Energy savings of office buildings by the use of semi-transparent solar cells for windows

The study investigated a PV window that consists of a double glazed window with semi-transparent solar cells. The window provides natural light transmission as well as electricity production. The effect of the PV window on energy consumption of office buildings was analyzed in terms of heating and cooling loads, daylighting, and electricity production. The purposes of the study were to find the optimum solar cell transmittance and window to wall ratio (WWR), and to estimate energy savings of the building. A standard floor of an office building was modeled to run computer simulation, and annual energy simulation was performed with EnergyPlus. The results showed that the solar cell transmittance of 40% and WWR of 50% achieved the minimum electricity consumption in the building when artificial lighting was controlled with daylighting. The optimum solar cell transmittance for PV windows in different orientation was also presented. By using the optimum PV window, the electricity consumption was reduced by 55% compared to the single glazed window with WWR of 30% and no lighting control.     

Novel glazing technologies to mitigate energy consumption in low‐carbon buildings: a comparative experimental investigation

Buildings play a key role in total world energy consumption as a consequence of poor thermal insulation characteristics of facade materials. Among the elements of a typical building envelope, windows are responsible for the greatest energy loss because of their notably high overall heat transfer coefficients. About 60% of heat loss through the building fabric can be attributed to the glazed areas. In this respect, novel cost‐effective glazing technologies are needed to mitigate energy consumption, and thus to achieve the latest targets toward low/zero carbon buildings. Therefore in this study, three unique glazing products called vacuum tube window, heat insulation solar glass and solar pond window which have recently been developed at the University of Nottingham are introduced, and thermal performance analysis of each glazing technology is done through a comparative experimental investigation for the first time in literature. Standardized co‐heating test methodology is performed, and overall heat transfer coefficient (U‐value) is determined for each glazing product following the tests carried out in a calibrated environmental chamber. The research essentially aims at developing cost‐effective solutions to mitigate energy consumption because of windows. The results indicate that each glazing technology provides very promising U‐values which are incomparable with conventional commercial glazing products. Among the samples tested, the lowest U‐value is obtained from the vacuum tube window by 0.40 W/m²K, which corresponds to five times better thermal insulation ability compared to standard air filled double glazed windows. Copyright © 2015 John Wiley & Sons, Ltd.     

Optimization of building window system in Asian regions by analyzing solar heat gain and daylighting elements

This paper presents and optimizes the annual heating, cooling and lighting energy consumption associated with applying different types and properties of window systems in a building envelope. Through using building simulation modeling, various window properties such as U-value, solar heat gain coefficient (SHGC), and visible transmittance (Tvis) are evaluated with different window wall ratios (WWRs) and orientations in five typical Asian climates: Manila, Taipei, Shanghai, Seoul and Sapporo. By means of a regression analysis, simple charts for the relationship between window properties and building energy performance are presented as a function of U-value, SHGC, Tvis, WWR, solar aperture, effective aperture, and orientation. As a design guideline in selecting energy saving windows, an optimized window system for each climate is plotted in detailed charts and tables.     

建筑形式对太阳能热利用的影响研究

以上海地区的住宅建筑为研究对象,通过模拟分析的方法,采用DeST软件计算确定建筑逐时的采暖、空调能耗,研究分析窗墙比对建筑全年采暖能耗、全年空调能耗以及全年采暖、空调总能耗的影响规律,研究分析太阳辐射热增加所导致采暖能耗的降低幅度与外围护结构保温性能两者之间的定量关系。计算结果表示在夏季外窗遮阳和夜间通风的条件下,加大南向窗墙比可增强太阳能的热利用效率,降低建筑全年的采暖、空调总能耗;而外围护结构保温性能的增强则可降低室内向室外散热的程度,相应提高对冬季太阳辐射的热利用程度,从而达到降低采暖能耗的目的。     

Lighting and energy performance of solar film coating in air-conditioned cellular offices

In subtropical Hong Kong, the principal objectives of fenestration design include eliminating direct sunlight and decreasing cooling loads. To avoid the problems of glare, excessive brightness and thermal discomfort, occupants may block the windows with internal shading devices, resulting in poor daylighting performance and very small amount of electric lighting energy savings. Recently, the advances in thin film coatings for window glass products provide a means of substantially reducing heat gain without proportionally reducing daylight transmittance. It has been suggested that film coatings together with photoelectric lighting control systems could minimise the electric lighting and cooling requirements without causing undue visual and thermal discomfort to the occupants. This paper presents field measurements on solar control film coatings in fully air-conditioned offices in Hong Kong. Solar heat gains, indoor illuminance levels and the electricity consumption by the fluorescent luminaires were systematically recorded and analysed. Measurements were made for two cellular offices, one with solar control film coating on the window glass and the other without. The findings showed that the solar film coating could cut down energy expenditures for air-conditioned buildings, especially for spaces with large glazing areas subject to substantial amount of direct sunlight. Results are presented and the design implications discussed.     

The impact of shading design and control on building cooling and lighting demand

Shading should be considered as an integral part of fenestration system design for commercial and office buildings, in order to balance daylighting requirements versus the need to reduce solar gains. In this paper, the simultaneous impact of glazing area, shading device properties and shading control on building cooling and lighting demand was calculated using a coupled lighting and thermal simulation module. The interactions between cooling and lighting energy use in perimeter spaces were evaluated as a function of window-to-wall ratio and shading parameters. An exterior roller shade was used as an example. The impact of shading device type, properties and control on building cooling and lighting energy demand was quantified and analyzed. The simulation results indicate that, if an integrated approach for automatic control of motorized shading is used in conjunction with controllable electric lighting systems, substantial reduction of energy demand for cooling and lighting could be achieved in perimeter spaces, depending on climatic conditions and orientation.     

Energy efficient design of building: A review

Energy saving is a high-priority in developed countries. For this reason, energy-efficient measures are being increasingly implemented in all sectors. The residential sector is responsible for an important part of the energy consumption in the world. Most of this energy is used in heating, cooling, and artificial ventilation systems.With a view to developing energy-efficient structures, this article provides an overview of building design criteria that can reduce the energy demand for the heating and cooling of residential buildings. These criteria are based on the adoption of suitable parameters for building orientation, shape, envelope system, passive heating and cooling mechanisms, shading, and glazing. An analysis was made of previous studies that evaluated the influence of these parameters on the total energy demand and suggested the best design options. This study is useful for professionals who are responsible for decision-making during the design phase of energy-efficient residential buildings.     

遮阳板在建筑节能中的应用研究

太阳辐射对供暖、空调负荷有重要的影响。在中国北方地区的建筑物的窗和墙面上，夏天希望尽可能的减少太阳照射，而冬天则希望多得到太阳照射。因此，遮阳板的合理设计就成为建筑节能的一个重要途径。该文根据天津地区典型气象年的气象资料，对该地区建筑南向窗遮阳板的构造进行建模分析，并对其节能前景进行预测。考虑到建筑物美观、遮阳板自身材料及建筑的层高等因素，在保证冬季的日照的前提下，天津地区南向窗遮阳板宽度宜取0．5～0．9m，距窗户上沿距离0．5～0．8m。安装遮阳板后，南向窗每个夏季可减少太阳辐射得热80～110MJ/m^2。     

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.     

Integration of smart windows into building design for reduction of yearly overall energy consumption and peak loads

The purpose of this work is to investigate the potential of diminishing the energy consumed by typical low thermal mass office buildings for heating, cooling and lighting by using smart windows. The windows considered consisted of a double pane glazing unit in which a controllable absorbing layer is added on the interior surface of the exterior glass pane. This absorbing layer allows to change the optical properties of the window, resulting in a direct potential of control of the incident solar heat flux entering the building through the windows. A corresponding numerical model is developed showing that optimizing the solar heat flux absorption rate of the absorbing layer in regard of the necessary heating, cooling and lighting needs helps reducing significantly the total yearly energy consumption, and cooling peak loads. The simulations were done considering a building located in Quebec City, Canada.     

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.     

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.     

A review on applying ventilated double-skin facade to buildings in hot-summer and cold-winter zone in China

The need to energy conservation and sustainable development in buildings is causing a new interest towards passive solar systems. Among them, double-skin facade (DSF) proves to be extremely attractive and promising. DSF is building envelope formed by two layers of different glazing facades which are separated by a ventilated air cavity. The cavity of DSF is used to collect or evacuate the solar radiation absorbed by the facades, thereby improving the thermal comfort and the indoor air quality while conserving energy for heating and cooling. Being a technique developed for colder climates, DSF has been widely applied in commercial buildings across Europe. Nowadays buildings with DSF also appear in the hot-summer and cold-winter zone in China where the weather conditions in summer seem to be not so good for the application. In fact, the thermal analysis of the DSF system is essential to its application in such hot-summer zone. This paper seeks to describe the existing main research methods on the thermal performance of DSF and the shading devices. Problems and possibilities are concomitant. Applying ventilated DSF with controlled shading device system would be a new efficient way for the commercial buildings in the hot-summer and cold-winter zone to meet the task of sustainable building design in China.     

Economic performance assessment of residential building retrofits: a case study of Istanbul

This paper presents an approach proposed for assessing economic performance of retrofit applications performed at functional building element level in residential buildings and exemplifies it for the city of Istanbul, which has a temperate-humid climate. This approach covers three steps. In the first step, typical buildings and their properties are determined by analysing the residential buildings in Istanbul, and then the retrofit alternatives are generated. In the second step, an economic impact analysis is performed for typical buildings by considering the costs during their life cycle phases. In the third step, a life cycle cost assessment is done according to the variables, which are defined as window-to-wall ratio (WWR), window system, thermal insulation material, orientation and building age. The most beneficial retrofits are lastly determined depending on their economic performance ratios (EPRs). In conclusion, insulating exterior wall, projected floors, floors above unheated space and roof floors separately provide the highest benefits at the age of 30, and the lowest benefits occur at the age of 15 for all WWRs. The highest EPRs are obtained for the WWR of 10% at all ages. The renewal with PVC frame provides benefit while the renewal with wooden frame causes losses. Orientation slightly affects the EPRs. The use of stone wool in the retrofit causes a reduction in the EPRs due to high investment cost of stone wool. When the 20-year-old building which has the WWR of 10% is insulated with extruded polystyrene and renewed with PVC, it provides the highest benefit of about 45%.     

Daylighting performance of an electrochromic window in a small scale test-cell

In this paper, the results of an experimental investigation aimed at assessing the performance of electrochromic (EC) windows with respect to daylighting control in buildings are presented. The research is performed under real weather conditions by a small scale test-cell equipped with a small area double glazing unit (DGU) where one pane consists of an EC device with visible transmittance τ_v ranging from 6.2 to 68.1% and the other of an ordinary clear float glass (τ_v ≈90%). Experimental tests are carried out as a function of time, weather conditions, test-cell orientation and switching strategies. These data are integrated with spectrophotometric measurements. Results show that the angle selectivity of the glazing combined with its active switching effect allows a wide range of selectable transmission states to suit the latitude and orientation of a building in relation to the local climatic conditions. For south facing windows and under the involved climatic conditions EC glazing driven by a dynamic control strategy can be very effective in reducing discomfort glare caused by high window brightness. Glare reduction can be realized contemporarily maintaining the work plane illuminance to adequate level for computer based office tasks so without compromising much of the available daylight. Furthermore, since EC glazing is never switched to heavily darkened states (τ_v >20%), colour rendering of inside objects should be always acceptable, although internal illuminance level could be slightly lower than to what users prefer in relation to the correlated colour temperature of the incoming light. These results change when considering west orientation for which high-luminance direct sunlight patches are registered on the work-plane even for EC glazing switched to its lowest transmitting state letting suppose that EC windows cannot provide full control of uncomfortable direct sunlight effects without integration of additional shading devices.     

Switchable glazing with a large dynamic range in total solar energy transmittance (TSET)

Modern, energy-saving buildings incorporate large areas of highly insulating glazing. The resulting solar gains lead to major savings in heating energy during winter, but protection against overheating in summer is also needed. Usually this problem is solved by using mechanical shading devices, with the disadvantages of high cost and low durability.The work on switchable glazing at Fraunhofer Institute for Solar Energy Systems, in cooperation with industrial partners, aims to present new and viable alternatives. Two types of switching layers, which are quite different in their structure and function, but are similar in having a large dynamic range in TSET, are being investigated—gasochromic and thermotropic.Gasochromic windows are actively switched between a clear and a coloured (but image-preserving) state by alternately introducing strongly diluted O₂ and H₂ gases. In contrast to classic electrochromic configurations, only one tungsten oxide film with a very thin catalyst coating is needed. At present, prototype windows with an area of 1.1×0.6 m² are being produced by sputtering. Careful adjustment of the layer structure, the gas concentration and its flow velocity is needed to obtain the desired switching rate. Homogeneous colouring of the whole area within seconds has been achieved. In addition to information on the colouring kinetics, the paper also discusses system aspects of these windows.Thermotropic layers switch reversibly and automatically, from a clear state with high transmittance to a milky white state with high diffuse reflectance, when their temperature rises. Depending on the composition of the material, the switching temperature can be chosen in the range needed. The measured optical properties of glass laminates with a thermotropic layer are presented. These are combined with the measured values for further panes to calculate the characteristic data for thermotropic insulated glazing units. The results are compared with those measured on a 1.1×1.65 m² prototype window. Stability results are also included.The effect of the two different types of switchable glazing on building energy savings is explored for a residential building model, using the TRNSYS building energy simulation program.     

Lighting and cooling energy consumption in an open-plan office using solar film coating

In subtropical Hong Kong, solar heat gain via glazing contributes to a significant proportion of the building envelope cooling load. The principal fenestration design includes eliminating direct sunlight and reducing cooling requirements. Daylighting is an effective approach to allow a flexible building façade design strategy, and to enhance an energy-efficient and green building development. This paper studies the lighting and cooling energy performances for a fully air-conditioned open-plan office when solar control films together with daylight-linked lighting controls are being used. Measurements were undertaken at two stages including the electricity expenditures for the office using photoelectric dimming controls only (first stage) and together with the solar control film coatings on the windows (second stage). Electric lighting and cooling energy consumption, transmitted daylight illuminance and solar radiation were systematically recorded and analysed. The measured data were also used for conducting and validating the building energy simulations. The findings showed that the solar film coatings coupled with lighting dimming controls cut down 21.2% electric lighting and 6.9% cooling energy consumption for the open-plan office.     

Shading effect of eggcrate devices on vertical windows of arbitrary orientation

A combination of horizontal and vertical elements forming an eggcrate-type shading system is often used in the architectural design of building facades. An analytical-numerical method is presented to estimate the shading effect of such a structure on vertical windows of arbitrary azimuthal orientation. The effect of reflections from aggregate walls is accounted for in the analysis. Daily and monthly average solar radiation data are presented for shaded windows in Venice (45.40° northen latitude). The effects of the window geometry and the eggcrate projection are discussed.     

The thermal performance of window coverings in a whole house test facility with single-glazed sash windows

The residential sector is responsible for 29% of the total energy consumption of the UK, with 62% of this energy being used for space heating. Heat loss through the fabric of building elements is a crucial factor in the energy efficiency of homes, and a wide number of studies have looked at physical interventions to improve the energy efficiency of existing buildings, commonly called retrofit. This research considers the impact of window coverings on reducing heat loss from homes, a measure that is not commonly considered an energy efficiency intervention. Although the amount of glazing varies widely between homes, all windows are a significant factor contributing to heat loss. While physical changes such as double and triple glazing can improve the energy performance of buildings, the impact of curtains and blinds is not well characterised. Previous research into window coverings has been undertaken using laboratory tests, such as hotbox and small climatic chamber environments. This study presents the impact of window coverings on heat loss within a unique whole house test facility. This allows for a better replication of a real heating system and the effects that it has on localised heat transfer. This gives a more detailed picture of in situ performance, similar to that which may be found in the field.