Thermal behavior and Energetic Dispersals of the Human Body under Various Indoor Air Temperatures at 50% Relative Humidity Hakan CALISKAN Usak University, Department of Mechanical Engineering, Usak, Turkey ABSTRACT In this paper, thermal behavior and energetic dispersals of the human body under various indoor air temperatures at 50% relative humidity are investigated. The indoor air temperatures are considered as 21 C, 22 C, 23 C, 24 C and 25 C, while the room is 6x6x2.9m. The energy analysis of the human body and the effects of indoor air temperature on human body are studied. As a result, it is found that the Predicted Mean Vote (PMV) rate of the human body is near to zero at 23 C, and the Predicted Percentage of Dissatisfied (PPD) of the human body is minimum at 23 C. Considering the PMV and PPD rates, the best indoor air condition is found at 23 C indoor air temperature for human body. On the other hand, the maximum energy loss is found by radiation, while minimum energy loss is determined by evaporation of sweat. Also, the stored energy of the human body is directly proportional to the indoor air temperature. Keywords Thermal comfort; PMV; PPD; Human body; Energy; Thermodynamics INTRODUCTION Climatic factors affect the organisms. Hence, human body tries to adjust itself to the surrounding ambient condition. So, thermal comfort, which establishes a connection between the external thermal stress and the human thermoregulation capacities such as heat loss and heat storage is necessary [1]. Thermal comfort is a combination of a subjective sensation and several objective interactions with the environment. In other words, it is about how we feel and heat and mass transfer to the environment. So, comfort depends on person and environmental related situations. Human body core temperature is considered as 37ºC. It may depart a few degrees under unhealthy circumstances, particularly above that value, as with fever, or during heavy prolonged physical exercise. So, the heat is necessary to be evacuated through our skin to the environment to compensate our metabolic dissipation, with a baseline rate of about 1 W/kg, increasing with physical activity up to 5 W/kg; e.g. it is around 100 W for an adult in office-work. Generally, temperature of skin is below 33 ºC, allowing the heat outflow, but it depends a lot on external conditions, clothing, and actual and previous activity levels. Air temperature, background radiant temperature (of walls, sky, sun, etc.), air relative humidity, and wind speed are environmental effects on thermal comfort. Non-thermal environmental variables like ambient light and noise may affect the thermal sensation too [2]. If the human body is too warm, the blood vessels vasodilate and the blood flow increases through the skin, and then people begin to sweat. Sweating is a kind of cooling method. The energy required for the sweat to evaporate is obtained from the skin. A few tenths of a degrees increase in the core body temperature can stimulate a sweat production which quadruples the body s heat loss. When the human body is cold, the blood vessels begin to vasoconstriction, and the blood flow reduces through the skin. Then the shivering occurs due to the internal heat generation increasing by stimulating the muscles. Thus, human body heat production increases. This system is also very effective, and it can increase the body s heat production dramatically. The human body temperature control is a complex process. It is still not understood clearly. However, the two sensors are generally accepted to use to control the human body temperature. They are located in the skin and 109 Hakan CALISKAN
in the hypothalamus. The hypothalamus-sensor is a heat sensor which starts the body s cooling function when the body s core temperature exceeds 37 C. The skin-sensor is cold sensors which start the body s defense against cooling down when the skin temperature falls below 34 C. If the hot and cold sensors output signals at the same time, our brain will inhibit one or both of the body s defense reactions [3]. Energy efficient buildings can be considered effectively constructed if the occupants are comfortable. If they are not comfortable, then they can take alternative means of heating or cooling a space such as space heaters or window-mounted air conditioners that could be substantially worse than typical Heating, Ventilation and Air Conditioning (HVAC) systems. Thermal comfort is hard to measure because it is very subjective. As explained above, it generally depends on the air temperature, humidity, radiant temperature, air velocity, metabolic rates, clothing levels, humans physiology, etc. According to American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), thermal comfort is defined as that condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation. It is also called to be human comfort. As a result, thermal comfort is the satisfaction of the occupants with the surrounding thermal conditions. In the thermal comfort assessment, generally the following factors are considered [4]: Metabolic rate (met): Human body The generated energy by human body. Clothing insulation (clo): Thermal insulation of the human body clothes Air temperature: The surrounding air temperature of the human body. Radiant temperature: The weighted average of all the temperatures from surfaces surrounding the human body. Air velocity: Air movement rate per time. Relative humidity: Water vapor percentage in the air. The heat transfer occurs between the environment and the human body area. If the heat leaving the occupant is greater than the heat entering the occupant, the thermal perception is cold. If the heat entering the occupant is greater than the heat leaving the occupant, the thermal perception is warm or hot. A method of describing thermal comfort was developed by Fanger [5] in 1973 and is referred to as Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD) [4]. The PMV is used for the thermal sensation of the human body and the rate changes between -3 (cold) to +3 (hot). The PMV sensation scale is tabulated in Table 1. The recommended acceptable PMV range for thermal comfort from ASHRAE is between -0.5 and +0.5 for an interior space. Table 1. Predicted Mean Vote (PMV) sensation scale Value Sensation -3 Cold -2 Cool -1 Slightly cool 0 Neutral 1 Slightly warm 2 Warm 3 Hot The PPD gives information on thermal dissatisfaction by predicted the percentage of people feeling very warm or cold in the surrounding ambient condition. These most commonly used indexes are found by Fanger [5] and adapted by (International Organization for Standardization) ISO Standard 7730 [6]. The PPD is a function of PMV, given that as PMV moves further from 0, or neutral, PPD increases. The maximum number of people dissatisfied with their comfort conditions is 100% and, as you can never please all of the 110 Hakan CALISKAN
people all of the time, the recommended acceptable PPD range for thermal comfort from ASHRAE standard is less than 10% persons dissatisfied for an interior space [4]. In this study, thermal behavior and energetic dispersals of the human body under various indoor air temperatures at 50% relative humidity are studied. The temperatures are considered as 21 C, 22 C, 23 C, 24 C, and 25 C. SYSTEM DESCRIPTION A human body in a room is considered as a system, while the room width and depth are 6m and height is 2.9m. In the human body, the hypothalamus of brain controls the heat balance. It takes information about temperatures of body, while thermo-receptors, such as muscle, skin, send the temperature changing to the brain. So, internal body temperature keeps constant [1,7]. The human body external heat transfer occurs by conduction, convection, radiation and evaporation of perspiration. On the other hand, the human body generates heat by metabolism and loses heat generally by evaporation and diffusion of body liquids. Some necessary system data is given in Table 2. Table 2. System data Parameter Symbol Rate Room air temperature T ra 21 C~25 C Mean radiant temperature T m 21 C~25 C Relative humidity of room air RH r 50% Air speed V air 0.1 m/s Clothing Clo 1 clo Activity Act 1 met Temperature of the cloth T cl 27.10 C Temperature of the core T cr 36.82 C Temperature of skin T sk 33.77 C Clothing area factor f cl 1.15 Convective heat transfer coefficient h c 3.826 W/m 2 K The ratio of the effective area of the human body f ef 0.725 Absorption coefficient a i 0.42731 Radiative heat transfer coefficient of a black surface h rb 6.3 W/m 2 K Emittance of clothing surface ε cl 0.955 Temperature of surface T i 33.77 C Specific heat capacity of dry air c p,a 1.005 J/gK Molar mass of dry air R a 28.97 g/mol Gas constant R 8.314 J/molK Atmospheric air pressure P 101325 Pa Specific heat capacity of water vapor c p,v 1.846 J/gK Molar mass of water molecules R w 18.05 g/mol Velocity of the inhaled air V 6.99 (10-5 ) m/s 111 Hakan CALISKAN Velocity of liquid water generated in body core V in w, core 1.95 (10-9 ) m/s Specific heat capacity of liquid water c p,w 4.186 J/gK Density of liquid water ρ w 1000 kg/m 3 Velocity of liquid water generated in body shell as sweat 4.79 (10-9 ) m/s Velocity of the exhaled air Average convective heat transfer coefficient overclothed body surface Vw, shell V out h ccl 4.28 (10-5 ) m/s 3.0825 W/m 2 K
ANALYSIS The energy balance of the human body can be given as follows [1]: En En En En En En En En (1) M W Dif Sw Re s Loss, T Hx S where En ", M En, En W Dif, En Sw, En Res, En Loss, T, En Hx and En S are the generated energy rate by metabolism, the external work rate, the energy loss rate by water vapor diffusion through the skin, the energy loss rate by evaporation of sweat, the energy loss rate by respiration, the energy loss rate due to difference in temperature, the total heat exchange rate with radiation, convection and conduction, and the stored energy rate in the body, respectively. The generated energy rate by metabolism ( En M ) is the total of the metabolic energy rate by the person s activity ( En M, act ) and the metabolic energy rate for shivering ( En M, shiv ). En En En (2) M M, act M, shiv The metabolic energy rate by the person s activity ( En M, act ) is accepted to be 58.33 W/m 2 [7]. Also, the metabolic energy rate for shivering ( En M, shiv ) is determined by; En M, shiv (19.4) 34 Tsk 37 Tcr (3) where T sk and T cr are the skin and core temperatures of the human body, respectively. The metabolic energy rate for shivering and the external work rate ( En ) are accepted to be zero [8]. The energy loss rate by water vapor diffusion through the skin ( En Dif sv, Tsk v, a W Dif ) is calculated from En 0.00305 P P (4) where P sv, T sk is the saturated water vapor pressure at skin temperature, and P va, is the water vapor pressure in the ambient air. The energy loss rate by evaporation of sweat ( En ) is found from En (0.42) En En 58.15 (5) Sw M W The energy loss rate by respiration ( En 0.0000172 5867 Re s M v, a Res Sw ) is determined by; En En P (6) The energy loss rate due to difference in temperature ( En Loss, T ) can be calculated from En Loss, T 0.0014 En M 34 Ta (7) where T a is the ambient temperature. The total heat exchange rate with radiation, convection and conduction ( EnHx ) is determined by Tsk Tcl En Hx (8) (0.155) I cl T where sk is the skin temperature, T cl is the clothing surface temperature, I cl is the thermal insulation of the clothing. 112 Hakan CALISKAN
The PMV rate is found as follows: 0.114 En PMV (3.155) 0.303e M 0.028 L PMV where L PMV is the thermal load acting on the human body. 0.00305,, (0.42) 58.15 LPMV EnM EnW Psv T P sk v a EnM En W Tsk T cl 0.0000172 En M 5867 Pv, a 0.0014 En M 34 Ta (0.155) Icl The PPD rate is calculated by PPD 100 95 e PMV PMV 4 2 0.03353 0.2179 (9) (10) (11) RESULTS AND DISCUSSION The energy analysis is performed to the human body in a 50% relative humidity and 21 C, 22 C, 23 C, 24 C, 25 C room conditions. The energy analysis results are given in Table 3. Table 3. Energy analysis results Indoor air temperature ( C) 21 22 23 24 25 Energy rate by metabolism (W/m 2 ) 58.33 58.33 58.33 58.33 58.33 Energy loss rate by water vapor diffusion through the skin (W/m 2 ) 12.45 12.21 11.96 11.69 11.41 Energy loss rate by evaporation of sweat (W/m 2 ) 0.07 0.07 0.07 0.07 0.07 Energy loss rate by respiration (W/m 2 ) 4.58 4.51 4.42 4.34 4.25 Energy loss rate due to difference in temperature (W/m 2 ) 1.06 0.98 0.90 0.82 0.73 Energy loss rate by radiation (W/m 2 ) 29.16 24.67 20.02 15.30 10.54 Stored energy (W/m 2 ) 11.00 15.88 20.95 26.10 31.32 PMV (-) -1.03-0.44 0.18 0.80 1.42 PPD (%) 27.45 8.97 5.65 18.38 46.62 The energy rate by metabolism is the total generated energy rate of the human body and it is found that 58.33 W/m 2. The energy loss by radiation is the major heat loss of the human body for all of the indoor air conditions. The maximum energy loss rate by radiation is found at 21 C to be 29.16 W/m 2, while minimum one determined at 25 C as 10.54 W/m 2. Generally, the losses are inversely proportional to indoor air temperatures. On the other hand, the stored energy of the human body is directly proportional to the indoor air temperature. The maximum stored energy is calculated to be 31.32 W/m 2 at 25 C, while minimum one is 11 W/m 2 at 21 C. The PMV rate is directly proportional to indoor air temperature. The changing of PMV is given in Figure 1. As is seen, the PMV rate is 0.18 at 23 C which is the best option of the human body. Because, the best PMV rate is near to zero which is neutral rate for the human body. The maximum PMV rate is determined as 1.42 at 25 C. So, the human body feels almost warm at 25 C. 113 Hakan CALISKAN
Fig 1: Changing of PMV. On the other hand, the PPD rate is very variable. The changing of PPD is shown in Figure 2. The PPD rate is minimum at 23 C to be 5.65% which is the minimum dissatisfied percentage of the humans. The maximum PPD value is found as 46.62% at 25 C. So, the human body fells dissatisfied at 25 C. Fig 2: Changing of PPD. CONCLUSIONS Thermal behavior and energetic dispersals of the human body under various indoor air temperatures at 50% relative humidity are investigated. The indoor air temperatures are considered as 21 C, 22 C, 23 C, 24 C and 25 C, while the room is 6x6x2.9m. The energy analysis of the human body and the effect of indoor air temperature on human body are studied. As a result, the following main conclusions can be drawn from the study: 114 Hakan CALISKAN
The Predicted Mean Vote (PMV) rate of the human body is near to zero at 23 C. So, the best thermal comfort condition considering PMV rate is obtained at 23 C. Because, human body feels neutral at zero PMV rate, warm at higher than zero PMV rate, and cool lower than zero PMV rate. The Predicted Percentage of Dissatisfied (PPD) of the human body is minimum at 23 C. In this regard, the best thermal comfort condition PPD rate is obtained at 23 C. Because, PPD shows the dissatisfied people percentage in given condition. It is better to be minimum. Considering the PMV and PPD rates, the best indoor air condition is determined to be 23 C for human body. Maximum energy loss is found by radiation, while minimum energy loss is obtained by evaporation of sweat. Generally, the losses are inversely proportional to indoor air temperatures. The stored energy of the human body is directly proportional to the indoor air temperature. It is maximum at 25 C, and minimum at 21 C. REFERENCES [1] H. Caliskan, Energetic and exergetic comparison of the human body for the summer season, Energy Conversion and Management, Vol. 76, pp. 169-176, 2013. [2] Webserver, 2016, http://webserver.dmt.upm.es/~isidoro/env/human%20thermal%20comfort.pdf [3] Labee, 2016, http://www.labeee.ufsc.br/antigo/arquivos/publicacoes/thermal_booklet.pdf [4] Sustainability workshop, 2016, http://sustainabilityworkshop.autodesk.com/buildings/human-thermal-comfort [5] P. O. Fanger, Thermal Comfort, McGraw-Hill, New York, 1973. [6] ISO 7730, Ergonomics of The Thermal Environment-Analytical Determination and Interpretation of Thermal Comfort Using Calculation of The PMV and PPD Indices and Local Thermal Comfort Criteria, International Standard ISO 7730, Third edition, 2005:11-5. [7] B. W. Olesen, Thermal Comfort, 1982, http://aldebaran. feld.cvut.cz/vyuka/ environmental_ engineering/lectures/l10%20thermal%20comfort.pdf [8] X. Wu, J. Zhao, B. W. Olesen, L. Fang, A novel human body exergy consumption formula to determine indoor thermal conditions for optimal human performance in office buildings. Energy and Buildings, Vol. 56, pp. 48-55, 2013. 115 Hakan CALISKAN