why determining the window is such a pane

eColi BR i u M • oC toBe R 2010 34

Why determining thermal loads through windows is such a pane

By Murray Mason B.e. Mech., l.AiRAH, f.ieAust., fAie., and trevor kingston, M.AiRAH

F O R U M

tHe ReleVAnt signifiCAnCe of tHe solAR And ConduCtion loAdsThe objective of this paper is to shed some light on the complex calculations required to accurately calculate the heat gain through windows and review the development of the relevant formulae used by the industry as glass technology developed. It also discusses the difficulties in applying accurate calculation methods at the design stage when the glass type and window framing is not reliably known. To this end it provides some empirical adjustment factors that can be used with readily available Centre of Glass properties.

in tHe BeginningEver since people began air conditioning their home or workplace, there has been conjecture on how best to calculate the heat load through windows in a practical way. With the advent of double glazing and heat absorbing glasses things became a little more difficult. In the “Carrier” method as now documented in AIRAH’s Application Manual DA91, U values of 5.89 for single glazing and 2.89 to 3.35 for double glazing (depending on the air gap thickness) were traditionally used. Shade factors were readily calculated from the absorptivity, transmisivity and reflectivity of the glass using relatively simple formula.

With the advent in more recent times of laminated and low e glasses, and the interest in energy consumption, more complex methods have been developed for use in estimating the energy consumption of buildings. These estimates of energy consumption use hourly meteorological data from the climatic data files available. However, for load estimation and equipment sizing, statistically derived “design temperatures” (with associated parameters such as wind speed) are still used when determining Shade Coefficients and U-values.

The AIRAH Application Manual DA91 (which originally was a rewrite of the Carrier Manual to convert it to metric units) provides formulas for calculating the Shade Coefficient which determines the amount of solar gain. For single glazing the formula is:

SC = (0.4 α + τ) / 0.884 eqn 1

where: α is the glass absorption coefficient

τ is the glass transmission coefficient

0.4 is the proportion of the absorbed heat that is transmitted inwards assumed to be the ratio of the outside film coefficient divided by the sum of the inside and outside film coefficients.

And for double glazing the following formula can be derived:

SC = ((0.21 x αo) + (0.66 x αi x αo) + (τi x τo) + (ρo x τo x ρi x τi)

+ (0.21 + 0.66 x ρo)(αi x τo x ρi)) / 0.884 eqn 2

where: α is the glass absorption coefficient

τ is the glass transmission coefficient

ρ is the glass reflection coefficient

subscript o is the outside pane and subscript i is the inside pane

0.21 is the proportion of the absorbed heat in the outer pane that is transmitted into the room assumed to be the ratio of the thermal resistance of the outside film divided by, the sum of the thermal resistance of the outside film, the air gap and the inside film.

0.66 is the proportion of the absorbed heat in the inner pane that is transmitted into the room assumed to be the ratio of the sum of the thermal resistance of the outside film and the airgap, divided by, the sum of the thermal resistance of the outside film, the air gap and the inside film.

The inside and outside film coefficients are a function of the wind speed across the surface of the window. Hence the shade coefficient is dependent on the assumed inside and outside wind speed.

In calculating film coefficients, Carrier (DA91) assumes wind speeds of 2.5 outside and 1.0 m/s inside. Glass manufacturers generally quoted Shade Coefficients based on ASHRAE2 conditions which assumed wind speeds of 2.8 outside and 0.0 inside. These wind speeds change:

the 0.4 in equation 1 to 0.34 and

the 0.21 and 0.66 in equation 2 to 0.19 and 0.61 respectively

This in turn results in a different Shade Coefficient. The question is and has always been, which is the correct value to use?

A manual conversion from “ASHRAE” values to “Carrier” values based on the glass absorption coefficient often used was to add the following to the “ASHRAE” value:

Glass absorption coefficient

Value to add to the ASHRAE2 value

Less than 15% + 0.01

15 to 30% + 0.02

30 to 50% + 0.04

50 to 70% + 0.05

70% + 0.06

35oC toBe R 2010 • eColi B R i u M

F O R U M

www.vaisala.com/hvac

Reliable and accurate HVAC solutions / humidity & CO2

www.bigfootsystems.com.au

the complete solution

Pro Pipe Supplies Pty Ltd, 13-15 Main Street, Beverly, SA5009t: +61 8 8268 8633 [email protected]

Glass Type Double Single

Eqn 1 & 2 (Carrier1)

Eqn 1 & 2 (ASHRAE2)

Manualy Converted

ASHRAE NFRC3

(Summer)

WINDOW54 Using Carrier wind speeds

Pilkington 6mm clear Single 0.94 0.93 0.95 0.94 0.95

G.James Solarplus TS21 on clear Single 0.45 0.41 0.43 0.36 0.38

Pilkington 6mm Eclipse Advantage Single 0.61 0.57 0.59 0.54 0.53

G.James HL219 Single 0.66 0.62 0.67 0.54 0.61

G.James Solarplus SL20 Low E Single 0.71 0.68 0.73 0.60 0.66

G.James Solarplus SL60 Low E Single 0.48 0.44 0.49 0.35 0.42

Pilkington 6mm Clear + 6mm Clear Double 0.80 0.79 0.81 0.81 0.81

Pilkington Low E + 6mm clear Double 0.71 0.70 0.73 0.71 0.71

Pilkington Eclipse Advantage Artic blue + 6mm Clear

Double 0.39 0.37 0.42 0.33 0.34

Table 1: Shade Coefficient for different glasses using different methods of calculation.

sofia

Text Box


F O R U M

This correction can be applied to the value for single glazing, but whether it can be used for double glazing, low e glasses, laminated glasses, etc is debatable.

Table 1 lists the Shade Coefficient for a sample list of 6 single and 3 double pane glasses using the different calculation methods. It indicates that the ASHRAE2 values are generally lower than the “Carrier” values and that the manual corrections based on absorption coefficient are not to far off the mark.

The last two columns are further alternative values; the ASHRAE NFRC3 (National Fenestration Rating Council) Summer values and the value obtained using the NBNL (Lawrence Berkely National Laboratory) WINDOW54 program with the DA91 wind speeds. These are different again and are discussed later in this paper.

U values which determine the conduction load through a window, even more so, depend on the inside and outside film coefficient and hence the assumed wind speed.

Hence the inside and outside wind speeds are very important factors when determining the conduction and solar gain through windows.

tHe ReleVAnt signifiCAnCe of tHe solAR And ConduCtion loAdsIn an office building in Melbourne the percentage break-up of the lighting and equipment energy consumption and the cooling energy consumption would typically be approximately:

• electricalinputforlightsplustheairconditioningcoolingenergy for the lighting load 28%

• electricalinputforequipmentplustheairconditioningcooling energy for the equipment load 30%

• thecoolingenergyforthesolargainthrough the windows 26%

• thecoolingenergyfortheconductiongainthrough the windows 2.5%

the remaining 13.5% being the cooling energy for the heat from people, heat gain through the walls and roof and outside air.

The first thing to note is that the cooling energy consumption associated with the heat gain through windows is primarily due to the solar gain and is relatively large whilst the conduction load is typically only about 10% of the total window load and 2.5% of the total building energy consumption so the variation in U value with wind speed is not that important in the overall scheme of things but the impact on the shade coefficient is very important. In cooler climates the window conduction load for heating will be more significant, but even then the solar gain is still dominant.

glAss tHeRMAl PRoPeRtiesTo calculate the conduction and solar heat gain through windows, the thermal properties of the glass have to be known:

• Fortheconductionload,theUvalue.

• Forthesolargain,theabsorption,transmissionandreflectioncoefficient or the Solar Heat Gain Coefficient (SHGC) and/or the Shade coefficient (SC)

The properties used depend on whether the air conditioning loads and plant capacities are being estimated or whether the building energy consumption is being estimated. Load calculations, usually employ simplified calculation methods based on industry accepted “design conditions”. For example for the solar gain through windows, the Shade Coefficient is normally taken as the value at an angle of incidence of 30 degrees whereas for energy calculations, values at varying angles of incidence are used. Inside and external wind speeds, which significantly affect U values and to a lesser extent Shade Coefficient and SHGC, are usually assumed fixed for load estimation whereas hourly values are used when estimating energy consumption.

Glass manufacturers publish glass properties for their range of glass types but these are at a given set of industry accepted reference conditions set by (and agreed to by glass manufacturers), the National Fenestration Rating Council in Maryland USA (NFRC3 conditions). The LBNL (Lawerence Berkely National Laboratory) WINDOW54 program, which contains a data base of glasses from manufacturers around the world, can be used to calculate glass properties under different conditions and, in the case of double and triple glazing, different combinations of glass types.

It is important to realise that these NFRC3 reference conditions are for the purpose of comparing glasses and are not design conditions, which can vary with location and the type of air conditioning system.

tHe PHYsiCAl PARAMeteRs tHAt AffeCt tHe glAss tHeRMAl PRoPeRtiesThe parameters that affect the glass thermal properties include at any given time:

• Theinsideairtemperature

• Theinsideairvelocity

• Theeffectiveroomradianttemperature

• Theeffectiveroomemmisivity

020

Pilkington 6mm clear

25 30 35 40

1

2

3

4

5

6

U V

alue

Ambient Temperature

G. James HL219Pilkington 6mm clear + 6mm clear Pilkington Low E + 6mm clear

Figure 1: U-Value variation with temperature


F O R U M

• Theoutsideairtemperature

• Thedirectsolarradiation

• Theoutsidewindspeed

• Theoutsidewinddirection(windwardorleeward)

• Theeffectiveskyradianttemperature

• Theeffectiveskyemmisivity

Of these, the parameters that have the most significant affect are the inside and outside wind speeds. Many of the other parameters

Stand out from the troop

The AIRAH members always stand out . . . They are BETTER trained, BETTER informed, BETTER connected and their post nominals identify them as professionals.

To join visit www.airah.org.au

01


1.5 2 2.5 3 4

1

2

3

4

5

6

U V

alue

Wind Speed m/s


00


0.5 1 1.5

1

2

3

5

4

6

7

U V

alue

Inside Wind Speed


Pilkington 6mm clear Pilkington Low E + 6mm clear

Figure 2: U-Value variation with outdoor wind speedFigure 3: U-Value variation with inside wind speed


F O R U M

will either be unknown by designers or very difficult to ascertain. Figure 1 to 3 illustrate the affect of temperature, external wind speed and internal wind speed on the U value of glass for a number of single glazed and double glazed windows. For these comparisons the WINDOW54 program was used to calculate the glass U values. The full lines are (four) single glazed windows and the broken lines are (two) double glazed windows and it can be seen that the most significant variable is the inside wind speed.

Glass manufacturers data is usually based on the standard NFRC3 Winter conditions (including an ambient temperature of – 18 degrees). Fortunately the ambient temperature has little affect otherwise, with load estimation, a different U value would be required for each hour of the design day in each month.

The outside wind speed does have an affect and for cooling load estimation, DA91 suggests a value of 2.5 while with the NFRC3 Summer conditions a value of 2.8 is stipulated. For NFRC3 Winter conditions the outside wind speed is 5.5 m/s.

The inside wind speed has a far greater affect and while DA91 suggests a value of 1 m/s, the NFRC3 summer condition is based on a value of 0.0 m/s and this significantly reduces the U value (and for the glass manufacturer, gives an apparently better thermal performance of the glass).

The Shading Coefficient is virtually unaffected by ambient temperature, but is affected by the inside wind speed and to a lesser extent the external wind speed as illustrated in Fig 4 and 5 where the same glasses as in figures 1 to 3 are compared.

Conditions on WHiCH glAss tHeRMAl PRoPeRties ARe BAsedFor air conditioning load estimation, industry accepted monthly design dry and wet bulb temperatures as a function of location are readily available. For example in the AIRAH Application Manual DA91 or on the ASHRAE Design conditions CD5.

The suggested or accepted wind speeds are however a single value independent of location, time of year, time of day, etc and differ in the various literatures. So in ASHRAE2 an external wind

speed of 2.8 m/s and an internal wind speed of 0.0 m/s is used. In DA91 an external wind speed of 2.5 m/s and an inside wind speed of 1.0 m/s is implied in the worked examples although calculations at other wind speeds are illustrated.

For load estimation, design values for all the other parameters identified above are not listed in DA91 and in ASHRAE2 only the NFRC3 conditions are listed which as stated earlier are reference conditions for use in comparing different glasses not for designing.

For energy calculations most building energy simulation programs adjust the entered U value and solar properties each hour on the basis of the external wind speed in the hourly weather data being used and the user’s estimate of the internal wind speed (as input to the program). The ambient wind speed in the weather file will normally be that at the Metreological Bureau recording station. This is usually a 10 minute average for each hour and could well be quite different to the wind speed at the project location and on the surface of each window.

The BCA has a Glass Calculator to assist with checking compliance of glazing. This uses the NFRC3 Winter U-value and the NFRC3 Summer SHGC.

It is important to recognise that the NFRC3 “standard” conditions are reference values used by glass manufacturers as a basis for comparing glass types and listing their glass properties. These conditions are not necessarily conditions to be used for estimating air conditioning plant loads and in energy simulation programs, they need to be adjusted each hour particularly for wind speed. For U values, the NFRC3 winter ambient temperature reference condition of –18°C is certainly not appropriate for Australia although fortunately the ambient temperature only has a minor affect.

Appropriate values of design Conditions when using the WINDOW54 program to determine Shade Coefficient and Summer U values based on DA91 for use in load estimation programs, such as the ACADS-BSG program CAMEL are suggested as:

00


0.5 1 1.5

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

1

Shad

e co

effici

ent

Inside Wind Speed



01 1.5 2 2.5 3 4

0.2

0.4

0.6

0.8

1

1.2

Shad

e C

oeffi

cien

t

Wind Speed

Pilkington 6mm clear G. James HL219Pilkington 6mm clear + 6mm clear Pilkington Low E + 6mm clear


Figure 4: Shade coefficient variation with inside wind speed

Figure 5: Shade coefficient variation with outside wind speed


F O R U M

The NFRC3 Conditions used by glass manufacturers when publishing glass properties are:

These are the values that were used in determining the Shade Coefficients in the last two columns in Table 1

tHe WindoW fRAMeYet another important parameter when determining the Shade Coefficient and U value of windows is the frame. A frame with a significantly higher U value than the glass can degrade the thermal properties of the window. The affect is a function of

the U value of the frame itself, the width of the frame (as this changes the percentage area of the glass). The most significant impact is on the U value. The absorption coefficient of the external surface of the frame also has an affect on the Shade Coefficient.

Values of Shading Coefficient and U value of the combined glass and frame can be determined using the LBNL THERM program (for calculating the U value of the frame) in association with the LBNL WINDOW54 (for calculating the overall value for the glass and frame).

Unfortunately in practice the details (properties) of the window frame are often unknown (particularly at the planning or early design stage of the project) and for load estimation calculations, empirical corrections will usually suffice and indeed is often all that can be used.

600.00 1.000.50 1.50 2.00 2.50

Centre of glass

70

80

90

100

110

Win

dow

SH

GC

/ C

of G

SH

GC

(%)

Ratio of frame to glass U value

Large % glass to window 90Small % glass to window 75Small % glass to window 70

Medium % glass to window 80

600.00 2.001.00 3.00 4.00 5.00 6.00

Centre of glass

70

80

90

100

110

Win

dow

SH

GC

/ C

of G

SH

GC

(%)




Figure 6: Window SHGC / C of G SHGC (%) vs Ratio frame to glass U-value for various glass to window areas (single glazing)

Figure 7: Window SHGC / C of G SHGC (%) vs Ratio frame to glass U-value for various glass to window areas (double glazing)

DA91 Summer

Inside air room temperature. (deg°C) 24

Effective room temperature. (deg°C) 24

Outside wind speed (m/s) 2.5

Effective Room and sky Emmissivity 1.00

Outside air and effective sky temp . . . (deg°C) 32.0

Direct Solar Radiation (W/sq.m) 783.0

Inside air velocity (m/s) (Convective Coeff 8.0 W/m2.K) 1.0

NFRC3 Shade Coefficient (Summer)

NFRC3 U value (Winter)

Inside air room temperature (deg°C)

24 21

Effective room temperature (deg°C)

24 21

Outside wind speed (m/s) 2.8 5.5

Effective Room & sky Emmissivity

1.00 1.00

Outside air & effective sky temp. (deg°C)

32.0 -18.0

Direct Solar Radiation (W/sq.m)

783.0 0.0

Inside air velocity (m/s) 0.0 0.0


F O R U M

Figures 6 to 8, illustrate the affects of various frame types expressed (x axis) as the ratio of the frame U value to the glass U value. They are a plot of the ratio of the SHGC or the U-value of the window divided by the Centre of Glass value (no frame) as a percentage for a selection of single and double glazed glass types with varying percent area of glass to window.

Wooden frames have a low U value so that for single glazed windows the frame to glass U value ratio is approximately of 0.4 and for double glazing it is approximately 0.8. At the other end of the graphs, values of 2.0 (single glazing) and 5.6 (double glazing) represent frames without a thermal break which is typical of the frames most commonly used in Australia.

From Figs 6 and 7 it can be seen that with a high U value frame, the SHGC (and hence Shade Coefficient) approaches the Centre

of Glass value and for both single and double glazing, is virtually independent of the % glass to window ratio.

From Figs 8 and 9 it can be seen that with a high U value frame the U value departs significantly from the Centre of Glass value and is very much dependent on the % glass to window ratio particularly with double glazing where the glass U value is much lower.

Tables 2 and 3 are empirical frame and glass to window area adjustment factors for shade factor derived from Figures 7 and 8 for use when only the centre of glass values are available. The factors in Tables 2 and 3 are for use with the Carrier Method as the storage load factors for glass with this method already include an allowance of 85% for wooden frames. For other methods and for energy simulation programs these values need to be multiplied by 0.85

600.00 1.000.50 1.50 2.00 2.50

Centre of glass

70

80

90

100

110

120

130

Win

dow

U V

alue

/ C

of G

U-V

alue

(%)




500.00 2.001.00 3.00 4.00 5.00 6.00

Centre of glass

90

130

170

210

250

Win

dow

SH

GC

/ C

of G

SH

GC

(%)




Figure 8: Window U-value / C of G U-value (%) vs Ratio frame to glass U-value for various glass to window areas (single glazing)

Figure 9: Window U-value / C of G U-value (%) vs Ratio frame to glass U-value for various glass to window areas (double glazing)

Glass/Sash Frame to Centre of Glass U value Ratio

Area % 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

95 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17

90 1.05 1.07 1.08 1.10 1.11 1.12 1.14 1.15 1.17

80 0.93 0.96 0.99 1.02 1.05 1.07 1.10 1.13 1.16

70 0.81 0.85 0.90 0.94 0.98 1.03 1.07 1.11 1.15

60 0.60 0.67 0.74 0.80 0.87 0.94 1.00 1.07 1.14

Table 2: Empirical Shade Coefficient Adjustment for Varying Frames and Glass to Window Area Ratios (Single Glazing)

this is the first part of “Why determining thermal loads though windows is such a pane”.

Part 2 will run in november ecolibrium.

HVAC&R’s

Blue-ribbon Event

2010 AIRAH Awards Presentation DinnerNovember 18, 7.00pm @ the Crystal Palace, Luna Park, Sydney

The AIRAH awards recognise those members, individuals and companies who have exhibited outstanding achievements

in the previous year.

For booking enquiries, email [email protected]

Award Sponsors:

why determining the window is such a pane

Documents