Morad R. Atif

Energy performance of daylight-linked automatic lighting control systems in large atrium spaces: report on two field-monitored case studies

Abstract This paper presents the field-measured energy performance of two common types of daylight-linked lighting control systems, continuous dimming and automatic on/off installed in two existing large atrium spaces located in Canada. The daylighting performance was evaluated based on the daylight contribution to the indoor illuminance and the amount of electrical lighting displaced by daylighting via the daylight-linked lighting control systems. Measured daylighting contribution to the space indicates that significant lighting energy savings can be achieved in atrium spaces if the daylight-linked lighting control system is appropriately selected, installed and commissioned throughout its existence. Results extrapolated from measurements collected during summer and winter indicate that as installed, the continuous dimming lighting control system provides 46% annual savings in electrical lighting consumption, while the automatic on/off saves between 11and 17% in lighting energy. These savings account for 68% of the lighting energy consumed during main occupancy for the continuous dimming system, and 31.5% for the automatic on/off. Operation irregularities such as a reduced dimming linearity and an incorrect adjustment of the phases of the dimming control system, as well as the inadequate location of the photocell controlling the automatic on/off lighting system, the improper maintenance of the skylight during winter and the oversizing of the lighting system reduced the energy efficiency of the lighting control systems by 30–65%.

Applicability of daylighting computer modeling in real case studies: comparison between measured and simulated daylight availability and lighting consumption

The objective of this work is to investigate the accuracy of the Adeline lighting software in simulating the illuminance distribution from daylighting and the electrical lighting consumption of an existing atrium building. The case study is an enclosed atrium space equipped with an automatic on/off lighting control system and roofed by a skylight made of several glazing systems. Field monitoring of the atrium space was conducted during summer and winter and included measurements of horizontal indoor and outdoor illuminance, solar radiation and electrical lighting system time-of-use. The computer simulation included the creation of one simple and one complex computer model used to simulate the daylighting performance of the atrium space. The software accuracy was evaluated based on comparisons between the predicted and the on-site measured illuminance distribution and lighting energy consumption.

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.

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.

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.

Prediction Model of Optical Characteristics for Barrel Vault Skylights

The topic of this paper is part of a project to develop software to analyze the optical characteristics and daylighting performance of conventional and tubular skylights. Skylights are found in many building types, such as commercial and institutional buildings, houses, shopping malls, hotels, etc. However, skylight manufacturers lack design tools to predict the optical performance of skylight products. Prediction of skylight optical characteristics is a difficult task, due to the complexity of skylight shapes that change with design requirements. This paper presents an analytical model to compute the overall optical characteristics of barrel vault skylights under direct beam light. The model is based on a ray-tracing technique, and can handle skylights with different glazing types (gables opaque, or glazed with different glazing types from the top surface of the skylight), different shapes (short/long with high/low-rise profiles) and different orientation (north-south, west-east, or any direction). Applications of the model showed that barrel vault skylights transmit much more light at high incidence angle on a horizontal surface than similar flat skylights. The skylight length-to-radius ratio (L/R) has a significant impact on the optical characteristics. Short skylights (L/R = 2) with uniform glazing transmit approximately the same amount of light at any sun position. However, long skylights (L/R = 5) transmit about 64 percent more light when the sun is perpendicular to the skylight axis than when the sun is parallel to the skylight axis. Short skylights with clear gables and tinted top transmit substantially more light than flat skylights with similar tinted glazing, particularly when the sun is parallel to the skylight axis.

Tracer Gas Measurements for Ventilation, Air Movement and Air Infiltration in a Four-Sided Atrium Office Building

Abstract The research described in this paper is part of a project aimed at improving energy costs and the indoor environment of atrium buildings. Tracer gas techniques were used to assess the ventilation performance in terms of air distribution and contaminant migration patterns and to measure the air infiltration rate of a three-storey atrium. This atrium serves as an entrance to a large office-laboratory complex. The results indicate that even though the HVAC systems of the test atrium are connected to the building complex, tracer gas techniques were successfully used to determine the air distribution and contaminant migration patterns, and the air infiltration rate. A detailed description of the methods used and the test results are presented.