Humidification-dehumidification Process Research Articles

Numerical study on the impact of fluid distribution on a counter-current direct contact evaporator

Humidification-Dehumidification (HDH) desalination units have been extensively studied during the past decades using macro-scale modeling, which relies on empirical closure correlations. Nonetheless, the HDH process remains relatively inefficient in terms of total energy requirements when compared with other desalination technologies such as Reverse Osmosis (RO) or Multi-Stage Flash (MSF). To improve the performance of direct contact evaporation, modeling of the transport phenomena at the small local scale is required. The objective of the present paper is to accurately model the hydrodynamic as well as heat and mass transfer as it typically occurs in a HDH counter-current direct contact evaporator. The numerical model is based on the Volume-of-Fluid (VOF) method included in ANSYS Fluent v19.2, which is modified to properly model diffusion driven evaporation. The study with multiple fluid distribution patterns shows a high increase in liquid hold-up and evaporated mass with an increase in water spray density, while the gas distribution does not have a noticeable impact on the performance of the evaporator column. The insight provided by the CFD model highlights the importance of considering the water distributor in conjunction with the packed column design for improving the performance.

Performance improvement of a water-purifying absorption cooler through humidification-dehumidification

An absorption heat pump that achieves water purification and air-conditioning with secondary water purification through humidification-dehumidification is developed. A thermodynamic model of this cycle is formulated and used to predict cycle performance. The cycle with humidification-dehumidification outperforms similar cycles using forward osmosis and membrane distillation at baseline operating conditions by 19% and 12%, respectively, because of the additional evaporator loads imposed by the humidification-dehumidification process. The performance of the cycle is insensitive to ambient temperature and humidity due to the high air flow rate associated with air conditioning. However, cycle performance is sensitive to the water load once the heat transfer fluid flow rates and heat exchanger sizes have been fixed, demonstrating a 12% decrease in performance when the water output ratio is changed from 7% below the baseline to 7% above the baseline. Designs that allow for on-design or below-design operation will allow excellent cycle performance.

Performance Evaluation of a Novel Hydrophobic Membrane Used in a Desalination System: A Comparison between Static and Moving Configurations

Water scarcity is growing and in particularly in regions where population is high. It is estimated by world wild life organization that two thirds of human population may face water shortage by 2025. However, the amount of water available on earth covers approximately two thirds of the total surface area, but most of the water is seawater. Seawater cannot be used for any human use due to the high salinity levels. Desalination processes have been implemented on various scales whereby reverse osmosis is the most successful. However, such system is too complex and expensive. An alternative system utilizing humidification-dehumidification process for desalination is proposed in this paper. The process involves the use of a novel hydrophobic membrane allowing the humidification. Two configurations have been tested in a closed loop cycle, namely: static and moving membrane. The results from the experiments have shown that the efficiency of the moving membrane configuration is higher than the static by 46%. And based on 1 Litre brine feed, 50% of the volume has been successfully desalinated.

A Novel Humidification Technique Used in Water Desalination Systems Based on the Humidification–Dehumidification Process: Experimentally and Theoretically

In this paper, an experimental and theoretical investigation is performed on a novel water desalination system based on a humidification–dehumidification technique using a heat pump. An ultrasonic water atomizer is used in the humidification process in order to improve the humidification system. In addition to that, a new configuration is employed in the humidification process (hybrid atomization system), which combines the traditional spraying atomization system and the ultrasonic water atomizer. The new humidification system performance is investigated and compared with the spraying water atomizer system in terms of humidification effectiveness and freshwater productivity. The results show that the ultrasonic water atomizer has enhanced and improved humidification effectiveness, and consequently, the productivity of freshwater. The maximum humidification effectiveness and productivity achieved by the ultrasonic water atomizer are 94.9% and 4.9 kg/h, respectively, meaning an increase of 25.2% and 18.8%, compared to the traditional spraying atomization system. The hybrid system increases humidification effectiveness and productivity by 3.8% and 8.2%, respectively, in comparison with the stand-alone ultrasonic water atomizer. A cost analysis was also carried out in this paper in order to perform an economic comparison of different humidification processes (spraying, ultrasonic; and hybrid atomization systems). The minimum production cost of one liter of freshwater amounts to $0.0311 with the spraying system, $0.0251 with the ultrasonic system, and $0.0250 with the hybrid atomization system. These results reveal the profitability of the new configuration.

Investigation of heat pump-driven humidification–dehumidification desalination system with energy recovery option

This paper presents a vapor compression heat pump-driven humidification–dehumidification process with energy recovery option. Two systems with different applications of recovered energy are proposed. System A recovers energy for preheating inlet feed water to the heat pump condenser, while system B recovers energy for preheating the humidifier inlet air. The system performance metrics such as gained output ratio (GOR), recovery ratio (RR), productivity, specific electrical energy consumption (SEEC) and the cost of freshwater production are investigated based on variations in mass flowrate ratios, humidifier effectiveness, seawater temperature, brine heat exchanger effectiveness, unit cost of electricity and expected plant life. The thermo-economic performance of the proposed system is compared with system “C” without energy recovery option. Findings reveal the need to recover thermal energy associated with the discharged brine for humidification and dehumidification desalination system having ineffective/poor component effectiveness, especially the humidifier. The proposed system shows that the cost of freshwater for system C can be reduced by about 15.23% with energy recovery in system A. The GOR of system C is improved by about 23.1%, whereas the RR, productivity and SEEC of system C are improved by about 23.3% each. The conditions at which the energy recover is effective are also shown in the results.

Experimental analysis of the humidification of air in bubble columns for thermal water treatment systems

The humidification-dehumidification process (HDH) for desalination is a promising technology to address water scarcity issues in rural regions. However, a low humidifier efficiency is a weakness of the process. Bubble column humidifiers (BCH) are promising for HDH, as they provide enhanced heat and mass transfer and have low maintenance requirements. Previous studies of HDH-systems with BCHs draw different conclusions regarding the impact of superficial air velocity and liquid height on the humidification. Furthermore, the impact of flow characteristics has never been investigated systematically at all. In this study, an optimized BCH test setup that allows for optical analysis of the humidifier is used and evaluated. Our test setup is validated, since the influence of water temperature on the humidification, which is exponential, is reproduced. Measurements with seawater show that the normalised system productivity is increased by about 56% with an increase in superficial air velocity from 0.5cm/s to 5cm/s. Furthermore, the system productivity is increased by around 29% with an increase in liquid height from 60mm to 378mm. While the impact of superficial air velocity can be traced back to temperature changes at the humidifier and dehumidifier outlets, the impact of liquid height is shown to be caused by a smaller heat loss surface in the humidifier with an increase in liquid height. For the impact of sieve plate orifice diameter, a clear influence on the humidification is not apparent, this parameter needs to be investigated further. Finally, our new test setup allows for analysing the humidification of air (1) in a systematic way, (2) in relevant measurement ranges and (3) in comparison with optical analyses of the flow characteristics.

Theoretical and experimental study of a seawater desalination system based on humidification-dehumidification technique

The technique of desalination seawater by humidification dehumidification process is considered a promising method for small capacity production plants. This process has many advantages such as operation at low temperature, ability to use the renewable energy sources and requirements of low technology level. The current work presents the results of a study on humidification dehumidification desalination system using the solar energy, in addition to a developed mathematical model consisting of the energy equations of each component to simulate the experimental work. The results show that there is a good agreement between the theoretical and experimental work. The results also reveal that there are effective values for the mass flow rate of air, mass flow rate ratio, and mass flow rate of feed water. The average productivity of the desalination system is 2.45 kg/h and the estimated cost per 1 L of fresh water is 0.047 US$. Finally, the results obtained from the present work are acceptable among those of previous studies.

Water desalination using solar continuous humidification–dehumidification process using hygroscopic solutions and rotating belt

In this study, solar energy is utilized for providing fresh water by purification of tab water using rotating belt utilizing the addition of hygroscopic solutions containing kaolin or calcium chloride. The function of the hygroscopic solution is to increase the surface area and thus the percentage of evaporation on the rotating belt. It was shown that the use of hygroscopic material enhances the production of fresh water to a significant level. Additionally, the use of kaolin has been shown to be superior to the use of calcium chloride. The use of kaolin in the proposed solar desalination device has increased solar desalination from 10.8 kg/m2·day of using pure tap water to 32.1 kg/m2·day. A mathematical model was established to the use of hygroscopic material in this solar desalination using rotating belt. It was found that the maximum system efficiency was 61% after utilizing kaolinite; approximately seven times more comparing to the efficiency without using kaolinite (8.7%).

Freshwater and cooling production via integration of an ethane ejector expander transcritical refrigeration cycle and a humidification-dehumidification unit

Despite the fact that humidification-dehumidification (HDH) desalination systems can be driven by various renewable or waste heat energies, many of the available renewable technologies are expensive in some parts of the globe. Hence, proposing and developing high-efficient mechanical-based HDH units can be an encouraging alternative which is more highlighted in recent investigations. In pursuance of this objective, three innovative mechanical-driven HDH units are simulated and the results are compared with each other. The simulated hybrid desalination system consists of a HDH unit and an ethane ejector expander transcritical refrigeration cycle (ethane-EETRC). The simulated hybrid systems can produce freshwater and cooling load, simultaneously. The main difference between each system is the proposal of using two-stage compression and humidification-dehumidification processes at different scenarios. The results display that a maximum freshwater of 17.3 m3/day can be achieved when a two-stage HDH unit is used with a single-stage compression ethane-EETRC, resulting in CGOR (cogeneration-based gain output ratio), exergy efficiency, and cooling load of 6.56, 17.13% and146.9 kW, respectively. However, to achieve as high cooling load as 161.8 kW - with CGOR, exergy efficiency, and freshwater of 5.19, 18.1% and 11.97 m3/day, respectively - a configuration with two HDH units coupled with a two-stage ethane-EETRC is highly commendable. Moreover, the cost evaluation results indicated that unit overall cost of the product (UOCP) of the hybrid system with two HDH units integrated with a two-stage ethane-EETRC and the hybrid set-up with a two-stage HDH unit integrated with a single-stage ethane-EETRC is calculated 8.2 $/GJ and 8.93 $/GJ, respectively. At last, an intensive parametric evaluation of some influential parameters is presented.

Humidification-dehumidification process used for the concentration and nutrient recovery of biogas slurry

Biogas slurry, the by-product of anaerobic digestion, represents 90%–95% of the input feed for digesters. Treatments of biogas slurry to reduce its volume and add value to it are difficult and costly. This study used the humidification-dehumidification process to concentrate biogas slurry and find a more economical and feasible method to provide appropriate conditions for sustainable utilization of biogas slurry. The effect of heating temperature, heating time, air flow rate and initial pH on the efficiency of water removal and nutrients (ammonia nitrogen, total phosphorus, chemical oxygen demand, soluble salt) recovery was investigated. The results showed that the humidification-dehumidification system could be effectively used to concentrate the biogas slurry. The recovery of ammonia nitrogen, total phosphorus, and soluble salt in the condensed phase reached 96% when the initial pH of biogas slurry was 6, the heating temperature was 70 °C, the air flow rate was 10 L/min, and the heating time was 30 min. Under these conditions, the mass concentrations of ammonia nitrogen, total phosphorus, chemical oxygen demand, and electrical conductivity in the dilute phase were 157.49 mg/L, 0.66 mg/L, 8.70 mg/L, and 0.55 mS/cm, respectively, which was far below the mass concentrations of them in raw biogas slurry. Water removal efficiency of 34.12% and ammonia nitrogen recovery of 98.04% were obtained by the response surface methodology optimization when the heating temperature was 61.92 °C, heating time was 48.54 min, air flow rate was 10 L/min, and initial pH was 4.80. Treatment of biogas slurry by the humidification-dehumidification system can result in a higher efficiency of water removal and nutrient recovery in a relatively short time. Moreover, the simple process, low operation cost and requirement for influent quality is conducive to engineering applications of the humidification-dehumidification process and the value-added utilization of biogas slurry.

Pressure effect on an ocean-based humidification-dehumidification desalination process

A new humidification-dehumidification (HDH) desalination process is proposed and analyzed. Being ocean based, the process does not produce any brine. It is largely powered jointly by solar energy, wind energy, and various types of ocean energies in a nearly natural way. A vacuum pump is employed to drive the air circulation throughout the HDH process. It is the only unit that consumes electricity. The HDH process is analyzed under various conditions, including using a low pressure (as low as to 0.2 atm) for humidification and the ambient pressure for dehumidification, running the entire HDH process around a low pressure (as low as to 0.2 atm), and running it around the ambient pressure. The results from case studies show that applying different pressure levels to humidification and dehumidification would lead to a prohibitively high electric energy consumption of the vacuum pump. Being the most favorable operating condition, running the HDH process around the ambient pressure yields a freshwater production rate at the level of 4 to 11 l/h per HDH line, depending on the pipe sizing and weather conditions. The associated minimum electric energy consumption of the vacuum pump is at the level of 0.9 to 1.6 kWh/m3-water.

Water productivity enhancement in variable pressure humidification dehumidification (HDH) desalination systems using heat pump

Variable pressure humidification dehumidification (VPHDH), humidification compression (HC), and conventional humidification dehumidification (HDH) processes are coupled to a heat pump (HP) for enhancement of their performances and desalination rates. Process simulation of the modified processes reveals the superiority of the proposed VPHDH-HP process to HC-HP, HDH-HP and the conventional HDH processes. Due to the superiority of VPHDH processes over conventional HDH processes, coupling them to HP units as a novel idea improves the desalination rate and energy utilization. The effects of dehumidifier to humidifier pressure ratio (PD/PH), humidifier pressure, and the inlet water to air mass flow rate ratio (w/a) on desalination rate and performance ratio (PR) are studied for the VPHDH-HP process as the superior process. Increasing PD/PH within 2.0 to 3.0 range, leads to an increase in desalination rate by 215%. Increasing w/a from 1.5 to 2.0 increases the desalination rate by 15%, while the same from 2.0 to 3 leads to a 53% decrease in the desalination rate for humidifier pressure of 0.5 bara. A cost analysis yields a water cost of US $4.68/m3 in VPHDH-HP system, indicating its performance superiority against its counterparts.

Solar-Driven Humidification–Dehumidification Process for Water Desalination Analyzed and Optimized via Equilibrium Theory

This study investigates a desalination system based on a humidification–dehumidification process driven by solar modules and proposes a solution within the framework of equilibrium theory. Although...

Parameter analysis and optimization of a two-stage solar assisted heat pump desalination system based on humidification-dehumidification process

A combination of heat pump and humidification-dehumidification (HDH) process is a suitable choice to obtain fresh water for small-scale desalination applications, especially when the solar energy is used as the auxiliary heat source. In this paper, a novel two-stage solar assisted heat pump (SAHP) desalination system based on HDH, in which the humidifiers are connected in parallel, is proposed. A mathematic model is developed to improve the system performance by optimizing the operating parameters such as process air flow rate and cooling seawater flow rate, and it is also validated by the experimental results. Analysis results indicate that there exists an optimal process air flow rate in the desalination system, which does not vary with the hot seawater flow rate. However, it will be increased with the increase of cooling seawater flow rate. When the flow rates of process air and cooling seawater are 350 m3/h and 0.55 m3/h, respectively, the maximum fresh water yield is 17.94 kg/h. The corresponding gained-output-ratio (GOR) is 2.02. However, the system performance is constrained by a bottleneck: increasing dehumidifying capacity can result in a reduction in the performance of lower-temperature (LT) humidifier. Consequently, a modified system is then proposed to solve this bottleneck effectively. The maximum fresh water yield can be increased by 16.70% to 20.54 kg/h, and the corresponding maximum GOR is also increased by 18.05% to 2.42.

Biogas fueled combined cooling, desalinated water and power generation systems

This paper designs a novel biogas fueled combined cooling, fresh water and power (CCWP) generation system, and investigates its performance under extremely-hot summer days. In this tri-generation system, biogas is used as clean resource to reduce emissions and consumption of fossil fuels. Six electrical chillers are used for cooling demand supply of a benchmark building located in Ahwaz, Iran. In addition, expected required power of chillers and building is supplied either by gas turbine cycle or by main grid (If total power consumed by chillers and electrical equipment is larger than output power of gas turbine, building owner pays for purchasing electricity from local power system to satisfy not-supplied demand). A low-grade waste heat is extracted from the flue gases for preheating purpose and closed-air open-water humidification dehumidification (HDH) desalination process. In addition, a mixed integer non-linear programming problem is solved to minimize an objective function composed of daily electricity cost, total capital investment, operation and maintenance cost of CCWP generation system considering all operational limitations of gas turbine cycle, chillers, HDH process, and strategic cooling and electrical demand response programs (DRPs). If ambient air decreases, gas turbine power and mas flow rate of potable water will be increased due to lower compressor power and higher absolute humidity of air, respectively. Application of DRPs reduces total electricity cost from 31.2$ to 16.4$. Utilization of biogas and waste heat recovery of flue gases with no need to combustion of fossil fuels results in emission reduction. Hence, this tri-generation plant is called a net-zero emission system.

Experimental investigation on a coconut coir packed humidifier for a solar desalination plant

Desalination by humidification dehumidification processes is a promising simple, cost effective and low maintenance concept. It works on the principle of air humidification at high temperature and dehumidification at relatively low temperatures. The present study is for evaluating the performance of the humidifier with coconut coir as packing material used for air humidification in the solar humidification dehumidification (HDH) desalination plant. Experiments were carried out on the humidifier packing for different flow rates of water and its temperature. Humidifier range, effectiveness and efficiency, evaporation rate of water, heat and mass transfer rates are studied by experimentally for different flow rates of hot water and its temperature. The hot water flow rate is varied from 125 to 225 LPH with an increment of 25 LPH and water temperature from 38 to 54°C. It is found that at water flow rate of 200 LPH and at 0.55 m3/s volume flow rate of air, effectiveness of humidifier found to be maximum of 0.74.

Combined water desalination and electricity generation through a humidification-dehumidification process integrated with photovoltaic-thermal modules: Design, performance analysis and techno-economic assessment

Abstract Humidification-dehumidification (HDH) processes have proved to be a promising solution for small-scale desalination, appropriate for water production in off-grid locations where the water demand does not justify the installation of conventional large-scale systems. With this contribution, we investigate the design and operation of an HDH process coupled with photovoltaic-thermal (PVT) solar modules for the simultaneous generation of clean water and electricity. The HDH system consists of established technologies, namely a humidification column and a heat exchanger, and is simulated through a commercial software accounting for heat and mass transfer limitations. On the other hand, PVT modules are a relatively new technology and their performance is determined experimentally to characterize the simultaneous generation of electricity and heat under realistic operating conditions. Based on the maximum-efficiency ratio of water to air mass flow rates, the optimal design of the system is determined for a wide range of ambient conditions by evaluating the impact of saline water flow rate, PVT configuration and HDH size on the amount of clean water produced. Then, the optimal operation of the system is characterized as function of the ambient conditions for a fixed system design. The state of the system is represented by the maximum process temperature, at the outlet of the PVT modules, whereas the performance is evaluated in terms of clean water and electricity generation. Finally, a techno-economic assessment is carried out to compare the proposed technology against conventional solar-driven HDH units, which use thermal and photovoltaic panels, for a wide range of electricity price, ambient conditions, amount of yearly water produced and size of the HDH process.

Thermodynamic study on an enhanced humidification-dehumidification solar desalination system with weakly compressed air and internal heat recovery

Humidification-dehumidification (HDH) process has proved to be a promising technology for obtaining drinkable fresh water. Traditional HDH process exhibits limitations in steady working due to the dependence in the stability of heat and cold sources. A combination of heat pump and HDH has been proposed to solve effectively above challenge. However, the energy equalization between heat source and cold source must be guaranteed to maintain the continuing water production. A novel enhanced HDH method with weakly compressed air and internal heat recovery based on traditional mechanical vapor compression is therefore developed, in which moist air instead of vapor is used as the work fluid. Solar energy is required only for the system start-up, and cooling seawater and additional heat sources are no longer needed during steady operation stage. Meanwhile, the restrictions of evaporation and condensation temperature in the HDH process driven by heat pump can also be completely solved. It is found that the system performance can be significantly improved by increasing spraying seawater temperature. When the spraying seawater temperature is 70 °C, the maximum specific energy consumption and gained-out-ratio (GOR) are 18.35 kg/kWh and 12.24 respectively, and GOR is much larger than that of tradition HDH process (normally less than 6) driven by heat pump or solar energy. This new approach provides the possibility for high-efficient, steady and small-scale drinkable water production.

Experimental study on a solar assisted heat pump desalination unit with internal heat recovery based on humidification-dehumidification process

On-going study on the humidification-dehumidification (HDH) desalination has proved that internal heat recovery is a significant and potential method for improving the system performance and reducing the fresh water cost. In this paper, a novel solar assisted heat pump (SAHP) with internal heat recovery desalination system based on HDH process is proposed and studied experimentally. The objective of this study is to investigate the ability of heat recovery in improving the system performance and obtain the optimal operating parameters. Based on the experimental results, it can be concluded that the fresh water cost of the two-stage HDH desalination system is reduced by 17.36%, and system productivity and gained-output-ratio (GOR) are increased by 15.51% and 55.64%, respectively compared with those of the single-stage. Particularly, benefiting from energy cascade utilization, the distribution ratio is introduced to further improve the system performance, which is affected strongly by the cooling seawater flow rate. Furthermore, a comparison of the humidifier materials between plastic polyhedron empty balls (PPEBs) and honey-comb paper indicates that the increment in system productivity by PPEBs is 27.76% lower than that by honey-comb paper with a reasonable specific surface area.

Predicting the water production of a solar seawater greenhouse desalination unit using multi-layer perceptron model

A solar seawater greenhouse is a type of desalination plant that uses solar energy and seawater to humidify the interior of the greenhouse and produce fresh water from the humid air (humidification-dehumidification process). The produced water is used both for irrigating agricultural crops and for drinking. Many parameters affect the performance of seawater greenhouses. The present study employed an artificial neural network to examine the effective parameters of the greenhouse on the fresh water production such as width, length, the height of the front evaporator, and roof transparency. A suitable structure was obtained for the multi-layer perceptron (MLP) method and the mathematical statistics % AARE, RMSE, and R2, were used to evaluate network performance. The method showed good agreement with the experimental data. Using the optimized created network, the effect of each parameter on the produced fresh water was assessed. Finally, a 125 m wide and 200 m long of greenhouse with a 4 m height of front evaporator and roof transparency of 0.6 that produced 161.6 m3/day of fresh water was introduced as the optimal seawater greenhouse.