Development and testing of thermochromic coatings for buildings and urban structures

The present study reports the development and comparative testing of thermochromic coating to be used in buildings and urban structures. Experimental results from an extensive comparative analysis of the thermal and physical behaviour of thermochromic, highly reflective (cool), and common coatings are reported and analyzed. The surface temperature was monitored on 24 h basis from August to mid-September 2007. It was revealing that the temperature of thermochromic coatings was lower than cool and common coatings. Measurements of spectral reflectance indicated that the thermochromic coatings at the colored phase (below the transition temperature of 30 °C) are energy-absorbing while at the colorless phase (above the transition temperature of 30 °C) are energy-reflecting. The data obtained was used for the calculation of solar reflectance. The results showed that the solar reflectance of the thermochromic samples was significally higher compared to the cool and common ones. A 10-day period test was also performed showing the impact of solar radiation on thermochromism.The comparative results demonstrate that the use of thermochromic coatings can both contribute to energy savings in buildings, providing a thermally comfortable indoor environment, while can contribute highly to improve the urban microclimate.     

Effect of decoration film on mold surface temperature during in-mold decoration injection molding process

In-mold decoration (IMD) during injection molding is a relatively new injection molding technique and has been employed for plastic products to improve surface quality and achieving colorful surface design, etc. During IMD processing, the film is preformed as the shape of mold cavity and attached to one side of the mold wall (usually cavity surface), then molten polymer is filled into the cavity. Heat transfer toward the mold cavity side during molding IMD part is significantly retarded because the film is much less thermal conductive than metal mold. To investigate the effect of film on temperature field, polycarbonate (PC) was injection molded under various conditions including coolant temperature, melt temperature, film material and film thickness. Simulations were also conducted to evaluate the melt–film interface temperature and its influence from film initial temperature and film thermal properties. For PC film, it was found that the heat transfer retardation results in the mold temperature drop in cavity surface and the maximum temperature drop as compared to that of conventional injection molding without film may be as high as 17.7 °C. For PET film, this maximum mold temperature drop is about 13 °C. As PC film thickness increases, the retardation-induced mold temperature difference also increases. The initial film temperature (30 °C and 95 °C) may affect the melt–film interface temperature at the contact instant of melt and film by about 12 °C to 17 °C. When thermal conductivity of film increases from 0.1 W/(m–k) to 0.2 W/(m–k), melt–film interface temperature may vary by 22.9 °C. The simulated mold temperature field showed reasonable agreement with experimental results.     

Plasma-induced TCO texture of ZnO:Ga back contacts on silicon thin film solar cells

This paper considers texturing of ZnO:Ga (GZO) films used as back contacts in amorphous silicon (a-Si) thin film solar cells. GZO thin films are first prepared by conventional methods. The as-deposited GZO surface properties are modified so that their use as back contacts on a-Si solar cells is enhanced. Texturing is performed by simple dry plasma etching in a CVD process chamber,at power=100 W, substrate temperature=190 °C (temperature is held at 190 °C because thin film solar cells are damaged above 200 °C), pressure=400 Pa and process gas H₂ flow=700 sccm. Conventional a-Si solar cells are fabricated with and without GZO back contact surface treatment. Comparison of the with/without texturing GZO films shows that plasma etching increases optical scattering reflectance and reflection haze. SEM and TEM are used to evaluate the morphological treatment-induced changes in the films. Comparison of the a-Si solar cells with/without texturing shows that the plasma treatment increases both the short-circuit current density and fill factor. Consequently, a-Si solar cell efficiency is relatively improved by 4.6%.     

TMAH-textured, a-Si/c-Si, heterojunction solar cells with 10% reflectance

Single-polished c-Si (1 0 0) wafers were textured in aqueous solutions with varying concentrations of tetra-methyl ammonium hydroxide (TMAH). The resulting surface reflectance and morphology were examined as a function of etching time and temperature, TMAH concentration, and addition of isopropyl alcohol to the solution.The lowest reflectance, 9.8% at a 600-nm wavelength with 0.3% scattering over a 4″ wafer surface, was obtained after 40 min of etching in a 2% TMAH solution at 80 °C under 700 rpm magnetic stirring. Upon adding isopropyl alcohol to the solution, the resulting pyramids were round-edged, and 12% sample reflectance was obtained.The results are interpreted in terms of micro-masking formation and temperature-dependent crystallographic selectivity.The compatibility of the treatment with photovoltaic applications was evaluated by studying the performance of heterojunction solar cells, which are particularly sensitive to surface quality. A degradation of the open circuit voltage was observed in devices fabricated on surfaces featuring crooked pyramid sides. Optimised process conditions led to smooth pyramid sides and no degradation of the open circuit voltage, which indicates no sign of increased surface recombination-centre concentration. The reduced reflectance resulted in a 16% increase of the short circuit current of the solar cell device.     

Self-consistent optical parameters of intrinsic silicon at 300 K including temperature coefficients

An updated tabulation is presented of the optical properties of intrinsic silicon, of particular interest in solar cell calculations. Improved values of absorption coefficient, refractive index and extinction coefficient at 300 K are tabulated over the 0.25-1.45 μm wavelength range at 0.01 μm intervals. The self-consistent tabulation was derived from Kramers-Kronig analysis of updated reflectance data deduced from the literature. The inclusion of normalised temperature coefficients allows extrapolation over a wide temperature range, with accuracy similar to that of available experimental data demonstrated over the −24 °C to 200 °C range.     

Prediction of thermophysical properties of gas turbine combustion gases using the Mansour-Najjar equation of state

A number of thermophysical properties of combustion gases occurring in turbine engines have been compared with experimental data by using the Mansour-Najjar equation of state. The values obtained are shown to be more accurate than those derived from the virial equation of state. The following properties were studied: density, specific heat, enthalpy, entropy, viscosity, and thermal conductivity in the temperature range 200–2600 K at pressures of 3–12 atm.     

Through-plane thermal conductivity of the microporous layer in a polymer electrolyte membrane fuel cell

Understanding the thermal properties of the microporous layer (MPL) is critical for accurate thermal analysis and improving the performance of proton exchange membrane (PEM) fuel cells operating at high current densities. In this study, the effective through-plane thermal conductivity and contact resistance of the MPL have been investigated. Gas diffusion layer (GDL) samples, coated with 5%-wt. PTFE, with and without an MPL are measured using the guarded steady-state heat flow technique described in the ASTM standard E 1225-04. Thermal contact resistance of the MPL with the iron clamping surface was found to be negligible, owing to the high surface contact area. Effective thermal conductivity and thickness of the MPL remained constant for compression pressures up to 15 bar at 0.30 W/m°K and 55 μm, respectively. The effective thermal conductivity of the GDL substrate containing 5%-wt. PTFE varied from 0.30 to 0.56 W/m°K as compression was increased from 4 to 15 bar. As a result, GDL containing MPL had a lower effective thermal conductivity at high compression than the GDL without MPL. At low compression, differences were negligible. The constant thickness of the MPL suggests that the porosity, as well as heat and mass transport properties, remain independent of the inhomogeneous compression by the bipolar plate. Despite the low effective thermal conductivity of the MPL, thermal performance of the GDL can be improved by exploiting the excellent surface contact resistance of the MPL.     

Band-approximated radiative heat transfer analysis of a solar chemical reactor for the thermal dissociation of zinc oxide

A solar chemical reactor for the thermal dissociation of ZnO is modeled by means of a detailed heat transfer analysis that couples radiative transport to the reaction kinetics. An extended band-approximated radiosity method enables the analysis of directional and wavelength depended radiation exchange. Boundary conditions included the incident concentrated solar radiation, determined by the Monte Carlo ray-tracing technique, and the hemispherical and band-approximated optical properties derived for the quartz window. Validation was accomplished by comparing the numerically modeled and experimentally measured window temperatures, reaction rates, and energy conversion efficiencies. The experimentally measured solar-to-chemical energy conversion efficiency increased with temperature, peaked at 14% for a reactor temperature of 1900 K and ZnO dissociation rate of 12 g/min, and decreased as the reactor approached its stagnation temperature. The conditions for which this efficiency can be augmented are discussed.     

On the development, optical properties and thermal performance of cool colored coatings for the urban environment

This paper reports the measured solar spectral properties and the thermal performance of 10 prototype cool colored coatings, developed at the National and Kapodistrian University of Athens, using near-infrared reflective color pigments in comparison to color-matched, conventionally pigmented coatings. These coatings are developed to be used in the urban environment to fight the heat island effect. The spectral reflectance and the infrared emittance were measured and the solar reflectance of the samples was calculated. The surface temperature of the coatings when applied to concrete tiles was monitored, using surface temperature sensors and a data logging system, on 24 h basis from August to December 2005 in an effort to investigate the ability of the cool colored coatings to maintain lower surface temperatures than conventionally pigmented color-matched coatings. The data obtained has been extensively analysed and indicate significant success in the development of these cool colored coatings. It was found that all the coatings containing infrared reflective pigments have solar reflectance values higher than those of standard coatings. Furthermore, it was demonstrated that cool colored coatings maintain lower surface temperatures than color-matched conventionally pigmented coatings. This temperature difference is mainly due to differences in solar reflectance. These cool colored coatings can be used on buildings (roofs and walls) and other surfaces in the urban environment. Thus, at building scale, the use of cool colored coatings with increased solar reflectance can improve building comfort and reduce cooling energy use, and at city city-scale it can contribute to the reduction of the air temperature due to the heat-transfer phenomena and therefore improve outdoor thermal comfort and reduce the heat-island effect.     

Preparation of new titanium oxy nitride based electro catalysts using an anhydrous sol-gel method for water electrolysis in acid medium

Titanium oxy nitride has been fabricated for the first time from anhydrous Sol-gel Method and used as anode for water electrolysis in acid medium. It is shown that the properties of this material are highly dependent on the calcination temperature. The conductivity increases from insulator (titanium dioxide type) to metallic (titanium nitride type) when the calcination temperature rose from 500 °C to 1000 °C. When the catalyst material is calcinated at 700 °C, the rutile structure of titanium dioxide is obtained and agrees well with those obtained with the TGA-DSC results. These results also show that the structure of the osbornite of titanium nitride appears at 1000 °C. The BET surface area also increases with the calcination temperature. The morphology of the calcinated samples gradually breaks into smaller conchoidal fracture particles when the sample preparation temperature increases. Electrochemical voltage–current polarization curves of the oxygen evolution reaction in sulphuric acid on these anodes electrodes confirm that the materials calcinated at 800 °C or even at higher temperatures exhibit a low Tafel slope and a high electrochemistry surface area which are both known as good values for the oxygen evolution in acid medium. The results of this study suggest that our titanium oxy nitride prepared by an anhydrous sol-gel method followed by calcination is an interesting anode catalyst for water electrolysis. Its performance for this reaction is compared to those of some noble metal oxides.     

The preparation and properties of multi-component molten salts

This paper was focused on thermal stability of molten salts and their thermo-physical properties at high temperature. In this experiment, multi-component molten salts composed of potassium nitrate, sodium nitrite and sodium nitrate with 5% additive A of the chlorides were prepared by statical mixing method. The experiments found molten salt with 5% additive A had higher thermal stability and its best operating temperature would be increased to 550 °C from 500 °C when comparing to ternary nitrate salt. Meanwhile, thermal stability and thermal cycling analysis showed molten salts with 5% additive A had lower freezing point and loss of nitrite content and deterioration time of molten salts were reduced at the same time. DSC tests also indicated loss of latent heat in molten salts with 5% additive A was decreased. Besides, thermo-physical properties measured showed molten salt with 5% additive A had a heat capacity of 2.32 kJ/kg °C, lower than 4.19 kJ/kg °C for water between 0 °C and 100 °C and a low viscosity range from 3.0 to 1.4 cp between 150 °C and 500 °C, analogous with 1.8–0.3 cp for water between 0 °C and 100 °C. Other thermo-physical properties, such as thermal conductivity, density and linear thermal expansion, were also determined here.     

Development of a model for calculating the longwave optical properties and surface temperature of a curved venetian blind

This article is about the development of a mathematical model for calculating the longwave optical properties of a curved venetian blind. The calculated optical properties are used to determine the performance of the glass window installed with a venetian blind in terms of thermal comfort. The blind, whose optical properties are considered nonspecular, is modeled as an effective layer. The effect of slat curvature is included in the developed model. A six surface enclosure formed by two consecutive slats is used to analyze for the longwave optical properties of the effective layer. The longwave optical properties, transmittance, reflectance, absorptance and emittance are developed by using the radiosity method. The steady state energy balance method along with the developed longwave optical properties are used to determine the surface temperature of the effective layer. The empirical expression for the total heat flux from the indoor glass window surface with an adjacent venetian blind is adopted in the developed model. The surface temperature of the blind, which is the key parameter for calculating the thermal performance of glass windows with venetian blinds with respect to thermal comfort, is chosen as the parameter used for the model validation. The predicted surface temperature of the venetian blind is compared with the surface temperature of the venetian blind obtained from the measurement. The agreement between the predicted temperature and the measured temperature is good.     

Improved high temperature performance of La0.8Sr0.2MnO3 cathode by addition of CeO2

Ceria is proposed as an additive for La_0.8Sr_0.2MnO₃ (LSM) cathodes in order to increase both their thermal stability and electrochemical properties after co-sintering with an yttria-stabilized zirconia (YSZ) electrolyte at 1350 °C. Results show that LSM without CeO₂ addition is unstable at 1350 °C, whereas the thermal stability of LSM is drastically improved after addition of CeO₂. In addition, results show a correlation between CeO₂ addition and the maximum power density obtained in 300 μm thick electrolyte-supported single cells in which the anode and modified cathode have been co-sintered at 1350 °C. Single cells with cathodes not containing CeO₂ produce only 7 mW cm⁻² at 800 °C, whereas the power density increases to 117 mW cm⁻² for a CeO₂ addition of 12 mol%. Preliminary results suggest that CeO₂ could increase the power density by at least two mechanisms: (1) incorporation of cerium into the LSM crystal structure, and (2) by modification or reduction of La₂Zr₂O₇ formation at high temperature. This approach permits the highest LSM-YSZ co-sintering temperature so far reported, providing power densities of hundreds of mW cm⁻² without the need for a buffer layer between the LSM cathode and YSZ electrolyte. Therefore, this method simplifies the co-sintering of SOFC cells at high temperature and improves their electrochemical performance.     

Nanostructured hematite photoelectrochemical electrodes prepared by the low temperature thermal oxidation of iron

We report on a facile low temperature method for the preparation of high surface area, nanostructured α-Fe₂O₃ (hematite) thin films and their application as photoelectrochemical (PEC) water splitting electrodes. The hematite films are fabricated by thermal oxidation in air of DC sputter deposited iron films at temperatures as low as 255 °C. This method results in films with a higher surface area than typically obtained by directly sputtering α-Fe₂O₃. It is shown that beyond a minimum iron thickness, α-Fe₂O₃ nanowires result upon thermal treatment in atmospheric conditions. Structural and optical characteristics of the resulting films are analyzed. The oxidation process is studied in detail and correlated to the photoelectrical properties. The Fe films oxidize in stages via Fe-oxide layers of increasing oxidation states. Resulting photoelectrochemical performance of fully oxidized films is a balance between optical absorption and charge collection, which varies with film thickness. The optimum film achieved a net photocurrent density of 0.18 mA/cm² in 1 M NaOH at 1.23 V vs. RHE under simulated AM1.5 sunlight, amongst the highest values reported for undoped hematite films produced at low temperature.     

KOH modified Nafion112 membrane for high performance alkaline direct ethanol fuel cell

A polymer electrolyte membrane for alkaline direct ethanol fuel cell (ADEFC) was prepared by dipping Nafion112 membrane into KOH solution for some time at room temperature. The obtained membrane (Nafion112/KOH) exhibited higher mechanical properties and thermal stability than Nafion112 membrane. The ionic conductivity of Nafion112/KOH in 1 M, 2 M and 6 M KOH solutions was 0.011 S/cm, 0.026 S/cm, 0.032 S/cm, respectively, depending on internal OH⁻ concentration and the volume fraction of the internal aqueous phase. Single cell performance suggested that active ADEFC with Nafion112/KOH membrane can deliver a peak power density of 58.87 mW/cm² at 90 °C, meanwhile, it can stably run for at least 12 h above 0.2 V. On the other hand, Pt-free air breathing ADEFC with Nafion112/KOH can output a peak power density of 11.5 mW/cm² at 60 °C, and the corresponding lifetime was as long as 473 h above 0.3 V.     

Preparation and characterization of phosphotungstic acid-derived salt/Nafion nanocomposite membranes for proton exchange membrane fuel cells

Nafion/Cs_2.5H_0.5PW₁₂O₄₀ nanocomposite membranes are prepared and characterized as alternate materials for PEMFC operation at high temperature/low humidity. The Cs_2.5H_0.5PW₁₂O₄₀ solid acid particles (hereafter CsPWA) have the high surface area, the high hygroscopic property and the ability to generate proton in the presence of water molecules. The results of prepared membranes at three levels (0, 10 and 15%) indicate that the CsPWA particles have influence on the water content, ion exchange capacity, thermal properties (TGA and DSC), proton conductivity and PEM fuel cell performance. Particles agglomeration and Nafion active sites (sulfonic groups) covering are seen in the nanocomposite membranes. The conductivity of nanocomposite membranes at high temperatures (110 and 120 °C) is higher than plain Nafion and may be related to the additional water within the nanocomposite membrane and/or the additional surface functional site provide by CsPWA. The fuel cell responses show that in the fully hydrated state and at the higher current densities, the prepared MEAs with nanocomposite membranes possess better response compared with the plain Nafion. In partially hydrated cell, at both low and high current densities, the superior performance of the MEA prepared by nanocomposite membranes is observed.     

Preparation and characterisation of Ti oxide based catalyst supports for low temperature fuel cells

Bare and doped Ti-oxides were prepared by using various synthesis routes and investigated in terms of structure, morphology and electrochemical properties for application as catalyst supports in low temperature fuel cells. A crystalline Anatase phase was obtained for most of the doped and undoped Ti-oxide supports after an air treatment in a range 300°–600 °C. Rutile structure was observed for treatments at higher temperatures. BET surface area varied as a function of the preparation route, the doping agent and the thermal treatment. BET surface area decreased passing from bare TiO₂ to Ta-doped TiO₂ and further decreased in the presence of Nb doping. A sulphite complex route allowed to achieve crystalline materials with crystallite domain sizes of about 3–4 nm upon thermal treatment at 400 °C. The corresponding BET surface area for TiO₂ was around 250 m²/g whereas it decreased to 175 m²/g for the doped support. Electrochemical oxidation tests at 1.4 V RHE showed that oxide supports were significantly more stable in terms of electrochemical corrosion than conventional carbon black supports used in fuel cells. This comparison was extended to Ti-suboxides with Magneli phase obtained by high temperature reduction. The latter material appeared also very stable but characterised by low surface area (about one order of magnitude lower than bare TiO₂) due to the high temperature treatment. Oxygen reduction reaction studies (ORR) showed that suitable performance can be achieved in the presence of a proper combination of electronic conductivity and dispersion for the oxide supported catalyst nanoparticles.     

Development of non-perfluorinated hybrid materials for single-cell proton exchange membrane fuel cells

Development of low temperature fuel cells that operate under 100 °C are needed to reduce the costs, to design a class of hybrid membranes and to construct various structures of membrane-electrode-assembles (MEAs) for proton exchange membrane fuel cells (PEMFC). In this work, PVA/PMA/SiO₂ hybrid composite membranes were synthesized and their conductivities were determined by impedance measurements. We found a maximum conductivity value of 4.2 × 10⁻³ S/cm at 80 °C and 100% relative humidity (RH). A fuel cell test evaluation for various MEAs was conducted by the potentiodynamic analysis and the current density values were determined from the current–voltage (I–V) curves. A maximum current density of 635 mA/cm² was obtained at 80 °C and 100% RH. To the best of our knowledge, this is the first time that a high current density of PVA-based electrolytes for PEMFCs operating at low temperature is reported. The structural characters were examined using of XRD and FTIR methods, and thermal properties were studied using DSC and TGA techniques and the results were discussed (cf. supplementation). The present study revealed that the single cell performance depends mainly on the temperature, relative humidity and chemical compositions of the membranes.     

Alcohol electrooxidation at Pt and Pt–Ru sputtered electrodes under elevated temperature and pressurized conditions

The electrooxidation properties of methanol and 2-propanol, which are both promising candidates for direct alcohol fuel cells (DAFCs), have been studied under elevated temperature and pressurized conditions. Sputter-deposited Pt and Pt–Ru electrodes were well-characterized and utilized for the electrochemical measurement of the alcohol oxidation at 25–100 °C. The Pt electrode prepared at 600 °C had a flat surface, and the Pt–Ru formed an alloy. The electrochemical measurements were carried out in a gas-tight cell under elevated temperature, which accompanies the pressurized condition. This is a representative example of the DAFC rising temperature operation. As a result, at 25 °C, the onset potential of the 2-propanol oxidation is about 400 mV more negative than that of the methanol oxidation, and current density of the 2-propanol oxidation exceeds that of the methanol oxidation. Conversely, at 100 °C, the methanol oxidation current density overcomes that of 2-propanol, and the onset potentials of the two are almost the same. The highest current density for the methanol oxidation is obtained at the Pt:Ru = 50:50 electrode, whereas at the Pt:Ru = 35:65 for the 2-propanol oxidation. A Tafel plot analysis was employed to investigate the reaction mechanism. For the methanol oxidation, the number of electrons transferred during the rate-determining process is estimated to be 1 at 25 °C and 2 at 100 °C. This suggests that the methanol reaction mechanism differs at 25 and 100 °C. In contrast, the rate-determining process of the 2-propanol oxidation at 25 and 100 °C was expected to be 1-electron transfer which accompanies the proton-elimination reaction to produce acetone. Consequently, it is deduced that methanol and 2-propanol have an advantage under the rising temperature and room temperature operation, respectively.     

Characteristic of hydrogen and syngas evolution from gasification and pyrolysis of rubber

The characteristics of syngas evolution during pyrolysis and gasification of waste rubber have been investigated. A semi-batch reactor was used for the thermal decomposition of the material under various conditions of pyrolysis and high temperature steam gasification. The results are reported at two different reactor temperatures of 800 and 900 °C and at constant steam gasifying agent flow rate of 7.0 g/min and a fixed sample mass. The characteristics of syngas were evaluated in terms of syngas flow rate, hydrogen flow rate, syngas yield, hydrogen yield and energy yield. Gasification resulted in 500% increase in hydrogen yield as compared to pyrolysis at 800 °C. However, at 900 °C the increase in hydrogen was more than 700% as compared to pyrolysis. For pyrolysis conditions, increase in reactor temperature from 800 to 900 °C resulted in 64% increase in hydrogen yield while for gasification conditions a 124% increase in hydrogen yield was obtained. Results of syngas yield, hydrogen yield and energy yield from the rubber sample are evaluated with that obtained from woody biomass samples, namely hard wood and wood chips. Rubber gasification yielded more energy at the 900 °C as compared to biomass feedstock samples. However, less syngas and less hydrogen were obtained from rubber than the biomass samples at both the temperatures reported here.