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Energy and Buildings 63 (2013) 108–118

Contents lists available at SciVerse ScienceDirect

Energy and Buildings

j ourna l ho me pa g e: www.elsev ier .com/ locate /enbui ld

eview

uning control of buildings glazing’s transmittance dependence onhe solar radiation wavelength to optimize daylighting and building’snergy efficiency

eong Tai Kima, Marija S. Todorovica,b,∗

Department of Architectural Engineering, Kyung Hee University, Yongin 446-701, Republic of KoreaMultidisciplinary Studies Program, University of Belgrade, Serbia

r t i c l e i n f o

rticle history:eceived 15 March 2013ccepted 19 March 2013

eywords:ealthy buildingsnergy efficiencyaylightingynamic control mathematical model

a b s t r a c t

Further advance of glazed, healthy building’s energy efficiency and sustainability is inextricable linked tothe building’s envelopes/facades fundamental physics study related to the dynamic control of sunlightand optimal control of solar heat gains. Relevant mathematical models and algorithms, as well as infras-tructure/hardware and software integrated performance prediction and validation are studied. Reviewedis the most recent analytical and experimental research, current state of science and art, as well as someof the on-going R&D at the edge of new breakthroughs of the healthy buildings daylighting dynamiccontrol’s performance prediction and validation. It has been shown that, concerning the variability ofthe solar radiation spectra incident on the building’s envelope, and also variability of outdoor and indoor

uning control of glazing’s solar radiationransmittanceuilding performance prediction

air temperature differences, it is necessary tuning control of glazing’s transmittance dependence on thesolar radiation wavelength, with an aim to optimize daylighting with the reference to people needs (theirhealth and comfort), and energy (thermal and electrical loads minimization). Finally, presented are ele-ments of an analytical modeling approach, as initial results of study, aimed to reach a challenging researchgoal – Tuning control of buildings Glazing’s transmittance dependence on the solar radiation wavelengthto optimize daylighting and building’s energy efficiency.

© 2013 Elsevier B.V. All rights reserved.

ontents

1. Introduction – daylighting relevance for building’s health and energy efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1082. Building energy performance and daylighting simulation optimization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

2.1. Dynamic climate responsive fenestration and fac ade systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1112.2. Current research and development work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

3. Daylighting health impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1134. Tuning control of glazing’s daylight transmittance via biomimetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135. Basic elements of the optimization modeling approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.1. Unshaded window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1165.2. Transmitted flux from sky and ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1165.3. Solar gains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1165.4. Switchable glazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

∗ Corresponding author at: Multidisciplinary Studies Program, University of Bel-rade, Serbia. Tel.: +381 11 2667775; fax: +381 11 2660532.

E-mail addresses: deresmt@eunet.rs, deres@agrif.bg.ac.rs (M.S. Todorovic).

378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.enbuild.2013.03.036

1. Introduction – daylighting relevance for building’shealth and energy efficiency

In order to stop the global climatic changes and its moreand more obvious consequences, current irreversible destructionprocesses are to be replaced by the intensive growth of energy

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fficiency and RES utilization, especially in building sector – respon-ible for more than one third of total energy use and, in mostountries in the world, is the largest greenhouse gas emissionsource [1,2]. Energy is mainly consumed during the use stagef buildings, for heating, cooling, ventilation, lighting, appliances,tc. A smaller percentage (approx. 10–20%) of the energy use isor materials manufacturing, construction and demolition. Energy-elated impacts of buildings must be considered in their life-cyclenvironmental analysis focusing factors that affect energy con-umption: facades concepts/building envelope alternatives, glazingnd fenestration, types of building structure thermal mass andnsulating materials, lighting and day-lighting control, natural ven-ilation and energy-recovery opportunities, and HVAC systemsegimes and operational modes such as temperature control, airolume control, motors and pump types of control, indoor andutdoor air quality and environmental protection. All of these con-iderations have an impact on the buildings energy efficiency, HVACnd refrigeration requirements and resulting CO2 emission, as wells on the built environment indoor and outdoor health [1–4].

A holistic approach to the sustainable – healthy Zero Energyuilding design requires a method to determine and optimize totaluilding performance, to optimize all relevant energy and differ-nt working fluids flows and interactions between all technicalomains of buildings – HVAC and all other technical systems. In theame time occupant comfort is not to be neglected but the multiple-omain comfort assessment is required for all aspects of indoornvironment quality IEQ (thermal, light, air quality, acoustics/noisend electromagnetic radiation, smell).

For buildings in four season countries, in order to create theonditions for comfort during winter, it is necessary to “resist heatoss” and to “promote heat gain” by using insulation and other tech-iques to retain heat inside the occupied space (minimize heat

osses), by constructing an airtight building envelope to reduceeat loss by air infiltration (and ex-filtration) and by application ofind breaks to shield the building from dominating winter winds

5]. Promoting heat gain can be achieved by application of passiveolar technologies combined with buffer zones and high buildinghermal mass to reduce the daily temperature variations.

Depending on the geographic position and climatic conditions,o provide comfort during summer conditions in many regionsorldwide, it is necessary to reduce heat gains and to increaseeat losses. To resist heat gains is important to apply passive cool-

ng techniques as reduction of internal heat loads, application ofhermal mass to dampen temperature variations and applicationf solar shading to reduce the solar heat gain but at the same timensure a satisfactory daylight level. Increasing heat loss can bechieved by utilizing natural cooling technologies (earth coolingither by conductive heat loss through the building construction ory pre-cooling of outside air, by using vegetation and water evap-ration techniques, to cool outside and inside air, by exposing theuilding construction to the cold “night sky” radiation and by uti-

izing natural ventilation during night time with cool outside air –alled natural cooling) [5].

However, perfect balance between natural resources and com-ort requirements can rarely be achieved, except under very specificnvironmental circumstances, and the performance of climaticesigned buildings vary throughout the year depending uponhether the prevailing climatic condition is “under-heated” com-ared to what is required for comfort (in winter) or “over-heated”in summer). Also, it is necessary to stress, that as a result oflobal warming, in weather conditions known as “hot waves”,uilding’s bioclimatic adaptability and particularly passive and/or

atural cooling technologies, in many regions in the world, areot any more able neither to provide people with appropriate

ndoor environment and comfort, nor in some cases to save theirives. However, bioclimatic approach, or also called passive solar

Buildings 63 (2013) 108–118 109

buildings approach, is important basic step toward energy effi-cient building’s constructions, which are offering, via integratedsustainable building design, opportunity to become fully energysustainable Zero Energy buildings, air-conditioned and cost effec-tively supplied exclusively with the renewable energy sources(RES).

In addition, to the building’s comfort and Zero energy features,buildings are to be health. Among the first focusing that fact wereTurner [6] and Chen et al. [7] (outlining strong relationship betweenIAQ, people productivity and people health). Methodology of theindoor environment assessment specifying indoor environmentindicators (IEI) for acoustics, vibration, illumination, thermal andvisual comfort, indoor air quality and electromagnetic environ-ment has been developed. It has been based on setting a weightingamong the physical categories and is carried out through the ana-lytic hierarchy process (AHP) method [8]. The weighting of essentialcategories determined by AHP method and the evaluated scalescorresponding to the field-measured values did show feasibility ofall IEQ’s relevant IEC (including daylight and illumination), assess-ment on the buildings users health benefits.

Built environment has been studied, more in depth, as archi-tectural field of light and relevant elements for the healthy lightdetermination and promotion in [8], providing clear differencesbetween the healthy light, and light which certain features can beharmful for people. Based on the outlined needs, to block too exces-sive penetration of sunlight, too much light, to discomfort glare,and UV penetration (aimed to protect human health), the scheme ofwhole research related to light and healthy living environment hadbeen organized and proceeded with the definitions of the relevantdesign elements in terms of light [8].

Optical issues related to the harmfulness of light and surveysof already built examples of architectural and optical solutions toattenuate the harmfulness of light and improve the visual satisfac-tion have been analyzed and their performances are provided in [9]along with the citations of medical and biological research whichrevealed that light entering the human eyes has, apart from a visualeffect, also an important non-visual biological effect on the humanbody.

2. Building energy performance and daylighting simulationoptimization

Solar radiation, visible to the naked eye, range or daylight spans380–780 nm. Understanding the properties of spectral distribu-tions of daylight and its dynamical changes at different sites withvarying atmospheric conditions is an active research area.

Understanding the solar–terrestrial interaction, including cli-matology and space weather, starts within the context of Earth’sconnection to the Sun’s activity. The energy from nuclear fusion inthe Sun’s core is released and transported to the solar visible sur-face. This energy manifests in thermal and magnetic processes inthe solar photosphere, chromosphere, and corona, giving rise to ahighly ionized plasma in distinct features and layers and produc-ing radiation throughout the electromagnetic spectrum. Throughinteractions with the terrestrial environment, this radiation helpscreate short timescale space weather as well as the longer-termglobal climate [10].

Variations of the total solar irradiance, as well as variations of itsall segmental ranges, are the fundamental forcing mechanisms tothe terrestrial atmosphere, land, and oceans. The Sun varies on alltime scales and the magnitude of variability is a strong function of

wavelength. On its way to the Earth surface solar radiation is pass-ing through the terrestrial atmosphere and related solar–terrestrialinteraction is responsible for the additional variability of the inci-dent solar radiation on the Earth and/or objects on its surface.

110 J.T. Kim, M.S. Todorovic / Energy and Buildings 63 (2013) 108–118

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xtraterrestrial solar spectral irradiance curves (air mass = 0 left,nd of few air masses > 0 right) are visible in Fig. 1.

Consequently, performance requirements of glaz-ng/fenestration systems, and their daylighting features, areependent on the geographical location, climate, buildingype/purpose, orientation and internal spatial design, exteriorbstructions/shading, as well as on the occupants visual needs.aylighting’s energy relevant glazing performance (desired

lluminance and luminance levels, total luminous flux, savingsotentials, daylight efficacy and cooling loads, thermal impacts,aylight efficacy and cooling loads) short descriptions given byelkowitz [11] are Desired illuminance and luminance levels, Totaluminous flux, Savings potentials. Thermal impacts. Daylight efficacynd cooling loads and Lighting controls.

Lighting controls encompasses daylighting design as a crucialtep in integrated sustainable building design. It includes productesign (sensors and electronics), architectural design (positioning),

nstallation, calibration, and operational adjustments. The wholeystem must maintain the desired illuminance levels under a wideariety of daylight conditions and overall lighting must create aisually appealing space as the illuminance levels of electric light-ng are dimmed responding to the instantaneous daylight.

Characteristics of current glazing and fenestration systems,articularly in office buildings are more and more dynamic. Inpreading use are electrochromic and thermochromic glazings,ontrol of which is linked to the whole building energy manage-ent systems – BMS. However, recent R&D results and current

esearch, particularly in material science show that limits in tech-ological development of glazing’s special features are not reached,nd new breakthroughs are to be further expected.

Daylighting design, as an integral part of lighting design tasksithin the “Zero energy approach” strategy in the integrated

ustainable building design (ISBD) methodology, addressed in4,12,13] foresees: determination of the function of the buildingnvelope ratio of transparent and opaque area and glazing proper-ies related to visible light transmission and solar heat gains as therimary climatic modifier to minimize heat loss in winter, to min-

mize heat gain in summer by optimizing use of natural light andearching in general a “simple” approach promoting good instal-ation, operation and maintenance. By “simple” is meant conflictsetween design services are to be avoided and not left to the hopehat the control system will resolve the conflicts.

Daylighting assessment and optimization encompasses: win-ows, glazing, interior finishes, skylight, light shelves, light wells

nd light pipes, as well as aimed to the harmonization of the day-ighting and lighting interaction covered are to be fixtures, locationsf fixtures and Lamps. Further, glazed systems can be classified inwo categories: static and dynamic. Static systems include not only

mass = 0 left, and of few air masses > 0 right).

glazing, such as spectrally selective and holographic glazing, butspecialized designs of light-shelves and light-pipes, while dynamicsystems cover automatically operated Venetian blinds and elec-trochromic glazing [11].

Important knowledge and performance data on the building’senvelopes including daylighting are IEA ECBCS Annex 32 – IntegralBuilding Envelope Performance Assessment Technical SynthesisReport [14] and the IEA – ECBCS Annex 44 – Integrating Environ-mentally Responsive Elements in Buildings [15].

More than three decades, different of the above listed systems,are in a very intensive research and development, particularly indomain of material sciences and technologies, as well as in the areaof building physics and building’s performance simulation – BPS(algorithms/software development for glazing, envelope and wholebuilding dynamic behavior analysis and optimization) [13].

As a result, even for large glazed areas, presumed as not appro-priate for very hot or very cold climates, are today available nearlycountless kinds of glasses with so many combinations of verylow heat transfer coefficients, and very different solar and visiblelight transmittance, that even huge glass areas (including wholeglazed building) can be relatively energy efficient if their design orredesign is based on all year round dynamic behavior simulationstudy and optimization, as for example Belgrade Contemporary artMuseum’s energy refurbishment described in [12].

Constructed 25 years ago New Belgrade Contemporary artMuseum architecturally had been designed as a today’s architec-tural master peace of work (Fig. 2). With huge glazed envelopesurfaces it deserved very careful surplus of solar radiation and solargains control, as well as glare-effected day-lighting control. It hasbeen redesigned to approach energy efficient public building status.Its envelope and construction have been studied by BPS – BuildingPerformance Simulations for Belgrade’s TMY – typical meteoro-logical year. Analyzed were and quality (glare taking in account),dynamics of its thermal behavior, heating and cooling loads andenergy demand, indoor spaces air quality and particuloarly day-lighting availability and artificial lighting energy demand.

In an approach to refurbishment, to select optimized glazingfeatures, dynamic day-lighting study via RADIANCE, DAYSIM andADELINE software, and BPS (Building performance simulation) tooptimize whole Museum’s envelope and structure integrally withits HVAC and other technical systems was of crucial importance(see a few day lighting study results in Figs. 3 and 4). Optimiza-tioon parameters were glazing’s visual light transmittance andsolar heat gain coeficient. Obtained percentage reductions in heat-

ing loads/heating energy annual demand comparing to the existingbuilding status before refurbishment and glazing replacement are34.7/53.7, reductions in cooling loads/cooling energy demand arerespectively 18.6/30.4.

J.T. Kim, M.S. Todorovic / Energy and Buildings 63 (2013) 108–118 111

Fig. 2. Contemporary art Museum in New Belgrade (left) in reconstruction (right) [12].

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ig. 3. Day-lighting in the plane 90 cm above the 1st floor, for glazing visible light t0lux [12].

.1. Dynamic climate responsive fenestration and fac ade systems

Intensive development and commercialization of complex glaz-ng, fenestration and fac ade systems in late 90-ties of last century,equired standards, measurements and testing procedures devel-pment. J.L.J Rosenfeld et al. presented in [16] results of the ALTSETroject (part of the European Commission’s Standards, Measure-ent and Testing program), which objective was the development

f European standard test procedures for the determination ofngular-dependent light and total solar energy transmittance foromplex glazing and integrated shading elements. The measure-ent of these quantities is difficult and laborious and therefore

xpensive. Hence, it was of interest to consider whether it is possi-le to derive the properties of a complex glazing using models. The

nput data for such models are generally the optical and thermal

roperties of the components that make up the complex glazing.

For many components, such as uncoated and coated glassnd plastic films, the required properties can be measured with

Fig. 4. Day-lighting distribution in the office on 1st floor, for glazing visible ligh

ittance 72% and sunny sky, left max 7010lux, min: 140lux, right max: 350lux, min:

relatively little effort, using standard laboratory equipment. Formore complex components, it is generally necessary to develop asub-model. This can itself be time consuming, so it was not evidenta priori that a modeling approach will always be practicable orpreferable to direct measurement. A disadvantage of the model-ing approach is that samples of the components must be madeavailable, and there is always some residual doubt whether theassembly into the complex glazing might affect the overall prop-erties. An advantage of modeling is that, once the model has beenvalidated, it is very easy to study the effect of varying the propertiesof some particular component. This is valuable for product devel-opment. It is also easy to vary the boundary conditions, such asthe direction or spectral distribution of the incident light. Modelsof complex glazing are also useful in that they can be incorporatedinto other models, for example building or daylighting simulations.

Within that project, in parallel to the development of test pro-cedures, models to predict the properties of a variety of complexglazing, were developed and validated. Paper [16] reviewed the

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rogress made during that ALTSET collaborative program. Modelsave been developed for a wide range of complex glazing, includ-

ng those incorporating solar control films, transparent insulationnd both fixed and variable blinds. The models were based on theoncept that a complex glazing can be represented by a stack ofayers, each layer representing one of the component elements ofhe glazing. Project results did show that, if the optical and ther-

al properties of each layer are known, methods are availableo calculate the corresponding properties of the stack. Conducted

easurements demonstrated, that used carefully, the models areble to predict the luminous and total solar energy transmittance ton accuracy comparable to that obtainable experimentally, henceodels validation result was “models are suitable for integration

nto building energy simulation tools”.Currently, further intensive engineering work is devoted to the

mprovements and innovations of both dynamic facades and alsotatic facades but of the dynamic features. Further advanced facadesill require enhanced automation and better sensors and controls

or optimal operations. In the case of dynamic facades of residentialuildings the opening of a window or lowering of a shade may beone by the occupants themselves based on their comfort needs.

n office buildings a design strategy is involving more and moreredictive algorithms, integration of fac ade and lighting systemsith a wide range of building sensors (wired and wireless) as well

s anticipatory signals for predicted, for example evening wind andemperature, day ahead utility price signals and next day expecteduilding occupancy [11].

Also, new lower cost sensors with communications based onnternet protocols are searched for motorized blinds and elec-rochromic windows [11]. In addition, design of advanced facadesequires advanced simulation and design tools appropriate toescribe innovative solutions and holistic – fully integrated build-

ng’s dynamics (co-simulation), as well as adequate software andools for building’s dynamic daylighting/lighting operation optimalontrol.

However, it is necessary to stress that although application ofighting control technologies has increased the public interest, ashese technologies have been commercialized and promoted dur-ng the last years, their successful use in buildings has been realizedtill in relative small percentage of new projects. One reason is theifficulty in quantifying the energy savings and thus the subsequentayback period [17]. Majority of existing simulation tools (whichre embedded in building energy codes) are needed during initialesign are based on the estimation of the potential energy savingsue to daylight, and paper [17] focuses the limitations of currentimulation approaches comparing their results, in order to assessheir accuracy.

Lighting control systems are a complex technology which ismploying a variety of currently available controllers, software,ensors and devices, but there is a lack of information concerninghe actual performance of these systems and control strategies. Inrder to fully exploit their capabilities and implement the mostnergy efficient control strategies, relevant simulation softwarere needed during the initial design phase. This will improve theesign parameters such as optimal ceiling or wall positions for thehoto sensor, shielding configurations from electric lighting andaylighting sources and the control algorithms to be defined accu-ately. In the paper [17] three stand-alone programs have beenested while the analysis of the results revealed that simple andasic scenarios are capable of identifying weak areas in a givenrogram.

In addition, recently new simulation tools have been developed

iming at the calculation of energy savings as a result of the day-ight control (SPOT, DAYSIM). As the paper [17] states, currentlys really needed a well defined case study as benchmark and fur-her developments in algorithms, particularly with the reference

Buildings 63 (2013) 108–118

to the following optimization algorithm for the best position of thephotosensor, and control optimization of various types of shadingsystems

The objective of the document [18] is to promote the use ofdynamic daylight performance measures for sustainable buildingdesign. The paper review the shortcomings of static daylight perfor-mance metrics which concentrate on individual sky conditions andthe common daylight factor, and is proceeding with review of previ-ously suggested dynamic daylight performance metrics, discussingthe capability of these metrics to lead to superior daylightingdesigns and their accessibility also to the nonsimulation experts. Inaddition, several example offices are examined to demonstrate thebenefit of basing design decisions on dynamic performance metricsas opposed to the daylight factor.

2.2. Current research and development work

Currently, worldwide search for sustainable NZEB (Net Zeroenergy buildings) created numerous R&D programs aimed to fur-ther building’s energy demand reduction to the level that could besatisfied exclusively by the RES supply (building’s integrated and/orin the building’s yard and/or by the municipality at the annual netzero balance). Current research challenges are to make facades,optimized with the reference to all performance requirements andconstrains.

NZEB goal did renew requests for more innovations andadvanced research on the smart glass – switchable glass refer-ring exactly to the electrochromic – electrically switchable glass,feature of which is changeable light transmission. Beside elec-trochromic devices, to the same group belong suspended particledevices, micro-blinds and liquid crystal devices [19,20].

Saeli et al. [21] are drawing attention to the thermochromicglazing, and by conducting a series of BPS are demonstrating howthermochromic glazing has significant potential to reduce energyconsumption in buildings by allowing visible light for day light-ing, reducing unwanted solar gain during the cooling season, butallowing useful solar gain in the heating season. Thermochromic(TC) materials have active, reversible optical properties that varywith temperature. Thermochromic windows are adaptive windowsystems for incorporation into building envelopes. Thermochromicwindows respond by absorbing sunlight and turning the sunlightenergy into heat. As the thermochromic film warms it changesits light transmission level from less absorbing to more absorb-ing. The more sunlight it absorbs the lower the light level goingthrough it. Thermochromic materials will normally reduce opti-cal transparency by absorption and/or reflection, and are specular(maintaining vision).

This review on the status of the R&D on the new generation ofelectrochromic glazing can be ended stating that it is still focused onfurther improvement of the glazing’s adaptive response on the sun-light changes. Consequently there is a need for renewed searchingon basic physics related to smart glazing, with coatings that couldperform climate responsively and dynamically change from clearto spectrally absorbing and/or spectrally reflective to reduce solargain and control glare. Selkowitz’s statement given in [11] “Deliver-ing dynamic, responsive control of solar gain and glare, but permittingdaylight use, is still the holy grail of fac ade technology” is still valid.In addition, within the process of optimization of building design,sometimes are neglected human factors issues and the fact thatmost buildings exist to house the activities and living of people andtherefore are to accommodate their needs as well as their wantsand perhaps even their whims [11].

The review of the complex multi layered build-up of stateof the art glazing products shows that the integration of mostrequirements of modern building envelopes: thermal insulation,adjustable shading by means of electrochromic coatings and

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ven energy generation through integrated photovoltaics [22–25].owever, because these systems require during manufacturing sig-ificant embodied energy, and in addition most of the materialesources would be lost at the end of its life, the multi-layeredomposite glazing build-ups contradict the basic principles of sus-ainability. Therefore, there is a need for smarter concept fordaptive – to the climate and indoor loads responsive solution –lazing with intrinsic tuning control of buildings glazing’s transmit-ance dependence on the solar radiation wavelength to optimizeaylighting and building’s energy efficiency.

The utilization of bio-chemical processes represents one alter-ative. Photosynthesis generates biomass by absorbing day-lightnd CO2. Because cell division rates respond directly to the exter-al conditions, trees and plants have long been used in landscapings smart shading devices. Higher plants go through a relativelylow yearly cycle, but micro-organisms such as algae respond tohanging conditions within hours. But the question is: can micro-rganisms be utilized to shade buildings? With such goal, since009 Arup Germany has been leading a research collaboration withOLT International and SSC, a small enterprise specialized in hydro-iological processes. Project is about the integration into buildingsf photobioreactors (PBR) [24].

Based on the photobioreactors, transparent containers is basedevelopment of the Flat Panel Uplift Bioreactor, in which air bubblesise up inside the cavity creating strong turbulence which helps thelgae growth. The shading factor of this system depends on the den-ity of cells. The system represents an interactive, adaptive shadingystem. During times of high solar radiation the density of the cellsncreases, blocking the light. In contrast, the light transmission cane increased by an intensified harvesting process. First generationrototype of external louvers with integrated photobioreactors ishown in Fig. 5.

. Daylighting health impact

Review of the daylight health impact study at the edge of “newreakthroughs” has been presented in [3]. Corresponding study andesults on the visual health as one of the components of a healthynvironment presents paper [26] stressing that visual health is

vital question because of both the amount of illuminance andhe quality of light, that is, the distribution and the glare. There-ore, making the most use of natural light is one of the promisingpproaches to visual health. Based on photometric, medical, psy-hological, and physiological criteria, these various human needshould be accommodated: visibility, activity, health, safety, com-ort, social contact, communication, esthetic appreciation, and soorth. A series of computer simulations were used in evaluating theisually severe hardships.

In order to curb the excessive penetration of UV rays, opticallyunctional glasses are proposed and the performance assessmentn the glazed materials has been measured. Various pair-glassesonsisting of UV control glass and conventional glasses have beenanufactured and evaluated, providing photometric data in terms

f wavelength. Relations between the visual criteria and healthhould be considered with the following lighting parameters: illu-inance uniformity, luminance, pleasantness, modeling, glare and

ight color aspect or color temperature. Meeting biological lightingeeds might be very different from visual needs. This strengthenshe idea that present indoor lighting levels (and standards) are tooow for biological stimulation. Medical research has shown that arolonged lack of the “light vitamin” can cause health problems,anging from minor sleep and performance difficulties to major

epression.

Results of the conducted research presented in series of paper26–36] elucidate synergetic effects of interdependence of healthnd the whole cluster of issues daylighting related: overview of new

Buildings 63 (2013) 108–118 113

developments in optical daylighting systems for building a healthyindoor environment [27]; optimization of photovoltaic integratedshading devices [28], monitoring and evaluation of a light-pipe sys-tem used in Korea [29]; luminous impact of balcony floor at atriumspaces with different well geometries [30]; UV-ray filtering capa-bility of transparent glazing materials for built environments [31];a distribution chart of glare sensation over the whole visual field[32]; effect of background luminance on discomfort glare in relationto the glare source size [33]; the position index of a glare source atthe borderline between comfort and discomfort (bcd) in the wholevisual field [34]; a first approach to discomfort glare in the pres-ence of non-uniform luminance [35]; application of high-densitydaylight for indoor illumination [36].

Summarized concluding statement of the listed voluminousresearch work was clearly and simple formulated in [3]: an energy-saving and sustainable design strategy could be effectively appliedin exploiting daylight, which not only decreases energy demandand CO2 emissions but promotes occupant wellbeing by creatingthe most ideal condition possible for a space occupants comfortand health.

4. Tuning control of glazing’s daylight transmittance viabiomimetics

At the building envelope glazing are occurring interactively sev-eral physical phenomena and processes. At the outdoor ambientside of envelope are acting: air temperature and humidity, “sky”equivalent temperature, wind velocity, solar radiation and otherouter electromagnetic fields. All these quantities are permanentlychangeable, including the spectral distribution of the solar radi-ation, hence sunlight and daylight are permanently variable. Tooptimize building’s glazing’s features relevant for building’s energyefficiency and indoor environment quality, it is necessary to deter-mine the functional dependence of building’s energy efficiency (allyear round) on the relevant glazing’s features visual light transmit-tance and solar heat gain factor/coefficient. As the solar radiationis spectrally distributed and changeable, optimal glazing featurewill be also spectrally distributed, and its optimal control couldbe obtained only by fine spectral tuning. The most of approachescurrently in use, even in the most developed smart glazing tech-nologies are based on the separation of solar radiation in two partsdaylight and solar heat gains.

Our approach toward optimal dynamic control of the glazingfeatures is determination of glazing with inherently tunable fea-tures with the reference to the spectral distribution of the incidentradiation. Such glazing’s intrinsic characteristic would be possi-bility to be responsive to the incident solar radiation spectral –wavelength distribution.

In our previous study of the architects interests on biomimet-ics, and more general on the attractive recognition of potentialroles of biomimetics in scientific search [2], was introduced a newtuning potential: biomimetic dynamically responsive self-adaptiveglazing feature to the radiation spectra changes as a biologicallyinspired design search aimed to dynamically optimal control solarradiation and daylighting, In this study we are proceeding with itsimplementation.

In our searching approach bio-mimetic means approachingbio-functional tuning, adaptability phenomena and bio-optimalcontrol. Fig. 3 shown the biomimetic shading of the Esplanade-Theaters in Singapore (left) and Lycaena butterfly (right) knownon its color vision via spectral tuning of receptor arrays – theireyes contain four spectral types of visual pigments but the dis-

tribution of these pigments within the receptor mosaic is quitedifferent between species. Photochemical and physiological studiesare revealing great inter-specific diversity in the spectral propertiesof butterfly visual pigments and photoreceptors.

114 J.T. Kim, M.S. Todorovic / Energy and Buildings 63 (2013) 108–118

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b

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Fig. 5. First generation prototype of extern

Very little is known, as yet, about how chromatic information isrocessed by their visual systems. Questionable is understandingn the physical phenomena governing sensory capabilities and lim-tations related to the requirements of visually mediated behaviors,s well as explanation of the color vision in Lycaena butterflies viapectral tuning of receptor arrays? Physically sound answers, onhese questions, could be fine basis to invent new type of glazing

aterial’s with a special feature – ability for a tuning control of glaz-ng’s transmittance dependence on the solar radiation wavelength2].

In [2] a structure with set of tasks and activities of the researchroject on the healthy buildings daylighting dynamic control’serformance prediction and validation has been defined. In itsnal section study [2] presents the potential scheme of the con-rol problem which the tuning control of glazing’s transmittanceependence on the solar radiation wavelength is to solve. Presentedcheme items follow:

. Based on the time dependent measurement of the outdoor solarradiation and daylight data are to be determined time dependentspectral distributions of solar radiation and of the daylight versustheir wavelengths.

. Building’s physics and integrally building and HVAC as well as theother technical systems are to be optimized with the referenceto the total building performance – energy efficiency implemen-ting BPS and measured relevant meteorological parameters, aswell as indoor spaces by the standards defined comfort con-ditions. This optimization is to be done with variable featuresof the tuning control of glazing’s transmittance dependence onthe solar radiation wavelength and taking in account their timedependence.

c. As a result of optimization (item b) obtained will be the timedependence of the optimal spectral distribution of the glazing’stransmittance and SHGC – glazing’s dynamic features relevantfor the optimal tuning control with the reference to the energyefficiency.

. By the measurement/monitoring/audit obtained optimal indoorenvironment and health related quality (IE&HQ) data set (for

the relevant parameters and variables) are to be expressed inmathematical form implementing “inverse” method.

. Finally the control loop is to be closed by comparing obtainedglazing’s feature under item c. with the data set on the IE&HQ

ers with integrated photobioreactors [24].

determined under item d. and based on the comparison results ifit is concerning comfort and/or health necessary glazing featuresare to be corrected (Fig. 6).

It has to be mentioned that concerning “Tuning control ofglazing’s daylight transmittance via biomimetics” our explorationon applicable concepts is has not been exhausted in this study,For example an attention might be deserve for example studies[37–39]. Yang et al. in their study “Porous biomimetic microlensarrays as multifunctional optical structures” presented an attrac-tive material research on the microlenses as important opticalcomponents that image, detect and couple light [37]. Most syn-thetic microlenses, however, have fixed position and shape oncethey are fabricated and therefore, the attainable range of their tun-ability and complexity is rather limited. In comparison, biologicalworld provides a multitude of varied, new paradigms for the devel-opment of adaptive optical networks. The review [37] discussesa few inspirational examples of biological lenses and their syn-thetic analogs. We focus on the fabrication and characterization ofbiomimetic microlens arrays with integrated pores, whose appear-ance and function are similar to a highly efficient optical elementformed by brittlestars. Authors are wandering about the possibilityto develop a tunable optical device coupled with the microfluidicsystem, in which the replacement of rigid microlenses with softhydrogels could provide means for changing the lens geometryand refractive index continuously in response to external stimuli,resulting in intelligent, multifunctional, tunable optics. Hong et al.in [39] are proceeding with investigation and development of theapproach presented in [38]

5. Basic elements of the optimization modeling approach

Finally, study is proceeding with assessment of the basic ele-ments of the modeling approach, aimed to enable achievingdetermined challenging research goal – Tuning control of buildingsglazing’s transmittance dependence on the solar radiation wave-length to optimize daylighting and building’s energy efficiency.

As the approach to establish relevant mathematical opti-

mization model for the tuning control of buildings glazing’gtransmittance dependence on the solar radiation wavelength tooptimize daylighting and buildiing’s energy efficiency, have beenselected co-simulation and combined use of the available software

J.T. Kim, M.S. Todorovic / Energy and Buildings 63 (2013) 108–118 115

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RNSYS or EnergyPlus and combination of WINDOW, DAYSIM, andADIANCE [40–42].

A. TRNSYS based optimization problem and determination ofhe resulting function of the glazing’s transmittance dependencen the solar radiation wavelength (outdoor solar radiation spectra)oreseen solution steps based on the TRNSYS use are

In existing equations sets for building’s energy analysis glaz-ing’s transmittance, absorbance and reflexivity of solar radiation,including all its relevant wavelength parts, are to be defined as afunction of the radiation wavelength and not as a certain constantvalue.Building performance simulation is to be performed for a seriesof assumed functional forms and proceeded with the determina-tion of the optimal one for which the building’s energy efficiencyis optimal in synergy with control of the accessible daylightingvalues.Determination of the final optimal control function or set of theoptimal control functions implementing neural network or fuzzylogic method.

Follows relevant equations set.The luminous transmittance, �vis(cϑ) for light incident at an

ngle ϑ is defined as

vis(ϑ) =∫

D65(�)V(�)�(�, ϑ)d� (1)

here D65(�) is the relative spectral power distribution of the CIEtandard Illuminant D65 and V(�) is the standard observer photopicuminous efficiency function (CIE, 1986). Thus, to calculate �vis(ϑ) its necessary to know the spectral distribution of the transmittance,(�, ϑ) of the glazing.

The total solar energy transmittance, g(ϑ), is the sum of theransmittance under solar irradiation, �s(ϑ) and the secondarynternal heat transfer factor, qi, caused by the heating of the glazings it absorbs a fraction of the incident light

(ϑ) = �(ϑ) + qi(ϑ) (2)

he solar transmittance is defined as

s(ϑ) =∫

Es(�)�(�, ϑ)d� (3)

here Es(�) is the normalized distribution of solar radiation. Inonformity with existing standards, we have used the air mass 1.5lobal distribution (ISO, 1990).

) and Lycaena butterfly (spectral tuning of receptor arrays-right).

The solar absorptance, ˛s(ϑ) is defined analogously to the solartransmittance, as

˛s(ϑ) =∫

Es(�)˛(�, ϑ)d� (4)

A detailed window model has been incorporated into the TYPE56 component using output data from the WINDOW 4.1 programdeveloped by Lawrence Berkeley Laboratory, USA [41]. This win-dow model calculates transmission, reflection and absorption ofsolar radiation in detail for windows with up to six panes. Externaland internal shading devices and an edge correction for differentglazing spacer types are considered. The optical and thermal win-dow model [41] is described below.

With Version 16 a 2-Band-Solar-Radiation-Window-Model wasintroduced. The model is only different in regard to the shortwavesolar radiation. The model splits the external solar radiation into avisual part and a non visual part. The fraction of the visual part canbe calculated with the radiation of a black body at a temperature of5800 K for a wave length band between 380 nm and 780 nm relatedto the total radiation of a black body at the same temperature. Thisleads to the following equations for the visual radiation [41]:

Idif,visual = 0.466Idif,solar (5)

Idir,visual = 0.466Idir,solar (6)

Concerning the given above description of the calculation proce-dure, and stressed facts about the solar spectra variability, it is to beperformed additional analysis about the validity of these two equa-tions. The same is relevant for the visual part of the solar radiationand the expressions (7) and (8) as used in [41]:

Idif,non visual = (1 − 0.466)Idif,solar (7)

Idir,non visual = (1 − 0.466)Idir,solar (8)

The wavelength dependence of the absorption of short-wave radi-ation (direct and diffuse solar radiation, diffuse reflected radiationfrom the all the surfaces of the zone and the optional inner shadingdevice) on the glazing system of the window is to be taken inaccount in farther calculation of a heat flux from the pane to the cer-tain zone. Consequently, in the equation (9) absorption coefficientsand direct and diffuse radiation are to be introduced as the variablewavelength function.

˙∑[

(Ri−1 + Ra)]

Qabs,i =i→n

(Idir absdir,i + Idif absdif,i + (Iref,z + Iref,sh)absdif,i,b)Rtot

(9)

B. EnergyPlus [43] based optimization problem and deter-mination of the resulting function of the glazing’s transmittance

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16 J.T. Kim, M.S. Todorovic / Energ

ependence on the solar radiation wavelength (outdoor solar radi-tion spectra) foreseen solution steps based on the EnergyPlus usere

.1. Unshaded window

For the unshaded window case, the luminance of the windowlement is found by projecting the ray from reference point to win-ow element and determining whether it intersects the sky or anxterior obstruction such as an overhang. If L is the correspondinguminance of the sky or obstruction, the window luminance is

w = L�vis(cos B) (10)

here �vis is the spectrally dependent visible transmittance of thelass for incidence angle B.

Exterior obstructions are generally opaque (like fins, overhangs,eighboring buildings, and the building’s own wall and roof sur-

aces) but can be transmitting (like a tree or translucent awning).xterior obstructions are assumed to be non-reflecting. If Lsky ishe sky luminance and �obs is the transmittance of the obstructionassumed independent of incidence angle), then L = Lsky�obs. Interiorbstructions are assumed to be opaque (�obs = 0).

.2. Transmitted flux from sky and ground

The luminous flux incident on the center of the window from luminous element of sky or ground at angular position (�, �), ofuminance L (�, �), and subtending a solid angle cos�d�d� is

�inc = AwL(�, �) cos cos �d�d� (11)

he transmitted flux is d� = d�incT(ˇ) (12)

here T(ˇ) is the window spectral transmittance for light at inci-ence angle ˇ. This transmittance depends on whether or not theindow has a shade.

For an unshaded window the total downgoing transmitted fluxs obtained by integrating over the part of the exterior hemisphereeen by the window that lies above the window midplane. Thisives

FW,unshaded = Aw

�max∫

�min

�(2)∫

0

L(�, �)T(ˇ) cos cos �d�d� (13)

he upgoing flux is obtained similarly by integrating over the partf the exterior hemisphere that lies below the window midplane:

CW,unshaded = Aw

�max∫

�min

0∫

�/2−�w

L(�, �)T(ˇ) cos cos �d�d� (14)

here �w is the angle the window outward normal makes with theorizontal plane.

For a window with a diffusing shade the total transmitted fluxs

sh = Aw

�max∫

�min

�/2∫

�/2−�w

L(�, �)T(ˇ) cos cos �d�d� (15)

he downgoing and upgoing portions of this flux are

FW,sh = �(1 − f ); �CW,sh = �f (16)

here f, the fraction of the hemisphere seen by the inside ofhe window that lies above the window midplane, is given by

= 0.5 − �w/�.

Buildings 63 (2013) 108–118

For a vertical window (�w) the up- and down-going transmittedfluxes are: �FW,sh = �CW,sh = �/2.

For a horizontal skylight(

�w = �

2

): �FW,sh = �, �CW,sh = 0.

(17)

The limits of integration of � in Eqs. (153)–(155) depend on �. From[Fig. 12 – Winkelmann, 1983] we have sin = sin (A − �/2) = (sin �tan �w)/cos �, which gives

A = cos−1(tan � tan �w) (18)

Thus �min = −| cos−1(tan � tan �w)|;and �max = cos−1(tan � tan �w)|. (19)

The flux incident on the window from direct sun is �inc

= AwEDN cos fsunlit . (20)

The transmitted flux is � = T(ˇ) �inc (21)

where T is the net transmittance of the window glazing (plus shade,if present).

For an unshaded window all of the transmitted flux is downwardsince the sun always lies above the window midplane. Therefore�FW,unsh = � and �CW,unsh = 0.

5.3. Solar gains

Solar radiation incident on a window is calculated separately assun, sky, and ground radiation. A different transmittance must beapplied for each type of radiation.

For beam radiation the TDD beam transmittance �TDD(�) for thesolar spectrum is used as described above. For sky and ground radi-ation a diffuse transmittance for the TDD must be developed. Thetransmittance of diffuse radiation can be defined as the total trans-mitted flux divided by the total incident flux.

�diff =∑

Itrans∑Iinc

(22)

For a given pipe or TDD, �diff,iso is a constant. The program calculates�diff,iso once during initialization using a numerical integration.The diffuse isotropic transmittance is useful, but not sufficient, fordetermining the transmittance of sky radiation. As described in theSky Radiance Model section, sky radiation has an anisotropic distri-bution modeled as the superposition of three simple distributions:a diffuse isotropic background, a circumsolar brightening near thesun, and a horizon brightening. While the daylighting model iscapable of calculating the luminance of any position in the sky, thesolar code only calculates the ultimate irradiance on a surface. Forthis reason it is not possible to integrate over an angular distribu-tion function for sky radiance. Instead the three sky distributionsmust be handled piecewise.

It is important to note that transmittance above is for the totalTDD. The transmittance of the dome and diffuser must be includedto account for their angular dependence as well. The beam trans-mittance is used as an approximation for all circumsolar radiation.Since the radiance of the horizon is isotropic, and therefore constantacross the entire horizon, the actual value of the radiance cancels

out. The result is a constant that is calculated once during initial-ization. Ground radiation is assumed to be isotropic diffuse. Thetransmittance of ground radiation is the diffuse isotropic transmit-tance �diff,gnd = �diff,iso.

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J.T. Kim, M.S. Todorovic / Energ

The solar flux transmitted by a TDD due to beam, sky, and groundadiation is calculated as normal for a window but uses the respec-ive transmittances for the TDD.

′′TDD−trans,beam = (Isun cos �)fsunlit�TDD(�) (23)

′′TDD−trans,sky = Ih,sky fskymult�TDD,diff,aniso (24)

′′TDD−trans,gnd = (Isun cos � + Ih,sky)Fsk �TDD,diff,iso (25)

here, Isun = solar beam intensity of the sun; Ih,sky = total hori-ontal diffuse solar radiation due to the sky; � = incident anglef the beam on the dome; fsunlit = sunlit beam fraction of theome area; fskymult = anisotropic sky view multiplier (see AnisoSky-ult); Fsg = view from ground to dome; �TDD(�) = TDD beam

ransmittance; �TDD,diff,aniso = TDD anisotropic sky transmittance;TDD,diff,iso = TDD isotropic diffuse transmittance

.4. Switchable glazing

For switchable glazing, such as electrochromics, the solar andisible optical properties of the glazing can switch from a light stateo a dark state. The switching factor, fswitch, determines what statehe glazing is in. An optical property, p, such as transmittance orlass layer absorptance, for this state is given by

= (1 − fswitch)plight + fswitch pdark (26)

here plight is the property value for the unswitched, or light state,nd pdark is the property value for the fully switched, or dark state.he value of the switching factor in a particular time step dependsn what type of switching control has been specified: “schedule,”trigger,” or “daylighting.” If “schedule,” fswitch = schedule value,hich can be 0 or 1.

. Conclusions

Reviewed is the most recent analytical and experimentalesearch, current state of knowledge, science and art, as well asome of the on-going R&D on the healthy buildings daylightingynamic control’s performance optimization.

It has been shown that, concerning the variability of the solaradiation spectra incident on the building’s envelope, and also vari-bility of outdoor and indoor air temperature difference, with andim to optimize daylighting with the reference to people needstheir health and comfort), and energy (thermal and electrical loads

inimization), it is necessary to realize tuning control of glazing’sransmittance dependence on the solar radiation wavelength.

Presented are basic elements for the development of an analyt-cal model, aimed to be used for the invention of might be a newptimal control method, and/or new material and technology.

Understanding the properties of spectral distributions of solarnd daylight spectra and its dynamical changes with varyingtmospheric conditions is to be improved. For their precise deter-ination (mathematically and measuring, as well as vice-versa via

nverse modeling) a need for new research approach is outlined.Outlined are as necessary: advanced biomimetically inven-

ive research in material science, nano level materials intrinsichenomena relevant for the fine DL natural wavelength tuning

nd related mathematical description – modeling (experimentallyerified); enriched innovative co-simulation/modeling, and syner-etic inter-operative experimenting/on-line co-simulation basedynamic control tools development.

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Buildings 63 (2013) 108–118 117

Acknowledgement

This work was supported by the National Research Foundationof Korea (NRF) grant funded by the Korea government (MEST) (No.2008-0061908).

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30] G. Kim, J.T. Kim, Luminous impact of balcony floor at atrium spaces with dif-ferent well geometries, Building and Environment 45 (2010) 304–310.

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18 J.T. Kim, M.S. Todorovic / Energ

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