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4.3 Heat losses 失热量

4.3 Heat losses 失热量. 4.3.1 Factors affecting heat loss 影响建筑失热量的因素. Figure 3.1 Heat losses from a building. Fabric heat loss 结构失热量 Ventilation loss 通风失热量. Factors affecting heat losses 影响建筑失热量的因素. 1) insulation of shell 建筑外围护结构的保温 Good insulation decreases the heat losses

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4.3 Heat losses 失热量

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  1. 4.3 Heat losses 失热量 4.3.1 Factors affecting heat loss影响建筑失热量的因素 Figure 3.1 Heat losses from a building Fabric heat loss结构失热量 Ventilation loss 通风失热量

  2. Factors affecting heat losses影响建筑失热量的因素 1) insulation of shell 建筑外围护结构的保温 Good insulation decreases the heat losses 好的保温减少建筑失热量 Poorly-insulated increase the heat losses 保温不好增加建筑的失热量

  3. 2 ) area of the shell 建筑外围护结构的面积 A terraced house排房and a detached house独立式住宅 Which one loses less heat ?? Table 4.7

  4. 3）temperature difference between inside and outside建筑内外的温差 • Large temperature difference increases the heat losses by conduction and ventilation. • Mainly depend on the design temperature for the inside air. • Recommended comfort temperature for different types of buildings are given in table 4.5

  5. 4) Air change rate换气次数 • Warm air leaving a building carries heat and is replaced by cold air • Table 4.5 gives typical rates of air infiltration

  6. 5) Exposure to climate 建筑的暴露情况 External surface resistance 外表面热阻 Three types of exposure • Sheltered: 受遮挡 Buildings up to 3 storeys in city centres. • Normal: 正常 Most suburban and country buildings • Severe:严重 Buildings on exposed hills or coastal sites. 山坡或海边的建筑 Floors above the fifth in suburban or country sites. 市郊或农村的5层以上建筑 Floors above the ninth in city centres. 市中心九层以上建筑

  7. 6）efficiency of services设备的效率 • Flue is positioned inside the building Flue is positioned inside the building • Flue is positioned on an external wall

  8. 7) Patterns of use 建筑使用模式 • The number of hours per day and the days per year that a building is used have a large effect on the energy consumption of a buildings • Each parts needs to be considered as a separated building for the heating calculations.

  9. Fabric heat loss结构失热量 walls, windows, roofs and floors. steady state conditions,稳态条件 4.3.2 Calculation of heat loss失热量计算 稳态计算法：不考虑建筑物以前时刻传热过程的影响，只采用室内外瞬时或平均温差与围护结构的传热系数、传热面积的积来求负荷

  10. Applied in the following cases: 蓄热性能小的轻型、简易围护结构的传热过程，可用逐时室内外温差乘以传热系数和传热面积近似计算 室内外温差的平均值远远大于室内外温度的波动值时，采用平均温差的稳态计算带来的误差也比较小，在工程设计中是可以接受的。

  11. Ventilation loss 通风失热量

  12. External temperature 室外温度 • In winter: = outside air temperature for design purpose • it is necessary to take account of solar radiation as well as air temperature. • In summer: = sol-air temperature teo

  13. 太阳直射辐射 大气长波辐射 对流换热 太空散射辐射 壁体得热 环境长波辐射 地面长波辐射 地面反射辐射 • Sol-air temperature（室外空气综合温度） • 1 围护结构外表面的热平衡图 • 1）对流换热量 • 2）太阳辐射 • 太阳直射辐射 • 天空散射辐射 • 地面反射辐射 • 3）长波辐射换热量 • 与大气之间的长波辐射 • 与环境表面之间的长波辐射 • 与地面的长波辐射

  14. 太阳直射辐射 大气长波辐射 对流换热 太空散射辐射 壁体得热 环境长波辐射 地面长波辐射 地面反射辐射 2 建筑物外表面单位 面积上得到的热量： 式中 —建筑物外表面单位面积上得到的热量，W/m2 —围护结构外表面的对流换热系数，W/ m2℃ —室外空气温度，℃ —围护结构外表面温度，℃ —围护结构外表面对太阳辐射的吸收率 —太阳辐射照度，W/ m2 —围护结构外表面与环境表面的长波辐射换热量，W/ m2

  15. 白天：太阳辐射强度〉〉长波辐射，可忽略长波辐射作用白天：太阳辐射强度〉〉长波辐射，可忽略长波辐射作用 夜间：没有太阳辐射的作用，天空的背景温度<< 空气温度，不 可忽略建筑物向天空的长波辐射 特别是冬季夜里忽略天空辐射可能导致对热负荷的估计 过低 Qlw 也被称为夜间辐射或有效辐射

  16. Worked example 4.1 A window measuring 2 m by 1.25 m has an average U-value, including the frame, of 6.2 W/m2K. Calculate the rate of fabric heat loss through this window when the inside comfort temperature is 20℃ and the out side air temperature is 4 ℃. know U= 6.2 W/m2K A=2X1.25=2.5m2⊿t=20-4=16 ℃ using So fabric loss=248W

  17. Worked example 4.2 A simple building is 4 m long by 3 m wide by 2.5 m high. In the walls there are two windows, each 1 m by 0.6 m, and there is one large door 1.75 m by o.8 m. The construction has the following U-values in W/m2K: windows 5.6, door 2.0, roof 3.0, floor 1.5. The inside environmental or comfort temperature is maintained at 18 ℃while the outside air temperature is 6 ℃. The volumetric specific heat capacity of the air is taken to be 1300J/m3 ℃. There are 1.5 air change per hour. Calculate the total rate of heat loss for the building under the above conditions.

  18. Step1:sketch the building with its dimensions, as infigure 3.2. calculate the areas and the temperature difference. Step 2:tabulate the information and calculate the rate of fabric heat losses using Step3: calculate the ventilation heat loss. CV= 1300J/m3 ℃, N=1.5/h V=4X3X2.5=30m3, ⊿t=18-6=12 ℃ using So rate of ventilation heat loss = 195W Step4: total rate of heat loss = fabric heat loss + ventilation heat loss= 1734.24+195=1929.24W

  19. 4.3.3 Non-steady condition 非稳定条件 • For situations where the steady state assumption is invalid it is necessary to consider the effects of daily variations in the outside temperature Variations in solar radiation Changes in the internal heat input • Thermal admittance（蓄热系数）or Y-value • Unit： W/m2K Thermal transmittance 传热系数 For very thin units, such as glass, the admittance becomes the same as the U-value. 传热系数和蓄热系数是相反的概念。传热系数表示热传导的能力，蓄热系数表示储存热量的能力。

  20. Figure 4.3 Thermal response Heavyweight structures have smaller temperature swings(温度波动 ) than lightweight structures. damping, 衰减 McMullan

  21. It is too difficult to solve 《空气调节》

  22. 4.4 Heat gains 建筑得热量 Figure 4.4 Typical heat gains in a building McMullan

  23. typical heat gains in a building 1）Solar heat gains from the sun太阳辐射得热量 2）Casual heat gains from occupants and equipment in the building 室内人员和设备形成的一般得热量

  24. 1）Solar heat gains from the sun太阳辐射得热量 Depends on many factors Table 4.9 seasonal solar gain through windows Sun controls to Prevent excessive heat gain and glare（眩光） caused by direct sunshine. External controls （外遮阳） Internal controls （内遮阳） Special glasses （特殊玻璃）

  25. 公共建筑可调节的金属材质遮阳装置

  26. External controls

  27. External controls

  28. Special glasses （特殊玻璃）

  29. 2）Casual heat gains from occupants and equipment in the building室内人员和设备形成的一般得热量 Heat from people Heat from lighting Heat from cooking and water heating Heat from machinery, refrigerators , electrical appliances Table 4.11 domestic seasonal heat gains

  30. 4.5 Heat balance Energy for heating or cooling Fabric Heat Losses casual Heat gains ventilation Heat Losses solar Heat gains + + + + = This is a general expression of balance which is true for summer and winter conditions.

  31. Seasonal energy requirements季节性能耗 • Temperature : average temperature • are valid for calculating total energy consumption and can be used to predict the quantity of fuel required in a season and how much it will cost. • can not be used to predict the size of the heating or cooling plant required; • such a prediction needs consideration of the coldest and hottest days.

  32. Worked example 4.3 Over a heating season of 33 weeks the average rate of heat loss from a certain semi-detached house（半独立式住宅） is 2500W for the fabric loss and 1300W for the ventilation loss. The windows have areas: 6m2 south-facing, 5m2 east-facing, 6m2 north-facing. The house is occupied by three people and cooking is by gas. Use the values for seasonal heat gains given in table 3.7 and 3.9 and calculate : (a) The seasonal heat losses (b) The seasonal heat gains; and (c) The seasonal heat requirements.

  33. (a) total rate of heat loss= fabric loss+ ventilation loss = 2500W+1300W=3800W heat energy lost= rate of heat loss × time taken =3800W × (33×7 ×24 ×60 ×60)s = 75.842GJ(giga joules)千兆焦 so seasonal heat loss = 75.842GJ =75842MJ(mega joules) 兆焦

  34. (b) Heat gains solar window gain ( table 3.7) south (680MJ/m2×6) 4080 east (410MJ/m2×5) 2050 north (250MJ/m2×6) 1500 casual gains ( table 3.9) body heat ( 1000MJ×3) 3000 cooking (gas) 6500 water heating 2000 electrical 3000 total 22130MJ So seasonal heat gain=22130MJ

  35. (c) Seasonal heat requirement = heat loss- heat gain =75842-22130 =53712MJ(megajoules兆焦) =53.712GJ(giga joules千兆焦)

  36. Efficiency 效率 Efficiency is a measure of the effectiveness of a system which converts energy from one form to another • Domestic heating efficiency →table 4.12 • Delivered energy（供给能量） • Useful energy（有用能）

  37. Worked example 4.4 The seasonal heat requirement of a house is 54GJ, which is to be supplied by a heating system with an overall house efficiency of 67%. The solid fuel used has a calorific value of 31MJ/kg. calculate the mass of fuel required for one heating season. Efficiency = 67/100, output= 54MJ, input energy=? Using

  38. Input energy = 80597MJ Mass of fuel needed=

  39. 4.6 Energy regulations 能源规范 Why do we need energy regulations? Can help to minimise energy use in buildings Regulation about thermal insulation control heat loss from buildings Minimise the heat load for heating in winter Minmise the cold load for air conditioning in summer

  40. there are many regulations4.6.1 Building regulations 建筑规范 How to realize energy efficiency in buildings by regulations? (1) Heat loss by transmission through the fabric (2) Heat loss by air leakage around openings and through the fabric (3) Control system for space heating and hot water (4) Heat loss from vessels and pipes used for water (5) Heat loss from hot water pipes and hot air ducts used for space heating (6) energy-efficient lighting sources and switching for the lighting

  41. 4.6 Energy regulations 能源规范4.6.1 Building regulations建筑规范4.6.2 Energy rating, SAP 建筑能耗评级4.6.3 Carbon Index，CI 碳指数4.6.4 Insulation of the building fabric围护结构保温

  42. 4.6.5 Other measurements for energy conservation 其他节能措施 • Controlling the insulation of the building fabric 控制建筑围护结构的保温 • Thermal bridging around openings 开孔处的热桥 • Infiltration 空气渗透 • Space heating control 分区供暖控制 • Hot water controls and insulation of storage 热水控制和储罐的保温 • lighting 采光

  43. Important words in chapter 4 metabolic rate Stack effect Non-renewable energy Renewable energy Primary energy Secondary energy Calorific values Dry resultant temperature sol-air temperature

  44. Air change rate • Thermal admittance

  45. 1 SAP Energy Ratings are expressed on a scale of ( ) A 0 to 1.0 B 0 to 10 C 0 to 100 D 0 to 100%

  46. 2 Carbon Index energy ratings are calculated using the information for a SAP rating and expressed on a scale of ( ) A 0 to 1.0 B 0 to 10 C 0 to 100 D 0 to 100%

  47. 3 ( ) may contribute to energy efficiency A Controlling the insulation of the building fabric B avoid Thermal bridging around openings C mimise Infiltration D Space heating control E Hot water controls and insulation of storage F energy-efficient lighting source and control

  48. 4 Casual heat gains in a building include ( ) A heat from people B heat from lighting C heat from sun D heat from cooking and water heating E heat from machinery, refrigerators F heat from electrical appliances

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