Search Results (26)

Aiming to improve cooling and dehumidification performance in air conditioning systems and to meet the trend toward environmentally friendly refrigerants, this study proposes a coupled system that combines a CO₂ transcritical refrigeration cycle (CTRC) with a liquid desiccant dehumidification cycle. The system takes advantage of high-grade waste heat from the exothermic side of the CTRC to drive the regenerating process of the liquid desiccant dehumidification. A cooling evaporator is adopted to cool indoor air, while another evaporator (i.e., Evaporator II) is utilized to cool the concentrated solution, improving dehumidification capacity and enabling independent control of sensible and latent heat loads. Through thermodynamic modeling and the exergy analysis model, a mathematical model of the system is developed to examine how key parameters (such discharge pressure and the CO₂ mass flow rate ratio in Evaporator II (λ)) affect performance and to analyze exergy loss features. Results show that the system’s coefficient of performance (COP) and dehumidification coefficient of performance (COP_deh) initially rise and then fall with increasing CTRC discharge pressure, achieving an optimal pressure of around 10,500 kPa (COP up to 4.32) under a specific working condition, surpassing those of standalone CTRC systems. Properly increasing λ enhances dehumidification capacity and energy efficiency, with a low specific dehumidification energy (SDE) of 0.2033 kWh/kg, indicating high economic efficiency. Most exergy losses occur in the CO₂-solution heat exchanger and dehumidifier (over 60% of total losses). The system’s maximum exergy efficiency reaches 12.4%, leaving room for further improvements. This coupled system offers an efficient, eco-friendly way for air conditioning in high-humidity environments, combining cooling and dehumidification with the potential for energy recovery. Full article

A multi-stage TG-DSC approach consisting of five heating/holding and five cooling/holding stages within one experiment in the temperature range 20–320 °C was applied to investigate the dehydration/hydration processes in K⁺, Na⁺, Ca²⁺, and Mg²⁺ clinoptilolite forms. The influence of extra-framework cations on the parameters characterizing these processes (such as mass changes, dehydration and hydration heats calculated per gram zeolite, amounts of water molecules leaving and entering the structure, and enthalpy values calculated per mol water) was established. The values of molar enthalpy of dehydration for different cationic clinoptilolite forms increase in different ways with temperature increasing (within the framework of 50–120 kJ mol⁻¹). The data on the molar enthalpy are in good agreement with the distributions of the two types of water molecules—weakly bound to cations and water molecules coordinating cations in the applied crystal chemical models of the cationic exchange samples. The data obtained for water molecules and their molar enthalpies of dehydration for the various cationic forms are useful in studying the sorption of water vapor and other sorbates, in choosing a desiccant and an object to dry at room conditions, etc. The first data on the hydration energy of sequentially added water molecules in a dynamic cooling mode in the temperature range 320–20 °C were obtained. Full article

This study investigated the performance of a hybrid desiccant dehumidification system integrated with a post-cooling mechanism, focusing on its application to energy-efficient indoor climate control. A liquid desiccant system using magnesium chloride (MgCl₂) was tested in its pure form and in combination with silica gel at 10% and 20% concentrations to enhance its moisture removal capabilities. The key parameters, including the air velocity (3–6 m/s), desiccant flow rate (1–3 LPM), and desiccant composition, were varied to analyze their effects on the dehumidification efficiency, moisture removal rate (MRR), temperature reduction after post-cooling, and coefficient of performance (COP). The results show that post-cooling using a crossflow heat exchanger effectively lowered the exit air temperature, ensuring thermal comfort. Addition of silica gel significantly improved system performance. The MgCl₂ + 20% silica gel mixture achieved the highest dehumidification efficiency of 0.86, the greatest temperature drop of 1.95 °C, and the maximum COP of 2.36 at optimal flow conditions. While the dehumidification efficiency declined with increasing air velocity due to reduced contact time, the COP increased owing to the higher thermal processing of the air stream. This study highlights the potential of optimized hybrid desiccant systems as sustainable solutions for building air conditioning, aligning with the key Sustainable Development Goals (SDGs) related to clean energy, climate action, and sustainable infrastructure. Full article

The building sector accounts for nearly 40% of global primary energy consumption, with heating, ventilation, and air conditioning (HVAC) systems contributing significantly to energy use and greenhouse gas emissions. Conventional HVAC systems face challenges in addressing humidity control and efficiency, particularly in cold and dry climates. This research demonstrates the development and transient simulation of a novel solar-assisted desiccant wheel-based system for heating and humidification (SDHH) in Taxila, Pakistan. The proposed system includes a desiccant wheel, heat wheel, water-to-air heat exchanger, and a direct evaporative cooler. An array of flat plate collectors supply hot water to the heat exchanger. TRNSYS simulations investigated the performance of SDHH by evaluating heating capacity, humidification, and indoor temperature and humidity values in winter. Results show that the SDHH system maintained the required temperature in the zone and improved the zone humidity level. The desiccant wheel increased the absolute humidity of product air by 0.003 kg/kg. The average indoor temperature was 21 °C, and the average absolute humidity was around 0.008 kg/kg. These results justify using the proposed system in dry and cold climate conditions. Full article

Deep dehumidification is crucial for industrial applications requiring ultra-low humidity levels. Traditional cooling-based dehumidification struggles to achieve low dew points efficiently due to excessive energy consumption and frost formation risks. As an alternative, desiccant-based methods, particularly solid desiccant systems, offer improved performance with lower energy demands. This study experimentally investigates a fixed-bed dehumidification system utilizing a plate-fin heat exchanger filled with a silica gel/calcium chloride composite material. The performance evaluation focuses on the influence of ambient conditions and operating parameters, including air velocity and cooling fluid temperature. Among these, the most influential parameter was the velocity of air. For the tested heat exchanger, an optimum value in the range of 0.4–0.6 m/s was identified. Under optimal conditions, the tested HEX was able to reduce the dew point of air down to −2 °C, achieving a reduction in the humidity ratio up to 13 g/kg. The results indicate that air velocity significantly impacts also heat and mass transfer, with coefficients ranging from 80 to 140 W/(m² K) and 0.015 to 0.060 kg/(m² s), respectively. The findings highlight the potential of composite desiccant fixed-bed systems for efficient deep dehumidification, outperforming conventional lab-scale components in heat and mass transfer effectiveness. A comparison with other works in the literature indicated that up to 30% increased mass transfer coefficient was achieved and up to seven times higher heat transfer coefficient was measured. Full article

Indoor comfort has become a major factor with advancements in science and technology. This also leads to an increase in greenhouse gases as well as energy consumption. Desiccant-coated heat exchangers are one of the common solutions to these risks and to lower energy usage. In the present work, the capability of a solid composite desiccant blend prepared from coconut shell-based activated carbon and bio char was studied. Aluminum plates have been coated with the prepared solid desiccants. Desiccant-coated heat exchangers were cooled by the cerium oxide nanofluid passing through the pipes connected along the length of the heat exchanger. Air was blown through the plates where dehumidification occurs due to the vapor pressure difference between the air and the desiccant-coated plate. The experiments were conducted by varying the air velocity, water flow rate, and nanoparticle concentration. The nanoparticle volume fraction varied from 0.05% to 0.3%. Different performance parameters such as the moisture removal rate, dehumidification efficiency, cooling capacity, and coefficient of performance (COP) were calculated. Results showed that the performance parameters were enhanced with an increase in the water flow rate as well as the air flow rate. Furthermore, it was seen that with the addition and increase in nanoparticle concentration, the moisture removal rate and dehumidification efficiency were enhanced. In comparison to no addition of nanoparticles, a 0.3% addition of nanoparticles demonstrated a maximum increase in MRR of 53% and dehumidification efficiency of 57%. A maximum reduction of 6.1% in the dehumidification area was achieved by using 0.3% nanoparticles with water. It is recommended to use nanofluids for dehumidification using solid desiccants, which can enhance the performance without having negative influence on the environment. Full article

This work is motivated by the difficulty of cultivating crops in horticulture greenhouses under hot and arid climate conditions. The main challenge is to provide a suitable greenhouse indoor environment, with sufficiently low costs and low environmental impacts. The climate control inside the greenhouse constitutes an efficient methodology for maintaining a satisfactory environment that fulfills the requirements of high-yield crops and reduced energy and water resource consumption. In hot climates, the cooling systems, which are assisted by an effective control technique, constitute a suitable path for maintaining an appropriate climate inside the greenhouse, where the required temperature and humidity distribution is maintained. Nevertheless, most of the commonly used systems are either highly energy or water consuming. Hence, the main objective of this work is to provide a detailed review of the research studies that have been carried out during the last few years, with a specific focus on the technologies that allow for the enhancement of the system effectiveness under hot and arid conditions, and that decrease the energy and water consumption. Climate control processes in the greenhouse by means of manual and smart control systems are investigated first. Subsequently, the different cooling technologies that provide the required ranges of temperature and humidity inside the greenhouse are detailed, namely, the systems using heat exchangers, ventilation, evaporation, and desiccants. Finally, the recommended energy-efficient approaches of the desiccant dehumidification systems for greenhouse farming are pointed out, and the future trends in cooling systems, which include water recovery using the method of combined evaporation–condensation, as well as the opportunities for further research and development, are identified as a contribution to future research work. Full article

Air conditioning energy consumption accounts for most building energy consumption, indoor dehumidification is the main cause of air conditioning energy consumption. Optimize the dehumidification methods of air conditioning systems have great significance to the development of green buildings and people’s pursuit of comfort. Improvement of fins on air conditioning heat exchangers is a hot topic of current research and has achieved considerable results in terms of indoor dehumidification and energy saving compared to traditional air conditioners. This paper reviews two kinds of heat exchangers modified by coating, including desiccant-coated heat exchangers and hydrophobic/hydrophilic coated heat exchangers. For desiccant-coated heat exchangers, the preparation methods of advanced desiccant materials and the possibilities of using this material to achieve excellent energy efficiencies were presented, and the operating parameters that affect thermal performance and dehumidification are determined, including airflow temperature, air velocity, inlet air relative humidity, and regeneration temperature. For hydrophobic/hydrophilic coated heat exchangers, different kinds of hybrid hydrophobic-hydrophilic surfaces are highlighted for they are a high water droplet nucleation rate and surface heat transfer efficiency. In addition, the challenges and future works are explained at last. This paper will provide a valuable reference for the follow-up research, which will be helpful for indoor humidity control and reducing the energy consumption of air conditioning. Full article

Heat storage with thermochemical (TC) materials is a promising technology for solar energy storage. In this paper, a solar-driven desiccant evaporative cooling (DEC) system for air-conditioning is proposed, which converts solar heat energy into cooling with built-in daily storage. The system utilises thermochemical heat storage along with the DEC technology in a unique way. Magnesium Chloride (MgCl₂·6H₂O) has been used, which serves as both a desiccant and a thermochemical heat storage medium. The system has been designed for the subtropical climate of Lahore, Pakistan, for a bedroom with 8 h of cooling requirements during the night. MATLAB has been employed for modelling the system. The simulation results show that 57 kg of magnesium chloride is sufficient to meet 98.8% of cooling demand for the entire month of July at an elevated cooling requirement. It was found that the cooling output of the system increased with increasing heat exchanger effectiveness. The heat exchangers’ effectiveness was increased from 0.7 to 0.8, with the solar fraction increased from 70.4% to 82.44%. The cooled air supplied to the building meets the fresh air requirements for proper ventilation. Full article

Great variation in shape and size between primary (juvenile) and secondary (adult) needles, so-called leaf-heteroblasty, occurs in several Pinus species. Most of them loss primary needles during the juvenile-to-adult transition of the tree. An exception to this is Pinus canariensis (a Canary Islands endemism) in which basal resprouting twigs of adult trees frequently wear both primary and secondary needles. Taking advantage of this extraordinary study-case-species, we conducted an exhaustive comparison of both needle types through quantitative analyses of needle anatomy, photochemical performance, gas exchange, and resistance to extreme dehydration and to extreme needle temperature. We hypothesized that primary needles would show lower investment to leaf structure but higher photosynthetical efficiency. Primary needles had less stomatal density and thicker and less wettable cuticles. In cross section, primary needles showed smaller structural fraction (e.g., percent of hypodermis, endodermis and vascular tissue) and higher fraction of photosynthetic parenchyma. Significant differences between primary and secondary needles were not found in net carbon assimilation not in their leaf mass area values. Interestingly, secondary needles showed higher electron transport rate, and they were additionally much more efficient in retaining water under severe and controlled desiccant conditions. When subjected to extreme temperatures (−10° to +50 °C), primary needles recovered better their photochemical efficiency than secondary needles, after +46° and +48 °C heat-shock treatments. Our results indicate that both needle types broaden the diversity of physiological responses against environmental constrains in basal twigs of adult P. canariensis trees. Considering that this is a fire-resistant and resprouting species, this advantage could be particularly useful after a drastic environmental change such a fire or a gap opening in the forest. Full article

This study provides insights into the feasibility of a desiccant dehumidification-based Maisotsenko cycle evaporative cooling (M-DAC) system for greenhouse air-conditioning application. Conventional cooling techniques include direct evaporative cooling, refrigeration systems, and passive/active ventilation. which are commonly used in Pakistan; however, they are either not feasible due to their energy cost, or they cannot efficiently provide an optimum microclimate depending on the regions, the growing seasons, and the crop being cultivated. The M-DAC system was therefore proposed and evaluated as an alternative solution for air conditioning to achieve optimum levels of vapor pressure deficit (VPD) for greenhouse crop production. The objective of this study was to investigate the thermodynamic performance of the proposed system from the viewpoints of the temperature gradient, relative humidity level, VPD, and dehumidification gradient. Results showed that the standalone desiccant air-conditioning (DAC) system created maximum dehumidification gradient (i.e., 16.8 g/kg) and maximum temperature gradient (i.e., 8.4 °C) at 24.3 g/kg and 38.6 °C ambient air conditions, respectively. The DAC coupled with a heat exchanger (DAC+HX) created a temperature gradient nearly equal to ambient air conditions, which is not in the optimal range for greenhouse growing conditions. Analysis of the M-DAC system showed that a maximum air temperature gradient, i.e., 21.9 °C at 39.2 °C ambient air condition, can be achieved, and is considered optimal for most greenhouse crops. Results were validated with two microclimate models (OptDeg and Cft) by taking into account the optimality of VPD at different growth stages of tomato plants. This study suggests that the M-DAC system is a feasible method to be considered as an efficient solution for greenhouse air-conditioning under the climate conditions of Multan (Pakistan). Full article

The objective of this work is to analyze a gas injection borehole heat exchanger coupled with a desiccant assisted air conditioning system during cooling and heating operation. A common problem that occurs in air conditioning systems is peak loads, during which the cooling or heating power of the soil can be exceeded. To counteract this drawback, a gas injection borehole heat exchanger, which is capable of creating artificial groundwater flow along the heat exchanger by inducing a pressure difference inside the well, is used. Experimental results of the performance differences between a conventional and a gas injection borehole heat exchanger are presented. Under the same inlet conditions, a reduction in the outlet temperature of up to 2 °C is achieved compared with an equivalent conventional borehole heat exchanger in cooling mode. The maximum cooling power is increased by 26%. As a result, a fast and dynamic responding control of the heat transfer between the heat exchanger and the soil is possible. During winter operation, despite the lower drilling depth of the gas injection borehole heat exchanger system, the performance is within the range of a conventional system. The power increase is limited to around $0.2{ \mathrm{kW}}_{\mathrm{th}}$ at a steady state. In conclusion, gas injection borehole heat exchangers can be promising in terms of reliable peak load handling within large geothermal fields. Full article

Solar thermal-powered desiccant dehumidification systems are attracting attention for cooling load-dominated climates. However, their performance varies substantially from place to place depending on climatic conditions, which therefore warrants a tailored design and specification at each geographical location. The current article attempted to investigate the feasibility of extending an existing solar thermal system in a school building in Abu Dhabi to provide dehumidification for the existing air condition system through a desiccant system. The system performance was predicted through a Transient System (TRNSYS) Simulation model to determine the energy savings achieved by the solar-assisted dehumidification system. The current articles determined the effect of fluid flow rate, solar radiation concentration, and heat exchanger effectiveness at the dehumidification of the fresh air as well as energy saved by the proposed system. It was concluded that the system can remove 35% moisture from the air, simultaneously saving 10% of the building’s energy. The system cost and benefit analysis revealed a payback period of 7.5 years, considered slightly higher for an attractive investment in such systems. Full article

This study investigated a composite polymer desiccant material’s performance, which is prepared by impregnating solid desiccant such as sodium polyacrylate (SAP) on to hygroscopic salts such as lithium chloride (LiCl). Dehumidification performance of the proposed composite polymer desiccant (SAP-LiCl) was analyzed by coating the suitable weight percentage (wt %) of the desiccant onto a single fin-tube heat exchanger (FTHE) system and testing the desiccant-coated heat exchanger (DCHE) in a testing tunnel under various operating conditions. Net dehumidification efficacy of DCHE in terms of sorption and desorption amount and thermal performance (COP_th) were analyzed. For instance, with processed air inflow temperature, relative humidity and regeneration temperature setting of 30 °C, 80% RH and 70 °C, DCHE’s sorption, desorption amount and COP_th were recorded as high as 945.1 g, 1115.1 g, and 0.39, respectively. It was further realized that the performance of the DCHE could be enhanced by modulating the cyclic switching time for dehumidification and regeneration processes. For instance, with the aforementioned processed airflow conditions, when the cyclic switching time tuned as 60 min instead of 10 min for a total time period of 120 min, there is a net 58% improvement to the COP_th of the system. It was further observed that, under the same time period corresponding to the increase in cyclic switching time, the overall COP_th can be enhanced; however, the water vapor sorption and desorption amounts of desiccant were decreased. Full article

In this study, silica gel and sodium polyacrylate desiccants are coated onto a finned tube heat exchanger (Desiccant Coating Heat Exchanger, DCHE), which can absorb the vapor in the process air for dehumidification. In the experiments, the desiccant is coated on fins using the dense coating method, which causes the fixed fin area to be coated with greater amounts of desiccants for a better dehumidification performance. This study discusses the dehumidification performances of a single stage DCHE and two-stage DCHEs in series under different relative humidity conditions of the inlet process air and different regeneration water temperatures. The results show that the thermal coefficient of performance (COP_th) of the DCHEs for the two desiccants prepared by the dense coating method is better than that of DCHEs with the general immersing coating method by a factor of 2–2.4. The two-stage DCHEs in series have a lower supply humidity ratio than a single stage DCHE at different inlet humidity levels, and they can be used in the industry when a special low humidity manufacturing process is required. The overall dehumidifying capacities of two-stage series-connected DCHEs at regeneration temperatures of 50 °C and 70 °C are approximately twice as high as those of a single stage DCHE. The COP_th value of a single stage or two stages increases with an increase in the inlet humidity of the process air. The COP_th values of the sodium polyacrylate single stage and two-stage DCHEs are 1–1.3 times greater than those of the silica gel single stage and two-stage DCHEs at a high inlet air humidity. Finally, the effects of different regeneration water temperatures on the performance of DCHEs are investigated. With an increase in the regeneration water temperature, the COP_th value, dehumidifying capacity and regeneration capacity of single stage or two-stage DCHEs increase as well. Full article