Abdelaziz Laouadi

COMPARISON BETWEEN COMPUTED AND FIELD MEASURED THERMAL PARAMETERS IN AN ATRIUM BUILDING

Abstract This paper presents a comparison study between simulation and filed measurements of thermal parameters of an atrium building in Ottawa, Canada. The selected atrium was an enclosed three-storey building with a pyramidal skylight. The atrium was fully conditioned and has open corridors at each storey connecting it to adjacent spaces. The atrium space was used for circulation and reception while adjacent spaces are offices and meeting rooms. The atrium was monitored in June 1995 and in December 1995 to consider extreme conditions of the outdoor climate. The simulation results were obtained using ESP-r computer program. The comparison included those of predicted and measured solar radiation entering the atrium space at the rooftop, and predicted and measured indoor temperatures of the atrium floors. Results for the solar radiation showed good agreement between the measured and predicted values. When the mechanical system was turned off, the predicted temperatures were within ±2°C of the measured temperatures in winter. In summer, however, the predicted temperatures were 2–3°C higher than the measured temperatures.

Towards developing skylight design tools for thermal and energy performance of atriums in cold climates

This paper presents an analysis of the impact of selected design alternatives on the thermal and energy performance of atriums based on the methodology outlined in the accompanying paper. Computer simulation programs were used to predict the impact of the selected design alternatives on the design performance outputs of atriums. Design alternatives focused on fenestration glazing types, fenestration surface area, skylight shape, atrium type, and interaction of the atrium with its adjacent spaces. Design performance outputs, evaluated with respect to a basecase design, included seasonal solar heat gain ratio, cooling and heating peak load ratios and annual cooling, heating and total energy ratios. Design tools were developed to quantify the impact of the design alternatives on the performance outputs. The design tools were cast into two-dimensional linear relationships with the glazing U-value and SHGC ratios as independent parameters. The results for enclosed atriums showed that the annual cooling energy ratio increased at a rate of 1.196 per unit of SHGC ratio and decreased at a rate of 0.382 per unit of U-value ratio. However, the annual heating energy ratio increased at a rate of 1.954 per unit of U-value ratio and decreased at a rate of 1.081 per unit of SHGC ratio. Similar trends were also found for the three-sided and linear atriums. Pyramidal/pitched skylights increased the solar heat gain ratio by up to 25% in the heating season compared to flat skylights. The effect of the skylight shape on the annual cooling and heating energy may be positive or negative, depending on the glazing U-value and SHGC ratios and the atrium type. Atriums open to their adjacent spaces reduced the annual cooling energy ratio by up to 76% compared to closed atrium spaces. However, open atrium spaces increased the annual heating energy ratio by up to 19%.

Natural convection heat transfer within multi-layer domes

Abstract Domes have become increasingly popular in modern building designs. Glazed domes are used to bring daylight and solar heat into the indoor space. For domes with multiple spaced layers of glazings, there is little information available on natural convection heat transfer within these layers. This information is required for the evaluation of the dome thermal performance (e.g., the U-factor). This paper presents a numerical study on heat transfer by laminar natural convection within multi-layer domes with uniform spacing heated from the outside. The flow and temperature fields within the domed enclosure were obtained using the control volume approach combined with the fully implicit scheme. Correlations for the heat transfer as a function of the dome shape and the gap spacing between the layers were developed under steady-state conditions. The results showed that the convection heat transfer for fully hemispheric domes (half of spheres) may reach more than 13% higher than that for low profile domes (hemispherical caps) for small gap spacings (gap spacing-to-radius ratio δ δ >0.3). The critical gap spacing that yields the maximum heat transfer was quantified for each dome shape.

Efficient calculation of daylight coefficients for rooms with dissimilar complex fenestration systems

The daylight coefficient (DC) method is a powerful and efficient method to perform annual daylight illuminance simulation. A set of coefficients are calculated for a given room space and static fenestration systems prior to simulation start. Time series of indoor daylight illuminances are obtained by only knowing the sky luminance. However, for rooms with dissimilar dynamic complex fenestration systems (such as windows with movable shadings) whose optical behaviour (transmission, reflection and scattering) may change during simulation, the efficiency of the DC method may be compromised as another whole set of coefficients must be re-calculated. This study presents the development of a new methodology to compute the DC set for rooms with dissimilar complex fenestration components only once prior to simulation start. A validation study is carried out, in which the daylight illuminances in an office space equipped with a clear window and internal Venetian blinds are compared using predictions from the present model, the Radiance program, as a benchmark model employing detailed optical model of Venetian blinds, and the Daysim program employing a simple engineering blinds model. Findings from the validation study show that the present model yields overall accurate results when compared with the benchmark model for any window orientation, although some local illuminance differences are observed in areas under direct sunlight exposure.

Methodology towards developing skylight design tools for thermal and energy performance of atriums in cold climates

Abstract This is one of two papers that outlines the methodology used to develop, through computer simulation, skylight design tools for thermal and energy performance of atriums in cold climates. The methodology identified important design alternatives that included skylight and atrium physical variables, and a series of thermal and energy performance outputs that may serve as selection criteria for an energy-efficient design. New prediction models were developed to overcome some computer-simulation limitations, which included models to deal with airflow between an atrium and its adjacent spaces and temperature stratification within an atrium space. The developed airflow network model is a technique used to predict the mutual influence between the atrium and its surrounding spaces without requiring additional geometrical information on the surrounding spaces. The developed temperature stratification model is consistent with airflow network models and the zone concept used in building thermal-simulation programs since it takes into account radiation (overlooked in airflow network models) and convection heat transfer at the same time. This was done through treating fictitious surfaces that separate the thermal zones in a similar way as real surfaces. Fictitious surfaces were assigned a high emissivity, high solar transmittance, high thermal conductivity and convection film coefficient of 10 W / m 2 ° C . These values were found to yield reasonable solar radiation absorption, convection and radiation heat balances for the real surfaces irrespective of the number of the thermal zones. The developed models were integrated into a simulation computer program, and then validated against field measurements of a case study atrium. The predicted indoor temperatures were within ±2°C of the measured ones in both winter and summer days.

Daylight availability in top-lit atria: prediction of skylight transmittance and daylight factor

Atrium and skylight shapes are important architectural design elements that influence daylight availability within the space and, therefore, lighting energy consumption. There is a lack of prediction models for skylight transmittance. and daylight availability in atria. A new concept was developed to predict the diffuse transmittance of skylights. A skylight shape is converted into a representative shape through a shape parameter. Generic formulae for the skylight diffuse transmittance were developed under different sky conditions. A zonal model combined with the flux transfer method was developed to predict daylight availability in top-lit atria through the predictions of the average daylight factor (DF) at the atrium floor and ceiling (non-glazed portion of the roof), and the local DF normal to walls. The DF model was compared with currently available models derived from theory and experiments under artificial skies. The results showed that the computed e transmittance for translucent skylights. under real partly cloudy or dear skies may reach up to 33% in summer and 56% in winter higher than that under CIE overcast skies. The developed zonal model yielded very dose results to the models based on the nnite-dement method. However, models based on physical scale measurements lack general consensus among themselves, and may produce average DF values at floor level up to 43% higher than those produced by the zonal model. Physical scale models may also yield local DF values normal to walls up to 50% lower than those predicted by the zonal model.

Complex fenestration systems: towards product ratings for indoor environment quality

Complex fenestration systems (CFS) include windows featuring complex glazing such as translucent and transparent insulation, solar control films, patterned or decorative glass, in-between pane shades, etc. CFS are believed to exhibit superior energy performance, but may have adverse effects on environmental features important for building-occupant satisfaction requirements such as the outdoor view (connection to outside), indoor view (feeling of privacy), luminance (major factor for discomfort glare), and light diffusion quality (relates to uniformity of illuminance on work plane). This article is part of a larger effort to rate complex fenestration systems for energy performance and indoor environment quality (IEQ). In the end, IEQ ratings must be derived from direct studies of how building occupants perceive the indoor environment conditions created by the installed fenestration product. As a first step, this work tackles the theoretical development of new metrics to rate CFS with regards to IEQ, namely the view impairment index, luminance index, and light diffusion quality index. The new indices are applied to some typical CFS, namely a diffuse window, and a clear window combined with an interior shading screen, and integrated perforated Venetian blinds. The results show that the diffuse window may increase the luminance by more than 100% under clear sky conditions when compared with a clear window with a similar light transmittance. White colored Venetian blinds may increase the window luminance by up to 50% and reduce the outdoor view by up to 66% as compared with a clear window with a similar light transmittance.

Transparent domed skylights: Optical model for predicting transmittance, absorptance and reflectance

Daylighting and thermal loads are very important design issues for skylight design, especially in large spaces such as atria. However, the trade-off between daylighting and thermal performance of skylights has been difficult to solve, due to a lack of daylighting and thermal design tools. A mathematical model was developed to predict the visible/solar transmittance, absorptance and reflectance of multi-glazed domed skylights for both direct and diffuse radiation- The model is based on tracking the beam and diffuse radiation transmission through the dome surface. Since all building energy simulation and fenestration rating tools are limited to planar skylights, the model was translated into a simple method in which domed skylights were substituted by optically equivalent planar skylights. The results showed that domed skylights yield slightly lower visible/solar transmittance at low sun zenith angles, and substantially higher visible/solar transmittance at high sun zenith angles, or near the horizon, than do planar skylights having the same aperture. The absorptance of domed skylights is higher than that of planar skylights, particularly at high sun zenith angles, or near the horizon. The model was compared with the IESNA transmittance calculation procedure for domed skylights and with the Wilkinson model. The IESNA transmittance calculation procedure overestimates by 19% the transmittance of a dome at low sun zenith angles and significantly underestimates the transmittance of a dome at high sun zenith angles, or near the horizon. However, the Wilkinson model significantly underestimates the transmittance of a dome for both low and high sun zenith angles.

Thermal performance modelling of complex fenestration systems

Complex fenestration systems (CFS) have become standard elements in facade design of high performance buildings. They include, for example, shading devices to control illumination, solar heat gains, glare and view-out, and photovoltaic elements imbedded in glazing layers to produce electrical energy on site. However, current methodologies to evaluate the thermal performance of CFS are limited to few products and types. This article develops a general methodology to compute the thermal performance of CFS. The methodology assumes each system layer as porous with calculated effective radiation and thermal properties. A new thermal penetration length model was developed to account for the effects of porous layers on the convective film coefficients of adjacent gas spaces, and applied to various types of shading devices. This methodology is validated using the available measurement and computational fluid dynamics (CFD) simulation results for the U-factor of double-glazed windows with between-pane and internal blinds.

The energy performance of the Central Sunlighting System

This article presents a simulation study to predict the energy performance of the Central Sunlighting System (CSS) installed in open-plan offices. Several simulation tools are combined to conduct the simulations. SkyVision calculates the daylight luminous flux and the lighting and solar heat gains of the CSS. A set of coefficients pre-calculated using Radiance relates the desktop illuminances to the CSS luminous fluxes. DaySim is used to compute the daylight illuminance from the perimeter windows. ESP-r is used to compute the heating and cooling energy use of the office spaces. The results show that the CSS may save a significant amount of energy in North American climates. Energy savings from the combination of daylighting from windows and the CSS for typical, four-cubicle, open-plan offices range from 44% to 57% for lighting, 8% for cooling and from 14% to 23% for the total (lighting, cooling and heating) energy.