Experimental study of space cooling using ceiling panels equipped with capillary mats

Transcription

1 Experimental study of space cooling using ceiling panels equipped with capillary mats Tiberiu Catalina and Joseph Virgone University of Lyon, France Corresponding SUMMARY The purpose of this paper is to investigate the thermal performances and the effect on the thermal comfort of a cooling ceiling installed in an experimental test room called Minibat. With a 9.6m 2 surface and controlling the temperatures on all the exterior walls of the cell, Minibat was the perfect environment for this experimental study. The studied ceiling panels were equipped with capillary mats using polypropylene as material. During the experiment we have analyzed different cases where the ceiling surface temperature varied between 15 C and 19 C. Different parameters like air temperature, humidity or surface temperature were measured during the experiment. To evaluate the thermal comfort we have calculated the PMV value for different chilled ceiling surface temperatures. The results indicate that the cooling ceiling could assure the indoor thermal comfort and with good thermal performances in terms of specific cooling rate or vertical temperature asymmetry. INTRODUCTION The majority of air-conditioning devices function on the principle of pulsated air, where the hot air of the room is recycled, cooled and returned into the room. The increase of the thermal loads in the buildings, mainly due on arrival of data processing and of office computers, the installation of air-conditioning systems was necessary to neutralize these loads and to create a good indoor thermal comfort. Air conditioning systems, which consume large quantities of energy, have become a necessity for almost all the buildings [1] to provide a comfortable indoor environment. Currently the evacuation of these quantities of latent and sensible heats is done mainly with air treated introduced by air diffusers. To maintain comfort under these conditions, a greater volume of cooled air must be provided to the working area. Disadvantages such as noise, cold-drafts, air temperature differences between the human head and foot or energy wasting in certain cases, show that a new cooling system needs to be proposed. The use of water as a coolant to cool surfaces of buildings (ceilings or walls) is consequently a tempting alternative solution. It is even more appealing as water cooling requires much lower flow rates and thus much smaller areas of piping. The chilled ceiling radiant panels are room cooling systems for placement in the ceiling zone. Their cooling surfaces are connected with closed circuit heat conducting pipework containing flowing chilled water. With a cooled ceiling, the temperatures of a room s surfaces are lower than with air-conditioning solutions and the same is true when other partition surfaces are

2 cooled. The principles of cooling ceilings are not very different from those of a radiator with tubes or hotplates. The hot air arriving in contact with the cooled surface is cooled below the average temperature of the room and therefore descends at low speed into the occupation zone [2, 3]. The main difference between cooling ceilings and air-conditioning systems is the mechanism of heat transportation. Air-conditioning using air employs convection only, while cooled ceilings employ a combination of radiation and convection. With cold ceilings, the transfer of radiant heat occurs by a clear emission of electromagnetic waves from the hot occupants and their environments to the radiant ceiling. The sensation of comfort produced by a cold ceiling can be compared to that felt during a summer night where one feels the freshness of the sky even if the ambient air temperature can be higher than 26 C so when using a chilled ceiling it is possible to get the same conditions of comfort with more elevated air temperatures than a classical air-conditioning system [5]. With a cooling ceiling the temperature of water or its flow rate can be varied so that the surface temperature is adapted to the desired conditions. However the many advantages on thermal comfort [4] or energy reduction [5], the cooling ceiling has not reached its full potential because of the condensation risk on the chilled surface. To prevent the risk of condensation on the chilled water pipes or radiant surfaces, the water flow temperature should not be below 16 C. The dew point temperature of the indoor air must always be lower than the surface temperature of the cooling ceiling, this being a reliable way to avoid condensation. For a more precise protection dew point detectors can be installed at the coldest point on the flow pipe of the cooling ceiling installation. They indicate the beginning of condensation at an early stage and trigger an increase in supply water temperature or a chilled water supply shutoff. To reduce the dew point temperature and assure the hygienic quality of the indoor air it is necessary to have a ventilation system to bring fresh air in the occupancy area. This article purpose is to present an experimental study on a chilled ceiling installed in a test cell called Minibat. The main aims were to analyze the vertical temperature asymmetry and the thermal comfort. By taking measurements of air temperature, relative humidity or surface temperature we have been able to give relevant and realistic conclusions on this subject. THE EXPERIMENTAL SET-UP The experimental cell Minibat, presented in Figure 1, is composed of two identical parts which dimensions are 3.10m x 3.10m x 2.50m respectively to (x, y, z). In our study we have used the test cell number 2 where we have installed the radiant cooling ceiling panels. The glazed façade separates Minibat from a climatic chamber whose temperature is controlled and can vary between -10 C and 40 C. A thermal guard allows keeping the five others faces at a constant temperature, which can vary between 5 C and 30 C. During our tests, the thermal guard temperature was kept at 26 C. A battery of 12 spotlights, of 1000W each one, not used in this study, makes it possible to simulate an artificial sunning (gas-discharge lamps with metal halides which spectrum is similar to the sun one). All the faces temperatures are measured using thermocouples of resolution ±0.4 C, and each face with 9 thermocouples. The temperatures of the climatic chamber and the various parts of the thermal guard are measured using Pt100 probes which resolution is of ± 0.3 C.

3 z y Cell 1 Cell 2 Φ VERTICAL VIEW Figure 1. Experimental test cell Minibat HORIZONTAL VIEW 23 Table 1 presents the structure of the wall, ceiling and floor of the experimental test chamber. The cooling ceiling panels are metallic and are insulated with 8 cm foam to reduce the heat losses. These one have been added on the lower face of the existing ceiling. Table 1. Composition of the test cell Minibat (before adding the cooling ceiling) Wall Material λ ρ Thickness [W/m C] [kg/m³] [mm] Floor concrete Wall plaster plate insulated material plaster plate wood plate Ceiling plaster plate wood plate insulated material wood For the data acquisition we have used a computer and a multifunction board with more than 100 connections. To measure the indoor humidity we have used a humidity sensor placed at the centre of the room with a precision of ± 2.5% for measurement of relative humidity between 10 and 90%. A globe temperature is also measured at the center of the room. Concerning the cooling ceiling, we have used 9 panels equipped with capillary mats and being placed only in cell 2. The exterior diameter of the tubes is 3mm and they are spaced by 1.5cm. An indoor heat gain of 250W was installed in the test cell which has a surface of 9.61m 2. To measure the vertical asymmetry of temperature we have taken measurements for six points of height. Several tests were done by modifying the surface temperature of the cooling ceiling in order to see the effect on the thermal comfort or the vertical asymmetry.

4 RESULTS The temperature of the thermal guard was set to 26 C during the whole experimentation. It was observed that there were variations but with a maximum of 1 C due to the system controller but which we have considered totally acceptable. Figure 2 presents the temperature for different heights on the exterior and interior walls surfaces of the test cell. Exterior walls (thermal guard) Interior walls (cooling ceiling activated) Figure 2. Temperature variation against the walls during the experiment Being surrounded by surfaces that have large temperature differences (cooling ceiling and the walls in our case) may be a cause of discomfort, even when the air temperature is considered in the comfort zone. These conditions of discomfort are frequently caused by cold windows, un-insulated walls or direct sunlight. In general, people are more sensitive to asymmetric radiation caused by a warm ceiling than that caused by warm vertical surfaces [7]. Figure 3. Thermal screenshots of the cooling ceiling during the experiment The temperature on the surface of the chilled ceiling is relative constant on each panel, except the connection between them (see Figure 3). A reason that temperature on the surface is not perfectly uniform could be that a gap of air was formed between the tubes and the metallic panels and acting like an insulation on some areas.

5 Figure 4. Specific cooling power of the cooling ceiling in function of the difference between the mean water temperature and the air temperature The cooling power of a radiant ceiling (RC) system is a function of the heat transfer between the room and the cooled ceiling. This heat transfer has two components: radiation and convection which can be calculated. In Figure 4 is presented the specific cooling power of the chilled ceiling depending on the difference of the mean water temperature which was measured during the experiment and the air temperature. The specific cooling power of a cooled ceiling can be expressed by the following empirical equation: q = 8.92 (t air -t cold surface ) 1.1 where q is the sum of the convective and radiant heat transfer [W/m 2 ] t air is the air temperature of the room [ C] t coldsurface is the temperature of the cooling ceiling [ C] A large vertical air temperature difference between the head and ankles may be also a discomfort cause. Based on criteria of 5% dissatisfied, the allowable temperature difference is 3 C, which applies for situations where the temperature increases with height from the floor (i.e., the head is warmer than the feet). With a cooling ceiling the temperature decreases when we are approaching the chilled ceiling height and when the temperature of the ceiling is low enough we can even sense the refreshment on the body skin. The results indicate that in the centre of the test cell, the stratification is acceptable and is smaller than the 3 C limit imposed by standards. In Figure 5 it can be observed the vertical temperature asymmetry for three different cooling ceiling temperatures. In all the cases the difference between the ankle and the head is about 1 C. The data presented in Figure 4 was obtained by measuring the temperature in six points of height, from 0.4m to 2.3m. The time step of one measure was of 45 seconds which correspond to the minimum imposed by the acquisition system to pass throw all the measurements including air and surface temperatures, water temperature, relative humidity or globe temperature. We can conclude on this part that the cooling ceiling don t create discomfort in the working area but the need of taking

6 measurements also near the walls seems necessary to be done. All the temperatures were measured by thermocouples type K with a precision of ± 0.3 C. Figure 5. Vertical temperature asymmetry in the centre of the test room for different ceiling temperatures Thermal comfort is very difficult to define because we need to take into account a range of environmental and personal factors when deciding on the temperatures or the ventilation that will make a comfortable indoor space. The best that we can realistically hope to achieve is a thermal environment which satisfies the majority of people or the so called reasonable comfort. In order to analyze the thermal comfort in a room cooled by a chilled ceiling we have taken measurements of air temperature, globe temperature, relative humidity and surface temperatures (9 thermocouples per internal surface of the room). By simulating the test cell using the computational fluid dynamics (CFD) we have estimated the air velocity inside the room. The PMV (Predicted Mean Vote) [6] was calculated by using the data obtained after the tests. The PMV index establishes a thermal strain based on steady-state heat transfer between the body and the environment and assigns a comfort vote to that amount of strain. Figure 6. Relative humidity and globe temperature when using the cooling ceiling at 18.8 C In Figure 6 are presented the evolutions of relative humidity and the globe temperature during more than 12 hours of experimentation. We can observed that the relative humidity was stabilized at a value of 43.5% and the globe temperature at 25 C when using the chilled ceiling at 17.4 C. The PMV equation for thermal comfort is a steady-state model. It is an empirical equation for predicting the mean vote on an ordinal category rating scale of thermal

7 comfort of a population of people. The air velocity was considered after the CFD simulations of 0.15 m/s, the rest of parameters influencing the thermal comfort being measured. In Figure 7 it can be observed that the PMV index for different clothing insulations when using the cooling ceiling surface at different temperatures. We have only presented two cases of ceiling surface temperature which were the more relevant, knowing that in general the cooling ceiling temperature must be kept at higher values than the dew point temperature of the indoor air in order to avoid the condensation risk. Figure 7. PMV for different clothing insulation when using the cooling ceiling at different temperatures The values presented in Figure 7 show that the cooling is a very attractive solution for the thermal comfort. It can be observed that when the cooling ceiling temperature is at 17.4 C and when the metabolism is 1.2 met (70W/m 2 ) and the clothing insulation is 0.9 clo the obtained value of PMV is in the limit proposed by the norms. DISCUSSION The results obtained after the experiment showed that with a cooling ceiling the vertical temperature asymmetry is less than 1.1 C, acceptable value which not create a discomfort in the occupancy zone. Another important aspect of the test was the effect of the chilled ceiling on the thermal comfort. The temperature of the ceiling was changed in order to see the effect on the thermal comfort, the results being very satisfying. The PMV was calculated for different clothing insulation, its values fulfilling the limits imposed by the standards. These discussions prove that ceiling radiant cooling systems can be more comfortable than conventional air cooling systems, due to less air movement, minimal vertical air temperature difference and with good results on the thermal comfort even for higher values of the ceiling temperature. However the big amount of data obtained more extensive experimental test research are sorely needed especially on the condensation risk problems or the effect of hygroscopic materials on the thermal performances.

8 REFERENCES 1. L.Z. Zhang, J.L. Niu, Indoor humidity behaviors associated with decoupled cooling in hot and humid climates, Building and Environment 38 (2003) Plafonds froids et introduction d air laminaire (Cold Ceilings and Laminar Introduction of Air), P.Pepinster, Division St-Group Technical Facilities Management (ST/TFM) CERN, Geneva (Switzerland) 3. Systèmes de rafraîchissement par le plafond (Systems of cooling by the ceiling), Chaud, froid, plomberie (Heat, cold, plumbing) N 607 November 1998; 4. T. Catalina, J.Virgone, J.J. Martin, B. White, F. Kuznik, Simulation of the ambient comfort of a cooling ceiling integrated into a room, Aicaar, Milano, April T. Catalina, J.Virgone, Evaluation of performances, thermal comfort and energy consumption of a reversible radiant ceiling by capillary mat: application for the prefabricated buildings, EPIC Lyon, November ISO 7730 (1994) Moderate Thermal Environments - Determination of the indices PMV and PPD and specifications of the conditions of thermal comfort 7. C. Huizenga, H. Zhang, P. Mattelaer, T. Yu, E. Arens, P. Lyons, Window performance for human thermal comfort, University of California, Berkley, Arup Façade Engineering BIBLIOGRAPHY 1. ASHRAE (1992) Thermal environmental conditions for human occupancy ANSI-ASHRAE, Standard ASHRAE , Heat Transfer and Thermodynamics, F.Kreith 1967, University of Colorado 3. ASHRAE (1993) Thermal Physiological Principles and Comfort, ASHRAE Fundamentals Handbook Chapter 8, Atlanta (USA): American Society of Heating, Refrigerating and Air- Conditioning Engineers Inc, 1993