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The control of CO2 emissions released in the atmosphere is an indisputable need in order to avoid environmental and health consequences, as well as to reach European and global targets for climate change confrontation. During the last decades, various methods have been proposed towards this direction. As fossil fuels remain dominant in the energy field, technologies focused on CO2 capture have been developed, known as Carbon Capture Systems (CCS). During the last decade, CCS have managed to become cost-competitive in relation to other technologies. However, CCS need to receive strong policy support in order to develop solid and applicable business cases. This work studies reactive absorption, a post-combustion method to remove CO2 from flue gases using aqueous chemical solutions that react with CO2 and capture it. In specific, this paper focuses on two alkanolamines as absorbents, Monoethanolamine (MEA) and 2-amino-2-methyl-1-propanol (AMP). Following previous work, a dynamic model was developed in Matlab®, simulating transient behavior of a packed column for reactive absorption. The methodology followed to solve the set of partial differential equations describing the problem was the Finite Differences Method (FDM). Based on experimental data from a pilot plant various scenarios were tested in order to investigate the reaction of the system in different operational perturbations, such as flow disturbances, changes in temperature and in the solution’s molarity. These factors were studied parametrically and the outcome of these simulations was used to perform a Sensitivity Analysis (SA) in order to determine the important parameters influencing CO2 absorption performance. The parametric analysis showed that the inlet gas flow rate was the most influential factor determining the CO2 absorption level for both solvents. In following, the temperature of the liquid entering the column was also statistically important having strong impact on absorption. Surprisingly, the flow of the lean solvent wetting the column seems to have lower impact than the gas flow. Thus, increased solvent flow does not ensure respective enhancement of the CO2 absorption. Molarity is also very important as the quantity of amine in the solution determines the quantity of the captured CO2. In terms of reaction time towards disturbances, changes in liquid molar flow, liquid temperature and molarity seem to lead to higher reaction time until the system reaches a new steady state value (2–5 min). Finally, the results showed that there is a maximum capture level, beyond which the absorption cannot be further enhanced for specific conditions.

Solar driven cooling systems can be a promising alternative to traditional electrical grid powered air conditioning plants. These systems use either thermal or electrical processes to convert solar radiation into cooling. Despite the numerous theoretical and experimental researches that have been carried out in recent years on solar cooling it is still not clear which system presents the best performance. In this study the performance of three different solar cooling systems is examined, namely: 1) a solar electrical, 2) a solar thermal and 3) a hybrid solar electrical-thermal cooling system. The first one system uses PV collectors in order to supply a conventional electrical vapor compression chiller. The second one utilizes thermal collectors in order to power a heat driven adsorption chiller and the third one utilizes PV/T collectors to feed both an electrical and an adsorption chiller. The study is performed for Athens, Greece climatic conditions during an assumed summer period from May to September. The optimum tilt angle of the solar collectors is calculated for maximization of the solar radiation during summer period. It is found to be 14°. The best performance is observed for the solar electrical cooling system utilizing the mono-Si PV modules with a maximum Solar COP of around 0.47 while the lowest is presented by the a-Si PV modules powered cooling system with a minimum Solar COP of about 0.13.

Chemical absorption (or reactive absorption) is a common technique used for gas sweetening purposes, capturing acid components from point source emissions or even from ambient air (Cheng and Tan, 2011). CO2 reactive absorption is extensively studied as it is a very effective process, targeting to the emissions reduction of this dangerous greenhouse gas, with wide perspective for further optimization. Reactive absorption combines physical absorption and reaction between acid components and basic solvents, thus the modelling of these mechanisms is challenging and requires a multi-component design (Koronaki et al., 2013). In this work, a rate-based, steady state model, based on the two-film theory, is presented and it is validated for two alkanolamine based aqueous solvents, Mono-ethanolamine (MEA) and 2-Amino-2-methyl-1-propanol (AMP). The model predictions are validated against data extracted by experiments conducted in a pilot scale absorption column with random packing. The comparisons between the model results and the experimental data reveal satisfactory predictions of the various parameters’ profiles along the column. A parametric analysis is performed, investigating the effect of inlet gas and liquid flow rates, the initial liquid loading, the type of packing, the initial amine solution concentration and the gas and liquid inlet temperatures on the CO2 absorption efficiency and on the temperature of the liquid getting out the column. The sensitivity analysis shows that the gas flow rate entering the column is the most important factor influencing the CO2 removal efficiency. In addition, inlet liquid temperature has the greater statistical significance on the outlet liquid temperature. The comparison of the two solvents under the same reference conditions shows that MEA is more efficient as solvent for CO2 absorption.

Thermally driven chillers offer a reliable and environmentally friendly alternative to conventional electrically powered chillers. Solar energy can be utilized in order to provide the necessary heat to these heat driven cooling cycles. This work examines the application of an adsorption chiller cooling system powered by solar energy under climatic conditions of eastern Mediterranean area. Athens and Nicosia the capitals of Greece and Cyprus respectively and Alexandria of Egypt are the three examined cities. A single-stage, two bed adsorption chiller is selected for investigation. A lumped parameter model is used to simulate the performance of the cooling system. The maximum cooling capacity of the system is about 14.7 kW and is observed for Nicosia climatic conditions in July. Subsequently, the performance of the system is examined for various types of solar collectors under Athens climatic data. The utilization of hybrid photovoltaic-thermal (PV/T) collectors is studied and the production of electric power is calculated. The overall energy and exergy efficiencies are also calculated for the combined systems. Following the analysis of the adsorption chiller performance (cooling capacity, COP), the selective solar thermal collector is the optimum collector for the cooling system with maximum cooling capacity and COP around 16 kW and 0.51 respectively. The overall energy efficiency of the system is also maximized when the selective solar collector is utilized (21.7%). However the overall exergy efficiency is maximized when the unglazed PV/T solar collector is used (11.35%). Finally, the effect of the cooling tower's cooling water outlet temperature on the adsorption chillers performance is investigated.

Liquid desiccant dehumidification systems have been used for many years in specialized applications. The performance of liquid desiccant systems relies heavily on heat and mass transfer characteristics of two critical components: the dehumidifier and the regenerator. The purpose of this study is to provide a comprehensive review of heat and mass transfer correlations developed to mathematically model the adiabatic absorption and desorption process. There has been an attempt to describe the most important characteristics of each research, such as the method adopted, the assumptions used, validation of the data, as well as the most important results. It was found that most work considers the desiccant solution flowing counter currently with air and the use of structured packing instead of random. Fewer researchers considered co-flow configuration or desiccants other than salts.

Liquid desiccant systems have been proposed as energy saving alternatives to the conventional vapor compression systems for handling the latent load. This paper presents the results from a study of the performance of a counter flow liquid desiccant dehumidifier. A heat and mass transfer theoretical model of an adiabatic packed column has been developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under various operating conditions. Good agreement was found between experimental tests and the theoretical model, with the maximum deviation being ±2.9% in air outlet temperature, ±15.9% in air outlet humidity ratio and ±2.8% in solution outlet temperature. Following the model validation, the rate and the efficiency of the dehumidification process were assessed under the effects of variables, such as air temperature and humidity, desiccant temperature and humidity and air and desiccant flow rates. The three most commonly used liquid desiccant solutions, namely LiCl, LiBr and CaCl 2 were evaluated against each other. The results show that high absorber efficiency and system efficiency could be achieved under humid conditions, low air mass flow rates and LiCl as the desiccant solution.

A thermodynamic model was developed and used to assess the sensitivity of thermal performance characteristics of a closed wet cooling tower to inlet air conditions. In the present study, three cases of different ambient conditions are considered: In the first case, the average mid-winter and mid-summer conditions as well as the extreme case of high temperature and relative humidity, in Athens (Greece) during summer are considered according to the Greek Regulation for Buildings Energy Performance. In the second case, the varied inlet air relative humidity while the inlet air dry bulb temperature remains constant were taken into account. In the last case, the effects on cooling tower thermal behaviour when the inlet air wet bulb temperature remains constant were examined. The proposed model is capable of predicting the variation of air thermodynamic properties, sprayed water and serpentine water temperature inside the closed wet cooling tower along its height. The reliability of simulations was tested against experimental data, which were obtained from literature. Thus, the proposed model could be used for the design of industrial and domestic applications of conventional air-conditioning systems as well as for sorption cooling systems with solid and liquid desiccants where closed wet cooling towers are used for precooling the liquid solutions. The most important result of this theoretical investigation is that the highest fall of serpentine water temperature and losses of sprayed water are observed for the lowest value of inlet wet bulb temperature. Hence, the thermal effectiveness, which is associated with the temperature reduction of serpentine water as well as the operational cost, which is related to the sprayed water loss due to evaporation, of a closed wet cooling tower depend predominantly on the degree of saturation of inlet air.

A detailed model was developed and employed to examine the thermal performance of a closed wet cooling tower. The model is capable of predicting the variation of air thermodynamic properties, sprayed and serpentine water temperature as well as heat transfer rates exchanged between air and falling water stream inside the indirect wet cooling tower. The reliability of simulations was tested against experimental data obtained from the literature. A parametric study was conducted to evaluate the thermal behaviour of the indirect cooling tower under various air mass flowrates, serpentine water mass flowrates and inlet temperatures. The results of the theoretical investigation revealed an increase in cooling capacity and percentage loss of sprayed water due to evaporation, with increasing air mass flowrate. On the other hand, the increase of serpentine water mass flowrate resulted in slight increase in the overall temperature reduction of serpentine water. The effect of variable serpentine water inlet temperature on thermal performance of the indirect wet cooling tower was insignificant compared to other cases.

Thermally driven chillers also known as sorption heat pumps have drawn considerable attention in recent years. They can be divided into two main categories: absorption (liquid-vapor) and adsorption (solid-vapor) systems. Even though adsorption cycles have relatively lower coefficient of performance compared to absorption cycles, however they prevail in terms of heat source, electric consumption for moving parts, crystallization etc. In order to overcome the drawback of low COP and specific cooling capacity, nanofluids, i.e. mixtures of nanometer size particles well-dispersed in a base fluid, can be used as heat transfer fluids as recent experimental and theoretical research has proved that nanofluids can exhibit a significant increase on heat transfer. In this study a two bed, single-stage adsorption chiller which utilizes the silica gel-water pair as adsorbent-refrigerant is simulated. The cooling capacity and the COP of the chiller are calculated for various cycle times. The usage of nanofluids as heat transfer fluids in the chiller evaporator and condenser and their effect on chiller performance and size is investigated. It is proved that the presence of nanofluids at different volume concentrations will enhance the cooling capacity and the COP of the adsorption chiller and therefore will lead to smaller, in terms of size, heat exchangers.

A detailed model is presented for the analysis of the thermodynamics and heat and mass transfer in LiBr–H2O absorption on a cooled horizontal tube. The hydrodynamic description is simplified by using Nusselt’s equations. A consistent thermodynamic formulation is developed demonstrating the real nature of water-vapour and the electrolytic nature of the aqueous solution. In terms of the transport phenomena description, a two-dimensional model is proposed for the heat and mass transfer inside the LiBr–H2O film and an exact energy balance is applied in the coolant stream. An analysis of the dependence of the absorption process on the main operating and geometric parameters is presented for the case of the LiBr–H2O absorption on a single cooled horizontal tube. The agreement of the proposed model results with existing measurement data in the non-wavy flow regime is verified. � 2006 Elsevier Ltd. All rights reserved.

An analytical model was developed to describe thermodynamically the water evaporation process inside a counter-flow wet cooling tower, where the air stream is in direct contact with the falling water, based on the implementation of the energy and mass balance between air and water stream describing thus, the rate of change of air temperature, humidity ratio, water temperature and evaporated water mass along tower height. The reliability of model predictions was ensured by comparisons made with pertinent experimental data, which were obtained from the literature. The paper elaborated the effect of atmospheric conditions, water mass flow rate and water inlet temperature on the variation of the thermodynamic properties of moist air inside the cooling tower and on its thermal performance characteristics. The analysis of the theoretical results revealed that the thermal performance of the cooling tower is sensitive to the degree of saturation of inlet air. Hence, the cooling capacity of the cooling tower increases with decreasing inlet air wet bulb temperature whereas the overall water temperature fall is curtailed with increasing water to air mass ratio. The change of inlet water temperature does not affect seriously the thermal behaviour of the cooling tower. Copyright © 2005 John Wiley & Sons, Ltd.

A detailed model is developed for treating the 2-D water vapour absorption into an aqueous solution of LiBr, which is flowing over a vertical tube. Numerical predictions are presented for the spatial variation of solution temperature and concentration in the axial direction and within the LiBr–H2O falling film taking into account the flow and temperature rise of coolant inside the tube. Special care was given to the adequate description of the thermodynamic properties of superheated water vapour, which is treated as a real gas. Emphasis was also given in demonstrating the electrolytic nature of the aqueous solution, introducing thus the effect of variable heat of absorption on the temperature field. A parametric study for examining the effect of absorption pressure, solution mass flow rate and inlet coolant temperature on the absorption process was carried out implementing the model under laminar flow conditions. A good coincidence with experimental observations obtained for solution flows with Re

Liquid desiccant air conditioning systems have recently been attracting attention due to their capability of handling the latent load without super-cooling and then reheating, as happens in the conventional compression-type air conditioning systems. In liquid desiccant cooling cycles, a sorbent solution is employed to dehumidify the air, circulating between the two critical components; the dehumidifier and the regenerator. As the strong desiccant solution is sprayed on top of the internally cooled dehumidifier, it flows down by gravity and comes in contact with the process air. The desiccant solution which, by definition, has a strong affinity for water vapor absorbs moisture from the air. The end of the process finds the air cool and dehumidified and the solution diluted. The diluted desiccant solution enters the regenerator in order to retrieve its initial concentration. Hot water derived from a low temperature source supplies the necessary heat to the solution and the excessive water content is evaporated. At the end of the process, the hot humid air is rejected to the ambient and the concentrated solution is driven to the dehumidifier. The complex heat and mass transfer phenomena, occurring both in the dehumidifier and regenerator, has been the subject of earlier work by the authors. Based on the knowledge gained, a liquid desiccant system was installed at the National Technical University of Athens, Laboratory of Applied Thermodynamics, for experimental purposes. The liquid desiccant system was constructed by the German company L-DCS [1]. The main components of the system are the dehumidifier, the regenerator and the evaporative cooler. The system uses water as the cooling medium and LiCl solution as the desiccant. It also employs two storage tanks, one for the concentrated solution and one for the diluted. The purpose of this publication is to present the newly installed liquid desiccant system, to predict the performance of the dehumidifier and to carry out preliminary design optimization.

Liquid desiccant air conditioning systems have recently been attracting attention, owing to their merits in handling the latent heat. Desiccant systems avoid not only the energy penalty caused by overcooling and reheating, but also the bacteria generation caused by condensed water. They can also significantly reduce the electricity peak load caused by conventional compression type air conditioning systems, especially in hot and humid regions. Desiccant systems are thus more energy efficient, healthy and environmentally friendly than conventional mechanical cooling. This paper presents the results from a theoretical study of a liquid desiccant system that provides air conditioning to a typical office building. A coupled heat and mass transfer analytical model was developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under Mediterranean conditions. A parametric analysis was implemented to investigate the effects of ambient temperature and humidity ratio on the dehumidification mass rate, the load coverage and the thermal COP of the system. Simulation results showed that under hot and humid weather, the COP reaches its maximum value, 1.075. However, as the weather becomes more humid, the latent load coverage of the system is decreased and as it becomes hotter, the sensible load coverage of the system is decreased. The maximum latent load coverage, 91.8%, happened at 40°C, 0.011kgw/kgdα. Results can be useful for researchers and engineers.Copyright © 2012 by ASME

Liquid desiccant systems are emerging as promising alternatives to achieve humidity control in a variety of applications with high latent loads and low humidity requirements. Their advantage lies on their ability to handle the latent load without super-cooling and then reheating the air, as happens in a conventional compression-type air conditioning system. This paper presents the results from a study of the performance of a counter flow internally heated liquid desiccant regenerator. A tubular heat exchanger is proposed as the internally heated element of the regenerator and water as the heating fluid. The desiccant solution is sprayed into the internally heated regenerator from the top and flows down by gravity. At the same time, ambient air is blown from the bottom, counter-flowing with the desiccant solution. The desiccant is in direct contact with the air, allowing for heat and mass transfer. The water, flowing inside the tubes of the regenerator, provides the necessary heat for regeneration. A heat and mass transfer theoretical model has been developed, based on the RungeKutta fixed step method, to predict the performance of the device under various operating conditions. Experimental data from previous literature have been used to validate the model. Excellent agreement has been found between experimental tests and the theoretical model, with the deviation not exceeding ±6.1%. Following the validation of the mathematical model, the dominating effects on the desorption process have been discussed in detail. The three most commonly used liquid desiccant solutions (LiCl, LiBr, CaCl2) and two different flows (DDU: water downward – desiccant downward – air upward, UDU: water upward – desiccant downward – air upward) have been also evaluated against each other. Considering the flow analysis, the type of flow does not affect the regeneration capacity as much as the type of the desiccant solution. It has been concluded that high regeneration rate can be achieved under DDU flow (water downward – desiccant downward – air upward), low solution concentration, high air inlet temperature, high solution inlet temperature, low air inlet humidity ratio and CaCl2 as the desiccant solution.

Liquid desiccant air conditioning systems have recently been attracting attention due to their capability of handling the latent load without super-cooling and then reheating the air, as happens in a conventional compression-type air conditioning system. This paper presents the results from a study of the performance of an internally cooled liquid desiccant dehumidifier. A plate heat exchanger is proposed as the internally cooled element of the dehumidifier and water as the cooling fluid. The desiccant solution is sprayed into the internally cooled dehumidifier from the top and flows down by gravity. At the same time, fresh humid air is blown from the bottom or top, counter-flowing or co-flowing with the desiccant solution. The desiccant is in direct contact with the air, allowing for heat and mass transfer. The cooling water, flowing inside the plates of the dehumidifier, carries out the heat of the crossed air and solution. A heat and mass transfer theoretical model has been developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under various operating conditions. Experimental data from previous literature have been used to validate the model. Excellent agreement has been found between experimental tests and the theoretical model, with the deviation not exceeding ±4.1% for outlet air temperature and ±4.0% for outlet humidity ratio. Following the validation of the mathematical model, the dominating effects on the absorption process have been discussed in detail. Namely, effects of flow configuration, air inlet temperature, humidity and flow rate, as well as desiccant inlet temperature, concentration and flow rate have been investigated against the dehumidification rate and the cooling efficiency. The two most commonly used liquid desiccant solutions, namely LiCl and LiBr have been also evaluated against each other. The results suggested that high dehumidification mass rate can be achieved under counter flow between air and solution, low air mass flow rates, low cooling water temperature, low desiccant temperature and LiCl as the desiccant solution.Copyright © 2012 by ASME