I.P. Koronaki

ORME : A multicriteria rating methodology for buildings

Abstract To check the compliance of a building with regulations, evaluate the efficiency of retrofit, or even label a building one would in most cases perform a comparison of a number of building qualities. Within the framework of the European Joule–Thermie OFFICE project, a multicriteria rating methodology was developed for this purpose, based on a rating method that uses principal component analysis, and a ranking method that uses a partial aggregation technique. The aim of this methodology is to rate or to rank office buildings and retrofit scenarios of the same building according to an extended list of parameters, including: • energy use for heating, cooling and other appliances, • impact on external environment, • indoor environment quality, • cost. The paper presents the principles used in the methodology, and some examples of application to actual buildings. More information is given in a complete report (ORME—Office building rating methodology for Europe, Office Project Report, University of Athens, 1999).

Experimental assessment and thermodynamic analysis of a solar desiccant cooling system

The objectives of this study were to evaluate the performance of a solar desiccant cooling system with LiCl desiccant wheel as well as to obtain useful data and experiences for practical application in Mediterranean areas with higher solar fractions. An experimental set-up was built and used to test the system performance under typical summer environmental conditions. It has been found that the required regeneration temperature of the system is low while the COP th of the system is high. Regeneration temperatures up to 65°C were recommended for each environmental condition. In addition, the effects of some important operating parameters, such as inlet temperature and humidity ratio of process and regeneration air, on system performance were also investigated in this study. Useful experiences from international standards were taken into account for the evaluation of desiccant wheels’ performance. The analysed procedure and the developed performance indicators provide useful information about the sustainability of solar desiccant cooling systems with solid desiccant wheels. This information in combination with several set of experiments was the basis for the development of nomographs, which are very helpful for the sustainability and the design analysis of solid desiccant cooling systems. This novel diagrams (nomographs) can provide information on the outlet conditions of process and regeneration air streams of a solar cooling unit with LiCl desiccant wheel and therefore the dehumidification capacity of these systems as a function of ambient conditions. The model development was followed by a sensitivity analysis performed on these predicted results. The key parameters were the thermodynamic condition of process and regeneration air streams, the thermal coefficient of performance and the mass air flow ratio of the regeneration and process streams.

Critical review of coupled heat and mass transfer models for a liquid desiccant adiabatic dehumidifier and regenerator

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.

HYBRID LIQUID DESICCANT / VAPOUR COMPRESSION AIR-CONDITIONING SYSTEMS: A CRITICAL REVIEW

Hybrid Liquid Desiccant Cooling / Vapour Compression Systems is an environmentally friendly technology used to condition the internal environment of buildings. In contrast to conventional vapor compression air conditioning systems, in which the electrical energy drives the cooling cycle, desiccant cooling is heat driven; therefore, hybrid LDC/VCS have the potential to utilise cleaner energy sources such as gas, hot water, waste heat or solar thermal energy. In hybrid LDC/VCS, the latent cooling load is handled by the desiccant dehumidifier, while the sensible is handled by a conventional VCS. Hybrid systems combining liquid desiccant cooling with Vapor Compression Systems, Vapor Absorption Systems and Solar Collectors use less electrical energy compared to conventional air-conditioning alone, while these savings rise as the latent load increases. Unlike other surveys on desiccant cooling, this review focuses on a detailed coverage of the hybrid LDC/VC systems. Commonly used liquid desiccants are compared towards their physical properties. Hybrid LDC/VCS employing various components and features are summarized, while different system configurations are schematically presented. Key factors for the hybrid system performance are the desiccant material, the design variables and the conduction of experiments prior to operation.Copyright

Thermodynamic Analysis of an Internally Heated Regenerator Using Three Liquid Desiccant Solutions

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.

Thermodynamic Analysis of a Liquid Desiccant Cooling System Under Mediterranean Climatic Conditions

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

Parametric Study of a Single-Stage Two-Bed Adsorption Chiller

AbstractThis study examines the influence of operating conditions and heat exchangers’ characteristics on the overall performance of a single-stage, dual-bed adsorption chiller. A simple lumped par...

Performance Analysis of a Silica Gel-Water Adsorption Cooler: Impact of Nanofluids on Cooler Performance and Size

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.Copyright

Preliminary Investigation of a Liquid Desiccant System for Dehumidification and Cooling in Athens

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.© 2013 ASME

Thermodynamic Analysis and Performance Investigation of an Alpha-Type Stirling Engine

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.Copyright