Thermal comfort: b ibasic concepts
Erik JohanssonErik JohanssonHDM
Contents
Variables that affect thermal comfort Variables that affect thermal comfort Indoor thermal comfort Outdoor thermal comfort Comfort zones Comfort zones Thermal comfort indices Subjectively perceived comfort
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Thermal comfort – definition
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Thermal comfort – definition
“a condition of mind that expresses satisfaction with the thermal environment” (American Society of Heating, Refrigerating and Air-conditioning Engineers, ASHRAE)
“The absence of discomfort: a person feels neither too warm nor too cold” (McIntyre 1980)
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( y )
Thermal comfort – thermalThermal comfort thermal stress
Cold or heat stress
Light discomfort
Comfort
Light discomfort
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Consequences of qpoor thermal comfort
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Consequences of qpoor thermal comfort
Efficiency: Decreased mental performance Decreased physical performance
Health: Heat strokes (heat) Heart diseases (heat)
D h d ti (h t) Dehydration (heat) Frostbite (cold)
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Parameters affectingParameters affecting thermal comfort
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Parameters affectingParameters affecting thermal comfort
Air temperature
Exchange of radiation
Air movement
Humidity Humidity
Activity
Cl thiErik Johansson, HDM 9
Clothing
Air temperature
Dry bulb temperature The temperature in the shade
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Exchange of radiation The radiation exchange between the
h b d d di fhuman body and surrounding surfaces (heat loss or heat gain)
Mean Radiant Temperature (MRT): the p ( )average temperature of the surrounding surfaces (floor walls ceiling)surfaces (floor, walls, ceiling)
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Mean radiant temperature p(MRT)
MRT Average temperature of the room surfacesMRT Average temperature of the room surfaces
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MRT Average temperature of the room surfacesMRT Average temperature of the room surfaces
Wind speedp The wind transports heat from the body by
convection and will therefore cool the bodyconvection and will therefore cool the body
The wind also makes the water in the skin The wind also makes the water in the skin evaporate and the energy needed is taken from the moist body thus creating coolingfrom the moist body, thus creating cooling
Skin
MoistEvaporation
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Evaporation heat is taken from the body
Humidity
Too low humidity causes breathing oo o u d y causes b ea gproblems
High humidity prevents evaporative cooling by transpirationcooling by transpiration
Hi h t t diffi lt t High temperatures are more difficult to endure if the humidity is also high
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Relative vs. absolute humidity
Ab l tAbsolute humidity (g/kg)(g/kg)
Temperature (°C)Erik Johansson, HDM 15
Temperature (°C)
The combined effect of temperature and humidity
4000100% 90% 80% 70% 60% 50%
40%
3000
30%DH
2000Pre
ssur
e (P
a)30%
V2000
Vap
our P
20%
C
INV
100010%
C
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Temperature (°C)
Activity levels
Activity Metabolic rate(W) (met)
Sleeping 75 0 7Sleeping 75 0.7Sitting 105 1.0Standing 125 1.2House cleaning 210–350 2.0-3.4Walking (3–6 km/h) 210–400 2.0-3.8Heavy physical work 420–500 4 0-4 8
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Heavy physical work 420 500 4.0 4.8
ClothinggThe insulating property of clothing is measured by the clo value (0 clo means no clothes)
Clothing cloShorts + short-sleeve shirt 0.4Trousers/skirt + shirt 0.6Trousers/skirt + shirt + jacket 1.0
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Thermal indices
Combines two or more variables into one i dindex
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Thermal indices
Combines two or more variables into one i dindex:
Thermal index VariablesThermal index Variables
Wind chill temperature index wind speed, temperature
Temperature-humidity index Temperature, humidity
Operative temperature Temperature MRTOperative temperature Temperature, MRT
ET*, SET*, PMV, PET Temp., humidity, MRT, air movements
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air movements
Heat balance of the humanHeat balance of the human bodyy
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Heat balance of the humanHeat balance of the human bodyy
radiative heat loss (or i )gain)
+
Metabolic heat =convective heat loss
+ evaporative heat loss
++ respiration heat loss
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Thermal comfort indices
Thermal indices based on the heat e a d ces based o e eabalance of the body: PMV = predicted mean vote ET* = effective temperatureET effective temperature SET* = standard effective temperature PET = physiologically equivalent temperature
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Indoor thermal comfort standards
ISO 7730 (based on PMV) ASHRAE 55 (based on ET*)
Calibration of PMV and ET*Calibration of PMV and ET*: extensive climate chamber research extensive climate chamber research
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Climate chamber
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Calibration: comfort votesThe seven point thermal sensation scale
(ASHRAE):( )
cold cool slightly coolg y neutral (comfort temperature)
slightly warm slightly warm warm
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hot
Comfort limits of PMVSensation PMVcold –3cool –2slightly cool –1neutral 0neutral 0slightly warm +1warm +2hot +3
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PMV – Predicted mean vote
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ASHRAE f t t d d 55 (ET*)ASHRAE comfort standard 55 (ET*)(80% of population experience comfort)
Summer:Summer:Activity: 1.2 met
Clothing: 0.5 cloClothing: 0.5 clo
Air speed: 0.25 m/s
Winter:Winter:Activity: 1.2 met
Clothing: 0 9 metClothing: 0.9 met
Air speed: 0.15 m/s
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How reliable are the current thermal comfort standards?
Previously PMV and ET* were assumed to be valid for:
all building types all building types all climate zones all populations
L t h h h th t thi i Later research has shown that this is not true
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Weaknesses of the existing gcomfort standards
Comfort indices such as PMV and ET* are ignoring :
Cultural and social aspects Cultural and social aspects
Behavioural aspects Behavioural aspects
Influence of outdoor climate
Thermal adaptation
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Field surveys
People’s subjective thermal People s subjective thermal perception (at home, at work,
td t )outdoors, etc.)
St d d ti i Standard questionnaires
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Subjective comfort votesThe seven point thermal sensation scale used in
field surveys (ASHRAE):y ( )
cold cool slightly coolg y neutral (comfort temperature)
slightly warm slightly warm warm
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hot
Calculated PMV vs. actual comfort votes
Japan summer (Tanabe 1988)
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Japan, summer (Tanabe 1988)
Field study in Thailand
Comparison between air-conditioned and naturally ventilated offices in warm-humid Bangkok (Busch 1990)
Tneutral Tmax
Air-conditioned offices 24.5C 27C
Naturally ventilated offices 27.5C 31C
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offices
Comfort zone for Havana, CubaComfort zone for Havana, Cuba (naturally ventilated buildings)
Havana, summer (Tablada 2006)
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Psycho-physiological aspects Acclimatization:
Changes in the thermoregulatory system of the body (days, weeks)
B h i l d t ti Behavioural adaptation: Adjustment of clothing
O i / l i i d Opening/closing windows
Psychological factors: Expectations Thermal history
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Neutral temperature pvs. outdoor temp.
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Neutral vs. outdoor temp: pPakistan and Tunisia
Neutral temperature vs. Outdoor temp. 35
30
atur
ePakistan (Nicol 1995)
Pakistan (Nicol et al 1999)
Tunisia (Bouden & Ghrab 2005)
20
25
utra
l tem
pera
15
20
Neu
105 10 15 20 25 30 35
Mean outdoor temperature (°C)
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Mean outdoor temperature ( C)
ASHRAE adaptive comfort stdASHRAE adaptive comfort std (naturally ventilated buildings)
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Outdoor thermal conditions
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Outdoor thermal conditions
Large variations in temperature, humidity, and g p , y,wind speeds
30
32
T (C)
80
100
(%)Temp. Relative humidity
28
30
60
80
24
26
20
40
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0 3 6 9 12 15 18 21 24hour
0
Outdoor thermal conditions
Large Large variations in radiation (solar,radiation (solar, long-wave, reflected))
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Outdoor thermal conditions
Different clothing and activityg y Large micro-
climaticclimatic variations within shortwithin short distances
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Thermal comfort:Thermal comfort: psycho-physiological aspects
Acclimatization Acclimatization Adaptation Bigger impact
d hBigger impact
d hAdaptation
Expectationsoutdoors than indoorsoutdoors than indoors
Thermal history
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Outdoor thermal indices
Wind chill temperature index Temperature-humidity index PMV, ET*, SET*
Ph sologicall eq i alent temperat re Physologically equivalent temperature (PET)
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Wind chill temperature
A cold stress index Combines wind and temperature For temperate and cold climates
Updated 2003 (Canada & USA) Updated 2003 (Canada & USA)
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New wind chill index (2003)
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Physologically equivalentPhysologically equivalent temperature (PET)
Air temperature Mean radiant
temperatureHeat balance p Humidity
Heat balance equation
Wind
Assumed comfort zone: PET = 18 – 23°C
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Solar radiation
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Solar radiation
Direct (beam) radiation Diffuse radiation
Solar radiation scattered because of particles in the sky and clouds
Global (total) radiation: direct + diffuse
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Solar radiation:1000W/m² Global Diffuse Direct
Solar radiation: direct and diffuse600
800
Fez, Morocco (hot dry)200
400
00 3 6 9 12 15 18 21 24
Time (h)
Guayaquil, Ecuador (h h id)
1000W/m² Global Direct Diffuse
(hot humid)600
800
200
400
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00 3 6 9 12 15 18 21 24
Time (h)
Mean Radiant TemperatureMean Radiant Temperature (MRT)( )
Di t l di ti Direct solar radiation Diffuse solar radiation Reflected radiation Long-wave radiation
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MRT on a65
Temp. (°C)
Mean radiant temperature (MRT) MRT on a clear day 55
60Mean radiant temperature (MRT)Air temperature
Colombo Sri50
Colombo, Sri Lanka (7°N)
40
45
30
35
25
30
15
20
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1500 03 06 09 12 15 18 21 24
time (h)
How to determine MRT
Radiation measurements Radiation measurements
Short- and Short- and S o t a dlong-wave radiation in
S o t a dlong-wave radiation in six directionssix directions
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How to determine MRT
Globe thermometer
Measures the ”globe temperature” from
Measures the ”globe temperature” fromtemperature from which MRT can be
calculated
temperature from which MRT can be
calculatedcalculatedcalculated
PiPiPing-pong ball globe thermometer
Ping-pong ball globe thermometer
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thermometerthermometer
Calculation of MRT from the globe temperature
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Calculations of MRT without measurements
Complicated calculations Complicated calculations Simplified models/softwares, for example
Ra ManRayMan
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Thermal comfort study in yGuayaquil
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Microclimat measurements -cloudy afternoon
70
50
60
Wi d d ( / )
30
40Wind speed (m/s)Air temp. (°C)Rel humidity (%)
10
20
Rel. humidity (%)Globe temp. (°C)
0
10
0 0 0 0 0 0 0 0 0 0 0 020 jan 2010 Erik Johansson, HDM 6014
:50
15:0
0
15:1
0
15:2
0
15:3
0
15:4
0
15:5
0
16:0
0
16:1
0
16:2
0
16:3
0
16:4
0
Air temp. – MRT – PET
Temp. (°C) Air temp. MRT PET
45
50
40
45
30
35
25
30
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15:00
15:10
15:20
15:30
15:40
15:50
16:00
16:10
16:20
16:30
16:40
PET
Increases with: decreased wind speed increased humidity
Decreases with: Decreases with: increased wind speed decreased humidity
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Physiologically Equivalent Temperature (PET) - Lund
-15--5 -5-5 5-15 15-25
22
2425-35 35-45 45-55
18
20
14
16
ur
10
12 Hou
4
6
8
0
2
4
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