FUNDAMENTAL OF BUILDING PHYSICS SOURCE OF HEAT GAIN IN BUILDING

HEAT GAIN IN BUILDING Heat; is a form of energy, appearing as molecular motion in substances or as radiation in space. Measured in Joule, J Temperature; considered as presence of heat in a substance. Thermodynamics; the science of the flow of heat. The first law of thermodynamics/energy is the principle of conservation of energy. Energy cannot be created or destroyed, only changed from one form to another. The second law of thermodynamics, (Clausius 1850) states that heat (or energy), transfer can take place in one direction only. i.e. from hotter to a cooler body. Heat transferred from hot to cold in 3 basic ways: Conduction Convection radiation FUNDAMENTAL OF BUILDING PHYSICS

HEAT GAIN IN BUILDING 50° C 30° C Heat transfer by conduction 45° C Heat transfer by convection 35° C 35° C 50° C sun 20,000° C Radiation through vacuum Heat transfer by radiation 20° C earth FUNDAMENTAL OF BUILDING PHYSICS : Method of Heat Transfer

HEAT GAIN IN BUILDING FUNDAMENTAL OF BUILDING PHYSICS: Sources of Heat Gain

HEAT GAIN IN BUILDING Qi= internal heat gain, heat from human bodies, lamps, appliances Qs=solar heat gain Qc=conduction heat gain Qv=ventilation heat gain Qe=evaporative cooling Qm=mechanical cooling FUNDAMENTAL OF BUILDING PHYSICS: Sources of Heat Gain

HEAT GAIN IN BUILDING Why we study Heat? To achieve THERMAL COMFORT. Thermal balance exists when the sum of all heat flow is zero i.e.; When this sum is greater than 0(+), temperature indoor will heat up. When less than 0(-), temperature indoor will cooling down. FUNDAMENTAL OF BUILDING PHYSICS: Sources of Heat Gain

HEAT GAIN IN BUILDING Result of the differing properties of heat radiation when it is generated by bodies at different temperature. High temperature (sun) emits radiation of short wavelength pass through glass. Heat absorbed (inside building) by objects, which then re-radiate the heat. Object inside the green house (lower temperature) radiated heat (longer wavelengths) not able to penetrate the glass. Re-radiate heat trapped and rise up the indoor temperature. e.g: entering the closed car that has been sitting under the sun. FUNDAMENTAL OF BUILDING PHYSICS: The Greenhouse Effect

HEAT GAIN IN BUILDING Building envelope/enclosure/shell is the part of the building which physically separates the exterior environment from the interior environment/s. Prevents air, moisture, heat/cool from freely flow in/out from a building. Three parts of building envelope: interior, exterior and the system. FUNDAMENTAL OF BUILDING PHYSICS: Building Envelope

HEAT GAIN IN BUILDING Building envelope-critical component of any building (protects building occupants and regulate indoor environment). Controls the flow energy between interior and exterior. High performance building; envelope must be able control the heat gain in summer and heat loss in winter. Consists of: Roof, Floor slabs Walls Windows Doors. Optimal design of the building fabric provide significant reductions in heating and cooling loads-which in turn allowing downsizing of mechanical equipment Good design; extra cost justified by savings (building operations) FUNDAMENTAL OF BUILDING PHYSICS: Building Envelope

Art School, Nanyang Technological University, Singapore The glass facade provides a high performance building envelope that reduces solar gain and heat load while allowing the benefits of natural views and daylight into creative spaces. The glass walls provide a visual exchange between indoors and the surrounding landscape or interior plaza as fluid spaces. The diffused natural daylight is abundant throughout studios and classrooms, thus making them productive spaces for young creators. The curving green roofs distinguish the building from among the other structures on campus but the line between landscape and building is blurred. The roofs serve as informal gathering spaces. Besides that purpose, the roofs serve as open space, insulate the building, cool the surrounding air and harvest rainwater for the landscape irrigation. This amazing design is surely going to be used more widely because it provides better and healthier surrounding. In this particular example it offers a brand new experience in many perspectives, fulfilling the intent that a school for art should inspire creativity, while solving the green surface deficiency.

HEAT GAIN IN BUILDING To control the three (3) components of heat transmittance through building fabric: Conduction of heat through building fabric Convection via air movement Radiant transmission, (through glasses and other building fabric). Good insulator reduce heat flow. Consideration in determining insulation solutions: Effect on building design Heavyweight and lightweight construction balance. Performance in use and longevity Buildability and the risk Sustainability implications FUNDAMENTAL OF BUILDING PHYSICS: Thermal Insulation

HEAT GAIN IN BUILDING Thermal Conductivity (λ value or k value) the measure of the rate at which heat is conducted through a particular material under specified conditions property of a material that indicates its ability to conduct heat. Measured as the heat flow in watts across a thickness of 1 m of material for a temperature difference of 1 degree K and a surface area of 1 m² Unit : W/m K Thermal resistivity (r)= 1/λ m.K/W r = thermal resistivity (moC/W, hr ft2 oF/Btu) λ= thermal conductivity (W/moC, Btu in/hr ft2 oF) FUNDAMENTAL OF BUILDING PHYSICS: Thermal Conductivity

HEAT GAIN IN BUILDING Thermal conductivity for a material calculated using: insulation Measured heat flow Heat supply Sample material insulation - Coefficient of thermal conductivity from the sample material (W/m K) - rate of heat flow between the faces (J/s=W) - Cross sectional area of the sample (m2) - Temperature difference between the faces (°C or °K) - Distance between the faces (m) FUNDAMENTAL OF BUILDING PHYSICS: Thermal Conductivity

FUNDAMENTAL OF BUILDING PHYSICS: Thermal Conductivity

HEAT GAIN IN BUILDING Happens if there exist a temperature gradient. Conductive heat flow occurs in direction of the decreasing temperature (higher temperature=higher molecule energy) Fourier’s Law stated that CHT as: - Heat transferred per unit time (W, Btu/hr) - Thermal conductivity of the material (W/m.K or W/m °C, Btu/(hr °F ft2/ft)) - Heat transfer area (m2, ft2) - Temperature difference across the material (K or °C, °F) - Material thickness (m,ft) FUNDAMENTAL OF BUILDING PHYSICS: Conductive Heat Transfer

HEAT GAIN IN BUILDING Example; A plane wall constructed of solid with thermal conductivity 70 W/m °C, thickness 50mm and with surface area 1m by 1m, temperature 150 °C on one side and 80 °C on the other. Conductive heat transfer can be calculated as: q = (70 W/m°C)(1m)(1m)((150°C)- (80°C))/(0.05) = 98,000 W = 98 kW FUNDAMENTAL OF BUILDING PHYSICS: Conductive Heat Transfer

HEAT GAIN IN BUILDING or is the relative power of material surface to emit heat by radiation. Rough black surfaces absorb most heat and emit least heat. Color of most building materials has an important effect on the heat absorbed by the building from the sun. Surface coefficients for building materials Surface Emissivity Absorptivity Aluminum 0.05 0.2 Asphalt 0.95 0.9 Brick-dark 0.9 0.6 Brick-black 0.9 0.9 Paint 0.9 0.3 Slate 0.9 0.0 FUNDAMENTAL OF BUILDING PHYSICS: Emissivity and absorption

HEAT GAIN IN BUILDING Thermal transmittance (U-value) and thermal resistance (R-value) indicate the design thermal performance of a building material or assembly. R-value; resistance of heat flow through a building material (m2 K/W) bigger the value, better insulation (greater resistance). Material Resistance Thermal resistance of each layer of material depends on the rate at which the material conduct heat and thickness of the material; Alternatively; -thermal resistance of that component (mK/W) -thickness of the material (m) -thermal conductivity of the material (W/mK) -resistivity of material = (mK/W) 1/λ FUNDAMENTAL OF BUILDING PHYSICS: Thermal Transmittance and Resistance

HEAT GAIN IN BUILDING Example Find the thermal resistance of a 100mm thickness of lightweight concrete block. Solution: value for from table given = 0.19W/m K for the block = 100mm @ 0.1m Therefore; FUNDAMENTAL OF BUILDING PHYSICS: Thermal Transmittance and Resistance

HEAT GAIN IN BUILDING Airspace Resistances Surface Resistances Depends on conduction, convection and radiation of the surface. Factors affect surface resistance are: Direction of heat flow; upward and downward Climatic affects; sheltered or exposed Surface properties; high or low emmissivity Airspace Resistances Depends on the nature of any conduction, convection and radiation within the cavity. Factors affect airspace resistances: Thickness or airspace Flow of air in airspace; ventilated or unventilated Lining of airspace; normal surfaces of reflective surfaces of low emmissivity. FUNDAMENTAL OF BUILDING PHYSICS: Thermal Transmittance and Resistance

HEAT GAIN IN BUILDING Total thermal Resistance (RT) is the sum of thermal resistances of all the components of the structure elements RT Example of brickwall resistances; RT= Rsi +R1+R2+Rso FUNDAMENTAL OF BUILDING PHYSICS: Thermal Transmittance and Resistance

HEAT GAIN IN BUILDING Thermal Mass Materials that have the capacity to storage thermal energy for extended periods. Absorb daytime heat gains (reducing cooling load) and release heat during night (reduce heat load). Lower initial temperature than the surrounding air (act as heat sink). Beneficial for country which had a big different between day and night outdoor temperature. (e.g. UAE). FUNDAMENTAL OF BUILDING PHYSICS: Thermal Transmittance and Resistance

HEAT GAIN IN BUILDING U-value of a construction is defined as the quantity of heat that flows through a unit area of a building section under steady-state conditions. Unit: W/m2 K -Total thermal resistance. FUNDAMENTAL OF BUILDING PHYSICS: Thermal Transmittance and Resistance

HEAT GAIN IN BUILDING Average U-Values When a wall is composed of different construction materials with different U-value. Overall insulation of the wall depends upon the relative areas of constructions; FUNDAMENTAL OF BUILDING PHYSICS: Thermal Transmittance and Resistance

HEAT GAIN IN BUILDING EE means use less energy for heating, cooling, lighting and equipments. The building envelope play an important roles Typical Malaysian office building consume 250kWh/m2/year of energy. Building Energy Index (BEI); i) PTM GEO; 50 ii) Kettha LEO; 114 iii) Typical KL; 300 iv) MS1525:2001; 14 FUNDAMENTAL OF BUILDING PHYSICS: Energy Efficient in Buildings

HEAT GAIN IN BUILDING Typical house Office building FUNDAMENTAL OF BUILDING PHYSICS: Building Energy Index

FUNDAMENTAL OF BUILDING PHYSICS: Energy Efficient in Buildings

FUNDAMENTAL OF BUILDING PHYSICS: Energy Efficient in Buildings