The Influence Of Window Type And Orientation On Energy-Saving In Buildings Application To A Single Family Dwelling

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The Influence Of Window Type And Orientation On Energy-Saving In

Buildings – Application To A Single Family Dwelling

Urbikain M. K., Mvuama M. C., García Gáfaro C. and Sala Lizarraga J. M.

The University of the Basque Country, Alameda Urquijo s/n, 48013, Bilbao, Spain Correspondence e-mail: [email protected]

ABSTRACT

The trend to reduce energy losses through enclosures (walls and windows) is an increasingly important part of improving the energy efficiency of a building. Adequate window design and orientation can reduce the heating and cooling requirements of a building.

An optimization process was undertaken for a single-zone building envelope, based on the climate of the Basque Country and using a parametric study of annual heating requirements, according to glazing type and orientation. Two software were used in the analysis: WINDOW 5.2 for window thermal performance and TRNSYS 16 for building thermal simulation. The results of the study were applied to the case of a single-family dwelling to predict energy savings for heating, based on the recommended window type for each orientation.

INTRODUCTION

Windows play a key role in the energy efficiency of both residential and commercial

buildings. At least one-fourth of domestic heating requirements in OECD countries (Muneer T. et al, 2000) are considered to be the result of energy losses through windows. In recent years, major efforts have been made to improve the thermal behavior of windows using various technologies, such as low-emissivity (low-E) coatings, inert gas fill, insulating edge spacers, low-conductivity frames, etc. This approach has led to windows with a thermal transmittance as good as a wall.

Energy consumption in buildings depends largely on the type of building, whether residential or commercial. In the latter case, 90% of the heating and cooling is linked to losses and gains through the façade, particularly through windows, which are the most sensitive to heat flows. Energy requirements are a function of various factors such as climate, building location, season of the year, and operational and functional conditions.

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NOMENCLATURE

A Area (m2₎

cp Specific heat (J/kg-K)

g Solar heat gain coefficient

R Thermal resistance (m2K/W)

Rfsol Solar reflectance of the exterior-facing surface of the glazing system Rbsol Solar reflectance of the interior-facing surface of the glazing system Rfvis Visible reflectance of the exterior-facing surface of the glazing system Rbvis Visible reflectance of the interior-facing surface of the glazing system

Tsol Solar transmittance

Tvis Visible transmittance

U Thermal transmittance or U-factor (W/m2_K)

α1 Absorptance of the first pane in the glazing system, counting from the exterior-facing surface

α2 Absorptance of the second pane in the glazing system, counting from the exterior-facing surface

α3 Absorptance of the third pane in the glazing system, counting from the exterior-facing surface

ρ Density (kg/m3₎

DESCRIPTION OF BUILDING ENVELOPE AND STANDARD WINDOWS

The building envelope considered is similar to the one described in ANSI/ASHRAE Standard 140/2004, consisting of a single-zone enclosure of 8.0 m x 6.0 m x 2.7 m, fitted with two windows of 3.0 m x 2.4 m, as shown in Figure 1. The thermophysical properties of the materials used for the model are listed in Table 1, which correspond to the configuration of the high-mass enclosure described in the ANSI/ASHRAE Standard 140/2004 mentioned above. 6.0 m 8.0 m 2.7 m 3.0 m 2.4 m 1.0 m 2.4 m 3.0 m 0.5 m 0.15 m

Figure 1. Simulated building envelope for determination of heating requirements

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compared to tinted glass, this glazing system has the property of greater visible transmittance (in this case, 0.70). A common measure of the efficiency of spectrally selective glass is given by the light-to-solar ratio. This is the visible light transmission divided by the solar heat gain coefficient for the glazing system.(in this case, 1.76, with a maximum possible ratio of 2). Low-E coatings reduce radiative heat transfer. If a lower conductive-convective heat transfer in the air-gap is also desired, then gases with a lower thermal conductivity than air can be used, as occurs with windows Type 5. If heat flow through the system is to be further

decreased, triple glazing with two low-E coatings and a low-conductance gas can be used; this is the configuration chosen for the window Type 6 .

Table 1. Physical properties of the enclosure

Element _(w/mK)K Thickness_(m) _(W/mU 2_K) _(m2_K/W)R _(kg/mρ 3₎_(J/kgK)cp

Int. surface coeff. 8.30 0.12

Concrete block 0.510 0.100 5.10 0.196 1400 1000

Foam insulation 0.040 0.0615 0.65 1.54 10 1400

Wood 0.140 0.009 15.56 0.06 530 900

Exterior wall

Ext. surface coeff. 29.30 0.03

Concrete slab 1,130 0,080 14.12 0.07 1400 1000

Floor

Insulation 0.040 1.007 0.04 25.17

Plasterboard 0.160 0.010 16.00 0.06 950 840

Fiberglass 0.040 0.118 0.36 2.79 12 840

Ceiling

Roof deck 0.140 0.019 7.37 0.14 530 900

Summary

Element Area (m2) A.U. (W/K)

Wall 63.600 32.58

Floor 48.000 1.89 Ceiling 48.000 15.25

Table 2. Types of glazings

Number of panes

Glass/air-gap thickness

(mm)

Coating position Coating Gas fill

Type 1 Double 3.9/12.7/3.9 None Air

Type 2 Double 3.9/12.7/3.9 Surface 3 Low-E, 0.155 Air

Type 5 Double 3.9/12.7/3.8 Surface 3 Low-E, 0.066 Argon

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Table 3. Visible and solar energy parameters of the window Types of windows U (W/m2K) g Tvis Type 1 2.757 0.712 0.729 Type 2 1.921 0.660 0.667 Type 3 1.749 0.558 0.710 Type 4 1.666 0.370 0.630 Type 5 1.467 0.559 0.710 Type 6 0.974 0.447 0.627 Table 4. Optical properties of glazing system for a normal incident angle

Tsol Rfsol Rbsol Tvis Rfvis Rbvis α1 α2 α3 g

Type 1 0.720 0.128 0.128 0.812 0.145 0.145 0.086 0.066 0.780 Type2 0.579 0.160 0.141 0.742 0.172 0.158 0.089 0.172 0.722 Type 3 0.525 0.277 0.282 0.791 0.115 0.115 0.108 0.090 0.609 Type 4 0.350 0.336 0.381 0.702 0.097 0.112 0.295 0.018 0.399 Type 5 0.525 0.277 0.282 0.791 0.115 0.115 0.108 0.090 0.610 Type 6 0.394 0.324 0.324 0.699 0.144 0.144 0.163 0.062 0.058 0.485

BUILDING ENVELOPE MODELING AND SIMULATION TOOL

The energy analysis of the building envelope was performed using TRNSYS 16, a transient simulation software of thermal systems created by the Solar Energy Laboratory of the University of Wisconsin. The TYPE 56 multi-zone model was used for both the model envelope and the single-family dwelling.

The most important heat flows considered in this TYPE are a long-wave radiative exchange with the exterior environment, a convective heat exchange and a fraction of absorbed solar radiation. There is also a transient heat exchange through the wall layers, modeled using transfer functions (Mitalas and Stephenson, 1971). Long-wave radiation heat exchange between the interior surfaces of the building envelope are estimated using the star network method implemented by Seem (1987).

The TRNSYS software allows importing a file with the optical data and thermal and solar properties of windows from the WINDOW (LBNL) software. The WINDOW calculation procedure (Finlayson et al., 1993) used to obtain an overall window property is based on computing an area-weighted average of the properties of each component of the window (ISO 15099). The total heat flow through a glazing system can be divided into the heat loss flow depending only on temperature difference and the heat flow depending on the intensity of the short-wave radiation.

SIMULATION OF MODEL ENVELOPE

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of global horizontal radiation is 146 W/m2 and 164 W/m2, respectively. Since the TRNSYS building model requires hourly data to estimate energy requirements, Meteonorm 5.1

(Meteotest, Edition 2003) was used to generate files with typical meteorological year data for the cities under consideration.

The building envelope operating conditions described in ANSI/ASHRAE Standard 140-2004 were used, i.e. continual infiltration throughout the year of 0.5 air changes per hour, an internal gain of constant sensitive heat of 200 W, with a 60% radiant fraction and 40% convective fraction. A temperature setting of 20ºC for heating was used as a control strategy to estimate energy requirements.

Based on the results obtained, an analysis was then carried out to analyze the sensitivity of annual energy requirements to the following factors: type of glazing and orientation. The effect of window orientation was analyzed by turning the building envelope in steps of 90º, such that the only wall with windows was oriented toward the north, south, east and west. The heating season is considered to be from October to May. The heating requirements for the cases studied according to orientation are shown in Tables 5 and 6.

Table 5. Heating requirements according to orientation and window type (kW.h) for Bilbao Type 1 Type 2 Type 3 Type 4 Type 5 Type 6 N 2493.28 1915.78 1908.07 2112.7 1718.94 1594.17 S 1109.83 783.95 850.7 1283.1 726.40 755.07 E 1898.60 1435.11 1468.2 1812.7 1309.08 1270.85 O 1910.06 1449.53 1480.95 1819.6 1315.58 1272.89 Table 6. Heating requirements according to orientation and window type (kW.h) for Vitoria

Type 1 Type 2 Type 3 Type 4 Type 5 Type 6 N 4026.20 3202.44 3196.80 3466.45 2893.81 2696.41 S 1904.77 1346.13 1485.83 2234.19 1274.49 1353.61 E 3072.65 2376.32 2450.00 2934.62 2206.13 2152.26 O 3117.02 2404.50 2472.00 2954.25 2227.40 2158.40 Type 1, which corresponds to an uncoated double-pane glazing, was taken as the reference case, and the heating energy savings obtained by replacing this by other glazing types were then analyzed. The conclusions obtained for the Basque cities were similar, since their climates are not much different. Figures 2 and 3 show the results obtained for Bilbao and Vitoria, respectively.

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C6/C1 C5/C1 C4/C1 C3/C1 C2/C1 Orientation W E S N 40.00 30.00 20.00 10.00 0.00 -10.00 -20.00

Percent difference with respect to Case 1

Heating requirements (kW.h) 1000.00 1500.00 2000.00 2500.00 3000.00 0.00 500.00 N S E Orientation W CASE 6 CASE 5 CASE 4 CASE 3 CASE 2 CASE 1 (a) (b)

Figure 2. For Bilbao: (a) heating consumption for each window type, (b) % differences with respect to Type 1. (b) C6/C1 C5/C1 C4/C1 C3/C1 C2/C1 Orientation S E W N 40.00 30.00 20.00 10.00 0.00 -10.00 -20.00

Percent difference with respect to Case 1

CASE 6 CASE 5 CASE 4 CASE 3 CASE 2 CASE 1 S E Orientation W N 0.00 500.00 1000.00 1500.00 2000.00 2500.00 3000.00 3500.00 4000.00 4500.00 Heating requirements (kW.h) (a)

Figure 3. Vitoria: (a) heating consumption for each window type, (b) % differences with respect to Type 1.

COMMENTS

If low-E double glazing Type 3 is used, the heating requirements are lower in all directions considered, making this an appealing option. However, in the southern direction, the effect is less noticeable than with glazing Type 2, because Type 3 has a moderate solar gain (0.61) with a lower solar transmittance (0.53). For both Bilbao and Vitoria, the heating requirements are lower with windows Type 2 than Type 3, except on the northern side. Therefore, glazings Type 2 are recommended for all orientations, and glazings Type 3 are strongly recommended for the northern direction.

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The glazing Type 5 is a version of glazing Type 3 in which the air gap is replaced by a gas of a lower conductivity. In addition to reducing radiation heat exchange, this approach reduces heat transfer by conduction-convection with the low-E coating. This type of glazing results in considerably lower heating requirements than those achieved with uncoated double glazing and similar to those obtained with triple glazing with low-E coatings. In particular, 31% savings is achieved for Bilbao in the northern, eastern and western directions, and 34% in the southern direction.

Triple glazing Type 6 represents a good option from the energy point of view, since it decreases the heating requirements considerably, achieving 36% savings in the northern direction and 32% in the southern for Bilbao for the model envelope.

ENERGY SAVINGS IN A SINGLE-FAMILY DWELLING

A heating consumption study of a single-family dwelling located in Bilbao was conducted by varying the glazing type. Figure 4 shows the single-family dwelling that was simulated. The window with glazing Type 1 was taken as a reference. Considering a mean system efficiency of 70%, the heating requirements were determined by performing the following simulations: with windows Type 1, with windows type 3 for northern orientation and type 2 for all other orientations and with windows Type 5 for all orientations

The results are shown in Table 7. The savings obtained when replacing the reference window with others are lower than when doing so with the ASHRAE envelope. This is primarily due to the fact that the model envelope has large windows and its walls are better insulated. In addition, whereas in the physical model, the effect of glazing surfaces has been analyzed for each orientation separately, in the actual case, a combination of glazing areas in the various orientations was used. Replacing Type 1 by a combination of Type 2 and 3 according to the orientation, 9.1% savings in heating requirements were obtained. When Type 5 was used, similar savings were achieved.

Figure 4. (a) Cross-section of the simulated home, (b) thermal areas defined for Trnsys

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Table 7. Results obtained when varying window types in a single-dwelling home. Heating consumption, kW.h % Type 1 16984.04 Type 2/3 15440.26 9.09 Type 5 15432.61 9.13 CONCLUSIONS

Double glazings Type 4 are not interesting from the heating viewpoint. In south-facing windows, there is a loss with respect to the glazing taken as the reference. In other directions, the savings that would be produced are insignificant compared to other low-E windows. Cases of interest for the climate of the Basque Country are glazings Type 2 and 3, e.g., low-E glass of 0.155 and 0.066, with high and moderate solar gain, respectively. With these panes, the heating requirements decrease for all orientations, making them truly of interest for our climate conditions. In the southern, eastern and western directions, glazing Type 2 is prefered, whereas Type 3 is prefered for the northern direction, due to higher longwave reflectivity. Therefore, these types of glazing are suitable for residential buildings. In this study, 9% savings was obtained by using a combination of glazings Type 2 and 3.

Type 5 is similar to Type 3, in which the air-gap has been replaced with argon, thus

decreasing the conductive-convective exchange. This represents a slight improvement over Types 2 and 3, but may be important in the case of large windows, depending also on the type of enclosure.

The application of the results to the actual case confirms the trends observed with the model envelope. The extent of percent savings will depend on the specific characteristics of the building, in particular, the relative area of glazing surface and the inertia and insulation of the enclosure.

REFERENCES

D.G. Stephenson and G.P. Mitalas (1971). Calculation of heat conduction transfer functions for multilayer slabs. ASHRAE Trans. 77 2, pp. 117–126.

E.U. Finlayson et al., Window 4: documentation of calculation procedures. Lawrence Berkeley Laboratory, Energy and Environment Division, Berkeley, 1993.

ISO 15099:2003, Thermal Performance of Windows, Doors and Shading Devices – Detailed Calculations.

J.E. Seem (1987). Modelling of heat transfer in buildings, PhD Thesis. Department of Mechanical Engineering, University of Wisconsin-Madison, Madison.

T. Muneer, N. Abodahab, G. Weir and J. Kubie (2000). Windows in Buildings: Thermal, Acoustic,