AR/IA 241 LN 231 Lecture 4: Fundamental of Energy

Transcription

1 Faculty of Architecture and Planning Thammasat University A/IA 24 LN 23 Lecture 4: Fundamental of Energy Author: Asst. Prof. Chalermwat Tantasavasdi. Heat For a specific substance, the heat given to the substance is proportional to the mass of the substance (m) and the temperature change of the substance (ΔT). Therefore, the equation for heat can be expressed as Q m c p ΔT = () where Q = heat given to the substance (J, BTU) m = mass of the substance (kg, lb) c p = specific heat of the substance (J/kg-K, BTU/ lb-f) ΔT = temperature difference (K or C, F)

2 The specific heat (c p ) tells us how much heat is needed to warm up a mass. for water, c p = BTU/ lb-f (4.8 x 0 3 J/kg-K) for air, c p = 0.24 BTU/ lb-f (.0 x 0 3 J/kg-K) Mass is the product of Density (ρ) and Volume (V); equation () becomes Q = ρ V c p ΔT (2) Heat Capacity (ρ x c p ) determines how much heat can be stored in a given volume. Since designers deal with dimensions of built environments in terms of width, length, and height, volume is therefore easier to use than mass. As a result, heat capacity is very useful number when comparing substances with the same volume. Application involves building material selection. If the designers want to keep the heat in the buildings, material with high heat capacity, e.g. water and steel should be selected. On the other hand, if the heat is not needed, material with low heat capacity, e.g. foam and fiberglass should be selected. 2

3 2. Heat Transfer Thermodynamics deal with the magnitude of heat exchange. Heat Transfer, however, determines the time for a heat exchange process, or the size of a surface to achieve the rate of heat exchange. In heat transfer, the symbol of Q (heat) becomes q (rate of heat transfer). Energy transfers because of the differences of temperatures. There are three modes of heat transfer: Conduction: through connected molecules of solid Convection: through movable molecules of liquid or gas adiation: hot molecules radiate more heat than the cold ones without media 3. Conduction The amount of conduction heat transfer is proportional to the substance surface area (A), surface temperature difference (ΔT) but is an inverse of the substance thickness (x). Therefore, conduction equation can be expressed as q k A ΔT x = (3) where q = conduction heat transfer (J/s, BTU/hr) k = thermal conductivity (W/m-C, BTU-in/ft 2 -hr-f) A = surface area (m 2, ft 2 ) ΔT = surface temperature difference (C, F) x = thickness of the substance (m, in) Thermal Conductivity (k) is a specific property of a substance that explains how fast the heat would transfer through a substance. Since thermal wave is part of the electromagnetic waves, thermal property is similar to electrical property. Material that has good electrical conductivity such as metal also has good thermal conductivity. 3

4 Conductance (C = k/x) gives the rate of heat transfer regardless of the substance thickness. esistance ( = /C = x/k) is the inverse of conductance. It explains how difficult heat can transfer though a substance and is known as -value. Equation (3) becomes q A ΔT = (4) The knowledge of conduction has numerous applications in built environments. Building form selection Outdoor environmental creation and indoor condition selection Building envelop design A building section is generally comprised of more than one material. For calculation of the total heat flow through a building section, the results of the resistance need to be determined. A useful method is comparing to electrical flow: Ohm s Law. Category Thermal Flow Electrical Flow ate of Energy q i Driving Force ΔT ΔV esistance Equation q = A ΔT / i = ΔV / The overall resistance is comparable to the electric series circuit where = All 2 3 n (5) Thermal Transmittance (U = / All ) represents the effectiveness of the whole section of construction. Equation (4) could become q = U A ΔT (6) 4

5 In real life, a wall could be comprised of various materials and does not have a uniform section. It is called a composite wall. The overall resistance of such complex section could be comparable to the electrical parallel circuit where = (7) All 2 3 n A difference between electrical flow and thermal flow is that for thermal overall resistance, the surface areas have to be taken into account. Equation (7) then becomes A All All A A2 A3 A = n n (8) The knowledge of total heat flow also shows a significant application. In construction detailing, heat travels through a weaker path (lower -value) than a stronger path (higher -value). Material with low thermal resistance could drop the performance of the whole section by more than half. This is sometimes known as thermal bridge. It is therefore desirable to avoid any weak link in a construction section. 4. Convection Heat transfer via is proportional to the surface area and the surface temperature difference. Therefore, the equation can be expressed as q Conv = h A ΔT () c 5

6 where q Conv = heat transfer (W, BTU/hr) h c = coefficient (W/m 2 -C, BTU/hr-ft 2 -F) A = surface area (m 2, ft 2 ) ΔT = surface temperature difference (C, F) Convection coefficient (h c ) is a function of velocity (v) and length of surface (l). It explains how fast heat transfer would be. For natural, h c = BTU/hr-ft 2 -F For forced, h c = 2-50 BTU/hr-ft 2 -F -value of air film (=/h c ) can be included in the term of conduction equation, using electrical analogy. For glass, of the air film dominates the overall -value because the glass has a very low thermal resistance. In general, -value of a single pane of glass is approximately hr-ft 2 -F/BTU. Still air has lower h c (higher -value) than moving air. Therefore, double glazing with still air cavity should improve thermal resistance of the single glazing. The major question is the appropriate distance between the two panes of glass. Larger distance between the two panes of glass should benefit the -value. However, hot air naturally rises due to its lower density. It can bring the heat from hot side to colder side, increasing the heat transfer. The optimum distance between panes of glass is found to be ½-¾. -values of double and triple glazing are approximately 2 and 3 hr-ft 2 -F/BTU, respectively. Filling the cavity with special inert gas, e.g. Argon, increases the -value further. On the other hand, infiltration will considerably reduce the -value. 5. adiation The major radiation related to built environments is from the sun. The following table demonstrates the spectrum of solar radiation that has wavelength (λ) of μm. 6

7 Type λ (μm) % of solar energy Effect Ultraviolet (UV) skin tan, color fade Visible daylight Infrared (I) heat Short wave refers to the visible (light) wave due to its shorter λ comparing to infrared. Long wave refers to the infrared (heat) wave due to its longer λ comparing to visible. When solar energy strikes a substance, it is reflected, absorbed, and transmitted. Therefore, ρ + α + τ = (2) where ρ = eflectivity α = Absorptivity τ = Transmissivity (=0 for opaque) Emissivity (ε) is the portion of solar energy that emits to the environment after its absorption. For most building materials, ε α Solar radiation of transparent material Transparent materials can cause green house effect. The transmissivity of glass is generally high for short waves as light easily transmits through glass. It is low for long waves as heat transfer through glass is more difficult than light. When sunlight energy transmits through glass, it is absorbed by interior material. Then it is irradiates back in the form of heat, which is trapped in the room. The result is the room gets warmer. The effect is similar to that from CO 2 and green house gases in the earth atmosphere. The application for a green house design is appropriate for cold climate where the temperature is too low for human comfort. However it is very inappropriate for warm climate because it will heat up the already hot interior space. 7

8 Solar radiation of opaque material Opaque materials have different response to solar radiation. Two major material properties involve color and shininess. Color affects short wave absorptivity. Light color reflects more light energy than darker one. That is why we see the color the way it is. Shininess affects long wave emissivity. Shiny surface emits less heat than textured one. These have effects on surface temperature. Light color and textured surface should be selected if low temperature is needed. On the contrary, dark and shiny surface gives high temperature. In addition, shiny surface material, e.g. aluminum foil, in a wall/roof cavity improves the thermal performance of the section. However, it needs air gap between the surface and the cavity in order to work well. 6. Combined Conduction, Convection, and adiation Opaque material In general, outdoor surface temperature has to be determined. It is complicated because and radiation depend on many parameters, e.g. air velocity, sun angles, cloud conditions, building orientation and surface material properties. Sol-air temperature (T sol-air ) is a convenient way to identify outdoor surface temperature by combining the effect of and radiation. The information derives from measurements in various climatic conditions and can be found in most heat transfer handbooks. Then, heat transfer can be calculated using general electrical analogy. 8

9 Transparent material The radiation heat transfer equation can be expressed as q ad A SHGF SC = (3) where q ad = adiation heat transfer (W, BTU/hr) A = Surface area (m 2, ft 2 ) SHGF = Solar heat gain factor (W/m 2, BTU/ft 2 -hr) SC = Shading coefficient = Solar gain / Solar gain of clear glass Solar heat gain factor is the amount of solar energy on the surface as a function of sun angles cloud conditions, and building orientation. The information derives from measurements in various climatic conditions and can be found in most heat transfer handbooks. Shading coefficient is the comparison of solar gain on the particular type of glass and that of clear glass. It also accounts for the effect of shading devices. When combining the effect of conduction and, equation (3) becomes q = A[( SHGF SC) + ( U ΔT )] (4) 7. Heat Balance in Buildings To understand the heat balance in buildings, first we need to define system and surrounding. Generally, the indoor air represents the system while buildings and outdoor environments represent the surrounding. Then apply the first law of thermodynamics: heat gain = heat loss. Heat gain refers to the heat given to the indoor air while heat loss refers to the heat lost from the indoor air. Normally there are six types of heat gain and loss: Conduction via building envelope Convection heat gain from people and appliances Convection of heating or air-conditioning systems Convection of infiltration adiation heat gain through fenestration adiation through solid parts Equations for conduction are already given. Convection and radiation could be combined into conduction equations. However, equation for of heating/air-conditioning systems and infiltration can be separately expressed as 9

10 q Conv = ρ V c p ΔT (5) where V = Volumetric flow (m 3 /s, ft 3 /m: CFM) The followings show the examples of major heat balance in buildings in warm climates. They represent both air-conditioned and naturally ventilated buildings, and also separated by the time of usage to day and night. Infiltration is not included while radiation of solid and fenestration are combined. solar radiation night sky radiation of A/C A/C building Day conduction via building envelope of A/C A/C building Night conduction via building envelope from people and appliances from people and appliances solar radiation night sky radiation of N/V N/V building Day conduction via building envelope of N/V N/V building Night conduction via building envelope from people and appliances from people and appliances Bibliography American Society of Heating, efrigerating and Air-conditioning Engineers. (997). 997 ASHAE handbook fundamentals (I-P ed.). Atlanta, GA: ASHAE. Cowan, H. J. (99). Handbook of architectural technology. New York: Van Nostrand einhold. Lechner, N. (200). Heating, cooling, lighting: Design methods for architects (2 nd ed.). New York: John Wiley & Sons. Moore, F. (993). Environmental control systems: Heating cooling lighting. Singapore: McGraw- Hill. Stein, B. & eynolds, J. S. (2000). Mechanical and electrical equipment for building (9 th ed.). New York: John Wiley & Sons. 0