Absorption system have been investigated for many years. However, coefficient of performance COP or heat gain factor HGF for absorption systems are significantly lower than those for conventional compression systems. This has restricted their wide application. This paper discusses the behavior of mixture NH 3 -H 2 O through of an ejector, operating in an absorption heat pump system. This combination improves the performance of conventional absorption system and with the phasing out of ozone-damaging refrigerants, absorption refrigerators, heat pumps and air-conditioning now provide a potential alternative. For the detailed calculation of the proposed system a method has been developed, which employs analytical functions describing the thermodynamic properties of die mixture. The influence of three major parameters: generator, condenser and evaporator temperature, on ejector efficiency and heat gain factor of the system is discussed. Also the maximum value of HGF was estimated by correlation of above three temperatures.

In the present work a new vapor pressure equation is proposed which combines all advantages of the most significant equations, i.e. • As Wagner equation, it is capable of representing the correct shape of the vapor pressure curve for expanded ranges of the temperature. • As Antoine equation, it can be inversed expressing boiling point as a function of the pressure. • As in Lee-Kesler and two reference fluids methods, it has been found a very useful and more accurate correlation in terms of the reduced temperature. Another advantage of the new equation is its fundamental, single and generalized form for all substances. Extensive test of the proposed equation using experimental/smoothed data for twenty-two pure fluids indicates that it normally predicts very well the vapor pressure from the critical down to the triple point. (Overall Average Mean and Overall Average Max. Error 0.0526% and 0.1195% respectively). A predictive method too for the triple point pressure has been developed (Mean Error 1.1553%).

The aim of the research in this paper is a second law analysis of a Stirling cryocooler. A one-dimensional model is proposed for the simulation of the gas flow in the expansion space, the regenerator, the warm-end, the compression space and the compressor. Helium gas is selected as the working medium. An algorithm has been developed considering parametrically the most from the main operational tasks of the thermodynamic cycle. Performance indices such as heat input, efficiency, external dimensions of the engine and technical requirements are taken into account as constraints. Engine operating parameters i.e. speed, external temperature, mean pressure are fixed. The regenerator loss has a critical influence on the cryocooler efficiency and the reduction of this kind of internal irreversibilities is extremely difficult due to the generator is subject to rapidly cycling flows accompanied by steep temperature gradients and large pressure variations. The second flow analysis of the regenerator identifies two principal losses, the irreversible internal heat transfer into the solid matrix and the hydraulic resistance. An optimization technique leads to entropy generation charts, extremely useful for a good design of the regenerator. Finally the main thermodynamic characteristics (net refrigeration, power input and the coefficient of performance) of the cryocooler are given both cases with and without external and internal irreversibilities.

It is a general trend today, the old centrifugal machines to be replaced by new absorption machines. The mass flow rate of the cooling water in the centrifugal machines is normally 30% less than that in the two-stage absorption chiller for the same refrigerating capacity. Some absorption chillers manufacturers have updated and improved the double-effect technology increasing the cooling water temperature difference from the typical value of 5.5°C to 7.4°C and reducing the cooling water flow rate by about 30%. Using such a modern double effect absorption unit to replace a centrifugal chiller the same cooling water circuit can be used and the total cost of the retrofit is minimized. In this case a new flow pattern of the cooling tower is developed, and in this paper the design of a new tower fill is predicted taking into account the new factors characterizing the operating conditions and the required performance of the tower. As an example, the operational curves of a modified cooling tower (1500 KW cooling power) used by a 240 RT double-effect absorption chiller are presented.

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.

In recent years different equations of state for environmentally friendly refrigerants have been developed from different sources of experimental data which, in many cases, use a very large number of auxiliary functions and coefficients. A rather interesting equation that has not been recollected lately, proposed by Wilsak and Thodos in 1985, appears to have a convenient form for effective and significantly accurate calculations of the thermophysical properties of substances. Wilsak-Thodos equation of state presents a relatively simple form as it correlates experimental data with the corresponding ones at saturation and contains only three density dependent functions. For this reason, this study attempts to identify the coefficients of the equation approximating accurately the experimental data and attributing in an integrated manner with auxiliary equations for some of the most common of the environmental friendly refrigerants.

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, 0.011kg w /kg dα . Results can be useful for researchers and engineers.

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.

The Stirling engine, as an external combustion engine, can be powered using a variety of heat sources including the continuous combustion process thus achieving significantly reduced emissions. Energy systems powered by a Stirling engines meet the needs of various applications not only in the domestic and industrial sections but in military and space gadgets as well. Stirling engines can also be used as cryocoolers in medical applications where they are called to achieve very low temperatures. Each energy system using Stirling Engine optimizes its performance in specific operating conditions. The system capacity depends on the geometric and structural characteristics, the design of the unit, the environment in which the engine is allowed to it works as well as the size of the load. In order to study the function and the efficiency of Stirling energy systems a CHP SOLO 161V -ALPHA TYPE STIRLING ENGINE was installed in the Laboratory of Applied Thermodynamics of NTUA. A thermodynamic analysis has been conducted using appropriate computing codes. The effect of each independent variable on the system performance was investigated. The study was divided into distinct levels of detail, bringing out each variable. Initially, the performance of the heat engine was examined assuming an ideal regenerator. Then, the effectiveness of the regenerator was evaluated as well as its effect on the engine performance, while the effect of the pressure drop and the energy dissipation on the engine efficiency was also investigated. Measurements were conducted using different operational conditions concerning the heating load of the engine. The effect of the geometrical characteristics of the regenerator on power output and engine performance was examined based on the results of a simulation analysis. Moreover, the power output and the efficiency of the machine in relation to the thermal load of the unit and the average pressure of the working medium were investigated. Major performance input characters affecting geometrical and operational parameters of the unit were identified leading to unit optimization with specific combinations leading to increased system performance. Simulation results were validated by comparison to corresponding values obtained by relative experiments conducted with the SOLO unit. Finally, a sensitivity analysis was performed in order to investigate the effect of the operating conditions on the performance of an alpha type Stirling Engine.

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 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 ±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, CaCl 2 ) 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 CaCl 2 as the desiccant solution.

One of the most important components of an absorption air-conditioning/heat pump system is the absorber, where the refrigerant water vapour is absorbed into the liquid solution. While absorption systems have been in use for several years, the complex transport phenomena occurring in the absorber are not fully elucidated yet. Thus, an attempt is made to model the absorption process of water vapour in aqueous solutions of lithium bromide considering a falling-film, vertical-tube absorber. The proposed analysis is based on the formulation of four differential equations describing the spatial variation (parallel to the tube-axis) of solution mass, temperature, mass fraction and coolant temperature. The system of ordinary differential equations is numerically solved using a non-stiff numerical method. Thermophysical properties and especially, heat and mass transfer coefficients are calculated using widely-accepted and reliable relationships, which are extracted from the literature using recently published information on wavy-laminar flows. In the present study, the questionable assumption of treating the water vapour as an ideal gas is heavily modified utilizing. Consequently, the hypothesis of saturated water vapour at the steam-solution interaction surface is revised by introducing an energy difference between the superheated steam and the liquid water within the binary solution. The last correction encouraged us to compare theoretical results for solution temperature, mass fraction and mass flow rate, which were obtained using both assumptions. It was proved that the initial treatment causes an underestimation of the absorbed steam mass and correspondingly, an underestimation of solution temperature and mass fraction at the mass exchange interface. An attempt is made also to identify the effect of mass transfer coefficient on the effectiveness of the absorption process and on the energy differences between the superheated steam and the liquid water either as pure substance or as component of the binary mixture. It was shown that the increase of mass transfer coefficient leads to an increase of steam mass transfer rate and to a corresponding decrease of solution temperature slope at the entrance of a tube. Correspondingly, the increase of mass transfer coefficient results in an increase of heat of absorption and heat of dilution at the same variation range of the solution mass fraction.

Linear dynamics theory adequately describes the behavior of Free Piston Stirling Engines (FPSEs). The aim of this paper is to predict the conditions for stable operation of FPSEs and the modeling of FPSEs. The linearization technique of the dynamic balance equations proposed recently by F. de Monte and G. Benvenuto has been applied using RE-1000 of Sunpower Inc. for a typical well known FPSE. The equations of motion are solved analytically in terms of the stiffness and damping coefficients of the machine. Using the criterion of the stable engine cyclic steady operation a rigorous mathematical form is obtained for the main parameters of the engine. The proposed model gives results close to the data coming from the literature and can be used to predict the thermal performance, the piston stroke and the delivered power. Furthermore, using for reasons of simplicity Schmidt Analysis (Isothermal model) the indicated output power is obtained. In addition, a reference is given to some of the most important thermal losses in the engine decreasing the theoretical performance up to experimental level.