Energy Efficient Buildings



Energy Efficient BuildingsWindows and Solar Heat GainIntroductionA major pathway of heat loss and gains into buildings is through windows. Heat is lost and gained through windows through:conductionsolar radiationair infiltration through cracks/leaksThis chapter discusses:physical mechanisms of these pathwayshigh performance window constructionUwin (conduction)SHGC (solar heat gain coefficient)Thermal Properties of Windows Single PaneSingle Pane & Storm Window Rules of Thumb:Single Glaze: R = 1 (hr-ft2-F/Btu)Double Glaze: R = 2 (hr-ft2-F/Btu)Triple Glaze: R = 3 (hr-ft2-F/Btu)These calculations show that heat loss/gain through windows is 5 to 15 times greater than through walls. This provides strong motivation to develop high performance windowsHigh Performance Windows1st task is to stop air movement between panesAir movement from leakage reduces Rspace = 1.01 above, and essentially makes storm windows the same as a single pane windowAir movement between panes causes a convection cell which increases heat transferWhen the space between windows < 1-inch then Fviscous > Fbuoyant air doesn’t move and have conduction instead of convectionIncreasing space decreases conduction, but increases convectionOptimum appears to be 0.5-1.0 inches (Pella windows are ~)2nd task is to find & stop greatest heat lossCase 1 Regular glass, air Case 2 Low ε, air (ε = .10) Case 3 Low ε, argon (ε = .10, kargon = .108) The center-of-glass U value would be:Q/A = U dTU = Q/A dt = 13.1 Btu/ft2-hr / 50 F = 0.26 Btu/hr-ft2-F Source: ASHRAE FundamentalsTotal Conductive Heat Loss/Gain through WindowsThermal resistances of:center-of-glass (cg)edge-of-glass (eg)frame (f)are different.Thus, the total conductance of the window must include all three. In general, Ueg > Ucg > Uframe. Thus, the conduction heat transfer through a window is Use Toa instead of Tsa since and α << 1.Window TypesWindow type influences air leakage and frame fraction. For passive solar, look for windows that seal shut, such as casement or awning windows rather than sliding windows to reduce air leakage. Also look for windows with high ratios of Aglass / Awin to maximize Qsol and reduce edge-of-glass heat loss. Finally, choose high quality wood or vinyl frames since low-quality frames can warp and allow air and moisture (condensation) between glazings.Window Thermal Performance DataData Source: PellaSolar Heat Gain Coefficient All solar radiation incident on a window is absorbed, transmitted or reflected. From an energy balance, the sum of the absorbtivity, transmittance and reflectivity equals 1.0Solar transmittance of glass is a function of wavelength and the angle of incidence. In general, glass transmits visible light, but is opaque to infrared light. Since most of the radiation from the sun lies in the visible part of the spectrum, windows transmit most of the energy in solar radiation into the building. After the solar radiation is absorbed by building surfaces, the surfaces reradiate the heat as low temperature infrared radiation. Since glass is opaque to infrared radiation, this radiation remains inside the building The transmittance of glass is also highly dependent of the angle of incidence; at perpendicular angles most of the light is transmitted and at low angles of incidence most of the light is reflected. Solar heat gain coefficient, SHGC, is the fraction of solar radiation incident on fenestration that is transmitted into the building. SHGC includes fraction of solar transmitted plus fraction of solar that is absorbed by glass and radiated into the building:Solar heat gain coefficients are shown in the tables below.In the Tables above, note that the transmittance of visible light, labeled as “Center Glazing Tv” is different than the “Center of Glazing” SHGCs. Most daylighting applications seek to maximize the transmittance of visible light. In the Tables above, note that “Total Window” SHGC is proportionally less than the “Center of Glazing” SHGC by the proportion of glass area to total window area (Aglass / Awin). Thus, to calculate solar heat gain using the total window area Awin use the “Total Window” SHGC. The SHGC reported on window labels in the “Total Window SHGC at Normal Incidence”. The SHGC is different for diffuse solar radiation, which is spread evenly over the sky, and beam solar radiation which is directional. Thus, the total solar energy into a building, Qsol, is:In the northern hemisphere, most solar radiation is from the south. Thus, to simplify calculations, the average solar heat gain coefficient can be approximated as:Thus, using the average solar heat gain coefficient, the solar heat gain through a window can be calculated as:Selective and Non-Selective Low-e Coatings While low-e coatings and multiple glazings significantly reduce U, they also significantly reduce SHGC. This is good for commercial buildings, warm climates, and E,W windows that allow more Qsol in summer than winter. But it is bad for passive-solar heating applications. Original low-e coatings had higher SHGCs. But new low-e coatings are “selective” and reduce transmittance in the IR range, thus reducing SHGC. Fortunately, old-style, non-selective, low-e coatings are available if specified in the US, or as the standard, in Canada. Selective data from: Ellison, Tim, American Solar Energy Society Annual Conference, Madison WI, 2000.The Efficient Windows Collaborative () characterizes window performance into three groups according to solar gain: low, mid and high. In many temperature U.S. climates, engineers often specify high SHGC windows for south-facing exposures and low SHCG windows for all other exposures. This enhances solar gain during winter when the sun Is primarily in the south, and reduces solar gain during summer when the sun hits north, east and west exposures. Center-of-glass properties are:Whole window properties are:National Fenestration Rating Council ()right0The National Fenestration Rating Council (NFRC) energy performance label can help you determine how well a product will perform the functions of helping to cool your building in the summer, warm your building in the winter, keep out wind, and resist condensation. All energy performance values on the label represent the rating of windows/doors as whole systems (glazing and frame) for 48-in by 59-in gross window size.U-Factor The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. In U.S. units U-Factor ratings generally fall between 0.20 and 1.20 Btu/hr-ft2-F. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.Solar Heat Gain Coefficient The SHGC is the fraction of incident solar radiation admitted through a window (both directly transmitted and absorbed) and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits in the house.Visible Transmittance Visible Transmittance (VT) measures how much light is transmitted through a product. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted. Air Leakage* Air Leakage (AL) is indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). The lower the AL, the less air will pass through cracks in the window assembly. Condensation Resistance* Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. CR is a number between 0 and Heat Transfer Through WindowsThus, the net heat transfer through a window, not including air leakage due to infiltration, can be calculated as the sum of the solar and conduction heat transfer: Example: Calculate the net heat gain into a building through a south-facing window. The total area of the window is 6 ft2, the total U value of the window is 0.30 Btu/hr-ft2-F, the total normal SHCG of the window is 0.70. The outdoor air temperature is 30 F. The indoor air temperature is 70 F. The total solar radiation on each of a south exposure is 150 Btu/hr-ft2.Windows and OverhangsIn the Northern hemisphere, the sun makes a path across the southern sky; thus, south-facing exposures on buildings get much sunlight than other exposures. In addition, in the summer the sun takes a higher path across the sky than in the winter. The seasonal variation in the path of the sun across the sky can be utilized to maximize solar gain during winter and minimize it during summer. In the Northern Hemisphere, overhangs on south facing windows create shade in the summer, but allow direct sunlight to hit the windows in the winter. Thus, overhangs on south facing windows are highly effective at reducing solar heat gain during summer and facilitating solar heat gain during winter.Source: )The path of the sun across the sky varies with latitude as well as with the season (see Figure below). At locations near the equator, the sun path is high in the sky and the sunset and sunrise are about 12 hours apart all year long. At locations far from the equator, the sun path is low in the sky and the number of daylight hours are large in the summer and small in the winter. (CollectorSpacing_SunPosition.xlsx)To estimate shading, it is necessary to know the position of the sun in the sky. The position of the sun in the sky can be calculated for any latitude at any given date and time (see for example: Duffie and Beckman, Solar Engineering of Thermal Processes, John Wiley and Sons, 2013). This position can be plotted in sun path charts. For example, at 40 north latitude, the sun has an altitude angle of about 57 at 60 west of south on July 21 at 2 pm (see chart below).The position of the sun in the sky can be calculated for any latitude at any given date and time. WeaTran calculates this position, and uses it to determine solar gain on any exposure. WeaTran can also calculate net solar gain minus shading from overhangs and wings when the geometry of the overhang or wing is specified by enabling the “Advanced Solar Options button” If the windows are protected by horizontal overhangs or vertical fins along their sides, enter the height of the window “hc”, the protrusion of the overhang or wing “po”, and the gap between overhang/fin and window “gapo” as shown in the figure below. ................
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