1. P.1 Book E3 Section 2.1 Energy performance of buildings 2.1Energy performance of buildings The Integer Hong Kong Pavilion Factors affecting the energy performance of buildings Heat transfer by conduction Check-point 1 U-value of building materials Overall Thermal Transfer Value (OTTV) Check-point 2 Controlling heat flow through windows Improving energy efficiency of buildings Check-point 3

2. P.2 Book E3 Section 2.1 Energy performance of buildings Lamps controlled automatically according to amount of natural light. Double-glazed windows with low-e coating. Solar panel. The Integer Hong Kong Pavilion Features of the Integer Hong Kong Pavilion once displaced in Admiralty: What is the significance of those features?

3. P.3 Book E3 Section 2.1 Energy performance of buildings 1 Factors affecting the energy performance of buildings Energy consumption in Hong Kong: In a building, energy is mainly used for air-conditioning (space conditioning).

4. P.4 Book E3 Section 2.1 Energy performance of buildings 1 Factors affecting the energy performance of buildings In HK, air-conditioning produces a cooler interior.  Natural flow of heat: surroundings  interior (through the building envelope, i.e. windows, walls & roofs)  The amount of energy to keep a cooler interior depends on the rate of heat flow from the surroundings.  The rate is related to the thermal properties of the building.

5. P.5 Book E3 Section 2.1 Energy performance of buildings 1 Factors affecting the energy performance of buildings Two ways of heat transfer through building envelope: Conduction through the walls, roof and glass windows Radiation through glass windows 2.1 Energy saving apartment Simulation

6. P.6 Book E3 Section 2.1 Energy performance of buildings 2 Heat transfer by conduction Consider heat being transferred from one end of a material to the other end by conduction. The rate of conduction depends on 4 factors: 1.Temperature difference between the two ends, T hot – T cold

7. P.7 Book E3 Section 2.1 Energy performance of buildings 2 Heat transfer by conduction 2.Distance between the two ends, d 3.Type of material

8. P.8 Book E3 Section 2.1 Energy performance of buildings 2 Heat transfer by conduction 4.Cross-sectional area of the conductor, A 2.2 Factors affecting heat conduction Simulation

9. P.9 Book E3 Section 2.1 Energy performance of buildings 2 Heat transfer by conduction QtQt 1.  T hot – T cold 2.  1d1d QtQt 3.  A QtQt The relationship between the rate of conduction and the above factors can be expressed as: QtQt area A heat transferred Q in time interval t colder end T cold hotter end T hot d

10. P.10 Book E3 Section 2.1 Energy performance of buildings 2 Heat transfer by conduction QtQt  A (T hot – T cold ) d or Combining these relationships, QtQt =  A (T hot – T cold ) d  Depends on the conducting property of the material  Larger value  better conduction Estimating the thermal conductivity of glass Expt 2a : thermal conductivity (unit: W m –1 K –1 ) 

11. P.11 Book E3 Section 2.1 Energy performance of buildings Experiment 2a Estimating the thermal conductivity of glass 1.Insulate a container box and put a stirrer in it. 2.Fill the container with hot water at about 70  C to an almost overflow level. 3.Place a sheet of glass on top of the hot water in full contact. Cover the box with a polystyrene lid. stirrer

12. P.12 Book E3 Section 2.1 Energy performance of buildings Estimating the thermal conductivity of glass 4.Measure the temperature of water with a temperature sensor. temperature sensor data-logging interface Experiment 2a

13. P.13 Book E3 Section 2.1 Energy performance of buildings Estimating the thermal conductivity of glass 2.1 Expt 2a - Estimating the thermal conductivity of glass Video 6.The thermal conductivity of the glass can be determined by  QtQt =  A (T hot – T cold ) d QtQt 5.When the temperature drops to a certain value (T hot ), calculate the rate of energy loss ( ) of water. At the same time, measure the temperature of the upper surface of the glass (T cold ). Experiment 2a

14. P.14 Book E3 Section 2.1 Energy performance of buildings 2 Heat transfer by conduction Rate of conduction Example 1 The thermal conductivity of some common materials:

15. P.15 Book E3 Section 2.1 Energy performance of buildings Example 1 Rate of conduction Temperatures inside and outside an air-conditioned room are 25  C and 33  C respectively. Area of the 10-cm thick concrete wall = 9 m 2 Thermal conductivity of concrete = 1.10 W m –1 K –1 Find the rate of conduction through the wall. Rate of conduction =  A (T hot – T cold ) d = 1.10  9  (33 – 25) 0.1 = 792 W

16. P.16 Book E3 Section 2.1 Energy performance of buildings Check-point 1 – Q1 (a)Find the rate of conduction through the concrete wall. (Thermal conductivity of concrete = 1.10 W m –1 K –1 ) Rate of conduction =  A (T hot – T cold ) d = 1.10  (2.8  3.5)  (35 – 22) 0.15 = 934 W

17. P.17 Book E3 Section 2.1 Energy performance of buildings Check-point 1 – Q1 (b)Find the heat transferred by conduction through the wall in 1 hr. Heat transferred = rate of conduction  time = 934  3600 = 3.36 MJ

18. P.18 Book E3 Section 2.1 Energy performance of buildings 3 U-value of building materials Some building materials are made of composite substances and some have complex structures.  More convenient to consider their thermal transmittance instead of thermal conductivity Thermal transmittance or U-value (U ) is defined as dd U =U =

19. P.19 Book E3 Section 2.1 Energy performance of buildings 3 U-value of building materials  U-value represents the conduction rate per unit cross-sectional area per unit temperature difference (Unit: W m –2 K –1 )  Lower U-value  lower rate of heat transmitted through conduction  Rate of heat transfer by conduction: QtQt = UA (T hot – T cold )

20. P.20 Book E3 Section 2.1 Energy performance of buildings 3 U-value of building materials Typical U-values of some building components: Window conduction Example 2

21. P.21 Book E3 Section 2.1 Energy performance of buildings Example 2 Window conduction A building installs a double-glazed window with frames (U-value = 3.3 W m –2 K –1 ). Size of the window = 0.8 m  0.7 m Temperature is 31  C outside and 23  C inside. Find the rate of heat transfer by conduction. Area of the window A = 0.8  0.7 = 0.56 m 2 Rate of energy transfer ( ) = UA (T hot – T cold ) = 14.8 W = 3.3  0.56  (31 – 23) QtQt

22. P.22 Book E3 Section 2.1 Energy performance of buildings 3 U-value of building materials Average U-value Example 3

23. P.23 Book E3 Section 2.1 Energy performance of buildings Example 3 Average U-value A building has four walls.  Each of area 20 m wide and 60 m tall  Made of concrete; has 400 identical single-glazed windows  U-value of the concrete part = 1.5 W m –2 K –1  Area of each window = 2 m 2  U-value of the window = 5.7 W m –2 K –1 Average U-value for the four walls = ? Example 5

24. P.24 Book E3 Section 2.1 Energy performance of buildings Example 3 Average U-value Total area of windows and concrete part = 20  60  4 = 4800 m 2 Total area of the windows = 400  2 = 800 m 2 Total area of the concrete part = 4800 – 800 = 4000 m 2 Average U-value (5.7  800) + (1.5  4000) 4800 = = 2.2 W m –2 K –1 Example 5

25. P.25 Book E3 Section 2.1 Energy performance of buildings 3 U-value of building materials Single-glazed and double-glazed windows Example 4

26. P.26 Book E3 Section 2.1 Energy performance of buildings An experiment to record the temperature rise inside a box: The box has an opening surface for fitting a window. When fitted tightly with a window, the box is completely enclosed. Example 4 Single-glazed and double-glazed windows

27. P.27 Book E3 Section 2.1 Energy performance of buildings Example 4 Single-glazed and double-glazed windows (a)State all the processes involved in transferring heat through the window. Conduction and radiation (b)A single-glazed window and a double-glazed window are attached to the opening in turn. Although the double-glazed window has a lower U-value, temperature rises in both cases are almost the same. Explain for the result.

28. P.28 Book E3 Section 2.1 Energy performance of buildings Example 4 Single-glazed and double-glazed windows (b)U-value is only related to heat transfer by conduction. The doubled-glazed window only reduces the amount of heat transfer through conduction but not radiation. In this experiment, heat is transferred by radiation predominantly.  Temperature rises in both cases are almost the same.

29. P.29 Book E3 Section 2.1 Energy performance of buildings 4 Overall Thermal Transfer Value (OTTV) Overall Thermal Transfer Value (OTTV):  Average rate of heat transfer to the interior of the building per unit wall area.  For an ‘external wall’ (opaque walls with windows): Q c : heat transfer due to conduction Q r : heat transfer due to radiation Unit of OTTV: W m –2 OTTV = +  A total QctQct QrtQrt

30. P.30 Book E3 Section 2.1 Energy performance of buildings Some factors that affect the OTTV of external walls (the same principles apply to the OTTV of roofs): 4 Overall Thermal Transfer Value (OTTV) Heat transfer by conductionHeat transfer by radiation  U-values of walls and windows  colour of paint and building materials  orientation and thickness of the wall  type of glass  shading from the building  orientation of windows and locate climate

31. P.31 Book E3 Section 2.1 Energy performance of buildings According to the regulations in Hong Kong:  OTTV  30 W m –2 for a building tower  OTTV  70 W m –2 for a podium The design of the buildings and the building materials for the walls, the windows and the roof should comply with this regulation. OTTV of an envelope Example 5 4 Overall Thermal Transfer Value (OTTV)

32. P.32 Book E3 Section 2.1 Energy performance of buildings Example 5 OTTV of an envelope Refer to Example 3. The building is air-conditioned and the temperature of the interior is 7 K lower than the exterior.Example 3 (Assume: heat flow through the roof is insignificant.) (a)Find the rate of heat transfer by conduction through the walls and windows. Rate of heat transfer by conduction = UA (T hot – T cold ) = 2.2  4800  7 = 73 920 W

33. P.33 Book E3 Section 2.1 Energy performance of buildings Example 5 OTTV of an envelope (b)Rate of heat transfer by radiation per unit area = 160 W m –2. Find the rate of heat transfer by radiation through the windows. Rate of heat transfer by radiation = 160  800 = 128 000 W

34. P.34 Book E3 Section 2.1 Energy performance of buildings Example 5 OTTV of an envelope (c) OTTV of the exterior walls = ? = (73 920 + 128 000)  4800 OTTV = +  A total QctQct QrtQrt (d)In Hong Kong, the OTTV for a building should not exceed 30 W m –2. Does the design of this building comply with government regulations? OTTV of the building > 30 W m –2  Fails to comply with the regulations = 42.1 W m –2

35. P.35 Book E3 Section 2.1 Energy performance of buildings ∵ Total area of walls in a building is normally large ∴ Good insulation is essential to keep the OTTV to a low value 4 Overall Thermal Transfer Value (OTTV) A well-insulated wall may have a U-value as low as 0.25 W m –2 K –1.

36. P.36 Book E3 Section 2.1 Energy performance of buildings Check-point 2 – Q1 The four walls of the house are 15 cm thick. U-value of wall P (made of brick) = ? (Thermal conductivity of brick = 0.5 W m –1 K –1 ) dd U-value of wall P = 0.5 0.15 = = 3.33 W m –2 K –1

37. P.37 Book E3 Section 2.1 Energy performance of buildings Check-point 2 – Q2 OTTV of the house = 34.4 W m –2 Rate of heat transfer through the envelope = ? (All exterior walls and the roof should be included in calculating OTTV.) The four walls of the house are 15 cm thick.

38. P.38 Book E3 Section 2.1 Energy performance of buildings Check-point 2 – Q2 Total surface area of the house = 68 m 2 rate of heat transfer = OTTV  A total = 34.4  68 = 2340 W 4  3  4 +  4  2.5 1212 = OTTV = +  A total QctQct QrtQrt By,

39. P.39 Book E3 Section 2.1 Energy performance of buildings 5 Controlling heat flow through windows Buildings containing large windows  Let more natural light into the interior  use of energy for lighting However,  U-value of glass is high.  The heat transfer by solar radiation greatly increases the OTTV.  energy usage in air-conditioning How to cut down the energy transfer through windows?

40. P.40 Book E3 Section 2.1 Energy performance of buildings 5 Controlling heat flow through windows a Tinted glass Tinted glass has a uniform colour (normally bronze or grey) absorbs infra-red radiation   the amount of heat entering the building

41. P.41 Book E3 Section 2.1 Energy performance of buildings a Tinted glass Drawbacks:  The absorption increases the temperature of glass.  At least half of the subsequent radiation from the glass enters the interior.  Not efficient for providing shade.  Some types of tinted glass absorb natural light.

42. P.42 Book E3 Section 2.1 Energy performance of buildings 5 Controlling heat flow through windows b Reflective glass Reflective glass Coated with a reflective film Blocks and reflects solar heat and visible light  heat entering the building Drawbacks:  natural light entering the building  Reflected heat and light affect the surroundings.  Heat accumulation and glare problems

43. P.43 Book E3 Section 2.1 Energy performance of buildings 5 Controlling heat flow through windows c Double-glazing Air has a much lower conductivity than glass. Double-glazed window  traps air between two panes of glass  low U-value The layer of embedded air should be thin so that the effect of convection is minimized.

44. P.44 Book E3 Section 2.1 Energy performance of buildings c Double-glazing The experiment below investigates the heat transfer through a window mainly by conduction.

45. P.45 Book E3 Section 2.1 Energy performance of buildings c Double-glazing The experiment results: Double-glazed window  Has a lower conductivity than single-glazed window  Has much better heat insulation when heat transfer through radiation is negligible

46. P.46 Book E3 Section 2.1 Energy performance of buildings c Double-glazing Examples: 1.Triple-glazed windows employ three panes of glass with two layers of air. 2.Using inert gases instead of dry air  U-value of windows employing argon gas can be further dropped by 30%. ( ∵ Argon gas has a lower conductivity than dry air.) Common variations based on insulated glazing include using more layers.

47. P.47 Book E3 Section 2.1 Energy performance of buildings 5 Controlling heat flow through windows d Low-e glass Infra-red radiation from the sun can pass through common glass into the interior of buildings. Windows made of low-emissivity glass (low-e glass) can transmit visible light while reflecting the invisible infra-red radiation from outside.  Have plenty of natural light but low energy consumption for air-conditioning

48. P.48 Book E3 Section 2.1 Energy performance of buildings d Low-e glass Low-e glass has an almost invisible thin metal/metal oxide coating:  Placed on the outer window surface  Allows visible light (shorter wavelength) to be transmitted through the window  Reflects infra-red radiation (longer wavelength) Double-glazing employing low-e glass can effectively reduce heat transfer through both conduction and radiation.

49. P.49 Book E3 Section 2.1 Energy performance of buildings 5 Controlling heat flow through windows e Solar control window film Principle similar to that of low-e glass Reflects over 90% solar heat Lets through most visible light Blocks UV radiation Solar control window film: reflects heat and ultra-violet radiation sunlight allows visible light to enter

50. P.50 Book E3 Section 2.1 Energy performance of buildings 5 Controlling heat flow through windows e Solar control window film Solar control window film: reflects heat and ultra-violet radiation sunlight allows visible light to enter  Low cost and convenient option of improving energy efficiency of an existing system A solar film can be easily installed by sticking the film to a clean window with a spray of water.

51. P.51 Book E3 Section 2.1 Energy performance of buildings e Solar control window film 1.Visible transmittance:  Shows how much visible light is transmitted  Value from 0–1 (Higher value  more light transmitted)  Ordinary glass has a value of 0.8 to 0.9. Solar control window film has a value of 0.75. Two main properties to consider when choosing solar control window film:

52. P.52 Book E3 Section 2.1 Energy performance of buildings e Solar control window film 2.Shading coefficient:  Shows how much solar heat (infra-red radiation) is transmitted.  Value from 0–1 (Lower value  less solar heat transmitted)  Solar control window film has a value of ~0.5. Two main properties to consider when choosing solar control window film: Rates of heat transfer by conduction through windows Expt 2b

53. P.53 Book E3 Section 2.1 Energy performance of buildings Experiment 2b Rates of heat transfer by conduction through windows 1.Insulate a container box and put a stirrer in it. 2.Fill the container with hot water at about 70  C to an almost overflow level. 3.Place single-glazed glass on top of the water in full contact. Cover the box with a polystyrene lid. stirrer

54. P.54 Book E3 Section 2.1 Energy performance of buildings Experiment 2b Rates of heat transfer by conduction through windows 4.Measure the temperature of water with a temperature sensor for 20 minutes. Sketch the temperature-time graph. temperature sensor data-logging interface

55. P.55 Book E3 Section 2.1 Energy performance of buildings Experiment 2b Rates of heat transfer by conduction through windows 2.2 Expt 2b - Rates of heat transfer by conduction through windows Video 7.Determine in each case the rate of heat loss for the water for the same drop of 2  C using QtQt = mc (T 2 – T 1 ) t 2 – t 1 5.Measure the mass of water used. 6.Repeat with a sheet of double-glazed glass of the same dimensions.

56. P.56 Book E3 Section 2.1 Energy performance of buildings 5 Controlling heat flow through windows Rates of heat transfer by radiation through windows Expt 2c

57. P.57 Book E3 Section 2.1 Energy performance of buildings Experiment 2c Rates of heat transfer by radiation through windows 1.Cover the interior surfaces of a wooden box with black polystyrene foam boards. 2.Secure a sheet of single-glazed glass on the window opening of the wooden box. Put a temperature sensor into the box so that its tip is at the centre of the box. data-logging interface temperature sensor single-glazed glass 200-W incandescent lamp wooden box

58. P.58 Book E3 Section 2.1 Energy performance of buildings Experiment 2c Rates of heat transfer by radiation through windows 3.Place a 200-W lamp at around 10 cm in front of the glass. The centre of the lamp and the tip of the sensor should be at the same level. 4.Switch on the lamp. Record the temperature inside the box by a data-logging interface and sketch the temperature-time graph. data-logging interface temperature sensor single-glazed glass 200-W incandescent lamp wooden box

59. P.59 Book E3 Section 2.1 Energy performance of buildings Experiment 2c Rates of heat transfer by radiation through windows 2.3 Expt 2c - Rates of heat transfer by radiation through windows Video 5.Repeat with low-e glass and glass with solar control window film in turn. data-logging interface temperature sensor single-glazed glass 200-W incandescent lamp wooden box

60. P.60 Book E3 Section 2.1 Energy performance of buildings 6 Improving energy efficiency of buildings a Natural ventilation A natural ventilation system  operation costs and energy consumption for air-conditioning.  Involves the following factors: building shape, orientation and location; wind direction; the shape, size and location of openings in the buildings

61. P.61 Book E3 Section 2.1 Energy performance of buildings 6 Improving energy efficiency of buildings b Natural lighting Make good use of natural daylight : Save energy for lighting Apart from windows, daylight can be brought indoor by light tubes and skylights. Factors include building orientation and window location.

62. P.62 Book E3 Section 2.1 Energy performance of buildings b Natural lighting Sometimes, a balance is needed. Windows facing the southwest  demand for air-conditioning  need for electrical lighting  integrated approach to optimize energy efficiency

63. P.63 Book E3 Section 2.1 Energy performance of buildings 6 Improving energy efficiency of buildings c Renewable energy supplement Making good use of renewable energy can make buildings more energy efficient. Examples:  Solar cells are used to generate electricity

64. P.64 Book E3 Section 2.1 Energy performance of buildings c Renewable energy supplement Solar heaters or the heat generated by air- conditioning can be used for pre-warming water. Wind energy can produce natural ventilation or it can be captured by wind turbines to generate electricity.

65. P.65 Book E3 Section 2.1 Energy performance of buildings 6 Improving energy efficiency of buildings d Energy management and control systems Automation systems in buildings monitors energy demand and controls the operation of equipment.   energy consumptions E.g. The service-on-demand escalator  Pauses when there is no passenger  Re-starts automatically when a passenger has been detected  energy consumption by 50%

66. P.66 Book E3 Section 2.1 Energy performance of buildings d Energy management and control systems In a building with a number of elevators,  The automatic control system can minimize energy use without causing inconvenience.  The elevators controlled by ‘intelligent’ systems can determine the optimum operation mode.

67. P.67 Book E3 Section 2.1 Energy performance of buildings Solar control window film cuts down the transmission of A visible light. B infra-red radiation. C ultra-violet radiation. D All of the above Check-point 3 – Q1

68. P.68 Book E3 Section 2.1 Energy performance of buildings In a double-glazed window, which of the following materials is embedded between the panes of glass? A Water B Air C Plastic D Glass with lower U-value Check-point 3 – Q2

69. P.69 Book E3 Section 2.1 Energy performance of buildings The End