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Daylighting Systems: Development of Techniques for Optical Characterization and Performance Evaluation
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Successful integration of daylighting systems requires the ability to predict their performance for given climates. In this dissertation, a bottom-up approach is applied to evaluate the optical performance of a selection of daylighting systems. The evaluations are based on the optical properties of the included materials, and part of the dissertation focuses on developing new optical characterization methods.

The work on characterization techniques uses an integrating sphere method to characterize the transmittance of light scattering samples more accurately. The method's principle is to reduce the discrepancy in light distribution between the reference and the sample scans by using an entry port beam diffuser. For samples exhibiting distinct light scattering patterns, the benefits of improved uniformity outweigh the errors introduced by the diffusing material. The method is applicable to any integrating sphere instrument, and its simplicity makes it suitable for standard measurements.

In addition to normal-hemispherical properties, many daylighting applications require knowledge of the system's spatial light distribution. This dissertation presents a method combining experimental techniques and ray tracing simulations to assess the light distribution from a Venetian blind system. The method indicates that ray tracing based on simplified optical data is inadequate to predict the light distribution for slat materials exhibiting both specular and diffuse properties.

Ray tracing is a promising complement to experimental methods used to characterize light guiding or light redirecting systems. Here, spectrophotometric measurements of a scaled mirror light pipe validate a ray tracing model. The model shows excellent agreement with experimental results for both direct and diffuse incident light. The spectral evaluation shows no dramatic color changes for the transmitted light. The ray tracing model is used to evaluate four daylighting systems for a selection of Swedish locations. The percentage of occupied time when the studied systems achieve full design illuminance is relatively low, but the systems provide a valuable contribution to the required illuminance.

Additionally, this dissertation provides an overview of available energy efficient windows and illustrates the importance of including the solar energy transmittance when evaluating window energy performance.

Overall, this dissertation presents optical characterization techniques for improved performance evaluations of daylighting systems.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , 157 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 987
Keyword [en]
Optical spectroscopy, integrating sphere, ray tracing, light scattering, window physics, Venetian blinds, core daylighting, optical characterization
National Category
Engineering and Technology
Research subject
Engineering Science
Identifiers
URN: urn:nbn:se:uu:diva-183307ISBN: 978-91-554-8512-2 (print)OAI: oai:DiVA.org:uu-183307DiVA: diva2:562538
Public defence
2012-12-07, Polhemsalen, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2012-11-16 Created: 2012-10-24 Last updated: 2013-01-23Bibliographically approved
List of papers
1. Method for more accurate transmittance measurements of low-angle scattering samples using an integrating sphere with an entry port beam diffuser
Open this publication in new window or tab >>Method for more accurate transmittance measurements of low-angle scattering samples using an integrating sphere with an entry port beam diffuser
2011 (English)In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 50, no 7, 999-1006 p.Article in journal (Refereed) Published
Abstract [en]

For most integrating sphere measurements, the difference in light distribution between a specular reference beam and a diffused sample beam can result in significant errors. The problem becomes especially pronounced in integrating spheres that include a port for reflectance or diffuse transmittance measurements. The port is included in many standard spectrophotometers to facilitate a multipurpose instrument, however, absorption around the port edge can result in a detected signal that is too low. The absorption effect is especially apparent for low-angle scattering samples, because a significant portion of the light is scattered directly onto that edge. In this paper, a method for more accurate transmittance measurements of low-angle light-scattering samples is presented. The method uses a standard integrating sphere spectrophotometer, and the problem with increased absorption around the port edge is addressed by introducing a diffuser between the sample and the integrating sphere during both reference and sample scan. This reduces the discrepancy between the two scans and spreads the scattered light over a greater portion of the sphere wall. The problem with multiple reflections between the sample and diffuser is successfully addressed using a correction factor. The method is tested for two patterned glass samples with low-angle scattering and in both cases the transmittance accuracy is significantly improved.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-149785 (URN)10.1364/AO.50.000999 (DOI)000287851800019 ()
Available from: 2011-03-23 Created: 2011-03-23 Last updated: 2017-12-11Bibliographically approved
2. Transmittance characterization of anisotropically light scattering samples using an integrating sphere with an entry port beam diffuser
Open this publication in new window or tab >>Transmittance characterization of anisotropically light scattering samples using an integrating sphere with an entry port beam diffuser
(English)In: Optical materials (Amsterdam), ISSN 0925-3467, E-ISSN 1873-1252Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-183304 (URN)
Available from: 2012-10-24 Created: 2012-10-24 Last updated: 2017-12-07Bibliographically approved
3. Light-scattering properties of a Venetian blind slat used for daylighting applications
Open this publication in new window or tab >>Light-scattering properties of a Venetian blind slat used for daylighting applications
2010 (English)In: Solar Energy, ISSN 0038-092X, E-ISSN 1471-1257, Vol. 84, no 12, 2103-2111 p.Article in journal (Refereed) Published
Abstract [en]

The low cost, simplicity, and aesthetic appearance of external and internal shading devices, make them commonly used for daylighting and glare-control applications. Shading devices, such as Venetian blinds, screens, and roller shades, generally exhibit light scattering and/or light redirecting properties. This requires the bi-directional scattering distribution function (BSDF) of the material to be known in order to accurately predict the daylight distribution and energy flow through the fenestration system. Acquiring the complete BSDF is not a straightforward task, and to complete the process it is often required that a model is used to complement the measured data. In this project, a Venetian blind slat with a white top surface and a brushed aluminum bottom surface was optically characterized. A goniophotometer and an integrating sphere spectrophotometer were used to determine the angle resolved and hemispherical reflectance of the sample, respectively. The acquired data were fitted to a scattering model providing one Lambertian and one angle dependent description of the surface properties. These were used in combination with raytracing to obtain the complete BSDFs of the Venetian blind system.

Keyword
ABg-model, BSDF, Raytracing, Venetian blinds
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-139621 (URN)10.1016/j.solener.2010.09.005 (DOI)000285125900013 ()
Available from: 2010-12-28 Created: 2010-12-28 Last updated: 2017-12-11Bibliographically approved
4. Spectrophotometric measurements and ray tracing simulations of mirror light pipes to evaluate the color of the transmitted light
Open this publication in new window or tab >>Spectrophotometric measurements and ray tracing simulations of mirror light pipes to evaluate the color of the transmitted light
2014 (English)In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 124, 172-179 p.Article in journal (Refereed) Published
Abstract [en]

Tubular daylighting systems are designed to guide light to the building's core using a highly reflective pipe. The intensity of the transmitted light is essential for the performance of the system. For the qualitative perception of the provided illumination, the color of the delivered light is also an important aspect. For highly reflective mirror light pipes, spectral variations are generally assumed not to affect the color of the transmitted light. Here, spectrophotometric measurements and ray tracing simulations of mirror light pipes are used to verify this commonly made assumption. The characterization methods employ spectral evaluations for both direct and diffuse incident light. The color properties are evaluated for mirror light pipes with a length to diameter aspect ratio of up to 16, using the CIE chromaticity diagram and CIELAB coordinates. For the xy chromaticity diagram, a larger color shift was noted for different illuminants than as a result of the optical properties of the reflective material. Using the CIELAB coordinates, a small color shift was noted for light incident at low solar altitudes. Overall, highly reflective films with spectral variations of a few percent do not markedly affect the color of the transmitted light.

National Category
Energy Engineering
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-183305 (URN)10.1016/j.solmat.2014.01.049 (DOI)000335111000024 ()
Available from: 2012-10-24 Created: 2012-10-24 Last updated: 2017-12-07Bibliographically approved
5. Evaluation of optical and thermal properties of coatings for energy efficient windows
Open this publication in new window or tab >>Evaluation of optical and thermal properties of coatings for energy efficient windows
2009 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 517, no 10, 3173-3177 p.Article in journal (Refereed) Published
Abstract [en]

Over the past couple of decades a number of coated glazing products have appeared on the market, converting the window from an energy drain to a possible resource in the building's energy supply system. In this paper the light transmittance, total solar energy transmittance, and thermal transmittance of coated glazing currently available on the market and future dynamic electrochromic coatings are reviewed and their function in heating or cooling dominated climates discussed. Electrochromic coatings can switch between a transparent state and an absorbing or reflecting state when a small electrical potential is applied, and hence adapt their optical properties to the present condition. Energy balance calculations for the window component stress the importance of selecting the window depending on climate. In a heating dominated climate a low-e coating with a high g-value can result in a net energy gain on a south facing facade and thus be better than an ideal wall.

Keyword
Coated glazing, Climate, Energy efficiency, Electrochromics
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-139620 (URN)10.1016/j.tsf.2008.11.083 (DOI)000265178700031 ()
Available from: 2010-12-28 Created: 2010-12-28 Last updated: 2017-12-11Bibliographically approved
6. Optical and thermal properties of functional coatings for future high performance windows
Open this publication in new window or tab >>Optical and thermal properties of functional coatings for future high performance windows
2008 (English)Conference paper, Published paper (Refereed)
Abstract [en]

With modern society facing the task of reducing energy consumption in all areas of life, modern windows provide an enormous potential to reduce energy consumption for the heating, cooling and lighting of buildings. For future buildings and for the retrofitting of older buildings the window is more and more becoming an integral part of the building’s energy system. In this paper we present the optical properties for a selection of different window coatings and discuss their impact on the performance of the window. Special emphasis is put on switchable glazing. Optimum performance for switchable glazing is often a trade off between minimum energy consumption for cooling heating and lighting. This can sometimes be in conflict with occupancy preferences. As an example we show how different control strategies for electrochromic windows can influence the energy balance of the window, and that small variations in the control algorithm can lead to improvements. The results were obtained by using the WinSel window simulation tool.

 

Keyword
Switchable glazing, control strategies, energy balance calculations
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-125235 (URN)
Conference
EuroSun, Lisbon, Portugal, 7-10 October, 2008
Available from: 2010-05-11 Created: 2010-05-11 Last updated: 2016-04-13Bibliographically approved

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