Thermal Comfort Indices

Although thermal perception depends highly on the individual objectifying thermal

comfort (or heat stress respectively) is

possible by calculating the human energy

balance (gains and losses of energy) and

calibrate indices in combination with

surveys with locally adapted persons.

The empirical formulas can then predict thermal sensations for the majority of the population (up to 90%) and indicate more sustainable urban morphologies.

All indices are composites achieved by weighting the impact of the meteorological parameters. In open spaces comfort indices allow to calculate human biometeorological thermal comfort in the canyon air volume at near-surface height of usually at 1.1m to 1.50m above ground, which refers to the average height of a standing person’s center of gravity in Europe (Matzarakis et. al. 1999).

The main variables to determine thermal comfort are

• Microclimatic variables (in order of

importance)

- Mean radiant temperature (MRT) - Air temperature (T_air)

- Air velocity (V_air) - Air humidity (RH_air)

• Individual variables (metabolism rate,

clothing insulation and albedo)

• Subjective perceptions (the

preference of thermal sensations, e.g. “Some like it hot”)

Generally thermal comfort in open spaces is mainly influenced by (direct) solar radiation (increase of heat stredd) and of air movement (ventilation, decrease of heat stress) represent high variations especially (in tropical latitudes), while air temperature and humidity vary less and are more homogeneous in spatial terms. In the following five indices to assess thermal comfort in the tropical outdoor environments analyzed are shortly outlined. Three of them (according to necessities of different models) will be

Fig. 6.2: Relative frequency of accidents related to air temperature, Riveiro, (1985) (left) Correlation between air temperature and urban criminality in São Paulo, analyzing mean air temperatures 1961-1991 and 1979-1995, Mendonça (2002) (right) Fig. 6.3: Brainmap shows the complex interactions of microclimatic parameters and the urban environment necessary to be analyzed to assess thermal comfort in a simplified way 100 120 140

Relative Frequency of accidents of work

Criminality

Air temperature

Criminality

Mean monthly air temperaturt

e

Jan Feb MarA br MayJunJulAugSepO ut NovD ec Jan Jan 50 150 250 350 450 550 10 15 20 25 30 35 10 Ambient temperature [°C] 15 20 25

Wind

Air Temperature

Humidity

City

MAN

COMFORT

94

used as indicators to estimate thermal benefits of tree canopies (and urban typologies). A more complete list of calibrated of indices for São Paulo can be found in Monteiro and Alucci (2005).

PMV - Predicted Mean Vote Index

(Fanger, 1970)

According to Bruse the index was originally developed for indoor situations and subsequently adapted for outdoor climate by Jendritzky (1993, unpublished). PMV is still the base for ISO 7730 and ASHRAE standards. However various authors, like Grimme

et al. (2003) and Spangenberg (2004)

have pointed out that especially the cooling effect of air velocity is not calculated in an appropriate way in tropical situations.

PET - Physiologically Equivalent

Temperature (Höppe, 1999)

The MEMI (Munich Energy Balance Model for Individuals) was developed for outdoor conditions and is fully described in the German VDI-Guidelines7_{. The}

approach calculates a temperature analogy, the perceived (or effective) temperatures (given, like e.g. air temperature, in °C).

A common tool to calculate the Physiologically Equivalent Temperature is Rayman (Matzarakis, et. al 2002). PET was calibrated by Monteiro and

Fig 6.4: Meditating man on Paulista Avenue in São Paulo, picture to illustrate environmental and especially thermal discomfort (Advertisement 2006) (left) Scheme of impacts to calculate thermal comfort in urban situations near surface (right)

Alucci (2005) for São Paulo climatic conditions and occupants adaptation and is being calibrated by Katzschner (2007) for various global places. The neutral temperatures given for temperate climate in Kassel (Germany) are 15-22°C and for example 21-28°C for subtropical climate like Athens (Greece).

HL – Heat Load (Blazejczyk, 2002)

Blazejczyk (2002) also applies the MEMI (Man Environment heat exchange model, see Höppe 1998) but with a modified approach to take solar radiation into account. Because the concept of thermal comfort may be misleading Alucci and Monteiro (2007) also prefer the concept of Heat Load, proposed in this concept and applied in Tensil (Alucci, 2005).

dPET – Dynamic Physiologically

Equivalent Temperature (Bruse,

2007)

In a state-of-art approach Bruse (2007) proposed multi-agent system for assessing dynamical urban thermal conditions using the indicator PET (see above). The climBot model is a plug-in model for ENVI-met (applied here) which takes the thermal short term history of the individuals moving in urban structures into account (heating and cooling of the human body, clothing, metabolism rate, etc).

TEP - Temperature of Equivalent

Perceptive (Monteiro, 2008)

The indicator Temperature of Equivalent

Perceptive (TEP) is the only locally calibrated indicator for climatic conditions in São Paulo yet and makes an analogy to air temperature like PET. The index is originally named

Temperatura efetiva percebida. To

calibrate the indicator Monteiro (2008) carried out surveys of more than 2000 individuals in three locations: open sky, overhead shading membrane and overhead shading tree on the campus of the University of São Paulo.

TEP=-3.777+0.4828* T_Air +0.5172* MRT +0.0802* RH_Air -2.322* V_Air (4) where

T_Air = Air Temperature in ºC MRT = Mean Radiant Temperature in ºC RH_Air = Relative Humidity of air in % V_Air = Velocity of air (m/s)

95 A sensibility study of TEP showed that

decreases of 1.94°C MRT, 2.07ºC of air temperature and 12.5% of humidity are necessary to lead to a decrease to an increased comfort of 1ºC TEP, as well as an increase of wind speed of 0.43 m/sec. Since it was found that tree shade can reduce MRT by up to 16°C, while air temperature and humidity is difficult to decrease, and wind speed difficult to increase through the aerodynamic optimization of the urban structure. Thus shade is the most effective strategy to increase thermal comfort in predominantly hot cities.

In document Nature in Megacities: São Paulo/Brazil - A Case Study (Page 93-95)