||Building physics 3: Energy Part 1
Energy in Buildings
Dr. Kristina Orehounig
1
These buildings are examples from Austria of very low energy buildings built according to the passive house concept.
||
Energy use in buildings
1. Heat gains/losses in a building
2. Steady state calculations
3. Influencing factors
27.02.2019 3
This lecture…
||Building physics 3: Energy Part 1
Energie in Gebäuden - Ziele
Energy issues in the context of buildings
Building stock responsible for about 50% of energy demand
40% of CO2-Emissions
Goal to reduce energy consumption by 50% until 2050
Strategies to improve the buildings energy performance
Low-Energy/low emission buildings
Insights into standards, calculation methods, and simulation
methods
Energy in Buildings
4
||Building physics 3: Energy Part 1
5
Content
Introduction
Definitions
Heat load calculation
Annual heating energy demand
-----------------------------
Influencing factors
Simplified methods
Situation in Switzerland
||Building physics 3: Energy Part 1
Motivation
Building
Micro climate
Systems
Occupant
Thermal
Performance
6
||Building physics 3: Energy Part 1
Energie in Gebäuden
Energy demand in private households
Source: Prognos 2013
Energy in Buildings
Space
heating
Domestic hot
water
||Building physics 3: Energy Part 1
Energy in Buildings
Change from 2000 to 2012
Source: Prognos 20138
||Building physics 3: Energy Part 1
New tendencies of energy efficient construction
9
Until 1975
conventional
approx. 220 kWh.m-2.a-1
Cantonal requirementsapprox. 48 kWh.m-2.a-1
9
||Building physics 3: Energy Part 1
Conventional
Low-energy (Minergie)
Passiv (Minergie-P)
Energie-autark
(Net-zero-energy)
E-Plus10
New tendencies of energy efficient construction
||Building physics 3: Energy Part 1
11
Content
Introduction
Heat load calculation
Annual heating energy demand
Influencing factors
Simplified methods
Situation in Switzerland
||Building physics 3: Energy Part 1
Heizwärmebedarf (Net energy demand)
Heizwärmebedarf
Definition
The energy which is required to
heat a building (or a zone).
It is independent from the
heating system or energy
carrier.
||Building physics 3: Energy Part 1
Heizwärmebedarf
Definition
Heizenergiebedarf (Brut energy demand)
Heizenergiebedarf
Erzeugung
Speicherung
Verteilung
Takes production, storage,
transmission and distribution of
heat into account.
Is equal to the net energy
demand divided by the
efficiency of the heating system
13
||Building physics 3: Energy Part 1
Heizwärmebedarf
14
Definition
Heizenergiebedarf (Brut energy demand)
Heizenergiebedarf
Erzeugung
Speicherung
Verteilung
Example:
Net energy demand for a
building: 38 kWh.m-2.a-1
100m²: 3800 kWh.a-1
Brut energy demand with e.g. oil
heating system:
3800/0.7 = 5430 kWh.a-1
||Building physics 3: Energy Part 1
15
Definition
Endenergiebedarf
Heizwärmebedarf
Heizenergiebedarf
Erzeugung
Speicherung
Verteilung
Final energy consumption
Final energy consumption:
• Domestic hot water
• Lighting
• Electrical appliances
+
||Building physics 3: Energy Part 1
16
Definition
Endenergiebedarf
Primary energy
Heizwärmebedarf
Heizenergiebedarf
Erzeugung
Speicherung
Verteilung
Primary energy
Generation,
conversion and
distribution of the
energy carrier
+
||Building physics 3: Energy Part 1
17
Definition
Net energy demand for heating
The energy needed for a zone / building when the total efficiency of the
installation for heating equals 100%
Brut energy demand for heating
The net energy demand divided by the efficiency of the heating system
(distribution and heat release)
||Building physics 3: Energy Part 1
18
Definition
Energy use
The energy used in a building for heating, domestic warm water, lighting and electric appliances . Also the energy used to make the heating system operational (pumps, fans, control system) is included
Primary energy use
The total energy used to produce and transport the energy necessary for the building
||Building physics 3: Energy Part 1
Heating demand calculations involves three
different types of calculations:
Heat load calculations – Used to design the
heating system
Annual heating energy demand – Used to determine the amount of energyneeded
Transients – Used to investigate time
dependent response of house and heating
system
19
Definition
||Building physics 3: Energy Part 1
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Outside Dry-Bulb Temperature [°C]
Heat load calculation Annual heating energydemand
Transient
For dimensioning theheating system
For calculating theannual heating energy
demand
To calculate the time dependent behaviour
Reference temperature
Typical year Hourly values
20
Definition
HDDor
monthlyvalues
||Building physics 3: Energy Part 1
-25
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Outside Dry-Bulb Temperature [°C]
Heat load calculation Annual heating energydemand
Transient
For dimensioning theheating system
For calculating theannual heating energy
demand
To calculate the time dependent behaviour
Reference temperature
Typical year Hourly values
21
Definition
HDDor
monthlyvalues
||Building physics 3: Energy Part 1
-25
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03
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24
.12
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Outside Dry-Bulb Temperature [°C]
Heat load calculation Annual heating energydemand
Transient
For dimensioning theheating system
For calculating theannual heating energy
demand
To calculate the time dependent behaviour
Reference temperature
Typical year Hourly values
22
Definition
HDDor
monthlyvalues
||Building physics 3: Energy Part 1
23
Content
Introduction
Heat load calculation
Annual heating energy demand
Influencing factors
Simplified methods
Situation in Switzerland
||Building physics 3: Energy Part 1
24
Heat load calculation
To dimension the heating system
What is the maximum heat load which the heating
system has to deliver based on a standardized indoor
temperature?
Gebäude-
kategorieRaumnutzung
Norm-
Innentemperatur
[°C]
Wohnen Schlafraum 20
Wohnraum 20
Küche 20
Industrie Arbeiten im Sitzen 20
leichter Aktivitätsgrad 20
mittlerer Aktivitätsgrad 18
Schwere körperliche
Arbeit15
||Building physics 3: Energy Part 1
25
Heat load calculation
To dimension the heating system
What is the maximum heat load which the heating
system has to deliver based on a standardized indoor
temperature?
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
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.01
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22
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05
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26
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16
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07
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03
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Outside Dry-Bulb Temperature [°C]
||Building physics 3: Energy Part 1
26
Heat load calculations
How many radiators
How large radiators
Heat source requirement
||Building physics 3: Energy Part 1
27
Content
Introduction
Heat load calculation
Annual heating energy demand
Influencing factors
Simplified methods
||Building physics 3: Energy Part 1
Amount of heat, which is required over a year, to
maintain a certain indoor temperature within the
building
28
Annual heating energy demand
||Building physics 3: Energy Part 1
Energy performance of a building:
29
Heating demand
Solar
gains
Ventilation losses
Transmission-
losses
Internal
gains
Infiltration losses
Heating
load
tQ ISVTH
||Building physics 3: Energy Part 1
Transmission losses – Through walls, floor,
roof, windows, doors
30
What kind of heat losses does a house have?
Solar
gains
Transmission-
losses
Internal
gains
Heating
load
||Building physics 3: Energy Part 1
Ventilation losses – Heat lost by introduction
of cooler ambient air to the heated space
31
What kind of heat losses does a house have?
Ventilation lossesSolar
gains
Transmission-
losses
Internal
gains
Heating
load
||Building physics 3: Energy Part 1
Infiltration losses – Heat lost by leakage of
cooler ambient air into the house / heated air
outside the house
32
What kind of heat losses does a house have?
Infiltration-
losses
Solar
gains
Transmission-
losses
Internal
gains
Heating
load
Ventilation losses
||Building physics 3: Energy Part 1
What do losses depend on?
Source: Willems 2013
33
Indoor and outdoor temperature
Ventilation and infiltration flow rates
Area of walls, floor, roof, windows, doors
Insulation level of walls, floor, roof, windows, doors
U-values [W.m-2.K-1]
Building component Very bad bad middle good Very good
Roof >1.0 0.6 0.3 0.22 <0.15
Wall >1.5 0.8 0.4 0.3 <0.2
Window 5.2 3.5 1.8 1.4 <1.2
Typical U-values of building components
Source: hornbach.ch
||Building physics 3: Energy Part 1
34
Terminology
Protected (conditioned) volume V
The volume in a building, where thermal comfort is required and thus heating /
cooling
In residential buildings the protected volume
equals the habited volume.
External dimensions are used to calculate the
protected volume.
Reference area
Floor area in a building which is heated Quelle: Energieatlas
||Building physics 3: Energy Part 1
35
Definitions
Energy reference area (Energiebezugsfläche)
alle ober- und unterirdischen Geschossflächen, die innerhalb der
thermischen Gebäudehülle liegen
Building envelope area (Gebäudehüllfläche)
Fläche der thermischen Gebäudehülle (Aussenabmessungen)
Setzt sich zusammen aus Flächen gegen aussen, gegen
unbeheizte Räume und gegen Erdreich
Reduktionsfaktoren für Flächen gegen unbeheizt und Erdreich
||Building physics 3: Energy Part 1
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Transmission heat losses
The transmission heat losses in a zone
Transmission heat losses to the outside environment
Transmission heat losses to other zones at different temperature (e.g. non-
heated spaces)
||Building physics 3: Energy Part 1
37
Building envelope area - dimensioning (SIA 416)
Building envelope area
Reduction factors for areas against unheated or to the ground
||Building physics 3: Energy Part 1
38
Building envelope area - dimensioning (SIA 416)
Building envelope area
Details
||Building physics 3: Energy Part 1
39
Building envelope area – an example
unbeheizt
||Building physics 3: Energy Part 1
40
AUT
The transmission heat loss for a building
component is given by
Heat transfer
coefficient
U-value
W/m2K
Heat flow
W, J/s
Surface m2
Temperature
difference
Transmission heat losses
||Building physics 3: Energy Part 1
Transmission heat losses
41
Heat transfer coefficient for the
thermal envelope
in
n
e h
ddd
h
U1
...1
1
2
2
1
1
Heat transfer
coefficient outside
W/m2K
Heat transfer
coefficient inside
W/m2K
Thermal
conductivity
W/mK
thickness
m
Number of layers
||Building physics 3: Energy Part 1
Heat transfer coefficient
42
Heat exchange between the environment and wall surface area is called the heat transfer coefficient(combined for radiation and convection).
||Building physics 3: Energy Part 1
Heat transfer coefficients - According SIA 180
43
Transmission heat losses
||Building physics 3: Energy Part 1
44
Transmission heat lossesU-values according to SIA 380-1
||Building physics 3: Energy Part 1
Calculate the required thickness of an insulation material for ceiling construction of a
living room against a non heated attica space according to SIA.
1. Transmission heat losses
Mineral wool d=?, λ =0,04 W/mK
reinforced concrete ceiling d=140mm, λ =2,1 W/mK
gypsum plaster 15mm, λ=0,7 W/mK
||Building physics 3: Energy Part 1
Calculate the required thickness of an insulation material for ceiling construction of a
living room against a non heated attica space according to SIA.
1. Transmission heat losses
in
n
e h
ddd
h
U1
...1
1
2
2
1
1
Mineral wool d=?, λ =0,04 W/mK
reinforced concrete ceiling d=140mm, λ =2,1 W/mK
gypsum plaster 15mm, λ=0,7 W/mK
||Building physics 3: Energy Part 1
Transmission heat losses
47
The building envelope is composed of different
components, like walls, windows, glazing, roofs. The
transmission heat loss factor HT describes the total
heat loss through the building envelope.
Heat flow
WTemperature
difference
TT H
Transmission
heat loss factor
W/K
||Building physics 3: Energy Part 1
Transmission heat losses
48
When the building is composed of N surfaces A1, A2, A3,
… and U values U1, U2, U3, … the heat loss factor is
given by
N
i
iiT UAUAUAUAH1
332211 ...
mTT UAH
the transmission heat loss factor is also given by
with Um the average U-value and AT the total surface
covering the protected volume
||Building physics 3: Energy Part 1
Transmission heat losses
49
Thermal bridges (linear, point) are taken into account
using a linear heat transfer coefficient (W/mK) or
point heat transfer coefficient (W/K)
l
k
kk
m
j
jj
n
i
iiT zLUAH111
with Lj the length of the jth type of linear thermal bridges,
zk the number of repeating point thermal bridges
lijnvormig geconcentreerd
linear form point form
||Building physics 3: Energy Part 1
50
Thermal bridges according to SIA 380-1
Transmission heat losses
||Building physics 3: Energy Part 1
51
Thermal bridges according to SIA 380-1
||Building physics 3: Energy Part 1
Transmission heat losses
53
l
k
kk
m
j
jj
n
i
iiT zLUAH111
Transmission heat
transfer coefficient
W/K
Thermal bridgesTransmission heat losses
||Building physics 3: Energy Part 1
ΦT
2. Transmission heat losses
6m
3m
3m
2m
Uw
1.1 W / m2 K
Uwall
0.2 W / m2 K
i 20C
e
1.1C
22°C
N
i
iiT UAUAUAUAH1
332211 ...
TT H
T HT ?
||Building physics 3: Energy Part 1
Limit and target values according to SIA 380-1 (Ausgabe 2009)
ΦT
2. Transmission heat losses
Target valuesLimit values
||Building physics 3: Energy Part 1
59
What kind of heat losses does a house have?
Ventilation lossesSolar
gains
Transmission-
losses
Internal
gains
Heating
load
Infiltration-
losses
||Building physics 3: Energy Part 1
60
Ventilation heat (enthalpy) losses
The enthalpy heat loss due to ventilation can be due to air
exchange between the zone and the outside environment,
between different zones and due to air infiltration or
exfiltration of the ventilation (air heating) system
||Building physics 3: Energy Part 1
Natural ventilation
Mechanical ventilation
Air conditioning unit
Heat recovery ventilation
61
Ventilation heat (enthalpy) losses
window ventilation duct ventilation
||Building physics 3: Energy Part 1
Natural ventilation: window ventilation
Ventilation through open
window. Usually, high, narrow
windows lead to a more
efficient air exchange than low,
wide windows.
Short time, fast ventilation=
Stoßlüftung/shock ventilation
62
||Building physics 3: Energy Part 1
Natural ventilation: duct ventilation
Vertical ducts, which
(specifically in winter) use the
thermal stack-effect. Additional
inlet air openings have to be
provided.
Suitable for rest rooms and
bathrooms.
||Building physics 3: Energy Part 1
Mechanical ventilationLow pressure ventilation (Unterdrucklüftung):
The air is sucked via a fan into the vent pipes. (Negative pressure in the interior, fresh air is sucked in
through joints and / or supply air openings)
Applications: sanitary area, large kitchens, rooms with high concentration of pollutant, targeted ventilation
possible (for example, „exhaust (kitchen) hood")
High pressure ventilation (Überdrucklüftung):
By generating an overpressure (fan in the supply air shaft) in the interior air escapes through joints and / or
exhaust ducts. Problematic: dust uptakes
Zentrale Ventilationsanlage im
Dachraum
Einzelventilatoren
64
||Building physics 3: Energy Part 1
65
Ventilation heat (enthalpy) losses
eiaaV Vcn
3600
ca the specific heat (1000 – 1030 J/Kg.K),
n the air change rate per hour (1/h) also denoted ACH,
a the density of air (1.2 kg/m3),
V the volume (m3)
qV outside air flow rate
V
qn
V
Ventilation heat
losses
||Building physics 3: Energy Part 1
66
Ventilation heat (enthalpy) losses
eiaaeiaaV Vcn
cG 3600
Ga the air flow (kg/s),
ca the specific heat (1000 – 1030 J/Kg.K),
n the air change rate per hour (1/h) also denoted ACH,
a the density of air (1.2 kg/m3),
V the volume (m3)
qV outside air flow rate
||Building physics 3: Energy Part 1
Ventilation heat (enthalpy) losses
Typical ventilation and infiltration rates
67
Source: Gross et al. 2007
Window
tilted
Window
½ open
Window
fully openCross-
ventilation
Mech.
Vent.
with fans
Mech.
ventilation
||Building physics 3: Energy Part 1
68
Infiltration losses
Pressure difference between the indoors and outdoors cause leakage
through cracks near windows, doors, and corners of the house.
Usually the leakage is around 0.1- 0.3 ACH for new houses, and 0.5-1.5 ACH
for old houses.
The heat losses are calculated as the ventilation heat losses with the new
volumetric flow rate.
||Building physics 3: Energy Part 1
• For ventilation losses we introduce a heat loss factor HV
with rec the efficiency of the heat recovery system, when
using a infiltration / exfiltration ventilation system.
• Frequently we use the following simplified equation
69
recaaV Vcn
H 13600
VnHV 34.0
Ventilation heat (enthalpy) losses
||Building physics 3: Energy Part 1
70
Heat recovery systems
||Building physics 3: Energy Part 1
V nL
3600V ca a i e
ΦV
3. Ventilation losses
nL
0.5 /h
a
1.2 kg/m3
ca
1000 J/kgK
QH T V g S I t
||Building physics 3: Energy Part 1
72
What heat gains does a building have?
Ventilation losses
Transmission
losses
Solar
gains
Internal
gains
Infiltration losses
heating
load
||Building physics 3: Energy Part 1
73
Internal gains
Solar radiation – heat which is gained through transmission of sun light
through windows which is absorbed by the indoor space, stored and released.
People – every person produces around 100-120 W/Person
Electrical appliances and lighting – thermal energy released from appliances
which is given to the environment.
||Building physics 3: Energy Part 1
75
The total solar irradiation on a surface with angle b,
consists of a direct and diffusive part. We consider the
diffusive radiation of the sky to be isotropic.
Direct radiation
Diffuse radiation
from the sky
Diffuse radiation
from the ground
Solar gains
||Building physics 3: Energy Part 1
76
Solar gains
The total irradiation depends highly on weather conditions especially on the cloudiness
||Building physics 3: Energy Part 1
Solar gains
77
The total irradiated solar energy depends on the time of the year(summer vs winter), the orientation and the inclination of the surface.
Example of a South oriented surface
Low
sun
altitude
during
winter
21th december
21th december
21th june
||Building physics 3: Energy Part 1
78
Solar gains The total irradiated solar energy depends on the time of the
year (summer vs winter), the orientation and the inclinationof the surface.
Example of vertical surfaces with different orientation
||Building physics 3: Energy Part 1
Solar gains
79
South Facing FacadesFor a predominately South facing facade,
effective solar shading can be achieved
using a fixed horizontal solar shading
system.
During the day in both summer and
spring/autumn, a fixed horizontal system
projecting out from the window can be
designed to shade the building during
office hours.
In the winter such a device cannot stop
direct rays of the sun penetrating the
building since the sun is much lower.
However the heat gain and solar glare is
greatly reduced in winter and therefore
this may not considered to be a major
problem.
summer
winter
Spring /
autumn
||Building physics 3: Energy Part 1
80
east west
Solar gains
East or West Facing FacadesWith a predominantly East or West facing facade, a
fixed system will not perform well throughout the
whole day as the altitude of the sun is much lower.
Sunlight will pass directly under most horizontal
shading systems as shown in the illustration below.
To overcome this problem, effective solar shading
can be achieved using a movable solar shading
system
||Building physics 3: Energy Part 1
81
STEsingle glazing
Transparent
component
irradiation EsT : the total solar
irradiation including
direct and diffuse parts
on the transparent
component (W/m2)
Solar gains through transparent components
||Building physics 3: Energy Part 1
82
STE
STS E
One part of the irradiation
is reflected with ρS the
reflection coefficient
one part is transmitted,
with τS the transmission
coefficient
one part is absorbed, with
αS the absorption
coefficient
reflection
Transparent
component
irradiation
STS Eabsorption
STS E
transmission
single glazing
Solar gains through transparent components
||Building physics 3: Energy Part 1
83
STS E
STE Due to absorption the
glass pane heats up and
releases heat by
convection and radiation
to the environment
according to the heat
transfer coefficient hi for
inside and he for outside
Transparent
component
absorption
irradiation
single glazing
Solar gains through transparent components
||Building physics 3: Energy Part 1
84
STE
The total heat admission through
glass is the sum of
the radiation transmitted through
glass
the inward convective /radiative
flow from the heated glass due to
absorbed solar radiation
the heat flow due to outdoor-
indoor temperature differences
Transparent
component
irradiation
STS E
transmission
STS Eabsorption
ie
single glazing
Solar gains through transparent components
||Building physics 3: Energy Part 1
85
STE
Transparent
component
irradiation
STS E
transmission
STS Eabsorption
ie
Solar heat gain coefficient
or G-value
is the fraction of incident solar
radiation admitted through a
window, both directly
transmitted and absorbed and
subsequently released inward.
SHGC is expressed as a number
between 0 and 1. The lower a
window's solar heat gain
coefficient, the less solar heat it
transmits.
Solar gains through transparent components
single glazing
||Building physics 3: Energy Part 1
86
STE
STS E
Transparent
component
irradiation
ie
SiS
hhhg
Overall solar heat
gain coeffcient for
single glazing or ‘g’
valuetransmission
STS Eabsorption
Solar gains through transparent components
||Building physics 3: Energy Part 1
88
STE
STS E
Transparent
component
irradiation
Transmission coefficient
for double glazingtransmission
21
21
1
S
1 2
Solar gains through transparent components
||Building physics 3: Energy Part 1
||Building physics 3: Energy Part 1
||Building physics 3: Energy Part 1
91
Solar gains
sTrs EgAF
The glass reduction factor Fr takes into account the
surface reduction due to the window frames
Other influencing factors
Shadowing (protection by the horizon, shadowing devices)
Soiling of the glass, use of curtains
g-value or solar
heat gain
coefficientHeat flow
W Surface m2
Solar irradiation
W/m2
Glass
reduction factor
Solar irradiation – Heat gained by mainly transmission of sunlight through
windows which is then captured by inside capacity, stored and emitted.
||Building physics 3: Energy Part 1
92
Solar gains
Reduction factor due to solar shading devices
Without shading
system
internal shading
system
external shading
system
||Building physics 3: Energy Part 1
Calculate the solar gains of a south oriented 2m2 big window in vertical wall
at the location of Vienna. (frame 20%, g=0,6)
4. Solar gains
sign desciption value unit
IS Irradiation- south 356 kWh.m2.a-1
IN Irradiation- north 150 kWh.m2.a-1
IO Irradiation- east 210 kWh.m2.a-1
IW Irradiation- west 210 kWh.m2.a-1
Ihorizontal Irradiation- horizontal 368 kWh.m2.a-1
||Building physics 3: Energy Part 1
Calculate the solar gains of a south oriented 2m2 big window in vertical wall
at the location of Vienna. (frame 20%, g=0,6)
4. Solar gains sTrs EgAF
sign desciption value unit
IS Irradiation- south 356 kWh.m2.a-1
IN Irradiation- north 150 kWh.m2.a-1
IO Irradiation- east 210 kWh.m2.a-1
IW Irradiation- west 210 kWh.m2.a-1
Ihorizontal Irradiation- horizontal 368 kWh.m2.a-1
||Building physics 3: Energy Part 1
95
What heat gains does a building have?
Ventilation losses
Transmission
losses
Solar
gains
Internal
gains
Infiltration losses
heating
load
||Building physics 3: Energy Part 1
96
Internal heat gains
Internal gains I based on SIA (Appliances, lighting, people)
People
Appliances
||Building physics 3: Energy Part 1
97
Electric appliances
||Building physics 3: Energy Part 1
98
Monthly heat balance for a zone (room)
tQ ISVTH
T : transmission heat losses
V : ventilation heat losses
s : Solar heat gains
I : Internal heat gains
t : the time of heating in a month
: the use factor
Net energy
demand J
||Building physics 3: Energy Part 1
Energy performance of a building:
99
Heating demand
Solar
gains
Ventilation losses
Transmission-
losses
Internal
gains
Infiltration losses
Heating
load
tQ ISVTH
||Building physics 3: Energy Part 1
100
Monthly heat balance for a zone (room)
tQ ISVTH
T : transmission heat losses
V : ventilation heat losses
s : Solar heat gains
I : Internal heat gains
t : the time of heating in a month
: the use factor
Net energy
demand J
||Building physics 3: Energy Part 1
How much of heat gains can be utilised within the building
When the losses are low and the gains are high, the inside temperature can rise above the comfort temperature, and gains become useless, or even cost energy (cooling)
101
The use factor
Small lossesHigh gains
||Building physics 3: Energy Part 1
102
The use factor
The use factor depends on the ratio between
heat gains and heat losses using the factor g
The use factor depends on the heat capacity or
heat storage of the building
00,
VT
VT
IS ifgg
||Building physics 3: Energy Part 1
103
Capacity of a building
The diurnal variations of the outside temperature (green line) result in heat flows into the building during the day, where part of the heat is stored in the material.
During the night, the heat flow is reversed (from the building to the environment).
||Building physics 3: Energy Part 1
104
Capacity of a building
The higher the
thermal mass, the
greater the time lag
and the smaller the
ratio between the
maximal variation of
internal and external
temperatures
(Timax/T0max).
Thus thermal mass
leads to increased
thermal comfort and
to reduced peak
loads for technical
systems.
||Building physics 3: Energy Part 1
105
Capacity of a building
||Building physics 3: Energy Part 1
106
Capacity of a building
The use factor depends on the capacity of a
building, which is expressed by the time
constant.
The time constant is given by the ratio between
capacity and loss factor
VT HH
C
||Building physics 3: Energy Part 1
107
Capacity of a building
The capacity is the capacity of the layers
situated at the inside from the insulation layer
(up to a certain thickness, e.g. 0.1 m)
layers
jjji
walls
ii dcCCAC ,
||Building physics 3: Energy Part 1
The use factor
108
The use factor as a function of the factor g and
the time constant
Use
facto
r
Gain/Loss ratio
Time constant
||Building physics 3: Energy Part 1
Used only for certain parts of the
day (e.g. schools, retail,
restaurants,…)
Always in use
Use factor for thermal gains
||Building physics 3: Energy Part 1
g 1 g a
1 g a1
5. The use factor
ΦT
ΦV
ΦS
QHΦI
g 151 338
208 347 0.88
QH T V g S I t
Use factor
a ao
o
Parameter for
Use factor
(thermal inertia)
g S I
T V
Gain/Loss-ratio
||Building physics 3: Energy Part 1
111
The energy demand for heating
Determine the net energy demand for heating for every zone of the building, for every month and sum up the values over the year
Use monthly averages
When the monthly average is negative, we assume the energy demand for heating zero
||Building physics 3: Energy Part 1
112
The energy demand for heating
Determine the brut energy demand Divide the monthly net energy demand by the monthly average efficiency of the heating
system (distribution, heat release by radiators, convectors, …)
Heizwärmebedarf
Heizenergiebedarf
Erzeugung
Speicherung
Verteilung +
When solar energy systems are present, we
subtract the useful contribution of the solar
system from the total energy use
||Building physics 3: Energy Part 1
6. Energy performance number
ΦT
ΦV
ΦS
QHΦI
QH T V g S I t
QH T V g S I t
QH Januar 208 347 0.61 151 338 (31 24 60 60)
QHJanuar 693MJ
g 1 g a
1 g a11 0.881.32
1 0.881.321 0.61
||Building physics 3: Energy Part 1
QH Januar
AE693
5 6 23.1MJ / m2
ΦT
ΦV
ΦS
QHΦI
6. Energy performance number
QH T V g S I t
QH
AE