Thermal transmittance of multiple glazing: computational fluid dynamics prediction

Computational fluid dynamics (CFD) is applied for predicting the convective heat transfer coefficient, thermal resistance and thermal transmittance for a double glazing unit. The predicted thermal resistance of glazing is compared with reference data and good agreement is achieved. The convective heat transfer coefficient and thermal transmittance vary with the air space width and the temperature difference across glazing. The CFD technique can be used to gain insight into multiple glazing performance and also optimise the design and operation of novel multiple glazing systems such as air flow windows or double skin facades in terms of energy efficiency and thermal comfort.     

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.     

Heat transfer analysis of boreholes in vertical ground heat exchangers

A ground heat exchanger (GHE) is devised for extraction or injection of thermal energy from/into the ground. Bearing strong impact on GHE performance, the borehole thermal resistance is defined by the thermal properties of the construction materials and the arrangement of flow channels of the GHEs. Taking the fluid axial convective heat transfer and thermal “short-circuiting” among U-tube legs into account, a new quasi-three-dimensional model for vertical GHEs is established in this paper, which provides a better understanding of the heat transfer processes in the GHEs. Analytical solutions of the fluid temperature profiles along the borehole depth have been obtained. On this basis analytical expressions of the borehole resistance have been derived for different configurations of single and double U-tube boreholes. Then, different borehole configurations and flow circuit arrangements are assessed in regard to their borehole resistance. Calculations show that the double U-tubes boreholes are superior to those of the single U-tube with reduction in borehole resistance of 30-90%. And double U-tubes in parallel demonstrate better performance than those in series.     

Potential application of “see-through” solar cells in ventilated glazing in Hong Kong

This paper reports the findings on the energy performance of “see-through” PV glazing as applied to a typical open-plan office environment of Hong Kong. An experimental system was first set up and the measurements were used to verify the theoretical models developed via the ESP-r simulation platform. The validated models were subsequently used to evaluate the annual variations in thermal loads and electricity generation for two PV glazing systems as compared to the common absorptive glazing provisions. The results indicate that the innovative natural-ventilated PV double-glazing technology could significantly cut down the air-conditioning power consumption by 28%, as compared to the conventional single absorptive glazing system.     

Influence of thermal emittance on the performance of laminated solar control glazing

Influence of thermal emittance on the performance of laminated solar control glazing is presented. A transient one-dimensional mathematical model allowing the prediction of conductive heat transfer within the glazing and convective and radiative heat transfer from the glazing towards the interior and exterior are considered separately. A constant normal incidence of air mass 2 solar radiation of 750 W/m² was assumed. The redistribution of the component of the solar radiation absorbed by the laminated glass and the shading coefficient (SC) were calculated for solar transmittance, 0.05 to 0.35; thermal emittance of the inner surface of the glazing, 0.15 to 0.85; convective heat transfer coefficient for the exterior surface, 10–100 W/m² K and exterior ambient temperatures of 15°C, 32°C and 45°C. The results indicate that as the emittance decreases, the SC decreases by 10–20% for all cases of ambient temperatures considered. The contribution from the convective mechanisms to the heat transfer to the interior is always higher than that from radiative process in the range of ambient temperatures considered. The results presented in this paper would help to decide whether for a given location of interest, the incorporation of a heat mirror glazing would make a meaningful reduction in the cooling load in enclosures with single glazed windows.     

南京地区冬季窗节能研究

根据南京市的典型年逐时气象参数，对于该市的同一座建筑物，应用自编的逐时能耗模拟程序对围护结构冬季采暖期能耗（窗户，墙体和屋顶）进行逐时动态负荷模拟计算，通过逐时负荷计算累加得到不同窗户，墙体和屋顶的冬季采暖期间能耗，结果表明，塑窗比钢窗节能，双层窗比单层窗节能，对单层窗而言，双玻窗比单玻窗节能。     

Evaluation of the thermal performance indices of a ventilated double window through experimental and analytical procedures: Uw-values

Simulations to evaluate energy demand for heating and cooling and thermal comfort are becoming more and more common place in the building design process, at least in the most complex cases. In all detailed or simplified calculations, to analyse heat transfer to and from a building, several input parameters are needed. The inputs for the simulation of a whole building are at least the building geometry, the building envelope thermal indices (like thermal transmittance or the solar heat gain coefficient) and typical local climatic data. In a ventilated double window, the air flow through the channel between the two windows makes its thermal performance highly dynamic and dependent on the air flow characteristics. For a whole building simulation, single coefficients or easily calculated coefficients are needed for each facade system, including ventilated systems. In this paper, equivalent thermal transmittance coefficients for a ventilated double window are assessed and presented. For that, experimental measurements in the absence of solar radiation (night period) were used to identify tendencies and validate calculations. Furthermore, simulations were done in order to estimate the U_w-values of the ventilated double window under different windows configuration and different air flow rates. These values can then be used in whole building simulation programmes.     

Highly insulating aerogel glazing for solar energy usage

Granular silica aerogels have been integrated into highly-insulating translucent glazing. This work was performed within the large R&D project ISOTEG pursued by the ZAE Bayern. To avoid settlement of the granules, which often occurred in earlier glazing concepts and even caused destruction of the glazing, the granules were sandwiched between a double skin sheet made of PMMA. The sheet was mounted between two low-e coated glass panes. To optimize the thermal insulation, krypton was used as filling gas. This construction allows to achieve heat transfer coefficients of less than 0.4 W/(m² K). Optimized granular layers provide high solar transmittance of 65% for a thickness of 20 mm. Thus a total solar energy transmittance of 35% for the whole glazing unit is achieved. The glazing has a thickness of less than 50 mm. Such aerogel glazings can be integrated into solar wall systems or used as lightscattering daylighting elements with vanishing energy losses over the heating period even for north facade integration. Optical and thermal properties of the developed granular aerogels as well as the thermal properties of the whole glazing unit are reported.     

An experimental and analytical evaluation of the insulative properties of double glazing and thick curtains

Theoretical and experimental evaluations are made of thermal curtains applied to double-glazed greenhouse structures. The reflectance and emittance of the curtain is permitted to be different on the two surfaces: It may be partially transparent or opaque to thermal radiation, and it may have thick blanket insulative properties. The effect of cloud cover upon the night-sky radiation temperature is included in the model. Convection coefficients are prescribed for the curtain and glazed surfaces, and the effect of air leakage into the space between the curtain and inner layer of glazing is included as an empirically defined parameter. The thermal balance for the system is obtained in terms of the radiative, convective and air-leakage parameters relevant to each component. The resulting set of thermal balance equations are solved iteratively using existing matrix-solution programs at the University of Waterloo. An experimental rig was constructed to determine nighttime heat loss from a conventional double-walled polycarbonate glazing material which is partially transparent to thermal radiation. The rig was operated with and without an insulating curtain. An opaque multilayer curtain and a semitransparent bubble film curtain were tested. The agreement between the thermal model and the test rig was within ±5% for all cases. Cloud cover was found to be a significant parameter that alters heat loss upwards by 11% from fully cloudy to clear-sky conditions. Intermediate cloud cover conditions were investigated as well and were found to be well predicted by an existing correlation for sky temperature vs cloud cover. The analysis is extended to provide a simple resistance coefficient for heat loss from the glazed structure based upon inside and outside temperatures for a specified cloud cover. It is found that the semiopaque curtain and double-glazed combination has 3.74 times the thermal resistance of a single glass structure as compared to a value of 3.81 for the opaque curtain and double-glazed arrangement. In addition the semiopaque curtain can be used for solar shading during the summer time, thereby serving a dual purpose that significantly increases its economic advantage.     

Heating performance investigation of a bidirectional partition fluid thermal diode

A novel thermal diode, bidirectional partition fluid thermal diode (BPFTD) that is fabricated by integrating a thermal insulation partition and a movable control blade into a water tank, is proposed. The bidirectional configuration allows the BPFTD to serve both passive solar heating in winter and passive cooling in summer. BPFTD heating performances are tested with two side-by-side hot boxes and compared experimentally with a water-wall having optimum thickness. Two stages of experiments are conducted. The first stage is to investigate an appropriate position of BPFTD partition, and the second compares the BPFTD with the water-wall. The test results show that the BPFTD has much better heating performances than the water-wall. Analysis indicates that the BPFTD may increase heat supply by around 140% when a single glazing cover without night insulation is used and by around 70% in case of using a double glazing cover without night insulation.     

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.     

Solar heat gains through train windows: a non-negligible contribution to the energy balance

The sector of transportation accounts for about one third of the total energy consumption in Switzerland. A monitoring campaign of the energy consumption of a regional train revealed the critical energy-consuming systems. Heating, cooling and ventilation were identified as major consumers. Windows are a source of non-controlled heat transfer. In summer, it may result in overheating leading to larger cooling loads while in winter, it is an important source of thermal losses. Selective double glazing and solar protection coatings can reduce these effects. Angular-dependent optical properties of a selective double glazing have been measured, and the solar heat gain coefficient (g value) was determined. An estimation of the solar gains received by a panoramic waggon was performed using the monitored solar irradiation and the measured properties of the glazing. These data were compared to the heating and cooling energy consumption monitored in this waggon. Solar gains were found to be in the same order of magnitude that the heating energy during some sunny days. They were also compared to the estimated thermal losses through the glazing and the entire envelope. These results show that the solar gains play a non-negligible role in the energy balance of the waggon. Furthermore, thermal simulations were performed to evaluate the solar gains in different conditions. It showed that 7 to 13% of energy can be saved using the glazing adapted to the climatic conditions. In addition, improving the thermal insulation of the train envelope or equipping the train with an efficient heat recovery system can lead to significant energy savings.     

The use of a thermal energy recycle unit in conjunction with a basin-type solar still for enhanced productivity

A recently developed thermal energy recycling unit operating under forced air circulation was attached to a conventional, basin-type solar still to enhance overall still productivity. In this unit, a relatively large fraction of the latent heat of condensation of the distillate is utilized to preheat and evaporate the feedstock. The system performance was tested in the laboratory using a solar simulator. The solar still was double glazed and no condensation was observed on the inner glazing when operating in the thermal energy recycling mode. The overall system productivity was about three times that of a conventional (single-effect) basin-type solar still. The advantages of the proposed system design are the following: (i) the solar still productivity can be enhanced significantly and at a reasonable cost; (ii) non-wetting glazings (e.g. certain plastic glazings) can be utilized, since in this mode of operation the glazing does not function as a condensation surface; (iii) as a result, the thermal losses from the outer surface of the glazing to the ambient can be reduced significantly by the use of double glazings; (iv) the system is very adaptable to the utilization of an external waste energy source (e.g. wet steam or hot saturated air) for nocturnal distillation, viz. operation in the absence of solar radiation.     

Thermal performance of an architectural window with chemically deposited SnS---CuxS solar control coating

A mathematical model enabling the prediction of the thermal performance of solar control glazings employing chemically deposited solar control coatings with or without a transparent protective polymer coating is presented. Differential energy balance for the glazing is set up assuming one-dimensional steady state case for normal incidence of air mass 2 solar radiation and by considering conductive heat transfer within the glazing and convective and radiative heat transfer into the interior and exterior of the building. Using the specific example of the optical properties of the already reported SnS---Cu_xS solar control coatings, the redistribution of the absorbed component of the solar radiation is evaluated for constant convective heat transfer coefficient and temperature in the interior and for exterior temperatures in the 0–50°C range. The results yield shading coefficient versus exterior temperature curves for two specific SnS---Cu_xS coatings without and with a protective transparent varnish and offering transmittance in the visible region of 27 and 21%.     

The thermal performance of window coverings in a whole house test facility with single-glazed sash windows

The residential sector is responsible for 29% of the total energy consumption of the UK, with 62% of this energy being used for space heating. Heat loss through the fabric of building elements is a crucial factor in the energy efficiency of homes, and a wide number of studies have looked at physical interventions to improve the energy efficiency of existing buildings, commonly called retrofit. This research considers the impact of window coverings on reducing heat loss from homes, a measure that is not commonly considered an energy efficiency intervention. Although the amount of glazing varies widely between homes, all windows are a significant factor contributing to heat loss. While physical changes such as double and triple glazing can improve the energy performance of buildings, the impact of curtains and blinds is not well characterised. Previous research into window coverings has been undertaken using laboratory tests, such as hotbox and small climatic chamber environments. This study presents the impact of window coverings on heat loss within a unique whole house test facility. This allows for a better replication of a real heating system and the effects that it has on localised heat transfer. This gives a more detailed picture of in situ performance, similar to that which may be found in the field.     

Analysis of a ventilated solar evaporator for concentrating dilute hazardous waste

A mathematical model of a ventilated solar evaporator for concentration of dilute waste is presented. The design is similar to conventional solar still design, but allows for air flow in order to enhance the evaporation rate. The model consists of energy and mass balances on the basin, liquid concentrate, air and glazing. The model shows that the thermal efficiency of the evaporator is enhanced by convective air flow. Single cover evaporators are more efficient than two cover evaporators. Energy storage in the basin reduces efficiency.     

Complex multimaterial insulating frames for windows with evacuated glazing

The thermal performance of a complex multimaterial frame consisting of an exoskeleton framework and cavities filled with insulant materials enclosing an evacuated glazing was simulated using a two-dimensional finite element model and the results were validated experimentally using a guarded hot box calorimeter. The analysed 0.5 m by 0.5 m evacuated glazing consisted of two low-emittance film coated glass panes supported by an array of 0.32 mm diameter pillars spaced 25 mm apart, contiguously sealed by a 10 mm wide metal edge seal. Thermal performance of windows employing evacuated glazing set in various complex multimaterial frames were analysed in detail. Very good agreement was found between simulations and experimental measurements of surface temperatures of the evacuated glazing window system. The heat loss from a window with an evacuated glazing and a complex multimaterial frame is about 80% of that for a window comprised of an evacuated glazing set in a single material solid frame.     

Triple vacuum glazing: Heat transfer and basic mechanical design constraints

Given the major role played by windows with regard to energy losses from buildings in cold climates, low thermal transmittance is an indispensable property of glazing in low-energy buildings. Evacuation offers the only means of achieving negligible gaseous conduction in glazing cavities. Application of low-emittance coatings to glass sheet surfaces inside the cavity reduces the radiative heat transfer. The feasibility of double vacuum glazing using arrays of support pillars between the glass sheets has been shown by other authors. This type of glazing is commercially manufactured today. Based on these achievements, our study set out to investigate heat transfer in triple vacuum glazing by means of (i) an analytical thermal network model and (ii) a numerical finite difference model. The study focused on the impact of the following parameters on thermal transmittance: emittances of glass sheet surfaces inside the cavity, support pillar radius, support pillar separation and thermal conductivity of support pillar material. The design procedure for triple vacuum glazing taking into account not only thermal but also mechanical stresses due to atmospheric pressure, i.e., to enable identification of favourable parameter sets, is presented. Our findings suggest that use of the triple vacuum glazing concept can significantly reduce the thermal transmittances achieved by the best insulation glazing units currently on the market. E.g., a centre-of-glazing thermal transmittance of less than 0.2 W m⁻² K⁻¹ is achievable using stainless steel support pillars, 6 mm/4 mm/6 mm sheets of untempered soda-lime glass and four low-emittance coatings (ε = 0.03).     

Solar sorption refrigerator

The performance of a solar sorption refrigerator is studied experimentally for three configurations of its collector cover: with single glazing, with double glazing and with single glazing plus transparent insulation. The collector consists of fifteen stainless steel tubes having a selective surface (with good thermal absorption coefficient) and contains granular activated carbon adsorbent with ammonia refrigerant. The collector surface area is approximately 1.43 m² and contains about 17 kg of carbon. The collector operates with good efficiency (36 to 47% depending on conditions) when in the single glazing configuration.     

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。