Results 1 - 10 of 11
Results 1 - 10 of 11. Search took: 0.014 seconds
|Sort by: date | relevance|
[en] Highlights: • Multi-objective optimization of desiccant wheels is investigated. • Response surface method is used for establishing novel regression models. • Effects of operating variables on corresponding responses are comprehensively evaluated. • Optimum values of input variables have been derived to minimize process outlet temperature and humidity ratio. • A valuable equation for determination of Pareto-optimal points has been proposed. - Abstract: A two-step computational framework based on the combination of response surface methodology and multi-objective optimization is proposed to model the outlet-air state of desiccant wheels and subsequently optimize their operation. Regeneration temperature, surface area ratio, rotational speed, and wheel diameter are considered as decision parameters in the genetic algorithm. The central composite design and response surface methods have been employed to design experiments, establish predictive empirical models, and determine interactive effects of decision variables on response variables—process outlet temperature and humidity ratio. Several experiments have been performed to verify applicability of the proposed methodology and validate obtained results. A value of the coefficient of determination exceeding 0.95 demonstrates high reliability and accuracy of the modeling process involved in the proposed methodology. Results obtained demonstrate greater dominance of the surface area ratio compared to other decision variables in terms of their influence on response variables. After successful validation against experimental data, the developed models have been considered as a combination of two objective functions. A fast and elitist non-dominated sorted genetic algorithm II-based optimization technique has been employed to simultaneously determine optimum values of decision variables. A Pareto-optimum front has been presented to select the best value of each decision parameter from available points of optimum operation, and a valuable equation for Pareto-optimal points has been deduced for each material to assist designers develop an optimum design of desiccant cooling systems.
[en] Highlights: • Preparation and characterization of clay additives based CaCl2 composite desiccant is described. • The exit air humidity ratio relative to inlet air humidity ratio is expressed in terms of percentage reduction in moisture. • Process air relative humidity, air velocity and bed weight influences the dehumidification performance. • Experimental results for percentage reduction in moisture are compared with theoretical mass transfer model. - Abstract: Transported clay suitable for pot making is used as desiccant carrier. Additives like saw dust and horse dung are considered in particle preparation. Particles nearly spherical in shape are prepared manually and are dried under shadow and subsequently the particles are dried at different temperatures. These burnt particles are characterized for pore volume and surface area. The BET test reveals that clay particles subjected to 500 °C possess higher pore volume but clay-horse dung particles exhibit higher surface area. Heat treated particles of clay with additives are impregnated with CaCl2 solution of 50% concentration. The ratio of desiccant water content to surrounding layer water content varies from 14.09 to 75.34 for CaCl2 based composite desiccants. One dimensional PGC mass transfer model for process air through burnt clay – additives - CaCl2 desiccant bed is adopted. The RMSE of measured and predicted results for reduction of moisture content from the process air by composite desiccant beds are in the range of 3.26–13.2%.
[en] Highlights: • The experimental operation of a solar driven hybrid DEC system is presented. • In situ monitoring data are analysed and compared to the system’s expected performance. • Drawbacks affecting the system’s performance are explained. • Fine-tuning of the control and maintenance strategy of the main system components is suggested. - Abstract: The aim of this paper is to report on a two-year operational experience with a solar driven desiccant and evaporative cooling (SDEC) system coupled with a vapour compression heat pump. The main objectives are to analyse the benefits and drawbacks of this innovative hybrid SDEC system, to compare the monitoring results against the expected theoretical ones, and to assess the system’s performance with respect to a reference air handling unit. The comparison focuses on the summer key operation modes using Primary Energy Ratio (PER) as indicator of the entire system performance. The results of the detailed analysis lead to the following conclusions: the specific design of the hybrid SDEC leads to high air quality, simpler control process and low electricity consumption for partial load conditions. The monitoring results show a summer mode PER 20% lower than expected due to underperformance of the desiccant wheel. Nevertheless, this innovative system is still very efficient as its PER is twice as high as the one of the considered reference system. Lastly, suggestions for optimization of the existing system through the fine-tuning of the control strategy of its main components are presented.
[en] Highlights: • An adjacently internally-cooled plate membrane liquid desiccant dehumidifier (AIMLDD) is applied. • The AIMLDD is used for liquid desiccant air dehumidification. • A lumped parameter model is established to study the heat and mass transports in the AIMLDD. • An analytical solution of the performances are obtained and experimentally validated. • The performances of the AIMLDD are about 3.3–9.1% larger than those of the cooling tube type. - Abstract: An adjacently internally-cooled plate membrane liquid desiccant dehumidifier (AIMLDD) is like a four-fluid heat and mass exchanger. The feed air and the solution streams flow in the neighboring channels formed by plate membranes. The water falling film and the sweeping air stream flow in the cooling channel formed by two plastic plates. Absorption heat generated in the solution by absorbing the water vapor transferred from the feed air across the membranes can be taken away by the water. A lumped parameter model is established in a unit cell containing half of a feed air channel, a membrane, a solution channel, and half of a cooling channel, to study the heat and mass transports in the AIMLDD. An analytical solution of the normalized governing equations is obtained. Cooling effectiveness, dehumidification effectiveness, dehumidification rate, energy transfer rate of the feed air, and the ratio of the sensible heat transfer rate of the water to the energy transfer rate are calculated and experimentally validated. The performances of the AIMLDD are compared with those of an internally-cooled plate membrane liquid desiccant dehumidifier with cooling tubes inside the solution channels (IMLDD). The performances of the AIMLDD are about 3.3–9.1% larger than those of the IMLDD.
[en] Polyamide 6-LiCl (PA 6-LiCl) electrospun nanofibrous membranes (NFMs) have been successfully prepared as novel solid desiccant materials. The PA 6 NFM with 20% LiCl mass ratio had a sorption capacity of 1.8 g g−1 at 25 °C and 95% relative humidity, which was 4 times more than that of silica gels. The desorption isobars of the NFMs indicated that over 85% of sorbed water in the NFMs can be desorbed at about 50 °C, and the low regeneration temperature made it promising as an energy-saving desiccant material. The experimental results manifested that the sorption/desorption kinetics of the NFMs better fit the pseudo-second order model. According to scanning electron microscope images and the cycle experiment, the NFMs were also found to possess notably improved stability against moisture and could be recycled with little degradation of performance, which confirmed the practicability of the new desiccant membranes. (paper)
[en] Highlights: • A hybrid liquid desiccant-heat pump for cooling in humid regions was modeled. • Modeling was performed in energy, exergy, economic and environmental aspects. • By two objective functions and eight system design parameters system was optimized. • Using LD-HP system decreased the electricity consumption for 45.6% with COP 4.83. • With payback period 3.37 years, CO2 production was decreased for 3.12 (45.6%) kg. - Abstract: A hybrid liquid desiccant-heat pump (LD-HP) system for cooling in hot and humid regions is modeled and optimized in this paper. This hybrid LD-HP system contained dehumidifying and cooling sections. The whole system was modeled and analyzed in four energy, exergy, economic and environmental aspects. Then the system was optimized using multi-objective Genetic Algorithm (GA) method. With two objective functions (total annual cost and exergy efficiency) and eight system design parameters the optimum values of design parameters were estimated. Results for our case study showed that the proposed optimized LD-HP system decreased the electricity consumption for 33.2% in comparison with that for an electrical HP system during seven months of operation in a year (18.9% due to using desiccant dehumidifying system and 81.1% due to using a heat exchanger instead of an electrical heater). This amount of lower electricity consumption also provided 1.85/year lower CO2 production (33.2%) in comparison with that for a conventional HP system. The COP of LD-HP system at the optimum point was also about 4.83 (in comparison with 2.74 for the conventional case in which heat pump and electrical heater were used). Finally, added equipment to the traditional HP system (dehumidifier, regenerator, heat exchangers, pumps and fans) had 3.04 years payback period.
[en] Highlights: • A novel hybrid desiccant cooling system with a condenser is studied. • Thermal balancing technique is applied for zero and single extraction systems. • A mathematical model of the proposed system using enthalpy pinch method is explained. • The coefficient of performance is improved by 85% using the single extraction. • The investigated hybrid system also produces fresh water as a by-product. - Abstract: This paper focuses on thermodynamic balancing of the regeneration process in a hybrid liquid desiccant cooling/desalination system. The thermal balancing technique is investigated by adding a single extraction between the system regenerator and condenser. This technique is considered as a potential method to reduce energy consumption and increase the hybrid system performance, dramatically. The mathematical procedure to model the proposed system using enthalpy pinch is outlined to study the effect of extraction on the hybrid system performance. The hybrid system, with a single extraction and without extraction (zero extraction), is analyzed in terms of the normalized entropy generation, local enthalpy losses, the coefficient of performance, and proper extraction location. The results show that at enthalpy pinch of 20 kJ per kg of dry air, single extraction system performance is 85.7% better than zero extraction. In addition, the single extraction produces 103.2 kg of fresh water per hour as a by-product compared to 94.2 kg per hour for the zero extraction system.
[en] The aim of the present work was to study Argentinean natural clays with different modifications and their use as desiccants. To this end, bentonite clays collected from different regions were characterized and modified by heat treatment and calcium addition, and the desiccant capacity, in the form of powder and pellets, was studied. In addition, the hydration degree reversibility of these materials was estimated. The mineralogical and structural characterization was performed by X-ray diffraction, Fourier-transform infrared spectroscopy, differential thermal analysis, and thermogravimetric analysis. The textural characterization was performed by adsorption-desorption of N2 , chemical composition by X-ray fluorescence and cation exchange capacity. The water adsorption was determined at 25 °C between 11-98% relative humidity. The results indicated that the desiccant power has a close relationship with the degree of clay purity, drying of the sample and the method of calcium incorporation. (author)
[en] Highlights: • The energy potential and desiccant capacity of two HVAC systems was analysed. • Both HVAC systems served air to a spa room for 6 different climate zones. • The energy consumption of the DW-IEC system was lower than that of the DX system. • High energy savings were obtained with the DW-IEC system for hot climate zones. • These energy savings resulted in better SCOP values for the DW-IEC system. - Abstract: Air handling in buildings with high latent loads usually requires a high-energy cost to satisfy the user’s thermal comfort needs. Hybrid systems composed of desiccant wheels, DW, and indirect evaporative coolers, IEC, could be an alternative to direct expansion conventional systems, DX systems. The main objective of this work was to determine the annual energy consumption of a hybrid system with a DW activated at low temperatures and an IEC, DW-IEC system, compared to a DX system to serve air in a small building with high latent loads, such as spas. Several annual energy simulations for 6 climate zones were performed, analysing electric energy consumption, seasonal mean coefficient of performance, SCOP, and energy consumption per unit of dehumidified water, Econs, of each system. The simulations were based on experimentally validated models. The annual energy consumption of the DW-IEC system was lower than that of the DX system for the 6 climate zones, achieving significant energy savings, up to 46.8%. These energy savings resulted in better SCOP values for the DW-IEC system. Therefore, the proposed DW-IEC system has high potential to reduce energy costs, achieving the user’s thermal comfort.
[en] Highlights: • Performance of closed-type heat-source tower (CCHT) is investigated experimentally. • Correlations of heat and mass transfer coefficients for the CCHT are developed. • Mass transfer coefficient of CCHT is larger than that of liquid dehumidifier. • Latent heat ratio of CCHT is 6–31%, while that of liquid dehumidifier is 42–124%. - Abstract: A heat-source tower heat pump (HTHP) is a potential heating source for space heating due to frost-free characteristic. The heat and mass transfer between the antifreeze solution and air at low ambient temperatures is a key issue for performance enhancement. In this study, a test bench for evaluating the cross-flow closed-type heat-source tower (CCHT) is fabricated to investigate its performance under variable working conditions. Based on the experimental results, correlations of heat and mass transfer coefficients are developed, which can be used to predict the performance of the CCHT. As a result, the mass transfer coefficient between the solution and the air of the CCHT is 0.015–0.051 kg m−2 s−1, while that of the liquid desiccant dehumidifier is 0.0037–0.015 kg m−2 s−1. However, the latent heat ratio of the CCHT is 6–31%, while that of the liquid desiccant dehumidifier is 42–124%.