Environmentally Sustainable Design - Building's Thermal Performance and Heat Flow in the Wall - Lab Report Example

Name: ID No: 3916526 Course: Title: The calculation of heating and cooling loads Date: Table of Contents Name: 1 Course: 1 Date: 1 Table of Contents 2 Introduction 4 Estimating thermal heat 5 Heat flow in the wall 7 Heat transfer 7 Conduction 7 Convection 8 Radiation 8 Evaporation 9 Solar radiation 9 Radiation on the roof 10 Heat conduction 12 Internal heat gain 14 Heat lost due to evaporation 15 Ventilation 15 Conclusion 19 This presented here summarizes the steady stead calculation for heat loss and heat gain in a building. However, the method may only be useful in a single room. This is because the algebra becomes complex when a multistoreyd building is involved. In addition, there are other factors that need to be considered such as the effects of shading by objects or self shading, the ability of the building to store or release heat to the environment, and the variation solar radiation and the environmental temperatures makes the estimation to be hard. Therefore computer simulation can provide the best solution which captures all the aspects in the building that affects heat flow. Thermal calculation is used by the designer to determine the energy consumption for a building. This can be used in the selection of materials to be used in a building and in selecting appropriate retrofitting of the existing building. Therefore, if the computer simulation is integrated in determination of the thermal performance of a building, the building design can meet the optimum energy efficiency. 19 References 20 Introduction The building envelops that consists of the floor, walls, roofs, windows and doors are important in maintaining of the preferred interior comfortable conditions for the occupants (Zubir & Brebbia, 2013). Thus, they are crucial elements in energy performance of a building and are important in the performance of low energy buildings. Energy conservation, passive and lighting design has impact on the building envelop through different factors like ventilation and sunlight. Sunlight affects the heat gains, but the use of HVAC and lighting systems resulting into high energy consumption. The objective of determination of thermal performance is to reduce thermal gain during summer and thermal loss in winter. Optimum performing building with acoustic, visual and thermal comfort for the occupants takes care of the desired heat, sound and light. Thermal energy loads in a building can be generated from the exterior environment through different modes like radiation, conduction and convection. Effective thermal design requires control of both the internal heat gain loads and the perimeter thermal loads, mainly because of the exterior environment. The main factors which influence the transmission of heat in a building are the difference in temperature between the exterior and the interior surfaces of the building and the ability of the building materials to control the flow of heat (Zubir & Brebbia, 2013). The U-value for the materials used in the building is one of the crucial factors in design in order to reduce the overall thermal transfer value which is the average heat gain into the building to reduce cooling loads and conserve energy. The main component in the transfer of heat includes conduction through the walls and windows, and solar radiation through transparent windows and roofs. The building should be constructed with low U-value for the floor, walls and the roof. It indicates the thermal energy transferred per unit area per unit temperature difference. R-value is the resistance to the transfer of heat which determines the thermal transmittance and resistance. These values are critical when reducing the heat flow through the building assemblies (Zubir & Brebbia, 2013; Badea, 2014). Estimating thermal heat The thermal performance of any building can be obtained by determining the energy transfer between the building and its surrounding environment. It involves estimation of cooling and heating load and thus, the selection of the materials used in the building can be made correctly (Zubir & Brebbia, 2013). The technique can also be used to calculate the temperature variation in the building over a period of time. Thus, it can be used in estimating the uncomfortable time duration. The calculation allows determination of effectiveness and efficiency of the building design and assist in improving the building designs in order to realize efficient energy of the building with excellent conditions inside the building. Building architects need to have the knowledge about the performance of the building design in order to look for a suitable alternative. This paper discusses methods for estimation of thermal performance of a building (Underwood &Yik, 2004). There are various ways through which heat flow between the building and the environment. Heat transfer through building elements like ceilings, floor, walls, and windows. Heat can also flow through the elements by radiation and convection. The heat from solar radiation can pass through the glass windows and absorb on the surfaces inside the building. Heat is increased inside the building by the equipment, lights and occupants, but it is lost through evaporation resulting in cooling (Badea, 2014). The interaction between these processes is shown in the figure below. (Badea, 2014) Human body produces heat through metabolic activities in the body. Part of the heat produce by the body is dissipated into the surrounding through radiation, conduction, convection and evaporation. The body cools by loosing heat into the environment, and warms by gain heat (Badea, 2014; Underwood & Yik, 2004). The building’s thermal performance is affected by the properties of materials in the building such as the glass walls, equipments and lighting systems; design such as the geometry of roofs, walls, and windows; weather condition that include the humidity, temperature of the surrounding and solar radiation; usage of the building such as the lighting and the number of occupants which can be simplified in the following diagram. (Hui 2012) Different methods have been proposed to study the influence of these factors on the building’s performance. These techniques include correlation, dynamics and steady state methods. Some of them are complex and require detailed information input to produce accurate output on regular basis. Other techniques are simple and can be performed using hand calculations (Badea, 2014). Heat flow in the wall The solar radiation from the sun can either be reflected or absorbed by the wall and converted into heat energy. The heat is transferred through convection or conduction as shown below. Part of the heat gained by the wall is lost through radiation and convection form the outer surface, and the rest is conducted into the wall. Some heat is stored in the wall and the rest enters into the interior surface, where it is transferred through radiation or convection into the room, resulting to temperature rise in the room. These exchange processes occurs on the roofs and walls. The exchange of heat through radiation between the roof and the wall also occurs. All these processes influence the temperature inside the room and a result affect the occupants’ thermal comfort (Underwood &Yik, 2004). Heat transfer Heat can be transferred through radiation, conduction, evaporation and convection. Conduction This is the transfer of heat from the part of the body with high temperature to the other part with low temperatures. The heat transfer occurs through direct contact of the molecules with each other. Conduction occurs in the gases, solids or liquids. The rate of conduction can be calculated from the following equation. Quantity of heat, Where k – is the thermal conductivity of material A is the area L is the thickness of the wall Th reperesnts the temperature of the hot surface and Tc is the temperature of the colder surface This means that materials with high conductivity transfer more heat (Hui 2012). Convection This is heat transfer within the liquid which can either be air of liquid, from one part with higher temperature to another part with lower temperatures. The kind of heat transfer occurs on the floors, roofs or walls. The density variation within the liquid results in buoyancy, which lead to exchange of heat between the surface and the fluid. Forced convection occurs if the fluid motion is due to external forces like the wind. The rate of transfer of heat through convection is calculated from the following equation (Hui 2012). Where Ts is the surface temperature Tf is the fluid temperature h is heat transfer coefficient in W/m2-k which is influenced by the fluid velocity, surface orientation, nature of heat flow and the physical properties of the fluid. Radiation This is the transfer of heat away from the body as the body temperature increases. This heat energy travels without the medium. The heat from the body either be absorbed or reflected by the body. The following equation is used to calculate the exchange of radiation between two parallel surface planes with their respective temperatures. The radiation that occurs between two surfaces, Where σ is Stefan-Boltzmann constant A is the surface area ε is the emissivity of the surface T2 is the temperature of surface 2 T1 is the temperature of surface 1 The radiation from the atmosphere is given by Where Ts and Tsky are the temperatures of the surface exposed to the atmosphere and the temperature at the sky respectively (Lienhard & Lienhard, 2011). Evaporation This is the removal of water through vaporization. The process occurs at any temperature but can increase as the temperature increase. The rate of evaporation is increased by increase in the speed of the wind, increased surface area and low atmospheric pressure. The heat energy in from of latent heat of vaporization is taken from the liquid and from the surrounding surface, which causes cooling. Solar radiation The sun produces light and heat for the whole solar system. The sun exists in form of hot gases whose heat is produced from fusion reactions. Its diameter is 1.39 x 106 km and 1.496 x 108 km away from the earth whose diameter is 1.27 x 104 km .The disc of the sun subtends an angle of 32’ at a given point on the surface of the sun. Since the angle is very small, it is assumed that the lines of radiation are parallel (Lienhard & Lienhard, 2011). The figure below represents the geometry between the earth and the sun. The earth takes one year to revolve around the sun in an elliptical path, but also rotate about its own axis in 24hrs. The energy flux received at the atmosphere per second in a unit surface area perpendicular to the rays of the sun is known as solar constant whose value is 1367 W/m2. But due to the fact that the earth revolves around the sun in an elliptical circle, the extraterrestrial radiation varies. Therefore, the intensity of the radiation on any day is given by the following equation (Badea, 2014). Iext = Isc [1.0 + 0.033cos (360n/365)] Where is between 1 and 365 Radiation on the roof Most surfaces of the roofs are tilted, except some which are horizontal. A tilted roof receives three solar radiation types which include the bema radiation which comes directly from the sun, reflected rays which come from the neighbouring objects, and diffuse rays that comes from the sky dome. However, it is very complicated to estimate the radiation that comes from the neighbouring objects, and in many cases its value is negligible. If the surface is tilted at an angle of say β, then the hourly incident solar radiation is given by: IT = Igr Where r is global radiation tilt factor r Cos θ=sin φ (Cos β. Sin δ + Cos δ. Cos γ. Cos ω. Sin β) + Cos φ (Cos δ. Cos ω. Cos β – sin δ. Cos γ. Sin β + Cos δ. Sin γ. Sin ω. Sin β) Cos θz= Cos φ. Cos δ. Cos ω + Sin φ. Sin δ Where Ig – average hourly global radiation Id average hourly diffuse solar radiation Ρ – Ground reflectivity Φ – Location latitude, which is +ve for the northern hemisphere Δ – Is the angle between the line that joins the centres of the earth and the sun with the projection from the equator plane. It is given by n is day in a year γ – It is the angle of surface azimuth. It represents the angle between the project of normal to the horizontal plane surface and the line due south. It is negative for west of south and positive in east of south. β – The angle between the horizontal with the plane surface ω – It is called the hour angle. It is the angular measurement of time and is equal to 150 in one hr. it is based on local time, and is +ve in the morning and –ve in the evening. Solar heat gain through the transparent window The solar gain through the window is given by αs – is the average absorptivity of the space Ai = surface area of the ith element Sgi – mean value of solar radiation in a day in the window Τi – the transmissivity of the window M number of windows Heat conduction Heat conducted through walls, floors and roofs in a steady state is given by Qcond = AUΔT U is the surface area of the wall, floor or the roof U is the thermal transmittance whose S.I. units is given by W/m2-K ΔT – the change in temperature between inner and the outer surface The steady state technique is not dependent on the type of material in the building. Thermal transmittance, U = RT - the sum of thermal resistance which is given by ho & hi are the outside and inside heat transfer coefficients. kj is the thermal conductivity of the material and Lj is thickness of jth layer. Thermal transmittance shows the amount of heat transmitted through the roof or wall per unit area at a given time. Low value of thermal transmittance indicates high insulating value of the material. Therefore, this can be used to compare the insulation values for different materials. The values obtained from each of the element such as walls, floor, windows and doors are added together to obtain the heat flow rate through conduction in the building (Badea, 2014). The change in temperature as a result of the exposure of the wall to solar radiation is calculated as follows ΔT = Tso – Ti Where Tso = sol – air temperature Ti = indoor temperature Where Tso is calculated from the following equation Tso = To = hourly ambient temperature in K ST = hourly solar radiation incident on the surface on W/m2 α = Absorptance of the surface solar radiation ho = heat transfer co-efficient outside in W/m2-K ε = surface emissivity ΔR = the difference between the radiation on the surface from the sky and the radiation emitted by black objects Internal heat gain The building can gain internal heat from the occupants and/or the equipment which can be determined as follows. The heat produced by the occupants depends on the level of activities of the people inside. The heat outputs from human body due to different activities are shown in the table below. Activity Heat rate generated by the occupants W W/m2 Resting 80 45 Sleeping 60 35 Sitting 100 55 Typing 150 85 Waking 200 110 Digging or cutting 300 or more 300 or more (Bansal, Hauser & Minke, 1994) Internal heat is also produced by lamps. Most of the energy produced by electric lamps such as fluorescent lamps is from of heat, and small part is emitted as light that can also change to heat when it strikes a surface of an object (Badea, 2014). Internal heat is also gained from the appliances and equipment such as radios, iron box, computer, and television. In case the electric motor used to drive these appliances is within the room, the total heat gain in the building can be obtained by summing all the heat produced in the building (Badea, 2014). The common heat energy produced by appliances is shown in the table below. Equipment Load (W) Radio 15 TV 250 Vacuum cleaner 800 Water heater 3000 The total heat flow due to internal gain is given by the follow. Qi = (Number of people x heat out per person) + rate of heat for lighting system + load for appliances Heat lost due to evaporation Evaporation causes cooling effect and can be calculated as follows. Qevap = mL Where L is latent heat of vaporization in J/kg-k and m is the evaporation rate in kg/s Ventilation The rate of air exchange between the outside and interior of the building determine the rate of heat flow of the building. Heat flow rate, Qv is given by Qv = ρVrCΔT Ρ – is the air density in kg/m3 Vr is the rate of ventilation in m3/s C is the specific heat of air in J/kg-K T is the difference in temperature (T2-T1) in K (Lienhard & Lienhard, 2011) Example Given that the dimensions of a single room is 6 m long by 5 m wide by 4m high, as shown below. The room is maintain at a temperature of 230C using an air conditioner. The load on the air conditioner can be calculated using the steady state method as follows. The following information has been provided. The location: Melbourne Month: October The rate of ventilation: 2/hr Lighting systems: 3 fluorescent lamps with a rating of 100W Occupants: 4 people occupancy, working in an office on 24hr basis. Door: 2m x 1.2m facing north Window: 3m X 1.5m facing south U door = 3.18 W/m2-K Uwall = 3 W/m2-K Uroof = 3 W/m2-K Uglazing = 5.77 w/m2-K Outside temperature in October = 32.220C Inside temperature = 230C Heat transfer co-efficiency on the outside = 22.7 W/m2-K External wall absorptance = 0.6 Solar radiation on the south facing wall, north facing wall, east facing wall and the west facing wall are 111.3, 101.1, 158.2 and 155.2 W/m2 respectively The solar radiation on the roof and on the window is 303.1 and 111.3 W/m2 respectively. Window transmissivity = 0.86 Absorptivity of the space = 0.6 Absorptivity of glazing and wood are 0.06 and 0 respectively Specific heat of air = 1005 J/kg-K Air density = 1.2 kg/m3 The heat in the room is written as Qtotal = Qc + Qs + Qi + Qv Using equation, Tso = 35.160C =36.40C, =34.890C, =36.320C, =330C and =32.220C, ΔR for the roof = 63 W/m2, Thus, = 38.70C QC = 3(15-4.5) (35.16 - 23) + 3 x 12 (36.4 – 23) + 3 x (15 – 2.4) (34.89 – 23) + 3 x 12(36.32 – 23) + 3.18 x 2.4(32.22 – 23) + 2.31 x 20(38.7 – 23) + 5.77 x 4.5(33.0 – 23) = 2921.099 W Qi = 3 x 100 +4 x 100 =700W Qs = 0.6 x 4.5 x 111.3 x 0.86 = 258.4 W Qv = Therefore, Qm = 2921.099W + 700W + 258.4W + 370.644W = 4250.143W = 4. 25kW Since the total heat gain is positive, it means that more heat is entering the house. The appliance is required to reduce the temperature. Given that the air conditioner has a standard cooling capacity of 2.8, the power needed by the appliance is 4.25/2.8 = 1.52 kW. If the machine will be used for 12 hrs in a day, then the power consumption per day = 1.52 x 12 = 18.24kWh = 18.24 units of electricity. If the cost of one unit = 16c per kW/h, then, the expense is equal to 2.92 A$. Conclusion This presented here summarizes the steady stead calculation for heat loss and heat gain in a building. However, the method may only be useful in a single room. This is because the algebra becomes complex when a multistoreyd building is involved. In addition, there are other factors that need to be considered such as the effects of shading by objects or self shading, the ability of the building to store or release heat to the environment, and the variation solar radiation and the environmental temperatures makes the estimation to be hard. Therefore computer simulation can provide the best solution which captures all the aspects in the building that affects heat flow. Thermal calculation is used by the designer to determine the energy consumption for a building. This can be used in the selection of materials to be used in a building and in selecting appropriate retrofitting of the existing building. Therefore, if the computer simulation is integrated in determination of the thermal performance of a building, the building design can meet the optimum energy efficiency. References Badea, N. (2014). Design for micro-combined cooling, heating and power systems: Stirling engines and renewable power systems. Bansal, N. K., Hauser, G., & Minke, G. (1994). Passive building design: A handbook of natural climatic control. Amsterdam: Elsevier Science B.V. Cebeci, Tuncer. (2013). Convective Heat Transfer: Solutions Manual and Computer Programs. Springer Verlag. Hui Y. H., (2012). Handbook of Meat and Meat Processing, Second Edition, CRC Press Lienhard, J. H., & Lienhard, J. H. (2011). A heat transfer textbook. Mineola, N.Y: Dover Publications. Underwood, C. P., Yik, F. W. H., & Wiley InterScience. (2004). Modelling methods for energy in buildings. Oxford: Blackwell Science. Zubir S, S., & Brebbia, C. A., International Conference on Urban Regeneration and Sustainability, (2013). The sustainable city VIII. Read More