Numerical analysis of the thermal behaviour of glazed ventilated facades in Mediterranean climates. Part II: applications and analysis of results

Double-skin envelopes are an attractive design option for high-end buildings. They allow architects to combine completely transparent external facades with an internal skin, which can also be transparent, or partially opaque. Between both skins there is an air channel that is used to collect or evacuate the solar radiation absorbed by the facade. This design may be a solution for reducing thermal overheating, which is common in completely transparent facades in Mediterranean climates. A numerical code was developed to analyze and predict the thermal behaviour of these facades. This code was applied to the analysis of the ‘standard’ geometry of a double-skin facade. A parametric study was carried out for typical Mediterranean climates to determine the influence of different variables, such as the position of blinds, the introduction of solid–liquid phase-change materials (PCM) and the use of low-ε glazing, etc. In order to characterize the thermal performance of the facades, two coefficients were defined that relate the indoor and enthalpic gains to the solar radiation. If they are designed carefully, ventilated facades will exhibit a significantly better passive behaviour than conventional glazed facades and also allow energy to be collected in the form of hot air to be used to reduce energy consumption in winter.     

Guidelines for improving natural daytime ventilation in an office building with a double-skin facade

In recent years, there has been a great deal of interest in double-skin facades due to the advantages claimed for this technology in terms of energy saving in the cold season, protection from external noise and wind loads and their high-tech image.The advent of computers and other office equipment has increased the internal heat gains in most offices. Highly glazed facades, together with the extra heat gains from the electric lighting made necessary by deep floor plans and the wider use of false ceilings, have increased the risk of overheating. To preserve comfort and reduce cooling loads, it is important to apply natural cooling strategies, including natural ventilation.Some argue that double-skin facades are designed to improve natural ventilation in buildings by the stack effect, and to allow this even in situation in which it is generally not possible due to high outdoor noise levels and/or high wind speeds.But poor operation of the double-skin facade openings can generate disastrous scenarios such as the injection of hot air from the double-skin facade into the offices and the contamination of offices on the upper floors by used air from the offices on the lower floors.This article examines how natural ventilation can be utilised in an office building with a double-skin facade during a sunny summer’s day. It mainly considers natural daytime ventilation in relation to the orientation of the double skin and the speed and direction of the wind.     

Experimental study of a mechanically ventilated double-skin façade with venetian sun-shading device: A full-scale investigation in controlled environment

The aim of this article is to present results of an experimental campaign performed on a full-scale facility provided with a double-skin façade. The behaviour of this architectural concept is tested under controlled climatic conditions. A summer case is scrutinised under different configurations: variation of the airflow through the double-skin façade and different angle of the solar shading device. This paper describes the experimental conditions, as well the test facility and the tested façade element. The results show the temperatures of the test cell and the façade and how they depend on the climatic conditions and the sun-shading device blade angles. One objective of this research was to measure and provide extensive data set detailing air and surface temperatures on the double-skin façade, together with airflow rates and air velocities. The experiments are fully described so that the results can be used for the validation of numerical models dealing with ventilated double-skin façades with venetian sun-shading device.     

Numerical analysis of the thermal behaviour of ventilated glazed facades in Mediterranean climates. Part I: development and validation of a numerical model

Ventilated glazed facades are formed by two layers of different materials, opaque or transparent, that are separated by an air channel, used to collect or evacuate the solar radiation that is absorbed by the facade. For architectonic reasons, the outer layer is usually made entirely of glass, while the indoor layer may be partially opaque. This allows direct solar gains to be reduced and increases the thermal inertia of the building. This paper is a presentation of a code for the numerical simulation of ventilated and conventional facades. It is based on time-accurate, one-dimensional discretizations for the channel and the different solid zones, and allows heat fluxes and temperature distributions in the facade to be obtained over the course of one year. The numerical code allows advanced elements to be integrated into the facade, such as phase change materials, selective surfaces and improved glasses. The code has been validated by comparing it with analytical solutions where possible, with reference situations and with experimental measurements obtained in real-site test facilities in different climatic conditions. The numerical code is a useful tool for optimising the design of facades so as to take advantage of different materials, orientations, geometries and to address different climatic conditions.     

Experimental evaluation of ventilated walls with an external clay cladding

The paper presents the results of an experimental study on ventilated walls with an external clay cladding in a temperate Mediterranean climate.The aim of this work is to assess the actual thermal performance of the ventilated façade with a complete thermo-fluid dynamic analysis.The paper describes the experimental work carried out on walls with different exposure and various heights of the ventilation channel (6 m and 12 m) with controlled internal air temperature. The air velocities, temperatures and heat fluxes in the ventilated facade were monitored and correlations between the thermo-physical parameters were identified.The study showed that, on sunny days, the external surface temperatures, temperatures in the air cavity and air velocity in the gap are considerably higher for the 12 m wall, while during the nighttime lower temperatures are found for the various layers of the wall.The data obtained allowed us to calculate the Reynolds number and to identify the airflow rate in the channel. Correlations were found between the air velocity in the gap, the airflow rate, the Reynolds number and the sun–air temperature. These correlations are useful for modelling ventilated walls, which are generally very complex, with simplified equivalent walls. Finally it was possible to identify the peak temperatures with respect to exposure and thermal lag. The thermo-graphic analysis showed that the behaviour of the walls is almost uniform without thermal bridges.     

A review on applying ventilated double-skin facade to buildings in hot-summer and cold-winter zone in China

The need to energy conservation and sustainable development in buildings is causing a new interest towards passive solar systems. Among them, double-skin facade (DSF) proves to be extremely attractive and promising. DSF is building envelope formed by two layers of different glazing facades which are separated by a ventilated air cavity. The cavity of DSF is used to collect or evacuate the solar radiation absorbed by the facades, thereby improving the thermal comfort and the indoor air quality while conserving energy for heating and cooling. Being a technique developed for colder climates, DSF has been widely applied in commercial buildings across Europe. Nowadays buildings with DSF also appear in the hot-summer and cold-winter zone in China where the weather conditions in summer seem to be not so good for the application. In fact, the thermal analysis of the DSF system is essential to its application in such hot-summer zone. This paper seeks to describe the existing main research methods on the thermal performance of DSF and the shading devices. Problems and possibilities are concomitant. Applying ventilated DSF with controlled shading device system would be a new efficient way for the commercial buildings in the hot-summer and cold-winter zone to meet the task of sustainable building design in China.     

Green vertical systems for buildings as passive systems for energy savings

This paper presents a classification of green vertical systems for buildings. The aim of this classification is to facilitate the identification and differentiation between systems. This classification is also essential to compare future research results relating to their operation. In addition, the mechanisms by which green facades can be used as passive energy savings systems are reviewed: shadow produced by the vegetation, insulation provided by vegetation and substrate, evaporative cooling by evapotranspiration, and the barrier effect to the wind. Finally, the paper describes the first results about the behaviour of a double-skin green facade or green curtain in Dry Mediterranean Continental conditions. It is verified that a microclimate between the wall of the building and the green curtain is created, and it is characterized by slightly lower temperatures and higher relative humidity. This means that the green screen acts as a wind barrier and confirms the evapotranspiration effect of the plants.     

Combined photovoltaic and solar thermal systems for facade integration and building insulation

Most photovoltaic (PV) facades are built as curtain facades in front of thermally insulated buildings, with air ducts in between. This causes additional costs for support structure and installation, while heat dissipation from the solar cells is often not optimal. Measurements carried out are facing both concerns: integration of a thermal insulating layer (which meets the latest German heat-preserving regulation, WSV 95) into the PV facade, plus additional cooling by active ventilation or water flow. Active ventilation at conventional curtain PV facades allows a reduction of cell operating temperatures of 18 K, resulting in an 8% increase in electrical energy output at an airspeed of about 2 m/s. Cell temperatures increase by 20.7 K at thermal insulating PV facade elements (TIPVE) without cooling, which causes a 9.3% loss of electrical yield, but installation costs can be reduced by 20% (all related to a conventional PV curtain plus a heat-insulating facade at a building). HYTIPVE, a hybrid thermal insulating PV facade element combined with a water cooling system, which could also serve for heating up water, lowers the operating cell temperature by 20 K and increases electrical yield by 9% (compared with conventional curtain PV facades). Further economic investigations of such a HYTIPVE, including its operational costs and substitution effect, related to the electrical and thermal yield, are in progress.     

Influence of glass properties on the performance of double-glazed facades

In this work the influence of the glass properties on the performance of double-glazed facades has been studied. The total heat rate into the building has been calculated for ten different facades formed by different glass combinations. To obtain this thermal load into the building it has been necessary to solve the fluid field within the channel formed by the two layers of glass. The mass flow rate in the channel as well as the mean temperature increase of the air passing through the channel have also been obtained. In order to reduce the thermal loads into the building, the cooling effect resulting of blowing the air through the channel has also been analysed.     

Daylight optimization of multifunctional solar facades

Multifunctional solar facades consisting of a transparent window and an opaque photovoltaic section are analyzed and optimized. Employing numerical daylight estimation techniques, the optimal shape, position, and area of the window section is determined. Maximum yearly average daylight availability is achieved with a similarly shaped window as the facade which is placed near the centre of the facade. For non-residential buildings, the yearly average useful interior daylight illuminance does not increase significantly for windows larger than 30% of the total facade area. Considering both the artificial lighting requirement replaced by daylight through the window and the electricity produced by the PV section of the facade, the maximum electricity benefit for a south-facing facade is achieved with a window area of about 10% of the total facade area in Southern Europe (38° N) and 15% in Northern Europe (60° N).     

Thermal performance estimation for ventilated PV facades

This paper explores different approaches to thermal performance estimation for ventilated photovoltaic (PV) facades. In particular, an extension of the familiar heat loss and radiation gain factors (U and g values respectively) has been employed to take account of the energy transfer to the facade ventilation air. In total, four terms describing ventilation gains and transmission losses in terms of irradiance and temperature components are defined which characterise the performance of the facade in total. Steady state analysis has been applied in order to express these four parameters in terms of the detailed heat transfer process within the facade. This approach has been applied to the ventilated PV facade of the public library at Mataró, Spain. Monthly U and g values have been derived and the associated thermal energy gains calculated for various climates. An alternative, simpler approach views the ventilated façade as a single solar air collector. The applicability of the standard expression for solar collector efficiency has been investigated through examination of the Mataró data. Summer and winter energy yield calculations carried out on this basis have been compared to the four parameter approach.     

INTELLIGENT FACADES: OCCUPANT CONTROL AND SATISFACTION

This paper, through the discussion of recently completed doctoral research, addresses the influence of occupant control in achieving occupant satisfaction in buildings with intelligent facades. The focus of the study is the most common form of intelligent facade which the research has termed automated glazed facades. These are defined by this research as highly glazed facades capable of independent physical reaction to either external or internal conditions or presets. The findings of a series of occupant satisfaction surveys and interviews in automated glazed facade buildings in the UK, Germany, Switzerland, France and the USA are discussed

Occupant control is found to be the dominant issue affecting occupant satisfaction in buildings with intelligent facades. The importance of occupant control is shown to be rooted in the occupants ability to achieve their desired conditions. The main elements of occupant concern are identified and the potential of the automated provision of occupant requirements is discussed in light of the findings.     

Improvements on PASSYS test cells

PASSYS test cells are outdoor test facilities for the evaluation of the thermophysical parameters of real scale samples of facades and building components. The most important application is the testing of passive solar facades. While we have the advantage of measurements under real weather conditions (real solar radiation) there is the disadvantage of non steady state boundary conditions. Various strategies are presented to enhance measurement accuracy. A so-called pseudo adiabatic shell (PAS) was introduced to reduce the heat losses through the test cell envelope and the time constant by using a controlled electric heating foil. A similar approach is described here using heat flux sensitive tiles (HFS). A special heat flux sensor was designed and tested at ITW to meet the requirements of an accurate measurement of the overall heat flux passing the test cell envelope. By the application of a movable cold box in front of the tested facade steady state outside conditions can be provided for the evaluation of the U-value according to the requirements proposed by CEN/TC 89/WG7.     

新型齿结构三通道喷燃器空气动力特性研究

提出一种新型的齿结构喷燃器，并采用冷态模化方法考察该结构对流场和混合强度场的影响。结果表明：齿结构喷燃器，对于增大内外回流区和提高整体的混合强度的作用是十分明显的。     

Photovoltaic-integrated building facades

Photovoltaic (PV) cells, when integrated within a building facade, offer the possibility of generating electric power and heat for local use or export. This paper reports on a project to investigate the practical operational efficiencies that might be delivered from such facades. The results from laboratory experiments and computer simulations are presented: the former were used to develop an empirical relationship between cell temperature and power output; the latter were undertaken to assess operational efficiencies under a range of climate conditions representative of the UK.     

Cooling potential of ventilated PV façade and solar air heaters combined with a desiccant cooling machine

Thermal energy collected from a PV-solar air heating system is being used to provide cooling for the Mataro Library, near Barcelona. The system is designed to utilise surplus heat available from the ventilated PV facade and PV shed elements during the summer season to provide building cooling. A desiccant cooling machine was installed on the library roof with an additional solar air collector and connected to the existing ventilated PV façade and PV sheds. The desiccant cooling cycle is a novel open heat driven system that can be used to condition the air supplied to the building interior. Cooling power is supplied to the room space within the building by evaporative cooling of the fresh air supply, and the solar heat from the PV-solar air heating system provides the necessary regeneration air temperature for the desiccant machine. This paper describes the system and gives the main technical details. The cooling performance of the solar powered desiccant cooling system is evaluated by the detailed modelling of the complete cooling process. It is shown that air temperature level of the PV-solar air heating system of 70 °C or more can be efficiently used to regenerate the sorption wheel in the desiccant cooling machine. A solar fraction of 75% can be achieved by such an innovative system and the average COP of the cooling machine over the summer season is approximate 0.518.     

3D coupled CFD/FEM modelling and experimental validation of a planar type air pre-heater used in SOFC technology

In this present paper, a coupled 3D thermo fluid-thermo mechanical modelling approach of a plate type air pre-heater is introduced. The conjugate heat transfer within the pre-heater is numerically solved using 3D computational fluid dynamics. The model considers the discrete structure of the whole pre-heater assembly including the air channels, plates and manifold ports. Wall surface and gas temperature measurements are performed using thermocouples to validate the numerically predicted calculations. The results show good agreement, implying that the proposed model can be used in the design and process optimisation of the air pre-heater. Moreover, a submodel created from the thermofluid analysis has been used to investigate the thermomechanical behaviour of the air pre-heater manifold region using the nonlinear finite element method. The nonlinear elastic-plastic behaviour of the material has been considered. The component locations susceptible to stress could be determined. The approach results in reduced prototype costs and product development time in the design and optimisation of efficient air pre-heater designs.     

On the behaviour of hygroscopic wheels: Part I – channel modelling

In this paper a detailed mathematical formulation is developed for the numerical modelling of the behaviour of a channel of a hygroscopic compact matrix. A comparison between the detailed version and a simplified one is performed considering a two-dimensional airflow between desiccant parallel plates. The distinct heat and mass transfer phenomena are strongly coupled, and some properties of the airflow and of the desiccant medium exhibit important changes during the sorption processes. Both physical models take into account the gas side and solid side resistances to heat and mass transfer. The wall domain is treated similarly in both models, by taking into account the simultaneous heat and mass transfer together with the water adsorption/desorption process. Two phases co-exist in equilibrium inside the desiccant porous medium, the equilibrium being characterized by sorption isotherms without hysteresis. The detailed model is based on the solution of the differential equations for the conservation of mass, energy and momentum, assuming that no momentum transport exists in the porous wall domain. In the simplified model, the airflow is treated as a bulk flow, the interaction with the wall being evaluated by using appropriated convective coefficients.Both models are compared in the simulation of a parallel plate channel during an adsorption process. The results show a good agreement for channel lengths greater than 0.1 m. In part II of the paper, the simplified model is adapted to the simulation of the three-dimensional problem in the channel of a hygroscopic rotor, and it is used to perform parametric studies.     

非均等配风下的风水冷选择性冷渣器冷态排渣特性

以某电厂450t/h CFB锅炉的风水冷选择性冷渣器为原型,通过冷态模化实验研究了各仓室在均等和非均等配风下的排渣特性,分析了各仓室的风量比关系对排渣速率的影响规律。实验结果表明,冷渣器的实际流化风量下限值高于设计值,在送风机总风量基本不变甚至减小的情况下,通过调配各仓室的风量比关系可明显提高冷渣器的排渣速率。所得实验数据对冷渣器的安全运行具有一定的指导意义和参考价值。     

Numerical Prediction of Natural Convection Occurring in Building Components: A Double-Population Lattice Boltzmann Method

In this article, a double-population thermal lattice Boltzmann method is proposed to solve the problem of the heated cavity with imposed temperatures. This family of problems can be considered as a test model for building physics application. A Taylor series expansion- and least-square-based lattice Boltzmann method (TLLBM) has been implemented in order to use a non-uniform mesh. This allowed us to investigate, at reasonable computational cost, the laminar and transitional flow fields (10³ ≤ Ra ≤ 10⁸). The numerical results, concerning the heat and mass transfers in the cases tested, are in good agreement with those from the literature. In order to demonstrate the possibilities of the method described in the article, applications are described covering double-skin facades and solar collectors or local heaters.