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.     

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.     

Durability of electrochromic glazing

Fundamental properties of all-solid-state electrochromic windows to control the solar energy have been investigated. This system comprises of multilayer represented as Glass/ITO/NiO/Inorganic Electrolyte (Ta₂O₅, etc.)/ WO₃/ITO/ Adhesive Film/ Glass. Of the various electrochromic systems examined so far, the most important features are their environmental stability and the possibility of large area applications. Our system can control the visible transmittance between 72.6% and 17.6% and has a cyclic life over 100000 cycles at 60°C. Based on the accelerated weathering tests, the stability of the system is estimated to be over five years for outdoor applications. For the problem of scaling up, some technical aspect is given and the prototype window of size 40×60 cm is exemplified. The present system could be more suitable for architectural and automobile applications in the near future by developing production technology.     

Environmental assessment of the Greek transport sector

Transport constitutes a crucial factor to the quality of life, since many people depend greatly on access to a reliable transport system. However, there are concerns about the impacts of the transport system on the quality of life, since it constitutes one of the main sources of greenhouse gases and also gives rise to significant air pollution stemming from acidifying pollutants, ozone precursors and particulate matter. During the last decade, the demand for transport services in Greece has rapidly grown following the European trend. Transport policies have recognised the need to restrain transport growth and to improve the various transport modes. Technology and fuel improvements have resulted in decreases of emissions of certain pollutants. Taking into account the major role of road transport in Greece for both passenger and goods transport, this work is focused on the assessment of the Greek transport sector. The changes made in the Greek transport sector during the past decade as well as the adverse environmental impacts of the Greek transport sector are presented and analysed. This work aims to present, assess and investigate the progress of the Greek transport sector—over the past decade—in relation to its sustainability. The scope is to examine the effectiveness of various emission reduction measures, in terms of their effectiveness in reducing emissions from transport.     

Sol-gel deposited electrochromic devices

This paper describes the current status of fabrication of electrochromic devices based on sol-gel deposited electrochromic and counter-electrode thin films. Sol-gel film deposition is being developed by Sustainable Technologies Australia as a technique for fabrication of switchable architectural glazings, which have the potential to lead to significant energy reductions in commercial buildings, especially in warm and hot climates. The paper describes the performance of these devices, with particular emphasis on the performance of sol-gel deposited electrodes, including estimates of the energy performance of the devices.     

Environmental payback time analysis of a roof-mounted building-integrated photovoltaic (BIPV) system in Hong Kong

This paper reports the investigation results of the energy payback time (EPBT) and greenhouse-gas payback time (GPBT) of a rooftop BIPV system (grid-connected) in Hong Kong to measure its sustainability. The 22 kWp PV array is facing south with inclined angle of 22.5°. The hourly solar irradiance and ambient air temperature from 1996 to 2000 were used as weather data input. The annual power output was found to be 28,154 kWh. The embodied energy for the whole system in the lifespan was 205,816 kWh, including 71% from PV modules and 29% from balance of system (BOS). The percentage of embodied energy for silicon purification and processing reached 46%. The EPBT of the PV system was 7.3 years, and the GPBT was estimated to be 5.2 years considering fuel mixture composition of local power stations. This paper also discussed the EPBTs for different orientations, ranging from 7.1 years (optimal orientation) to 20.0 years (west-facing vertical PV façade). The results show that the ‘sustainability’ of a PV system is affected by its installation orientation and location. Choosing locations and orientations with higher incident solar irradiance is one key for the sustainability of BIPV technology applications.     

Experimental evaluation of ventilated glazing performance in Hong Kong

Window glazing affects much the indoor environment and the energy use in buildings. While double glazing has better thermal performance than single glazing, the airflow window options carry additional advantage of directly removing the absorbed solar energy in glass panes. This paper reports an experimental study in Hong Kong in evaluating the thermal/energy performance of the above‐mentioned glazing systems. A new approach of using numerical simulation technique to improve the quality of experimental analysis was introduced. Our findings show that the natural‐ventilated glazing system has a better thermal performance than the double‐glazing system since the latter received 13.6% more convective heat gain. The main advantage of the exhaust‐ventilated glazing system lies in the decrease of convective heat transfer to 34.3% of the double glazing, and 19.4% of the single‐glazing types. The results showed that the ventilated glazing schemes in association with daylight utilization could lead to substantial electricity savings in the office environment. Copyright © 2008 John Wiley & Sons, Ltd.     

Transmittance-absorptance product of solar glazing with transparent insulation materials

The transmittance-absorptance product of solar glazing containing the transparent insulation material (TIM) of square celled honeycomb is investigated. A method is developed for the determination of transmittance-absorptance product of beam, sky and ground diffuse solar radiations using the individual transmittances of cellular array and encapsulating covers; the internal reflections are taken into account. Three practical cases; cellular array, cellular array with top cover, and cellular array with top and bottom covers are considered. The results are presented for beam radiation as a function of angle of incidence and sky and ground diffuse radiation as a function of tilt angle. The predicted results are tested by measuring the global radiation transmittance of commercial TIM; the predicted results deviate from the measurements by an average of 2.0%.     

Electro-optical analysis of PEDOT symmetrical electrochromic devices

New symmetrical electrochromic devices were constructed, with PEDOT acting as electrochromic layer or as counter electrode layer depending on the polarity of the applied voltage. Devices of around 500 mm² were analysed in this work. They can switch from 0.5 to 2 V, obtaining a spectral change of around 21% mainly in the red wavelength interval. Measured electrochemical impedance was fitted to an equivalent circuit, made by a Randles circuit, with Warburg impedance simulating ionic diffusion at low frequencies. Results show problems in the contact layers, not seen in normal operation. This will serve as a method to improve the devices construction.     

The effect of glass coating emittance and frame rebate on heat transfer through vacuum and electrochromic vacuum glazed windows

The thermal performance of an electrochromic vacuum glazing and a vacuum glazing with a range of low-emittance coatings and frame rebate depths were simulated for insolations between 0 and 1000 W m⁻² using a three-dimensional finite volume model. The vacuum glazing simulated comprised two 0.4 m×0.4 m glass panes separated by a 0.12 mm wide evacuated space supported by a 0.32 mm diameter pillar array spaced at 25 mm. The two glass sheets were sealed contiguously by a 6 mm wide metal edge seal and had either one or two low-emittance coatings. For the electrochromic vacuum glazing, a third glass pane on which an electrochromic layer was deposited was assumed to be sealed to an evacuated glass unit, to enable control of visible light transmittance and solar gain and thus improve occupant thermal comfort. It is shown that for both vacuum glazing and electrochromic vacuum glazings, when the coating emittance value is very low (close to 0.02), the use of two low-emittance coatings only gives limited improvement in glazing performance. The use of a single currently expensive low-emittance coating in both systems provided acceptable performance. Deeper frame rebate depths gave significant improvements in thermal performance for both glazing systems.     

Predicted thermal performance of triple vacuum glazing

The simulated triple vacuum glazing (TVG) consists of three 4 mm thick glass panes with two vacuum gaps, with each internal glass surface coated with a low-emittance coating with an emittance of 0.03. The two vacuum gaps are sealed by an indium based sealant and separated by a stainless steel pillar array with a height of 0.12 mm and a pillar diameter of 0.3 mm spaced at 25 mm. The thermal transmission at the centre-of-glazing area of the TVG was predicted to be 0.26 W m⁻² K⁻¹. The simulation results show that although the thermal conductivity of solder glass (1 W m⁻¹ K⁻¹) and indium (83.7 W m⁻¹ K⁻¹) are very different, the difference in thermal transmission of TVGs resulting from the use of an indium and a solder glass edge seal was 0.01 W m⁻² K⁻¹. This is because the edge seal is so thin (0.12 mm), consequently there is a negligible temperature drop across it irrespective of the material that the seal is made from relative to the total temperature difference across the glazing. The results also show that there is a relatively large increase in the overall thermal conductance of glazings without a frame when the width of the indium edge seal is increased. Increasing the rebate depth in a solid wood frame decreased the heat transmission of the TVG. The overall heat transmission of the simulated 0.5 m by 0.5 m TVG was 32.6% greater than that of the 1 m by 1 m TVG, since heat conduction through the edge seal of the small glazing has a larger contribution to the total glazing heat transfer than that of the larger glazing system.     

Stresses in vacuum glazing fabricated at low temperature

This paper reports an experimental and theoretical study of the stresses in and durability of vacuum glazing fabricated at low temperature using an indium based edge seal. For the first time a finite-element model with support pillars incorporated directly, enabled the stresses in the whole structure to be explicitly calculated. Experimental validations of the finite element model predictions were undertaken. Modelling results are presented for a case with American Society of Testing and Materials standard winter boundary conditions. It was found that, for the particular system studied, the predicted stress level in the structure is essentially the same for indium sealed and solder glass sealed vacuum glazing, and the magnitude of stress values in the indium seal is comparable with that dictated by the indium strength characteristics.     

Environmental impacts of a standalone solar water splitting system for sustainable hydrogen production: A life cycle assessment

Hydrogen is broadly utilized in various industries. It can also be considered as a future clean energy carrier. Currently, hydrogen is mainly produced from typical fuels such as coal; however, there exist some other clean alternatives which use water decomposition techniques. Water splitting via the copper-chlorine (Cu–Cl) thermochemical cycle is a superb option for producing clean carbon-free fuel. Here, the life cycle assessment (LCA) technique is used to investigate the environmental consequences of an integrated solar Cu–Cl fuel production facility for large-scale hydrogen production. The impact of varying important input parameters including irradiation level, plant lifetime, and solar-to-hydrogen efficiency on various environmental impacts are investigated next. For instance, an improve in the solar-to-hydrogen efficiency from 15% to 30%, results in a reduction in the GWP from 1.25 to 6.27E-01 kg CO₂ eq. An uncertainty analysis using Monte Carlo simulation is conducted to deal with the study uncertainties. The results of the LCA show that the potential of acidification and global warming potential (GWP) of the current system are 8.27E-03 kg SO₂ eq. and 0.91 kg CO₂ eq./kg H₂, respectively. According to the sensitivity analysis, the plant lifetime has the highest effect on the total GWP of the plant with a range of 0.63–1.88 kg of CO₂ eq./kg H₂. Results comparison with past thermochemical-based studies shows that the GWP of the current integrated system is 7% smaller than that of a solar sulfur-iodine thermochemical cycle.     

Optical properties of electrochromic all-solid-state devices

We have investigated the optical properties of an all-thin-film electrochromic device, with a thin film of ZrO₂ acting as an ion conductor. The device also employed electrochromic layers of amorphous or crystalline WO₃ and NiV_xO_yH_z. Transmission (T) and reflection (R) spectra were recorded in the wavelength range 300–2500 nm at different intercalation levels, both for single films and complete devices. The results show that T decreases significantly upon intercalation in the WO₃ thin films as well as in the devices. The reflectance only shows minor changes.     

Status of reversible electrodeposition electrochromic devices

The status of electrochromic devices based upon the reversible electrodeposition of thin bismuth–copper films is reported. The electrochemistry and interface chemistry of the system relevant to information display applications are briefly described. Low information content displays are currently being produced in small quantities with saturated black and white contrast ratio of 25 : 1 and lifetimes of greater than ten million cycles. Electrochemical scanning tunneling microscopy (ECSTM) studies on highly ordered pyrolytic graphite (HOPG) substrates were undertaken as a first step in the unraveling of the detailed nucleation and growth behavior of the Bi–Cu system at the atomic level. The results obtained support previous studies using scanning electron microscopy and surface spectroscopy. Preliminary results on the plating current efficiency of the system using a quartz crystal microbalance (QCM) are reported.     

Optical indices of lithiated electrochromic oxides

Optical indices have been determined for thin films of several electrochromic oxide materials. One of the most important materials in electrochromic devices, WO₃, was thoroughly characterized for a range of electrochromic states by sequential injection of Li ions. Another promising material, Li_0.5Ni_0.5O, was also studied in detail. Less detailed results are presented for three other common lithium-intercalating electrochromic electrode materials: V₂O₅, LiCoO₂, and CeO₂–TiO₂. The films were grown by sputtering, pulsed laser deposition (PLD) and sol–gel techniques. Measurements were made using a combination of variable-angle spectroscopic ellipsometry and spectroradiometry. The optical constants were then extracted using physical and spectral models appropriate to each material. Optical indices of the underlying transparent conductors, determined in separate studies, were fixed in the models of this work. The optical models frequently agree well with independent physical measurements of film structure, particularly surface roughness by atomic force microscopy. Inhomogeneity due to surface roughness, gradient composition, and phase separation are common in both the transparent conductors and electrochromics, resulting sometimes in particularly complex models for these materials. Complete sets of data are presented over the entire solar spectrum for a range of colored states. These data are suitable for prediction of additional optical properties such as oblique transmittance and design of complete electrochromic devices.     

The effect of glazing shape upon the thermal performance of buildings

The great incidence that glazing has in a building energy conservation makes it one of the most important parameters to be taken into account especially in commercial buildings, where the surface occupied by glass areas is very important. So, different shapes of glass areas and their influence in the energy consumption of a commercial building are studied in this paper. Horizontal glazing (with different heights) and vertical glazing (with the same area as the horizontal ones), separated by opaque areas are considered in a base case building. A traditional wall and a curtain-wall are considered, and the different annual consumptions per conditions unit surface, both in winter and summer, are obtained.     

双层玻璃空气层厚度变化对热阻影响的实验研究

针对目前对双层玻璃空气热阻最佳厚度利用解释模糊的问题,通过建立实验模型,对空气层分别为10～70mm等12组厚度的双层玻璃内表面温度等数据进行测量,按照有关规程中的方法进行理论分析,最终得出:当空气层厚度小于5mm时,传热过程以导热和对流换热为主;大于5mm时,以辐射换热为主;0～20mm时,导热率变化非常显著,25～70mm时,导热率变化较为缓慢,且在20mm左右出现一个峰值;工程设计时可以把空气层厚度控制在20mm左右,这样有利于发挥空气层热阻的作用。     

Durability of polymeric glazing materials for solar applications

The economic viability of solar collector systems for domestic hot water (DHW) generation is strongly linked to the cost of such systems. Installation and hardware costs must be reduced by 50% to allow significant market penetration. An attractive approach to cost reduction is to replace glass and metal parts with less expensive, lighter weight polymeric components. Weight reduction decreases the cost of shipping, handling, and installation. The use of polymeric materials also allows the benefits and cost savings associated with well established manufacturing processes, along with savings associated with improved fastening, reduced part count, and overall assembly refinements. A key challenge is to maintain adequate system performance and assure requisite durability for extended lifetimes. Results of preliminary and ongoing screening tests for a large number of candidate polymeric glazing materials are presented. Based on these results, two specific glazings with moderate and poor weathering stability are selected to demonstrate how a service lifetime methodology can be applied to accurately predict the optical performance of these materials during in-service use. A summary is given for data obtained by outdoor exposure and indoor testing of polyvinyl chloride (PVC) and high temperature modified polycarbonate copolymer (coPC) materials, and an initial risk analysis is given for the two materials. Screening tests and analyses for service lifetime prediction are discussed. A methodology that provides a way to derive correlations between degradation experienced by materials exposed to controlled accelerated laboratory exposure conditions and materials exposed to in-service conditions is given, and a validation is presented for the methodology based upon durability test results for PVC and coPC.     

Structural and electrochromic properties of tungsten oxide prepared by surfactant-assisted process

By virtue of gemini surfactant template, nanostructured tungsten oxides thin films were prepared from the modified tungsten hexachloride sol-gel techniques. Temperature was varied as it is an important factor for crystallization, surface morphology and microstructure of tungsten oxides, from the studies of X-ray diffractions, scanning electron microscopy and transmission electron microscopy. The mesoporous sample calcined at 300 °C has tri-dimensional vermicular mesopores with nanocrystallites embedded in the pore wall, while such uniform structure would be destroyed by higher calcination temperature of about 400 °C. X-ray photoelectron spectroscopy was used for analyzing the surface-binding states and the stoichiometry for the oxides. Electrochromic characterization was implemented by simultaneous voltametric and spectrophotometric measurements of tungsten oxides/indium tin oxide (ITO) electrodes. The investigation results showed that organized pore-wall nanostructure has strong effects on the electrochemical and chromogenic properties depending on the specific surface area and the impacts from the evolved crystallization.