Energy performance of windows based on net energy gain

Transcription

1 Energy perforance of windows based on net energy gain Svend Svendsen, Professor, Technical University of Denark; Jesper Kragh, ssistant Professor, Technical University of Denark; Jacob B. Laustsen, ssistant Professor, Technical University of Denark; KEYWORDS: Window, glazing unit, frae, net energy gain, energy perforance. SUMMRY: The paper presents a new ethod to set up energy perforance requireents and energy classes for windows of all diensions and configurations. The net energy gain of windows is the solar gain inus the heat loss integrated over the heating season. The net energy gain can be calculated for one orientation or averaged over different orientations. The averaged value ay be used for energy labeling of windows of standard size. Requireents in building codes ay also be based on the net energy gain instead of the theral transittance of the window. The size and the configuration of the window, i.e. nuber of glazing units, have a very large effect on the net energy gain. Therefore the energy labeling or the requireents based on the standard size ay not give valid inforation on the energy perforance of windows of non-standard size. The paper presents a ethod to set up requireents and classes for energy perforance based on the net energy gain that includes the effect of window size and configuration. The net energy gain of windows can be divided into the net energy gain of the glazing unit and the heat losses of the frae and the assebly of the glazing unit and the frae. The glazing unit contributes proportionally to its area. The frae also contributes proportionally to its area, but as the area of the frae is the width ties the length, the heat losses of the frae and the assebly both contribute proportionally to the length of the frae. ccordingly, the net energy gain of the window can be expressed as a function of two paraeters representing the energy perforance and two paraeters representing the geoetry of the window. The two energy perforance paraeters are the net energy gain per area of the glazing unit and the su of the heat losses through the frae and the assebly per length of the frae. The two geoetry nubers are the area of the glazing unit relative to the window area and the length of the frae profiles relative to the window area. Requireents and classes for the energy perforance of the window can be given by assigning values to the two energy perforance paraeters. In this way the requireents and classes for the energy perforance of the window becoe a function of the geoetry of the window. Therefore the effect of the diension and the configuration of the actual window are taken into account and ake it possible to express the energy perforance of all windows in a correct anner. The ethod is useful in relation to coparison of window products for replaceent of existing windows, but ay also be useful for handling the energy perforance of windows in a general but realistic way in the early phase of the design of new buildings. 1

2 1. Introduction The net energy gain of windows is the solar gain inus the heat loss integrated over the heating season. The net energy gain expresses in a siple way the energy perforance of windows. In relation to the introduction of the directive on the energy perforance of buildings in EU there is a shift fro heat loss to used energy in the characterization of buildings with respect to energy perforance. The net energy gain of windows is useful in quantifying the energy perforance of windows. The net energy gain can be calculated for glazing units separately and for coplete windows based on the total solar energy transittance and the theral transittance of the two types of products. For glazing units center of glass values are used and therefore the net energy gains of glazing units do not depend on the size or shape of the unit. For windows, the total solar transittance and the theral transittance are averaged over the glazing unit and the frae. s these values depend on the size and configuration of the window the net energy gain will also change due to change of geoetry of the window. Therefore energy labeling or energy perforance requireents based on net energy gain of a standard size ay not give valid inforation on the energy perforance of windows of non-standard size. The paper presents a ethod to set up requireents and classes for energy labeling of windows based on the net energy gain that includes the effect of window size and configuration.. Net energy gain of windows The product standard pren (CEN, 005) allows the total window perforance to be based on easureents/calculations on a standard window, and these results are then claied to be valid for all other window sizes. This is not an appropriate approach, since the actual area and geoetry of a window has a great influence on the overall window energy perforance. This is due to the fact that a saller window has a relatively greater frae fraction than a bigger window if using the sae frae diension. sall window diension can therefore be expected to have a higher overall U-value and a lower total solar energy transittance since the relative glazing area has been reduced. This can ake significant changes to the result of the net energy gain of the window. National building codes typically include requireents for windows based on EN ISO (CEN, 000) and therefore need detailed inforation on energy properties and geoetry. Calculations have been perfored to docuent the consequences. In table 1 basic properties of different windows have been listed for 3 window sizes Large window, standard size + 5% according to specien size in pren14351: [ ] ² Standard size according to pren14351: [ ] ² Sall window, standard size - 50%: [ ] ² The glazing of the windows have the sae theral transittance, U g = 1. W/²K and the sae solar energy transittance, g g = 0.3. The used net energy gain equation is described in (Nielsen T. R., Duer K and Svendsen S, 000) and applies to Danish conditions and does not include air pereability: E ref = I g D U = 19 g 90 U [kwh/²] (1) w w w w Where I is the solar radiation calculated for a reference house and D is the degree hour nuber during the heating season in Denark. U w is the total theral transittance and g w is the total solar energy transittance of the window. It is seen that the net energy gain will be negative when energy is lost.

3 Table 1. Coparison of calculated energy properties of three window sizes for five different windows. Large: 1.54 x 1.85, Standard (Std.): 1.3 x 1.48 and Sall: 0.15 x Calculations are done with sae theral transittance of the glazing (1. W/²K) and sae solar energy transittance of the glazing (g g = 3%). U f is the theral transittance of the frae and Ψ is the linear theral transittance of the assebly between frae and glazing. U w and g w is the total theral transittance and solar energy transittance respectively, and E ref is the net energy gain of the window in a reference house. Frae and assebly data Solar transittance Theral transittance Net Energy Gain U w [W/²K] E ref [kwh/²] Width U f Ψ g w [] [W/²K] [W/K] [%] Window \ Size Large Std. Sall Large Std. Sall Large Std. Sall Wood Wood / luiniu PVC luiniu Mixed aterial s it can be seen fro table 1 there are big differences in the obtained theral transittance and total solar energy transittance and therefore also in the net energy gain. The difference in energy perforance is for all windows larger for different sized windows than for different products. The declared values of windows according to the proposed product standard ay not be directly coparable and can not be used to express the energy perforance of windows with different configurations and sizes. This is also illustrated in the following Fig. 1 where the product standard is used in three different ways. 1. U and g are calculated corresponding to the actual diensions.. U is fixed (based on easureents) and g is calculated corresponding to the actual diensions. 3. Both U and g are fixed and based on the standard size [ ] ². 0 Usage of fixed and variable theral transittance and vaiable total solar energy transittance The fixed theral transittance is based on [ ] Danish net energy gain [kwh/² year] Window properties Frae, U f = 1.8 W/²K Total window area [²] Variable Ut and g Ut based on fixed size fixed Ut and g Glazing: U g = 1. W/²K Edge construction:ψ = 0.10 W/K Width of frae: b = 110 FIG. 1. Fixed or variable U- value cobined with variable g-value s can be seen fro Fig. 1 using fixed values of U and/or g has a great influence on the net energy gain when looking at various window sizes. 3

4 Seen fro the consuer viewpoint, levels of docuentation are iportant. 1. Window energy properties declared for a nuber of typical configurations of standard size [ ] ² for a preliinary coparison of products.. Window energy properties declared for the actual configuration and size in question for a correct coparison and use of data for windows..1 Data on energy perforance of actual windows It cannot be recoended to apply pren (CEN, 005) in the atter of coparison of energy perforance of window sizes. It is recoended that the overall window energy perforance of the theral transittance and total solar energy transittance is based on the standard size for general coparison and on the actual type (configuration) and size for coparison of windows for a specific use. In both cases the U-value should be calculated in accordance with the calculation ethod described in EN ISO (CEN, 000). This way of docuenting the basic energy perforance data of windows should be based on data fro test or calculations of the individual docuentation and inforation on: 1. Radiation data of the glazing. Theral transittance of glazing 3. Theral transittance of frae 4. Linear theral transittance of edge construction in the assebly of the glazing and the frae, Ψ. The window anufacturers already use the declared values on the glazing unit fro the glazing anufacturer. The window anufacturer could in a siilar way declare the U f -value of the frae and the Ψ-value of the edge construction in the assebly based on table values in EN ISO (CEN, 000) or detailed calculations according to EN ISO (CEN, 003). Based on these energy perforance data on the glazing unit and the frae and edge construction, the window anufacturer could for any actual window configuration and size calculate the actual g- and U- value as well as the standard net energy gain. 3. Energy perforance classes of windows The net energy gain of windows can be divided into the net energy gain of the glazing unit and the heat losses of the frae and the assebly of the glazing unit and the frae. The glazing unit contributes proportionally to its area. The frae also contributes proportionally to its area, but as the area of the frae is the width ties the length, the heat losses of the frae and the assebly both contributes proportionally to the length of the frae. ccordingly the net energy gain of the window can be expressed as a function of two paraeters representing the energy perforance and two paraeters representing the geoetry of the window. The two energy perforance paraeters are the net energy gain per area of the glazing unit and the su of the heat losses through the frae and the assebly per length of the frae. The two geoetry nubers are the area of the glazing unit relative to the window area and the length of the frae profiles relative to the window area. Requireents and classes for the energy perforance of the window can be given by assigning values to the two energy perforance paraeters. In this way the requireents and classes for the energy perforance of the window becoe a function of the geoetry of the window. Therefore the effect of the diension and the configuration of the actual window are taken into account and ake it possible to express the energy perforance of all windows in a correct anner. 4

5 3.1 Net energy gain as a function of the windows diension and construction The solar and theral transittance of the window depends on the glazing and frae area and the assebly length between the glazing and frae profile. The net energy gain of the window is therefore also a function of the window geoetry. specification of the energy quality of a window can be established by using the data of each specific coponent and then calculate the net energy gain of the window using the actual geoetry. good energy window on the Danish arket can be based on the following data: Low energy glazing with net energy gain of 15 kwh/² Frae with a theral transittance of 1. W/²K, width 0.10 giving a product of 0.1 W/K Good assebly of the glazing and frae using a war edge construction. Linear theral transittance 0.0 W/K Spacer profiles inside sealed glazing units that ake the look as ulti-units. Linear theral transittance 0.0 W/K corresponding to the length of the internal spacer profile (0.0 W/K covers both sides of the spacer profile) The geoetrical data can be expressed by the following relative values: Glazing part: rea of the glazing units, g, in proportion to the window area, : g R = (1) ssebly part of the profiles: The length of the assebly between glazing and frae profile in proportion to the window area, : L ssebly P = () ssebly part of the window bars: The length of the window bars in proportion to the window area, : Lsseblywindowbars S = (3) Where L ssebly is the length of the assebly between the glazing and frae profile and L ssebly window-bar is the length of the window bars. The length of the window bar corresponds to the actual length and not both side lengths. The relative length of the frae profile is the sae as P. Hereby the corners of the windows are not included in the calculation. The four corners are therefore assued each to have an area of 0.01 ². The liit of the net energy gain of a window can then be expressed as a function of the geoetry. gain applied to Danish conditions with a degree hour nuber of 90 kkh gives: 1. E = 15 R P ( P + S) = 15 R 19.8 P 5.4 S (4) 15 R 0 P S It is suggested that today s standard windows fulfilling equation (4) is classified as B-windows. Iproved windows with triple glazing, 1+ glazing, better insulated frae profiles, and war edge construction could then be classified as -windows. Triple glazing or 1+ glazing with net energy gain of 5 kwh/² Frae with a theral transittance of 1.8 W/²K, width 0.05 giving 0.09 W/K Good assebly of the glazing and frae using a war edge construction. Linear theral transittance 0.0 W/K 5

6 Window bars with closed glazing units. Linear theral transittance 0.0 W/K corresponding to the length of the window bar (0.0 W/K covers both sides of the window bar) The net energy gain liit of windows classified as is then expressed as: E 5 R 14 P S (5) Exaple of calculation of the net energy gain deand for a new window of PVC with two different edge constructions. The window is shown in Fig. and the window data in Table FIG. : New window of PVC The net energy gain of the window with a war edge construction of plastic: Table : Window data Total theral transittance: Edge construction of plastic (war edge) U w 1.5 W/²K Edge construction of aluiniu U w 1.79 W/²K Total solar transittance g w Window diensions Height Width 1.30 Total area tot 1.80 ² Frae profile height Transo/Mullion height 0.15 Glazing area g 1.0 ² Frae area f 0. ² Glazing part (1) R 0.5 ssebly lengths: Frae profile (vertical).88 Frae profile (horizontal) Length ullion ( sides).88 Length transo ( sides) Total length l g 8.15 ssebly part of profiles () P 4.48 E = 19 g 90 U = = 80kWh / The net energy gain of the window with an edge construction of aluiniu: E = 19 g 90 U = = 93kWh / The liit of the net energy gain of the specific window using equation (4) (B-window): E = 15 R 0 P S = = 84kWh / The liit of the net energy gain of the specific window using equation (5) (-window): E = 5 R 14 P S = = 50kWh / The window is seen to be classified as a B-window if a war edge construction is used. () (7) (8) (9)

7 Exaple of calculation of the net energy gain deand for a sall bathroo window The sall bathroo window is shown in Fig. 3 and the window data in Table 3. Table 3: Window data Window diensions: Height 0.00 Width 0.00 Total area tot 0.30 ² Frae profile height Glazing area g 0.1 ² Frae area f 0. ² Glazing part (1) R 0.44 ssebly lengths: Frae profile (vertical) 0.8 Frae profile (horizontal) Total length l g 1.0 ssebly part of profiles () P 4.44 FIG. 3: Sall bathroo window Theral transittance: 0 U g g + U f f + ψ lg U = = = 1.7 W tot 0.3 Solar transittance: g glazing = 0.3; g window = 0.8 The net energy gain of the window: E = 19 g 90 U = = 98kWh / (11) The liit of the net energy gain of the specific window using equation (4) (B-window): E = 15 R 0 P = = 99kWh / The net energy gain of the sall bathroo window is seen to be just under the liit value of a B-window. Exaple of calculation of the net energy gain deand for a window classified as n iproved window with narrow frae profiles, glazing with low iron content increasing the solar transittance and war edge construction of plastic is developed and produced at the Technical University of Denark. The total theral transittance (U w ) of the window is 1.8 W/²K and the solar transittance (g w ) is The window is shown in Fig. 4 and the window data in Table 4. FIG. 4: Iproved window K (10) (1) Table 4: Window data Window diensions: Height 1.3 Width 1.48 Total area tot 1.8 ² Frae profile height Glazing area g 1.53 ² Frae area f 0.9 ² Glazing part (1) R 0.84 ssebly lengths Frae profile (vertical).744 Frae profile (horizontal).44 Total length l g ssebly part of profiles () P.74 7

8 The net energy gain of the window: E = 19 g 90 U = = 3.5kWh / The liit of the net energy gain of the specific window using equation (5) (-window): E = 5 R 14 P S = = 18.5kWh / The net energy gain of the iproved window is seen to be well under the liit value of the -window classification even though only double glazing is used. 4. Conclusion The proposed ethod of declaring the energy perforance of windows based on data on the glazing units and the frae profiles would ake it easier and better for both the anufacturer and the buyer or specifier of windows. The anufacturer could easier docuent his declaration of energy perforance of his product by use of the data on the profiles used in the windows. The buyer and specifier of windows could get the relevant energy perforance data on the actual window products to be used in new or existing buildings. The proposed ethod of energy labeling with classes based on energy perforance of the glazing unit, the frae profile and the edge construction would ake it possible to copare window products for an actual window geoetry. This would be useful in relation to coparison of window products for replaceent of existing windows, but ay also be useful for handling the energy perforance of windows in new buildings as the designer of the building would be able to see the effect of the geoetry and the basic energy perforance of windows separately. The two ethods are suited to be cobined and would offer a better and easier way of treating the energy perforance of windows in relation to the CE-arking of windows and energy labeling of buildings. 5. References Nielsen T. R., Duer K and Svendsen S, (000), Energy perforance of glazings and windows. Solar Energy, Vol. 9 (Suppl.), Nos. 1-, pp CEN (005), pren 14351, Windows and pedestrian doorsets - Product standard, perforance characteristics Part 1: Windows and external pedestrian doorsets without resistance to fire and soke leakage characteristics but including external fire perforance for roof windows. CEN (000), EN ISO , Theral perforance of windows, doors and shutters Calculation of theral transittance Part 1: Siplified ethod. CEN (003), EN ISO , Theral perforance of windows, doors and shutters Calculation of theral transittance Part : Nuerical ethod for fraes. (13) (14) 8