energy in buildings: steady state

PowerPoint PresentationEnergy in Buildings: Steady state Dr. Jonas Allegrini
1 29.10.2018 1 Jonas Allegrini
| | Building Physics: Theory and Applications 29.10.2018 Jonas Allegrini 2
| | Building Physics: Theory and Applications
Energy use in buildings
1. Heat gains/losses in a building 2. Steady state calculations 3. Influencing factors
This lecture…
Energie in Gebäuden - Ziele
Building stock responsible for about 50% of energy demand
40% of CO2-Emissions
Strategies to improve the buildings energy performance
Low-Energy/low emission buildings
Energy in Buildings
Content
29.10.2018 5 Jonas Allegrini
Motivation
Energie in Gebäuden
Quelle: Prognos 2013
Energy in Buildings
Energy in Buildings
Source: Prognos 2013 29.10.2018 8 Jonas Allegrini
New tendencies of energy efficient construction
Until 1975 conventional
Cantonal requirements ca. 48 kWh m-2 a-1
Conventional Low-energy (Minergie) Passiv (Minergie-P) Energie-autark (Net-zero-energy) E-Plus
New tendencies of energy efficient construction
Content
Introduction
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 energy
needed
Transients – Used to investigate time dependent response of house and heating system
Heat load calculations Used to design the heating system – What is the
maximum heat load the heating system has to be able to deliver?
How many radiators? How large radiators? Heat source requirement?
Content
Definitions
+
Heat energy demand
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)
Heat energy demand
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
Heating demand
Solar gains
Ventilation losses
Transmission- losses
Internal gains
Infiltration losses
Heating load
( )[ ] tQ ISVTH Φ+Φ−Φ+Φ= η 29.10.2018 18 Jonas Allegrini
Transmission losses – Through walls, floor, roof, windows, doors
What kind of heat losses does a house have?
Solar gains
Internal gains
Heating load
Ventilation losses – Heat lost by introduction of cooler ambient air to the heated space
Ventilation losses Solar gains
Infiltration losses – Heat lost by leakage of cooler ambient air into the house / heated air outside the house
Infiltration- losses
Solar gains
Internal gains
Heating load
Ventilation losses
Source: Willems 2013
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]
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 29.10.2018 22 Jonas Allegrini
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
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)
θ=Φ AUT
The transmission heat loss for a building component is given by
Heat transfer coefficient U-value W/m2K
Heat flow W, J/s
U λ = Thickness
Transmission heat losses U-values according to SIA 380-1
Heat transfer coefficients - According SIA 180
Transmission heat losses
Transmission heat losses 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 W
Transmission heat losses When the building is composed of N surfaces A1, A2, A3,
∑ =
the transmission heat loss factor is also given by
with Um the area weighted average U-value and AT the total surface covering the protected volume
Calculation example
Uw = 1.1 W / m2 K
Uwall = 0.2 W / m2 K θi = 20°C θe = − 1.1°C
22
Aglas = 5.04 m2
2 PC: 150 W during 8 h/day
1 printer: 100 W during 1h/day
Luminance: 6 W/m2
Grenzwerte gemäss SIA 380-1 (Ausgabe 2009)
ΦT
ΦT = HT ⋅ θ
mit HT = Ai i
ΦT = 208W
Uw = 1.1 W / m2 K
Uwall = 0.2 W / m2 K θi = 20°C θe = − 1.1°C
22
Transmission heat losses Thermal bridges (linear, point) are taken into account
∑∑∑ ===
111 χ
with Lj the length of the jth type of linear thermal bridges, zk the number of repeating point thermal bridges
lijnvormig geconcentreerdlinear form point form 29.10.2018 34 Jonas Allegrini
Values according to SIA 380-1 Transmission heat losses
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
Natural ventilation
Mechanical ventilation
Ventilation heat (enthalpy) losses
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, ρa the density of air (1.2 kg/m3), V the volume (m3) qV outside air flow rate (m3/h)
V qn V
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.
Heat recovery systems
with ηrec the efficiency of the heat recovery system, when using a infiltration / exfiltration ventilation system.
• For ventilation losses we introduce a heat loss factor HV
• Frequently we use the following simplified equation
( )recaaV VcnH ηρ −= 1 3600
VnHV 34.0=
Calculation example
Uw = 1.1 W / m2 K
Uwall = 0.2 W / m2 K θi = 20°C θe = − 1.1°C
22
Aglas = 5.04 m2
Luminance: 6 W/m2
ΦV = nL
ΦV = 0.5
ΦV
ΦV = 347 W
What heat gains does a building have?
Ventilation losses
Transmission losses
Solar gains
Internal gains
Infiltration losses
heating load
Solar gains
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 coefficient
Heat 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.
Solar gains
Without shading system
internal shading system
external shading system
Calculation example
Uw = 1.1 W / m2 K
Uwall = 0.2 W / m2 K θi = 20°C θe = − 1.1°C
22
Aglas = 5.04 m2
Luminance: 6 W/m2
ΦS = 5.04 ⋅ 0.6 ⋅ 50
Aglas = 5.04 m2
ΦS = 151W
= Aglas ⋅ g ⋅ E
Internal heat gains
People
Appliances
Electric appliances
Calculation example
Uw = 1.1 W / m2 K
Uwall = 0.2 W / m2 K θi = 20°C θe = − 1.1°C
22
Aglas = 5.04 m2
Luminance: 6 W/m2
ΦI = PP + PA + PL
ΦI = 2 ⋅ 80 ⋅ 8
PL PP PA
Luminance: 6 W/m2
Monthly heat balance for a zone (room)
( )[ ] tQ ISVTH Φ+Φ−Φ+Φ= η
ΦT : transmission heat losses ΦV : ventilation heat losses Φs : Solar heat gains ΦI : Internal heat gains t : the time of heating in a month η : the use factor
Net energy demand J
θ=Φ TT H
θ=Φ VV H
∑∑∑ ===
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)
The use factor η
Small losses High gains
The use factor η
The use factor η depends on the ratio between heat gains and heat losses using the factor γ
The use factor depends on the heat capacity or heat storage of the building
00, <Φ+Φ= Φ+Φ Φ+Φ
= VT VT
IS ifγγ
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).
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.
The use factor depends on the storage capacity of a building, which is expressed by the time constant.
The time constant is given by the ratio between heat capacity and loss factor
VT HH C +
The storage 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
The use factor The use factor η as a function of the factor γ and the time constant τ
U se
fa ct
Used only for certain parts of the day (e.g. schools, retail,
restaurants,…)
Calculation example
Uw = 1.1 W / m2 K
Uwall = 0.2 W / m2 K θi = 20°C θe = − 1.1°C
22
Aglas = 5.04 m2
Luminance: 6 W/m2
ηg = 1− γ a
1− γ a+1
= 0.88
Use factor
Parameter for Use factor (thermal inertia)
γ = ΦS + ΦI
The use factor
ΦT
ΦV
ΦS
QH = ΦT + ΦV − ηg ⋅ ΦS + ΦI( ) ⋅ t
= 1 3
= 0.8 + 36.6 70
1− γ a+1 = 1− 0.881.32
1− 0.881.32+1 = 0.61 C (internal walls, ceiling, floor) cinternal = 1 MJ/K C external wall cwall = 2.16 MJ/K
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
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, …)
The energy demand for heating
The energy use equals the brut energy demand divided by the monthly
production efficiency of the installation (e.g. the burner)
When solar energy systems are present, we subtract the useful contribution of the solar system from the total energy use
Calculation example
Uw = 1.1 W / m2 K
Uwall = 0.2 W / m2 K θi = 20°C θe = − 1.1°C
22
Aglas = 5.04 m2
Luminance: 6 W/m2
The use factor
QHJanuar = 208 + 347 − 0.61 ⋅ 151+ 338( ) ⋅ (31 ⋅ 24 ⋅ 60 ⋅ 60)
QHJanuar = 693 MJ
1− 0.881.32+1 = 0.61
QHJanuar
AE
QH
AE
Content
Influencing factors
Heating energy demand Outside climate Use of the building The building design
Heating energy consumption System
Outside climate
Temperature the lower the temperature the higher the net heating energy demand
Outside climate: temperature
The outside temperature evolution during a day is given for different months for Zurich. The temperature is lowest during January and highest in July. The temperature is lowest during nights and mornings and rises until 4 pm.
This figure shows the monthly average temperature and horizontal solar radiation for Zurich city.
Radiation is equal in November and January, the temperature in January is much lower, why?
Outside climate: temperature
Temperature the lower the temperature the higher the net heating energy demand
Solar Radiation the higher the solar radiation the lower the net heating energy demand
Outside climate
The table compares the total solar radiative energy for different places in the world
Also in moderate climates, solar energy shows a potential to contribute in the reduction of the total energy use in buildings by solar gains and solar energy
Outside climate: solar radiation
Outside climate: solar radiation
Outside climate – micro climate
We have only a limited influence on the local microclimate and also by these means on the heating demand.
For limiting the cooling load we can take much more mitigating
measures (see urban comfort) to limit heat islands effects
Shortwave radiation
Longwave radiation
Thermal mass
Sensible heat
Latent heat
Heat load
Anthropogenic heat
Influencing factors
Net energy demand Outside climate Use of the building The building design
Brut energy demand installation
Use of the building – internal temperature
The inside temperature has a direct effect on the net energy demand Control temperature
Use of the building – internal temperature Temperature control A good temperature control can lead to a substantial reduction of
the net energy demand
Heat only those zones which are needed Heat only during periods that it is needed which is called
‘intermittent’ heating
Due to adaptive comfort, the occupant has the possibility to adapt to internal temperatures.
Good insulation quality of the outside walls assures sufficient high surface wall temperatures, which allows lower air temperatures to be set (see comfort)
0
10
20
30
40
50
60
70
PP D
air temperature 20 air temperature 24
Clothing value=1clo; activity= 1met; air speed 0.15 m/s; relative humidity= 50% 29.10.2018 85 Jonas Allegrini
Due to hygienic and health reasons a minimum air change rate of 0.5 ACH has to be provided
Ventilation has a strong influence on the heating demand. Influence increases for very air tight and highly insulated buildings.
Use of the building – ventilation
A combination of appliances designed to supply interior spaces with outdoor air and to extract polluted indoor air.
Use of the building – ventilation system
Both mechanical and natural ventilation can be combined with openable windows.
A fan can extract air from humid zones (bathrooms) putting the building in underpressure, allowing for air comming in by openings in windows in the living spaces and sleeping rooms.
When having inlets and outlets, also openings for letting air pass by (air passages) are needed.
A continuous basic ventilation is needed in combination with peak ventilation using fans in kitchens and bathrooms
Use of the building – internal heat gains
We have to take internal gains into account as a boundary condition given by the functioning of the building
Internal gains have to be limited in summer times in order to limit the cooling load
Internal gains due to extreme electricity use have to be limited 29.10.2018 89 Jonas Allegrini
Influencing factors
Brut energy demand installation
Building design
Compactness
Compactness
Compactness
Compactness
TA VC =
The compactness equals to volume to surface ratio, with AT the surface where heat transmission losses occur
units (m)
( )ei m
Compactness
The compactness is not absolute, but depends on the volume. Higher volumes are more compact (higher compactness).
A sphere has the highest compactness, equal to R/3, with R the radius
A cube has a compactness, equal to a/6, with ‘a’
the side length
Compactness A cube of 500 m3 has a C = 1.32. Consider 5 of these cubes in a row. What
compactness do the three cubes in the middle have ?
Compactness A cube of 500 m3 has a C = 1.32. Consider 5 of these cubes in a row. What
compactness do the three cubes in the middle have ?
3 Va =
937,75003 ==a
Compactness
High rise building with length L=40 m, width W=40 m and height H=3m.N, with N the number of floors
HLW WLC 1
number of floors
Compactness often also defined in 1/m, so the surface area divided by the volume.
Influence of compactness on heating demand:
The depth of a building has an influence on the need for artificial lighting and on the possibility of passive cooling by ventilation by cross ventilation
Compactness
Building design
Average floor area per person rises continuously, and thus the total heating demand
Plan organisation
Plan organisation
Define different temperature zones in a building: e.g. living and sleeping area have different set temperatures
Orient those zones that can profit from solar
heat gains to the south. However, precautions for the summer comfort have to be taken.
Building design
Glazing U-value determines the thermal quality, while the g-value
defines the solar gains for the the building
system U-value g LTA W/m2K
single glass 5.9 0.81 0.9 double glass 3.0 0.72 0.8 low emission double glass 1.8 0.63 0.7 low emission double glass filled with argon 1.3 0.58 0.75 low emission double glass filled with krypton 1.0-1.1 0.58 0.75 low emission double glass filled with xenon 0.9-1.0 0.58 0.75 low emission double glass with foil filled with krypton 0.7 0.5 0.65 low emission double glass with foil filled with xenon 0.6 0.5 0.65
Light transmittance
Sheet1
system
U-value
g
LTA
W/m2K
1.3
0.58
0.75
1.0-1.1
0.58
0.75
0.9-1.0
0.58
0.75
0.7
0.5
0.65
0.6
0.5
0.65
Sheet2
Sheet3
Building design
Thermal insulation Good thermal insulation quality is the first step
towards a low net energy demand and good comfort
It is found by measurement that a linear relation
exists between the average U-value multiplied with the total surface of envelopes transmitting heat Um.AT and the net energy demand QH
Thermal insulation
For highly insulated houses, the thermal insulation becomes a less determining factor for further improving the net energy demand
… taken over by the energy efficient use of solar
gains, solar energy, heat recovery from ventilation, renewable energy, heat pumps …
Room temperatures should be between 23.5 and 26.5°C Internal gains should not be higher than 7 or 5 W/m² for working spaces and
living rooms Requirements for minimum insulation level especially in roof tops Reduction of solar gains in summer Time constant should be at least 100h
Thermal comfort in summer
Building design
The use factor The use factor η as a function of the factor γ and the time constant τ
U se
fa ct
Building design
Air tightness We do not count on infiltration leakage for
ventilation needs Air leakage leads to
Higher energy losses Low sound insulation Higher risk of moisture problems Reduction of the thermal quality and thermal capacity
The air tightness is measured by a blower door test.
Influencing factors
Brut energy demand Installation
Brut energy demand – Installations of systems
The brut energy demand is dependend on efficiency of the heating system.
Nutzungsgrade gemäss SIA 380/1
Content
Simplified methods: Heating Degree Days
The energy demand is proportional to the temperature difference outside – inside and the number of hours we have to heat.
=heating limit
( ) tei −θθ
( )∑ = etHD θ
Graphical representation
( ) tei −θθ
The energy demand is proportional to the temperature difference outside – inside and the number of hours we have to heat.
Suppose that the daily average outside temperature θe is x degrees lower than the inside temperature, then we say this day counts x heating degree days.
To take into account solar and internal gains, we define a limiting outside
temperature θe,limit. We will heat the building when the outside temperature is below this limit temperature, also called the heating limit θg.
Simplified methods: Heating Degree Days
Simplified methods: Heating Degree Days Based on the heating degrees days we can
estimate the total netto energy demand
QH=QT+QV: netto energy demand for heating taking into account internal and solar gains by the heating degrees days HDD
)(36002434.0 )(360024 JHDDVnQ
⋅⋅⋅⋅⋅= ⋅⋅⋅⋅=
Um : average U value, AT : total surface of protected volume, n ventilation rate per hour, V : volume
Climate classification on cooling and heating degrees days This figure
shows a comparison of the different energy consumptions of buildings resulting from climatic conditions.
Content
Only component certification is required (meeting certain U-values)
Overall certification of building envelope can be done if requirements for certain components can not be met
-> SIA 380-1 Goes back to the same equation Switzerland is devided in different climatic
zones
Standard in Switzerland
Requirements for new buildings based on SIA 380/1: (required values for heating demand per year for new buildings by 8.5º C mean annual temperature) Ath: weighted thermal building envelope AE : Reference area Values are increased/reduced by 8% per K higher or lower outdoor annual mean temperature (climate data see: SIA Merkblatt 2028). Targeted values for new buildings are 60% of requirements
Qh,li = Qh,li0 +
Ath: weighted thermal building envelope
Standard in Switzerland Climate stations Standard values based on SIA 2028
Climate stations Kantons define which category has to be taken into
account Inclination has to be taken into consideration
Occupancy data
Standard in Switzerland Example Transmission losses (based on 380/1)
= ∑ ( −)
QT Transmissionswärmeverluste [MJ/m2] Ai Fläche Bauteil (Anteil an thermischer Gebäudehüllfläche) [m2] Ui U-Wert Bauteil [W/m2K] θi Raumlufttemperatur mit Regelzuschlag [°C] θe monatl. Aussentemperatur [°C] Δt Zeitspanne (nd·86400s·106) nd Anzahl Tage pro Monat
Foliennummer 2
This lecture…
Foliennummer 10
What do losses depend on?
Terminology
Foliennummer 26
Foliennummer 33
Foliennummer 34
Solar gains
Solar gains
Calculation example
Foliennummer 49
The use factor h
The use factor h
Calculation example
Foliennummer 70
Foliennummer 71
Use of the building – ventilation
Use of the building – internal heat gains
Influencing factors
Building design
Content
Climate classification on cooling and heating degrees days
Content

energy in buildings: steady state

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