IBS 1-Envelope_2.pdf

Module 1 Envelope 2 Integrated Building Systems 1 Module1: Rawad El Kontar Announcements Previously on IBS 1 1. Modes of heat transfer: Convection, Conduction, Radiation 2. Definition of an Envelope: One widely used definition would be that of what envelopes the conditioned space of the buildin...

Module 1 Envelope 2 Integrated Building Systems 1 Module1: Rawad El Kontar Announcements Previously on IBS 1 1. Modes of heat transfer: Convection, Conduction, Radiation 2. Definition of an Envelope: One widely used definition would be that of what envelopes the conditioned space of the building 3. U-value & R-value : U-value is the rate of transfer of heat through a material through the 3 modes of heat transfer. R-value is the thermal resistance = 1/U-value 4. Calculate how much heat is transferred through the wall , calculate R- value and Uvalue 5. Solar Reflectance Index (SRI): expressed as a fraction which combines the material properties of emissivity and reflectivity. It indicates the ability of material to reflect heat Fourier’s Law of Conduction Heat transfer through the wall’s different layers What are the thermal processes affecting the Envelope? Outside (92 F) = thermal conductivity (W/m*K) = amount of heat transferred through a material = difference in temperature (K) Inside (68 F) k1 k2 k3 k4 are the thermal properties of a material? R value is mainly used for Walls and Roofs • Thermal resistance ( R ) is directly proportional to the thickness of a wall is the R Value [ 𝑚! 𝐾/𝑊 ] is Thermal Conductivity is the thickness of the material • Thermal resistance is inversely proportional to the thermal conductivity. What are the thermal processes affecting the Envelope? Resistance of Multiple Layers Outside (92 F) Rse R3 R2 R15 R3 Rsi Inside (68 F) R Value is additive is additive in R an electrical circuit R Total = Rsejust+asRCurrent + R + R + + R 1 2 3 4 si Rtotal=R1+R2+R3+R4 Rsi = Interior surface thin air thermal resistance (constant) Rse = Exterior surface thin air thermal resistance (constant) is the R Value is the U Value Windows Why are windows special? Selecting a Window The U Factor is how well the window resists heat The Solar Heat Gain Coefficient (SHGC) is the fraction of how much solar radiation is admitted through the window Visible Transmittance is the fraction of how much light comes through Windows • U-Value • Solar Heat Gain Coefficient (SHGC) • Visual Light Transmittance (VLT) • Window Wall Ratio (WWR) Windows U-value Tout = 40 ℃ 39 ℃ 36 ℃ 31 ℃ 24 ℃ SGU 4mm U = 5.8 DGU 4mm - 10mm – 4mm U = 5.8 Low -e 4mm - 10mm – 6mm U = 5.8 Low -e 4mm – 15 mm – 6mm U = 1.1 Windows SHGC • U-value refers to the flow of non-solar heat or heat due to temperature difference. • Another property of a Window is SHGC : the ration of solar heat gain entering the space through a window and the incident solar radiation ( value between 0 and 1) Windows SHGC 100 % Energy Reflection (ER) Directly Energy Transmitted (ET) Solar Heat Gain Coefficient (SHGC) Re-radiated Outside Re-radiated Inside Energy Absorption (EA) ER + EA + ET = 100 % SHGC vs U-value Solar Radiation Energy = 800 W Temperature difference = 20 ℃ Glass Area = 1 𝑚! SHGC = 0.3 U-value = 3.0 3 x 20 0.3 x 800 240 Watts SHGC contribution = 80 % + 60 Watts Total Heat Gain = 300 Watts U_value contribution = 20 % Windows allow Direct Solar radiation à SHGC is a very important factor to reduce Heat gain in hot and cold climates What SHGC to use in each climate ? • Low SHGC in hot climate • High SHGC in cold climate Shading devices can be characterized by Projection Factor Projection Factor Calculation Horizontal shading - Overhangs Vertical shading – Vertical fins Adjusted SHGC Adjusted SHGC = SHGC / SEF • SEF is the Shading Equivalent Factor • It is different for different latitudes and orientations To calculate Adjusted SHGC: 1. Determine the PF 2. Select SEF Coefficients 3. Calculate EF then Adjusted SHGC Calculating Adjusted SHGC Example Window to Wall Ratio (WWR) Visual Light Transmission (VLT) • VLT is and optical property of a light transmitting material which indicated the amount of visible light transmitted of the total incident light. It is expressed as a percentage or values between 0 and 1. Window choice is based on these 3 properties: • U-Value • SHGC : solar heat gain transmitted • VT: visible light transmitted Hot climate Cold Climate low U-value Low U-Value Lower SHGC Higher SHG Higher VLT Higher VLT Learning Objective 4 What is a thermal bridge? What is a thermal bridge? Outside (92 F) Inside (68 F) What is a thermal bridge? What is a thermal bridge? What is a thermal bridge? What is a thermal bridge? Learning Objective 1 In what states (phases) does water exist around us? In what states (phases) does water exist around us? Solid: Ice Liquid: Water Gas: Vapor Water is Mass unlike heat which is energy, and thus transfer of water is considered a form of mass transfer and not energy transfer Learning Objective 2 What are the processes that occur when the water phase is changing? What are the processes that occur when the water phase is changing? Gas Liquid Freeze Solid Thaw Water can change states through several processes depending on the conditions around it, mainly Temperature, and Pressure Learning Objective 3 When does phase change occur? When does phase change occur? Solid: Ice Liquid: Water 0 C⁰ 32 F ⁰ Gas: Vapor 100 C⁰ 212 F ⁰ When does phase change occur? Solid: Ice Liquid: Water 0 ⁰C 32 ⁰F Gas: Vapor 100 ⁰C 212 ⁰F Water is no different than any other object and the same rules of thermodynamics apply. Hotter objects will heat it up by giving it their energy and cooler objects will cool it down as the water gives off its energy to heat them up When does phase change occur? As energy received increases, the temperature should increase When does phase change occur? Boiling Point Assuming Pressure at 1 atm When does phase change occur? What is the phase at this point? When does phase change occur? At 100 C (212 F) and 1 atm pressure the Temperature- Energy Curve would exhibit a flat region When does phase change occur? What is the phase of water in this region? When does phase change occur? At this region water is undergoing phase change and is in a state between both When does phase change occur? • Sensible Energy is the energy received that raises the temperature of the substance • Latent Energy is the energy received that the substance uses to change phase. It does not change the temperature of the object. When does phase change occur? KEY TAKEAWAY: Phase Change will occur when a substance gains or losses enough sensible energy to reach the phase change point (ex. Boiling, Freezing etc) and then gains or losses enough latent energy to change its phase. The sum of Sensible and Latent Energy is called Enthalpy.