Smart switchable glazing for solar energy and daylight control

The exciting field of chromogenic materials for smart windows and other large-area applications is discussed. A selection of switchable glazing devices that change color electrically are detailed. The types of devices covered are the electrochromic which change color electrically, covering electrochromic, dispersed liquid crystal and dispersed particle glazing that switch under an applied electric field. Device structures and switching characteristics are compared. The status of prototype and commercial devices from commercial and university labs through out the world are covered. A discussion of the future of this technology is made including areas of necessary development for the realization of large area glazing in excess of 1m².     

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.     

Optimum glazing combination for solar space heating

A study is made for comparing the maximum seasonal energy yield obtainable by solar collectors for space heating application. Different glazing combinations with glass and plastic as glazing materials are considered. The study is made for four different locations. The performance of eight glazing combinations with covers ranging in number from one to three is compared to obtain the optimum combination for each location. The results show that selecting the optimum glazing combination improves the performance significantly. In general, plastic covers give higher yield. The study confirmed that the use of two covers is justified in cold, cloudy climates while a single cover is suitable for temperate climates. In most cases three covers lead to a significant reduction in the yield. Replacing plastic by glass as a top cover for longer life results in a small yield reduction. Some of the other conclusions are that the ratio of average to normal transmittance-absorptance product changes significantly with location and month of the year. However, the seasonal average value of this ratio is almost constant for any number of covers but changes with location.     

Vacuum glazing units and solar collectors

Possible application of vacuum glazing (VG) units for transparent insulation of solar collectors (SC) to reduce their heat losses is considered. Taking into account the SC operating conditions, the heat insulating parameters of VG samples measuring 0.3 × 0.3 m² and 0.5 × 0.5 m² are experimentally studied and compared to those of double glazing (DG) of the same size. Basing on the experimental data, the order of the vacuum in the VG is calculated. The contribution of the vacuum gap to the total heat transfer resistance of the VG is evaluated.     

Modelling and simulation of elements for solar heating and daylighting

Through the development of highly efficient transparent insulation materials (TIM), new opportunities are appearing in the field of daylighting and passive solar space heating. The simulation program WANDSIM, developed at the Fraunhofer-Institut für Solare Energiesysteme (ISE), models the dynamic performance of three important elements for daylighting and passive solar space heating: window glazing; transparently insulated masonry; transparently insulated glass wall. Selected simulation results of each type are represented and compared under thermal and daylighting aspects. The advantages of the transparently insulated glass wall, a new combined passive space heating and daylighting system, in economy and comfort are verified.     

Assessing industrial energy conservation by unit processes

This paper develops a methodological framework called cost-energy dynamics (CED) for assessing industrial energy conservation potential and cost. CED combines profitability and process analyses, looks at industrial energy consumption as consisting of end-use unit operations, links public and private decisions by internalizing the tax rate, and judges energy conservation measures in a continuous spectrum of cost effectiveness. CED provides a basis for estimating the price elasticity of energy demands, for quantifying the effectiveness of government measures, and for estimating the costs if the U.S. manufacturing industry is to save an additional 20% of energy per unit output between 1980 and 2000.     

Silicone glazing for solar applications in rural areas

Silicone glazing is a translucent glass fabric reinforced material which was produced by coating silicone resin 1-2577 on an open weave leno fabric in a coating tower constructed in Basaisa village, Al Sharkiya Governorate, Egypt. The unique feature of the tower used in this work is the utilization of solar energy for both powering its coating mechanism through the use of a photovoltaic module, and also heating its curing chamber. The optical and mechanical properties of silicone glazing were studied. Silicone glazing is found to have a solar transmission of 90%, an ultraviolet cut-off at 270 nm, and an infrared cut-off at 8.0 μm. The material has a high tensile strength, particularly along the fill and the wrap directions of the reinforcing fabric. The tensile strength tested at 0.8 strain rate is 50, and 80 pli (pounds per lineal inch) at the fill and the wrap directions, respectively. Silicone glazing was found suitable for many solar applications such as greenhouse screen, space solar heating, solar food driers, and skylights in buildings, especially in rural areas.     

Durability of polymeric glazing materials for solar applications

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

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.     

Passive solar systems for heating, daylighting and ventilation for rooms without an equator-facing facade

Spaces without northerly orientations have an impact on the ‘energy behaviour’ of a building. This paper outlines possible energy savings and better performance achieved by different zenithal solar passive strategies (skylights, roof monitors and clerestory roof windows) and element arrangements across the roof in zones of cold to temperate climates typical of the central and central-southern Argentina. Analyses were undertaken considering daylighting, thermal and ventilation performances of the different strategies. The results indicate that heating, ventilation and lighting loads in spaces without an equator-facing facade can be significantly reduced by implementing solar passive strategies. In the thermal aspect, the solar saving fraction reached for the different strategies were averaged 43.16% for clerestories, 41.4% for roof monitors and 38.86% for skylights for a glass area of 9% to the floor area. The results also indicate average illuminance levels above 500 lux for the different clerestory and monitor arrangements, uniformity ratios of 0.66–0.82 for the most distributed arrangements and daylighting factors between 11.78 and 20.30% for clear sky conditions, depending on the strategy. In addition, minimum air changes rates of 4 were reached for the most extreme conditions.     

An analysis of daylighting and solar heat for cooling-dominated office buildings

Computer simulation techniques were used to assess the energy performance of a generic commercial office building in Hong Kong. The simulation tool was DOE-2.1E. The thermal and energy performance of daylighting schemes were analysed in terms of the reduction in electric lighting requirement and the cooling penalty due to solar heat. Regression analysis was conducted to correlate the peak electricity demand and annual incremental electricity use with two fenestration variables, namely the solar aperture and the daylighting aperture. Contours of equal annual incremental electricity use were shown to be a function of the solar and daylighting apertures. It is envisaged that these simple charts can be a useful design tool for architects and engineers to assess the relative energy performance of different fenestration designs, particularly during the initial design stage when different building design schemes and concepts are being considered and developed.     

A demonstration site-built solar air collector using selective surface glazing, Boston, Massachusetts

A 450 ft ‘site-built’ air collector was completed in February 1981 in Boston, Massachusetss. The collector uses the Airco/Guardian ‘Passive Solar Glass’ with a selective surface coating on the inner lite of the collector's double glazing. This paper discusses the issues and theory that led to the building of the collector and estimates performance for the system. Preliminary data taken in late winter indicates that the system will perform seasonally as predicted.     

Heating of a biogas reactor using a solar energy system with temperature control unit

Maintaining a temperature level of about 40°C, the optimum temperature for mesophilic bacteria’s growth and activity, is crucial for obtaining the best results with respect to biogas production. This study investigates the utilization of solar energy for heating a bioreactor that was already developed at our laboratory and is currently in operation. For purposes of this study, a solar collector combined with a heat exchanger are designed, manufactured, and installed to provide the necessary heating requirements for a water jacket around the bioreactor. In addition, an inexpensive PID (proportional. integral, and differential) controller was also designed, installed, and tested to maintain a constant temperature of 40°C in the water-jacket around the bioreactor. The theoretical efficiency of the solar collector was computed and found to be about 61%. The controller action was seen to be very effective and a prompt response for little variation in the temperature of the water jacket (ΔT=1 K) was noticed. Moreover, the internal rate of return of the investment in such solar heating system was found to be 32.7%, which indicates that we have an attractive investment.     

An empirical validation of modeling solar gain through a glazing unit with external and internal shading screens

Empirical validations are integral components in assessing the overall accuracies of building energy simulation programs. Two test cell experiments were performed at the Swiss Federal Laboratories for Material Testing and Research’s (EMPA) campus in Duebendorf, Switzerland to evaluate the solar gain models with external and internal shading screens in four building energy simulation programs including: (1) EnergyPlus, (2) DOE-2.1E, (3) TRNSYS-TUD, and (4) ESP-r. Detailed information about the shading screen properties, modeling procedures, and thorough statistical and sensitivity analyses of simulation results are provided. For the external shading screen experiment, the mean percentage of the absolute difference between measured and simulated cooling power to maintain a near-constant cell air temperature for EnergyPlus, DOE-2.1E, TRNSYS-TUD and ESP-r were 3.7%, 5.5%, 10.6%, and 7.5%, respectively. EnergyPlus and DOE-2.1E were considered validated within 95% credible limits. For the internal shading screen experiment, the mean percentage of the absolute mean differences for EnergyPlus, DOE-2.1E, TRNSYS-TUD, and ESP-r were 6.7%. 13.8%, 5.7%, and 4.3%, respectively; only ESP-r was considered validated within 95% credible limits.     

Household energy conservation and locus of control: A research note

Five hypotheses were tested to investigate whether personality is a factor in household energy conservation beliefs and actions. Locus of control, the personality measure which distinguishes between individuals designated as internals or externals, was assessed in a survey of 383 households in Regina, Saskatchewan, Canada. The results indicate that only one hypothesis—obtaining information on energy issues from the media is a positive function of internality—reached significance, and one other hypothesis tended toward significance. We conclude that locus of control is a limited predictor of household energy conservation beliefs and actions, and recommend that energy conservation programme planners consider persuading externals to behave more internally.     

TIM–PCM external wall system for solar space heating and daylighting

An external wall system for solar space heating and daylighting composed of transparent insulation material (TIM) and translucent phase change material (PCM) is presented. This system enables selective optical transmittance of solar radiation. Visible light is mainly transmitted and invisible radiation is mainly absorbed and converted to heat, causing in particular phase change. The storage medium is also the absorber. The concept of the system is presented in detail together with the investigations carried out, including a brief outline of modeling, optical experiments on PCM samples and long-term experiments on a prototype wall as well as numerical simulations. The results indicate a promising thermal–optical behavior of the system. For instance in a Swiss lowland climate (Zurich-airport) a mean energy flux of 13 W m⁻² (system efficiency 0.27) was calculated through a south facing TIM–PCM wall into the building during the month with the lowest irradiation (December). The parameters of the prototype wall with a mean melting temperature of the PCM of 26.5°C were assumed. When considering the percentage of time in which the building does not lose energy through the south facing TIM–PCM wall, a maximum can be reached with a mean melting temperature of approximately 20 to 21°C. In this case energy losses through the façade occur only during 1% of the time. With regard to the practical application of the system in buildings, aspects of reliability and durability have to be further investigated.     

Workplane illuminance prediction method for daylighting control systems

A new prediction method with an interior light sensor for the workplane illuminance in daylighting control systems is introduced. Based on radiosity theory, the spatial distributions of daylight and electric light in the space are discussed and the prediction method is developed. An experimental verification of the technique is performed in an outdoor test-room for a double-glazed window system with a motorized venetian blind integrated between the two glazings. Electric light and daylight predictions were performed in accordance with the proposed method. The results showed that the electric light workplane illuminance can be predicted with high accuracy (±5 lux error with a linear correlation) and that the daylight workplane illuminance can be predicted within ±20 lux (with a linear correlation) for any sky conditions (overcast, clear sky, or variable) with the window system controlled to prevent direct daylight transmission. This method is useful for integrated control of motorized daylighting devices and dimmable electric lights.     

Switchable glazing with a large dynamic range in total solar energy transmittance (TSET)

Modern, energy-saving buildings incorporate large areas of highly insulating glazing. The resulting solar gains lead to major savings in heating energy during winter, but protection against overheating in summer is also needed. Usually this problem is solved by using mechanical shading devices, with the disadvantages of high cost and low durability.The work on switchable glazing at Fraunhofer Institute for Solar Energy Systems, in cooperation with industrial partners, aims to present new and viable alternatives. Two types of switching layers, which are quite different in their structure and function, but are similar in having a large dynamic range in TSET, are being investigated—gasochromic and thermotropic.Gasochromic windows are actively switched between a clear and a coloured (but image-preserving) state by alternately introducing strongly diluted O₂ and H₂ gases. In contrast to classic electrochromic configurations, only one tungsten oxide film with a very thin catalyst coating is needed. At present, prototype windows with an area of 1.1×0.6 m² are being produced by sputtering. Careful adjustment of the layer structure, the gas concentration and its flow velocity is needed to obtain the desired switching rate. Homogeneous colouring of the whole area within seconds has been achieved. In addition to information on the colouring kinetics, the paper also discusses system aspects of these windows.Thermotropic layers switch reversibly and automatically, from a clear state with high transmittance to a milky white state with high diffuse reflectance, when their temperature rises. Depending on the composition of the material, the switching temperature can be chosen in the range needed. The measured optical properties of glass laminates with a thermotropic layer are presented. These are combined with the measured values for further panes to calculate the characteristic data for thermotropic insulated glazing units. The results are compared with those measured on a 1.1×1.65 m² prototype window. Stability results are also included.The effect of the two different types of switchable glazing on building energy savings is explored for a residential building model, using the TRNSYS building energy simulation program.