Exergy performance and optimization potential of refrigeration plants in free cooling operation

Exergy analysis has been widely used to assess refrigeration systems by evaluating exergy losses or exergy efficiency. The latter is mostly used as an indicator to determine the system performance, which requires the comparison of the actual system with its idealized reversible version, but not the practical achievable efficiency. Therefore, a practice-oriented evaluation method for refrigeration plants in free cooling operation is proposed, based on exergy analysis and technical standards as baseline. By considering the exergy input of auxiliary devices, the overall design of hydraulic circuits can be assessed on subsystem level. The achievable optimization potential compared to the state of the art in technology and the performance is revealed with the introduced optimization potential index (OPI). The application is demonstrated on a case study, where the analysis reveals an adequate operation of the field plant in general. Most cooling locations show potential for improvement, which is indicated by an OPI superior to zero. Moreover, the auxiliary electrical exergy input shows the same magnitude as the thermal exergy input, which emphasizes the importance of reducing the electrical energy usage of auxiliary devices in refrigeration plants to increase the performance

Urban microclimate parameters for buildings energy strategies

[EN] Today, the cities need to increase energy efficiency, reduce polluting emissions and achieve a sufficient level of sustainability. The urban microclimate plays an important role on the buildings energy consumption and the feeling of comfort in the spaces. The microclimate parameters are of central importance for the activities that are carried out in the open spaces and to a large extent determine the use. The responses to the microclimate may be unconscious but very often result in a differentiated use of open spaces just as function of the different climatic conditions. For this reason, the aim of this work is focused on the environmental parameters and on the methodology of analysis aiming at the establishment of bioclimatic strategies for buildings on the basis of morpho-type of the components of the geometries and surface quality of the materials used in urban spaces and in function of the consequent microclimatic conditions obtained.Lepore, M. (2016). Urban microclimate parameters for buildings energy strategies. 1-10. doi:10.4995/vitruvio-ijats.2016.6944.SWORD11012AA.VV. Progetto RUROS, progettare gli spazi aperti nell'ambiente urbano: un approccio bioclimatico, CRES (Centre for Renewable Energy Sources, Department of Buildings), 2004Fanger P. O., Thermal comfort. MacGraw-Hill, 1972.Ghiaus C. and Allard F., Natural ventilation in the urban environment, Earthscan, London 2007.Mayer, E. A new correlation between predicted mean votes (PMV) and predicted percentages of dissatisfied (PPD). In: J. E. Woods, D. T. Grimsrud and N. Boschi, editors. Proc Healthy Buildings/IAQ '97, Bethesda, 2, 189- 194 (1997)Santamouris M., Environmental design of urban buildings, an integrated approach, Earthscan, London, 2006.Santamouris M., Energy and climate in the urban built environment, James&James, London, 2001

An overview of the microclimate conditions inside healing chambers

In the civil and industrial construction engineering, the comfort of occupants is ensured by achieving appropriate microclimate conditions. The human physical comfort (thermal, visual, acoustic) is achieved when the person from an enclosure is in a state of equilibrium and is able to perform tasks with maximum possible efficiency and without any kind of stress. Similarly, in the case of crops grown inside protected spaces, for plants, just as in the case of human being, specific microclimate conditions need to be ensured for their optimum development during the vegetation period. Plant development is a result of its own genetic features, but this is influenced by microclimate conditions inside of the greenhouses. Even more, in the case of healing and acclimatization rooms for grafted vegetables, the specific microclimate conditions must be strictly monitored. Technologically, it is recommended an optimum indoor temperature between 23°C and 25°C, with a maximum value that must not exceed 28°C, under conditions of relative humidity around 85% - 95%. Other two important aspects are the CO2 concentration and the level of solar radiation. The value of the CO2 concentration is an indicator of the healing process. For the grafting process, it is useful to know the time of the connection of the donor and receiver. Led by the the light levels, the photosynthetic activity of the grafted seedlings in the healing period, lead to a decreasing of CO2 concentration. This means that the two parts of the planting material are jointed, healed. So, the farmers can manipulate the environment in the healing rooms. This paper presents an overview of the scientific and technical issues that have to be achieved in order to reach suitable control and management of microclimate conditions in healing rooms for grafted vegetables

Influence of furniture arrangement on airflow distribution in open concept passive houses

This paper presents the analysis of airflow distribution in an open space room by means of CFD numerical simulation having as case study a pre-certified passive house, located in Cluj-Napoca. The ventilation system comprises two rectangular inlets, located on the floor near the glazing and six exhaust grilles of circular cross section located at the opposite side, at the kitchen's ceiling. An area of interest that represents the simulation/computation domain was selected. Numerical modeling was done through a commercial software, based on the finite element method that simulates a series of interconnected phenomena encountered in the field of applied engineering. Dry air was considered the fluid environment in the computing domain with thermo-physical properties taken from the program database. No slip boundary conditions were used for all the walls, volume flow rate of 215m3/h was specified for the inlets and exhaust nozzles were considered as open boundary. The following simplifying working hypotheses were established: stationary regime, incompressible fluid, constant temperature, isothermal jet, adiabatic system without internal heat sources, disregarding of mass forces. Airflow distribution analysis was performed for two distinct configurations: without furniture and in the presence of furniture. Simulation domain was divided into a large number of finite elements of variable size having a higher density near the boundary areas resulting tens of thousands of nodes. The purpose of the simulation was to evaluate the influence of furniture on the distribution of air currents in the occupied areas with direct impact on the occupants' comfort. Upon the comparative analysis for the two distinct cases, based on the distribution of streamlines, pressure and velocity vectors, the furniture's influence areas, the stationary areas and the air recirculation areas were identified

Thermal networks considering graph theory and thermodynamics

Heat transfer in solids may be dealt with the heat equation, which is a partial differential equation, from which different analytical solutions for the study of heat transfer throughout solids and at their surfaces may be found. This implies the resolution of a distributed parameter model. On the other hand, the possibility of considering the thermal-electrical analogy is usually assumed, this being based mainly on the similarity between Ohm’s and Fourier’s laws under the assumption that the different variables used in electrical networks may be regarded as analogues to the thermal network variables. This implies the use of a lumped parameter model, which may be represented as a system of differential and algebraic equations (DAE) linked to the graphical representation of the thermal network. In this latter case the limitations of such analogy for describing heat flow should be taken into account. Therefore, it would be important to consider thermal networks independently of the thermal-electrical analogy. For this, thermal networks may be built as particular cases of directed graphs, within graph theory, since thermal networks may have physical meaning without the electrical analogy. The interpretation of a graph as a thermal network may directly use physical principles of heat and thermodynamics. This enables us to propose an alternative to the use of the electrical analogy, since electrical networks are only a particular application of graph theory consistent with electromagnetic laws which are not analogous to thermodynamic laws. Furthermore, the construction and the use of thermal networks for analysing heat transfer problems may be simplified from this perspective.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Modèles de connaissance à paramètres identifiables expérimentalement pour les systèmes de refroidissement dessiccatif couplés à un système solaire

La Centrale de traitement d Air par Dessiccation (CAD) offre un contrôle complet de la température et de l'humidité dans les locaux climatisés. Son élément clé est la roue dessicante qui permet la dessiccation de l air et une régénération continue. A travers cette étude, nous nous intéressons au développement d une méthodologie pour obtenir un modèle dynamique de la roue utilisable dans les algorithmes de contrôle avancés de la CAD. La roue dessicante peut être considérée comme un système de type multi-entrées/multi-sorties (MIMO). La seconde partie de ce mémoire concerne l'identification expérimentale des paramètres des modèles d état de la roue dessicante pour deux types de modèles : boîte noire et boîte grise. Dans le cas de la boîte noire, tous les paramètres du modèle sont identifiés expérimentalement. Dans le cas de la boîte grise, certains paramètres sont dérivés de considérations physiques et les paramètres restants sont identifiés en utilisant les mesures expérimentales des entrées et des sorties. Les paramètres du modèle boîte grise ont une signification physique. En comparaison avec les modèles boîte noire, les modèles boîte grises sont moins précis sur le domaine sur lequel les paramètres ont été identifiés, mais beaucoup plus précis en dehors de ce domaine. Comme les paramètres ont une signification physique, leurs valeurs ne varient pas de manière significative avec le point de fonctionnement utilisé pour l identification. Dans l approche boîte grise, les valeurs des paramètres obtenues pour les modèles linéaires sont presque identiques pour tous les modèles locaux du coté dessiccation et pour tous les modèles locaux du coté régénération ; cela nous a permis de considérer qu un modèle local est valable pour tout le domaine de variation des variables d entrée. Le modèle final de la roue dessicante se compose de deux modèles globaux : un pour le côté de la dessiccation et l'autre pour le côté de la régénération. La troisième partie de ce travail consiste dans l'identification des coefficients de transfert de masse et de chaleur au sein de la roue dessicante en utilisant un modèle boîte grise. Le coefficient de transfert de masse, le coefficient de transfert convectif et le nombre de Nusselt ont été obtenus en écrivant les paramètres du modèle d état en fonction d une seule variable et en exprimant les paramètres en fonction des caractéristiques géométriques et des propriétés de matériaux de la roue. Ce travail contribue au développement d un modèle d état utilisable pour la synthèse des algorithmes de contrôle pour la roue dessicante.Desiccant Air Unit (DAU) offers a complete control of air temperature and humidity in the conditioned space. Its key component is the desiccant wheel which provides the functions of air desiccation and regeneration. The aim of this study is to develop a methodology for obtaining a dynamic model of the desiccant wheel which can be used for the model-based control algorithms of DAU. The desiccant wheel can be regarded as a multi-input/multi-output (MIMO) system. The first part of the thesis is devoted to the modeling of the desiccant wheel based on energy and mass balance equations. The resulting set of equations is formulated as a second order state-space system without delay. The second part of this thesis concerns the experimental identification of the parameters of the state-space model of the desiccant wheel by using a black-box and a gray-box approach. In the case of the black-box, all the parameters of the model are identified experimentally. The identified parameters have values which minimize the difference between the output of the model and the experimental values. The parameters of the black-box model do not have physical significance. Although precise in the range of variation of the inputs in which the parameters were identified, this model gives significant errors in other domains of variation of the inputs. The parameters of the gray-box model are physically significant. Compared with the black-box models, the gray-box model was less accurate for the domains for which the parameters were identified, but it was notably more robust when applied to other ranges of the inputs. Since the parameters are related to physical properties, their values do not vary significantly with changes of the operating point used for identification. For the gray-box approach, the parameter values obtained for the linear models are almost identical for all local models on the desiccation side and all the local models on the regeneration side, suggesting that a local model may be valid for all the complete range of input variables. Using the above results, a final model of the desiccant wheel was developed, comprising two global models: one for the desiccation side and another for the regeneration side. The third part of the thesis deals with the identification of mass and heat transfer coefficients of the air within the desiccant wheel using a gray-box model. The mass transfer coefficient, the convective heat transfer coefficient and the Nusselt number were obtained by defining the variable parameters of the model as a function of a single variable and by expressing the constant parameters as a function of the geometric and material properties of the wheel. This work contributes to the development of a state-space model used for the synthesis of control algorithms for the desiccant wheel.VILLEURBANNE-DOC'INSA-Bib. elec. (692669901) / SudocSudocFranceF

Natural ventilation in urban areas : results of the European Project URBVENT Part 1: urban environment

The application of natural ventilation is more difficult in urban than in rural environment, especially in street canyons due to reduced wind velocity, urban heat island, noise and pollution, which are considered to be important barriers to the application of natural ventilation. The wind, temperature, noise attenuation and outdoor-indoor pollution transfer were measured in a large range of variation and various types of urban configuration. The models obtained can be used in the initial stages of building design in order to assess the viability of natural ventilation in urban environment, especially in street canyons

Energy planning and forecasting approaches for supporting physical improvement strategies in the building sector: a review

The strict CO2 emission targets set to tackle the global climate change associated with greenhouse gas emission exerts so much pressure on our cities which contribute up to 75% of the global carbon dioxide emission level, with buildings being the largest contributor (UNEP, 2015). Premised on this fact, urban planners are required to implement proactive energy planning strategies not only to meet these targets but also ensure that future cities development is performed in a way that promotes energy-efficiency. This article gives an overview of the state-of-art of energy planning and forecasting approaches for aiding physical improvement strategies in the building sector. Unlike previous reviews, which have only addressed the strengths as well as weaknesses of some of the approaches while referring to some relevant examples from the literature, this article focuses on critically analysing more approaches namely; 2D GIS and 3DGIS (CityGML) based energy prediction approaches, based on their frequent intervention scale, applicability in the building life cycle, and conventional prediction process. This will be followed by unravelling the gaps and issues pertaining to the reviewed approaches. Finally, based on the identified problems, future research prospects are recommended

Linear fuzzy-discriminant analysis applied to forecast ozone concentration classes in sea-breeze regime

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