A Study on Application of Radiant Floor Heating in Large Space

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1 A Study on Application of Radiant Floor Heating in Large Space Min Hee Ahn 1, a, Chang Ho Choi 2, b, Hyun Woo Lee 3, c 1 Dept. of Architecture Eng., Kwangwoon University graduate school, Seoul, Korea, Dept. of Architecture Eng., Kwangwoon University, Seoul, Korea, Dept. of Architecture Eng., Kwangwoon University, Seoul, Korea, a belteshazzar@kw.ac.kr, b choi1967@kw.ac.kr, c hwlee@kw.ac.kr ABSTRACT This paper addresses the indoor air quality when radiant floor heating is applied in large space. Radiant heat exchange between surfaces depends on the orientation and the temperature of the surfaces. Also, the temperature and the radiant characteristic of the wall and the roof that face the floor have great influence on the indoor air environment due to the largeness of the wall and the roof in large spaces. In this study, we simulate a test-cell(25x20x10) using a ies VE. And using a CFD (microflo in VE), an indoor air environment was investigated to establish the optimum temperature of floor. At the first time of the heating, high floor temperature is demanded. At the middle of the heating, however, the temperature of the residential space was formed appropriately, although the temperature of the floor was set low. KEYWORDS: Large space, Radiant floor heating, Indoor air environment, ies VE, Floor temperature 1. INTRODUCTION The number of large space buildings such as international airports, sport facilities, atria, and convention centers has been increasing recently. Since the ceiling of large spaces is higher than a floor surface, the difference of temperature between the upper and lower is significant. The residential space is, however, limited to the lower area. When the mixed air method is applied in large spaces, the energy consumption increases because this method controls a great deal of air. So an air current and a distribution of temperature is studied at the residential area for protecting a consumption of energy and making a comfort indoor space. Therefore, our study proposes the radiant floor heating system to make an excessive current of air flow controlled and to maintain an appropriate indoor air environment at the residential space. When applying a radiant floor heating system, difference of temperature between the upper and lower is low, and exert comparatively comfort sensibility due to emitting radiant rays to human body. 2. THERMAL RADIATION FUNDAMENTALS Building surfaces emit thermal radiation by virtue of their absolute temperature. For small surface element (da) of a Lambertian emitter the radiation flux emitted into a small solid angle (dθ) lying in a direction making an angle θ to the surface normal is 1 4 dw = εσt cosθdωda (1) π 805

2 where dw is the radiation flux (W/m 2 ); ε is the surface emissivity (W/m 2 ); σ is the Stefan-Boltzmann constant (= x10-8 W/m 2 K 4 ); T is the absolute temperature of the surface (K); θ is the direction angle measured from the surface normal; dω is an element of solid angle; da is an element of surface area (m 2 ). Integrated over solid angle, the total radiation (W) emitted by a plane surface of area A is 4 W = εaσt (2) Surfaces also absorb a proportion of the radiation they intercept. By Kirchhoff s law the fraction of incident radiation that is absorbed by a surface is equal to its emissivity, ε. The emission and absorption of thermal radiation by building surfaces represents an important mechanism for heat transfer.. Also, the temperature and the radiant characteristic of the wall and the roof that face the floor have great influence on the indoor air environment due to the largeness of the wall and the roof in large spaces. 3. METHOD OF STUDY Radiant floor heating system is seriously affected by surface temperature of buildings. So we simulate a test-cell to deduce a surface temperature and indoor temperature like an actual environment. And then we set the boundary condition of CFD(indoor temperature, humidity, surface temperature, etc.). And then we investigate a comfort level by the indoor temperature and air current using a CFD when the beginning and the middle of heating. We only present a plan of a heating on this study. Because difference of temperature between upper and lower is increased by thermal stratification in large space during the heating. And the comfort level is decreased. 3.1 Modelings In this study, test-cell is an average size of five gyms in a primary school. The dimensions of the test-cell is a length 25m, width 10m, height 10m. Table 1 summarizes a condition of a shape, roof and wall of the test-cell. And a design temperature and insulation performance is based on the design standard of building energy saving of Korea in table 2. Table 1. Condition of Large Space Section Space shape Height of roof Construction of wall Construction of roof Table 2. Design condition of insulation & indoor temperature Section Condition A rectangle (25mX20m) 10m Concrete200mm, Insulation 65mm Concrete150mm, Insulation 110mm Condition External wall (when directly expose to the air) 0.47 W/ m2ㆍk Floor (when floor heating) 0.52 W/ m2ㆍk Roof (when directly expose to the air) 0.29 W/ m2ㆍk Designed indoor temperature 20~ Method of analysis We use a VE(Virtual Environment) made by ies4d. Engine of VE is a esp-r. Modules such as ApacheSim for thermal simulation, Radiance for lighting simulation, and SunCast for solar shading analysis are formed. Also it designs a HVAC system using a module of ApHVAC. So it calculates a similar energy consumption and indoor environment to actual environment. 806

3 Figure 1 shows a method of this study. First, design a radiant floor system using an ApHVAC and apply that in test-cell. And then calculate an indoor temperature, surface temperatures and heating loads of test-cell. And export the CFD boundary condition, then we import that boundary condition to CFD program named microflo. CFD program calculates a distribution of indoor temperature, air current and PPD. Table 3 summarizes the heating system condition, and Table 4 summarizes the condition and scheme applied to simulation. We use a standard weather data of Seoul for Simulation period is October to Figure 1. Method of this study March that is heating season of Korea. Table 3. Heating System Condition Section Heating system Output at max. control signal Heating time Set point Condition Radiant floor heating 40kW 16:00~22:00 20 Table 4. Condition & Scheme applied the simulation Section Weather data Simulation Period External convection model Internal convection model Long-wave radiation model CFD Condition Seoul2000 Oct.~Mar. McAdams Alamdari & Hammond CIBSE Power law scheme 4. RESULT & CONSIDERATION 4.1 Result of energy simulation (CFD boundary condition) We choose the 31th January that is the coldest day during the heating season for setting the CFD boundary condition. Figure 2 shows the variation of an indoor temperature, the temperature of wall and roof and outdoor temperature in the 31th January. We select a CFD boundary condition that is a surface and indoor temperature and humidity at 16:00 (beginning time of a heating) and 21:00(during a heating). 4.2 Result of CFD We examine an indoor air environment by floor temperature each 26, 30, 35. Set the section which is central vertical section. And then investigate a distribution of a temperature, air current and PPD Beginning of heating Figure 3 shows a distribution of indoor temperature by the floor temperature (26, 30, 35 ). Figure 2. A variation of temperature(indoor, wall, roof) 807

4 When floor temperature is 26, it shows 17.5 ~18 of the distribution of indoor temperature. When floor temperature is 30, indicate 18 ~19.5 of distribution. When floor temperature is 35, it shows 18.5~20.5 of the distribution. A temperature of the center Figure 3. Distribution of temperature by floor temperature (unit : ) Figure 4. Distribution of air current by floor temperature (unit : m/s) Figure 5. Distribution of PPD by floor temperature (unit : %) 808

5 of a test-cell is higher than near the wall in the all cases. We suppose the cause of this appearance that is the cold draft of the wall. Figure 4 is a distribution of indoor air current. The air velocity shows 0.05~0.5m/s at residential area regardless of a floor temperature, and it shows the similar distribution of air current in all cases. When the floor temperature gets higher, the air velocity minutely increases. The air velocity of the central upper zone is the fastest. Figure 5 shows the distribution of PPD for comparing the comfort by floor temperature. The distribution of PPD at residential area is shown 5.0~7.5% when floor temperature is 26, 5.0~6.5% when floor temperature is 30, 5.0~5.5 when floor temperature is 30. PPD is lower than 10% despite of low floor temperature. So a high indoor air quality is predicted Middle of heating Figure 6 shows a distribution of indoor temperature by the floor temperature. When floor temperature is 26, it shows 19 of the distribution of indoor temperature. When floor temperature is 30, indicate 19 ~ 20 of distribution. When floor temperature is 35, it shows ~22 of the distribution. A temperature of the center of a test-cell is higher than near the wall in the all cases like a beginning of a heating. Figure 7 is the distribution of air current. In all cases, air current at residential area is shown 0.05~0.4m/s. And the air velocity of middle zone is higher than near the wall. The higher floor temperature, the minutely larger air velocity is. And distribution of air current is loosened. Figure 8 is the distribution of PPD. When Floor temperature is 30, the distribution of PPD is shown the stablest distribution in all cases. It is shown distribution of PPD that is 5% as a whole regardless of a floor temperature. Figure 6. Distribution of temperature by floor temperature (unit : ) 809

6 Figure 7. Distribution of air current by floor temperature (unit : m/s) (a) floor temperature 26 (b) floor temperature 30 Figure 8. Distribution of PPD by floor temperature (unit : %) 5. CONCLUSION To investigate an effect of radiant floor heating system considering an intermittent heating environment in large space, floor temperature (26, 30, 35 ) is applied at the beginning and middle of a heating, and then calculate the distribution of indoor temperature, air current and PPD. (1) At the beginning of a heating, the distribution of indoor temperature is shown that is good distribution of 19~21 when floor temperature is 35. Temperature in the vicinity of wall is lower than center of the test-cell due to the cold draft of the wall. And the air current and PPD is a similar distribution of indoor temperature. (2) At the middle of a heating, the distribution of indoor temperature is shown that is good distribution of 19~21 when floor temperature is 30. The air current is shown stable distribution when floor temperature is 26. And the distribution of PPD is 5% in all cases. (3) Radiant floor heating system in large space needs a high floor temperature at the beginning of heating. At the middle of heating, however, it makes a comfort indoor air environment with a low floor temperature. And thermal stratification is not arisen. (4) In this study, radiant floor heating system makes a appropriate indoor air environment of the large space. But temperature in the vicinity of the wall is low owing to cold draft of the wall. It is expected that it is possible to solve it by zoning with the center of the room and the wall surroundings and setting the floor temperature respectively of the zone. REFERENCES Kim, D., H., Yook, I., S., Song, D., S., Kim, J., M., Study on the Integrated simulation method between thermal environment and IAQ in building. Proceedings of the AIK, Korea, pp. 630~632 Park, M., S., Lee, D., W., Kim, S., M., Park, Y., W., Kim K., H., Numerical Analysis for various ventilation type in large gymnasium. Proceedings of the SAREK, Korea, pp Kim, G., W., Park, T., S., Park, J., C., A Case Study on the Indoor Air Distribution in Large Spaces. Proceedings of the KSES. Korea, pp. 243~248 Lee, S., C., Evaluation on thermal environment installed ventilating fans in the rotunda at new national museum of Korea. SAREK Vol.16. pp. 303~309 ASHRAE, ASHRAE Handbook Fundamentals. 810