Thermal comfort: bibasic concepts · Thermal comfort – definition “a condition of mind that expresses satisfaction with the thermal environment” (American Society of Heating,

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

Erik Johansson, HDM 2

Thermal comfort – definition


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)


( y )

Thermal comfort – thermalThermal comfort thermal stress

Cold or heat stress

Light discomfort

Comfort

Light discomfort


Consequences of qpoor thermal comfort


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)


Parameters affectingParameters affecting thermal comfort


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


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)


Mean radiant temperature p(MRT)

MRT Average temperature of the room surfacesMRT Average temperature of the room surfaces


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


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


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

Erik Johansson, HDM 1600 5 10 15 20 25 30 35 40 45

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


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


Thermal indices

Combines two or more variables into one i dindex


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


air movements

Heat balance of the humanHeat balance of the human bodyy


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


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


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


Climate chamber


Calibration: comfort votesThe seven point thermal sensation scale

(ASHRAE):( )

cold cool slightly coolg y neutral (comfort temperature)

slightly warm slightly warm warm

20 jan 2010 Erik Johansson, HDM 26

hot

Comfort limits of PMVSensation PMVcold –3cool –2slightly cool –1neutral 0neutral 0slightly warm +1warm +2hot +3


PMV – Predicted mean vote


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


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


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


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


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


hot

Calculated PMV vs. actual comfort votes

Japan summer (Tanabe 1988)


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


offices

Comfort zone for Havana, CubaComfort zone for Havana, Cuba (naturally ventilated buildings)

Havana, summer (Tablada 2006)


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


Neutral temperature pvs. outdoor temp.


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)


Mean outdoor temperature ( C)

ASHRAE adaptive comfort stdASHRAE adaptive comfort std (naturally ventilated buildings)


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


0 3 6 9 12 15 18 21 24hour

0


Large Large variations in radiation (solar,radiation (solar, long-wave, reflected))



Different clothing and activityg y Large micro-

climaticclimatic variations within shortwithin short distances


Thermal comfort:Thermal comfort: psycho-physiological aspects

Acclimatization Acclimatization Adaptation Bigger impact

d hBigger impact

d hAdaptation

Expectationsoutdoors than indoorsoutdoors than indoors

Thermal history


Outdoor thermal indices

Wind chill temperature index Temperature-humidity index PMV, ET*, SET*

Ph sologicall eq i alent temperat re Physologically equivalent temperature (PET)


Wind chill temperature

A cold stress index Combines wind and temperature For temperate and cold climates

Updated 2003 (Canada & USA) Updated 2003 (Canada & USA)


New wind chill index (2003)


Physologically equivalentPhysologically equivalent temperature (PET)

Air temperature Mean radiant

temperatureHeat balance p Humidity

Heat balance equation

Wind

Assumed comfort zone: PET = 18 – 23°C


Solar radiation


Solar radiation

Direct (beam) radiation Diffuse radiation

Solar radiation scattered because of particles in the sky and clouds

Global (total) radiation: direct + diffuse


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


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


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


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


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


thermometerthermometer

Calculation of MRT from the globe temperature


Calculations of MRT without measurements

Complicated calculations Complicated calculations Simplified models/softwares, for example

Ra ManRayMan


Thermal comfort study in yGuayaquil


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

20 jan 2010 Erik Johansson, HDM 6114:50

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


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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Thermal comfort: bibasic concepts · Thermal comfort – definition “a condition of mind that expresses satisfaction with the thermal environment” (American Society of Heating,