热湿地区采用的辐射冷顶板加置换通风系统secret.docx

1、热湿地区采用的辐射冷顶板加置换通风系统secretRadiational panel cooling system with continuous naturalcrossventilation for hot and humid regionsAbstractThis paper investigates a hybrid cooling system, utilizing wind-driven cross ventilation and radiation panel cooling in an office setting .The characteristics of the ind

2、oor environment are examined using computational fluid dynamics (CFD) simulation, which is coupled with a radiation heat transfer simulation, and HVAC control in which the PMV value for a human model in the center of the room is controlled to attain the target value.The system is devised with an ene

3、rgy-saving strategy, which utilizes stratified room air with a vertical temperature gradient. The cooled air settles down within the lower part of the room, while the hot and humid air passes through the upper region of the room, sweeping out the heat and contaminants generated indoors. This strateg

4、y is found to be quite energy-efficient in the intermediate seasons of spring and autumn in Japan. Even under hot and humid outdoor conditions, the hybrid system coupled with radiation cooling would bring significant energy savings are possible compared with a hybrid system coupled with under floor

5、air-conditioning.1. IntroductionWind-driven cross ventilation is considered to be a promising energy-effective strategy, since it utilizes natures power. Adjusting the indoor environment by natural cross ventilation was the traditional method in hot and humid regions of Asia including Japan. However

6、, in modern buildings, where certainty, reliability, and efficiency are preferred, the use of wind-driven cross ventilation alone is inadequate.A hybrid air conditioning system with controlled natural ventilation, or a combination of natural ventilation with mechanical air conditioning is considered

7、 to be able to overcome the deficiency of wind-driven cross ventilation ,and has significant energy-reducing effects .Many studies have been conducted on natural ventilation systems and hybrid air-conditioning systems in Europe and Japan。In Japan in particular, a hybrid cooling system ,using under f

8、loor air conditioning incorporating natural ventilation is primarily used。But many studies have discovered that radiation cooling is more effective than conventional air-cooling systems in many areas。In this study, we advocate a hybrid cooling system using natural ventilation and radiation panel coo

9、ling. It is based on the concept of utilizing both natural ventilation and radiation panel cooling, aiming to introduce outdoor air into the indoor spaces by cross ventilation and to achieve comfortable indoor thermal conditions by the power of nature as far as possible.Even though it is impossible

10、for a higher outdoor temperature to cool a room by cross ventilation, outdoor air can still be introduced and pass through the upper part of the room, expelling out the heat and contaminants generated indoors. In the meantime, with the aid of a vertical thermal gradient, the lower part of the room c

11、an be further cooled by radiation cooling panels installed in the lower part of the room. This strategy is expected to be energy-efficient and to provide adequate thermal comfort in the room. In this paper, the concept of a hybrid cooling system is described and the measured results for the cases wh

12、ere outdoor air temperature and relative humidity are 21 at 60% and 30 at 70% are shown.1. Concept of radiational cooling system with continuous natural cross ventilationFig. 1is a conceptual diagram of the system suggested in this study. This cooling system has a ventilation opening in the upper pa

13、rt of the room to provide ventilation to the indoors by cross ventilation and to discharge internal heat generated in the room. Dehumidifying radiation panelsare installed in the lower part of the room to regulate indoor humidity and to provide cooling. Even when outdoor temperatures become too high

14、 to cool the indoors, heat and contaminants generated within the room maybe discharged by cross ventilation from the upper part of the room without significantly disturbing the occupied zone (or task zone) ,and radiation cooling panels installed in the lower part of the room would locally cool the o

15、ccupied zone only.Thus the proposed system does not ignore the possibility of adjusting the indoor environment using natural power, but maximizes its use to maintain the indoor environment at a comfortable level and to save energy. 3. Outline of CFD analysisAnalysis of room modelThe hybrid cooling s

16、ystem may be used in both residential and office buildings. Here, the case of an office building is examined. The office setting is shown in Fig. 3. The depth of the room is 10.8 m. The width of the analyzed area is set at half of the 3.6 m office module (1.8 m), to make a symmetrical configuration.

17、 Hybrid cooling is modeled as the outdoor air flows into the room from the upper opening of the window (0.5m 1.8m; Fig. 3, left), and is expelled through the opening on the other side (0.5m 1.8m; Fig. 3, right), while the radiation panel cooling is still operating. The radiation panels set in front

18、of the desks in the room serve as partitioning panels, and can be operated below the dew point temperature. Whereas in the case of a hybrid cooling system coupled with under floor air conditioning, the cooled air is supplied by diffusers from the floor and returns through out- let air grills in the

19、ceiling. People, lighting, personal computers, solar heat gains, etc to generate a cooling load, which is removed by natural cross ventilation, and radiation cooling or under floor air conditioning. A human model is placed in the center of the room as the thermal comfort sensor. 4. Results4.1. Flow

20、eldsThe flow fields for the different cases are shown in Fig. 7. In Case 1 (natural ventilation-radiation panel cooling), owing to the considerable temperature differences between the indoor air (26.6C) and outdoor air (21C), the negative buoyancy effect on the inflowing air is apparent. The inflowi

21、ng air flows downwards to the window side, and mixes with the indoor air in the lower part of the room. Though not shown here, the air cooled by the radiation panels flows down to the floor.In Case 2 (natural ventilation-underflow AC), the inflowing air shows the same tendency as in Case 1. However,

22、 this air mixes with the under floor AC and ascends toward the upper part of the room. Owing to the vertical temperature difference in the room, a clockwise circulating current is generated on the inner side of the room. In Case 3, since the temperature difference between the indoor and outdoor air

23、(30C), is small, the negative buoyancy effect of the inflowing air is not apparent. With the taller cooling panel, thermal plumes rising from the personal computers can clearly be seen (Fig. 7(c). This is because the average temperature in the task zone drops. But in Case 4, owing to the vertical te

24、mperature difference, the outdoor air descends to the floor.4.2. Temperature distributionsThe temperature distributions for the four cases are shown in Fig. 8. In Case 1, the average indoor temperature is 26.6C, somewhat higher than for Case 2 of 24.4C. However, the vertical temperature difference i

25、s slight at about 2C. In Case 2, due to the somewhat low velocity (0.8 m/s)of the under floor AC, the temperature near the floor drops to almost 20C. As a result, the vertical temperature difference in the roomis steep at about 6C, especially in the inner zone of the room. In both cases, due to the

26、descending currents of incoming air,a rather low-temperature region appears near the window. In Cases 3, the room average temperature is 29.5C. There are vertical temperature gradients in the room, and a high-temperature region is seen in the upper zone of the room because the incoming outdoor air s

27、tays there. In Case 4, the temperature distribution is similar to that for Case 2, but the temperature stratification becomes comparatively clear. Compared with the results for Case 3, the high-temperature zone (over 30C) disappears because the hot air is cooled by the under floor air conditioning.4

28、.3. Relative humidity distributionOnly the relative humidity distribution for Case 3 is shown in Fig. 9. In Case 1, since the radiation panel surface temperature (19.7C) is higher than the dew point temperature, no condensation occurs. The relative humidity in the room air is about 45% over the whol

29、e space and indicates a relatively uniform distribution compared to Case 2. In Case 2, the average relative humidity is about 50%.As with the temperature distribution, a sharp humidity stratification is formed. In Case 3, the surface temperature of the radiation cooling panel (18.3C) is lower than t

30、he dew point temperature, and condensation occurs on the surface of the radiation panel. Even after introducing high-humidity outdoor air, the average relative humidity in the room reaches about 61% because the radiation cooling panel dehumidifies the humid outdoor air. Compared with the results for

31、 Case 3, the average relative humidity in Case 4 (55%) drops because it is controlled by the under floor air conditioning.4.4. Age of air (Fig. 10)The age of the fresh outdoor air is calculated based on the resulting flow fields. The calculation results are non-dimensionalized by the nominal time co

32、nstant. In Cases 1 and 2,tnis 360 s, and in Cases 3 and 4 it is 1200 s. In Case 1, young air is observed in the lower part of the room because the fresh outdoor air moves into the bottom of the room. In Case 2, the same tendency is observed near the perimeter zone with Case 1. However, somewhat youn

33、ger air is distributed over the whole space, because the outdoor air mixes well with the indoor air due to the air current from the under floor AC. In both cases, the air in the inner part of the room becomes old, owing to the fresh outdoor air mixing with the indoor air.In Case 3, due to the outdoor air flow in the u