Daylight optimization of multifunctional solar facades

Multifunctional solar facades consisting of a transparent window and an opaque photovoltaic section are analyzed and optimized. Employing numerical daylight estimation techniques, the optimal shape, position, and area of the window section is determined. Maximum yearly average daylight availability is achieved with a similarly shaped window as the facade which is placed near the centre of the facade. For non-residential buildings, the yearly average useful interior daylight illuminance does not increase significantly for windows larger than 30% of the total facade area. Considering both the artificial lighting requirement replaced by daylight through the window and the electricity produced by the PV section of the facade, the maximum electricity benefit for a south-facing facade is achieved with a window area of about 10% of the total facade area in Southern Europe (38° N) and 15% in Northern Europe (60° N).     

Modern insulation requirements change the rules of architectural design in low-energy homes

In the design of very well-insulated homes, there is a need for a more nuanced design that takes into account winter and summer conditions. In this paper, we compare a traditional design for a typical Danish single-family house with large glazing areas oriented towards the south and smaller glazing areas towards the north, and a design with an even window distribution where the glazing-to-floor ratio is the same for each room. We found that the use of solar gains through south-oriented windows is not as important as is traditionally believed because, in well-insulated homes, space heating demand is not reduced much by having larger south-facing windows. Furthermore, we found that there is a g-value above which the additional solar gains through south-oriented windows do not help reduce space heating demand, and it becomes important to use solar shading or glazing with solar-control coating as a cheaper alternative to reduce overheating. Maximum window sizes from an overheating perspective were identified that are larger than the optimal window sizes for space heating demand. However, we show that the difference in space heating demand with optimal window size and with larger window sizes is small, so it is up to the building owner to decide whether or not he wants larger glazing areas to allow for more daylight. And windows can be positioned in the façade with considerable architectural freedom. However, we do recommend an even distribution of the glazing-to-floor ratio, because this generally provides an improved thermal indoor environment in south-oriented rooms and will ensure a better daylight level especially in north-oriented rooms. We also show that the optimal window size is influenced by thermal zone configuration and that there is a need for models in which a difference is made between zones with direct and with non-direct solar gains.     

Evaluation of control strategies for different smart window combinations using computer simulations

Several studies have shown that the use of switchable windows could lower the energy consumption of buildings. Since the main function of windows is to provide daylight and visual contact with the external world, high visible transmittance is needed. From an energy perspective it is always best to have the windows in their low-transparent state whenever there are cooling needs, but this is generally not preferable from a daylight and visual contact point of view. Therefore a control system, which can be based on user presence, is needed in connection with switchable windows. In this study the heating and cooling needs of the building, using different control mechanisms were evaluated. This was done for different locations and for different combinations of switchable windows, using electrochromic glazing in combination with either low-e or solar control glazing. Four control mechanisms were investigated; one that only optimizes the window to lower the need for heating and cooling, one that assumes that the office is in use during the daytime, one based on user presence and one limiting the perpendicular component of the incident solar irradiation to avoid glare and too strong daylight. The control mechanisms were compared using computer simulations. A simplified approach based on the balance temperature concept was used instead of performing complete building simulations. The results show that an occupancy-based control system is clearly beneficial and also that the best way to combine the panes in the switchable window differs depending on the balance temperature of the building and on the climate. It is also shown that it can be beneficial to have different window combinations for different orientations.     

Development of the nomo-graph for evaluation on discomfort glare of windows

Human beings have been using daylight for a long time. The daylight provides building occupants with psychological stability and also serves as lighting. However, the discomfort glare of windows produced from the daylight could be a severe problem as the working condition is getting more complicated and precise. Although numerous studies have been performed in order to solve this problem, most studies have been performed on Caucasians. The recent studies have revealed that there were differences between Caucasians and Asians in ocular activity, pigmentation, and color sensibility depending on the iris color. These data implies that it is difficult to directly apply the data from studies on the Caucasian to Asians whose physiological properties of eyes are different from those of the Caucasian. Therefore, it is necessary to develop an evaluation method of window discomfort glare, which may represent the visual feature of Asian appropriately. In the present study, we investigated the visual difference between Caucasian and Asians and established the nomo-graph to develop the method appropriate for the evaluation of the window discomfort glare for Asians.     

WinSim: A simple simulation program for evaluating the influence of windows on heating demand and risk of overheating

A two-node model of a room has been implemented in a computer program, WinSim, developed for evaluation of thermal performance of windows in new buildings and in case of retrofitting. The program calculates the annual heating demand and the number of hours with indoor temperatures higher than a user defined limit. WinSim is characterised by the limited amount of required input data. Guidelines for calculation of the effective thermal capacity of the room are given, and results obtained with WinSim have been compared to results from an advanced building simulation program. Good agreement has been found between the two programs with respect to calculated annual heating demand and energy savings due to window exchange, and also the calculated number of hours with overtemperature is similar. Based on the limited examples used for the comparison, it can be concluded that WinSim is well suited for a quick but realistic evaluation of thermal performance of windows.     

An open-loop venetian blind control to avoid direct sunlight and enhance daylight utilization

Shading devices, such as venetian blinds, are widely used to eliminate glare and maintain a comfortable working environment for occupants. This paper proposes an open-loop control of blind height and slat angle based on an analytic model of solar position, and geometry of venetian blinds and the windows. The algorithm enables completely blocking direct sunlight from entering the room beyond a certain user specified interior distance from the window while enhancing daylight utilization. We derive closed-form solutions for blind height and angle which are easily implemented in practice. Moreover, the proposed blind control algorithm is simulated with favorable performance. The algorithm has also been implemented and verified in real-time test-beds.     

The passive solar heated school in Wallasey. VII. Window opening behaviour and the microclimate of a classroom

Previous work on factors which influence the opening or closing of windows suggests that at low ambient temperatures movement might be associated with odour levels, at intermediate temperatures, with ambient humidity and at higher ambient temperatures with the need to cool buildings. The data on window position, together with other physical measures during the period of observation in the Wallasey School, has been examined to see what quantity is most closely associated with window position. It appears that in the classroom the number of open windows depends mainly upon air temperature, but it also depends markedly on time of day.     

Application of automated blind for daylighting in tropical region

This paper reports an experimental and simulation study on application of automated Venetian blind for daylighting in tropical climate. A horizontal blind system operating automatically under programmed control was constructed and integrated onto the glazed windows to form a window system with an automated blind in a room of a laboratory building. A dimming controller was also integrated to the lighting system of the room. Different operation schemes of the window system were devised and tested in the attempt to maximize energy savings while maintaining the quality of the visual environment in the room. Intensive measurement of illuminance of the interior space was undertaken during the experiments. A methodology for calculation of interior daylight illuminance and associated glare corresponding to the configurations of the experiments was adopted. The method was coded into a computer program. Results of calculation from the program agree well with those from experiments for all the schemes of operation conducted. The program was used to simulate the situation when each scheme of operation was implemented for a whole year. It was found that such window system with automated blind enabled energy savings of 80%, but a more sophisticated scheme also helped maintain the interior visual quality at high level.     

Daylight performance of anidolic ceiling under different sky conditions

The daylight performance of anidolic ceiling for diffuse daylight is the subject of this article. The performance is assessed over a wide range of sky conditions following the new CIE Standard General Sky and compared between locations in Singapore, Japan and the United Kingdom. The criteria illuminance ratio (IR) and daylight glare index (DGI) are used to quantify the daylight performance of a default ribbon window façade with and without an anidolic ceiling. The difference is expressed in new terms IR improvement factor (IR IF) and DGI reduction. These factors are charted over all 15 sky conditions and various sun altitudes and may serve as general references indicating conditions under which anidolic ceilings perform best. The application of these new terms are demonstrated for Singapore, Fukuoka and Sheffield, representing three main latitude bands and different sets of prevailing sky types. It can be concluded that the daylight performance improvement through anidolic ceilings is most significant in Singapore.     

The performance of innovative daylighting systems

Innovative daylighting systems have four key aims; to increase daylight levels deep within rooms, to improve daylight uniformity, to control direct sunlight and to reduce glare. Results from experimental studies at the Building Research Establishment (BRE) indicated that the systems tested (prismatic glazing, prismatic film, mirrored louvres and light shelves) usually reduced illuminances at the rear of rooms compared with unshaded windows. It is concluded that, in the UK at least, innovative daylighting systems should be designed primarily as shading devices, controlling glare while admitting more daylight than conventional blinds.     

Performance of Anidolic Daylighting Systems in tropical climates - Parametric studies for identification of main influencing factors

Making daylight more available in buildings is highly desirable, not only for reasons of energy-efficiency, but also for improvement of occupants’ health and well-being. Core-daylighting, that is daylight provision in areas situated at considerable distances from façades and windows, is currently one of the main challenges in sustainable building design. Anidolic Daylighting Systems (ADSs) are one very promising technology in the field of core-daylighting, but commercial solutions that are not only well-performing but also financially competitive are not yet widely available. This article presents results of parametric studies on Anidolic Integrated Ceilings (AICs), a special type of ADS, for identification of main influencing factors. The article describes a reliable method for simulating ADS and AIC performance under given sky conditions. Various simulation results for the example location Singapore are discussed in detail, it is concluded that the main influencing factors are coating material, system dimensions and external obstruction, and those influencing factors’ potential impacts are quantified. It is shown that AIC overall efficiencies can reach up to almost 50% in Singapore. The essentially new results presented in this article can be of great help to architects, engineers and scientists in the future, when it comes to precisely dimensioning ADS for various buildings and daylight conditions.     

Evaluation of electrochromic windows impact in the energy performance of buildings in Mediterranean climates

Old buildings refurbishment is essential for the global improvement of building energy indicators. Within this context, the paper focuses on the energy savings that may occur when using electrochromic (EC) windows, an interesting emerging technology alternative to shading devices to control solar gain in buildings located in Mediterranean climates. The EC windows technology is briefly presented and the optical properties adjustments of the glasses are discussed according to the operated range. The EC window dynamic behavior and the different control strategies are modeled and implemented in the ESP-r building simulation program. The EC window impact in the energy needs for heating and cooling is studied, considering different ambient parameters (exterior dry bulb temperature, interior dry bulb temperature and incident radiation) and set points for the EC control. A comparison of several windows solutions (single, double-glazing and EC windows), the type of building, internal gains from occupancy, lighting and equipment and the orientation of windows are considered for discussion through the analysis of the energy needs for heating and cooling. It is concluded that for this climate the best positive results are obtained when the EC are used in the west façade. For the south façade the results show no significant advantages in using EC windows.     

Daylighting and energy analysis of private offices with automated interior roller shades

This paper presents a comprehensive analysis to study the balance between daylighting benefits and energy requirements (control of solar gains) in perimeter private office spaces with interior roller shades taking into account glazing properties, shading properties and control together with window size, climate and orientation in an integrated daylighting and thermal manner. Daylight autonomy and useful daylight illuminances were computed as a function of façade design parameters. A thermal simulation module using the explicit finite difference thermal network approach runs at the same time step and calculates heating, cooling and lighting source energy consumption as well as surface temperatures and operative temperature. Based on the daylighting results, lighting internal gains (continuous dimming control) are simultaneously input to the thermal module. The model also considers the air in the gap between shade and interior glass as a separate thermal node.Detailed results for Chicago and Los Angeles showed that windows with visible transmittance higher than 50% have the ability to allow enough daylight into the space for all locations and orientations for window-to-wall ratios higher than 50%. Useful daylight illuminances between 500 and 1000 lux were considered in detail – it was found that this index can be maximized for specific window-to-wall ratios and that depends on the glazing properties and fabric properties for each orientation. Moreover, the complex interactions of the studied parameters and their impact on the heating, cooling and lighting energy performance revealed an interesting result: windows occupying 30–50% of the façade can actually result in lower total energy consumption for most cases with automated shading. This illustration of daylighting benefits can be realized only if the integration of daylighting and thermal climate-based analysis is modeled efficiently and depends on glazing and shading properties and control. Finally, best designs for each orientation and location were pointed out based on both daylighting and thermal results.     

Implementation of energy-efficient windows in Swedish single-family houses

A questionnaire survey of 1010 homeowners in Jämtland and Västernorrland, which are two counties in central Sweden, was conducted to understand the factors influencing their decision to install energy-efficient windows. We complemented this survey with an interview of 12 window sellers/installers in the county Jämtland. The annual energy cost reduction, age, and condition of the windows were the most important reasons for the window replacement decision. Approximately 80% of the respondents replaced their windows with energy-efficient windows with U-value of 1.2 W/m² K. Condensation problems, perceived higher prices, and lack of awareness about windows with lower U-values were important reasons for non-adoption of more energy-efficient windows. Window sellers/installers have a strong influence on homeowners’ window selection that was indicated by the 97% of homeowners who bought the windows that were recommended to them. Sellers/installers revealed that they did not recommend windows with U-value of less than 1.2 W/m² K because they thought that investing in such windows was not economical and because windows with U-value less than 1.2 W/m² K could cause water condensation on the external surface of window pane.     

Stochastic assessment of the energy performance of buildings

A building’s energy performance is a complex multi-dimensional metric consisting of a variety of parameters. Presented herein are the results of a stochastic analysis of the factors affecting a building’s energy performance. The analysis is based on the Dwelling Energy Assessment Procedure (DEAP) (amended for cooling loads) and the general guidelines prescribed by the European Energy Performance of Buildings (EPBD) Directive 2010/31/EU. Modifications to the DEAP model are made for investigating the effect of variable external weather conditions on a building’s energy performance, and to incorporate the additional energy requirement for cooling. Subsequently, a stochastic analysis for three dwelling types is performed to assess the impact of 68 factors on the energy performance of buildings, for 12 different regions in Europe. It is concluded that (1) the factors with the greatest impact on energy use are in descending order, the floor area, external weather conditions, dwelling’s envelope u value (roof, window, walls, and floors), the space heating system, ventilation, windows area and walls area; (2) the energy performance of a building follows a lognormal probability distribution function; (3) buildings in colder EU regions exhibit higher energy profiles and higher variability in their energy profiles than those in warmer regions.     

Simulation of airflow in one- and two-room enclosures containing a fire source

In this study, the airflow in a room that contains a heat source is simulated numerically. The flow is considered turbulent and buoyant. The results of the mathematical model are validated with available experimental data at specific locations in the domain. A simple geometry is adopted, consisting of a room with a door that plays the role of both inlet–outlet for the fluid (air). At the centre of the room a methane burner is placed to serve as a heat source. The problem is simulated using two turbulence models, the well-known standard k–ε model and the RNG k–ε model, both modified to account for buoyancy effects on turbulence. The burner is considered as a volumetric heat source. It is concluded that the fire plume development as well as the distributions of velocity and temperature are reasonably well predicted. Following this conclusion, both models are also applied to a different, more complex geometry that consisted of two rooms communicating via a door, while the heat source was placed in the first room. Unfortunately, there are no experimental data to compare with for this case, but the results appear plausible. Finally, important design factors, such as mass flow rates and neutral-plane heights, are calculated utilizing the CFD results, and are compared with those obtained by well-known empirical correlations. It is concluded that the bi-directional flow existing through the burning-room vent is similarly predicted by both turbulence models; the RNG k–ε model leading to higher, and more accurate predictions of temperature variations within the hot upper layer, at least for the single-room case.     

Three-state regulator for electrochromic windows

Electrochromic (EC) windows enable to change their optical transmittance by applying appropriate electrical signals to the window structure. In many applications, such as “smart” EC buildings’ windows is required, an automatic control of the window's transmittance, in order to assure a constant level of transmitted daylight for different outdoors’ illuminations. For this purpose we develop an analogue three-state electronic regulator which controls an “all sol–gel” EC device intensity of transmitted light on a constant pre-selected level. In this paper, first of all, the structure and the performances of an all sol–gel EC window is given. Then the electrical schematic of the regulator is represented and explained in detail. The performances of the regulator are demonstrated by electrical and optical measurements. At the end a demonstrational application—EC sun-glasses with an accompanying handy regulator—is presented.     

Glazing edge-seal using foamglass as spacer and frameless window design

This article presents a new design for the edge-seal of multiple-glazings with spacers made of foamglass and a new concept for frameless windows to reduce the heat loss through windows significantly. Thus the energy demand for heating is reduced or covered by solar energy gains through the window to a higher extent. The thermal performance of window assemblies with foamglass spacers and with and without frames is compared with that of the common window design. For the calculations of the heat flux a finite element analysis computer program has been used to account for the 2D-effects in the glazing, edge-seal and frame heat transfer patterns. The total heat transfer through an example window with a glazing 1 m×1 m is reduced by 45% using the window design presented. The objective of this article is not only to quantify the heat fluxes for different combinations of glazing, edge-seal and frame. The major part of the article focuses on practical aspects that are important for the durability of edge-seals, such as mechanical stress within the materials, water vapour and gas tightness, as well as on new design concepts of window–wall joints. A frameless window construction is an important aspect to enhance the thermal performance of windows. The costs for this kind of frameless windows are estimated to be less than or equal to windows commonly used now.