Solar control with thermotropic layers

Innovative means to control the light and energy flux according to demand are desirable for transparent façades and façade elements. One possibility to achieve this is presented by switchable layers that change their optical properties, either actively or passively, according to different control parameters. In addition to inorganic coatings on glass, there are several organic thermotropic systems that can be integrated into façade glazing. Above a certain temperature, thermotropic layers change from a clear to a translucent, light-diffusing state, thus switching from a highly transmitting to a diffusely reflecting state. We shall describe the basic principles of these systems and present an overview of some of the existing prototype systems. Installation options will be discussed and the results of an application in a retrofitted building presented.     

Facade systems with variable solar control using thermotropic polymer blends

Thermotropic layers integrated into glazing or transparent insulation systems reversibly reduce the total solar energy transmittance by becoming opaque when heated. Characteristics of both systems have been measured as a function of temperature and incidence angle, including the total solar energy transmittance for a thermotropic heat-mirror insulating glazing unit (IGU). Properties of complex thermotropic glazing systems have been simulated, starting from data characterising the thermotropic layer. The results show good agreement with measured data. A prototype system sample of a thermotropic heat-mirror IGU was integrated into an outdoor test facility and a thermotropic TEIF (transparent exterior insulation and finish) prototype system was installed into the facade of a demonstration house. Both systems have been monitored and showed the expected switching properties, resulting in the desired overheating protection. Computer simulations of the building integration of the thermotropic TEIF system have been performed, showing a strong dependence of the optimum switching temperature on the total solar and internal gains of the building.     

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.     

Smart switchable glazing for solar energy and daylight control

The exciting field of chromogenic materials for smart windows and other large-area applications is discussed. A selection of switchable glazing devices that change color electrically are detailed. The types of devices covered are the electrochromic which change color electrically, covering electrochromic, dispersed liquid crystal and dispersed particle glazing that switch under an applied electric field. Device structures and switching characteristics are compared. The status of prototype and commercial devices from commercial and university labs through out the world are covered. A discussion of the future of this technology is made including areas of necessary development for the realization of large area glazing in excess of 1m².     

Large-area smart glass and integrated photovoltaics

Several companies throughout the world are developing dynamic glazing and large-area flat panel displays. University and National Laboratory groups are researching new materials and processes to improve these products. The concept of a switchable glazing for building and vehicle application is very attractive. Conventional glazing only offers fixed transmittance and control of energy passing through it. Given the wide range of illumination conditions and glare, a dynamic glazing with adjustable transmittance offers the best solution. Photovoltaics can be integrated as power sources for smart windows. In this way a switchable window could be a completely stand alone smart system. A new range of large-area flat panel display including light-weight and flexible displays are being developed. These displays can be used for banner advertising, dynamic pricing in stores, electronic paper, and electronic books, to name only a few applications. This study covers selected switching technologies including electrochromism, suspended particles, and encapsulated liquid crystals.     

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.     

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.     

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.     

Copper based thin films to improve glazing for energy-savings in buildings

This paper presents research results for better window materials for energy saving purposes. Three alternatives of glazing treatments were developed in this project: Cu₂O, CuS + Cu₂O, and Cu + Cu₂O. The second and third alternatives present an adequate control in the infrared irradiance, achieving a significant reduction of the heat transfer through them. These windows allow the transmission of about half of the heat under hot weather condition, thus reducing the thermal load inside the building. The same windows also reduce the outwards flow of heat at half of the rate of windows without coating, improving comfort and reducing heat loss under cold weather conditions. The thin layers were produced using a sputtering technique with planar magnetrons. A high purity copper target (99.99%) was evaporated through plasma of argon–oxygen (70–30% respectively) in order to obtain Cu₂O. Soda-lime glass substrates of 600 × 400 mm were used. About 50% of the infrared is blocked by the Cu₂O window, when it is applied for thermal comfort. Thus, it was necessary to add another layer (CuS), which allowed a reduction of up to 20% in the NIR. The spectral behavior of the Cu + Cu₂O window showed a transmittance of about 50% in VIS range, while in the NIR, the transmittance decreased from 40% to 20%. The composite windows, which contained CuS, Cu, and Cu₂O are suitable solutions for solar control. Simulations of annual energy savings were carried out with the Energy-10 software for the Cu₂O + CuS window for two cities, Mexico City and Mexicali, the latter representing extreme weather conditions (over 35 °C in summer and below 6 °C in winter). The total energy saving was approximately 20% of the total energy demand for Mexicali. Thermal transmission measurements were also carried out at the center of these windows, giving values of about 4 W/m² K.     

Capacitive effect on the heat transfer through building glazing systems

In recent years, several intensive studies have been carried out in order to reduce the energy consumption of buildings. One solution lies on whole building energy simulation that permits to enable the heat (and moisture) transfer through the building envelope and, consequently, is a way to understand how to improve the building performance. This article aims to analyze the modeling level needed to successfully evaluate the heat transfer through glazing parts of windows in such whole-building simulations as it is well-known that windows are the thermally weakest elements of the building envelope.     

Silica aerogel granulate material for thermal insulation and daylighting

Silica aerogel granulate is a nanostructured material with high solar transmittance and low thermal conductivity. These properties offer exciting applications in building envelopes. One objective of the joint R&D project ISOTEG at ZAE Bayern was to develop and characterize a new glazing element based on granular silica aerogel. Heat transfer coefficients of less than 0.4 W/(m² K) and a total solar energy transmittance of 35% for the whole glazing unit were achieved. The glazing has a thickness of less than 50 mm. Another application for granular silica aerogel is, for example, in solar collectors.The thermal properties of the glazing as well as the optical and thermal properties of the granular aerogels are presented here. The solar transmittance of a 10 mm packed bed of silica aerogel was 53% for semi-translucent spheres and 88% for highly translucent granulate. In our heat transfer experiments the gas pressure, external pressure load, temperature and gas filling were varied. The various thermal conductivity values measured for the glazing and collector applications were compared to the values calculated using two different packed bed models. For the gas-dependent measurements the intergranular voids in the granulate were 1.0 ± 0.1 mm before loading the packed bed, 0.3 ± 0.1 mm at an external load of 3.2 bar (3.2 × 10⁵ Pa) and 0.6 ± 0.1 mm after release.A direct radiative conduction of λ_direct = 4.5 ± 0.5 × 10⁻³ W m⁻¹ K⁻¹ was obtained.     

Mg-Ni thin-film composition dependence of durability of electrochromic switchable mirror glass in simulated environment

Mg-Ni thin films as a switchable mirror material are promising for application in energy-saving windows because they can change their state between reflective and transparent by hydrogenation and dehydrogenation. In our previous work, we found that electrochromic switchable mirror glass based on a Mg₄Ni thin film is affected by environmental factors such as temperature and humidity. In this work, we investigated the effects of the environment on the optical switching properties of electrochromic switchable mirrors with Mg-Ni thin films of various compositions suitable for a broad range of applications and operating environments. When the mirror devices were kept in a simulated environment with a constant temperature 40 °C and a constant relative humidity of 80%, controlled by a thermostat/humidistat bath, their optical switching properties degraded. The degradation was found to be related to the change in the Mg-Ni thin films into nonmetallic states of oxides and hydroxides. The device with a Mg₆Ni thin film kept in the bath for 7 days showed no optical switching property. The mechanism of degradation in the bath was strongly affected by the composition of the Mg-Ni thin-film optical switching layer. The device constructed with an optical switching layer having high magnesium content degraded more rapidly in the test.     

Property and performance requirements for thermotropic layers to prevent overheating in an all polymeric flat-plate collector

Within this study the potential of thermotropic layers to prevent overheating of an all-polymeric solar collector is evaluated by theoretical modeling. The investigations show that collector stagnation temperatures can be reduced to temperatures between 80 and 90 °C by using thermotropic layers either in the glazing or on the absorber. For overheating protection to a maximum temperature of 80–90 °C a residual solar transmittance of the thermotropic layer below 0.25 in the opaque state (>0.85 in clear state) is effectual. The required switching temperature is dependent on the collector configuration.     

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.     

A passive solar building for ecological research in Argentina: the first two years experience

An energy-efficient building, featuring energy conservation, passive solar heating, and natural cooling strategies, was designed and built in La Pampa, a province in the temperate semi-arid region of central Argentina. Of compact design, it houses 350 m² of useful floor area in a roughly linear scheme, with the main spaces facing north and ancillary spaces (services) facing south. Solar windows running from above spandrel and up to ceiling height are provided for all the main spaces, and clerestory windows are provided for the solar gain to the south-facing spaces. An integrated sunspace is incorporated into the centre bay of the north facade, providing additional heat to inner spaces as well as functional and visual expansion. In the design stage, a simulation analysis was performed to assess the environmental and energy performance of the alternatives. The main energy features of the resulting building are a volumetric loss coefficient of 1.09 W m⁻³ °C⁻¹, and a predicted solar savings fraction of 70%. The summer cooling strategy includes the passive induction of exterior air into the building through earth-coupled ducts. Cooling by cross-ventilation is made possible during the night, but to preserve the security of the building from sudden storms, this occurs only when the building is occupied. Shading devices protect all windows in summer. Provisional monitoring, started during the 1995 winter period, showed encouraging possibilities of energy savings with adequate comfort conditions, demonstrating the technical feasibility of the scheme.     

Optimization of insulation thickness for different glazing areas in buildings for various climatic regions in Turkey

Insulation is one of the most effective methods intended for reducing energy consumption in both heating and cooling of buildings. Selecting the right materials and determining the optimum insulation thickness in building insulation application is an important issue. In 2000, the “Thermal Insulation Requirements for Buildings” was enacted in Turkey, energy saving by limiting the energy amount used for heating in buildings being the target. In this study, the effect of the alteration of windows and exterior wall areas on the heating energy requirement of the building and on the optimum insulation thickness has been examined by using P₁–P₂ method. The study has been carried out for four degree-day regions of Turkey for various insulation materials, glazing areas, and fuel types; the results have been presented in charts. In the rest of this study, effects of different insulation thicknesses and fuel on fuel consumption and thereby on emissions of pollutants such as CO₂ and SO₂ are evaluated. For example, in the building where XPS (extruded polystyrene foam) insulation material and natural gas are used and where the ratio of glazing area to exterior wall area is 0.2 (glazing area percentage), energy saving for the four regions has been found to be 13.996, 31.680, 46.613, and 63.071 $/m², respectively, and the payback period of investment has been found to be 2.023, 1.836, 1.498, and 1.346 years, respectively. The emissions of CO₂ are decreased by 50.91% for the cases in which optimum insulation material (XPS) and natural gas are used. The emissions of CO₂ and SO₂ are decreased by 54.67% for the cases in which optimum insulation material (XPS) and fuel oil are used.     

The passive solar heated school in Wallasey. V. Energy requirements for A passive school building

The near-south-facing glazed wall of the Wallasey School admits large solar gains in sunny weather, sufficient to meet in full the heat need in cold weather. It permits large heat losses, however, and during dull weather, and during the long winter nights there is little or no compensating solar gain. The net effect of such glazing over a season might be either to save, or to waste energy as compared with a windowless building, according to the sunniness and coldness of the climate and the window characteristics. To examine the action of the glazing, use was made of 50 years of daily mean ambient temperature, and contemporary sunshine hours, in conjunction with the solar gain factor for the translucent and pinboarded areas of the solar wall, and for certain values of design temperature and ventilation rate. It is concluded that such glazing leads to modest savings, of around 5 to 10 W/m² daily average. Most of the saving appears to be achievable by around 30 per cent glazing; further glazed area tends to supply unwanted solar gain in sunny periods while increasing the losses in sunless conditions. The annual electricity consumptions are noted for the 20 year life of the building. Their costs suggest that the building has been economical to heat.     

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.     

Electrochromic windows

Electrochromic windows can be described as devices based on electrochromic materials that work as reversible switching optical shutters, different device concepts and their electrochemical principles will be described.The relevance of electrochromic windows to low energy architecture will be discussed. A detailed analysis about the general requirements and the specific performances of electrochromic glazing for architectural applications will be introduced. The challenging issues of the present electrochromic technology approaching architectural glazing will be identified from the technological point of view and from the applications perspective.Emerging electrochromic glazing designs will be critically examined. A final discussion will be introduced about the different technical and marketing problems that slow down the time to market of electrochromic windows.     

The gasochromic properties of sol–gel WO3 films with sputtered Pt catalyst

Gasochromic films are receiving considerable attention, stimulated by the need for switchable windows that compete with more complex electrochromic ‘smart’ windows for building applications. The latest development of WO₃ films, prepared by the sol–gel route and dip-coating deposition overlaid by a thin layer of sputtered Pt metal, are presented. Colouring/bleaching kinetics of WO₃ films and WO₃ films in which a hybrid organic/inorganic sol–gel precursor (ormosil) has been added are evaluated. Results revealed that with respect to velocity of coloration, sol–gel WO₃ gasochromic films compete with the sputtered ones. Many aspects of the colouring/bleaching behaviour of the films resemble that of sputtered Pt/WO₃ films and thus confirm the similarity in the colouring/bleaching mechanisms. IR spectroscopy revealed the presence of well-defined W=O and the breaking of W–O bonds indicating the formation of H⁺… ⁻OW- and WO_3−x species in coloured films.