Chapter 2_building Physics Ii1819.pdf

FUNDAMENTAL OF BUILDING  PHYSICS

CHAPTER CONTENT  Basics of heat transfer   Sources of heat gain in buildings  Establishing thermal comfort  Mechanism of regulating thermal comfort

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INTRODUCTION • BUILDING: To control the immediate environment around  people. • BUILDING ENCLOSURE: provide shelter for the benefit of  human habitation, work or recreation. • BUILDING SERVICES: operates and control environment within  the building by enabling occupants to live and work  comfortably. • COMFORT: the state of being able to pursue some activity  without experiencing environmental distress.

BASIC OF HEAT TRANSFER • Heat; is a form of energy, appearing as molecular motion in substances or as radiation in space. Measured in Joule, J • According to the laws of thermodynamics, energy is neither created nor destroyed. • it merely changes form or is transferred from place to place; secondly, heat naturally goes from high temperature to low temperature locations and objects.

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• In building science, there are three primary ways that heat enter and move throughout a home or building: i conduction, ii convection, iii Radiation iv Others…source would be internal heat gain from occupants and appliances. Conduction • is the process by which heat contacts a surface and transfers that thermal energy to an adjoining surface or object. Heat flows by conduction through various building elements such as walls, roof, ceiling, floor, etc. • the sun heats a pane of glass and the heat spreads to the window frame and other surrounding objects.

• Heat can be conducted through solids, liquids and gases. • Some materials conduct more rapidly than others. • The basic equation of heat conduction of heat conduction is;

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Properties of Building Materials ? 273.15 K

Example 1 1. Material glasses 2. Outside temp = 380C 3. Inside temp = 250C 4. Surface exposed = 3m x 3m Find Qc ?

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Convection • is the transfer of heat through a medium or fluid, such as water or air. In uninsulated walls, for example, a current or cycle of warm and cool air will be generated and move heat from one area to another.

Radiation • occurs by direct i.e., line of sight exposure to heat or ultraviolet UV radiation. For example, when sunbathing, only the body surfaces directly facing the sun are affected. Another illustration is standing next to a camp fire, only the body surfaces or clothing facing the fire are directly warmed. • In case of buildings, external surfaces such as walls and roofs are always exposed to the atmosphere.

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• The radiation exchange Qradiation between the exposed parts of the building and atmosphere is given by

Internal heat gains • using heat‐generating appliances, such as the dishwasher, oven, stove, washer, and dryer.

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BASIC OF HEAT TRANSFER

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METHODS OF HEAT TRANSFER 50° C

30° C

45° C 35° C

35° C

Heat transfer by conduction

Heat transfer by convection

50° C 20,000° C

20° C

Heat transfer by radiation

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SOURCES OF HEAT GAIN FOR BUILDING

HEAT AND COLD ENTER AND MOVE THROUGHOUT A BUILDING

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HEAT GAIN IN  BUILDING Office building

Typical house

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SOURCES OF 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.

SOURCES OF 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

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THE GREENHOUSE EFFECT

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HEAT LOSS / GAIN

Building Energy Modeling

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SOURCES OF HEAT GAIN IN BUILDING • High performance building; envelope must be able control the heat gain in  summer and heat loss in winter. • Optimal design of the building envelop fabric provide significant reductions in  heating and cooling loads‐which in turn allowing downsizing of mechanical  equipment

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THERMAL COMFORT

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ESTABLISHING THERMAL COMFORT • PHYSICAL & PSYCOLOGICAL COMFORT depends on: – – – –

Temperature Quality of Air Lighting Environment Acoustic Environment

Thermal comfort - Visual Comfort - Aural Comfort

DEFINITION OF THERMAL COMFORT • The absence of discomfort • State/condition of mind which expresses  satisfaction with the thermal environment • The feeling of comfort(or discomfort) is based  on a network of sense organs: the eyes, nose,  tactile sensors, heat sensors, and brain • Bodily sensations, thermal comfort is a  sensation of hot, warm, slightly warmer,  neutral, slightly cooler, cool and cold

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ESTABLISHING THERMAL COMFORT • ASHRAE STANDARD 55: thermal comfort is define as the state  of mind that expresses satisfaction with the surrounding  environment. • Humans‐ thermal comfort is maintained when heat generated  by human metabolism allow to dissipate (maintaining the  thermal equilibrium with the surrounding). • Ways heat gain/transfer: 1. 2. 3.

convection,  radiation  evaporation.

• The body exchanges heat with its surroundings  by convection, radiation, evaporation and  conduction. If heat is lost, one feels cool. In case  of heat gain from surroundings, one feels hot  and begins to sweat.

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ESTABLISHING THERMAL COMFORT

BFC 32603 Sistem Mekanikal & Elektrikal Emedya Murniwaty Bt Samsudin [email protected]

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BODY’S HEAT BALANCE EQUATION  • The body is a state of thermal equilibrium  with its environment when it loses heat at  exactly the same rate as it heat gains. • The modes of heat exchange between the  body and environment, the body’s heat  balance equation as follows;

S = M ± W ± R ± C ± K – E – Res    

(equation 1)

S = bodily rate of heat storage (Wm‐2) M = Metabolic rate (Wm‐2) W = Rate of working (Wm‐2) R= Bodily rate of heat exchange by radition (Wm‐2) C= Bodily rate of heat exchange by convection (Wm‐2) K= Bodily rate of heat exchange by conduction(Wm‐2) E = Bodily rate of heat loss by evaporation (Wm‐2) Res = Bodily rate of heat exchange by respiration (Wm‐2)

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Physical  Variables (Air  Temperature,  Relative  Humidity, Air  Movement and  Ventilation)

Personal  Variables  (activity,  gender, age ,  clothing)

THERMAL COMFORT

Factors  Effecting

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FACTORS INFLUENCING COMFORT Environment

Individual  Factors

Acclimatization

Air Temperature Activity (met) Air Velocity

Gender Age

Mean Radiant  Temperature

Body built Clothing (clo)

Relative humidity

Others

Health conditions Food intake

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Air Temperature • Defined as ‘the dry‐bulb temperature (DBT) of  the air surrounding the occupant’. • Most important determinant of thermal  comfort. • The range of DBT established; 16 – 280C  = comfortable Below 160C = cool,  excessive clothing / high  activity rates required. Above 300C = hot, excessive air movement  and sweating is required to maintain comfort

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Relative Humidity (RH) • Humidity is the amount of water vapour in a given space. • RH defined as ‘the ratio of the mole fraction of water vapour present in the air to the mole fraction of water vapour present in saturated air at the same temperature. Normally expressed as a percentage. • Indication; RH 20% ‐ cause discomfort, excessive dryness RH 90% ‐ cause feeling clamminess/wetness • RH measured by a sling psychrometer or electronic hygrometer.

Mean Radiant Temperature (MRT) • Defined as ‘the uniform surface temperature  of an imaginary black enclosure in which an  occupant would exchange the same amount  of radiant heat as in the actual non‐uniform  space’ • Related to the mean temperature of the  surrounding surfaces • MRT measured by the globe thermometer

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Air Velocity (V) • V defined as ‘a movement of air through unit  length in unit time, meter per second (ms‐1) • Very important and related to the air  temperature and the part of the body exposed to  the air movement. • Indicator; Below 0.1 ms‐1 = feeling of stuffiness Up 1.0ms‐1 = comfortable • Measured by Kata Thermometer / electronic  anemometer

Activity & Metabolic Rate • The metabolism is the body’s motor, and the  amount of energy released by the metabolism  is dependent on the amount of muscular  activity.  • Measuring unit, Met;  (1 Met = 58.15W/m2 of body surface) • A normal adult has a surface area of 1.7 m2

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ESTABLISHING THERMAL COMFORT

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ESTABLISHING THERMAL COMFORT •

• •

Heat transmittance through the building fabric: – Conduction of heat through building fabric – Convection via air movement – Radiant transmission, typically through glass Good insulation reduces the flow of heat into a building when there are differences of  outside and inside air temperature Factors to be considered when determining the appropriate insulation solution: 1. Effect on building design‐ impact of external wall thickness on layouts, net 1.4 Sources of heat gain in buildings floor area and light penetration through window 2. Balance between heavyweight and lightweight construction, including  considerations related to exposed thermal mass. 3. Performance in use and longevity. 4. Buildability and the risk of on‐site work not meeting the required design standards. 5. Sustainability implications of the production process including sourcing of raw materials, ozone depletion, embodied energy and eventual disposal. BFC 32603 Sistem Mekanikal & Elektrikal Emedya Murniwaty Bt Samsudin [email protected]

ESTABLISHING THERMAL COMFORT

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ESTABLISHING THERMAL COMFORT

BFC 32603 Sistem Mekanikal & Elektrikal Emedya Murniwaty Bt Samsudin [email protected]

ESTABLISHING THERMAL COMFORT Thermal Conductivity (λ value or k value) and Resistivity (r) • 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 λ= thermal conductivity (W/moC, Btu in/hr ft2 oF) Thermal resistivity (r)= 1/λ m.K/W r = thermal resistivity (moC/W, hr ft2 oF/Btu)

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FUNDAMENTAL OF BUILDING PHYSICS: Thermal Conductivity

k or 

ESTABLISHING THERMAL COMFORT • 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:

q  kAdT / d q - Heat transferred per unit time

k or



(W, Btu/hr)

- Thermal conductivity of the material (W/m.K or W/m °C, Btu/(hr °F ft2/ft)) A - Heat transfer area (m2, ft2)

dT

- Temperature difference across the material (K or °C, °F)

d

- Material thickness (m,ft)

FUNDAMENTAL OF BUILDING PHYSICS: Conductive Heat Transfer

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ESTABLISHING THERMAL COMFORT • Thermal conductivity for a material calculated  d using:

H A(1   2 )  t d

insulation Heat supply

1

Sample material insulation



- Coefficient of thermal conductivity from the sample material (W/m K) H - rate of heat flow between the faces (J/s=W)

Measured heat flow

2

A

t

A - Cross sectional area of the sample (m2) (  1   2 ) - Temperature difference between the faces (°C or °K)

d - Distance between the faces (m) FUNDAMENTAL OF BUILDING PHYSICS: Thermal Conductivity

ESTABLISHING THERMAL COMFORT FUNDAMENTAL OF BUILDING PHYSICS: Conductive Heat Transfer

• 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 BFC 32603 Sistem Mekanikal & Elektrikal Emedya Murniwaty Bt Samsudin [email protected]

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ESTABLISHING THERMAL COMFORT FUNDAMENTAL OF BUILDING PHYSICS: Emissivity and absorption

•

e  or    is the relative power  Surface coefficients for building materials

of material surface to emit  heat by radiation. Surface Emissivity • Rough black surfaces  0.05 absorb most heat and emit  Aluminum least heat. Asphalt 0.95 • Color of most building  Brick-dark 0.9 materials has an important  Brick-black 0.9 effect on the heat  absorbed by the building  Paint 0.9 from the sun. Slate

Absorptivity 0.2 0.9 0.6 0.9 0.3

0.9

0.0

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ESTABLISHING THERMAL COMFORT • Thermal transmittance   (U‐value)  Material Resistance  Thermal resistance of each layer of and thermal resistance (R‐value)  material depends on the rate at indicate the design thermal  which the material conduct heat and d thickness of the material; performance of a building  R  material or assembly. • R‐value; resistance of heat flow  through a building material (m2 K/W) Alternatively; • bigger the value, better insulation  R  rd (greater resistance). 

R

-thermal resistance of that component (mK/W)

d

-thickness of the material (m)



r

-thermal conductivity of the material (W/mK) -resistivity of material =

1/λ

(mK/W)

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ESTABLISHING THERMAL COMFORT

BFC 32603 Sistem Mekanikal & Elektrikal Emedya Murniwaty Bt Samsudin [email protected]

ESTABLISHING THERMAL COMFORT • Example Find the thermal resistance of a 100mm thickness of  lightweight concrete block. Solution:  value for    given = 0.19W/m K

d Therefore;

for the block = 100mm @ 0.1m

0.1 2 m K /W 0.19  0.526m 2 K / W R

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ESTABLISHING THERMAL COMFORT 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 1. Depends on the nature of any conduction, convection and radiation within the cavity. 2. 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.

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ESTABLISHING THERMAL COMFORT 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 BFC 32603 Sistem Mekanikal & Elektrikal Emedya Murniwaty Bt Samsudin [email protected]

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ESTABLISHING THERMAL COMFORT 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).

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ESTABLISHING THERMAL COMFORT FUNDAMENTAL OF BUILDING PHYSICS: Thermal Transmittance and Resistance

• 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

U RT

1 RT

-Total thermal resistance.

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ESTABLISHING THERMAL COMFORT • 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; U (average) 

A1U1  A2U 2  ..  .. AnU n A1  A2  .. An

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BFC 32603 Sistem Mekanikal & Elektrikal Emedya Murniwaty Bt Samsudin [email protected]

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Exercise A portion of wall which is facing west in direction. Composition of the wall includes 20 mm glass window (R=1.1m2 oC/W) and 150 mm of brick wall (λ=0.77 W/m oC) covered with 15mm thick cement plaster (λ = 0.18 W/ m oC) finishes on both sides. Determine the average U-value for the wall.

1.5 m

3.0 m

2.5 m

4.0 m Plaster λ=0.18 W/ m oC d=15mm Brickwall λ= 0.77 W/m oC d=150 mm

Window glass R= 1.1 m2oC/W d=20 mm

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MECHANISM REGULATING THERMAL  COMFORT

• Energy efficient building/Green Building

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MECHANISM OF REGULATING  THERMAL COMFORT Roof ; Insulation serves to limit the conduction of heat through the building shell. Infiltration ; When outside air enters a building, it has to be cooled or heated to maintain comfort. The more unconditioned air entering the building, the greater the load on the heating and cooling system and the greater the cost. Windows ; Low-E windows provide excellent thermal insulation against weather extremes and can effectively reduce solar heat gain as well. Window tints and reflective films are efficient at reducing solar gain but can also reduce the visual connection with the outdoors. External window screens are excellent solar control devices for single- or two-story facilities, and architectural features such as awnings and overhangs allow year-round solar control without minimizing visual quality. BFC 32603 Sistem Mekanikal & Elektrikal Emedya Murniwaty Bt Samsudin [email protected]

MECHANISM OF REGULATING  THERMAL COMFORT Orientation ; long, narrow buildings facing south with their long axis running east/west will have lower peak cooling loads and electricity demand costs, and may be able to utilize smaller cooling equipment. Landscaping ; Well designed landscaping can reduce cooling costs from summer heat gains in building. Trees planted on the east, west and south sides of a one-or two-story building can effectively reduce summer solar heat gains through windows which is one of the major contributors to the cooling load on an air conditioning system. Trees also produce a natural cooling effect in the areas surrounding a building by evaporating water though their leaves. Daylighting ; Daylighting with skylights and other types of architectural glazing features can provide natural lighting creating a pleasant working atmosphere. Daylighting strategies may by particularly effective using skylights in large open areas such as warehouses and manufacturing plants, and in office spaces where the electrical lighting system output can be efficiently varied over a wide range of light levels. It is important to balance daylighting strategies with good solar heat control in order to keep cooling loads down. BFC 32603 Sistem Mekanikal & Elektrikal Emedya Murniwaty Bt Samsudin [email protected]

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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.

END OF CHAPTER 2 THANK YOU

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