Research paper 1: Comfort band in hot and humid climates

COMFORT BAND IN HOT AND HUMID CLIMATES

Term 1 Research Paper 2014-15 AA SED MSc/MArch Sustainable Environmental Design Architectural Association School of Architecture Graduate School Oindrila Ghosh January 2015

AA SED Term 1

MSc & MArch Sustainable Environmental Design 2014-15

Research paper

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AA SED SCHOOL

ARCHITECTURAL ASSOCIATION GRADUATE

PROGRAMME:

MSc / MArch SUSTAINABLE ENVIRONMENTAL DESIGN 2014-15

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RESEARCH PAPER 1

TITLE

COMFORT BAND IN HOT AND HUMID CLIMATES

NUMBER OF WORDS : 3220

STUDENT NAME: OINDRILA GHOSH

DECLARATION: ―I certify that the contents of this document are entirely my own work and that any quotation or paraphrase from the published or unpublished work of others is duly acknowledged.‖

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Date: 16 JANUARY 2015

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TABLE OF CONTENTS

1.0 Introduction ……………………………………………………………………….…. 3 2.0 Location,climate and comfort ……………………..……………………………... 4 3.0 Defining thermal comfort for hot and humid regions ………………………... 5 4.0 A review of research papers ………..…………………….………………………. 6 4.1 Tropical Summer Index (Sharma and Ali, 1986) 4.2 Thermal comfort in tropical climates: An investigation of comfort criteria for Bangladeshi subjects (F.H.Mallick, 1996).

4.3 Adaptive thermal comfort standards in hot and humid tropics (Fergus Nicol, 2004) 4.4 Development of an adaptive thermal comfort equation for naturally ventilated buildings in hot–humid climates using ASHRAE RP-884 database. (Doris Hooi Chyee Toe and Tetsu Kubota, 2013)

5.0 A comparative analysis ………………………………………………………….… 11 6.0 Conclusions ……………………………………………………………………..…… 12 7.0 References …………………………………………………………………….……… 13

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COMFORT BAND IN HOT AND HUMID CLIMATE Oindrila Ghosh AA SED Sustainable Environmental Design, Architectural Association Graduate School, London, UK MSc/March Sustainable Environmental Design 16 january,2015

Abstract: The objective of this paper was to understand thermal comfort and whether international organisations like ASHRAE 55-2010 effectively predict a comfort band for hot and humid climates.

Furthermore, four research papers are reviewed and a compared to understand their relevancy in predicting a comfort range for the hot and humid climate zone.

Keywords: Thermal comfort; Adaptive model; Hot–humid climate; Naturally ventilated building; ASHRAE

Introduction: Thermal comfort can be defined as the absence of irritation and discomfort due to heat or cold.

Thereby, the range when a person experiences ‗thermal comfort‘ is called as comfort zone (Givoni, 1998). This state of thermal comfort depends upon three factors:a) Physiological (quantitative) b) Psychological (qualitative) c) Environmental (quantitative) Various permutations of these factors help us to determine the comfort range for people spread across the wide variety of climatic zones in the world.

A lot of research has been undertaken to assign a certain band of values for comfort. Organizations like ASHRAE, ISO 7730, EN 15251 have tried to address this subject matter by coming up with certain standards of comfort range for indoors. These international standards work well for majority of the climatic zones but not for all. For example, ASHRAE standards are followed in India but the comfort range set by them are not at par with the adaptive behavior of the locals from eastern and southern region of the country. Therefore, when local architects and engineers use these standards, it can cause unnecessary consumption of energy resources.

The reason behind such a mismatch between standards and actual thermal acceptability is because people expect different thermal experiences in summer and winter and therefore, display adaptive behavior. A deep understanding about this relation between indoor comfort and outdoor temperatures in different climates can be used by architects to suggest appropriate measures for low energy design (Roaf,2012).

Therefore the question is what truly is the appropriate adaptive comfort range for indoor environments in tropical regions (like India)? Where the current standards are generally the weakest.

Location, climate and comfort: India is a vast country consisting of six different climatic zones according to Koppen climate classification. And this paper would concentrate on hot and humid (Type Aw) climate zone. This specific zone is spread across eastern India as well as the western coasts of India .And these are the areas were current standards don‘t work well. The city of Kolkata has one of the highest heat indexes (table 1) among all the cities lying in this zone and therefore was selected for further investigations.

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Table 1: shows the average heat index of indian cities located in the hot and humid climatic zone. (source: vagaries, wunderground,2015) Cities Heat Index o Chennai 43 C o Kolkata 41 C o Mumbai 36 C o Bhubaneshwar 46 C o Cochin 41 C Hot and humid zones are the most difficult zones to design for (Szkolay, 2004) because of two factors:1) High temperatures with low diurnal temperature differences (5K).

2) High relative humidity High relative humidity can inhibit effective evaporative cooling by loss of moisture through skin which leads to uncomfortable ‗sticky‘ feeling characteristic to hot and humid climates. Thus, it is understood that this sort of climate is most difficult to tackle using passive design. People in Kolkata (or locals of this region) wear light clothing (0.3 - 0.8 clo) and use simple mechanical devices like fans to provide comfort. People in these regions have expectations of comfort and show adaptive behavior corresponding to the high temperature and humidity levels (Mallick, 1996).

Defining thermal comfort for hot and humid regions: International standard – ASHRAE is presently followed in India. It uses two methods to define thermal comfort.

1) PPD – PMV method: This method takes into consideration six factors:a) operative temperature e) occupant clothing b) relative humidity f) metabolic rate c) mean radiant temperature d) wind velocity All these factors help determine the thermal comfort experienced by individuals under varying thermal conditions. After carrying out some analytical work with the ASHRAE model (figure 1), it was found that ASHRAE defines indoor comfort o o range of 22 C – 26 C. This range does not hold good for people living in this region (Nicol, 2004)

Figure1: The psychometric chart illustrates how ASHRAE 55 (PMV method) suggests a comfort band o of (22-26) C when the average summer conditions of Kolkata city are fed into the program.

(Source: after CBE thermal comfort tool)

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2) The adaptive model: ASHRAE had identified this above said problem and funded a project called the ASHRAE RP-884. The outcome is the adaptive model incorporated in the ASHRAE 55. The project was carried out in 160 different office buildings (naturally ventilated and HVAC both) spread across four different continents. It was observed that occupants of the naturally ventilated buildings were strongly affected by changing thermal expectations (Brager, de Dear, 2001).

Figure 2: ASHRAE 55(adaptive model applicable to naturally ventilated buildings) with average summer temperatures of Kolkata city as inputs for the web tool. (source: after CBE thermal comfort tool)

To further understand the thermal acceptability of occupants they had used the classic PMVo o PPD model. They had come up with a comfort band of 5 C with 90% acceptability and 7 C with 80% acceptability unit centered around equation 1. All this information is clearly illustrated in Figure 2. o

Tcomf = 0.31xTa,out +17.8 ( C)‌.. (eqn 1) o

But this model also had its own limitations. The graph sharply ends at 10-33 C and operative temperatures beyond this band cannot be extrapolated. Therefore, comfort temperatures for o cities in hot and humid regions which frequently experience temperatures above 33 C cannot be satisfactorily defined.

Review of Research papers Several studies have been carried out to determine what constitutes thermal comfort in these climates. A few of them will be discussed chronographically for a deeper understanding.

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Tropical Summer Index (Sharma and Ali, 1986): The main objective of this paper was to determine a simple index of thermal comfort as well as to figure out a range and optimum conditions prevailing in India.

The study was carried out on indian subjects wearing light clothes not exceeding 0.5 clo at the CBRI, Roorkee. The range of environmental variables and thermal sensations covered by the study was:o

Dry bulb temperature (Ta) = (20 – 41) C o Wet bulb temperature (Tw) = (13 – 30) C o Globe Temperature (Tg) = (19 – 43) C Air velocity (V) = (0 – 2.5) m/s Thermal sensation (S) = 2 – 7 (Bedford scale) After analyzing all the data, Sharma and Ali had come up with an equation:S = 0.067 Tw + 0.162 Tg – 0.449 V

1/2

– 1.917

……… (2)

―The globe temperature term in the equation takes account of the effect of air temperature and the radiant heat and, to a small extent, of air velocity. The humidity and air movement are separately 1/2 taken care of in the comfort equation by Tw and V terms. The equation thus takes into account all four environmental variables in proportion to their influence on the thermal sensation. This equation is therefore useful for constructing an index from which it should be possible to express the warmth of the environment in terms of one environmental variable, maintaining the others at predetermined fixed levels. The present index is termed as the 'Tropical Summer Index' (TSI). It is defined as the air / globe temperature of still air at 50% Relative humidity which produces the same overall thermal sensation as the environment under investigation.‖ (Sharma and Ali, The tropical summer index,1986) Furthermore, for the equation to be more practical, they had reduced equation (2) to 0.2174θ = S + 2.1

…………… (3)

On substituting S with the values of 3, 4 and 5 for thermal sensation of slightly cool, comfortable and o o o slightly warm, corresponding values of θ are obtained as 23.5 C, 25 C and 32.5 C. Further detailed analysis results in table 2.

Table 2: Ranges and optimum values of TSl for the three acceptable thermal sensations. (source: TSI, 1986) o

o

Thermal sensation

Range ( C)

Optimum value ( C)

Slightly cool

19.0 – 25.0

22.0

Comfortable

25.0 – 30.0

27.0

Slightly warm

30.0 – 34.0

32.0

For practical purposes, it was plotted on the psychometric chart as shown in figure 3. The lines o o between 25 C - 30 C is actually the comfort region but the hatched area between 40 – 70% RH is implied as the ‗comfortable‘ region. This is because the study intentionally chose to avoid extreme RH values.

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Figure 3: Tropical summer index and comfort zone. (source: TSI, 1986)

Thermal comfort in tropical climates: An investigation of comfort criteria for Bangladeshi subjects (F.H.Mallick, 1996).

The research was carried out in Dhaka, Bangladesh which has a hot and humid climate. The study considered recordings of Air temperature, humidity, Globe temperature, air movements and assessments were based on Bedford scale. The clo values of the participants were not above .5 clo and had a metabolic rate ranging between 0.8 - 1.2 met.

After collecting 400 observations, it was found that under no air movement, people experienced o o comfortable temperature ranges between 24 C to 33 C (figure 4). Inclusion of fan speed of .3 m/s o o increased the lower and upper limit of this range by 2.4 C to 2.2 C respectively (Table3).

Table 3: Comfort temperatures at different air velocities. (source: F.H. Mallick, 1996) Fan speed setting Air movement Comfort range Mean comfort temp. o

o

28.9 C

o

o

o

29.5 C

None

0 m/s

24 C - 33 C

Slow

0.15 m/s

24 C - 33 C

Medium

0.3 m/s

26.4 C – 35.2 C

o

30.9 C

Fast

0.45 m/s

27 C – 35.8 C

31.6 C

o

o

o

o o o

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Figure 4: Comfort conditions with no air movement. (source: F.H. Mallick, 1996)

It was also found that the participants had high tolerance for humidity due to long term conditioning and there are instances when participants reported feeling comfortable in humidities above 95%. No decrease in the comfort temperatures were found at higher humidities.

Finally, after all the observations, the research is supported by a equation involving comfort values and globe temperature as follows: C.V = Tg x 0.18 – 5.11

……….. (4)

where, C.V = Comfort vote.

Tg= Globe temperature. o

o

This equation gives us a comfort range of 22.3 C to 34 C within the three comfort sensations of slightly cool, comfortable and slightly warm. The study also suggested that there was no apparent changes in comfort sensations because of humidity.

Adaptive thermal comfort standards in hot and humid tropics (Fergus Nicol, 2004) The paper reflects on why International standards like ISO 7730 are failing to predict accurate comfort temperatures for hot and humid climatic regions. The study starts with the meta-analysis of three sets of data in order to draw a relation between comfort temperatures and humidity. The three datasets that was used were: ASHRAE 1998 database Humphrey‘s 1975 database Database from Pakistan (Nicol) After that, Nicol puts forward Humphrey‘s comfort equation for free running buildings as an international generalization of all field survey that had been carried out so far.

Tc = 0.534 To + 12.9

…………………… (5)

where, Tc = comfort temperature To = outdoor temperature Even though the paper does not produce its own definitive equation for comfort temperatures which takes into account humidity and air movements but the study mentions that the results from Humphrey‘s equation can be improved by conducting local surveys to accurately reflect the local climate and culture. Nonetheless, it upholds certain important facts that the resulting comfort range o will be 2-3 C on either side of the optimum comfort temperature derived from the equation. The study o suggested that introducing air velocity can add another 2 C into the comfort temperatures and that the

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o

occupants may require comfort temperatures approximately 1 C lower than that specified by the overall data when the outdoor relative humidity is greater than 75%.

Development of an adaptive thermal comfort equation for naturally ventilated buildings in hot– humid climates using ASHRAE RP-884 database. (Doris Hooi Chyee Toe and Tetsu Kubota, 2013) The study similarly like (Nicol, 2004) tries to employ a statistical meta-analysis of the extensive database of ASHRAE RP-884. They prepared the data, refined them and classified them climate wise. Therefore, leaving them with 1673 observations for hot-humid, 2776 observations for hot-dry and 3213 for moderate climatic regions.

Figure 4: Scatter diagram of indoor operative temperatures at thermal neutrality and daily mean outdoor air temperatures. Discontinuous lines denote linear regression models used in this study and represent adaptive equations for predicting neutral temperatures. (source: D.H.C. Toe, T. Kubota, 2013) The above graph (Figure 5), elaborates how occupants felt thermally comfortable during specifically observed indoor temperatures against outdoor temperatures. It was observed from these data that o o hot-humid climate had outdoor temperatures ranging between 20 C to 35 C. And using the regression analysis method, they had come up with an equation to define the neutral temperature in hot and humid climate.

Tneutop = 0.57 Toutdm + 13.8 ………………… (6) where, Tneutop = Neutral operative temperature Toutdm = outdoor mean temperature

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The study suggests that the adaptive equation for hot and humid regions differ markedly from current major adaptive comfort models like ASHRAE 2010. Its regression line is twice as steep as that of the ASHRAE‘s adaptive equation. Similarly like Nicol‘s study, that we had previously studied, this paper also does not include air velocity and humidity into its equation though it does talk about them individually and their effect on the comfort range. The study shows that we have steeper regression lines when wind speed increases. Figure 5, illustrates the effect of humidity on indoor operative temperature against outdoor temperature.

Figure 5: Scatter diagram of indoor operative temperatures at thermal neutrality and daily mean outdoor air temperatures for different percentages of relative humidity for hot and humid climates.

Here any value less than 60% represents low relative humidity and above 60% represents high relative humidity. (source: D.H.C. Toe, T. Kubota, 2013) Finally, Table 4 shows the comfort equation as well as its related criteria for naturally ventilated buildings in hot and humid climates.

Table 4: Proposed adaptive thermal comfort equation and related criteria for naturally ventilated buildings in hot–humid climate.

No. Aspect Criterion Note 1

Climate type

2

Neutral operative temperature, o Tneutop ( C) Daily mean outdoor air temperature, o Toutdm ( C)

3

All A climate types; and Summer season of Cfa climate type.

Tneutop = 0.57Toutdm + 13.8

Climate type refers to the Köppen–Geiger climate classification system.

Range from 19.4 to 30.5.

Recommended criterion no. (ii)

Toutdm is daily mean outdoor air o temperature ( C), i.e., the 24-h arithmetic mean for the day in question. applicable

range

for

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5

6

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Lower comfort operative temperature o limit, Tlower ( C) Upper comfort operative temperature o limit, Tupper ( C) Indoor air speed, v (m/s)

Indoor humidity, RH (%)

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No required limit.

-

Tupper = Tneutop − 0.7 for 80% comfortable thermal sensation votes.

-

<0.65 at and below neutral operative temperature; ≥0.65 above neutral operative temperature.

No required limit.

Recommended to provide non-still air and occupants' control to adjust the indoor air speeds according to their preferences.

-

Comparative analysis After analysis of the above mentioned papers, it was possible to come up with a comparison (table 5) for a deeper understanding of their approach to the problem of determining a comfort range for hothumid climate.

Table 5: A comparison amongst the above discussed papers.

Tropical Summer Index Thermal comfort in Adaptive thermal (Sharma and Ali, 1986) tropical climates: An comfort standards in hot investigation of comfort and humid tropics criteria for Bangladeshi (Fergus Nicol, 2004) subjects (F.H.Mallick, 1996).

0.2174θ = S + 2.1 1

Provides with ―index ‖ temperature based on thermal sensations and vice versa 50% RH considered for developing equation RH range considered for comfort (30-70)% Comfort range (25o 30) C [thermal sensation considered as 4 on Bedford scale]

C.V = Tg x 0.18 – 5.11

Tc = 0.534 To + 12.9

Provides with air/globe temperature based on thermal sensations and vice versa Rh range considered for comfort (50-90)%

Provides with a target comfort temperature with respect to outdoor temperature.

50% RH considered for developing equation. no specific range mentioned for comfort.

Comfort range is (2o 3) C on either side of optimum temperature. o In case of high RH, 1 C lower to remain comfortable.

Comfort o 32) C

range

(24-

Development of an adaptive thermal comfort equation for naturally ventilated buildings in hot–humid climates. (Doris Hooi Chyee Toe and Tetsu Kubota, 2013) Tneutop = 0.57 Toutdm + 13.8 Provides with a target comfort temperature with respect to outdoor temperature.

Does not have a specific limit to humidity.

Comfort o 31.2) C [when sensation upto 80%]

range

(24.9-

thermal votes are

1

index temperature can be defined as the temperature at a certain value of relative humidity and air velocity which will feel the same as the environment in question

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Limited usage as the study avoids the usage of the equation for hothumid climates with high heat index.

Has potential usage. It is simple and easy but cannot be completely depended upon as it draws conclusions from a small field survey.

Can be potentially used as the equation does not provide us with a set band of comfort temperature and is supported by simple set rules that help identify the effect of air velocity and humidity on the resultant temperature.

Very flexible.

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Can be potentially used as the equation does not provide us with a set band of comfort temperature and is supported by simple set rules that help identify the effect of air velocity and humidity on the resultant temperature

Conclusion All these studies had been carried out in naturally ventilated buildings and despite of their empirical o limitations, the comfort range predicted for the concerned climate was more or less similar (24 C o 33 C). It can be inferred from all these studies that hot and humid climatic regions will require a separate adaptive thermal comfort model of their own. Major international standards can be modified to incorporate new criteria for better applicability. It would also be helpful if further studies are carried out to determine suitable percentages of occupants in comfort, to quantify the effects of an increased air speed allowance, and to verify the applicability of these criterias to the driest months in the climate (D.H.C. Toe, T. Kubota, 2013). It can be said that quiet a formidable percentage of developing nations have hot and humid climatic regions and the locals of these regions are generally conditioned to such extreme temperatures. But it is found that there is an increasing use of HVAC and energy resources in these countries. If such adaptive models for ventilated buildings can be regularized, it can help vastly by cutting down energy consumption and would also encourage low energy design.

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References: Auliciems, A and S. Szokolay, 1997. Thermal comfort. PLEA Note 3. PLEA International / University of Queensland.

Givoni, B. (1998) Climate Considerations in Building and Urban Design. John Wiley & Sons, Inc., New York B. Ogwezi, G. Jeronimidis, G. Cook, J.Sakula, S. Gupta, Adaptive buildings façade for thermal comfort in hot and humid climate, online article, University of Reading, UK de Dear, R.J., Brager, G.S., 2002.Thermal comfort in naturally ventilated buildings :revisions to ASHRAE Standard55.Energy and Buildings 34 (6),549–561. (http://www.sciencedirect.com/) Givoni, Baruch 1976, Man, climate and architecture, 2nd ed, Applied Science Publishers, London Nicol, F., Humphreys, M., Roaf, S., 2012. Adaptive Thermal Comfort: Principles and Practice.

Routledge, London.

Mallick, F.H., 1996. Thermal comfort and building design in the tropical climates. Energy and Buildings 23(3),161–16, Elsevier Science, Lausanne.

M.R. Sharma, Sharafat Ali, Tropical summer index—a study of thermal comfort of Indian subjects, Building and Environment, Volume 21, Issue 1, 1986, Pages 11-24, Elsevier Science, Lausanne.

Nicol, F., 2004. Adaptive thermal comfort standards in the hot- humid tropics. Energy and Buildings 36(7) 628–637, Elsevier Science, Lausanne.

Nicol, J.F., Humphreys,M.A., 2002. Adaptive thermal comfort and sustainable thermal standards for buildings. Energy and Buildings 34(6), 563–572, Elsevier Science, Lausanne.

Toe,D.H.C., Kubota, T., 2013, Development of an adaptive thermal comfort equation for naturally ventilated buildings in hot–humid climates using ASHRAE RP-884 database, Frontiers of Architectural Research, Volume 2, Issue 3, September 2013, Pages 278-291. (http://www.sciencedirect.com/) Szokolay, SV 2008, Introduction to Architectural Science : The Basis of Sustainable Design, Elsevier/Architectural Press, Amsterdam.

Internet Sources:

http://en.wikipedia.org/wiki/Koppen_climate_classification http://www.wunderground.com/ accessed on 15 jan, 2015. http://www.vagaries.in/ accessed on 15 jan, 2015. http://smap.cbe.berkeley.edu/comforttool/ (for analytical work)

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