Application issues for large-area electrochromic windows in commercial buildings

Projections of performance from small-area devices to large-area windows and enterprise marketing have created high expectations for electrochromic glazings. As a result, this paper seeks to precipitate an objective dialog between material scientists and building-application scientists to determine whether actual large-area electrochromic devices will result in significant performance benefits and what material improvements are needed, if any, to make electrochromics more practical for commercial building applications.Few in situ tests have been conducted with large-area electrochromic windows applied in buildings. This study presents monitored results from a full-scale field test of large-area electrochromic windows to illustrate how this technology will perform in commercial buildings. The visible transmittance (T_v) of the installed electrochromic ranged from 0.11 to 0.38. The data are limited to the winter period for a south-east-facing window. The effect of actual device performance on lighting energy use, direct sun control, discomfort glare, and interior illumination is discussed. No mechanical system loads were monitored. These data demonstrate the use of electrochromics in a moderate climate and focus on the most restrictive visual task: computer use in offices.Through this small demonstration, we were able to determine that electrochromic windows can indeed provide unmitigated transparent views and a level of dynamic illumination control never before seen in architectural glazing materials. Daily lighting energy use was 6–24% less compared to the 11%-glazing, with improved interior brightness levels. Daily lighting energy use was 3% less to 13% more compared to the 38%-glazing, with improved window brightness control. The electrochromic window may not be able to fulfill both energy-efficiency and visual comfort objectives when low winter direct sun is present, particularly for computer tasks using cathode-ray tube (CRT) displays. However, window and architectural design as well as electrochromic control options are suggested as methods to broaden the applicability of electrochromics for commercial buildings. Without further modification, its applicability is expected to be limited during cold winter periods due to its slow switching speed.     

Durability issues and service lifetime prediction of electrochromic windows for buildings applications

In general, the purposes of this paper are to elucidate the crucial importance of durability and service lifetime prediction (SLP) for electrochromic windows (ECWs) and to present an outline for developing a SLP methodology for ECWs. The specific objectives are (a) to illustrate the generic nature of SLP for several types of solar energy conversion or energy conservation devices, (b) to summarize the major durability issues associated with ECWs, (c) to justify using SLP in the triad of cost, performance, and durability rather than just durability, (d) to define and explain the seven major elements that constitute a generic SLP methodology, (e) to provide background for implementing the SLP methodology for ECWs, including the complexity of the potential degradation mechanisms, and (f) to provide an outline of studies using ECWs for improving the durability of ECW materials and predicting a service lifetime for ECWs using the SLP methodology outlined in objective (d). Our major conclusions are that substantial R&Dis necessary to understand the factors that limit ECW durability, and that it is possible to predict the service lifetime of ECWs.     

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.     

Identification of the building parameters that influence heating and cooling energy loads for apartment buildings in hot-humid climates

Identifying the building parameters that significantly impact energy performance is an important step for enabling the reduction of the heating and cooling energy loads of apartment buildings in the design stage. Implementing passive design techniques for these buildings is not a simple task in most dense cities; their energy performance usually depends on uncertainties in the local climate and many building parameters, such as window size, zone height, and features of materials. For this paper, a sensitivity analysis was performed to determine the most significant parameters for buildings in hot-humid climates by considering the design of an existing apartment building in Izmir, Turkey. The Monte Carlo method is selected for sensitivity and uncertainty analyses with the Latin hypercube sampling (LHC) technique. The results show that the sensitivity of parameters in apartment buildings varies based on the purpose of the energy loads and locations in the building, such as the ground, intermediate, and top floors. In addition, the total window area, the heat transfer coefficient (U) and the solar heat gain coefficient (SHGC) of the glazing based on the orientation have the most considerable influence on the energy performance of apartment buildings in hot-humid climates.     

Saving energy in residential buildings: The use of fully reversible windows

New double-glazed window systems based on the possibility of reversing the window through its sash could be in a short time a reality.     

Thermal performance of an electrochromic vacuum glazing

Using a three-dimensional finite volume model, the thermal performance of an electrochromic vacuum glazing was simulated for insolation intensities between 0 and 1200 W m⁻². The electrochromic evacuated glazing simulated consisted of three glass panes 0.5 m by 0.5 m with a 0.12 mm wide evacuated space between two 4 mm thick panes supported by 0.32 mm diameter pillars spaced on a 25 mm square grid contiguously sealed by a 6 mm wide metal edge seal. The third glass pane on which the electrochromic layer was deposited was assumed to be sealed to the evacuated glass unit. The simulations indicate that when facing the indoor environment, the temperature of the glass pane with the electrochromic layer can reach 129.5 °C for an incident insolation of 600 W m⁻². At such temperatures unacceptable occupant comfort would ensue and the durability of the electrochromic glazing would be compromised. The glass pane with the electrochromic layer must therefore face the outdoor environment.     

Characterization of ion diffusion and transient electrochromic performance in PECVD grown tungsten oxide thin films

Electrochromic tungsten oxide thin films were synthesized by plasma-enhanced chemical vapor deposition (PECVD). Film density and electrochromic performance were controlled by the degree of ion bombardment. A moderate degree of ion bombardment was optimal, and the refractive index was shown to be a sensitive indicator of electrochromic performance. Chronoamperometry in concert with optical transmission was used to determine diffusion and absorption coefficients using both H⁺ and Li⁺ containing electrolytes. The absorption coefficients were similar for both ions, scaling with the degree of intercalation to 50,000 cm⁻¹ in the opaque state. The diffusion coefficients for optimized films were found to be relatively insensitive to the degree of ion intercalation, with values of 10⁻⁹ and 10⁻¹⁰ cm²/s for H⁺ and Li⁺, respectively. These values are about an order of magnitude greater than values reported for vacuum-deposited films, which was attributed to low relative density in the PECVD films. The diffusion and absorption coefficients were incorporated into a model that successfully reproduced transient optical performance.     

The energy saving index and the performance evaluation of thermochromic windows in passive buildings

The concepts of the energy saving equivalent (ESE) and energy saving index (ESI) are presented in this paper to evaluate the performance of new materials and components in passive buildings. The ESE represents the hypothetical energy that should be input to maintain a passive room at the same thermal state as that when a particular material or component is adopted. The ESI is the ratio of a particular material or component's energy saving equivalent to the corresponding value of the ideal material or component that can maintain the room at an ideal thermal state in passive mode. The former can be used to estimate the effect of the adoption of a certain building component or material on the building's thermal state from an energy standpoint, while the latter can be used to characterize the performance of the actual building component or material from a common standpoint and be used to evaluate the performance of components or materials in different climatic regions or under different operating situations. In this study, the ESI was used to evaluate the performance of a thermochromic window, represented by a single vanadium dioxide (VO₂) glazing, in passive residential buildings in three climatic regions of China (cold zone, hot summer and cold winter zone, and hot summer and warm winter zone).     

Visual and energy performance of switchable windows with antireflection coatings

The aim of this project was to investigate how the visual appearance and energy performance of switchable or smart windows can be improved by using antireflective coatings. For this study clear float glass, low-e glass and electrochromic glass were treated with antireflection (AR) coatings. Such a coating considerably increases the transmittance of solar radiation in general and the visible transmittance in particular. For switchable glazing based on absorptive electrochromic layers in their dark state it is necessary to use a low-emissivity coating on the inner pane of a double glazed window in order to reject the absorbed heat. In principle all surfaces can be coated with AR coatings, and it was shown that a thin AR coating on the low-e surface neither influences the thermal emissivity nor the U-value of the glazing. The study showed that the use of AR coatings in switchable glazing significantly increases the light transmittance in the transparent state. It is believed that this is important for a high level of user acceptance of such windows.     

Thermal performance analysis of an electrochromic vacuum glazing with low emittance coatings

Thermal performance of an electrochromic (EC) vacuum glazing (VG) was modelled under ASTM standard winter conditions. The EC VG comprised three 0.5 m by 0.5 m glass panes with a 0.12 mm wide evacuated space between two 4 mm thick panes sealed contiguously by a 6 mm wide indium based edge seal with either one or two low-emittance (low-e) coatings supported by a 0.32 mm diameter square pillar grid spaced at 25 mm. The third glass pane on which the 0.1 mm thick EC layer was deposited was sealed to the evacuated glass unit. The whole unit was rebated by 10 mm within a solid wood frame. The low-e coating absorbed 10% of solar energy incident on it. With the EC VG installed with the EC component facing the outdoor environment, for an incident solar radiation of 300 W m⁻², simulations demonstrated that when the EC layer is opaque for winter conditions, the temperature of the inside glass pane is higher than the indoor air temperature, due to solar radiation absorbed by the low-e coatings and the EC layer, the EC VG is a heat source with heat transferred from the glazing to the interior environment. When the emittance was lower to 0.02, the outdoor and indoor glass pane temperatures of the glazing with single and two low-e coatings are very close to each other. For an insolation of 1000 W m⁻², the outdoor glass pane temperature exceeds the indoor glass pane temperature, consequentially the outdoor glass pane transfers heat to the indoor glass pane.     

Effects of multi-parameter changes on energy use of large buildings

We describe the development of a methodology for predicting the effects of multi-parameter changes on the energy use of large buildings. The methodology is based on the third-order Taylor series expansion whose coefficients are evaluated for several key building parameters. The building was coded for energy performance simulation by the DOE-2 computer program. A database of simulation results was created from which the coefficients of the Taylor series expansion were derived. Test results using this energy predicting methodology provided reasonably accurate estimates of the total and cooling energy of a generic building in Singapore.     

Influence of glass properties on the performance of double-glazed facades

In this work the influence of the glass properties on the performance of double-glazed facades has been studied. The total heat rate into the building has been calculated for ten different facades formed by different glass combinations. To obtain this thermal load into the building it has been necessary to solve the fluid field within the channel formed by the two layers of glass. The mass flow rate in the channel as well as the mean temperature increase of the air passing through the channel have also been obtained. In order to reduce the thermal loads into the building, the cooling effect resulting of blowing the air through the channel has also been analysed.     

Influence of glass curtain walls on the building thermal energy consumption under Tunisian climatic conditions: The case of administrative buildings

The glass curtain walls have been recently introduced in Tunisia; they are seen as a new fashion and are highly appreciated by some for their pleasing aesthetics. The objective of this paper is to investigate whether the glass curtain walls are appropriate for the Tunisian local climate and context and if it is so, to give recommendations concerning the kind of glass to be used.A TRNSYS [Klein SA. TRNSYS a transient system simulation Program V5 14.2. Solar Energy Laboratory, University of Wisconsin Madison, July 1996] simulation was conducted on a typical administrative building. The investigation concerns only the building heating and cooling load. The building was split in five thermal zones; for each thermal zone, all the windows have the same orientation. The single zone model TYPE19 of TRNSYS [Klein, 1996] was used to model each thermal zone. An additional convection heat transfer between the different thermal zones of the building was modelled according to the Brown and Solvason law [Brown WG, Solvason KR. Natural convection through rectangular openings in partitions, Part 1: vertical partitions. Int. J. Heat Mass Transfer 1962; 5: 859–68]. This particular law was used because it has been validated in the Tunisian context by Bouden [Bouden C. Analyse du suivi thermique d’un pavillon solaire expérimental en région Tunisoise, Thèse de doctorat, Université de Paris 7, 1989]. We assume that the glass curtain wall will be implemented only on the main building facade; this is why it was simulated with different glazing sizes and glass types. The other facades remain unchanged. The results of this simulation have shown that, in relation to space heating, the glass curtain wall can be very interesting in the Tunisian context if the orientation as well as the kind of glazing are carefully selected.     

Impact of building forms on thermal performance and thermal comfort conditions in religious buildings in hot climates: a case study in Sharjah city

The common system used for thermal regulation in mosques of United Arab Emirates (UAE) is the heating, ventilating and air-conditioning (HVAC) system. This system increases demands on energy consumption and increases CO₂ emission. A passive design approach is one of the measures to reduce these problems. This study involved an analytical examination of building forms, followed by testing the impact of these forms on its thermal performance and indoor thermal comfort. The tests were conducted using energy simulations software packages. Passive parameters such as shading devices, thermal insulation and natural ventilation were applied in six cases, including the baseline case within each form. The obtained results showed a significant effect of mosque forms as well as passive design techniques on the thermal comfort within the structures. The findings confirmed that the use of passive design alone would not help achieve thermal comfort, but reduce the annual energy consumption by10%. By integrating a hybrid air-conditioning system as another supporting approach, the annual energy consumption could be reduced by 67.5%, which allows for the designing of a much smaller HVAC system.     

The passive solar heated school in wallasey. I foreword and introduction

St. George's School, Wallasey, situated in the U.K. at latitude 53.4° N was designed so that equitable thermal conditions should be achieved within it using solar gains, heat from the lighting system and body heat from the children without the use of a conventional heating system. The building opened in 1962 and evoked considerable comment, both favourable and unfavourable, in the mid-sixties. This article briefly notes some of the comments and provides an account of some of the features which the architect incorporated to control the solar gains that enter through the large south-facing solar wall. Later articles in this series describe the findings of observational surveys carried out in the building.     

Ventilated facades energy performance in summer cooling of buildings

The use of ventilated facades and roofs can help to reduce summer thermal loads and, therefore, the energy consumption due to air-conditioning systems. This paper discusses a simple analytical method for the calculation of the energy saving achievable by using ventilated facades in which the air flow inside the air duct is due to stack effect. Two particular cases of outstanding importance are investigated. The first in which the inner masonry wall is given, and the air duct and the outer facing have to be optimized. The second in which the outer facing is given, and the inner masonry wall and the air duct have to be optimized. The first case can occur in existing buildings renovation, while the second case can occur during the design process. Finally, the influence of the variation of some quantities necessary for calculation on the energy performance of ventilated facades is investigated. In particular, the energy performance of such facades results to be strongly influenced by the air duct width, the insulating material distribution, the solar radiation intensity, the wall outer surface thermal resistance and the roughness of the slabs delimiting the air duct.     

Dynamic characteristics and energy performance of buildings using phase change materials: A review

Thermal energy storage (TES) systems using phase change material (PCM) have been recognized as one of advanced energy technologies in enhancing energy efficiency and sustainability of buildings. The use of PCMs in buildings provides the potential for a better indoor thermal comfort for occupants due to the reduced indoor temperature fluctuations, and lower global energy consumption due to the load reduction/shifting. A good knowledge on dynamic characteristics and energy performance of buildings using PCMs is essential for building researchers and practitioners to better understand building temperature response characteristics and economic feasibility of using PCMs and take further proper actions to fully utilize PCMs to enhance indoor environmental quality and overall energy efficiency of buildings. This paper presents an overview of the previous research work on dynamic characteristics and energy performance of buildings due to the integration of PCMs. The research work on dynamic characteristics and energy performance of active and passive building applications is reviewed, respectively. Since the particular interest in using PCMs for free cooling and peak load shifting, the specific research efforts on both subjects are reviewed separately. A few useful conclusive remarks and recommendations for future work are presented.     

Reassessment of fenestration characteristics for residential buildings in hot climates: energy and economic analysis

This paper attempts to resolve the reported contradiction in the literature about the characteristics of high-performance/cost-effective fenestration of residential buildings, particularly in hot climates. The considered issues are the window glazing property (ten commercial glazing types), facade orientation (four main orientations), window-to-wall ratio (WWR) (0.2–0.8), and solar shading overhangs and side-fins (nine shading conditions). The results of the simulated runs reveal that the glazing quality has a superior effect over the other fenestration parameters and controls their effect on the energy consumption of residential buildings. Thus, using low-performance windows on buildings yields larger effects of WWR, facade orientation, and solar shading than high-performance windows. As the WWR increases from 0.2 to 0.8, the building energy consumption using the low-performance window increases 6.46 times than that using the high-performance window. The best facade orientation is changed from north to south according to the glazing properties. In addition, the solar shading need is correlated as a function of a window-glazing property and WWR. The cost analysis shows that the high-performance windows without solar shading are cost-effective as they have the largest net present cost compared to low-performance windows with or without solar shading. Accordingly, replacing low-performance windows with high-performance ones, in an existing residential building, saves about 12.7 MWh of electricity and 11.05 tons of CO₂ annually.     

Success factors of energy performance contracting (EPC) for sustainable building energy efficiency retrofit (BEER) of hotel buildings in China

Hotel building is a type of high-energy-consuming building and most existing hotel buildings need energy efficiency improvement in China. Energy performance contracting (EPC) is considered a win−win mechanism to organize building energy efficiency retrofit (BEER) project. However, EPC mechanism has been introduced into China relatively recently and many EPCs have not been successful in building energy efficiency retrofit projects. This research aims to develop a set of critical success factors (CSFs) of EPC for sustainable energy efficiency retrofit (BEER) of hotel buildings in China. Semi-structured interviews and a questionnaire survey with practitioners and other professionals were conducted. The findings reveal the relative importance of the 21 number of identified success factors. In order to explore the underlying relationship among the identified critical success factors (CSFs), factor analysis method was adopted for further investigation, which leads to grouping the 21 identified CSFs into six clusters. These are (1) project organization process, (2) EPC project financing for hotel retrofit, (3) knowledge and innovation of EPC, sustainable development (SD), and M&V, (4) implementation of sustainable development strategy, (5) contractual arrangement, and (6) external economic environment. Finally, several relevant policies were proposed to implement EPC successfully in sustainable BEER in hotel buildings.     

Carbon footprint of green roof installation on school buildings in Greek Mediterranean climatic region

Green roof installation in contemporary urban centres is increasing due to their numerous benefits, including microclimate improvement. However, the magnitudes of influence of the green roof design to energy savings is not fully clear, as well as the environmental benefit, in terms of reducing greenhouse gases emissions. The aim of this study was to estimate the effect of green roofs design on energy savings and their carbon footprint when installed on school buildings. The cooling and thermal insulation features of green roofs have been studied by using the TRNSYS simulation software. Different types of green roof systems (extensive and semi-intensive) and construction options are studied in four types of school buildings. Results showed that the estimated reduction in annual CO₂ emissions due to energy savings and CO₂ capture by plants was many times greater than the CO₂ emissions that caused from roof construction.