STUDY ON A FLOOR SUPPLY AIR CONDIGIONING SYSTEM WITH THERMAL ENERGY STORAGE USING GRANULATED PCM

Transcription

1 STUDY ON A FLOOR SULY AIR CONDIGIONING SYSTEM WITH THERMAL ENERGY STORAGE USING GRANULATED CM S. Takeda, K. Nagano, K. Nakayama, K. Shimakura Hokkaido University Sapporo, Japan Tel: sayakat@eng.hokudai.ac.jp T. Nakamura Shimizu Corporation Tokyo, Japan 1. INTRODUCTION The peak demand for electricity during hot summer is increasing year by year and the daytime load has become double that of nighttime in the Metropolitan area of Japan, [TECO, 2004]. The main reason for this is air conditioning of commercial buildings. Therefore one policy to reduce the peak demand would be to produce and use electricity at night and store cold thermal energy in order to reduce the daytime air conditioner load. This would not only reduce the cost to consumers by allowing them to use cheaper nighttime electricity but would also avoid the need for capital investment in new electricity power plants. Moreover this would bring reduction of CO 2 emission due to decrease of peak power generation supplied by oil-fired thermal power plants. Building mass thermal storage is a means of thermal energy storage, which makes use of concrete building slabs as a medium [ASHRAE, 2003]. This can avoid large storage tanks and space required by ice or water storage. Generally it uses the space above ceilings or below free access floors so is mainly used in conjunction with ceiling or floor air conditioning systems. The authors have focused on the floor supply air conditioning system in which air is supplied to a room through porous OA floor boards and a permeable carpet rather than via vents in the floor [Akimoto et al., 1995]. Demonstrated advantages of such systems are that they push air upward without blowing dust or other debris around and do not cause uncomfortable drafts. However, when a floor supply air conditioning system incorporates building mass thermal storage, owing to cooling of the floor boards during night, the region near the floor may be perceived as too cold when the system is first turned on in the morning. This is a disadvantage that needs to be improved. Furthermore, with current building mass thermal storage systems, the limited thermal capacity makes it difficult to maintain cooling during afternoons. To overcome this, we have proposed a novel building mass thermal storage based on a floor supply air conditioning system which incorporates latent heat storage to increase the total cold storage capacity [Nagano et al., 2006]. A concept diagram of the proposed system is shown in Figure 1. In this system, latent heat is stored in phase change material (abbreviated below as CM) that is embedded directly under OA floor boards in the form of granules several millimeters in diameter. This CM packed bed is permeable to air so is suitable for use in the floor supply air conditioning system. During night, circulation of cool air through the under floor space allows cold energy to be charged to the concrete slab, OA floor board and CM packed bed. On the other hand, they in turn can be used to remove the cooling load in the room during the daytime. In this paper, the authors describe a demonstration system that simulates an air conditioning system in office buildings. An appropriate experimental condition is determined by a numerical analysis with varying temperature conditions during night and comparing the thermal characteristics of each storage mass and thermal environment in the room space. Then possibility whether the whole cooling load can be covered only by the stored heat during night in the suitable condition is confirmed through the demonstration. Moreover, voting about thermal sensation and comfort sensation is carried out with some subjects, in which improvement of excessive cooling near the floor in previous systems is clarified by use of the granulated CM.

2 Ceiling space Ceiling space Room space Under floor construction Breathable carpet OA floor board Granulated CM Cooling load (Sensible heat) Room space Metal stand Under floor space FAN FAN Concrete slab Floor supply air conditioning [N-Mode: Cold storage during night] AHU Air filter [D-Mode: Discharge during the daytime] AHU Air filter Figure 1 Concept diagram of the proposed system 2. OUTLINE OF THE COOLING SYSTEM 2.1. Experimental testing room Figure 2 shows a plan of the environmental testing room. The room consists of a test room and a utility space. The floor, the walls and the ceiling are made of hard urethane foams, which the thicknesses are 89 mm and the heat conductivity is 28 W/m 2 /K. The floor space is 9.2 m 2. Cold water which is supplied by a hot and cold water generator is used for cooling of the testing room. Hot water is used for warming of the utility space. Extra heat is exhausted to the outdoor. Six sets of spiral line heaters, which generate heat to simulate the cooling load such as sensible heat from human bodies, OA equipments and solar radiation hourly, are hung from the ceiling in the testing room. Figure 3 is a perspective diagram of the room as seen from the A direction in Figure 2. The structure is a ceiling space, concrete slabs, a CM packed bed, OA floor boards, breathable carpets and a room space from the bottom up. Each thickness is 200 mm, 120 mm, 100 mm, 30 mm at the maximum, 23 mm, 5 mm and 2120 mm respectively. Six openings are provided on the wall. Ventilation routes which simulate ones during day and night can be changed by switching four dampers. Cold air is entered to O1 and flowed out from O2 during the period of cold energy Cold water G Outdoor Indoor CB S O3 7 Hot water Utility space 3520 Window 2120 Exhaust A F M Door Window Testing room 9.15 m 2 Door 2600 O2 O4 O6 O1 O Notes: CB: Control board, F: Fan, G: Hot and cold water generator, : Heat exchanger, M: Flow rate meter, : ump, S: ower supply Figure 2 Schematic diagram and plan of the room Notes: O: Opening, 1: Ceiling space, 2: Concrete slab, 3: Under floor space, 4: CM packed bed, 5: OA floor board, 6: Breathable carpet, 7: Room space Figure 3 erspective diagram of the room as seen from the direction A

3 storage, called N-Mode. O6 is another exhaust opening and is used under conditions in which air is let also into the room space in order to augment the stored heat for CM. On the other hand, during the daytime called D- Mode, air entered from O1 is completely flowed upward into the room space and exhausted from O3 at the top of the room. The air is returned to the ceiling space and exchanges heat between the back side of the concrete slab. 2.2 Characteristics of FMC-CM FMC (Flocculated Micro-Capsule)-CM has been applied to the system as a granulated CM in this system. FMC-CM which was introduced to the previous study showed lower phase change temperature than the appropriate one and couldn t provide improvement of thermal environment in the morning [Takeda et al., 2004]. Thus, FMC-CM which has suitable phase change characteristics is applied in this experiment. Figure 4 shows results of DSC measurements at different temperature changing rates. hase changes of the melting and solidification processes were started around 17.9 ºC to 18.3 ºC and 21.1 ºC to 21.4 ºC, respectively. The difference of specific enthalpy between 15 ºC and 21 ºC was 110 kj/kg on the average. The FMC-CM has a diameter of around 2 mm and an average length of 2 mm. A packed bed of the FMC-CM was supported by a wire netting basket and placed beneath OA floor boards as shown in Figure 5. The packed amount was 12.5 kg/m 2 and the OA floor board (23mm) Breathable carpet (5mm) Temperature changing rate: [K/min] 1500 DSC signal output [µw/mg] Melting process Temperature [ºC] Figure 4 DSC curves of the FMC-CM thickness was around 25 mm. Solidification process Granulated CM (25mm) Concrete slab (120mm) Under floor space (100mm) Figure 5 Construction under the floor in the experimental 3. DEMONSTRATION EXERIMENT USING FMC-CM 3.1 Discussion about storage condition by computer simulation Outline of the computer simulation program The computer simulation program reported in the previous paper solves heat balance equations for each storage mass: granulated CM, concrete and OA floor board, and for each space: room, under floor and ceiling [Takeda et al., 2003]. The program can be used for a two dimensional area as shown in Figure 1. The program also calculates radiative heat transfer between each wall, floor and ceiling surfaces. The angle factor is derived by the net energy exchange, which is the rate of emission of the surface minus the total rate of absorption at the surface from all radiant effects in its surroundings, including the return of some of its own emission by reflection off its surroundings. In addition, a rectangular parallelepiped human model is positioned to simulate a seated person in the center of the room space and radiant heat exchanges are calculated among the surfaces [Horikoshi et al., 1978].

4 Thermal comfort in the room is evaluated by the MV index with a thermal insulation of clothing of 0.7 clo and a relative humidity of 50 % as standard values in summer.

5 3.1.2 arameters used in the simulation program Here, suitable experimental conditions are verified by the computer simulation. Supply air temperature in the under floor space during the N-Mode (T 1in-N ) is varied from 10 ºC to 15 ºC as a parameter. The calculated subject is the experimental testing room as shown in Figure 2. N-Mode is for 9 hours from 22:00 to 7:00 with a ventilation time of 12 times/hour. 70% of the air is circulated in the under floor space and the rest is supplied to the upper direction in order to permeate air space between CM granules and to enhance the stored heat in CM, which is embedded upward to the under floor space. On the other hand, the D-Mode is set for 12 hours from 8:00 to 20:00. The internal heat generation is given as the same as that from the experiment. Air flow rate is changed according to the heat load so that average room temperature at 1.1m high from the floor surface is kept below a set temperature 27.5 ºC. An air conditioner starts to operate when the temperature increases over the set temperature. The characteristics of CM granules are determined from the DSC measurements, such as the phase change range from 18.0 to 21.3 ºC and latent heat amount of 120 kj/kg Results and discussion In each figure of Figure 6, variations of stored heat during night, air conditioner load, load shifting rate and MV at 9:00 are plotted according to T 1in. The top figure shows that stored heat runs short to cover the cooling load during the daytime and decreases with increasing T 1in. Air conditioner load appears in the daytime at T 1in values higher than 13 ºC. Load shifting effect also decreases to less than 0.8 at 15 ºC of T 1in. On the other hand, complete load shifting effect can be observed at lower T 1in such as 10 ºC or 11ºC, while MV drops gradually and results in a slightly cool thermal sensation because of excessive cooling of the sensible thermal storage masses during night. MV remains relatively steady around 0.7 to 0.6 at T 1in values higher than 12 ºC in these calculations. From a condensation point of view, T 1in should be kept at least above 11ºC. Therefore, T 1in conditions between 12 ºC and 13 ºC could provide sufficient storage performance as well as comfortable thermal environment in the following experiment. Stored heat, Air conditioner load [MJ/m 2 ] Load shifting rate [-] MV (9:00) [-] Daytime Nighttime T 1in [ C] Heat loss OA floor board CM Concrete Neutral Slightly cool Cool Cold Figure 6 Variations of stored heat during night, air conditioner load, load shifting rate and MV at 9:00 vs supply air temperature T 1in

6 3.2 Experimental results and discussion Storage performance Demonstration experiments are conducted under the same conditions as ones used in the calculation above. The following results are ones in the 3 rd day and almost in periodical steady states. One result indicates internal heat generation in the room of 2.42 MJ/m 2 and heat gain through surrounding walls of 5 MJ/m 2 and then the total amount of them, that is, daily cooling load, of 2.47 MJ/m 2. Air conditioner load doesn t cause at all during the daytime and the whole cooling load can be covered by the stored cold energy during night. Shown in Figure 7 and Figure 8 are hourly variations of stored heat in each storage mass and stored heat at the end of N-Mode, 7:00. The sum of the stored heat shows nearly equal storage and discharge amounts in one day. Stored heat in the nighttime is 1.11, 1.08 and 0.18 MJ/m 2 in concrete, granulated CM and OA floor board, respectively and is 2.38 MJ/m 2 in total, which nearly reaches the daily cooling load. Stored heat [MJ/m 2 ] OA floor board -0.5 CM Concrete Sum [N-Mode] [D-Mode] 0:00 4:00 8:00 12:00 16:00 20:00 24:00 Figure 7 Hourly variations of stored heat in each storage mass Stored heat during night [MJ/m 2 ] OA floor board CM Concrete 1.16 Experiment Calculation Figure 8 Stored heat during N-Mode Thermal environment in the room Shown in Figure 9 are hourly variations of supply air temperature (T 1in ), surface temperature of the carpet in the center of the room space and air temperature at 1,050 mm high from the floor, which are measured by T type thermocouples. Supply air temperature keeps 12.5 ºC on average during the N-Mode. It can be seen that the temperature falls in the suitable condition described above. The variation of the surface temperature always remains 1 to 3ºC lower than that of the room temperature due to influence of cooled air supplied from the under floor. Vertical distributions of air temperature in the room space are indicated in Figure 10. ISO7730 recommends a temperature difference between 0.1 m high around an ankle and 1.1 m high around a head for a seated person to be within 3 ºC [ISO, 1994]. In this experiment, temperature differences between 0.1 m high and 1.1 m high are 0.7 ºC, 2.4 ºC and 3.0 ºC at 8:00, 10:00 and 12:00 respectively. These values can meet the requirement. Figure 11 also indicates distributions of floor surface temperature taken by an infrared camera. It is seen that the floor surface temperature measured by a thermocouple is 22.1 ºC at 8:00, which is almost the same as the average one taken by the camera. However, the temperatures measured by thermocouples are 22.7ºC and 23.1 ºC at 10:00 and 12:00. These are 0.7 to 1.0 ºC lower than the average temperatures by the camera. It is possible that spiral line heaters, which simulate the cooling load, influence radiative environment on the floor surface Comparison with calculation Comparison between the calculation and the experiment is shown with respect to stored heat during night and the average floor temperature in Figure 8 and Figure 10, respectively. The stored heat from calculation almost agree with the measured one. The floor temperatures also show close values each other in the morning, in which the differences are within 0.2ºC.

7 Temperature [ºC] [N-Mode] Supply air (T 1in ) Room air [D-Mode] Floor surface 10 0:00 4:00 8:00 12:00 16:00 20:00 24:00 Figure 9 Hourly variations of temperatures 2.0 8:00 10:00 12:00 [8:00] Av.21.9ºC Height from the floor surface [m] :Measured temperatures by thermocouples :Calculated temperatures (8:00) (10:00)(12:00) 0.2ºC 0.1ºC 0.2ºC Temperature [ºC] [Enlarged illustration at the floor surface] Floor surface Temperature [ºC] Figure 10 Vertical distributions of measured temperature and comparison at the floor surface between experiment and calculation [10:00] Av.23.4ºC [12:00] Ave.24.1ºC Figure 11 Floor surface temperature taken by an infrared camera Thermal sensation vote in the room Finally, thermal comfort in this system is examined through thermal sensation vote in the room. The experiment was conducted in July 15th and 16th in Four Japanese young people, three women and a man, were adopted as subjects. Each subject experienced two kinds of condition as shown in Table 1. Case1 is conducted in the morning and Case2 is done in the afternoon. Room temperature in Case1 is 26.3ºC, while that in Case2 is 28.4ºC and relatively high. Table 1 Experimental conditions for thermal sensation vote Case1 Case2 Morning 9:00~10:00 Afternoon 15:30~16:30 Room temp. [ C] Relative humidity [%] Air flow rate [times/h]([cm/s]) 6.3 (0.4) 12.3 (0.8)

8 Two subjects enter the room at the same time for one experiment. Fresh air is supplied to the room at a rate of 40 m 3 /h, which is the amount that two subjects require at least. Women wear an underwear, nylon stockings, a blouse with short sleeves and a light skirt. A man also wears an underwear, socks, a short sleeves shirt, a necktie and a normal trouser. They wear canvas shoes in the room. Total thermal insulation of the clothes is around 0.7 clo per person, which is an ordinary value in summer. The subjects stand by for one hour at a next room air-conditioned at 27 ºC before the experiment and then are exposed for one hour in the testing room in both cases. Thermal sensation vote is given by the subjects every 15 min for one hour. The subjects evaluate thermal sensation and comfort sensation according to the 9-grade and 5-grade respectively. Shown in Figure 12 are distributions of thermal sensation and comfort sensation on average for the whole body in the both case. Figure 13 also indicates average thermal sensations of each part. In Case1, thermal sensations for the whole body range from slightly cool to neutral. The subjects also report that the room is neutral to comfortable at that time. Some of women tell slightly cool at calves but others do neutral sensation. It is likely that uncomfortable sensation near the feet in the morning just after thermal energy storage in the floor supply air conditioning system can be generally improved by the use of CM in this system. In Case2, room temperature at 1.1 m high exceeds 28ºC. This results in the votes of neutral to warm sensation for the whole body and those in the warmer side for all parts of the body. In particular, thermal sensation at the hands or the faces, which are exposed to the environment, seems to lead to slightly uncomfortable sensation from some subjects. Contrary, another subjects show slightly comfortable sensation for the same condition. Therefore, the system can provide acceptable thermal environment on the average also in the afternoon by only discharged coolness from the concrete, OA floor board and granulated CM. Warm 2 Comfortable 2 Slightly warm 1 Slightly comfortable 1 Neutral 0 Neutral 0 Slightly cool -1 Slightly uncomfortable -1 Cool -2 Case 1 Case 2 Morning Afternoon Uncomfortable -2 Case 1 Case 2 Morning Afternoon Figure 12 Distributions of thermal sensation (left) and comfort sensation (right) on the average of the whole body (24.4ºC) (26.3ºC) (23.4ºC) Face Shoulders Upper arms Forearms Hands Back Waist Abdomen Thighs Calves Soles of feet (25.9ºC) (24.9ºC) (28.4ºC) Cool Neutral Warm Cool Neutral Warm [Case1:Morning] [Case2:Afternoon] Figure 13 Thermal sensations of each part on the average of the subjects and air temperature near the position

9 4. CONCLUSIONS The following summarizes the results reported in this paper. 1) A demonstration system is constructed with a floor supply cooling system utilized thermal energy storage for granulated CM as well as for building structure during night in order to evaluate the effect of load shifting to the nighttime and thermal environment quantitatively in the proposed system. 2) Calculations intended for the environmental testing room of 9.2m 2 predict that the whole cooling load can be covered by discharged cold energy storing during night without operation of air conditioners during the daytime in conditions of supply air temperatures in the under floor space around 12 to 13ºC. 3) An experiment in the testing room with a supply air temperature of 12.5ºC on average can result in completion of load shifting. The authors confirmed that the calculation give good agreement with the experiment under the similar operating conditions with respect to stored heat in the nighttime and the floor and room air temperatures in the room space. 4) The results of thermal sensation votes from 4 subjects don t show neither cold nor uncomfortable sensations for both the whole body and each part of the body even in the morning. It is found that the proposed system using the granulated CM can improve the uncomfortable coldness especially near the floor in the previous systems. ACKNOWLEDGEMENT This work was supported in part by the Grant for Environmental Research rojects in the Sumitomo Foundation (Research representative: rof. M. Enai of Hokkaido University). Technical cooperation of Mr. Ishiguro at the Mitsubishi aper Mills Limited is greatly appreciated. REFERENCES Akimoto, T., Nobe, T. and Takebayashi, Y. (1995) Experimental Study on the Floor-supply Displacement Ventilation system. ASHRAE Transactions: Symposia SD , ASHRAE. (2003). ASHRAE Handbook HVAC Applications Horikoshi, T. and Kobayashi, Y. (1978). Configuration Factors between a Rectangular Solid as a Model of the Human Body and Rectangular lanes, for Evaluation of the Influence of Thermal Radiation on the Human Body II Characteristics of Configuration factors for the Rectangular Solids. Trans. of A.I.J. 267(5). ISO7730. (1994). Moderate Thermal Environments- Determination of the MV and D Indices and Specification of the Conditions for Thermal Comfort. International Organization for Standardization. Nagano, K., Takeda, S., Mochida, T., Shimakura, K. and Nakamura, T. (2006). Study of a Floor Supply Air Conditioning System using Granular hase Change Material to Augment Buildings Mass Thermal Storage Heat Response in Small Scale Experiments. Energy and Buildings, 38, Takeda, S., Nagano, K., Mochida, T. and Nakamura, T. (2003). Development of Floor Supply Air Conditioning System with Granulated hase Change Materials. roceedings of FUTURESTOCK th International Conference on Thermal Energy Storage, Warsaw, Takeda, S., Nagano, K., Mochida, T. and Shimakura, K. (2003). Thermal Characteristics in Direct Heat Exchange between Granulated hase Change Materials and Air. roceedings of FUTURESTOCK th International Conference on Thermal Energy Storage, Warsaw, Takeda, S., Nagano, K., Shimakura, K. and Nakamura, T. (2004) Application of Granules Including CM for the Floor Supply Air Conditioning System. IEA, 6 th Workshop of ECES IA Annex 17, Advanced thermal energy storage through phase change materials and chemical reactions feasibility studies and demonstration projects. Tokyo Electric ower Company. (2004). TECO ILLUSTRATED FY2004,