Effects of Latent/Sensible Heat Separation Air-Conditioning and Natural Ventilation on Indoor Thermal Environment in Environment-Friendly Office

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1 Effects of Latent/Sensible Heat Separation -Conditioning and Natural Ventilation on Indoor Thermal Environment in Environment-Friendly Office Shotaro Sakai 1,*, Takashi Akimoto 1, Yumiko Miyasaka 2 1 Shibaura Institute of Technology, Tokyo, Japan 2 Nikken Sekkei Ltd, Tokyo, Japan * Corresponding me1547@shibaura-it.ac.jp SUMMARY As the problem of global warming increases, the imperative to reduce the carbon dioxide emissions intensifies. The construction sector has the potential to significantly reduce these emissions, and prompt action by this sector is therefore recommended. Recently, various air conditioning systems have been developed that could realize energy conservation in buildings. The latent/sensible heat separation air-conditioning system and the natural ventilation system are important air conditioning systems that could reduce the environmental load efficiently. However, as these technologies have not been widely employed in high-rise buildings, not many studies have been conducted on the effectiveness of the systems. The purpose of this study is therefore to determine the effects of the latent/sensible heat separation airconditioning system and the natural ventilation system in an environment-friendly office building in Tokyo. We found that the air temperature in the inlets of the natural ventilation system dropped by approximately 1.2 K after mist spray had been applied. At the same time, the absolute humidity increased by approximately 3 kg/kg (DA) in this investigation. PRACTICAL IMPLICATIONS To uncover the effects of the latent/sensible heat separation air-conditioning system and the natural ventilation system in an environment-friendly office in Tokyo. KEYWORDS natural ventilation, latent/sensible heat, mist spray, office, indoor thermal environment. 1 INTRODUCTION The endeavor to minimize the effects of global warming requires the reduction of greenhouse gas emissions across all sectors of society. In this regard, the building sector has the potential to achieve significant reductions, particularly by undertaking energy-saving measures in existing buildings. The latent/sensible heat separation air-conditioning system and the natural ventilation system are important air conditioning developments that could reduce the environmental load of buildings efficiently. Therefore, quantifying the effects of such air-conditioning systems is an important factor to promote their use. In our research, we conducted field studies to assess the indoor thermal environment of an office building. We verified that the indoor comfort could be maintained if the latent/sensible heat separation air-conditioning systems were employed properly. The purpose of this study is to uncover the effects of the latent/sensible heat separation air-conditioning system and the natural ventilation system in an environment-friendly office in Tokyo.

2 2 SUMMARY OF THE BUILDING AND FIELD STUDY In our research, field studies were conducted during all four seasons to measure the indoor thermal environment in the headquarters building of the T Corporation. Table 1 shows a summary of the target building, whereas Figure 1 shows schematic views of the airconditioning system. In the target building, latent heat is treated with desiccant air conditioning and sensible heat with the multi-split type air conditioning systems. The sensible and latent treatment functions are separated in the desiccant systems. A desiccant material is used to remove the moisture by absorption or adsorption. Refrigeration is subsequently used to lower the temperature to the desired level before distribution. The refrigeration is done at a higher temperature than in a conventional air conditioning, thereby achieving a higher operating coefficient of performance (COP). We have assumed that by employing and managing more efficient systems, the research indicated that the energy cost of air conditioning could be reduced. The natural ventilation system with water mist sprayer is used during spring and autumn, providing cooling and reducing the temperature, with hardly any noticeable increase in the relative humidity. Water requires energy to evaporate. This energy exchange results in outdoor cooling, or reduced temperatures. Table 1. Summary of target building Headquarters building of T corporation Location Tokyo Use Office Scale Eleventh floors Completion date February, 214 Building area 571 m 2 Total floor area 5 83 m 2 Structure Steel construction Heat source equipment Multi-split type air conditioning systems -conditioning system Underfloor air-conditioning system, employing desiccant air-conditioning unit Other equipment Natural ventilation system with water mist sprayer Exhaust Desiccant airconditioning unit Multi-split type air conditioning systems Outdoor Indoor Outside Supply Return Mist sprayer Ventilator Underfloor A) B) Figure 1. Schematic view of the air-conditioning system. A) The latent/sensible heat separation air-conditioning system, B) Natural ventilation system, using water mist sprayer

3 mm mm Figure 2 shows the measuring locations. We measured the vertical temperature distribution, air velocity, air temperature and humidity, globe temperature, and the window opening. In spring and autumn, we measured the air velocity, air temperature, and humidity at the ventilation opening to verify the effect of the natural ventilation system that uses a water mist sprayer. Measurements during spring were conducted from Friday, June 6, 214 to Friday, June 2, 214 and from Friday, May 22, 215 to Friday, July 5, 215. In summer, measurements were conducted from Tuesday, August 19, 215 to Tuesday, September 2, 215. In autumn measurements were conducted from Monday, October 6, 215 to Monday, October 2, 215, whereas in winter they were conducted from Monday, December 8, 215 to Monday, December mm Ventilation opening mm Ceiling line FL+2 4 mm Pole FL+1 7 mm FL+1 1 mm FL+6 mm FL+1 mm Floor line Vertical temperature distribution temperature and humidity velocity Window opening sensor A) Measuring Measuring locations point of of floor floor plan plan B) temperature velocity Temperature and humidity Globe temperature Vertical measuring locations Figure 2. Measuring locations: A) Measuring locations of floor plan, B) Vertical measuring locations 3 RESULT OF A MEASUREMENT 3.1 RESULT OF A MEASUREMENT IN SUMMER Then relation between COP and the load factor The COP of a heat pump is a ratio of heating or cooling, depending on which is required. Higher COPs equate to lower operating costs. The COP could exceed 1, as instead of simply converting work to heat (which, if 1% efficient, would be a COP of 1), additional heat is pumped from a heat source to where the heat is required. Figure 3 shows the relation between COP and the load factor in the summer measurements. The load factor was calculated by the compressor curve method. The COP was obtained by dividing the air conditioner load by the amount of electric power consumed. The rated COP 3.3 was obtained by dividing the rated air conditioner load of 4 kw by the rated value (12 kw) of the amount of electric power consumed. It was found that if the load factor of the multi-split type air conditioning system were between.2 and.4, the COP would be

4 1: 2: 3: 4: 5: 7: 8: 9: 1 11: 13: 14: 15: 1 17: 19: 2 21: 22: 23: PMV more than 4.. Furthermore, if the load factor of the multi-split type air conditioning system were more than.4, the COP would be low. Nevertheless, even with a low load factor, the COP was often more than 3.3. It was therefore assumed that the latent/sensible heat separation air-conditioning system was highly efficient North HVAC South HVAC COP Load factor Figure 3. Relation between COP and the load factor in the summer measurements. Thermal comfort in summer Thermal comfort is difficult to measure because it is highly subjective. The environmental factors relevant to this measurement include temperature, radiant temperature, relative humidity, and air velocity. The personal factors are activity level (metabolic rate) and clothing. The recommended acceptable PMV range for thermal comfort an indoor space according to the ASHRAE scale is -.5 to +.5 (ASHRAE, 1992). In summer, the metabolic rate is 1.1 met (sedentary activity) and the clothing insulation is.57 clo (trousers and shirts). Figure 4 shows the PMV on August 25, 214. The PMV was found to drop from 1.5 to.5 when the running time of the air conditioner was from 8:3 to 9:3. The PMV was kept at the thermal comfort level during working hours. It was therefore assumed that the latent/sensible heat separation air-conditioning system could maintain a high level of comfort Figure 4. PMV on August 25, 214. SET* numerically represents the thermal strain experienced to a "standard" person in a "standard" environment. SET* has the advantage of allowing thermal comparisons between different environments with any combination of physical input variables. The ASHRAE recommended acceptable SET* range for thermal comfort is between 22.2 and Figure 5 shows the thermal environment on August 25, 214. The relative humidity was controlled at 4 5%, with the desiccant system in operation from 1 to 19:. The air conditioning system started operating at 8:, with the air temperature and SET* dropped by

5 3. K/4.5 K. This fall is attributable to the decline in the relative humidity associated with the desiccant system. 1: 2: 3: 4: 5: 7: 8: 9: 1 11: 13: 14: 15: 1 17: 19: 2 21: 22: 23: PMV temperature[ ] SET* temperature relative humidity 1: 2: 3: 4: 5: 7: 8: 9: 1 11: 13: 14: 15: 1 17: 19: 2 21: 22: 23: Figure 5. Thermal environment on August 25, relative humidity[%] 3.2 RESULT OF A MEASUREMENT IN WINTER Then relation between COP and the load factor Figure 6 shows the relation between COP and the load factor in the winter measurements. The rated COP of 3.75 was obtained by dividing the rated air conditioner load of 45 kw by the rated value (12 kw) of the amount of electric power consumed. The COP was maintained at approximately 2. 3., regardless of the load factor North HVAC South HVAC COP Load factor Figure 6. Relation between COP and the load factor in the winter measurement. Thermal comfort in winter In this winter measurement, the metabolic rate was 1.1 met (sedentary activity) and the clothing insulation was 1. clo (business suit). Figure 7 shows the PMV on December 16, 214. The PMV was found to rise from -1.5 to -.5 when the running time of the air conditioner was from 8:3 to 9:3. The PMV was maintained at the thermal comfort level during working hours Figure 7. PMV on December 16, 214.

6 Figure 8 shows the thermal environment on December 16, 214. The relative humidity was controlled at 4 5%, with the desiccant system operating from 8: to 19:. The air conditioning system started operating at 8: and the air temperature and SET* increased by 4.6 K/5.6 K. This rise is attributed to the increase in the relative humidity because of the desiccant system. temperature[ ] SET* temperature relative humidity 1: 2: 3: 4: 5: 7: 8: 9: 1 11: 13: 14: 15: 1 17: 19: 2 21: 22: 23: Figure 8. Thermal environment on December 16, relative humidity[%] 3.3 RESULT OF A MEASUREMENT IN AUTUMN/SPRING Natural ventilation is the process of supplying air to and removing air from an indoor space without using mechanical systems. It therefore alludes to the flow of external air to an indoor space as a result of pressure differences caused by natural forces. In this study, natural ventilation indicates wind-driven ventilation, which results from the different pressures created by the wind blowing around a building or structure, with openings being formed on the perimeter, which, subsequently, permit the flow of air through the building. The ventilation opening and mist spraying process is controlled by the BEMS (Building Energy Management System). Table 2 shows the operating conditions relevant to this process. Table 2. Operating conditions of ventilation opening and mist spraying condition Ventilation opening Mist spraying Outdoor air temperature 15 < air temperature < < air temperature < 26 Outdoor relative humidity relative humidity < 8% relative humidity < 7% Outdoor air velocity air velocity < 15 m/s air velocity < 15 m/s Rainfall No No The effects of natural ventilation system Figure 9 shows the amount of heat removed by natural ventilation and the air conditioning system. The average amount of heat removed by natural ventilation was MJ, whereas that removed by the air conditioning system was 89.2 MJ. The amount of heat removed from the air by natural ventilation only was 71%. It was therefore assumed that the natural ventilation system reduced the cooling load during midseason.

7 Amount of heat [MJ] /1/8 Removed heat by natural ventilation Removed heat by HVAC Outdoor air temperature 214/1/9 214/1/1 214/1/14 214/1/15 214/1/16 214/1/17 Figure 9. Amount of heat removed by natural ventilation and air conditioning. Figure 1 shows the air temperature and absolute humidity in the inlets of the natural ventilation system when mist was sprayed and when it was not sprayed. During mist spraying, the average air temperature was lower by 1.2 K compared with the no-spray condition. Moreover, the air temperature after spraying for five minutes was lower by approximately 1. K compared with the no-spray condition. It is therefore assumed that the natural ventilation system, with a water mist sprayer, reduced the cooling load during midseason. Figure 1. The air temperature and absolute humidity in the inlets of the natural ventilation system when mist was sprayed and not sprayed. Figure 11 shows the air temperature and relative humidity in the inlets of the natural ventilation system during mist spraying. The air temperature dropped to approximately 22 after the spraying. At the same time, the relative humidity increased by approximately 8%. 215/5/25 7: 8: 9: 1 215/5/26 11: 13: 14: 15: 1 17: Temperature[ ] Relative humidity[%] Absolute humidity [kg/kg(da)] Spraying After spraying 5minutes Not spraying Temperature[ ] 215/5/27 215/5/28 215/5/29 215/6/1 Average Tempeature [ ] Outdoor air temperature temperature Relative humidity Figure 11. temperature and relative humidity in the inlets of the natural ventilation system. Figure 12 shows the temperature difference between the indoors and the outdoors during spring 214. The temperature difference between the in- and outdoors indicates the difference between the average indoor air temperature and the outside air temperature. The outside air

8 temperature dropped at, while the temperature difference between the in- and outside increased. Temperature differnce [K] Temperature difference between inside and outside Figure 12. Temperature difference between inside and outside. The measurements for autumn were based on the results of the measurements for spring 214. We utilized the early-morning natural ventilation to reduce the rise in the indoor air temperature during the morning. Figure 13 shows the PMV of each point when the vent was opened at 5:. The metabolic rate was 1.1 met (sedentary activity) and the clothing insulation was 1. clo (business suit). Before the natural ventilation started (before 5:), the PMV was more than.5, whereas after the start of the natural ventilation (operating from 5: to 9:), the PMV was at an acceptable range (-.5 to +.5). The air conditioning systems did not operate on the particular day. It was therefore assumed that the PMV dropped because of the natural ventilation. 1.5 Pole 1 Pole 2 Pole 3 Pole 4 Outdoor air temperature 2 PMV : 4: 8: 1 14: : Figure 13. PMV of each point temperature[ ] 4 CONCLUSION The latent/sensible heat separation air-conditioning system is efficient and facilitates a highly comfortable indoor environment. The natural ventilation system, with the water mist sprayer, reduced the cooling load during midseason. The findings of the study are important to researchers, building designers, owners and operators, and occupants. ACKNOWLEDGEMENT We wish to express our sincerest appreciation to Messrs Hori and Watabe, whose comments and suggestions were invaluable throughout the course of my study. In addition, we wish to thank the T Corporation for their cooperation during the field study. 8 REFERENCES ASHRAE ANSI/ASHRAE Standard , Thermal Environmental Conditions for Human Occupancy. Atlanta: American Society of Heating, Refrigerating, and - Conditioning Engineers, Inc.