Comparing daylighting performances assessment of building within scale models and test modules

Physical models are commonly used to assess daylighting performance of buildings using sky simulators for purpose of research as well as practice. Recent studies have pointed out the general tendency of scale model assessments to overestimate the performance, usually expressed through work plane illuminance and daylight factor profiles, when compared to the real buildings. The cause of the discrepancy between buildings and scale models is due to several sources of experimental errors, such as modelling of building details, mocking-up of surface reflectances and glazing transmittance, as well as photometer features. To analyse the main sources of errors, a comparison of a full scale test module designed for experimentation of daylighting systems and its 1:10 scale model, placed within identical outdoor daylighting conditions, was undertaken. Several physical parameters were studied in order to determine their impact on the daylighting performance assessment. These include the accurate mocking-up of surface reflectances, the scale model location, as well as the photometric sensor properties. The experimental study shows that large discrepancies can occur between the performance figures. They lead, on average, to a relative divergence of + 60 % to + 105 % in favor of the scale model for different points located in the side lit room. Some of these discrepancies were caused by slight differences in surface reflectances and photometer cosine responses. These discrepancies were reduced to a + 30 % to + 35 % relative divergence, by putting in the effort to carefully mock up the geometrical and photometrical features of the test module. This included a sound calibration of photometric sensors, whose cosine-response appeared at the end to be responsible for the remaining relative divergence observed between the daylighting performance figures

Comparing the accuracy of daylighting physical and virtual models for complex fenestration systems

Nowadays, many new window components known as complex fenestration systems (CFS), such as laser-cut panels and prismatic films, are considered in order to improve the overall luminous properties of building spaces : detailed studies of CFS remain necessary however to validate their daylighting performance. Physical and virtual models are commonly used to assess the daylighting performance of more conventional daylighting strategies within buildings. Several recent studies have reported significant errors for both physical and virtual modelling procedures, 10% modelling errors leading in both cases to 15% up to 170 % inaccuracy in modelled daylight factors assessment: no similar error analysis was carried out in a systematic way for daylighting strategies involving CFS use. A side lit office room equipped with double glazing and a CFS (laser-cut panel and prismatic film) was mocked-up for that purpose in a daylighting test module. The office room was reproduced by way of a 1:10 scale physical model placed under a scanning sky simulator, as well as a virtual model built-up by the way of Radiance lighting program. Several model parameters were varied, leading to the evaluation of model inaccuracies through a sensitivity analysis. The most significant factor (internal surface reflectance) is considered in this paper, leading to a first set of modeling guidelines

Analysis of error sources within daylighting physical and virtual models of buildings

Recent studies have pointed out the general tendency of scale models to overestimate the daylighting performance of buildings, usually expressed through work plane illuminance and daylight factor distribution profiles. An analysis of the corresponding sources of error has allowed to identify the main parameters responsible for the overestimation - such as indoor surfaces reflectance, glazing transmittance and photometers features. It was shown that a careful mock-up of the real building characteristics can reduce the divergence of the scale models daylighting performance down to 30 %, even for locations situated away from the window side. An appropriate tuning of the numerical parameters involved in daylighting computer simulation models lead to comparable accuracies, as shown by different authors. Daylighting computer simulations of a real building (a 1:1 scale daylighting test module), together with a virtual model of the corresponding 1: 10 scale model placed in a scanning sky simulator, were used to carry out an in-depth analysis of the sources of error of both physical and virtual modeling techniques. Through a computer sensitivity analysis of the most significant parameters influencing the accuracy of the physical model, design rules and error calculation methods for scale models are expected to be drawn

Potential annual daylighting performance of a high-efficiency daylight redirecting slat system

While the primary role of window attachments is often to moderate glare and solar heat gains, they are also able to provide additional daylight to interior spaces. For this purpose, a variety of daylight-redirecting window systems have been developed over the past 150 years. Fixed reflective systems (slats/light shelves) or prismatic systems that rely on total internal reflection work well under specific solar conditions, but generally sacrifice performance over a much wider range of incident solar angles and sky conditions. Dynamic systems - typically reflective slats - are more responsive to sun angles but have not been able to achieve optimal performance for glare and daylight redirection efficiency. A previous investigation into an adjustable, reflective blind concept first conceived of in the late 1970s showed promise but was not reduced to practice due to lack of adequate simulation and analysis tools. In this paper, this concept is further developed and its energy and visual comfort performance evaluated for four mid-latitude, temperate climates using ray-tracing simulation techniques. Results indicate significant potential lighting energy savings when compared with conventional automated reflective blinds (2.1–4.9 kWh/(m ·a), or 14%–42%, depending on climate and orientation) or, especially, manually-operated matte white venetian blinds (1.4–7.9 kWh/(m ·a), or 9%–54%, depending on climate and orientation), while maintaining acceptable or better visual comfort conditions throughout the interior space. 2

Potential annual daylighting performance of a high-efficiency daylight redirecting slat system

While the primary role of window attachments is often to moderate glare and solar heat gains, they are also able to provide additional daylight to interior spaces. For this purpose, a variety of daylight-redirecting window systems have been developed over the past 150 years. Fixed reflective systems (slats/light shelves) or prismatic systems that rely on total internal reflection work well under specific solar conditions, but generally sacrifice performance over a much wider range of incident solar angles and sky conditions. Dynamic systems - typically reflective slats - are more responsive to sun angles but have not been able to achieve optimal performance for glare and daylight redirection efficiency. A previous investigation into an adjustable, reflective blind concept first conceived of in the late 1970s showed promise but was not reduced to practice due to lack of adequate simulation and analysis tools. In this paper, this concept is further developed and its energy and visual comfort performance evaluated for four mid-latitude, temperate climates using ray-tracing simulation techniques. Results indicate significant potential lighting energy savings when compared with conventional automated reflective blinds (2.1–4.9 kWh/(m2·a), or 14%–42%, depending on climate and orientation) or, especially, manually-operated matte white venetian blinds (1.4–7.9 kWh/(m2·a), or 9%–54%, depending on climate and orientation), while maintaining acceptable or better visual comfort conditions throughout the interior space

Balancing daylight, glare, and energy-efficiency goals: An evaluation of exterior coplanar shading systems using complex fenestration modeling tools

Exterior shades are the most effective way to control solar load in buildings. Twelve different coplanar shades with different geometry, material properties, and cut-off angles were investigated for two California climates: the moderate San Francisco Bay Area climate and a hot and dry Southern California climate. The presented results distinguish themselves from other simulation studies by a newly developed method that combines three research-grade software programs (Radiance, EnergyPlus, and Window 7) to calculate heat transfer, daylight, and glare resulting from optically-complex fenestration systems more accurately. Simulations were run for a case with constant electric lighting and a case with daylighting controls for a prototypical, internal load dominated office building. In the case of daylighting controls, the choice of slat angle and solar cut-off angle of a fixed exterior slat shading system is non trivial. An optimum slat angle was identified for the considered cases. Material properties (e.g., solar and visible reflectance) did not affect energy use if constant electric lighting was assumed, but they did have a significant influence on energy use intensity (EUI) when daylighting controls were assumed. Energy use increased substantially when an additional interior shade was used for glare control

Balancing daylight, glare, and energy-efficiency goals: An evaluation of exterior coplanar shading systems using complex fenestration modeling tools

Exterior shades are the most effective way to control solar load in buildings. Twelve different coplanar shades with different geometry, material properties, and cut-off angles were investigated for two California climates: the moderate San Francisco Bay Area climate and a hot and dry Southern California climate. The presented results distinguish themselves from other simulation studies by a newly developed method that combines three research-grade software programs (Radiance, EnergyPlus, and Window 7) to calculate heat transfer, daylight, and glare resulting from optically-complex fenestration systems more accurately. Simulations were run for a case with constant electric lighting and a case with daylighting controls for a prototypical, internal load dominated office building. In the case of daylighting controls, the choice of slat angle and solar cut-off angle of a fixed exterior slat shading system is non trivial. An optimum slat angle was identified for the considered cases. Material properties (e.g., solar and visible reflectance) did not affect energy use if constant electric lighting was assumed, but they did have a significant influence on energy use intensity (EUI) when daylighting controls were assumed. Energy use increased substantially when an additional interior shade was used for glare control

Comparative study on the overall energy performance between photovoltaic and Low-E insulated glass units

A novel semi-transparent building integrated photovoltaic (BIPV) laminate was developed and introduced in this paper. It was produced by cutting standard mono-crystalline silicon solar cells into small strips and then making electrical connections between each strip before laminating the cells between two layers of glass. The overall energy performance and energy saving potential of the BIPV insulated glass unit (IGU) under real world conditions were identified through a side by side comparative study. Compared to the reference IGU, the BIPV IGU had lower solar heat gain coefficient (SHGC) but much higher U-factor. The average HVAC electricity saving of the BIPV IGU was about 10% relative to the reference IGU. Daylighting measurement and analysis were carried out to evaluate the trade-offs associated with the BIPV IGU between daylight, glare, and lighting energy use. The results indicated that the BIPV IGU is better than the reference IGU in reducing discomfort glare. However, if the most conservative viewpoint near the window is used for the assessment, a lower transmittance BIPV IGU is required to bring the overall discomfort levels below the perceptible level. Lastly, the net energy saving potential associated with the novel BIPV IGU was identified based on the power, thermal and daylighting performance. On average, the BIPV IGU saved 16.8% of the total electricity use of the room. Further studies and improvement on the energy conversion efficiency of solar cells, the optimal transmittance as well as the thermal properties would make this technology more energy-efficient and affordable