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.     

Evaluation of predictive models for the angle-dependent total solar energy transmittance of glazing materials

The predictions of angle-dependent optical properties of glazings are discussed. A categorisation of windows depending on the type of coating on the glazing is discussed as a way of improving the accuracy in the predictive models. Four approximate ways to predict the angle dependence of the total solar energy transmittance are compared. The impact on the energy performance of windows with different angle dependence is assessed in a heating and a cooling dominated climate, respectively. Results imply that by simply using the clear glass angular profile for all types of windows gives quite low errors in the angle dependence prediction, lower than some other previously proposed models. By using a model with window category as input, the errors in angle dependence prediction can be further reduced. The impact on the energy performance from incorrect angle dependence is considerable in some cases but not necessarily critical.     

Thin film angular selective glazing

Angular selective glazing based on special obliquely deposited thin films, has considerable potential for optimizing the overall performance of a window in temperate to hot climates. In contrast to most currently used glazings for these environments it has the ability to control solar heat gain and glare while providing a clear view through the window and considerable but not excessive light gain. Various deposition techniques and film materials will be discussed and the spectral and integrated optical performance of single and double-layered systems presented for various directions of incidence. Full optical characterisation requires measurements for incidence directions over half a hemisphere. Some of this is presented but most data is confined to planes of incidence where angular selectivity is greatest as determined by deposition geometry. Two layer systems are designed to provide a low emittance and better solar control than the single layer systems while retaining good visible angular selectivity. The single layers are primarily to provide angular selective control of the visual radiation since they have moderately high transmittance in the near infrared and a high emittance. Theoretical models used to extract structural information and model the complex optical properties are introduced and their predictions assessed. Preliminary results on simulation of the performance of full size angular selective windows in comparison to other types of solar control glazing such as low e heavily tinted windows are discussed in terms of their relative impact on cooling load, lighting contribution, glare control and total energy savings.     

A single-thin-film model for the angle dependent optical properties of coated glazings

Technology for the manufacture of coated glazings with spectrally selective optical properties, such as low-e and solar-control glazings, has been developed in the last few decades. This is leading researchers to develop new optical and thermal models in order to ascertain glazing performance. These new models must accurately reproduce the optical properties for any incident solar angle by using the available experimental data, which often means only information for normal incident radiation. In this paper, a new model is presented that characterizes the angular dependence of coated glazings. To provide a simple, intuitive understanding, this model uses only one thin film to characterize optical performance. In addition an optimization algorithm has been developed to obtain the spectral optical properties of that equivalent film using spectral experimental data under normal incidence. Finally, the model is validated with experimental data and the results are compared with other known models.     

Facade systems with variable solar control using thermotropic polymer blends

Thermotropic layers integrated into glazing or transparent insulation systems reversibly reduce the total solar energy transmittance by becoming opaque when heated. Characteristics of both systems have been measured as a function of temperature and incidence angle, including the total solar energy transmittance for a thermotropic heat-mirror insulating glazing unit (IGU). Properties of complex thermotropic glazing systems have been simulated, starting from data characterising the thermotropic layer. The results show good agreement with measured data. A prototype system sample of a thermotropic heat-mirror IGU was integrated into an outdoor test facility and a thermotropic TEIF (transparent exterior insulation and finish) prototype system was installed into the facade of a demonstration house. Both systems have been monitored and showed the expected switching properties, resulting in the desired overheating protection. Computer simulations of the building integration of the thermotropic TEIF system have been performed, showing a strong dependence of the optimum switching temperature on the total solar and internal gains of the building.     

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.     

Effect of solar control glazings on human skin temperature

Human skin temperatures were measured under exposure to near-infrared radiation through solar control glazings containing SiWO_x films as solar control materials. It was found that the increase in skin temperature corresponds to the solar transmittance (T_s) value of the glazings. When the hand was exposed to radiation through a glazing containing solar films, the skin temperature after five minutes was less 0.9–1.8°C lower than when the corresponding glass without the films was used. The solar control glazings were effective in reducing the increase in skin temperature.     

A new correlation for direct beam solar radiation received by photovoltaic panel with sand dust accumulated on its surface

This work investigates experimentally and mathematically the effect of sand dust layer on beam light transmittance at a photovoltaic module glazing surface. A transmittance coefficient for beam light has been found with respect to the number of sand dust particles per unit area of glazing surface, size of the particles, beam light incidence angle and wavelength. It has been shown that the incidence wavelength has a negligible effect on the light transmittance in the visible region due to the fact that the size of the sand dust particles was much greater than the incidence wavelength. A new correlation has been found to help photovoltaic system designers to predict the amount of beam solar radiation that would reach the surface of the modules as a function of sand dust particles size and amount accumulated on the surface when they are used in dusty environments. Furthermore, the results from this work could improve the predictive capability of existing solar energy simulation models by incorporating the factors which account for sand dust accumulation on photovoltaic surfaces.     

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%.     

Angular dependent optical properties from outdoor measurements of solar glazings

A method for careful outdoor characterization of the angular dependent solar transmittance of solar glazing is developed and used for investigating the performance of structured and antireflection treated glass. Two identical collectors are series connected in order to get an identical flow. The reference collector is always covered with a thin Teflon foil and the test collector is furnished with the investigated covers. The method is carried out in six steps: (1) The collectors are long term tested in order to get the collector parameters, especially the U-values. (2) The performances of the collector at a temperature close to ambient are monitored during a clear day. (3) The effect of heat losses is eliminated from the knowledge of the U-values, and the optical efficiencies of the collectors are derived. (4) The ratio between optical efficiency of the test — and the reference collector is calculated. (5) Both collectors are tested with a Teflon film, with well-known optical properties, as a cover and the relative optical efficiency is derived. (6) From the relative transmittance between the investigated glazings and Teflon, the absolute value of the angular dependent transmittance is derived. The results show good agreement with conventional optical characterization. The commercial antireflection treatment increases the solar transmittance by 4% and the annual output by 9%. The structured glass shows similar angular performance as a flat glass if the structure is faced outwards, but shows a lower transmittance for incidence angles exceeding 40° if the structure is faced inwards. This means that the annual performance is decreased by 4% by facing the structure inwards instead of outwards.     

Monolithic silica aerogel in superinsulating glazings

Silica aerogel is an open-pored porous transparent material with optical and thermal properties that makes the material very interesting as an insulation material in windows. A number of different aerogels have been investigated for their optical and thermal performance. High thermal resistance of aerogel was found for all the investigated samples and the samples showed very high solar as well as light transmittance. However, all the investigated aerogel samples showed a tendency to scatter the transmitted light, resulting in a reduced optical quality when the aerogels are integrated in glazings. This phenomenon is considered as being the main obstacle to incorporating the material in clear glazings, but a significant improvement of the optical quality of aerogel has been observed during the last five years. A number of prototypical evacuated 500×500×28 mm³ aerogel double glazed units employing a new edge seal technique were manufactured and characterized for their optical and thermal properties. As expected, the same scattering of light was found in the aerogel glazings as in the aerogel samples, but excellent thermal performance was found, indicating a glazing type that, from a thermal point of view, is without competition in heating dominated climates.     

Energy performance of glazings and windows

The purpose of the work is to make it possible to compare the energy performance of different glazings or windows in an easy way. The energy performance is expressed by the net energy gain, which is given as the solar gain minus the heat loss. The net energy gain from glazings and windows both depend on the thermal transmittance (the U-value) and the total solar energy transmittance (the g-value). This fact makes it difficult to choose the glazings or windows with respect to energy performance in a given case. To compare different glazings or windows in an easy way, diagrams have been produced which give the net energy gain based on the orientation, the tilt, the U-value and the g-value of the glazings or windows. In addition, a single diagram showing the net energy gain in a one-family house has been produced which takes into account the orientation of the windows in the building. This makes it possible to evaluate the energy performance of different glazings or windows for a one-family house using a single diagram. The diagrams give an easy way of comparing different glazings or windows so the best solution in a given case can be chosen. In addition, the diagram showing the net energy gain in a one-family house is used in the Danish Energy Labelling and Rating system for classification of glazings.     

Cost-effective designs of solar air heaters

We discuss some aspects of solar air heaters, which could lead to more economical designs. It is shown that a partial flow channel between two glazings increases the efficiency of the system. This procedure may eliminate the need for insulation material, at least on the two sides of the space between two glazings. The effect of glazing absorptance on system performance is also investigated. We conclude that the upper glazing should be as transparent as possible while the lower glazing should have higher absorptance.     

A test method to evaluate the thermal performance of window glazings

An apparatus and a methodology to evaluate the thermal performance of window glazing are presented. Single glazings commercially available in Mexico are currently tested. During the test sequence, the glass samples are mounted in a specially designed calorimeter apparatus. The test is conducted in controlled laboratory conditions at the National Centre for Research and Technology Development in Mexico using a solar simulator test lamp. The calorimeter apparatus and the solar simulator test lamp were characterised and the overall heat loss coefficient U_c measured was of 1.7±0.1 W/m²°C. Overall heat transfer and shading coefficients are derived from the experimental results. The test method described allows the testing of practically any kind of glazing array. Glasses under investigation were of the reflective, absorbing coloured and common ones. This test method can be adequate to evaluate film coating glazing or multiple solar control coating glazing. Also, it can be of assistance to researchers, glass manufacturers and building designers in the development of rating and comparing of glazing options.     

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.     

Initial experimental tests on the use of small passive-solar test-boxes to model the thermal performance of passively solar-heated building designs

Small passive-solar test-boxes can be a useful tool in the thermal-modelling of a passively solar-heated building designs. Several cubic-meter sized test-boxes have been constructed that incorporate basic elements of passive-solar design: glazings to admit solar radiation, good thermal insulation, and large thermal storage capacity within the insulation-envelope. The experimental thermal behavior of these text-boxes has been accurately computer simulated using weather and solar radiation data inputs, and correlated with the performance of large passive test rooms nearby. Similar small test boxes can be constructed to perform as thermal models of a particular passive-solar building design, by normalizing the appropriate thermal design parameters to one critical parameter, such as south-facting glazing area, or building floor area. Thus a thermal model will not resemble a scaled architectural model of the building, but will be distorted in appearance so as to exhibit thermal behavior similar to the real building. For a particular passive-solar building design to be located at a given site, a small thermal model of the building can be constructed and placed at the site to investigate possible micro-climate effects, such as a south-facing slope or nearby trees. The use of small passive-solar test-boxes complements simulation analyses of the influence of macro-climate and geometry on the performance of passive solar heated buildings. The accuracy and limitations of using small test-boxes to predict passive-solar building performance are discussed.     

Angular-dependent optical properties of low-e and solar control windows—: Simulations versus measurements

The angular-dependent optical properties of low-e and solar control glazings have been investigated in a European project, ADOPT, within the Standards Measurements and Testing programme. The object of the project has been to identify reliable ways of predicting the angular dependency without having to perform measurements or detailed calculations. Two new predictive algorithms have been developed and validated. For the investigated coatings the accuracy of these predictive algorithms is mostly within 1% of the value obtained by measurements or Fresnel calculations. This is an improvement over previously used algorithms, which have failed to distinguish between different types of coatings. The improved accuracy is of importance in energy simulation of buildings and makes improved product specification possible.     

Improvement of solar stills by the surface treatment of the glass

Glass still glazings are treated with sodium silicate or hydrofluoric acid to make them more wettable. Consequently, the angle of inclination can be reduced to 1·5°. Stills of low glazing height are ammenable to diffusion mass transport which is expressed for the natural parameters of air temperature, wind, and solar energy intensity and for vapor length.     

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.