Thermal Comfort in Tropical Humid Climate to an Amendment of International Standards

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1 European Journal of Scientific Research ISSN X Vol.73 No.3 (2012), pp EuroJournals Publishing, Inc Thermal Comfort in Tropical Humid Climate to an Amendment of International Standards Kemajou Alexis Energy Laboratory of Cold and Air-Conditioning Advanced Teacher s Training School for Technical Education University of Douala, B.P Douala, Cameroun kemajoualexis@yahoo.fr Tel: (237) ; Fax: (237) Tseuyep Ange Energy Laboratory of Cold and Air-Conditioning Advanced Teacher s Training School for Technical Education University of Douala, Cameroun tseuyepa@yahoo.fr Tel: (237) ; Fax: (237) Egbewatt Ne Renewable Energy Laboratory, Department of Chemistry Faculty of Science, University of Douala, Cameroun negbewatt@yahoo.com Tel: (237) ; Fax: (237) Abstract This article deals with the conditions of thermal comfort in tropical humid climate. Several studies have indicated that in such climate, an overestimation of air-conditioned buildings is at the level of discomfort compared to that provided by international standards. We present the results of a survey carried out in air-conditioned buildings in the cities of Douala and Yaoundé in Cameroon in other to determine the conditions of optimal comfort and the acceptable thermal comfort zone. The experimental results found, are in agreement with other studies in air-conditioned atmospheres in tropical climate, allowing for assurance of thermal comfort with modest energy consumption without any resort to the use of active cooling. We recommend a reordering of international ergonomics standards ASHRAE and ISO 7730 for tropical climates, for them to claim universally validity. Keywords: Thermal comfort, humid tropical climate, international standards, thermoneutrality. 1. Introduction Thermal comfort is an important parameter in air-conditioned buildings because of its impact on the quality of indoor environments, health and productivity of the occupant. This parameter is regulated by

2 Thermal Comfort in Tropical Humid Climate to an Amendment of International Standards 339 international standards that ensure compliance with the requirements of indoor environments of thermal comfort. Audits carried out in air-conditioned buildings in tropical Africa show that the share of electricity consumption due to air conditioning is in the range of 40-80% [1] of the total power consumption of the building, thus putting this position at the core of energy saving actions. Several studies carried out in areas under humid tropical climate, have shown a significant difference between the optimal conditions of thermal comfort based on experimental measurements and predictions from international standards [2]. This is explained by the fact that these predictions were calculated according to an analytic approach without taking into consideration the adaptive phenomenon, that plays an important role in these regions. It is more appropriate to determine the conditions of comfort, under such climate, by taking into consideration the adaptive actions of the occupiers; which vary with the building characteristics and surrounding weather conditions [3]. This article has two objectives: Determine the optimal conditions of thermal comfort in the cities of Douala and Yaoundé (both in Cameroon) from an experimental investigation into the conditioned spaces Compare these experimental results with the predictions of international standards based on ASHRAE and ISO General Framework of the Investigation 2.1. Selection of Cities and Characteristics of Buildings The investigation was carried out in the two main cities of Cameroon: Yaoundé and Douala, given their high rate of urbanization, the dominant economic activity and the importance of air-conditioned buildings. The classical buildings considered fall into two groups: (a) old buildings with thick walls (25-30 cm), asbestos cement roofing with false ceiling and large windows, and (b) buildings relatively modern with walls made up of concrete block with cement plaster (15-20 cm), the roof slab in concrete or aluminums with false ceiling using plywood. The buildings visited in each city were generally one to many levels (storeys) and these met the standards of thermal building construction [4][5][6]. Taking this factor into account had the disadvantage of reducing the sample size of our investigation Collection of Data Data was collected in two phases: from January 25 th to February 11 th, 2010 in Douala, 15 th February to 3 rd March 2010 in Yaoundé. Data was collected in each office and reported in a specially design form: General information about the survey: age, weight, height and sex of occupant of building; date and time of the investigation and the location or identification of the inspected premises. Scales of judgments of the ambient air. It is the ASHRAE thermal sensation scale (J1) in seven points, the McIntyre (J2) scale in three points (Table 1). A section containing a list of clothing items that could be worn by those surveyed to calculate their clothing insulation. Table 1: Judgment scale of ASHRAE (J1) and McIntyre (J2) J1: At this particular moment, how do you feel the indoor air of the surrounding? J2: At this particular moment, I will like the indoor air to be : -3 : cold -1: cooler -2 : cool 0 : no change

3 340 Tseuyep Ange, Kemajou Alexis, Egbewatt Ne Table 1: Judgment scale of ASHRAE (J1) and McIntyre (J2) - continued -1 : slightly cool +1: hotter 0 : neutral +1: slightly warm +2: warm +3: hot 2.3. Measurement of Physical Parameters Environmental parameters measured were : air temperature, average radiant temperature, relative humidity and air velocity. On each occasion, measurements were done in the occupancy area of the premises in accordance with the AFNOR NFX [7]: The air temperature with an ambient electronic thermometer ±0.1 C The relative humidity of air using an electronic hygrometer ±5%, calibrated with a rotary psychrometer comprising two bulb thermometers (wet bulb and dry bulb) The speed of the air with a wind vane, at ± 0.05 ms -1 The average radiant temperature recorded by a mercury thermometer at ±0.1 C placed in a black cylinder. The characteristics of each locality visited were recorded: the nature of the material making up the walls and the roof, the walls color and the opening positions and the shade factor. The metabolic energy was not measure directly. However, the occupants were submitted to office activity; their metabolic energy is assumed to be 70W.m -2 ; the thermal isolation of the clothing worn, estimated according to the proposal of the standard ISO 9920 AP [8] for the two cities are on average of 0.53 clo for women, 0.7 clo average for men. This corresponds to an outfit composed of clothing elements such as follows: For men: light shirt with long sleeves, tie, light trouser, singlet, pant or short, socks and shoes. For women: blouse or bodice with short sleeves, light jacket, skirt, slip, petticoat, bra and shoes (a light dress that can replace the bodice and light jacket). The table below presents the median values of the thermal parameters in the air-conditioned buildings selected for the investigation Figure 1: Division of thermal parameters of conditioned spaces

4 Thermal Comfort in Tropical Humid Climate to an Amendment of International Standards 341 In our study, the calculations were made from the data collected in the field, the operative temperature defined as the arithmetical average of air temperature and radiation in smooth air (air velocity less than 0.4 ms -1 ) Progression of the Investigation The investigation generally took place as follow: We explain the purpose of the study and how the forms are to be filled, and there after we installed the measuring devices in an office. We shared the forms and while the occupants were filling out the forms, we proceed to the measurement of the relevant physical parameters (the occupants were told to be in place for at least half hour), The investigation was rounded off by the collection of the forms Each occupant was observed only once Profile of the Sample A sample of 293 people participated in the survey, 187 men and 106 women aged between 20 to 58 years (average age: 36.5 years), shared as follows: Yaounde : 92 men and 44 women Douala : 95 men and 62 women 2.6. Methodology Responses: The answers were coded on the algebraic forms and processed as quantitative data. We also calculated the average judgments per group of question occupant(s) and applied the linear regression analysis. A group is made up of question occupant(s) (01 to 10 people) who have expressed their judgments having experienced the same ambient conditions. The individual decision is an integer, the average judgment of a group may not be an integer (the sum of individual judgments divided by the number of subjects). We used the null hypothesis test of the regression coefficients [9][10] to verify that the linear correlation established is satisfactory. The regression line of thermal comfort established for each city, based on judgments issued by the ASHRAE scale, was used to determine the thermoneutral temperatures and acceptable thermal comfort zones. 3. Results of the Investigation Judgments According by ASHRAE Thermal Sensation Scale (J1) and Mcintyre (J2) We collected the occupants judgment on the seven points of ASHRAE thermal sensation scale (J1) and three points of McIntyre scale (J2) (Table 1) according to ambient conditions in air-conditioned offices. On the ASHRAE scale, the repartition of judgments is essentially identical to each city and throughout the two cities (Figure 2) or, in the latter case: 28.32% of judgments are about zero (neutral) 56.64% between "neither hot nor cold" and "cold" 15% between "neither hot nor cold" and "hot" On the McIntyre scale (Figure 3), across the two cities: 72% direct their opinion on "no change" 16.72% on "cooler" 11.6% for "warmer"

5 342 Tseuyep Ange, Kemajou Alexis, Egbewatt Ne These judgments are sensibly identical in each city. It is found that the general trend of perceptual judgments is a felling of cold in air-conditioned environments The Average Responses The average perceptual judgment on the ASHRAE scale by region is as follows: J1 = in Yaounde that is more "slightly cool" than "neutral" J1 = in Douala, ranging from "neutral" and "slightly cool" On the Mc Intyre scale, average judgment: J2 = 0.06 in Yaounde that is "no change" J2 = to Douala that is "no change" Figure 2: ASHRAE judgments frequency Figure 3: McIntyre judgments frequency 3.3. Regression Analyse Thermal comfort is a function of four ambient parameters (operative temperature: T o, average radiant temperature T r, relative air humidity: H r, air velocity: V a ) and two parameters related to people (clothing isolation and activity). T o do so, we carried out a simple and multiple linear regressions of perceptual judgment and average affective observed in T o on the one hand and on T o, H r and clo (clothing isolation of the investigated) on the other hand. The air velocity is considered to be low (V a

6 Thermal Comfort in Tropical Humid Climate to an Amendment of International Standards 343 <0.2 m.s -1 ) and the investigated activity is well known (office work). Each average judgement is affected by the weight of individual judgments Correlation Coefficient R and Linear Regression Equation J1 In T o Figures 4 and 5 show the regression equations found from individual judgments in each city. Yaoundé R² = J1 = To (1) Douala R² = J1 = To (2) Figure 4: Individual Perceptual judgment (ASHRAE) and linear regression line in the city of Yaoundé Figure 5: Individual Perceptual judgment (ASHRAE) and linear regression line in the city of Douala Correlation Coefficient R and Linear Regression Equation J2 In T o Figures 6 and 7 show the regression equations found from individual judgments in each city. Yaoundé R² = J2 = To (3) Douala R² = J2 = To (4)

7 344 Tseuyep Ange, Kemajou Alexis, Egbewatt Ne Figure 6: Individual Perceptual judgment (McIntyre) and linear regression line in the city of Yaoundé Figure 7: Individual Perceptual judgment (McIntyre) and linear regression line in the city of Douala 3.4. Thermoneutrality Temperature The temperature that a given group would consider "neutral" can also be determined from regressions of average perceptual judgments (OMV) according to the ASHRAE scale T o. Analytically, it is obtained by applying the ratio of the constant regression by the regression coefficient. The Table 2 below shows the conditions of optimal comfort for each region. We find a small difference between the thermoneutral temperatures of the two cities. Table 2: Optimal Condition of comfort in air-conditioned space City Yaoundé Douala Thermoneutral temperature ( C) C C relative Humidity (%) 59 % 51.4 %

8 Thermal Comfort in Tropical Humid Climate to an Amendment of International Standards Thermal Comfort Zone The convention notes that the judgments, within the central margin of seven points on the ASHRAE thermal sensation scale (i.e. between -1 to 1), consolidates the satisfaction of the thermal environment, and thus delimit the thermal comfort zone. ASHRAE standard [11] uses this criterion to determine the thermal comfort zone as the one that satisfies 80% of the occupants. In this study, we used the linear regression equation of the average perception of the operative temperature on the measured temperature of air to determine the thermal comfort zone (ASHRAE scale between -1 and 1). By respecting this limit, thermal zones of acceptability for different cities are reported in the Table 3 below: Table 3: Thermal confort Zone Cities Yaoundé Douala C < To < C 24 <.To < C Thermal Hr = 60 % at C Hr = 60 % at C Acceptability Hr = 70 % at C Hr = 69 % at 24 C Figure 8 represents the percentage of those unsatisfied for a given range of temperature in each city according to ASHRAE scale. The temperatures having low complain are within 24 and 27 0 C. The clothing insulation of those investigated are sensibly identical; the number of people investigated of most range of temperature is relatively low; because we had limited our investigation only to the buildings meeting with the regulations in force (windows regulatory, tight openings ) [4][5] Figure 8: Percentage of those unsatisfied by temperature range 3.6. Correlation Between the Scale of Thermal Comfort J1 and Scale J2 Table 4: Correlation Coefficient (R) between J1 scale and J2 scale City Yaoundé Douala J2 J2 J

9 346 Tseuyep Ange, Kemajou Alexis, Egbewatt Ne It can be seen from the data of Table 4 that J1 and J2 are strongly correlated. The scale J2 can thus help us as an affective criterion of judgment in air-conditioned spaces. If we use this scale to determine for example the optimal conditions of comfort condition in air-conditioned space (linear regression equation, we will find as thermoneutral temperature (T n ) in Yaounde: C and in Douala: C. 4. Comparison of Results Observed in Cameroon to the Predictions of International Standards 4.1. Prediction of the Standard ISO 7730 According to the standard ISO 7730[12], the ambient temperature of 25 C in smooth air is the ideal average temperature of thermal comfort for people dressed lightly (for indoors: summer clothes, thermal insulation of clothing worn: 0.093Wm -2 K -1 = 0.6 clo) in sitting position, involved in mental work of monitoring screens (metabolic heat produced: 70 Wm -2 = 1.2 Met). The predictive equation of ISO 7730 (ISO 1994) is: Y= 0.272T o 6.79 to75 % of relative humidity (5) We have determined the equations of multiple regressions between individual perceptual judgments and T o, H r and clo (cloth dressed in) for each city. By standardizing these perceptual judgments in a relative humidity of 75% and a cloth dressed of 0.6 clo we can find the following ideal comfort temperatures: Yaounde: C, Douala: 26.8 C On the whole, the average difference is 0.67 C, higher than that predicted by using the standard ISO relations. This difference can be explained by the experimental conditions. In fact, the ISO standard is based on research conducted in a controlled air-conditioned space ensuring an accurate control of the characteristic parameters of heat transfer, while, our study has be done in a real situation which implies some uncertainties on physical parameters measured and inaccuracy on the characteristics of heat exchange between the occupant and his environment. Further the sample size is reduced in the cities of Douala and Yaounde due to the limited number of air-conditioned buildings in accordance with the regulations in force as concerns building materials and furthermore the respect of ergonomic standards of thermal comfort [4][6]. If we had used for example the J2 scale, the results would be as follows: Yaoundé (24.28 C), Douala (24.36 C) and a combined value for the two cities (24.32 C). This allows us to deduce that our results are close to the predictions of the ISO standard and confirm the correlation established by this standard Prediction of ASHRAE This standard [11] recommends for summer, ambient temperatures between 22.6 and 26 C and, it predicts that the percentage of unsatisfied people will not exceed 20%. The relative humidity must not exceed 70% at 22.6 C or 60% at 26 C for indoor temperature. By respecting this limit, it seems that temperature range of C and C for Yaoundé and 24 C to C for Douala is accepted by Cameroonians. The prediction of this standard is not therefore verified. Our experimental results, can allow us to propose a large range of temperatures to be imposed in air conditioned space according to the need of energy saving Prediction of the AFNOR NF X The application of this standard [13] assumes ambient temperatures between 23 C and 26 C. Within this range, it predicts that the percentage of unsatisfied occupants will not exceed 20%. Furthermore, the maximum relative humidity should not exceed 60% at 26 C, the air velocity being less than 0.1ms - 1. The same remark concerning the ASHRAE [11] is still valid for the AFNOR standard [13].

10 Thermal Comfort in Tropical Humid Climate to an Amendment of International Standards Comparison with other Study Compared to other studies in tropical air-conditioned atmosphere, particularly those of KONE et al [14] in Ivory Coast, T n = 24.5 C and Bursh and Thailand T n = 25 C [14][15]. Arguably, the thermoneutral temperature of this study is classified on a good range. In the framework of a possible extension of this study, these results may contribute on one hand to the elaboration of a standard or regulation for indoor environments of air-conditioned locals in tropical countries and in setting up of energy performance code for conditioned building. 5. Conclusion Thermoneutral temperatures found are close to those predicted by international standards. As a first approximation, we can say that Cameroonians seem easier to accept hot environments compared to those living in temperate or cold climates. Thus they find temperatures between and C acceptable. In this framework, we can say that Cameroonians do not experience thermal discomfort in the temperature range between 27 to 28 C in air-conditioned room judged to be hot by international standard. This result can be explained by the adaptive phenomenon of the population survey living in hot area and who tolerate air conditioned ambient with temperature higher than those predicted by international standards. Furthermore, these results have to be confirmed by an investigation in a similar environment with a very large sample. These experimental results determined in real situations, represent a collection of important data relative to the perceptions of ambient conditions conditioned by Cameroonians. They can contribute to the control of energy consumption in air-conditioned buildings by setting acceptable ambient conditions for Cameroonians, which are higher than those imposed by the standards of thermal comfort. References [1] KEMAJOU A Les systèmes de ventilation et climatisation les problèmes habituellement observés et solutions Techniques Etudes de cas ; Editeur. IEPF (Québec) G1K4A1 Canada [2] NICOL J. Fergus, HUMPHREYS Michael A. (2002). Adaptive thermal comfort and sustainable thermal standards for buildings. Energy and buildings 34 (6) [3] HUMPHREYS (M.A.) Outdoor temperature and comfort indoors. Building Research and practice, Vol. 6, pp [4] CEBTP Aide à la conception thermique de l habitat courant en Afrique Tropical. Rapports présentés aux séminaires habitats climatiques et [5] C.S.T.B Règles de calcul des caractéristiques thermiques utiles des parois de construction. D.T.U. Règles TH k77. France. [6] Réglementation thermique 2000, France: [7] Norme AFNOR NF X , Spécifications relatives aux appareils et méthodes de mesure des caractéristiques physiques de l environnement. Association Française de Normalisation [8] Proposition de Norme ISO AP 9920 Estimation des caractéristiques thermiques d un ensemble vestimentaire, Organisation internationale de Normalisation, Genève. [9] SAPORTA(G) Probabilités Analyse des données statistiques. Editions Technip, Paris 1990, p [10] REGNIER (S.) Quelques aspects mathématiques des problèmes de classification automatique - Math. Sci. Hum. pg [11] ASHRAE standard Thermal environmental conditions for human occupancy. Amer. Soc. Heating Refrig. Air-Condit. Ing. Inc. Atlanta, 1981.

11 348 Tseuyep Ange, Kemajou Alexis, Egbewatt Ne [12] Norme ISO 7730 Spécification des conditions de confort thermique. Organisation Internationale de Normalisation, Genève, 1984 [13] Norme AFNOR NF X Détermination des indices PMV et PPD et spécifications des conditions de confort thermique. Association Française de Normalisation. [14] MOURTADA (A.) CAKPO (C.) KONE (M.L.) Diagnostic thermique dans le tertiaire en Côte d Ivoire. Normalisation et Conditions de confort optimales. LPBT, ENSTP. B.P. 1083, Yamoussoukro, Côte d Ivoire. Designing for Every one. Proceeding of Eleven Congress of the International Ergonomics Association. Paris [15] GRIVEL (F), DAH (C) et BERGER (X) Jugements portés par les ivoiriens sur les conditions climatiques rencontrées sur place comparaison avec les normes ergonomiques internationales en vigueur Colloque internationale l Ergonomie, Yamoussoukro, 1990