Determination of glazing area in direct gain systems for three different climatic zones

This study determines the glazing area in direct gain passive systems needed to ensure thermal comfort inside a building (room air temperature 20 ± 2°C). A 4 m × 4 m × 3 m single zone isolated house is analyzed in three different types of climates namely composite (8°C to 20°C, New Delhi), cold-cloudy (−2°C to 5°C, Srinagar), and cold-sunny (−14°C to −3°C, Leh). The analysis is based on the periodic solution of the heat conduction equations describing heat transmission in the building components, floor, walls, and roof, and the Fourier representation of the ambient temperature vnd the total solar radiation intercepted by the building envelope. Two types of construction are analyzed: the first type is a traditional construction with 22-cm-thick brick wall, plastered 15 mm on both the sides (U = 2.0 W m⁻² K⁻¹); and the second one is of the same type but with 10 cm of expanded polystyrene insulation on all the four walls and the roof (U = 0.31 W m⁻² K⁻¹). It is found that for traditional construction with U = 2.0 W m⁻² K⁻¹, the glazing U value has almost no effect on the room temperature even for large variation of the glazing area (10% to 40%, expressed in terms of percentage of floor area). For a well-insulated house (U = 0.31 W m⁻² K⁻¹), the glazing U value has no effect upon the room air temperature if the glazing area is small (less than 10%). The position of the insulation on the external surfaces is more effective in reducing large inroom air temperature. Finally, for an insulated house, we recommended glazing is 30%, 20%, and 10% for cold-sunny, cold-cloudy, and composite climates, respectively.     

On minimizing heat transport in architectural glazing

Significant reductions in the heating energy demand of buildings are achievable through minimization of the thermal transmittance of glazing. This paper reviews all the heat transport processes occurring in gas-filled and evacuated insulating glazing. The heat transfer mechanisms in gas-filled glazing cavities include radiative exchange between the glass sheet surfaces, convection and gaseous conduction. The application of two low-emissivity coatings (0.04) lowers the thermal conductance due to radiation between the glass pane surfaces to roughly 0.1 W m⁻² K⁻¹. At the same time, even where fill gases such as krypton and xenon are used, thermal conductance due to convection and conduction cannot be reduced to much below 1 W m⁻² K⁻¹. Heat transfer by convection and gaseous conduction only becomes negligible where the cavity is evacuated to approximately 10⁻² Pa. Heat transfer is then determined by radiation and, even more importantly, conduction in support pillars required to bear the atmospheric load on the external glass sheet surfaces. The fact that the average centre-of-glazing heat transfer rates achievable by evacuation of the cavity are some two to five times lower than those of gas-filled cavities increases the significance of heat transfer in the glazing edge regions. Consequently, in addition to the heat transfer in the cavity, the impact on glazing thermal transmittance of the edge seal and different frame constructions was also quantified. The possibilities and limitations of reducing total heat transfer in evacuated glazing are discussed on the basis of analytical and numerical methods. The results suggest that this concept offers significant advantages over current glazing technology in terms of overall thermal transmittance.     

A new transparently insulated, bifacially irradiated solar flat-plate collector

A new type of transparently insulated flat-plate collector was developed. It reaches higher efficiencies at low irradiation values or high operating temperatures than any other collector type known. Both sides of its absorber are covered with transparent insulation material and both sides are irradiated. Thus, the heat losses of the collector related to the total absorber area are distinctly reduced. An optical efficiency of η₀ = 0.72 and a temperature dependent U-value of U(ΔT) = (0.95 + 0.0076 ΔTK⁻¹) W m⁻² K⁻¹ were measured with an outdoor test facility. The bifacial-absorber collector is considered to be the best option for the DHW system of the energetically self-sufficient solar house in Freiburg because of its outstanding winter performance.     

Manufacture and cost of vacuum glazing

The vacuum glazing project at the University of Sydney has progressed to the point where the main features of the vacuum glazing design are determined well. Over 500 glazings with areas up to one square meter have been formed. The stresses to which these glazings are or may be exposed have been studied extensively. The durability of the glazing structure and the internal vacuum has been demonstrated. Vacuum glazing of the type designed and formed at the University of Sydney has a center-of-glazing thermal conductance as low as 0.85 and 1.2 Wm⁻²K⁻¹, for glazings with two and one internal low emittance coatings, respectively. A method for the manufacture of the vacuum glazing is outlined from which the cost to manufacture the glazing can be estimated. A cost at the factory of about $40 ± 7 m⁻² for vacuum glazing using two sheets of low-e glass and about $32 ± 6 m⁻² for glazing using one sheet of low-e glass is obtained, when production volume is approx. 10⁵ m²yr⁻¹ and is partially automated. This is about 25% higher than the estimated manufacturing cost of the high thermal resistance, argon filled, double glazing utilizing low-e glass, which are currently in production and being sold in the United States, Europe and Japan. These glazings typically have center-of-glazing thermal conductances of about 1.1 Wm⁻²K⁻¹ or more.     

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.     

Ex situ measurements of through-plane thermal conductivities in a polymer electrolyte fuel cell

In this paper thermal properties for materials typically used in the proton exchange membrane fuel cell (PEMFC) are reported. Thermal conductivities of Nafion membranes were measured ex situ at 20 °C to be 0.177 ± 0.008 and 0.254 ± 0.016 W K⁻¹ m⁻¹ for dry and maximally wetted membranes respectively. This paper also presents a methodology to determine the thermal conductivity of compressible materials as a function of applied load. This technique was used to measure the thermal conductivity of an uncoated SolviCore porous transport layer (PTL) at various compaction pressures. For the dry PTL at 4.6, 9.3 and 13.9 bar compaction pressures, the thermal conductivity was found to be 0.27, 0.36 and 0.40 W K⁻¹ m⁻¹ respectively and the thermal contact resistivity to the apparatus was determined to be 2.1, 1.8 and 1.1 × 10⁻⁴ m² K W⁻¹, respectively. It was shown that the thermal contact resistance between two PTLs is negligible compared to the apparatus’ thermal contact resistivity. For a humidified PTL, the thermal conductivity increases by up to 70% due to a residual liquid saturation of 25%.     

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.     

Battery effects in organic photovoltaics based on polybithiophene

Homopolymer photovoltaic devices based on thin films of polybithiophene, prepared by direct electrodeposition onto transparent fluorine-doped tin oxide electrodes followed by evaporation of an aluminium electrode to complete the device, were reported by Leguenza et al. [J. Solid State Electrochem. 11 (2007) 577.] to exhibit very high open-circuit voltages (V_oc) of up to 2 V at a very low light intensity of 6.6 W m⁻². In this letter, we report our attempts to reproduce the results. We achieved V_oc's0.8 V under a light intensity of 23 W m⁻². We also observed an unexpected V_oc in the dark suggesting that the high voltages previously reported might be attributed to the polarization of the polybithiophene layer, which lead to a battery effect in the dark and therefore not entirely a photovoltaic effect. We conclude that more work is needed before this observation can conceivably be applied in organic photovoltaics. Most notably, the application of this polarization effect depends on the demonstration of the high V_oc under high luminous intensities (i.e. >200 W m⁻²) and at high current densities (>1 mA cm⁻²) which has not been achieved yet.     

Application demonstration and performance of a cellulose triacetate polymer film based transparent insulation wall heating system

For the application demonstration of cellulose triacetate (CTA) polymer film based transparent insulation (TI) structures a technically and ecologically optimized TI facade system was developed and used to equip a south-oriented wall of a solar house meeting passive house standard in Graz, Austria. The demonstration building was equipped with an appropriate data recording system for solar irradiation, temperature, heat flux and humidity. The practical experiences within the heating periods 2002/03 and 2003/04 are reported in this paper. For the optimized TI facade system a solar energy efficiency of about 43% and a U-value of 0.76 W/(m² K) were obtained. Although CTA absorbs a high amount of water no adverse condensation phenomena were observable visually. The reasoning for these findings is explained and related to construction details.     

Heat transfer characteristics of gaseous flows in microtube with constant heat flux

Heat transfer characteristics of gaseous flows in a microtube with constant heat flux whose value is positive or negative are investigated on two-dimensional compressible laminar flow for no-slip regime. The numerical methodology is based on the Arbitrary–Lagrangian–Eulerian (ALE) method. The computations are performed for tubes with constant heat flux ranging from −10⁴ to 10⁴ W m⁻². The tube diameter ranges from 10 to 100 μm and the aspect ratio of the length and diameter is 200. The stagnation pressure, p_stg is chosen in such away that the Mach number at the exit ranges from 0.1 to 0.7. The outlet pressure is fixed at the atmosphere. The wall and bulk temperatures in microtubes with positive heat flux are compared with those of negative heat flux case and also compared with those of the incompressible flow in a conventional sized tube. In the case of fast flow, temperature profiles normalized by heat flux have different trends whether heat flux is positive or negative. A correlation for the prediction of the wall temperature of the gaseous flow in the microtube is proposed. Supplementary runs with slip boundary conditions for the case of D = 10 μm conducted and rarefaction effect is discussed. With increasing Ma number, the compressibility effect is more dominant and the rarefaction effect is relative insignificant where Kn number is less than Kn = 0.0096. And, the magnitudes of viscous dissipation term and compressibility term are investigated along the tube length.     

Using reduced data sets ISCCP-B2 from the Meteosat satellites to assess surface solar irradiance

This paper explores the capabilities of a combination of the reduced data set ISCCP-B2 from the Meteosat satellites and the recently developed method Heliosat-2 to assess the daily mean of the surface solar irradiance at any geographical site in Europe and Africa. Firstly, we discuss the implementation of the method Heliosat-2. Secondly, B2-derived irradiances are compared to coincident measurements made in meteorological networks for 90 stations from 1994 to 1997. Bias is less than 1 W m⁻² for the whole set. Larger bias may be observed at individual sites, ranging from −15 to +32 W m⁻². For the whole set, the root mean square difference is 35 W m⁻² (17%) for daily mean irradiance and 25 W m⁻² (12%) for monthly mean irradiance. These accuracies are close to those of similar data sets of irradiance, such as Medias and NASA Surface Radiation Budget. It is concluded that B2 data can be used in a reliable way to produce long-term time-series of irradiance for Europe, Africa and the Atlantic Ocean.     

Investigation of solar radiation in Botswana and some anomalous phenomena observed

Daily global insolation on a horizontal surface in Botswana is recorded continuously at several synoptic stations and at the University of Botswana's Physics Department. Over a number of years, daily total insolation on a tilted surface (β = −30°) was recorded at the Botswana Technology Centre. Hourly, and instantaneous direct normal, global, diffuse and UV-components are continuously recorded at the University of Botswana. All these measurements are done with standard EPLAB equipment.It is found out that the instantaneous direct normal radiation at Solar noon can be as high as 1150 W·m⁻²; and that at 30 min before sunset it can be above 600 W·m⁻²; and it can also be as high as 100 W·m⁻² at sunset or sunrise moments (i.e. with half of the solar disk under the horizon).Daily direct normal solar radiation can exceed 45 MJ·m⁻². Mean daily global radiation varies from 31 MJ·m⁻² in December to 16 MJ·m⁻² in June. Such big values of daily direct normal and global radiation are explained by low humidity and low turbidity.Cases of an anomalous phenomenon which lead to an abnormally big phase shift when direct normal radiation is increasing greatly after Solar noon are observed, and discussed. It is also found that when humidity is low and visibility is high, hourly I_g values recorded with a pyranometer can be less than I_bn (cosθ_z) + I_d-values. This discrepancy could be quite common for regions where humidity and turbidity are low. The trend in the behaviour of the UV-component during the last five years is also analyzed and discussed. The conclusion is made that the ozone layer over Botswana is continuously being depleted.     

Power generation from furfural using the microbial fuel cell

Furfural is a typical inhibitor in the ethanol fermentation process using lignocellulosic hydrolysates as raw materials. In the literature, no report has shown that furfural can be utilized as the fuel to produce electricity in the microbial fuel cell (MFC), a device that uses microbes to convert organic compounds to generate electricity. In this study, we demonstrated that electricity was successfully generated using furfural as the sole fuel in both the ferricyanide-cathode MFC and the air-cathode MFC. In the ferricyanide-cathode MFC, the maximum power densities reached 45.4, 81.4, and 103 W m⁻³, respectively, when 1000 mg L⁻¹ glucose, a mixture of 200 mg L⁻¹ glucose and 5 mM furfural, and 6.68 mM furfural were used as the fuels in the anode solution. The corresponding Coulombic efficiencies (CE) were 4.0, 7.1, and 10.2% for the three treatments, respectively. For pure furfural as the fuel, the removal efficiency of furfural reached up to 95% within 12 h. In the air-cathode MFC using 6.68 mM furfural as the fuel, the maximum values of power density and CE were 361 mW m⁻² (18 W m⁻³) and 30.3%, respectively, and the COD removal was about 68% at the end of the experiment (about 30 h). Increase in furfural concentrations from 6.68 to 20 mM resulted in increase in the maximum power densities from 361 to 368 mW m⁻², and decrease in CEs from 30.3 to 20.6%. These results indicated that some toxic and biorefractory organics such as furfural might still be suitable resources for electricity generation using the MFC technology.     

Performance analysis of non-isothermal solar reactors for methanol decomposition

A procedure for analyzing the performance of non-isothermal solar reactors for methanol decomposition was developed, based on a model of thermal loss from direct steam generation collector and a comprehensive kinetic model of methanol decomposition employing BASF K3-110 catalyst. It was found that catalytic bed temperature tends towards a certain value, which depends on the chemical reaction type, radiation intensity and collector structure mainly. For a beam incidence angle of 0°, system efficiency increases from 56% at a radiation intensity of 400 W m⁻² to almost 58% at a radiation intensity of 1000 W m⁻². For a radiation intensity of 400 W m⁻², beam incidence angle of 20°, absorber length of 10 m, feed temperature of 373 K and ratio of reaction section of 0.9, the mole flow rate of feed in the range of 0.21–0.23 mol s⁻¹ results in a maximum quantity of reacted methanol of 0.146 mol s⁻¹, while a mole flow rate of feed of 0.15 mol s⁻¹ leads to a maximum system efficiency of 54.2%. The research indicates that the pre-heating section should be as short as possible for effective use of solar energy.     

Fabrication of evacuated glazing at low temperature

An indium-based seal augmented with an adhesive, developed to maintain a vacuum between two sheets of glass, avoids the high temperatures required to produce a seal in evacuated glazings to date. An experimentally-validated three-dimensional transient model has been used to predict heat transfer for an indium/adhesive sealed 1 m² area evacuated window with a highly insulating frame. An overall heat loss coefficient of 0.9 W m⁻² K⁻¹, with a midplane value of 0.36 W m⁻² K⁻¹, can be achieved with 0.72 visible transmittance for two 6 mm panes separated by 0.2 mm diameter pillars 40 mm apart. The conduction through a 3 mm edge-seal was 1.14 W m⁻² K⁻¹. Detailed three-dimensional isothermal contour plots through the system are presented.     

Economic analysis of the profitability of energy-saving architectural measures for the achievement of the EPB-standard

Energy efficiency in buildings has become a key goal of any energy policy. Europe relies on the Energy Performance of Buildings Directive (EPBD), which has been converted by Flanders into the ‘Energy Performance and Interior Climate’ (EPB). Taking into account this Flemish EPB-standard (in terms of maximum U-values, E-level and K-value), this study seeks the economically most profitable combination of insulation – facade, roof, floor and glazing – for the Flemish citizen. For this purpose, a scenario-analysis is conducted using the EPB-software Flanders and a self-designed Excel file. Based on some important profitability criteria, the most profitable combination is determined for three representative types of dwellings studied. The scenario-analysis generates some well-founded guidelines for the Flemish citizen when building a house. It shows that in order to ensure the maximum profitability from investment in insulation, the key factor for the semi-detached dwelling is the insulation of roof and floor, whereas for a detached dwelling the key factor is the insulation of facade and floor. As a subsidiary consideration, the study also indicates that the U-values resulting from the more stringent E-level are still not sufficiently stringent because the U-values obtained for the most profitable combination are far below their maximum value. The same consideration applies in the case of the K-value.     

A potential interconnect material for solid oxide fuel cells: Nd0.75Ca0.25Cr0.98O3−δ

Chromium-deficient Nd_0.75Ca_0.25Cr_1−xO_3−δ (0.02 ≤ x ≤ 0.06) oxides are synthesized and assessed as a novel ceramic interconnect for solid oxide fuel cells (SOFCs). At room temperature, all the samples present single perovskite phase after sintering at 1600 °C for 10 h in air. Cr-deficiency significantly improves the electrical conductivity of Nd_0.75Ca_0.25Cr_1−xO_3−δ oxides. No structural transformation occurs in the Nd_0.75Ca_0.25Cr_1−xO_3−δ oxides in the temperature range studied. Among all the samples, the Nd_0.75Ca_0.25Cr_0.98O_3−δ sample with a relative density of 96.3% exhibits the best electrical conductivity of 39.0 and 1.6 S cm⁻¹ at 850 °C in air and hydrogen, respectively. The thermal expansion coefficient of Nd_0.75Ca_0.25Cr_0.98O_3−δ sample is 9.29 × 10⁻⁶ K⁻¹ in the temperature range from 30 to 1000 °C in air, which is close to that of 8 mol% yttria stabilized zirconia electrolyte (10.3 × 10⁻⁶ K⁻¹) and other cell components. The results indicate that Nd_0.75Ca_0.25Cr_0.98O_3−δ is a potential interconnect material for SOFCs.     

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.     

Energy rating of different glazings for Indian climates

In this study, energy rating of different window glazings, available in the Indian market, has been carried out. This rating is helpful in selecting the best window for a given building and a given climate. It is shown that savings by a window w.r.t. the base window (single glazed, clear glass, 6 mm thick), depend upon window type, its orientation, climatic conditions of the place, buildings dimensions and thermal transmittance of its walls and roof. The study has been performed for five different climatic zones of India. Ten types of windows have been studied which include clear glass, tinted glass, low-e coated and solar control windows. Three types of buildings are considered with U-value of their walls ranging 0.52–2.07 W/m²K and U-value of their roof ranging 0.54–2.34 W/m²K. Finally, regression analysis is performed to develop energy rating equations for different glazings, buildings and climates.     

Thermal conductivity measurements of magnesium hydride powder beds under operating conditions for heat storage applications

One of the major issues of the change in energy politics is the storage of renewable energy in order to facilitate a continuous energy supply to the grid. An efficient way to store energy (heat) is provided by the usage of Thermochemical Energy Storage (TES) in metal hydrides. Energy is stored in dehydrogenated metal hydrides and can be released by hydrogenation for consumption. One prominent candidate for high temperature (400 °C) heat storage is magnesium hydride. It is a well-known and investigated material which shows high cycling stability over hundreds of cycles. It is an abundant material, non-toxic and easy to prepare in bigger scales. One of the major drawbacks for heat storage applications is the low heat transfer capability of packed beds of magnesium hydrides. In this work we present results of effective thermal conductivity (ETC) which were measured under hydrogen pressure up to 25 bar and temperatures up to 410 °C in order to meet the operating conditions of magnesium hydride as a thermochemical heat storage material. We could show that the effective thermal conductivity of a magnesium hydride – hydrogen system at 410 °C and 25 bar hydrogen increases by 10% from 1.0 W m⁻¹ K⁻¹ to 1.1 W m⁻¹ K⁻¹ after 18 discharging and charging cycles. In dehydrogenated magnesium hydride this increase of the thermal conductivity was found to be at 50% from 1.20 W m⁻¹ K⁻¹ to 1.80 W m⁻¹ K⁻¹ at 21 bar hydrogen. These data are very important for the design and construction of heat storage tanks based on high temperature metal hydrides in the future.