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 lossFabric heat loss 结构失热量

Ventilation loss 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

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

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

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

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

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

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

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.

4.3.2 Calculation of heat loss 失热量计算

Fabric heat lossFabric heat loss 结构失热量 walls, windows, roofs and floors.

steady state conditions, 稳态条件

tUAfP

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

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。

tUAfP

Ventilation loss Ventilation loss 通风失热量通风失热量

3600

t NVCV VP

External temperature External temperature 室外温度室外温度

In winter:In winter: = outside air temperature for design purpose = outside air temperature for design purpose

it is necessary to take account of solar radiation as well as air te

mperature.

In summer:In summer: = sol-air temperature t = sol-air temperature teoeo

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

太阳太阳直射直射辐射辐射大气大气

长波长波辐射辐射

太空太空散射散射辐射辐射

对对流流换换热热

地面反射地面反射辐射辐射

环境长波辐射环境长波辐射

地面地面长波长波辐射辐射

壁体得热壁体得热

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

)()(

)(

wzoutwout

lw

outairout

lwwairout

tttQaI

t

QaIttq

qout

airt

wt

aIlwQ

太阳太阳直射直射辐射辐射大气大气

长波长波辐射辐射

太空太空散射散射辐射辐射

对对流流换换热热

地面反射地面反射辐射辐射

环境长波辐射环境长波辐射

地面地面长波长波辐射辐射

壁体得热壁体得热

2 建筑物外表面单位面积上得到的热量：

)()(

)(

wzoutwout

lw

outairout

lwwairout

tttQaI

t

QaIttq

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

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

248165.22.6Pf tUA

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.

Step1: sketch the building with its dimensions, as in figure 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=30m℃ 3, t=18-6=12 ⊿ ℃

using

tUAfP

1953600

12305.113003600

t NVCV

VP

So rate of ventilation heat loss = 195W

Step4: total rate of heat loss = fabric heat loss + ventilation heat loss= 1734.24+195=1929.24W

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 admittanceThermal admittance （蓄热系数）（蓄热系数） or Y-valueor Y-value UnitUnit ： W/m2K

Thermal transmittance Thermal transmittance 传热系数传热系数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。

Figure 4.3 Thermal response

McMullan

Heavyweight structures h

ave smaller temperature

swings(( 温度波动 温度波动 )) than l

ightweight structures.

damping, 衰减

It is too difficult to solve

《空气调节》

4.4 Heat gains 建筑得热量

Figure 4.4 Typical heat gains in a building

McMullan

typical heat gains in a building

11 ）） Solar heat gains from the sunSolar heat gains from the sun 太阳辐射得热量

22 ）） Casual heat gains from occupants and equipment in the Casual heat gains from occupants and equipment in the

building building 室内人员和设备形成的一般得热量

11 ）） Solar heat gains from the sunSolar 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 controlsSun 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）

公共建筑可调节的金属材质遮阳装置

External controls

External controls

Special glasses （特殊玻璃）

22 ）） Casual heat gains from occupants and equipCasual heat gains from occupants and equip

ment in the buildingment 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 gainsTable 4.11 domestic seasonal heat gains

4.5 Heat balance

Fabric

Heat

Losses

ventilation

Heat

Losses

solar

Heat

gains

casual

Heat

gains

Energy for heating or cooling

++ = + +

This is a general expression of balance which is true for summer and winter conditions.

PtE

Seasonal energy requirements季节性能耗

Temperature : average temperature are valid forare 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 predictcan 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.

Worked example 4.3

Over a heating season of 33 weeks the average rate of heat lo

ss from a certain semi-detached housesemi-detached house （半独立式住宅半独立式住宅） is

2500W for the fabric loss and 1300W for the ventilation loss. T

he 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.

(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) 千兆焦千兆焦

so seasonal heat loss = 75.842GJ

=75842MJ(mega joules) 兆焦兆焦

(b) Heat gains solar window gain ( table 3.7)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)casual gains ( table 3.9) body heat ( 1000MJ×3) 3000 cooking (gas) 6500 water heating 2000 electrical 3000 total 22130MJtotal 22130MJSo seasonal heat gain=22130MJ

(c) Seasonal heat requirement = heat loss- heat gainheat loss- heat gain

=75842-22130

=53712MJ(mega(mega joulesjoules 兆兆焦焦 ))

=53.712GJ(giga joules(giga joules 千兆千兆焦焦 ))

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 energyDelivered energy （供给能量）（供给能量） Useful energyUseful energy （有用能（有用能）

energy delivered

energy useful

100

%efficiency

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

energy delivered

energy useful

100

%efficiency

energyinput

54

100

67

Input energy = 80597MJ

Mass of fuel needed=

2600kg

31MJ/kg

80597MJ

valuecalorific

requiredenergy

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

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

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

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

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

Air change rate Thermal admittance

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%

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%

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

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

海陆风和山谷风

53

建筑的互遮挡情况

55