Chapter 5A: HEAT GAINS THRU WINDOWS Agami Reddy (rev Dec 2017) Basic considerations and window characteristics Window performance metrics Inter-pane convective and radiative interactions Overall U values of window Surface temperature of glazing Solar heat gains and SHGC Radiative properties of glass Thermal loads due to solar radiation HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Basic Considerations Window and glazing choices should be considered holistically. Issues to consider include: Heat gains and losses Visual requirements (privacy, glare, view) Shading and sun control Thermal comfort Condensation control Ultraviolet control Acoustic control Color effects Daylighting http://www.wbdg.org/design/windows.php Spacer material for windows with two or more glass panes are made of metal or insulators placed all along the perimeter to keep the panes apart. They act as undesirable thermal bridges and increase U value of center of pane Figure 5.1. Sketch of a double pane window HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Fig. 5.1(b) Right photo: Low-e double pane glass sash with stainless steel spacers and rollers at bottom (not visible). The I-section provides rigidity and also insulates the glass frame. Left photo: The frame for a double sliding window on which the sash slides. The several discrete compartments are meant to provide added rigidity and also increase thermal resistance. HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Fig. 5.2 Different Types of Residential Windows HCB-3 Chap 5A: Window Heat Gains

Window Performance Metrics Important characteristics needed to specify window system: Window U-value - typical aluminum frame single glazed window: 1.3 Btu/(h·ft²·°F) or 7.4 W/m2.K - multi-paned, high-performance window with low-emissivity coatings and insulated frames: 0.2 Btu/ (h·ft²·°F) or 1.1 W/m2.K Window Solar Heat Gain Coefficient (SHGC) of a particular window- ratio of solar heat gain to incident solar radiation Glass Visible Transmittance - same as above but limited to visible solar spectrum, this is proportional to the luminous efficacy of transmitted daylight Air leakage About 1/3rd of heat losses are thru windows HCB-3 Chap 5A: Window Heat Gains

Fig. 5.3 Example of National Fenestration Rating Council Label HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Solar Heat Gains Figure 5.6 Distribution of beam radiation falling on glazing product #1a of Table 5.1 at normal solar incidence. HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Notice that the sum adds up to 1.07. Explain why? HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Heat Gain Equations Total instantaneous heat flow thru glass = radiation transmitted through glass + inward flow of absorbed solar radiation + conduction heat gain Solar heat gains Could be inwards or outwards depending on season Q= UA (Tout – Tin) +A. (SHGC) IT Incident solar radiation Overall heat loss coefficient Solar Heat Gain Coefficient (non-dimensional) HCB-3 Chap 5A: Window Heat Gains

Conduction: Single Pane Window Figure 5.8 Energy balance of a single-pane window (neglecting heat capacity of glass). Physical configuration. Thermal network without solar radiation. Thermal network with solar radiation (and neglecting resistance in glass). Glass accounts for about 2% of total resistance - negligible BTU/h-ft-F HCB-3 Chap 5A: Window Heat Gains

Inter-Pane Heat Transfer from Chap 2: For infra-red radiation, glass behaves like an opaque material (2.69) Using the concept of radiative coefficient: Good approximation when surface temperatures are close or A.htotal =A.hcon+A.hrad HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Double Pane Window For double pane glass: convective heat loss from one pane to another roughly equal to radiation HT between panes can be reduced: - by suppressing convection: inert gases, small inter-pane gap - by reducing radiation: selective films HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Figure 5.9 Variation of center-of-glass U values with gap width for different window emissivities and gas fill materials. (a) Double-pane window. (b) Triple- pane window. Argon and krypton are less conductive gases than air One could try creating a vacuum between both panes, but that is not practical Experimental studies indicate that convection is suppressed when spacing < 13mm, and heat transfer is by conduction thru air. For larger spacing, increase in heat transfer is offset by thicker air layer. No point in increasing separation between panes > 10 mm (0.4 inches) HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Example 5.1: HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Table 5.3 HCB-3 Chap 5A: Window Heat Gains

Overall U-values for Windows Spacer material for windows with two or more glass panes are made of metal or insulators placed all along the perimeter to keep the panes apart. They act as undesirable thermal bridges and increase U value of center of pane. Area-weighted equation: The edge effect is not applicable for single pane windows Figure 5.10 Center-of-glass and edge-of-glass U values for a variety of spacer materials (“ideal” corresponds to a spacer with the same U value as the ­glazing). HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Example 5.2 Given: Center-of-glass U value (double glazed, airspace width 9 mm, ε = 0.10) and edge-of-glass U of for aluminum spacer Find: Uav Sketch: Figure 5.11 Lookup values: Ucg = 1.9 W/(m2 · K) from Figure 5.9 Ueg = 2.8 W/(m2 · K) from Figure 5.10 Uf = 10.8 W/(m2 · K) from Table 5.2 Figure 5.11 Dimensions of the frame, edge of glass (spacer), and center of glass for Example 5.2, drawn to scale. HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Dividing the sum of the UA values by the sum of the A values according to Equation 5.5 HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Interior Surface Temperature of glazing: Impacts comfort Surface temperature of glazing (Eq. 5.8): Assuming Ti = 20° C and hi = 8.29 W/(m2 .K). Figure 5.12 Interior glass surface temperature versus outdoor temperature, “Downdraft” actually caused by radiation heat loss from body to window surface HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Solar Heat Gains Figure 5.13 Components of solar heat gain with a double-pane window. HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains How should an ideal glazing behave spectrally when coupled to a conditioned space: In cold locations, it should have high solar radiation transmittance while acting as a good reflector in the long-wave infra-red spectrum emitted by the warm interior surfaces of the building. This is the principle of low e-coating (also called high-solar gain low-e) on window glass in cold climates (i.e., low emission over the long-wave spectrum). Figure 5.7 Conceptual illustration of two spectrally ideal selective glazing surfaces: one for hot climates and one for cold climates In hot locations, it should only let solar radiation in the visible portion of the solar spectrum, and block both solar radiation from the other spectral regions as well as infra-red radiation from outdoors to enter the conditioned space. This type of glass is referred to as selective low-e or low-solar-gain low-e glazing system. HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Commercially available glazing Fig. 5.4 Spectral transmittance for different types of glass HCB-3 Chap 5A: Window Heat Gains

Change of Radiative properties with Incidence Angle Figure 5.5 Variation of solar optical properties as a function of incidence angle θi, for three types of glasses: A = DSA (double-strength sheet), B = 6 mm (0.25 in.) clear glass, and C = 6 mm (0.25 in.) gray, bronze, or green tinted heat-absorbing glass. (a) Transmittance. (b) Reflectance. (c) Absorptance. HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Thermal Loads due to Solar Radiation Total instantaneous heat flow thru glass = radiation transmitted through glass + inward flow of absorbed solar radiation HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Center-of-Glass Total Window HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Eq. 5.11 HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Expression for SHGC Consider the simple one pane glazing configuration (Figure 5.8). So as to simplify the derivation, let us overlook the difference between beam and diffuse radiation effects. In the presence of solar radiation, the total heat gain becomes HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains SHGC approach combines the transmitted and absorbed solar radiation effects- used for simplified calculations. However, accurate cooling load calculations need to treat these separately since: transmitted solar radiation is absorbed by room walls and furnishings and released later in time absorbed solar radiation is released by convection almost instantaneously ASHRAE Fundamentals Handbook allows such a separation to be made using a software called WINDOWS 5.2 (we shall not discuss this in this class) HCB-3 Chap 5A: Window Heat Gains

HCB-3 Chap 5A: Window Heat Gains Outcomes Knowledge of various issues to be considered while selecting glazing Familiarity with different window designs in residential and commercial buildings Understanding of window metrics: U-value, SHGC, glass visible transmittance, air leakage Familiarity with ways to model instantaneous heat gains combining incident solar radiation and conduction heat gains Be able to calculate overall U-values for different window designs and be familiar with the use of lookup tables Familiarity with spectral properties of an ideal glazing either in a cold or hot location. Understanding of how radiative optical properties (transmissivity, reflectivity, absorptivity) vary with solar incidence angle Understanding of the concept of SHGC, basic modeling equations and use of lookup tables HCB-3 Chap 5A: Window Heat Gains