Transcript heat loss

4 Energy Use in Building
（建筑能耗）
All buildings use energy for lighting, heating or
cooling.
This chapter explains the use of energy in buildings
and includes the technical basis for defining thermal
comfort.
The mechanisms for the heat loss and gains
relating to buildings are analysed and calculated
so that you can understand the energy use of
particular buildings.
this chapter describes:
4.1 Energy use
4.2 Thermal comfort 热舒适
4.3 Heat losses 失热量
4.4 Heat gains 得热量
4.5 Heat balance 热平衡
4.6 Energy consumption 能耗
4.1 Energy use
4.1.1 Energy terms
（1） A fuel is a substance that is a source of energy
（2）fossil fuels 化石燃料
coal, crude oil and natural gas
（3）non-renewable energy 不可再生能源
is from sources which can only use once
fossil fuels
（4）renewable energy 可再生能源
is from sources which are replenishable可补给的
wind power, wave power潮汐能
（5）primary energy 一次能源
is the total energy contained in natural reserves
coal，oil，natural gas
（6）transformation 能源的转化
Is an activity that converts primary energy into another form
Fuels to electricity, crude oil to petroleum
（7）secondary energy 二次能源
Is the energy contained in a fuel which results from a
transformation process.
Electricity, manufactured gas, surplus hot water
（8）delivered energy 输出能源
Is energy content as it is received by the consumer.
Pay for money
（9）useful energy有用能
Is the energy required to perform a given task.
4.1.1 Energy units
The scientific unit of energy is the joule.
Megajoules 1MJ=106J
Gigajoules
1GJ=109J
Tonne of oil equivalent 吨石油当量
1 tonne of oil equivalent = 41.87gigajoules
= 107kilocalcories
= 11630 kWh
= 396.8 therms
4.4.1 calorific values 热值
Calorific value is a measure of the primary heat
energy content of a fuel expressed in terms of unit
mass or volume.
Some typical calorific values are quoted in table 4.1
Table 4.2 shows typical use of energy on the large
scale of national usage.
4.2 Thermal comfort
The
thermal comfort of human
beings governed by many
physiological(生理学
的)mechanisms of the
body ,Vary from person to
person.



The body constantly produces heat energy from the food
energy it consumes.
This heat needs to be dissipated （散失）at an
appropriate rate to keep the body at constant
temperature.
The transfer of the heat from the body is mainly by the
processes of convection, radiation and evaporation.



Are you or your employees feeling
uncomfortable with the temperature in the
workplace?
The term ‘thermal comfort’ describes a
person’s state of mind in terms of whether
they feel too hot or too cold.
There’s more to it than just room temperature.
4.2.1 Factors affecting thermal comfort
Personal variables
 Activity
 Clothing
 Age
 Sex
Physical variables
 Air temperature
 Surface temperature
 Air movement
 humidity
The
Six
Basic
Factors
4.2.1 Activity





The greater the activity of the body the more heat it
gives off.
The rate of heat emission depends upon the
individual metabolic rate （代谢率）of a person and
upon their surface area.
The average rate of heat emission decreases with
age.
Table 3.1 lists typical heat output from an adult male
for a number of different activities.
The output from adult females is about 85% that of
males.
4.2.2 Clothing




Clothes act as a thermal insulator for the body and
help to maintain the skin at a comfortable
temperature.
a scale of clothing（服装的数值体系） has been
developed: the clo-value.
1clo=0.155m2K/W of insulation and values range
form 0clo to 4clo.
Table 3.2 shows the value of different types of
clothing and indicates how the room temperature
required for comfort varies with clothing.
4.2.3 Room
temperature
When sitting near
the cold surface of
a window, do you feel
comfort? Why?
Different types of temperatures are described below
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Inside air temperature tai 室内空气温度
Mean radiant temperature tr 平均辐射温度
Inside environmental temperature tei室内环境温度
Dry resultant temperature tres ????温度
Room centre comfort temperature tc房间中心舒适温度
（1）Inside air temperature tai
室内空气温度
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
The inside air temperature is the average
temperature of the bulk air inside a room .
It is usually measured by an ordinary dry
bulb thermometer which is suspended in
the centre of the space and shielded from
radiation.
（2）Mean radiant temperature tr
平均辐射温度

The mean radiant temperature is the average effect
of radiation from surrounding surfaces.
A1t1  A 2 t 2  .....
tr 
A1  A 2  .....
The mean radiant temperature should be kept near
the air temperature but not more than 3 ℃ below it,
otherwise conditions are sensed as “stuffy”
（3）Inside environmental temperature tei
室内环境温度


Inside environmental temperature Is a
combination of air temperature and radiant
temperature. The exact value depends upon
convection and radiation effects.
For average conditions it can be derived from the
following formula
t ei  2 / 3t r  1 / 3t ai
Environmental temperature is recommended for
the calculation of heat loss and energy
requirements
（4）Dry resultant temperature tres
室内综合温度


Is a combination of air temperature,
radiant temperature and air movement.
When the air movement is low it can be
derived form the following formula
t res  1 / 2t r  1 / 2t ai
（5）Room centre comfort temperature tc
房间中心舒适温度





Is a measure of temperature which gives an
acceptable agreement with thermal comfort.
When air movement is low, the dry resultant
temperature at the centre of a room is commonly-used
comfort temperature
The globe thermometer黑球温度计 is a regular
thermometer fixed inside a blackened globe of
specified diameter
This globe temperature can be used to calculate other
temperatures and when air movement is small it
approximates to the comfort temperature.
球体, 地球仪, 地球, 世界
4.2.4 Air movement 空气流速
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The movement of air in a room helps to increase heat
lost from the body by convection
Can cause the sensation of draughts
Air movement above 0.1m/s in speed require higher
air temperature to give the same degree of comfort
Measure air movement
A hot-wire anemometer and a Kata thermometer
热线流速计
卡塔温度计（冷却温度表）
May be used to measure air movement.
Both devices make use of the cooling effect of moving
air upon a thermometer.
4.2.5 Humidity 湿度
Humidity is caused by moisture in the air
 Relative humidity within the range of 40-70% is required
for comfortable conditions
1 High humidities and high temperatures
→ feel oppressive (难以忍受的)
→ natural cooling by perspiration(排汗)is decreased
2 High humidity and low temperature
→ cause the air to feel chilly
3 Low humidity
→ dryness of throats and skin
Static electricity

4.2.6 Ventilation 通风

Ventilation Is necessary to provide oxygen and to
remove contaminated （受污染的）air

Ventilation Has a great effect on the heat loss from
buildings and condensation in buildings

A number of statutory regulations（法定规章）specify
minimum rates of air supply in occupied spaces.

Table 3.4 gives some typical fresh air-supply rates

Statutory 法令的, 法定的

Statutory regulations 法定规章
4.2 Thermal comfort
4.2.1
4.2 .2
4.2. 3
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
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
activity
Factors affecting
thermal comfort
clothing
room temperatures
Inside air temperature tai 室内空气温度
Mean radiant temperature tr 平均辐射温度
Inside environmental temperature tei室内环境温度
Dry resultant temperature tres 室内综合温度
Room centre comfort temperature tc房间中心舒适温度
4.2.4 air temperature
4.2.5 humidity
4.2.6 ventilation
Factors affecting
thermal comfort
4.3
Heat losses 失热量
4.3.1 Factors affecting heat loss
Figure 3.1 Heat losses from a building
Some important factors are listed below

Insulation of building

Area of the external shell 建】房屋的框架

Temperature difference

Air change rate 换气率

Exposure to climate

Efficiency of services

Use of building
4.3.2 Calculation of heat loss
失热量计算

Fabric heat loss 结构失热量

Fabric heat loss from a building is caused by the
transmission of heat through the materials of walls, roofs
and floors.
Assuming steady state conditions, the heat loss for each
element can be calculated by the following formula.

Pf  UAt
Ventilation loss 通风失热量
Ventilation heat loss from a building is caused by the
loss of warm air and its replacement by air that is
colder and has to be heated.
C V NVt
PV 
3600
External temperature 室外温度

When designing heating system of buildings it is necessary to
assume a temperature for the outside environment temperature

In winter: = outside air temperature for design purpose

For heat transfer calculation in summer it is necessary to take
account of solar radiation as well as air temperature.

In summer: = sol-air temperature teo
sol-air
temperature（室外空气综合温度）
is an environmental temperature for the outside air
which include the effect of solar radiation
1。围护结构外表面的热平衡图
太阳辐射
大气
长波
辐射
长波辐射换热量
对流换热量
太阳
直射
辐射
太空
散射
辐射
环境长波辐射
2。建筑物外表面单位面积上
得到的热量：
q   out (t air
地面
长波 地面反射
辐射
 t w )  aI  Q辐射
lw


Qlw
aI
  out (t air 

)  t w    out (t z  t w )
 out  out


对
流
换
热
壁体得
q   out (t air  t w )  aI  Qlw
大气
长波
辐射


Qlw
aI
  out (t air 

)  t w    out (t z  t w )
 out  out


式中
q
太阳
直射
辐射
太空
散射
辐射
环境长波辐射
对
流
换
热
壁体得热
地面
长波 地面反射
—建筑物外表面单位面积上得到的热量，W/m2
辐射 辐射
 out—围护结构外表面的对流换热系数，W/ m2℃
t air —室外空气温度，℃
t w —围护结构外表面温度，℃
a —围护结构外表面对太阳辐射的吸收率
I —太阳辐射照度，W/ m2
Qlw —围护结构外表面与环境表面的长波辐射换热量，W/ m2
Worked example 4.1
A window measuring 2 m by 1.25 m has an average Uvalue, 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
Pf  UAt  6.2  2.5 16  248
So fabric loss=248W
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 Pf  UAt
Step3: calculate the ventilation heat loss.
CV= 1300J/m3 ℃, N=1.5/h V=4X3X2.5=30m3, ⊿t=18-6=12 ℃
using P  C V NVt
V
3600
1300 1.5  30 12

 195
3600
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 Cyclic
(daily) variations in the outside temperature, Variations in
solar radiation and Changes in the internal heat input
Thermal admittance（蓄热系数）or Y-value is a
property of an element (构件）or a room which
controls fluctuations(波动) in the inside temperature.
Unit ： W/m2K
Thermal transmittance 传热系数

Heavyweight structures have smaller temperature
swings(温度波动 ) than lightweight structures.
Figure 3.3 thermal response

damping阻尼， 减幅, 衰减

Figure 4.3 Thermal response
McMullan

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。
4.3 Heat loss
4.3.1 factors affecting heat loss
 Insulation of building
 Area of the external shell 建】房屋的框架
 Temperature difference
 Air change rate 换气率
 Exposure to climate
 Efficiency of services
 Use of building
4.3.2 calculation of heat loss
This is about what we we
Fabric heat loss
did last lesson. Do you
P  UAt
f
Ventilation loss
C V NVt
PV 
3600
remember?
Now I want some one to
summary in Chinese.
4.3.3 Non-steady condition 非稳定条件
Thermal admittance（蓄热系数）or Y-value
4.4 Heat gains
Figure 4.4 Typical heat gains in a building
McMullan
typical heat gains in a building
1）Solar heat gains from the sun
2）Casual heat gains from occupants and
equipment in the building
1）Solar heat gains from the sun
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）
公共建筑可调节的金属材质遮阳装置
External controls
External controls
2）Casual heat gains from occupants and
equipment in the building
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
Now I want you to
turn to page 81
Now we will go on
4.5 Heat balance



The thermal comfort of humans requires that the inside
temperature of a building is kept constant at a specified
level, and the storage of goods also needs constant
temperatures.
In order to maintain constant temperature the building will
generally require heating or cooling, and both of these
process involve the consumption of energy.
Calculation of energy
E  Pt
Heat balance
Fabric
Heat
Losses
ventilation
+ + Heat
Losses
solar
casual
= Heat
+ Heat
gains
gains
Energy
for
+ heating
or
cooling
This is a general expression of balance which is
true for summer and winter conditions.
Seasonal energy requirements
季节性能耗




The energy requirement of a building at any particular time
depends on the state of the heat losses and the heat gains
at the same time.
These factors vary but it is useful to consider the total
effect over a standard heating season.
It is important to note that the calculation of seasonal heat
losses and gains assumes average temperature
conditions and 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.

Seasonal heat calculations 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.
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.
(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
(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
(c) Seasonal heat requirement = heat loss- heat gain
=75842-22130
=53712MJ(mega joules兆焦)
=53.712GJ(giga joules千兆焦)
Efficiency 效率

The heat energy required for buildings is commonly
obtained from fuels such as coal, gas and oil, even if the
energy delivered in the form of electricity.

Each type of fuel must be converted to heat in an
appropriate piece of equipment

The amount of heat finally obtained depends upon the
original heat content of the fuel and the efficiency of the
system in converting and distributing （分配）this heat.
Efficiency 效率
Efficiency is a measure of the effectiveness of
a system which converts energy from one
form to another
efficiency %
useful energy

100
delivered energy



Domestic heating efficiency →table 4.12
Delivered energy（供给能量）
Useful energy （有用能）
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
efficiency %
useful energy

100
delivered energy
67
54

100 input energy
Input energy = 80597MJ
Mass of fuel needed= energy required  80597MJ
calorific value
 2600kg
31MJ/kg
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 regulations
4.6.1 Building regulations 建筑规范
Encourage or enforce energy efficiency in buildings
How to realize energy efficiency in buildings by regulations?
The regulations achieve this aim by controlling the following
(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
Also should consider:

Other essential performances such as structural
stability, resistance to rain penetration and
overheating

The need for design details that are practical and
within the capabilities of the construction workforce

Building services that are easy for occupiers to
manage successfully

Regulations which are not too complex to interpret
and enforce
4.6.2 Energy rating, SAP
用能评级
The overall energy efficiency of a dwelling , such as a house,
can be given an Energy Rating by using a Standard
Assessment Procedure (SAP)
An SAP Energy Rating of a dwelling is found by using a
standard method method to calculate the annual energy
cost for space heating and water heating in the building
SAP Energy Ratings are expressed on a scale of 0 to 100,
The higher the SAP number the better the performance
4.6.3 Carbon Index，CI 碳指数
The Carbon Index (CI) is an energy rating
based on the overall carbon dioxide emission
figure for a building
Carbon Index energy ratings are calculated
using the information for a SAP rating and
expressed on a scale of 0 to 10,
The higher the CI the better the performance.
4.6.4 Insulation of the building fabric
围护结构保温
（1）Elemental method 构件法
Matching of standard U-values for individual
construction elements
Table 4.13 Elemental U-values for fabric insulation
（2）Target U-value method 目标U值法
Compare the average U-value of the whole
exposed fabric with a specified target U-value
This method offers greater design flexibility
than elemental method
They are subject to poorest acceptable Uvalues that are specified for each element
Also subject to limitations on thermal bridging
and air infiltration.
（3）Carbon index method 碳指标法
This method also offers design flexibility.
Buildings with higher carbon index values will
satisfy regulations
They are subject to poorest acceptable Uvalues that are specified for each element
Also subject to limitations on thermal bridging
and air infiltration.
（4）Commercial buildings 商业建筑
For buildings other than dwellings the design,
construction and operation, needs to
demonstrate that the building and its services
are energy efficient.
They are subject to poorest acceptable Uvalues that are specified for each element
Also subject to limitations on areas of openings.
4.6.5 Other measurements for energy
conservation
其他节能措施






Controlling the insulation of the building fabric
Thermal bridging around openings
Infiltration
Space heating control
Hot water controls and insulation of storage
lighting
Translate into Chinese
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
)