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.     

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.     

Self-adjusting smart windows based on polymer-dispersed liquid crystals

The control of sunlight can be achieved either by electrochromic or polymer-dispersed liquid crystal (PDLC) smart windows. We have recently shown that it is possible to homeotropically align fluid mixtures of low molecular mass liquid crystal with a negative dielectric anisotropy, and a liquid crystalline monomer, in order to obtain electrically switchable chromogenic devices. They are new materials useful for external glazing. In fact, they are not affected by the classical drawbacks of PDLCs. In this paper we present a new self-switchable glazing technology based on the light-controlled transmittance in a PDLC device. The self-adjusting chromogenic material, which we obtain, is able to self-increase its scattering as a function of the impinging light intensity. The relationship between the electro-optical response and the physical-chemical properties of the components has been also investigated.     

Properties, performance and current status of the laminated electrochromic glass of Gesimat

In the last 10 years Gesimat has developed a large-area electrochromic glazing with an advanced polymer electrolyte and tungsten oxide and Prussian Blue as complementary electrochromic layers. This electrochromic glass has a switching range between 75% and 8% visible transmittance and between 56% and 6% solar transmittance. The polymer electrolyte is based on polyvinyl butyral (PVB), a polymer in use as an interlayer for laminated safety glass since more than 60 years. The electrochromic films are deposited by a new large-area electrodeposition process. For lamination the standard methods of laminated safety glass production can be used.     

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.     

Angle-selective glazing for radiant heat control in buildings: theory

The theory and predicted performance of a new type of glazing exhibiting strong angle-selective reflectance and high viewing transparency are presented. The glazing comprises a fixed venetian blind style arrangement of laser-cut panels. The theory shows that the transition from high transmittance at near normal incidence to high reflectance at larger incident angles can be varied in magnitude and over a wide angular range by altering the laser-cut spacing and panel spacing. Calculated radiant transmission and daylighting performance for the glazing on southern and east/west facades in buildings at temperate and sub-tropical northern latitudes predicts excellent performance for a single practical configuration of the system.     

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.     

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.     

High-efficiency multistable switchable glazing using smectic A liquid crystals

Presently, nematic polymer dispersed liquid crystals (PDLCs) that are used in switchable glazing technology require constant power to operate and, moreover, exhibit unwanted haze at wide viewing angles. In this paper, a novel switchable glazing technology, based around a bistable electro-optic effect in the Smectic A liquid crystal phase, is described which does not require constant power to operate or exhibit haze. The application of a low-frequency (100 Hz) voltage induces an optically opaque state due to the motion of ionic material whereas the application of a higher frequency AC (1 kHz) voltage induces a haze-free clear state. Multistable (greyscale) operation is possible through the application of intermediate frequencies or voltages; the threshold voltages of the effect were found to range from 36 to 66 V rms. Any voltage-induced state is preserved indefinitely after removal of the voltage leading to low power consumption.     

Solar collectors with colored absorbers

The integration of solar collectors in buildings should be compatible with the architectural design, and solar collectors with colored absorbers would be aesthetically preferable. In our laboratory we constructed and tested flat plate solar collectors with colored absorbers for water heating applications. The study includes collectors in their typical form with the protective glazing, and also collectors without glazing. Unglazed solar collectors are not widely used, although they are cost effective solar devices, suitable for low temperature thermal applications. We tested outdoors the constructed models, glazed and unglazed, with black, blue and red brown absorbers. In order to overcome the high thermal losses of the unglazed collectors and the low optical efficiency of the colored absorbers, we used flat booster reflectors. The additional solar radiation input from the reflectors increases the thermal energy output of the collectors, improving their performance. Theoretical steady state efficiency curves are also given for collectors with or without glazing. The presented experimental and theoretical results determine the range of the effective operation of the proposed solar collector types, which can be used in a variety of applications, instead of glazed or unglazed solar collectors with a black absorber.     

Electrochromic characterization of sol–gel deposited coatings

Electrochromic devices have increasing application in large-area display devices, switchable mirrors and smart windows. A variety of vacuum deposition technologies have been used to make electrochromic devices. The sol–gel process offers an alternative approach to the synthesis of low-cost, high-quality electrochromic device layers. This paper discusses the developments in sol–gel-deposited electrochromic films and a prototype all sol–gel device with switching from 80% to 50% (550 nm). The sol–gel process involves the formation of oxide networks upon hydrolysis–condensation of alkoxide precursors. In this study, we cover sol–gel-deposited oxides of WO₃, TiO₂, Nb₂O₅, V₂O₅, Ni(OH)₂, Co(OH)₂ and CuO_x. The coloration reaction voltammetry and spectral optical properties are presented for selected films.     

On the effect of glazing and code compliance

The trend for modern buildings to be designed with large glazing areas in the exterior envelope is becoming more popular than ever before. At the same time, the power utilities in most of Kuwait demand code compliance through targeting of peak electric requirements for heating, ventilation and air-conditioning (HVAC) systems. The lack of detailed and accurate information on the thermal performance of glazing systems coupled with limits set by the code of practice can result in bad building design with respect to indoor comfort conditions. This paper presents a methodology for selecting glazing areas, while keeping the peak-load demand fixed. Provisions for the selection of large glazing areas are based on the type of glazing used. Finally, using a multi-storey building in Kuwait as an example, analysis has been conducted for seven different types of glazing. The choice of glazing offers the designer significant flexibility in opting for a glazing area of between 20 and 100%, and yet achieving full compliance with the code.     

Analysis of the top heat loss factor of flat plate solar collectors with single and double glazing

The dependence of the top heat loss factor of flat plate solar collectors with single and double glazing on the basic parameters was studied. An improved technique for calculation of the top heat loss factor of flat plate collectors with single glazing has recently been proposed by the authors. The present work, covering the flat plate collectors with single as well as double glazing, carefully examines the impact of the glass cover temperature(s) estimated by simple empirical relation(s) on the individual heat transfer coefficients and hence on the top heat loss factor. An analysis of the capability of the new method to accurately compute the top heat loss factor over an extensive number of combinations of the basic parameters has been carried out.     

Daylighting in the tropics

Traditional adaptations of tropical/sub tropical buildings to high ambient irradiance from high elevations are outlined. Generally, these adaptations result in severe shading of window apertures, greatly reducing access to daylight. Some examples of optical systems designed to improve daylighting in tropical buildings are discussed. These include angle selective glazing, light guiding shades, vertical and horizontal light pipes, switchable glazing and angle selective skylights. The simulation of these devices within packages such as RADIANCE is also discussed.     

Thermocirculation characteristics of a Trombe wall passive test cell

Passive heating, using Trombe wall elements for solar energy collection, can be readily integrated into new building structures and can be retrofitted to existing buildings.

Although the basic physical principles underlying the natural operation of such air thermosiphon devices have been well known for many years now, there is still insufficient design information available to allow sensible sizing and installation decisions to be undertaken by architects and thermal system designers. In this experimental investigation, flow visualisation studies have given a deeper insight into the fundamental flow mechanisms; whilst air velocity and temperature measurements have been used to explore the natural convection heat transfer processes involved in the thermocirculation flow. Particular attention has been paid to the effects on operational performance of the wall parameters such as wall/glazing distance and vent size.

Adequate data correlation of the experimental results has been achieved by using expressions derived for natural (free) convection processes occuring between vertical paralled plates; in this case the vertical heated wall surface and the cooler vertical glazing panel. It is felt that this type of fundamental heat transfer information is vital so that the overall performance of Trombe wall systems can be adequately modelled using the large range of simulation techniques currently available for thermal network analysis.     

The role of information and communication technologies (ICTs) in household energy consumption��prospects for the UK

Growing concerns about climate change and energy security have led to a strong focus on energy demand reduction and energy efficiency within United Kingdom (UK) energy policy. At the same time, information and communication technologies (ICTs) have become pervasive in society and this has brought with it new policy options which use them as enabling technologies. One such policy option planned for implementation in the UK is the use of smart meters and real-time displays to encourage people to become more aware of their energy consumption and possibly change their energy-related behaviours. Smart meters and display units by definition link individuals, technologies and society, and their effectiveness is influenced by a range of factors. Ten semi-structured stakeholder interviews with industry, government and academia and a review of literature were conducted in order to identify which factors are most likely to contribute to the effectiveness of implementing smart meters and real-time displays in the UK. Further analysis showed a number of key themes and perspectives on behavioural change, particularly as they relate to household electricity use and the role of smart meters in the UK energy policy, including the role of ICTs in energy demand reduction more generally.     

Numerical study of a ventilated facade panel

An energy-saving facade panel for non-residential buildings has been numerically investigated. Structured like a composite Trombe-Michel wall, the panel consists of a glazing, an absorber plate and insulation and contains a dead air space between glazing and absorber, as well as a convection channel between absorber and insulation. The influence of convection channel spacing on both recovery of solar energy during sunshine periods and on heat losses during night hours has been assessed. Two different options have been considered. First, the total panel thickness was maintained, which involves an increase of channel spacing having to be compensated by a corresponding decrease of the insulation thickness. Then, this constraint was removed so that an increase in channel spacing was allowed to entail an equivalent increase of the total panel thickness. The results indicate that large spacing favors energy recovery during sunshine periods for both options and reduces, although only slightly, heat losses during night hours for the second option. In the case of the first option, however, these losses tend to grow when channel spacing increases.     

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.     

Performance of various design of solar air heaters for crop drying applications

Different heat sources are employed for the drying of agricultural products. However, in many rural locations in most developing countries, supplies of non-renewable sources of energy are either unavailable, unreliable or, for many farmers, too expensive. In renewable energy sources, solar energy is the most appropriate for drying systems. This energy allows independent systems to be constructed and possesses a thermal conversion mode which necessitates a simple technology which is adapted to the rural regions for crop drying applications. These systems are all based on the air heating flat plate solar collectors.Therefore, six different types of natural circulation air heating solar collectors (Model-1: single plastic glazing, black painted hardboard absorber and front-pass; Model-2: single plastic glazing, black painted flat plate absorber and front-pass; Model-3: single plastic glazing, black painted zigzag plate absorber and front-pass; Model-4: single plastic glazing, black painted flate plate absorber and back-pass; Model-5: single plastic glazing, black painted zigzag plate absorber and back-pass; Model-6: double plastic glazing, black painted flat plate absorber and back-pass) were designed, constructed and analysed for their performance in this study. Each collector mainly consisted of a frame constructed from hardboard, vent holes, hardboard insulation, absorbing surface made of black coated aluminium sheet and clear plastic glazing.All solar air heaters were mounted on a stand facing south at an inclination angle, and they were tested simultaneously under the same environmental conditions. The experimental setup was instrumented for the measurement of solar radiation, temperature and relative humidity of the atmosphere air, outlet air temperature, surface temperature of the back and edge insulator and absorber plate, air speed and wind velocity.It is understood from the results of the investigation that the performances of Model-1, Model-2, Model-3, Model-4, Model-5 and Model-6 are 42.11, 45.88, 44.23, 39.76, 39.05 and 36.94% respectively, and the performance of the most efficient collector (Model-2) is aproximately 9% more than the least efficient one (Model-6). In addition, it is seen that unlike number of glazing sheet and air pass method, the effect of the shape of the absorbing surface on the performance is considerably less.     

V型多通道平板太阳能集热器的热性能研究

提出一种V型多通道平板太阳能集热器,对其建立稳态传热模型,利用Matlab软件编程进行求解,并进行实验测试验证模型的准确性.利用已验证的传热模型,模拟分析V型多通道平板太阳能集热器的结构和物性参数对其热性能的影响,结果表明当透明盖板和吸热体发射率变大时,集热器的集热效率会呈下降趋势;当V型槽顶角、吸热体长度及空气夹层厚...