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[en] Highlights: •Thermodynamic principles are applied to systematically compare three technologies. •Merits and limits of standalone versus integrated designs are identified. •Effect of climate conditions on performance and technology selection is evaluated. •Integrated desiccant/membrane technologies outperform current state-of-the-art VCS. -- Abstract: Recently, next-generation HVAC technologies have gained attention as potential alternatives to the conventional vapor-compression system (VCS) for dehumidification and cooling. Previous studies have primarily focused on analyzing a specific technology or its application to a particular climate. A comparison of these technologies is necessary to elucidate the reasons and conditions under which one technology might outperform the rest. In this study, we apply a uniform framework based on fundamental thermodynamic principles to assess and compare different HVAC technologies from an energy conversion standpoint. The thermodynamic least work of dehumidification and cooling is formally defined as a thermodynamic benchmark, while VCS performance is chosen as the industry benchmark against which other technologies, namely desiccant-based cooling system (DCS) and membrane-based cooling system (MCS), are compared. The effect of outdoor temperature and humidity on device performance is investigated, and key insights underlying the dehumidification and cooling process are elucidated. In spite of the great potential of DCS and MCS technologies, our results underscore the need for improved system-level design and integration if DCS or MCS are to compete with VCS. Our findings have significant implications for the design and operation of next-generation HVAC technologies and shed light on potential avenues to achieve higher efficiencies in dehumidification and cooling applications.
[en] Regenerator is major component of liquid desiccant regeneration system. The influence of operating parameters; air flow rate, solution flow rate and concentration of desiccant is investigated experimentally on the performance parameters; outlet specific humidity, evaporation rate, air outlet temperature, mass transfer coefficient and effectiveness of the regenerator. Air and desiccant solution flow in counter direction with celdek pads as packing material. It is concluded that evaporation rate increases with increasing solution temperature, air flow rate and solution flow rate whereas same decreases with increasing concentration of desiccant. The effectiveness of regenerator is increased by 99 % with increase in air flow rate. A comparison of present finding with those available in the literature is presented in the last. Simulation results have revealed good agreement between the present experimental results.
[en] Soil radon is employed to trace residual NAPL (Non-Aqueous Phase Liquid) contamination because it is very soluble in these substances and is strongly depleted over polluted volumes of the subsoil. The solubility of radon into NAPL vapors, generally poorly considered, is investigated here, either as growth of radon exhalation from a material contaminated with increasing volumes of kerosene, or as radon partition between liquid kerosene, water and total air, considered ad the sum of kerosene vapors plus air. - Highlights: • Laboratory simulation of recent NAPL spills was carried out • Kerosene vapors enhance soil radon exhalation and transport • The use of desiccants emphasizes radon preferential dissolution into NAPL vapors • Radon partitioning among liquid kerosene, water and total air is investigated • Radon partition coefficients between liquid and vaporized kerosene were estimated
[en] Highlights: • A PCM and desiccant packet is proposed for use in personal cooling vest to keep dry air next to skin. • A PCM-Desiccant model for clothed heated wet cylinder is developed and validated experimentally. • The microclimate air temperature was 0.6 °C higher in PCM-Desiccant case compared to PCM-only case. • Microclimate humidity content decreased due to desiccant from 21.23 to 19.74 g/kg dry air. • PCM melted fraction increased due to desiccant from 0.24 to 0.5. - Abstract: A novel combination of phase change material (PCM) and a solid desiccant layer is proposed for the aim of maintaining dry cool microclimate air adjacent to wet warm skin and hence improve PCM performance in cooling vests used in hot humid environment. A fabric-PCM-Desiccant model is developed to predict the temperature and moisture content of the microclimate air layer in the presence of a PCM-Desiccant packet. The developed model is validated through experiments conducted on a wet clothed heated cylinder for the two cases of using (i) a PCM only packet and (ii) a PCM-Desiccant packet. Microclimate air temperatures and humidity content as well as PCM and desiccant temperatures were measured experimentally and were compared with predicted values by the fabric-PCM-Desiccant model. Good agreement was attained with a maximum relative error of 7% in measured temperatures. A decrease is observed in the humidity content of the microclimate air in the presence of the solid desiccant from 21.23 g/kg dry air to 19.74 g/kg dry air and an increase in the melted fraction of the PCM at the end of the experiment from 0.24 to 0.5.
[en] Highlights: • A novel compression-absorption refrigeration THIC air-conditioning system is proposed. • All of the condensation heat of the ARS can be recovered for desiccant solution regeneration. • Effects of key parameters on the performance of the system are analyzed. • The PEE of the novel system is 34.97% higher than that of traditional system. • The generation temperature of system can decrease from 100 °C to 60 °C. - Abstract: The vapor compression system driven liquid desiccant dehumidification to realize temperature and humidity independence control (THIC) is a very good idea for the utilization of condensation heat. Absorption refrigeration system (ABS) is widely applied all over the world, due to its energy saving and environment protection; meanwhile, large amount of condensation heat is discharged to the environment. In this study, a hybrid compression-absorption refrigeration air-conditioning (AC) system combined with liquid desiccant dehumidification is proposed. All of the condensation heat from the condenser of the ABS can be recovered to regenerate the diluted liquid desiccant solution. Meanwhile the evaporation temperature in the ABS rises since the latent load is handled by liquid desiccant, which improves the performance of the ABS. The PEE of the proposed system is 34.97% higher than that of traditional absorption refrigeration AC system at the same operation condition. And compared with the traditional absorption refrigeration AC system, the generation temperature of the proposed system can decrease from 100 °C to 60 °C due to the existence of the compressor.
[en] Highlights: • The second law analysis of the desiccant cooling system is evaluated. • Effect of various operating parameters on the system performance was studied. • Effect of ambient air temperature and relative humidity was investigated. • The Carnot COP is between 9 and 25. - Abstract: In order to evaluate the potential of a desiccant based evaporative air conditioning system, a second law analysis of the systems was investigated. In this work, a wide range of working parameters that are regeneration temperature from 90 °C to 110 °C, volume flow rate from 2000 m"3 to 4000 m"3 and ambient air conditions which are relative humidity, ambient air temperature and wet-bulb temperature is carried out for performance calculation of the system. It is observed from results that the Carnot COP and the thermal COP of the system varies vice versa during the day. The highest value of Carnot COP and the thermal COP is about 25 and 0.62, respectively. Also, the cooling capacity has significant effect on the second law efficiency. The importance of this study shows that the second law analysis can provide useful information with respect to the theoretical upper limit of the system performance, which cannot be obtained from the first law of thermodynamic analysis alone.
[en] Highlights: • Purge angel analysis of the desiccant wheel considering operating and design parameters. • Mathematical model development and validation of desiccant wheel and purge angle. • Development of an optimal definition for purge angle to improve the performance. • Moisture removal and coefficient of performance enhanced by optimal purge air. - Abstract: Desiccant cooling systems are spreading as a promising technology to reduce the energy consumption and environmental impact of conventional electric driven vapour compression systems for air conditioning purposes. Desiccant wheels (DWs) are the key component of the desiccant cooling systems which have received substantial attention. Desiccant Wheel if equipped with a purge section will show better performance, however in most cases purge section is not considered or a fixed purge angle is assumed. In this study, analysis of the purge angle effects on energy and dehumidification performances of DW is carried out and a novel optimal purge angle definition is introduced. A mathematical model is developed and validated in order to model the coupled heat and mass transfer processes in a DW. In addition, the effect of process and regeneration air velocities, regeneration air temperature, rotational speed, desiccant layer thickness, channel length (DW length) and channel hydraulic diameter on the purge angle are studied. The results showed that purge angle is a function of outlet air humidity profile, while the process air velocity as an operating parameter and channel length as a design parameter presented the most substantial effect on the profile. Furthermore, implementation of the optimal purge angle, improves the DW coefficient performance (DCOP) and results in desired conditions of outlet process air without the necessity of substantial increase in the DW size.
[en] Highlights: • The study objective is to reduce the energy consumption of desiccant AC system. • Heat and mass losses are recovered in the proposed system using a condenser. • The conventional and the proposed systems are compared in terms of COP. • The proposed system performance is better than the conventional system. • The proposed system produces freshwater in addition to space cooling. - Abstract: Liquid desiccant air conditioning systems are cost-effective, environmentally friendly and energy efficient techniques, especially in coastal areas. In the conventional liquid desiccant air conditioning system, the scavenging air is expelled into the atmosphere carrying a considerable amount of energy and water vapor. Thus, there is plenty of room to improve the system performance by recovering these losses. The proposed system consists of a conventional liquid desiccant air conditioning system plus a condenser. The aim of this study is to reduce the energy consumption by recovering the heat from the scavenging air using the condenser while also producing freshwater in addition to space cooling. Lithium chloride (LiCl) is used as the liquid desiccant for this study. The mathematical formulation for simultaneous heat and mass transfer between the condenser and the regenerator was developed to establish a comparison between the performance of the conventional and modified systems. Using the generated model, it is found that the modified system performance is 11.25% better than the conventional system and that it produces 86.4 kg of freshwater per hour as a by-product under the given conditions.
[en] Highlights: • A selection of sorbents were tested for open thermochemical heat storage. • Sorbent performances were experimentally compared in two different reactors. • SIM-3a provided the best cyclic behaviour and thermal performance. • Using meshed tube air diffusers improves sorption heat storage performance. • A linear correlation between heat output and moisture uptake was obtained. - Abstract: In this study, the performance of three nano-composite energy storage absorbents; Vermiculite-CaCl_2 (SIM-3a), Vermiculite-CaCl_2-LiNO_3 (SIM-3f), and the desiccant Zeolite 13X were experimentally investigated for suitability to domestic scale thermal energy storage. A novel 3 kWh open thermochemical reactor consisting of new meshed tube air diffusers was built to experimentally examine performance. The results were compared to those obtained using a previously developed flatbed experimental reactor. SIM-3a has the best cyclic behaviour and thermal performance. It was found that 0.01 m"3 of SIM-3a can provide an average temperature lift of room air, ΔT = 20 °C over 180 min whereas for SIM-3f, ΔT < 15 °C was achieved. Zeolite provided high sorption heat in close approximation with SIM-3a, however, the higher desorption temperature requirements coupled with poor cyclic ability remain as obstacles to the roll out this material commercially. The study results clearly show that the concept of using perforated tubes embedded inside the heat storage material significantly improves performance by enhancing the contact surface area between air → absorbent whilst increasing vapour diffusion. The results suggest a linear correlation between thermal performance and moisture uptake, ΔT–Δw. Determining these operating lines will prove useful for predicting achievable temperature lift and also for effective design and control of thermochemical heat storage systems.
[en] A cross-flow heat exchanger type adsorber was investigated for a desiccant humidity control system. The aim of the adsorber was to improve dehumidification performance by forcibly cooling an adsorbent with flowing air. Aluminophosphate (AlPO) zeolite was coated on the heat exchanger, because it was expected to regenerate sufficiently even with a low-temperature heat source of around 333 K. Fundamental dehumidification behavior with the adsorber was experimentally investigated at various inlet absolute humidities, regeneration temperatures, and air flow velocities. Dehumidified water in an equilibrium state was kept even at a regeneration temperature of 333 K, indicating that the adsorber coated with AlPO zeolite could be driven using low-temperature heat at 333 K. The dehumidification rate was found to increase as the cooling and the process air velocities increased. However, the increase in the dehumidification rate decreased when the cooling air flowed at a velocity of 2 m/s or more. Heat removed by the cooling air increased as the cooling air velocity increased and the process air velocity decreased. It was also found that dehumidified air could be supplied for a longer period at a sufficiently low absolute humidity suitable for practical use as the cooling air velocity increased.