Thermal comfort

© G.Scudo, 2002 Gianni Scudo

Built Environment Sciences & Technology (BEST), Politecnico di Milano, Via Durando 10, 20158 Milano

Text of paper to the COST C 11 "Green structures and urban planning" - Milan Oct 2002

FIRST DRAFT

1. Introduction

2. Thermal comfort indexes.

3. Green urban structures as climate mitigating technology

4. Evaluation and design tools.

 

Appendixes (to be added)

a. "Green design" parameters

b. Urban green spaces categories

Review of programmes to evaluate outside comfort conditions

 Click button to return to COST C11 - WG1B home page
Click here to return to the Progress Report 2002

1. Introduction

 

The understanding and evaluation of thermal comfort in outdoor spaces is a basic need for bioclimatic urban design including the green urban structures contribution. Thermal comfort in urban space design was and still is an almost completely forgotten variable mainly in Mediterranean areas where the climatic conditions are (or were) moderate and the traditional urban architecture provided good bioclimatic solutions for open spaces evolved in a long "trail and error" processes. But in the contemporary urban design this unconscious traditional knowledge was not integrated or re-invented and the effect is ours contemporary urban environment which is often uncomfortable, due also to the progressive warming of climate at different scales.

Sensorial comfort is a very complex and multidimensional phenomena which involve processes of physical/psychological active adaptation to local culturally different urban environment.

Research in progress is trying to elaborate knowledge which integrates these two qualitative and quantitative aspects trying to overcome the discrepancy between people thermal perception and quantitative physiological evaluation (Nikoloupolou 2001, SaGAcités 2002) and to elaborate urban design tools. Also the important contributions of Environmental Psychology and Urban Research in the field of people activities and sensorial comfort perception in public spaces (Ghel, Jabobs and Bosselman, Appleyard) have to be revisited.

But anyway the knowledge of the interaction between physiological and psychological parameters of thermal comfort needs an up to date review of the state of the art of comfort evaluation methodology.

 

2. Thermal comfort evaluation and indexes.

 

To introduce outside thermal comfort indexes it is necessary to recall the basic difference between indoor and outdoor spaces. While indoor environment tend to have relatively steady and controllable ( by building and mechanical services) thermal, radiative and convective conditions, the outdoor one is defined by a great daily and seasonal variations of much less controllable microclimatic parameters ( Humidity, air temperature, surface temperatures, wind and radiation) which affect the energy budget of the body and therefore its thermal comfort.

The conventional heat balance equation which describes energy flows body/environment is:

 

M + W + R + C + ED + Ere + E Sw + S = 0

 

Where:

M = metabolic rate ( internal energy produced by food oxidation)

W = the physical work

R = the net radiant balance of the body

C = the convective heat flow

ED = the latent heat flow to evaporate water through skin ( perspiration)

ERe= heat flows for respiration ( air heating and humidifying)

Esw = heat flow for sweat evaporation

S = heat flow accumulated in body.

 

The single terms in the equation have positive signs, if they results in an energy gain of the body and negative sign in case they mean an energy loss (in this sense M is always positive; W, ED, and Esw always negative).The individual heat flows of the equation are influenced directly by the following climatic parameters:

 

air temperature C, ERe

air humidity ED, ESw, Ere

air velocity C, Esw

mean radiant temperature R

 

Human body does not have sensors to perceive the single climatic parameters, it can only consider for thermoregulation the temperatures of the skin and of the blood flow passing the hypotalamos. But these temperature are directly influenced by the integrated effect of all climatic parameters which strictly interact and affect .each others. As an example, in winter sunny environment with little wind the mean radiant temperature has the same importance as the air temperature, and its importance can grows in summer : in Mediterranean countries Mrt can easily reach 60-70°C with air temperature of about 30-35 °C. In windy environment air temp. Is far more important than radiant temperature because convective heat exchange dominates" (Höppe, 2000).

 

Outdoor thermal comfort is not only influenced by physiological response to highly variable microclimatic parameters but also by psychological and cultural adaptation which arranges a wide range of environmental stimuli fluctuation to avoid thermal stress and discomfort.

Many researches curried out in the last 15 years focused on role the of psychological adaptation based mainly on the influence of past experience, naturalness and expectation. Psychological variables can account up to more than 50% of the overall comfort evaluation ( Nikolopoulou, et al. 2001).

This discrepancy between thermal sensation expressed by people and thermal sensation evaluated by heat balance model is reported by many authors.

 

Fig.1: discrepancy between thermal sensation expressed by people and thermal sensation evaluated with PMV (Nikolopoulou et al., 2001)

 

Höppe in example (Höppe 2002) report about the role of psychological adaptation process either in cities, when people move for a short time from a green fresh area to a sunny warm street without any discomfort sensation, either in vacation beach resorts where motivated people expose themselves to very adverse thermal conditions (sunshine with Equivalent Temperature of about 40°C) without complaining because of the strong expectations they have their expressed thermal sensation is lightly warm.

 

But not only psychology is responsible of the difference between indoor and outdoor thermal comfort, there are also some thermo-physiological differences due to the different exposure time.

Many thermal indexes based on conventional steady state energy-balance one node model (PT, PMV modified ) seems to be inadeguate to asses thermal comfort in rapid changing environmental conditions, such as walking along a sunny and shaded street.

It is necessary to use dynamic adaptation models (two nodes models).

When moving from thermally neutral conditions to hot or cold environment, (from indoor to outdoor, or from fresh to warm/hot outdoor places), the homeostatic regulation system keeps the inner temperature of a person almost constant (with variation in the order of 0.1 &endash; 0.2 °C) for a certain period regardless of the climatic conditions , while the skin mean temperature changes significantly. In wintertime the variation is slower (hundreds of minutes), while in summertime is much faster (tens of minutes).

 

 

 

Figg. 2, 3: Model (IMEM) calculations of temporal course of body core temperature Tcore and mean skin temperature Tsk after entering a cold environment from thermally neutral conditions; horizontal lines represent steady state level. In fig 13 (Ta=Tmrt=0°C, VP=5 hPa, v= 1m/s) in fig 14 (Ta=30°C, T mrt=60°C, VP=15hPa, v= 0,5 m/s ) (Höppe 2002)

 

The practical design meaning of the research curried out by Höppe is that the above mentioned steady state outdoor thermal comfort models ( based on one node model of human body) cannot be used to evaluate comfort for moving activities mainly in cold contexts, while they can be used for settled activities in warm/hot climatic context.

To overcome these limitations the International Society of Biometeorology is working on the definition of new Thermal Index like the Universal Thermal Climate Index (UTCI) (Höppe 2002) in which both physical/physiological and psychological factors are to be considered.

This biometereological approach will give a relatively near future methodological contributions in many fields :health risk evaluations, regional planning, etc. But until these contributions will not be integrated into commonly used design tools within specific cultural contexts, the diffusion of bioclimatic urban design approach will be very limited.

In the last years many outdoor climatic and comfort indexes have been elaborated or are going to be developed.

 

In a very simple way they can be classified in the following four groups:

a. Empirical thermal indexes correlating only few climatic parameters and usually elaborated for specific climates. In example the Wind-cill Index, Discomfort Index (Sacré)

b. Psycho-sociological-climatic indexes, correlating subjective perception (Actual Sensation Vote, satisfaction indexes, etc) to microclimatic variables and comfort index (Nikolopoulos 2002, SAGAcitè 2002 )

c. Energy balance equation indexes based on two node model of the human body and on the evaluation of all thermal relevant climatic parameters. They generally couple an heat balance equation with a simplified model to evaluate Mean Radiant Temperature.

The most diffused indexes are:

Physiological Equivalent temperature PET ( Höppe 1999), New Effective Temperature ET new ( Gagge et al. 1971), Standard

Standard Effective Temperature SET end OUT_SET integrated with a model for outdoor radiant temperature OUT_MRT( J. Pickup, R. de Dear, 2000)

These model were elaborated to compare different climatic scenarios with standard clothing and activities.

d. Energy balance equation based on one node model of the human body : Perceived Temperature (PT ) model based on Fanger Equation plus an outdoor radiant evaluation model. ( Vinet, Jendrintzky)

Thermal Comfort Model , Grupo de Termotecnia, Univ. de Sevilla (Ciemat)

COMfort FormulA-COMFA, (Brown and Gillespie) a simplified developed mailny for Landscape use.

COMFA modified with simplified radiant evaluation model (work in progress BEST, Politecnico di Milano)

 

The validity of one node model for warm-hot climate

The index PET is going to be diffused in Europe. PET is defined as the air temperature at which in a typical indoor setting ( Without wind and solar radiation) the heat budget of the human body is balanced with the same core and skin temperature as under the complex outdoor conditions to be assessed. PET enables a common person to compare the integral effect of outside complex thermal conditions with its indoor own experience

 

 

fig.4: Heat balance modelling with MEMI (Munich Energy-Balance Model for individuals) for warm and sunny conditions (Ta= air Temperature, Tmrt =mean radiant temperature, RH =relative humidity, v= air velocity) (Höppe, 2000)

 

In the concrete case of warm sunny conditions described in Fig. 4 PET value of 43 °; this means that a person in a room with an air temperature of 43° reaches the same thermal state as at the warm and sunny outdoor conditions. Moving from direct solar radiation to shade will reduce PET to 29 °C.

 

 

 

Fig. 5: Weather charts for Germany, 11 November 1998. Left chart showing conventional weather information: minimum and maximum air temperatures, wind, cloud cover. Right chart showing the distribution of PET-values for midday. (Höppe 2000)

Ta

Tmrt

v

VP

PET

Typical room

21 °C

21 °C

0.1 m/s

12 hPa

21 °C

Winter, sunny

-5 °C

40 °C

0.5 m/s

2 hPa

10 °C

Winter, shade

-5 °C

-5 °C

5.0 m/s

2 hPa

-13 °C

Summer, sunny

30 °C

60 °C

1.0 m/s

21 hPa

43 °C

Summer, shade

30 °C

30 °C

1.0 m/s

21 hPa

29 °C

 

Tab.1: Examples of PET-values for different climate scenarios (Ta = air temperature, Tmrt = mean radiant temperature, v = air velocity, VP = water vapour pressure). (Höppe 2000)

 

Tab.2: Thermal sensation and Physiological stress related to PMV and PET

 

Anyway the main problem facing outdoor comfort index is Mean Radiant Temperature (in example shaded, semi - shaded, sunny conditions), which is difficult to do because it can change very much from a place to another .

Finally it is important to stress that the different thermal indexes are not an absolute evaluation of outside thermal comfort or strain because of the great variability of microclimatic parameters and of the complex interrelation between psychological ( not easy measurable) and physiological adaptation. Thermal indexes are a basis to asses the thermal environment which has then to be adjusted to physical adaptation ( clothing and metabolic heat) which depend on individual and are not site related.

updated 25 oct 2002