Energy and Environment
Energy and Environment ARCH 140
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ARCH 140 Architecture 140 Energy and Environmental Management College of Environmental Design UC Berkeley READING 9 HEAT TRANSFER MECHANISMS Thermodynamics Thermodynamics describes heat movement There is a certain order to the movement of heat energy It flows in a speci c manner it can be manipulated through the use of materials it can be stored borrowed bought and given away To design with it as opposed to against it one needs to know its laws First Law of Thermodynamics The rst and second laws of thermodynamics are basic to any discussion of energy The rst law is quotEnergy can neither be created nor destroyedquot Energy can be transformed from one form such as light into equivalent quantities of another form such as heat but the total amount of energy in any transaction and in the universe remains constant Energy can be converted absorbed stored and distributed Sunlight streaming through a window is absorbed by furniture floor and walls as heat Some is not absorbed it is reflected as light back into the space Electrical energy sent to a light bulb is converted into light and heat energy illuminating and heating the space Ultimately heat or light released or collected in a building either will be converted into work done eg maintenance of satisfactory environmental conditions or will be lost through the building skin Second Law of Thermodynamics The second law of thermodynamics states that when free interchange of heat takes place it is always the hotter of the two bodies that loses energy and the colder that gains energy see Figure 2112 A more general statement of this principle is that energy cannot spontaneously transform from a lower to a higher state Work can only be done in passing from a higher energy state such as light to a lower energy state such as heat For energy to ow in the opposite direction it must be forced additional energy must be applied and thus work must be done on the system The second law ofthermodynamics implies that all systems tend to achieve equilibrium Thus to establish a division between hot and cold in a substance that has reached a uniform temperature requires the use of either a heater to warm up part of the substance or a cooling device to cool down part of it Either action requires usage of additional energy Page 1 of Reading No 9 ARCH 140 Heating a building is similar Left alone the cold air outside and the warm air inside will reach a state of equilibrium with the warm molecules sharing their heat with the cold ones This will happen even with a completely airtight building because the warm air will heat the inside of the envelope and the cold air will cool the outside of the envelope until both bodies of air and the envelope are at the same temperature To keep the inside air warm then requires an addition of energy heat Cooling a building is basically similar except that energy is required to extract unwanted heat from the building Heat Transfer Heat flows or travels in different manners conduction convection and radiation Heat can also be absorbed or released in phase changes These types of heat transfer occur naturally outside the building at the skin of the building and within the building itself The manipulation of these forces enables the design professional to provide suitable interior conditions without requiring the unnecessary expenditure of energy resources an understanding of the laws of thermodynamics will help the designer to work in conjunction with these laws creating and manipulating desired thermal transfers throughout a building Conduction Conduction is the heat flow between adjacent molecules whether the molecules are within a single substance or in two separate bodies in direct contact such as air molecules in contact with a warm substance conduction occurs when the warmer molecules which are vibrating faster than the colder molecules transfer part of their energy to the colder molecules Convection Convection occurs when a fluid such as a gas or liquid is involved The uid is heated heat energy is added to the uid and then the fluid containing this heat moves from one place to another This movement can be forced mechanically such as by using a fan to move warm air from a furnace to room or more quotnaturallyquot without mechanical assistance as when smoke rises up a chimney or when warm air in a room rises to the ceiling allowing cool airto fall to the oor Page 2 of Reading No 9 ARCH 140 Radiation Radiation is the transport of energy in the form of electromagnetic waves Sunlight for instance is one form of electromagnetic waves As sunlight passes through the air and a window pane to reach the interior ofa room very little of its energy is absorbed When the light energy hits objects in the room some of it is absorbed as heat which is inferred energy The portion of light energy that is not immediately absorbed when it hits objects in the room is re ected Eventually this light energy is fully quotusedquot or absorbed either by objects in the room in the form of heat or by our eyes Materials absorb reflect and transmit radiation to varying degrees For the same transmission level high absorption means lower re ectance and vice versa Polished aluminum is a good re ector of both light and heat radiation asphalt paving is not Phase Changes Phase changes involve the relatively large amounts of energy needed to change the phase of substance ie from solid to liquid and liquid to gas The change of phase that occurs when water evaporates into water vapor for example requires almost a thousand times as much energy as it takes to raise water temperature one degree F This is the basis of evaporative cooling which adds water to warm air this process of evaporating water into air cools the air by the removing of heat required for evaporation HEAT TRANSFER THROUGH WALLS The amount of heat flowing from the interior to the exterior of a building envelope for a given temperature difference AT inout can be quanti ed in terms ofthe conductances of the materials making up the wall and the wall and the conductance of the wall39s surface air films The conductance of a single material measures the rate of heat ow per unit time through a given thickness ofthat substance The units of conductance are Btusqft F hr Page 3 of Reading No 9 ARCH 140 The conductance of a material is determined by a more basic property its conductivity which is the time rate of heat flow through a M thickness ofthat material The units of conductivity are Btu sqft F hr foot or inch thickness It can be seen that c kd where d thickness ofthe material EXAMPLE The conductivity of softwood such as pine is k 08 Btusqft F hrin while the conductance of a piece of plywood 34 inches thick made from a softwood is C 106 Btusqft F hr The lm or surface conductance is a measure of the time rate of heat ow across a surface as a function of the temperature difference between the building surface and the surrounding air It is an expression for the heat transferred from or to the surface by radiation and convection alone There are two types of lm conductances an inside and an outside This is because there is usually more heat transferred outside the surface ofa building due to wind Film conductance values are symbolized by hi inside air lm conductance h0 outside air lm conductance The convective heat transfer coef cient covers both convective and conductive heat transfer This is because when heat crosses a surface to a gas or liquid like air or water the conducted heat induces buoyancy differences in the fluid that then cause convective heat transfer by uid motion It is impossible to say where one process lets off and the other takes up so the two together are combined in the convective heat transfer coefficient HEAT TRANSFER IN SERIES In reality building surfaces are not composed ofa single material but of an assemblage of several materials each with its own conductivity Heat passing through assemblages is called heat transfer in series which is the conductive transfer from one material in the Page 4 of Reading No 9 assembly to another Therefore in order to find the combined conductive capacity of a building surface the total resistance to heat transfer must be determined The resistance or Rvalue ofa substance is the reciprocal of its conductance Rlor c gorl k h The units are sqft F hrBtu The overall resistance ofa construction assembly is equal to the sum ofthe resistances of each material component RtotalRl R2 39 R3 39 R4llll hi C1 02 he where l and 1 are the resistance values fortwo layers of the wall 02 Note Although hi and ho are terms frequently used in the literature in Arch 140 we mostly use their resistance equivalents Ri1lhi 068 hr ft2 F Btu for still air and R0 1ho two values here 017 hr ft2 F Btu for the 15 mph wind assumed typical for winter and 025 hr ft2 F Btu forthe 75 mph wind assumed typical for summer You should memorize these three numbers you will use them repeatedly in series heat transfer calculations THERMAL PROPERTIES lH VEAle FlLr lh3 WEWOF NF TE OF A WALL SECTION 9TuCCO I iH ULA OH AIR 7903 meonm ZEMF7 GH TEE OUT JIVE NH FlLM he gt E 5 ClU 1 3 EXTERIOK AIR TEN Thermal Pro ert ho Outside air film conductance Btuft2 F hr c Conductance Btuft2 F hr a Air space conductance Btuft2 F hr k Conductivity Btuft2 F hrin hi lnside air film conductance Btuft2 F hr R Resistance Btuft2 F hr U Coefficient of heat transmission Btuft2 F hr Uvalue Page 5 of 7 Reading No 9 ARCH140 The total Rvalue can then be inverted to determine the Uvalue the overall coefficient of heat transmission which quantifies the rate of heat ow through the various materials ofa wall assembly Ul Rtotal The units of U are Btusqft F hr Notice that the surface or film coefficients hi and ho are included in the Uvalue Although all materials have a characteristic Uvalue they cannot be added together to arrive at the overall conductance value for an entire wall assembly Therefore the R values must be summed and then inverted Now we can calculate the total heat transfer per hour through a surface using the calculated Uvalue in the following equation Q39 U X A X AT Btu X sqftX F hr sqft F hr where Q39 heat ow through the wall A area ofthe wall AT difference between the inside and outside temperatures U overall coefficient of heat transmission for this wall HEAT TRANSFER IN PARALLEL Most buildings have different types of wall surfaces sidebyside such as walls doors and windows Each of these different wall constructions has a different Uvalue and probably a different area The heat from the building39s interior ows through these in parallel meaning that the amount of heat owing through each type of wall goes through the same temperature drop the total interiorexterior temperature AT To get the total heat loss for a wall made up of several constructions in parallel you simply add up all the heat losses through the different constructions Total heat loss UXAXAT UXAXAT U XAXAT for walls windows doors etc Page 6 of Reading No 9 ARCH 140 If you factor out the AT which is the same in each case you get the familiar equation Total heat loss 2 U x A x AT where Z is the sum ofall the U x A39s This equation is used for roofs as well as walls in fact all building surfaces that divide the interior from the exterior air If you add up all the U x A39s for a whole building and divide by the total surface area of the building you will have an average Uvalue for the whole building This is termed the quoteffective Uvaluequot and is used in codes Equation U1 A1 U A7 U A2 UA effective A1 A2 A3 BUILDING HEAT LOSS HEAT FLOW IN PARALLEL wuovcnm HEAT L066 THFau 2w l f wuvuanm V HEM39 L066 TH ZOUQH wmvowe u upTvaiou tom201 u WaA L066 39 up T THZOU H Moe WHJWWS Aux I70amp26 wupuc au EAT THaauarl yv xx lt739lt 4 MUPUC HOLJ EAT L0 THZOUGH Fwolz aux Page 7 of 7 Reading No 9 Thermal Properties of Materials Arch 140 Lecture 7 Tuesday Feb 10 2009 CONDUCTION CONDUCTION xAxAT 392 heat ow Btuhr d thickness in AT temperature OF A area ftz quotquotquotquotI quotquotquotquotI quotquotquotquotI IIII hot is I I I39 3939 quotquot I 3939n39 k conductivity WEN CONDUCTION k 4quot 392 heat ow Btuhr d thickness in AT temperature 0F 33333 A A area ftz ThOt k d t39 39t gg 5gquot c011135111211l y Tcold hr ftZOF CON DUCTION XAXAT Btu 1 Btuin ft2 0F d thickness in AT temperature OF A area ftz k conductivity Btuinhr ft20F B uin hrtft20F CONDUCTION Q X A X AT f H B zLgtxltBtug r in hr f ZF d thickness in AT temperature OF A area ftz k conductivity Btuinhr ftZoF O6PrcpertiesGB2007 Alternative Measures of Conductive Properties of Conductive Properties Typical Building Materials Property Units Heat OW Q Material 2 Component conductivitv k BtuinhrftZOF A kd AT Insulation 03 512 insulation 190 conductance C Btuhrft2oF A C Wood soft 08 2x4 wood stud 43 Glass 54 Mt glass 005 regiStance R hr39ftZ39OFBtu A Brick 50 3 common brick 06 resistivity i hrft20FBtuin A lrd AT Stone 12 8 s eel 310 39 ivity property of a material independent of amount Azumjnum 1500 See Reader Appendix 5 ance property of an object depends on amount I for property values gt Air still 017 R M C lR kd ldr or R dk Water still 426 39 Example Example 4 thick wall of concrete k 12 Btuin hrftZOF 4 thick wall of concrete k 12 Btuin hrftZOF Find R dk Find R dk 4 in 12 Btu in hrft20F 4 in 12 Btu in hrft20F 033 hrftZoFBtu 033 hrftZOFBtu o k 39 NEW lNFO d Wall area A 10 ft2 I Temperature difference Q 391 quot AT 10 F Thot I Tcold AT Example Convection 4 thick wall of concrete k 12 Btuin hrftZOF Find R dk 4 in 12 Btu in hrft20F 033 hrft20FBtu If the wall was 10 ft2 and had a 5 F temperature difference across it how much heat would ow 392 A lR AT 10 ftz 1033 hrft20FBtu 5 0F 150 Btuhr O6PropertiesGB2007 CONVECTION air lms on surfaces Thot O heat ow Btuhr AT temperature F A area M h air lm conductance or coef cient F NOTE h is like a Conductance so 1 h lR or R h air lm resistance CONVECTION air lms on surfaces o 7 Q hXAXAT not O hea owB hr AT tem7erature F Z Q A z h EEKcgndgtance or coef cient NOTE39 h is like a Conductance so h lR or R 1h air lm resistance Air Film Conductance h amp Resistances Vertical Walls R 1h air lm resistance Indoors Ri 1hi 068 hr 2 FBtu Outdoors Ro 1ho 017 Winter 15 mph 025 summer 75 mph A Ro or ho Air Film Conductance h amp Resistances Vertical Walls R 1h air lm resistance Indoors Ri 1hi 068 hr 2 FBtu Outdoors Ro 1ho 017 Winter 15 mph 025 summer 75 mph 11ml Air Film Conductance h amp Resistances Ceiling vs Floor Winter example 0 heat ow across envelope Q cold Heat 10ng warm 2 cold 067Pr0pertiesiGB72007 Air Film Conductance h amp Resistances Ceiling vs Floor Think warm air rises cold air falls Winter example 0 interior air movement Q cold 1 Interior air movement at ceiling T Interior air movement 2 cold at oorsL Air Film Conductance h amp Resistances Ceiling vs Floor Radiation Winter example 0 Q cold Air movement T Resistance J h T warm R ow up 061 hI ZOFBtu A1r movement J a Resistance T h t cold R ow down Q V 092 hI ZOFBtu RADIATION SolarTerrestrial Spectrum Sun 10000 F Mm 05 pm yellow Why do wavelengths differ Spectrum of object RASOli iN Ea h 80 F Arm N 10 Hm Wien s Displacement Law temperatureT RADIATIO N 39 Wavelength at which RELATIVE ENERGV Q TEMPERATURE the maX1mum amount m R of energy is Lu emitted 0 D 3 52 0 5 2 5 WAVELENGT um hmax output Hm 2897 i UV VISIBLE SOLAR LONGVWAVE lNFRARED T K V39myelengthy L SPECTRUM NFRARED so TT means SOLAR RAD AT ON SHORTWAVE 1 Radiation Striking a Surface Radiation Material Properties 1 When radiation strikes a surface Ineldent Radlauon 2 How much radiation leaves is released or emitted from a surface Transmitted T or I Re ected R or p Absorbed A or X O6PropertiesGB2007 1 Radiation Striking a Surface lnc1dent Radiation Zero Opag ue Surfaces Re ected R or p Absorbed A or 0L Material Properties Opaque Surfaces 0L p 1 Absorptance and Re ectance of Common Ground Materials expressed as fraction OI Lolal incident solar energy Absomtance Re ectam M i Dry crass 07 03 Dry Snil 05 02 Asphalt 09 in Concrete 05 04 Show 01702 n94s Low Shrubs 07 u 3 Sand 03 o 2 Material Properties Opaque Surfaces 0L p 1 Absorpmnce and Re ectance of Common Ground Materials expressed as fraction OI Lolal incident Solar energy Am wince Renee Tater Dry Grass Dry Snil Asle Concrete Snow Low Shrubs Sand 03 0 2 Radiative Properties of Glass Heat Clear Absorbing Re ective Transmitted 87 43 30 ci39lcclcd 8 h39 Absorth 5 36 m u Arrmined m Isl 3 Q7 20 Total heal gain 10 80 50 Glass Transmittance Normal glass lets in light heat UV Visible Solar IR Energy intensity WW 1 5 2 2 5 Wavelenth mml Glass Transmittance Spectrally Selective Glass 7 ideal properties UV Visible Solar IR Energy intensity WW 1 5 2 2 5 Wavelenth mml 067Properties7GB72007 7th inning stretch 2 Radiation Leaving a Surface Q Total energy emitted from an object area under the curve Thot y g 9 Maximum amount perfect emitter QoA Real materials Wavelength Q 8 0 A T4 0 StephanBoltzman constant a very small number 5 67x 108 Vm rt 8 Emissivity a longwave property En ergy Tmust be expressed as absolute temp K or R What is Emissivity 8 Emissivity a amp LW Re ectivity p The ef ciency with which amaterial emits Opaque surfaces T Z 0 longwave radiation gt f 11 0f Independent of color I Using Kircho s Law 8 0t z 09 MOST materials except metals 00502 p 1 S for longwave radiation Kirchoff s Law Properties of metal and low 8 lms EmISSIVIly 8 AbSOthlVlly 0t Lowe low emissivity gt wits less LW for the same waveleng Lowe Highp high re ectivity re its more LW Practically speaking this means 8 any Low 3 High LW re ectivity LoweWindows Air Gaps Convectiveamp Radiative Exchange Simplest way to think my grimy about it Longwave radiation is re ected back 06PropertiesGB2007 Air Gaps Thermal Resistances convection radiation Thermal Resistance of Typical Air Spaces in Walls and Roofs NonReflective Reflective Surfaces on on One Reflective on Either Side Side Only Both Sides Vertical Air Space walls 0 14 1 Air Gaps Thermal Resistances convection radiation Thermal Resistance of Typical Air Spaces in Walls and Roofs Non Reflective Reflective Surfaces on on One Re ective on Either Side Side Only Both Sides Vertical Air Space walls 07 14 1 Horizontal Air Space roofs Winter 06 11 13 Summer 09 2 1 r on 415 Slope roofs Winter 06 11 14 Summer 08 1 22 41 See Reader Appendix 5 LW Emissivity e and Solar Absorptivity 0c Opaque surface sitting in the sun I Solar Reflectivity albedo p 1oc I Emissivity 10 09 08 07 06 05 04 03 02 01 0 is the efficiency with whicha i 39 material emits longwave LW a 01 infrared radiation I 5 02 Kirchoff s Law g Emissivity LWAbsorptivity 03 13 80 04 T Opaque Surfaces A 05 Absorptrvrty Reflectivrty 1 9 o 6 BI P 1 e 39 39 2 SO Z 07 T3 Shortwave a wig 39 OJ p 1 a E 39 08 m I 09 g Longwave 3 i 80le139PLw 10 0 01 02 03 04 05 06 07 08 09 10 p LW 1 8 Solar Absorptivity 0t Heat Gain LW Emissivity e and Solar Absorptivity 0c Perfect Dissipater I I R Ib d I 1 k body Absorbs zero I SQ ar e ECthty a e O p T 390 Abs rbs all Em39ts max3910 09 08 07 06 05 04 03 02 01 0 Em mix 10 0 393 09 01 8 i 08 02 quot39 03 g 07 03 39 I II a 06 04 E 05 05 II I2 04 06 e39 A U a 3 3 03 07 w m 2 3 02 08 E In 01 09 E 0 10 0 01 02 03 04 05 06 07 08 09 10 perfect Collector Solar Absorptivity ct Heat Gain Absorbs 3 Emits zero I Solar Reflectivity albedo p 1rx I 10 09 08 07 06 05 04 03 02 01 0 10 I 39 I I 39 White Rm IIIIIIIII Hunting tar I plaster paint 39 and gravel 9 43 09 Light green 01 paint Slant Black Light 08 39 G 02 I Whitetru39ashed Ashestn s 8 0 7 galvanized roo ng slate 0 3 I P39 d H Did aluminum me mg g 05 I paint I 04 of El 39 t 05 F EIITZE DENT 05 a New aluminum I 39 l 7 3 P A 04 39 Dam Stainless Ti id 05 S 53 steel 3m F39F39 E gt 03 315 New galvanized Oxidized 07 W u 39 galvanized 3 a 02 I 08 E Polished 7 Lu 0 1 aluminum Polished 0 9 3 39 full copper D Aluminum 39 l 7 sheet 0 I I I I I 10 0 01 02 03 6 08 09 10 Solar Absorptivity a Data Sources Florida Solar Energy Center ESource Lawrence Berkeley National Laboratory LBNLWindow5 Southwall Technologies Cardinal IG Energie Solaire SA Graph produced by Timothy Moore LW Emissivity e and Solar Absorptivity 0c Perfect Dissipater A k bod Absorbs zero I Solar Retiectwrty albedo p 1or Abs Ibs a Emits max 4 1 Emi s m x 10 0 4 09 01 U 3 08 02 03 4 39LI 3 07 03 II a I l 06 04 8 E 05 05 A t 8 04 06 B gt 45 03 07 l 2 393 02 08 LIEJ 01 09 E o 0 10 0 01 02 03 04 05 06 07 08 09 10 Perfect Conector Solar Absorptivity Transmissivityquot at t Absprbs 3 Emits zero For glass the X axis includes both the fraction of solar radiation that is absorbed and transmitted Data Sources International Glazing Database Southwall Technologies Cardinal IG Energie Solaire SA Graph produced by Timothy Moore O6PropertiesGB2007
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