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Advanced glazing technology for low energy buildings in the UK
Renewable Enersy, Vol.5, P m I, pp. 298-309, 1994 Elsevier Science Lid Printed in G r ~ Britain 0960-1481/94 $7.00+0.00
Pergamon
ADVANCED GLAZING TECHNOLOGY FOR LOW ENERGY BUILDINGS IN THE UK
P D Robinson BSc PhD MlnstE M G Hutchins BSc PhD Advanced Glazing Technology Group School of Engineering Oxford Brookes University Oxford OX30BP
Summary Significant advances have been made in glazing technology in recent years which will impact enormously on the way building facades and control systems interact with solar radiant heat and daylight, and which will change building heat loss characteristics. This paper provides background to the current research activities in the field of advanced glazing technology for reduced energy consumption in buildings. A report predicting potential energy and environmental benefits from the uptake of advanced glazing in the UK is outlined, a world collaborative research programme is described and details are given of an organisation established in the UK to disseminate advanced glazing research and development results and applications guidance to the fenestration and building design professions. Technologies of relevance to the context of UK buildings are discussed.
298
299
Introduction Significant advances have been made in glazing technology in recent years which impact enormously on the way building facades and control systems interact with solar radiant heat and daylight, and which change building heat loss characteristics. These changes have resulted from solar control glasses, insulating glass units, low emissivity coatings and gas cavity fills. The pace of change will accelerate over the next few years and will intimately affect the way that buildings receive and utilise solar radiant heat and daylight, as spectrally selective coatings improve and diversify in function, light redirection systems and glazings with switchable transmission properties become viable products, and more highly insulating windows become available using the technologies of transparent insulation, evacuated glazing, aerogels, and improved frame and spacer design. Advanced glazing materials such as these are being developed world-wide and will potentially realise large energy and environmental benefits in residential and commercial buildings. A study of the potential energy and environmental benefits of advanced glazing in the UK was carried out in 1992 (1) for the DTI's Energy Technology Support Unit. The main findings are outlined below. Energy and Environmental Benefits of Advanced Glazing in the UK DOMESTIC SECTOR ASSESSMENT The main criteria used to assess the energy and building environment implications were annual auxiliary heating energy and overheating (hours over 27oc). The assessment was carried out using the dynamic building simulation model SERI-RES (2) and a method of estimating national benefits developed by ETSU (1).
The simulation results for the specific advanced glazing technologies were interpreted into national benefits using conjectured market penetration curves. The technologies considered specifically excluded transparent insulation material applied to opaque walls which was being assessed in another study (3). The national implications of the adoption of advanced glazing systems in the UK domestic sector were predicted to be significant: By 2025 the annual saving in primary energy was predicted to be over 4MTCE/yr. (MCTE is million tonnes of coal equivalent). The net present value of the accumulated saving up to 2025 was predicted to be £2,511M. The predicted total accumulated saving in CO 2 by 2025 was 84MTonnes. The yearly saving of 7MT/yr. of CO2 is equivalent to 9% of the annual CO 2 emissions from space heating in the UK, estimated by the BRE from 1987 energy consumption data (4). OFFICE SECTOR ASSESSMENT The main criteria used to assess the energy and building environment implications of the advanced glazing systems in the office building sector were annual auxiliary heating energy (kWh/m 2 per annum), annual overheating (occupied hours over 27oc), annual lighting energy (kWh/m 2 per annum), annual cooling energy (kWh/m 2 per annum).
300
The national implications of the adoption of advanced glazing systems in the UK office sector are significant in the context of the market volume: By 2025 the annual saving in primary energy was predicted to be nearly 0.2MTCE/yr. The net present value of the accumulated saving up to 2025 was predicted to be £83Mo The predicted total accumulated saving in CO2 by 2025 was 5.2MTonnes. The assessment for the office sector is probably an underestimate as no account was taken of light redirection technologies which will improve daylight penetration into offices. Other areas of the non-domestic building sector were not addressed. CONCLUSIONS The study concludes that the benefits for the domestic and office sectors are unlikely to be achieved if the market is left to its own devices. It makes the point that the role of national and local government will be vital to set the framework to encourage the uptake of these new products. A number of approaches were suggested:
energy performance rating for buildings and building components such as windows (which should account for passive solar benefits) support for advanced glazing research, development and demonstration collaboration in international RD & D programmes in order to share experiences promotion of successful products within the framework of best and good practice legislation establishing energy targets for buildings and building components (windows should be double Iow-e glazed as a minimum standard) As a result of the promising predictions from the study, the UK DTI decided to fund the operating agency of the new International Energy Agency Task on Advanced Glazing Materials in the form of Prof MG Hutchins of Oxford Brookes University. lEA Task 18 Advanced Glazing Materials Task 18 of the International Energy Agency Solar Heating and Cooling Programme is the World's largest collaborative research and development project on Advanced Glazing Technology. Fifteen OECD countries have committed more than 100 person years of effort to this five year project which started in 1992. The objective of the Task is to develop the scientific, engineering and architectural basis which will support the appropriate use of advanced glazings and associated materials in buildings with the aim of realising significant energy and environmental benefits. Building energy analysis tools are being employed to identify appropriate applications and predict energy and environmental impacts which will derive from the use of advanced glazing products. Task 18 has a specific focus on the application
301
and technology transfer of new materials and components with an emphasis on nearmarket technologies. The Task aims to provide guidance for design engineers, building engineers and industry on the properties, use, performance and selection of advanced glazing materials. Necessary measurable parameters for specification of the thermal performance of advanced glazing materials are being identified and defined and appropriate measurement test procedures developed. The work of Task 18 is managed under two Sub tasks, each made up a number of activities:
Sub task A Applications Assessment and Technology Transfer A1 Applications, potentials and characteristics A2 Modelling A3 Control strategies A4 Environmental and energy impacts A5 Applications guidance Sub task B Technology Case Study Projects B1 Monolithic and granular aerogels B2 Geometric media (honeycombs and capillary structures) Chromogenic glazings B3 Low-emittance coatings B4 Evacuated glazings B5 B6 Advanced solar collector covers B7 Angular selective transmittance coatings B8 Daylighting materials and systems B9 Frame and edge seal technology B10 Advanced glazing materials properties database and technology summaries Bll Investigation of the optical properties and scattering behaviour of advanced glazing materials B12 Measurement of the total energy transmittance of advanced glazing systems B13 Directional optical properties measurements of advanced glazing materials B14 Measurement of the U-value of advanced glazing systems The goals of Sub task A are to identify appropriate applications, determine the energy and environmental implications and provide applications guidance in the area of advanced glazing materials. Work in Sub task A will enable: Investigation of the technical and economic potential of advanced glazing systems. Establishment of the physical properties of advanced glazing materials and control strategies critical to performance. Evaluation of design tools and provision of applications guidance to aid the selection of advanced glazing systems. Study of the climate dependence of performance for defined building types and reference zones. Comparison of performance obtained with different materials.
302
Assessment of cost and benefits and the setting of target costs against other solutions. Determination of the influence of degradation on thermal performance. In Sub task B each Case Study Project integrates a series of materials development and measurement activities to enable an extensive, in-depth determination and examination of materials properties and performance levels for potential use in advanced glazing systems. Each Project is undertaking measurement and characterisation work concerned with basic materials properties, e.g. optical, thermal, mechanical etc. In addition to providing basic characterisation, such measurements may be carried out to analyse degradation mechanisms and determine failure modes induced by ageing tests. Where possible measurements are being performed on large area samples of advanced glazing materials and there is particular emphasis on investigating the properties of whole windows comprising advanced glazing materials and their associated frames and sealant materials. For those materials which are near-market, prototype glazing systems are being constructed to enable the measurement of the gross optical and thermal properties necessary to specify the performance of advanced glazing systems. The influence of the boundary conditions is being investigated and the accuracy of appropriate values of key glazing parameters for use as input data for the modelling studies to be carried out in Sub task A is being established. Two key parameters, the total energy transmittance and the U-value, are regarded as necessary for characterisation and subsequent prediction of the energy performance of a glazing system and the building of which it forms part. Light redirection technologies are more problematic to characterize, with their directional properties. Advanced glazing materials present difficulties for the measurement of total energy transmittance because of internal absorption, scattering and incident angle effects. Total energy transmittance is being measured from radiation measurements and by calorimetric methods. The low thermal transmittance of advanced glazing materials mean that it is important to determine not just the U-value of such materials but the influence of the frame on the total heat transfer. Task 18 aims to determine the influence of centre versus edge effects and boundary conditions employed in the measurement of the U-value of advanced glazing systems. Calculated window performance parameters are being determined using the WINDOW 4 (5) VISION (6) and FRAME (7) computer programmes. Switchable glazing systems, based on electrochromic, thermochromic and liquid crystal materials, are being investigated together with control strategies necessary for their effective operation. Links with Industry Industry is represented in the Task through technical participation in many of the projects and through the provision of samples and the customising of windows for test purposes. Several participating countries are supported with the help of industrial funds. Industry clubs have been formed in several countries, most notably in Australia and the United Kingdom, to aid the dissemination of information arising from the work of Task 18.
303
UK Advanced Glazing Industry Club The UK AGIC was established in 1993 to promote awareness within the UK Fenestration Industry (i.e. glass and glazings manufacturers, designers, architects and end-users of the technology) of current international best practice and new developments for improved thermal and optical performance of windows. It allows exchange of information between the lEA Task's researchers from OECD countries and the UK Industry. AGIC will play an important role in the dissemination of knowledge and identification of appropriate applications of advanced glazing products. The UK, as it is funding the Operating Agency of the research programme, is in a pivotal position to influence the prioritisation of work to be undertaken in lEA Task 18, and the forms of applications guidance which will be issued to the intemational community in relation to the properties, performance, ranking, selection and use of advanced glazing systems. Through AGIC the members have a line of communication to the Operating Agent. AGIC is a non profit-making organisation administered by the Advanced Glazing Technology Group at Oxford Brookes University. All funds generated from the annual membership fee are used to:
provide technical and information services for AGIC members through the Advanced Glazing Technology Group subsidise the dissemination activities for members (production of technical literature, organization of seminars and workshops) ensure effective UK technical participation in Task 18 promote the knowledge and use of advanced glazing products for the benefit of UK interests. Detailed information on the research priorities of participating OECD countries is held by the AGIC, such as:
performance rating and energy labelling schemes of the National Fenestration Rating Council operating in the USA and Canada international standards work underway in the European Community, the USA and Australia technical reports identifying appropriate applications, energy benefits and market potential predictions Prototype advanced glazing products such as electrochromic devices (with switchable optical properties), aerogel windows and evacuated glazings (with Uvalues well below l W/m2K) have recently been distributed for characterisation and evaluation by the Task participants and results of these studies are starting to be published within Task 18. Members of the AGIC have access to all Working Documents produced within the Task, describing the activities of all Task participants around the world. Currently these working documents number some 100.
304
Advanced Glazing Technologies of Relevance to the UK Highly Insulating Glazing Systems The performance of the various advanced glazing technologies for low U-value windows are summarized in Figure 1. These technologies have been described in detail elsewhere [1]. Figure 1 (after Dengler, Wittwer [9]) shows, for each technology, the U-value versus total solar energy transmittance. For passive solar heating applications, materials with low U-value coupled with high total solar energy transmittance are most effective. There is always a trade off between U-value and solar transmission. Measures to reduce U-value also usually reduce solar transmission, for example: extra panes of glass, Iow-e coatings or greater thicknesses of transparent insulation material and aerogel. From Figure 1 monolithic aerogel (thickness of 20mm) between two panes of glass evacuated to 0.1 atmospheres performs extremely well with a U-value below 0.5 W/m2K yet retains a high total solar energy transmittance of qearly 70%. This technology has its problems though; it is expensive and awkward to manufacture and the product gives a hazy view and in appearance is slightly blue in reflection and yellow in transmission due to scattering. It continues to be developed. Transparent insulation materials (TIMs) also exhibit good performance but are, in fact, translucent and cannot be used for view windows. Multi paned glazing systems, or superwindows, comprise several panes of glass or plastic films, one or more low emissivity coatings, gas cavity fillings and sometimes insulating frames and spacers. These have a poorer performance with regard to Uvalue and solar transmission but do provide a clear view out. For triple glazing with two Iow-e coats of emissivity = 0.1, the U-value is approximately 0.9-1.0 W/m2K. For triple glazing with two Iow-e coats, emissivity of zero (i.e. the ideal case), the U-value is about 0.8 W/m2K, thus it can be seen that this technology is reaching its theoretical limit. However the durable and cheaper to manufacture hard Iow-e coats still have some catching up to do on the sputtered soft coats. Advancements are still to be made in lowering emissivity, though solar transmission properties are superior. These improvements in hard coatings evacuated glazing systems which use radiative heat flow. (Evacuated glazings an array of barely visible glass pillars of welded edge forms the vacuum seal.)
will also greatly enhance the U-value of one or two hard Iow-e coats to reduce comprise two panes of glass separated by a fraction of a mUlimetre in depth. A glass
By filling triple glazing cavities with gases of lower conductivity than air, the theoretical limit for triple glazing (e=0.1) is 0.2 W/m2K, but unless convection could be inhibited the practical lower limit is 0.5 W/m2K, with krypton.
~-
0
0.1
0
0.5 0.4t 0.3 0.2~
0.5
I
1
I
1.5
i-
2
2.5
I
U-value, W/m2K I--
[]
--J
3
H o n e y c o m b TIM
°
Granular aerogel
Capilliary TIM
Monolithic silica aerogel
•
--x-
-
Monolithic silica aerogel at 0.1 atm
4-
Triple, 2low-e, Kr
o
Evacuated, 21ow-e
Triple glazing
•
•
Double, low-e, Ar
Double glazing
i
D
Figure 1, Total solar energy transmittance plotted against heat loss coefficient for low U-value glazing systems.
0,7T 0.6~
0.8
r
0.9 T
Total solar energy transmittance
306
Edge effec~ Figure 1 shows U-values for the centre of the glazing systems. In reality these values are seriously degraded by the edge components of glazing systems: spacers and frames. New advanced glazings will require new types of frame and edge seal products. As the insulation properties of the glazing itself reach a performance close to that of well-insulated opaque walls, the thermal bridging caused by the spacer bars and the frames will be unacceptable, even for wooden windows [10]. The Tables 1 to 3 below show how the centre of glazing U-value, UCG is degraded by the presence of spaces (as represented by the insulating glass unit U-value, U~G) and frames (as represented by the overall window U-value, Uw). Data is shown for traditional and improved materials. Three different glazing systems, double glazing, double Iow-e with argon, and triple 2 Iow-e krypton, are used to illustrate the effect. (The WINDOW software [5] was used to calculate U-values.)
Table I. The effects of edge component heat flow on window U-value, Uw (W/m2K): Double glazed, I m 2 window, 3-12-3 Spacer material fibreglass glass butyl/metal aluminium
Frame material aluminium aluminium with thermal break 4.55 3.45 4.63 3.53 4.60 3.49 4,65 3.55
Wood
PVC
2,69 2.77 2.73 2,79
2.54 2,62 2.59 2.64
Insulatingglass Centre of only, UiG glass, UcG 2,84 2.81 2,94 2.81 2.89 2.81 2.96 2.81
Table 2. The effects of edge component heat flow on window U-value, Uw (W/m2K): Double low-e, argon, I m 2 window, 3-I 2-E3 Spacer material fibreglass glass ! butyl/metal aluminium
Frame material aluminium aluminium with thermal break 3.50 2.40 3.57 2.47 3.57 2.47 3,69 2,59
Wood
PVC
1.70 1.77 1.77 1.89
1.55 1.62 1.62 1,74
Insulatingglass Centre of only, UtG glass, UCG 1.50 1.43 1.59 1.43 1.59 1.43 1.75 1.43
Table 3. The effects of edge component heat flow on window U-value, Uw (W/m2K): Tdple, 2 low-e, krypton, I m 2 window, 3-10-E3-10-E3 Spacer material fibreglass glass butyl/metal aluminium
Frame material aluminium aluminium with thermal break 2,89 1.79 2.91 1.81 3.02 1.92 3.20 2.09
Wood
PVC
1.13 1.14 1.25 1.42
0.98 0.99 1.11 1.27
Insulatingglass Centre of only, UtG glass, UcG 0.73 0.65 0.75 0.65 0.90 0,65 1.12 0,65
307
It can be seen that, as the glazing technology becomes more advanced than double glazing, that it is vital to pay attention to edge heat loss effects if a low window Uvalue is to be achieved. Edge effects are more pronounced the smaller the dimensions of the windows. For high performance windows low profile frames and large glazing area units give the lowest U-values, especially when combined with insulating spacers and frames. The data from Tables 1 to 3 is shown graphically in Figures 2 to 4. Figure 2. Edge Effects: Double glazing, 3-12-3, Im2
5.00
Window U-value, W/m2K
4,50 4.00 3,50 3.00 2.50 2,00 1.50 1.0C
0,5( 0.01
kl el/metal
.gpac~ material
Fmrt~ mel~lel c
Figure 3. Edge Effects:Double, Iow-e (0.08), argon, 3-12-E3, 1m2
4
3,5 3 2.5 Window U-valuo, WIm2K
1.5
AI yl/metal
Spacer maledal
Frame mafedal
308 Figure 4. Edge Effects:Tdple, 2 low-e, Krypton; 3-10-e3-10-e3, Im2
3.5 3 2.5 Window U-value. Wlm2K
1.5 0.,~
AI h/I/metal
S~xlcef mclterlol
Frame material
Cool
Daylighting
Not all buildings in climates such as that of the UK or warmer benefit from passive solar gains, indeed in some non-domestic buildings, such as offices with high occupation and casual and computer heat gains, these may best be avoided. Daylight, however is almost universally desirable, as over reliance on electric lighting increases the heat input to the building and can result in an unpleasant luminous environment for occupants to work in. Spectrally selective Iow-e coatings exist which allow a high proportion of the visible light in the solar spectrum to be transmitted but block much of the other wavelengths responsible for solar heat gains The effect of these spectrally tuned soft Iow-e coated double glazed units, with the coating positioned on the inside surface of the outer panes, is to allow good daylighting without the penalties of overheating or increased cooling load. The positioning of the coating is important for reducing heat gain. With the coating on the outer pane, most of the absorbed energy will be dissipated to external ambient, rather than re-radiated inwards. Different products exist with different visible (Tv=s) and total solar energy transmittances. A key characteristic is the ratio of visible to solar transmittance -the higher this ratio the cooler the daylighting provided. The current best performing product is Tv~ =66% and total solar transmission = 34% (manufacturers figures). The physical theoretical limit is roughly T~s=60% and total solar transmission = 25%, so there is still room for development [11].
309
References
1. Fenestration 2000 Phase I1: Review of Advanced Glazing Technology and Study of Benefits for the UK, Halcrow Gilbert Associates, Report for ETSU, ETSU S 1342, Feb. 1992. 2. SERI-RES Building Thermal Simulation Model Version 1.2, P Haves, Report for ETSU, Nov. 1987. 3. Field test of the performance of an opaque wall clad with transparent insulation material, C Martin, M Watson of EMC, prepared for ETSU, Dec. 1990. 4. Greenhouse-gas emissions and buildings in the United Kingdom, G Henderson, L Shorrock, BRE Information Paper IP/2/90, Apr. 1990. 5. WINDOW 4 Program Description, Window and Daylighting Group, Lawrence Berkeley Laboratory, Berkeley, California, USA, Mar. 1992. 6 VISION3 User Manual, CANMET/University of Waterloo, Canada, Aug. 1992. 7. FRAME Version 3 Manual, CANMET/Enermodal Engineering, Canada, Mar. 1992. 8. UK Advanced Glazing Industry Club Newsletter Issues 1 and 2, 1993,1994, published by the Advanced Glazing Technology Group, Oxford Brookes University. 9. Glazing with Granular Aerogel, Dengler J, Wittwer V Final Report, CEC Project Joule-0057-C, 1993. 10. Frame and Edge Seal Technology: A State of the Art Survey, Aschehoug O, Thyholt M, et al, lEA18 Working Document T18/B9/WD1/94, 1994. 11. The Low-E Glazing Design Guide, Johnson T E, Butterworth Architecture, 1991, ISBN 0-7506-9147-6.
Pergamon
ADVANCED GLAZING TECHNOLOGY FOR LOW ENERGY BUILDINGS IN THE UK
P D Robinson BSc PhD MlnstE M G Hutchins BSc PhD Advanced Glazing Technology Group School of Engineering Oxford Brookes University Oxford OX30BP
Summary Significant advances have been made in glazing technology in recent years which will impact enormously on the way building facades and control systems interact with solar radiant heat and daylight, and which will change building heat loss characteristics. This paper provides background to the current research activities in the field of advanced glazing technology for reduced energy consumption in buildings. A report predicting potential energy and environmental benefits from the uptake of advanced glazing in the UK is outlined, a world collaborative research programme is described and details are given of an organisation established in the UK to disseminate advanced glazing research and development results and applications guidance to the fenestration and building design professions. Technologies of relevance to the context of UK buildings are discussed.
298
299
Introduction Significant advances have been made in glazing technology in recent years which impact enormously on the way building facades and control systems interact with solar radiant heat and daylight, and which change building heat loss characteristics. These changes have resulted from solar control glasses, insulating glass units, low emissivity coatings and gas cavity fills. The pace of change will accelerate over the next few years and will intimately affect the way that buildings receive and utilise solar radiant heat and daylight, as spectrally selective coatings improve and diversify in function, light redirection systems and glazings with switchable transmission properties become viable products, and more highly insulating windows become available using the technologies of transparent insulation, evacuated glazing, aerogels, and improved frame and spacer design. Advanced glazing materials such as these are being developed world-wide and will potentially realise large energy and environmental benefits in residential and commercial buildings. A study of the potential energy and environmental benefits of advanced glazing in the UK was carried out in 1992 (1) for the DTI's Energy Technology Support Unit. The main findings are outlined below. Energy and Environmental Benefits of Advanced Glazing in the UK DOMESTIC SECTOR ASSESSMENT The main criteria used to assess the energy and building environment implications were annual auxiliary heating energy and overheating (hours over 27oc). The assessment was carried out using the dynamic building simulation model SERI-RES (2) and a method of estimating national benefits developed by ETSU (1).
The simulation results for the specific advanced glazing technologies were interpreted into national benefits using conjectured market penetration curves. The technologies considered specifically excluded transparent insulation material applied to opaque walls which was being assessed in another study (3). The national implications of the adoption of advanced glazing systems in the UK domestic sector were predicted to be significant: By 2025 the annual saving in primary energy was predicted to be over 4MTCE/yr. (MCTE is million tonnes of coal equivalent). The net present value of the accumulated saving up to 2025 was predicted to be £2,511M. The predicted total accumulated saving in CO 2 by 2025 was 84MTonnes. The yearly saving of 7MT/yr. of CO2 is equivalent to 9% of the annual CO 2 emissions from space heating in the UK, estimated by the BRE from 1987 energy consumption data (4). OFFICE SECTOR ASSESSMENT The main criteria used to assess the energy and building environment implications of the advanced glazing systems in the office building sector were annual auxiliary heating energy (kWh/m 2 per annum), annual overheating (occupied hours over 27oc), annual lighting energy (kWh/m 2 per annum), annual cooling energy (kWh/m 2 per annum).
300
The national implications of the adoption of advanced glazing systems in the UK office sector are significant in the context of the market volume: By 2025 the annual saving in primary energy was predicted to be nearly 0.2MTCE/yr. The net present value of the accumulated saving up to 2025 was predicted to be £83Mo The predicted total accumulated saving in CO2 by 2025 was 5.2MTonnes. The assessment for the office sector is probably an underestimate as no account was taken of light redirection technologies which will improve daylight penetration into offices. Other areas of the non-domestic building sector were not addressed. CONCLUSIONS The study concludes that the benefits for the domestic and office sectors are unlikely to be achieved if the market is left to its own devices. It makes the point that the role of national and local government will be vital to set the framework to encourage the uptake of these new products. A number of approaches were suggested:
energy performance rating for buildings and building components such as windows (which should account for passive solar benefits) support for advanced glazing research, development and demonstration collaboration in international RD & D programmes in order to share experiences promotion of successful products within the framework of best and good practice legislation establishing energy targets for buildings and building components (windows should be double Iow-e glazed as a minimum standard) As a result of the promising predictions from the study, the UK DTI decided to fund the operating agency of the new International Energy Agency Task on Advanced Glazing Materials in the form of Prof MG Hutchins of Oxford Brookes University. lEA Task 18 Advanced Glazing Materials Task 18 of the International Energy Agency Solar Heating and Cooling Programme is the World's largest collaborative research and development project on Advanced Glazing Technology. Fifteen OECD countries have committed more than 100 person years of effort to this five year project which started in 1992. The objective of the Task is to develop the scientific, engineering and architectural basis which will support the appropriate use of advanced glazings and associated materials in buildings with the aim of realising significant energy and environmental benefits. Building energy analysis tools are being employed to identify appropriate applications and predict energy and environmental impacts which will derive from the use of advanced glazing products. Task 18 has a specific focus on the application
301
and technology transfer of new materials and components with an emphasis on nearmarket technologies. The Task aims to provide guidance for design engineers, building engineers and industry on the properties, use, performance and selection of advanced glazing materials. Necessary measurable parameters for specification of the thermal performance of advanced glazing materials are being identified and defined and appropriate measurement test procedures developed. The work of Task 18 is managed under two Sub tasks, each made up a number of activities:
Sub task A Applications Assessment and Technology Transfer A1 Applications, potentials and characteristics A2 Modelling A3 Control strategies A4 Environmental and energy impacts A5 Applications guidance Sub task B Technology Case Study Projects B1 Monolithic and granular aerogels B2 Geometric media (honeycombs and capillary structures) Chromogenic glazings B3 Low-emittance coatings B4 Evacuated glazings B5 B6 Advanced solar collector covers B7 Angular selective transmittance coatings B8 Daylighting materials and systems B9 Frame and edge seal technology B10 Advanced glazing materials properties database and technology summaries Bll Investigation of the optical properties and scattering behaviour of advanced glazing materials B12 Measurement of the total energy transmittance of advanced glazing systems B13 Directional optical properties measurements of advanced glazing materials B14 Measurement of the U-value of advanced glazing systems The goals of Sub task A are to identify appropriate applications, determine the energy and environmental implications and provide applications guidance in the area of advanced glazing materials. Work in Sub task A will enable: Investigation of the technical and economic potential of advanced glazing systems. Establishment of the physical properties of advanced glazing materials and control strategies critical to performance. Evaluation of design tools and provision of applications guidance to aid the selection of advanced glazing systems. Study of the climate dependence of performance for defined building types and reference zones. Comparison of performance obtained with different materials.
302
Assessment of cost and benefits and the setting of target costs against other solutions. Determination of the influence of degradation on thermal performance. In Sub task B each Case Study Project integrates a series of materials development and measurement activities to enable an extensive, in-depth determination and examination of materials properties and performance levels for potential use in advanced glazing systems. Each Project is undertaking measurement and characterisation work concerned with basic materials properties, e.g. optical, thermal, mechanical etc. In addition to providing basic characterisation, such measurements may be carried out to analyse degradation mechanisms and determine failure modes induced by ageing tests. Where possible measurements are being performed on large area samples of advanced glazing materials and there is particular emphasis on investigating the properties of whole windows comprising advanced glazing materials and their associated frames and sealant materials. For those materials which are near-market, prototype glazing systems are being constructed to enable the measurement of the gross optical and thermal properties necessary to specify the performance of advanced glazing systems. The influence of the boundary conditions is being investigated and the accuracy of appropriate values of key glazing parameters for use as input data for the modelling studies to be carried out in Sub task A is being established. Two key parameters, the total energy transmittance and the U-value, are regarded as necessary for characterisation and subsequent prediction of the energy performance of a glazing system and the building of which it forms part. Light redirection technologies are more problematic to characterize, with their directional properties. Advanced glazing materials present difficulties for the measurement of total energy transmittance because of internal absorption, scattering and incident angle effects. Total energy transmittance is being measured from radiation measurements and by calorimetric methods. The low thermal transmittance of advanced glazing materials mean that it is important to determine not just the U-value of such materials but the influence of the frame on the total heat transfer. Task 18 aims to determine the influence of centre versus edge effects and boundary conditions employed in the measurement of the U-value of advanced glazing systems. Calculated window performance parameters are being determined using the WINDOW 4 (5) VISION (6) and FRAME (7) computer programmes. Switchable glazing systems, based on electrochromic, thermochromic and liquid crystal materials, are being investigated together with control strategies necessary for their effective operation. Links with Industry Industry is represented in the Task through technical participation in many of the projects and through the provision of samples and the customising of windows for test purposes. Several participating countries are supported with the help of industrial funds. Industry clubs have been formed in several countries, most notably in Australia and the United Kingdom, to aid the dissemination of information arising from the work of Task 18.
303
UK Advanced Glazing Industry Club The UK AGIC was established in 1993 to promote awareness within the UK Fenestration Industry (i.e. glass and glazings manufacturers, designers, architects and end-users of the technology) of current international best practice and new developments for improved thermal and optical performance of windows. It allows exchange of information between the lEA Task's researchers from OECD countries and the UK Industry. AGIC will play an important role in the dissemination of knowledge and identification of appropriate applications of advanced glazing products. The UK, as it is funding the Operating Agency of the research programme, is in a pivotal position to influence the prioritisation of work to be undertaken in lEA Task 18, and the forms of applications guidance which will be issued to the intemational community in relation to the properties, performance, ranking, selection and use of advanced glazing systems. Through AGIC the members have a line of communication to the Operating Agent. AGIC is a non profit-making organisation administered by the Advanced Glazing Technology Group at Oxford Brookes University. All funds generated from the annual membership fee are used to:
provide technical and information services for AGIC members through the Advanced Glazing Technology Group subsidise the dissemination activities for members (production of technical literature, organization of seminars and workshops) ensure effective UK technical participation in Task 18 promote the knowledge and use of advanced glazing products for the benefit of UK interests. Detailed information on the research priorities of participating OECD countries is held by the AGIC, such as:
performance rating and energy labelling schemes of the National Fenestration Rating Council operating in the USA and Canada international standards work underway in the European Community, the USA and Australia technical reports identifying appropriate applications, energy benefits and market potential predictions Prototype advanced glazing products such as electrochromic devices (with switchable optical properties), aerogel windows and evacuated glazings (with Uvalues well below l W/m2K) have recently been distributed for characterisation and evaluation by the Task participants and results of these studies are starting to be published within Task 18. Members of the AGIC have access to all Working Documents produced within the Task, describing the activities of all Task participants around the world. Currently these working documents number some 100.
304
Advanced Glazing Technologies of Relevance to the UK Highly Insulating Glazing Systems The performance of the various advanced glazing technologies for low U-value windows are summarized in Figure 1. These technologies have been described in detail elsewhere [1]. Figure 1 (after Dengler, Wittwer [9]) shows, for each technology, the U-value versus total solar energy transmittance. For passive solar heating applications, materials with low U-value coupled with high total solar energy transmittance are most effective. There is always a trade off between U-value and solar transmission. Measures to reduce U-value also usually reduce solar transmission, for example: extra panes of glass, Iow-e coatings or greater thicknesses of transparent insulation material and aerogel. From Figure 1 monolithic aerogel (thickness of 20mm) between two panes of glass evacuated to 0.1 atmospheres performs extremely well with a U-value below 0.5 W/m2K yet retains a high total solar energy transmittance of qearly 70%. This technology has its problems though; it is expensive and awkward to manufacture and the product gives a hazy view and in appearance is slightly blue in reflection and yellow in transmission due to scattering. It continues to be developed. Transparent insulation materials (TIMs) also exhibit good performance but are, in fact, translucent and cannot be used for view windows. Multi paned glazing systems, or superwindows, comprise several panes of glass or plastic films, one or more low emissivity coatings, gas cavity fillings and sometimes insulating frames and spacers. These have a poorer performance with regard to Uvalue and solar transmission but do provide a clear view out. For triple glazing with two Iow-e coats of emissivity = 0.1, the U-value is approximately 0.9-1.0 W/m2K. For triple glazing with two Iow-e coats, emissivity of zero (i.e. the ideal case), the U-value is about 0.8 W/m2K, thus it can be seen that this technology is reaching its theoretical limit. However the durable and cheaper to manufacture hard Iow-e coats still have some catching up to do on the sputtered soft coats. Advancements are still to be made in lowering emissivity, though solar transmission properties are superior. These improvements in hard coatings evacuated glazing systems which use radiative heat flow. (Evacuated glazings an array of barely visible glass pillars of welded edge forms the vacuum seal.)
will also greatly enhance the U-value of one or two hard Iow-e coats to reduce comprise two panes of glass separated by a fraction of a mUlimetre in depth. A glass
By filling triple glazing cavities with gases of lower conductivity than air, the theoretical limit for triple glazing (e=0.1) is 0.2 W/m2K, but unless convection could be inhibited the practical lower limit is 0.5 W/m2K, with krypton.
~-
0
0.1
0
0.5 0.4t 0.3 0.2~
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I
1
I
1.5
i-
2
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I
U-value, W/m2K I--
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H o n e y c o m b TIM
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Granular aerogel
Capilliary TIM
Monolithic silica aerogel
•
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-
Monolithic silica aerogel at 0.1 atm
4-
Triple, 2low-e, Kr
o
Evacuated, 21ow-e
Triple glazing
•
•
Double, low-e, Ar
Double glazing
i
D
Figure 1, Total solar energy transmittance plotted against heat loss coefficient for low U-value glazing systems.
0,7T 0.6~
0.8
r
0.9 T
Total solar energy transmittance
306
Edge effec~ Figure 1 shows U-values for the centre of the glazing systems. In reality these values are seriously degraded by the edge components of glazing systems: spacers and frames. New advanced glazings will require new types of frame and edge seal products. As the insulation properties of the glazing itself reach a performance close to that of well-insulated opaque walls, the thermal bridging caused by the spacer bars and the frames will be unacceptable, even for wooden windows [10]. The Tables 1 to 3 below show how the centre of glazing U-value, UCG is degraded by the presence of spaces (as represented by the insulating glass unit U-value, U~G) and frames (as represented by the overall window U-value, Uw). Data is shown for traditional and improved materials. Three different glazing systems, double glazing, double Iow-e with argon, and triple 2 Iow-e krypton, are used to illustrate the effect. (The WINDOW software [5] was used to calculate U-values.)
Table I. The effects of edge component heat flow on window U-value, Uw (W/m2K): Double glazed, I m 2 window, 3-12-3 Spacer material fibreglass glass butyl/metal aluminium
Frame material aluminium aluminium with thermal break 4.55 3.45 4.63 3.53 4.60 3.49 4,65 3.55
Wood
PVC
2,69 2.77 2.73 2,79
2.54 2,62 2.59 2.64
Insulatingglass Centre of only, UiG glass, UcG 2,84 2.81 2,94 2.81 2.89 2.81 2.96 2.81
Table 2. The effects of edge component heat flow on window U-value, Uw (W/m2K): Double low-e, argon, I m 2 window, 3-I 2-E3 Spacer material fibreglass glass ! butyl/metal aluminium
Frame material aluminium aluminium with thermal break 3.50 2.40 3.57 2.47 3.57 2.47 3,69 2,59
Wood
PVC
1.70 1.77 1.77 1.89
1.55 1.62 1.62 1,74
Insulatingglass Centre of only, UtG glass, UCG 1.50 1.43 1.59 1.43 1.59 1.43 1.75 1.43
Table 3. The effects of edge component heat flow on window U-value, Uw (W/m2K): Tdple, 2 low-e, krypton, I m 2 window, 3-10-E3-10-E3 Spacer material fibreglass glass butyl/metal aluminium
Frame material aluminium aluminium with thermal break 2,89 1.79 2.91 1.81 3.02 1.92 3.20 2.09
Wood
PVC
1.13 1.14 1.25 1.42
0.98 0.99 1.11 1.27
Insulatingglass Centre of only, UtG glass, UcG 0.73 0.65 0.75 0.65 0.90 0,65 1.12 0,65
307
It can be seen that, as the glazing technology becomes more advanced than double glazing, that it is vital to pay attention to edge heat loss effects if a low window Uvalue is to be achieved. Edge effects are more pronounced the smaller the dimensions of the windows. For high performance windows low profile frames and large glazing area units give the lowest U-values, especially when combined with insulating spacers and frames. The data from Tables 1 to 3 is shown graphically in Figures 2 to 4. Figure 2. Edge Effects: Double glazing, 3-12-3, Im2
5.00
Window U-value, W/m2K
4,50 4.00 3,50 3.00 2.50 2,00 1.50 1.0C
0,5( 0.01
kl el/metal
.gpac~ material
Fmrt~ mel~lel c
Figure 3. Edge Effects:Double, Iow-e (0.08), argon, 3-12-E3, 1m2
4
3,5 3 2.5 Window U-valuo, WIm2K
1.5
AI yl/metal
Spacer maledal
Frame mafedal
308 Figure 4. Edge Effects:Tdple, 2 low-e, Krypton; 3-10-e3-10-e3, Im2
3.5 3 2.5 Window U-value. Wlm2K
1.5 0.,~
AI h/I/metal
S~xlcef mclterlol
Frame material
Cool
Daylighting
Not all buildings in climates such as that of the UK or warmer benefit from passive solar gains, indeed in some non-domestic buildings, such as offices with high occupation and casual and computer heat gains, these may best be avoided. Daylight, however is almost universally desirable, as over reliance on electric lighting increases the heat input to the building and can result in an unpleasant luminous environment for occupants to work in. Spectrally selective Iow-e coatings exist which allow a high proportion of the visible light in the solar spectrum to be transmitted but block much of the other wavelengths responsible for solar heat gains The effect of these spectrally tuned soft Iow-e coated double glazed units, with the coating positioned on the inside surface of the outer panes, is to allow good daylighting without the penalties of overheating or increased cooling load. The positioning of the coating is important for reducing heat gain. With the coating on the outer pane, most of the absorbed energy will be dissipated to external ambient, rather than re-radiated inwards. Different products exist with different visible (Tv=s) and total solar energy transmittances. A key characteristic is the ratio of visible to solar transmittance -the higher this ratio the cooler the daylighting provided. The current best performing product is Tv~ =66% and total solar transmission = 34% (manufacturers figures). The physical theoretical limit is roughly T~s=60% and total solar transmission = 25%, so there is still room for development [11].
309
References
1. Fenestration 2000 Phase I1: Review of Advanced Glazing Technology and Study of Benefits for the UK, Halcrow Gilbert Associates, Report for ETSU, ETSU S 1342, Feb. 1992. 2. SERI-RES Building Thermal Simulation Model Version 1.2, P Haves, Report for ETSU, Nov. 1987. 3. Field test of the performance of an opaque wall clad with transparent insulation material, C Martin, M Watson of EMC, prepared for ETSU, Dec. 1990. 4. Greenhouse-gas emissions and buildings in the United Kingdom, G Henderson, L Shorrock, BRE Information Paper IP/2/90, Apr. 1990. 5. WINDOW 4 Program Description, Window and Daylighting Group, Lawrence Berkeley Laboratory, Berkeley, California, USA, Mar. 1992. 6 VISION3 User Manual, CANMET/University of Waterloo, Canada, Aug. 1992. 7. FRAME Version 3 Manual, CANMET/Enermodal Engineering, Canada, Mar. 1992. 8. UK Advanced Glazing Industry Club Newsletter Issues 1 and 2, 1993,1994, published by the Advanced Glazing Technology Group, Oxford Brookes University. 9. Glazing with Granular Aerogel, Dengler J, Wittwer V Final Report, CEC Project Joule-0057-C, 1993. 10. Frame and Edge Seal Technology: A State of the Art Survey, Aschehoug O, Thyholt M, et al, lEA18 Working Document T18/B9/WD1/94, 1994. 11. The Low-E Glazing Design Guide, Johnson T E, Butterworth Architecture, 1991, ISBN 0-7506-9147-6.