optimal circulation structure of liquid desiccant dehumidification system based on composite model

Liquid desiccant dehumidification systems have demonstrated good energy savings and dehumidification effects in extremely hot and humid areas. However, to improve system performance, the matching problem between the desiccant-regeneration solution recirculation ratio and system performance has not been thoroughly investigated. Therefore, in this study, we constructed a liquid desiccant dehumidification system with solution recirculation and set four circulation modes: NS mode without solution recirculation, CS mode with only desiccant solution recirculation, RS mode with only regeneration solution recirculation, and CRS mode with both desiccant and regeneration solution recirculation. The orthogonal test method was used to determine the influence of solution recirculation on the evaluation indicators, and multi-objective optimization of the major parameters under the four circulation modes was performed based on the NSGA-II and TOPSIS algorithms. The results showed a clear conflict between the moisture removal rate (MRR) and energy consumption (Q). In extremely hot and humid areas, a high air flow rate is suitable for use in the regenerator to process small-flow-rate solutions or set larger sizes. Under typical environmental conditions, it is recommended to set up a liquid desiccant dehumidification system with a solution recirculation structure: when MRR demand is high, the CS mode should be used, where MRR is 6.72 g/s; when Q demand is high, the RS mode should be used where Q is 35.34 kW; considering both MRR and Q effects, the CRS mode should be used where unit energy consumption moisture removal rate is 0.1196 g/(s·kW).

Numerical investigation and comparison on energy performance and exergy destruction of cascading deep dehumidification systems with dehumidification amount exceeding 95%

Liquid desiccant and desiccant wheel systems are feasible approaches for the dehumidification process in modern buildings. Some specialized buildings and industrial fields have the demand for deep dehumidification process to produce supply air with low dew point temperature below 0 °C. In this study, the numerical investigation has been conducted to evaluate the energy and exergy performance of multiple cascading deep dehumidification systems assisted with liquid desiccant and desiccant wheel under a great dehumidification gradient. To process the air from 21 g/kg to 1 g/kg, the results indicate that the cascading liquid desiccant and desiccant wheel deep dehumidification system (LD-DW system) driven by two stages of heat pump exhibits the lowest demand of power consumption compared with the conventional dehumidification systems. It also shows less exergy loss and significantly higher exergy efficiency than other systems. The effects of influencing parameters on the dehumidification performance are also evaluated. The LD-DW system can effectively match the air state with the proper treatment dehumidification process, thus achieving a superior coefficient of system performance (COPsys) of 3.70, and it is an energy and exergy efficient method for advancing the deep dehumidification process.

Optimization of a multistage liquid desiccant dehumidifier by neural networks & particle swarm techniques

To overcome the inconsistent and limited dehumidification by single stage dehumidification system, a novel multistage, reciprocating dynamic liquid desiccant dehumidification system has been proposed to dehumidify larger space consistently. Experimental investigation on the proposed unit has been conducted by varying air velocity, cam shaft speed, and inlet relative humidity (RH). System's performance in terms of moisture removal rate (MRR), dehumidification efficiency (DE) and energy consumed has been compared with that of a single stage system. It is observed that multistage system performed better than the single stage system in terms higher MRR and efficiency. To optimise the system performance, neural network-based Particle Swarm optimization (PSO) has been employed. Inertia weight distribution for PSO has been obtained by the statistical Taguchi method. PSO optimized results for randomly distributed weights have been compared with the Taguchi given weights. It is found that the performance of the PSO with this method is better than the former in terms of optimized paramters. PSO predicted optimum input variables are found as air velocity of 8.2 m/s, inlet RH of 90% and cam speed of 15.16 rpm. The maximum performance parameters obtained are MRR of 7.549 g/s, Dehumidification efficiency of 0.762 and energy consumption of 2165.4 Watts. Confirmation experiments indicated that PSO predicted results are in well agreement with the experimental results with a maximum deviation of 1.8%. Current work predicts and optimizes the dehumidification performance which minimizes experimentation time and also the cost involved.

Study on the regeneration process and overall performance of a microencapsulated phase change material slurry dehumidification system

Building on previous findings that showed the efficacy of microencapsulated phase change materials (MicroPCM) in augmenting liquid desiccant dehumidification performance, this study endeavors to further explore the extent of the impact of MicroPCM in the regeneration and the entire dehumidification system. The results of the regeneration experiment on the microencapsulated phase change material slurry (MPCMS) reveal that the addition of MicroPCM impedes regeneration. Specifically, as the concentration of MicroPCM increases, both the regeneration rate and the MPCMS concentration after regeneration decrease, while the regeneration effectiveness exhibits an initial decline followed by an increase. An empirical correlation was proposed to predict the regeneration rate of MPCMS for their future application. The results showed that the addition of MicroPCM led to a decrease in the regeneration rate by as much as 48.1%, when compared to a pure LiCl solution. However, a simulation analysis of the entire system revealed that within a certain concentration range of MicroPCM addition, the coefficient of performance (COP) of the system could still be increased by up to 4.28%. This finding suggests that if a more suitable non-heating regeneration method were used, the addition of MicroPCM could further enhance the overall performance of the liquid desiccant dehumidification system.

Performance analysis and optimization of a coupled open and close liquid desiccant sorption system using LiCl solution

The efficient utilization of low-grade heat for air conditioning has been a topic of world concern, especially for low-grade heat below 80℃. In this study, a novel coupled open and close liquid desiccant sorption system driven by 70 ∼ 80℃ low-grade heat is proposed. This system employs a generator to provide strong solution for both the dehumidifier and absorber, achieving the air temperature and humidity independent control to reduce driving temperature and enhance system performance. A mathematical model is developed to simulate the proposed new system, and a comparative analysis is conducted with a traditional combined absorption refrigeration with liquid desiccant dehumidification system using LiCl solution. It is found that the COP of the new system is 0.76, which is higher than 0.65 of the traditional system when the hot water temperature is 75℃, the reason is that the new system avoids the sensible heat consumption of regenerated air which accounts for 17% of the total heat consumption in the traditional system. Furthermore, it is observed that the outlet concentration difference between the absorber and dehumidifier exists in the new system. The cycle is optimized to improve the concentration difference, leading to a 5.5% increase in the COP. After that, economic performance analysis shows a payback period of 2.65 years for a 150 kW cooling capacity system, with a reduction of 86.6 tons of CO2 emissions per year due to the decrease in power consumption.

A cleaner and more efficient energy system achieving a sustainable future for road transport

A novel integrated cooling system is developed for future medium to large electric vehicles by integrating the fuel cell, battery, metal-hydride, heat pump, and liquid desiccant dehumidification system to reduce power consumption and extend vehicle's driving range. The system benefits from the reuse of the normally wasted energy in the form of pressure difference and wasted thermal energy, from the hydrogen vessel, the fuel cell stack, and the battery pack. A numerical model for the proposed system and a finite element model for the dehumidifier and regenerator are developed and validated by experimental results from published data. A comprehensive evaluation of the impacts of the ambient air temperature and humidity, fuel cell current output, battery discharging C rate, and air mass flow rate on the Coefficient of Performance (COP), outlet air temperature, and cooling capacity is conducted. Two operating modes, namely non-compressive mode and heat pump supplemental mode are investigated and a detailed comparison between these two modes is undertaken. Furthermore, the proposed system under heat pump supplemental mode has been compared to other published cooling systems and dehumidification systems. Under non-compressive mode, results indicate that the proposed system can provide sufficient cooling capacity without the need of the compressor when the supply air mass flow rate is lower than 0.03 kg/s, under the specific operating situation. Under the heat pump supplemental mode, the proposed system can operate at 36 °C with a COP greater than 4, which is 56% higher than the cited published results, although the COP of the proposed system also considers battery cooling. Heat pump supplemental mode drastically reduces the 12-s insufficient cooling period that occurs at the beginning of the charging to discharging transition between the two metal hydrides to 2 s compared to non-compressed mode. Overall, this study provides a potential solution for future zero-emission vehicles by utilizing the heat and electric co-generation characteristic of the fuel cell, the isothermal characteristic of the metal hydride, and dehumidification and cooling characteristics of the liquid desiccant dehumidification system to extend the driving range of the electric vehicles and reduce energy consumption for cooling. Moreover, the proposed system can also provide domestic cooling loads and power by integrating the system into residential buildings.

Thermal modelling of potassium formate based cross-flow liquid desiccant dehumidification system

Thermal modelling of potassium formate based cross-flow liquid desiccant dehumidification system

Directional vapor transported and water-proof nanofibrous membranes for liquid desiccant dehumidification systems

The membrane-based liquid desiccant dehumidification system is a newly developed method in the field of air dehumidification. In this study, double-layer nanofibrous membranes (DLNMs) with directional vapor transport and water repellency for liquid dehumidification were fabricated by a simple electrospinning process. Specifically, the combination of thermoplastic polyurethane nanofibrous membrane and polyvinylidene fluoride (PVDF) nanofibrous membrane forms a cone-like structure in DLNMs, resulting in directional vapor transportation. The nanoporous structure and rough surface of PVDF nanofibrous membrane provide waterproof performance for DLNMs. Compare with the commercial membranes, the proposed DLNMs have a significantly higher water vapor permeability coefficient, which is as high as 539.67 g·μm m−2·24 h·Pa. This study not only provides a new route to construct a directional vapor transport and waterproof membrane, but also demonstrates the huge application prospect of the nanofibrous membrane formed by electrospinning in the field of solution dehumidification.

수용액 원심분사 기술이 적용된 액체식 제습 시스템의 제습 및 에너지 성능에 대한 실험적 연구

The main objective of this study was to analyze the dehumidification performance of a centrifugal atomization-based liquid desiccant (LD) dehumidification system and compare energy performance. Dehumidification rate and effectiveness were evaluated to determine dehumidification performance. A parametric study was performed experimentally according to five variables: air temperature, air humidity ratio, solution temperature, solution concentration, and liquid-to-gas ratio. Cooling load was also compared with a packed-bed-based LD dehumidification system via experiment results and simulations. Results indicated that the dehumidification performance was improved at low solution temperature and high solution concentration. The dehumidification rate was also higher in hot and humid air than in a mild air condition. As liquid-to-gas ratio increased, dehumidification effectiveness also increased while dehumidification rate decreased. Finally, the proposed centrifugal atomization-based LD system could achieve the same dehumidification performance at a lower liquid-to-gas ratio than packed-bed-based LD system while saving 72% system cooling load.

Steady-state performance assessment and geometrical optimization of packed-bed liquid desiccant air dehumidification systems

Steady-state performance assessment and geometrical optimization of packed-bed liquid desiccant air dehumidification systems

The design and experimental investigation of membrane-based tubular liquid desiccant dehumidification system

The design and experimental investigation of membrane-based tubular liquid desiccant dehumidification system

Performance assessment and optimization of liquid desiccant dehumidifier system using intelligent models and integration with solar dryer

Realistic performance predictions are necessary to control the liquid desiccant dehumidification systems effectively. In the present study, the application of artificial intelligence (AI) based artificial neural network (ANN), gene expression program (GEP) and adaptive neuro-fuzzy inference system (ANFIS) is investigated to estimate the dehumidifier effectiveness of desiccant dehumidifier systems while considering the effect of moisture removal rate and sensible heat factor as performance characteristics. It is found that the AI-based GEP model has the best prediction capability compared to the other developed AI models. In addition, the sensitivity analysis of independent parameters on the system performance parameters is estimated using the cosine amplitude method. The results demonstrate that the inlet desiccant temperature and specific humidity have a more substantial influence on the dehumidifier effectiveness and sensible heat ratio. Further, an algorithm involving the combination of multi-objective particle swarm optimization (MOPSO) with the GEP model is developed to optimize the input process parameters and to achieve better dehumidification performance. Lastly, based on obtained optimal operating conditions, a case study on a dehumidifier-integrated solar dryer is proposed for food/agriculture products drying applications using AI-based GEP-MOPSO model. It is also observed that the inlet water temperature and solar intensity have a significant impact on the useful heat gain of the solar collector.