In this paper, a new transient response theory is developed to predict the moisture or water transfer time constant and the fully developed flow moisture effectiveness of dehumidifier wheels and latent energy effectiveness of energy wheels. A new method to experimentally determine the latent energy or water transfer time constant is presented. This analytical model, which uses the new empirical time constant and its theoretical model, is then corrected for entrance length, carryover, sorption temperature changes, and manufacturing variations. The model permits the designer of these wheels and the HVAC applications engineer the opportunity to explore how each flow channel property and wheel operating condition changes the effectiveness. The theoretical latent and sensible effectiveness of energy wheels with molecular sieve and silica gel coatings are determined for two energy wheels, and these results are compared with the effectiveness calculated using experimental data at steady state ARI testing conditions. The comparisons are in agreement within the uncertainty bounds. The analytical model is also used to predict the latent effectiveness for two similar energy wheels where it is assumed that the desiccant coatings are interchanged. This comparison shows that the new hypothetical silica gel coating on the energy wheel would be more effective than the new molecular sieve coating on an energy wheel for the transfer of water vapor. The moisture transfer effectiveness is predicted for two dehumidifier wheels with the same design and desiccants as the tested energy wheels (i.e., the molecular sieve wheel and silica gel wheel). These wheels, which are used for supply air drying, are operated with the same ARI supply air condition and 5 rpm. Considering the comparisons for energy wheels, the molecular sieve and silica gel coatings are predicted to result in similar performance factors but with much lower effectiveness. This reduction in moisture effectiveness for the dehumidifier wheels is caused mostly by the reduction in wheel speed. Further research is required to investigate the performance of dehumidifier wheels at lower wheel speeds.
Wei Shang?Robert W. Besant
Combined heat and power (CHP) involves on-site or near-site generation of electricity along with utilization of thermal energy available from the power generation process. CHP has the potential of providing a 30% improvement over conventional power plant efficiency and a CO2 emissions reduction of 45% or more as compared to the US national average. In addition, an overall total system efficiency of 80% can be achieved because of the utilization of thermal energy that would be wasted if only the electric power were utilized, and because of the reduction of transmission, distribution, and energy conversion losses. The current research is being carried out in a four-story educational office building. This research focuses on the design, installation, and analysis of a modular CHP system consisting of a natural gas fired reciprocating engine generator with a liquid desiccant dehumidification system. The engine generator provides 75 kW of electric power to the building load bus while the combined waste heat from the exhaust gases and jacket water are used to regenerate the liquid desiccant. The liquid desiccant unit dehumidifies and cools the ventilation air to the building and supplies it to the mixed air section of the roof top unit. This paper discusses the various aspects involved in the design and installation of the system such as the heat recovery loop design and the electrical interconnection with the building load bus. Test results are also presented and the performance is compared to a traditional power plant with a conventional heating, ventilating, and air-conditioning system.
Sandeep M. Nayaka?Yunho HwangbEmail:firstname.lastname@example.org?Reinhard Radermacherb?
[a]EMCOR Energy Services 505 Sansome Street, Suite 1600, San Fransisco, CA 94111, USA; [b]Center for Environmental Energy Engineering, Department of Mechanical Engineering, 3163 Glenn Martin Hall Bldg., University of Maryland, College Park, MD 20742, USA;
Enhanced efficiency of the adsorption process in the dehumidifier is a key element for improved performance of desiccant cooling systems. Due to the exothermic nature of the adsorption process, the dehumidification and cooling capacity are limited by significant temperature changes in the adsorption column. In the present study, the effects of integration of sensible and latent heat storage particles in the desiccant bed for in situ management of released adsorption heat are investigated. For this purpose, column experiments are performed using an initially dry granular bed made of silica-gel particles or a homogeneous mixture of silica gel and inert sensible or latent heat storage particles. The packed bed is subject to a sudden uniform air flow at selected values of temperature and humidity. Also, a packed bed numerical model is developed that includes the coupled non-equilibrium heat and moisture transfer in the solid and gas phases. Investigations of the heat and mass transfer characteristics are reported using the composite structure and the results are compared with the base case of simple silica gel bed. Improved desiccant cooling system performance can be obtained by appropriate adjustment of desiccant cycle operation and proper choice of the volume ratio of thermal energy storage particles.
M. A. Rady1,2
BACKGROUND: Glufosinate-ammonium (GA) is used for chemical vine killing of potato crop before harvest. Over two years we tested the effect of GA, applied at 0.48 kg ha-1 of active ingredient, on desiccation of haulms, yield and some qualitative traits of tubers of three cultivars of early potatoes, a typical crop of southern Italy and large areas of the Mediterranean countries. RESULTS: Seven days after treatment desiccation reached 90% of leaves for all cultivars. At harvest, the highest desiccation percentage of haulms was 85% in Spunta. The desiccant did not affect yield in either year during which the analysis was carried out, but it increased the percentage of tubers in the range of 35-70 mm in diameter. In the first year, the treatment decreased dry matter and vitamin C content and increased free sugar concentration in tubers. In the second year, no negative effects on the nutritional traits were observed. Strong differences were found on yield and quality between the two years as a result of the different climatic conditions. CONCLUSION:In the two experiments GA had no influence on yield, but increased the percentage of potato tubers in the 35-70 mm range. From a qualitative standpoint in 2004 the chemical vine-killing treatment had some negative effects (decreased dry matter and vitamin C content, increased free sugar concentrations), whereas no such negative effects were observed in 2005.
Maria Gonnella1 *?Osman Ayala2?Annalisa Paradiso3?Vito Buono5?Laura De Gara3 4?Pietro Santamaria2?Francesco Serio1
Istituto di Scienze delle Produzioni Alimentari (CNR), via Amendola 122, 70126 Bari, Italy;Dipartimento di Scienze delle Produzioni Vegetali, Università di Bari, via Amendola 165/A, 70126 Bari, Italy;Dipartimento di Biologia e Patologia Vegetale, Università di Bari, via Orabona 4, 70125 Bari, Italy;Centro Integrato di Ricerca, Università Campus Bio-Medico, via Alvaro del Portillo 21, 00128 Rome, Italy;Associazione Produttori Patate, via M. Signorile 44, 70121 Bari, Italy
The interest in desiccant cooling cycles has been continuously growing in the last years, driven by its simplicity and flexibility of arrangements, as well as the claim for environmentally sound acclimatization techniques. Accordingly, the present work aims at developing and solving a numerical model that is able to represent the conjugated heat and mass transfer within a desiccant material. A mathematical model is constructed based on a number of simplifying assumptions, so as to achieve a relative mathematical simplicity while retaining physical reasoning. Heat and mass balances are applied to elementary control volumes, resulting in a system of four partial differential equations that account for the temperature and humidity content within the air flow channel and the sorbent layer, respectively, as well as an algebraic equation, which stands for the adsorption isotherm. Three different desiccant materials are represented by a generalized adsorption isotherm, which is characterized by a single parameter. The model is solved using a fully implicit finite-volume discretization technique. The results showed that a balance between the properties of commercially available adsorptive materials is desirable if the coefficient of performance is to be maximized.
Nóbrega, Carlos E. L.1Email: email@example.com?Brum, Nísio C. L.2
Mechanical Engineering Department, Centro Federal de Educação Tecnológica, Rio de Janeiro, Brazil;Mechanical Engineering Department, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil
During steady operation, regenerative wheels are subjected to cyclic step changes in inlet properties. The equation for the transient humidity step response of a desiccant coated rotary air-to-air wheel is developed from physical principles using a model similar to that used for the sensible energy or temperature response. This fully developed flow model is used to derive a characteristic moisture transfer time constant in terms of four dimensionless flow-channel desiccant-coating properties and the time duration of airflow through the wheel. Since the moisture diffusion time delays within the desiccant coating are significant for typical energy wheels and often desiccant dryer wheels, an empirical term is introduced with this time constant to account for the effects of the duration of exposure in the two types of wheels.
Wei Shang?Robert W. Besant
Radiant cooling has shown great advantages when it comes to improving thermal comfort and can also reduce energy demand. But the system may have a high condensation risk if used with a high latent load. This study proposes a new type of ventilation system that uses a radiant cooling panel and air supplied from a liquid desiccant dehumidification system, which provides very dry supply air and chilled water for radiant cooling. This study uses an office with a high latent load to demonstrate the application of such a system in a hot and humid climate. By using a validated computational fluid dynamics (CFD) program to calculate the distributions of air temperature, humidity, and velocity, as well as the predicted mean vote in the office, the results show that the new system could reduce the risk of condensation on the cooling panels and offer a comfortable indoor environment. The new system has great potential when it comes to use in hot and humid climate regions.
Yonggao Yin?Xiaosong Zhang?Qingyan Chen
The cosorption characteristics of water and toluene vapors in various concentrations of triethylene glycol (TEG) solution flowing through a packed-bed dehumidifier are investigated in this paper. A multi-component model was constructed using the reported equilibrium relationships of toluene and water vapors in TEG solutions together with the Krishna-Standart multi-component mass transfer correlation. The effects of liquid-to-air ratios, TEG inlet temperatures, air inlet temperatures were reported on the moisture and toluene removal rate as well as the moisture and toluene removal efficiency of the packed dehumidifier. Running the packed dehumidifier in a higher liquid-to-gas flow ratio generally increased the removal rates and efficiencies of both water vapor and toluene vapor from the airstream. Increasing inlet temperatures of the TEG solution led to a decrease in the removal rate of water vapor when running the packed dehumidifier at a high liquid-to-gas flow ratio. However, there was no significant change in the toluene vapor removal rate or toluene removal efficiency when the flow rate of the inlet TEG solution was increased.
C. K. Chau?W. M. Worek
In this work, the effect of design and operating parameters on the performance of a multilayer desiccant packed bed was theoretically and experimentally studied. In the experimental work, a silica gel packed bed of eight layers has been studied. The transient value of the mass of adsorbed water and desorbed water was measured for different values of the bed length. The theoretical model shows the dependence of the dimensionless value of water content in the bed on the dimensionless time. Also the model shows that the dimensionless temperature depends on the bed characteristics and bed water content. The effect of inlet air humidity and velocity on the adsorption process for each bed layer was studied at different inlet velocities and at different air humidities. The effect of inlet temperature on desorption process for each packed bed layer was also studied at different inlet temperatures. The theoretical model also introduces an equation which can be used to predict the optimum bed length. Also, the optimum length of the bed can be recommended from the experimental results according to the operation time. Good agreement between experimental and theoretical results was found.
Mechanical Power Department, Faculty of Engineering, Tanta University, Egypt
The regenerator is one of the key components in liquid desiccant air-conditioning systems, in which desiccant is concentrated and can be reused in the system. The regeneration heat is supplied into the regenerator by either hot air or hot desiccant. The heat and mass transfer performances of these two regeneration modes are analyzed and compared in detail. In the hot air driven regenerator, the parallel-flow regenerator has the best mass transfer performance and the counter-flow performs poorest under the same conditions, because the heat transfer process is the governing process and the mass transfer performance depends on the promotion of the heat transfer to the mass transfer process. In the hot desiccant driven regenerator, counter-flow configuration has the best mass transfer performance and parallel-flow is the poorest at the same conditions, since mass transfer is the governing process. Regeneration heat should be chosen to heat the desiccant instead of the air in the packed bed regenerator, since the hot desiccant driven regenerator has apparent better mass transfer performance. The proposed regeneration mode and flow pattern will be helpful in the design and optimization of the regenerators.
X.H. Liu?Y. Jiang?X.Q. Yi
Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, PR China