USE OF MICROENCAPSULATED PCM IN BUILDINGS AND THE EFFECT OF ADDING AWNINGS

Transcription

1 USE OF MICROENCAPSULATED PCM IN BUILDINGS AND THE EFFECT OF ADDING AWNINGS ABSTRACT C. Castellón, M. Nogués, G. Pérez, M. Medrano, L.F. Cabeza Centre GREA Innovació Concurrent Edifici CREA, Universitat de Lleida, Pere de Cabrera s/n, Lleida (Spain) Phone: , Fax: Since the 1970s, several researchers have tried to use Phase Change Materials (PCM) for thermal storage in buildings. The idea studied was to integrate a PCM in construction materials. An innovative concrete with commercial microencapsulated PCM was developed and a cubicle was constructed with this new PCM-concrete. A second cubicle with the same characteristics, but built with conventional concrete, was located next to the first as a reference system. During 05 to 08, the behaviour of both cubicles was tested. Temperature oscillations were reduced up to 4 ºC between both cubicles and peak temperatures in the PCM cubicle were shifted to later hours during spring and autumn. In summer, the high peak of outdoor temperatures and hot nights prevent the solidification of the PCM. This paper describes the effect of adding awnings in the cubicles to reduce the high temperatures of the walls and roof during summer. 1. BACKGROUND Energy is becoming one of the most important issues in our society. The shortage of primary energy, the cost of fossil fuels, and environmental concerns are the main factors that have prompted research into new and more efficient energy systems. Thermal energy storage systems have become a key technology because they provide several alternatives. Among the new alternatives for heating and cooling systems, thermal energy storage in phase change material (PCM) systems has gained importance in recent years. More detailed information can be found in Zalba (03) and Khudair (04). Phase change materials can absorb or release large amounts of latent heat working in narrow margins of temperature. A high energy density and isothermal behaviour during charging and discharging are the main reasons for PCMs to be preferred. PCMs have been considered for thermal storage in buildings since before There are three different ways to use PCM for heating and cooling in buildings: PCM in building walls, in building components and in heat or cold storage units. PCMs in buildings walls and PCM in other building components than walls are passive systems, where the heat or cold stored is automatically released when indoor or outdoor temperature rises or falls beyond the melting point. The third one, the PCM in heat and cold storage units, is an active system where the stored heat or cold is in a thermally container separated from the building by insulation. Therefore, in these cases the heat or cold is used only by demand. Depending on where and how the PCM is integrated, PCM with different melting points are applied. In the literature, development and testing were conducted for prototypes of PCM wallboard and PCM in concrete systems to enhance the thermal energy storage (TES) capacity of

2 standard gypsum wallboard and concrete blocks, with particular interest in peak load shifting and solar energy utilization. More detailed information can be found in Khudair (04) and Hauer (05). Several researchers have investigated methods for impregnating gypsum wallboard and other architectural materials with phase change materials. More detailed information can be found in Salyer (1985), Shapiro (1987), Babich (1994) and Banu (1998). 2. OVERVIEW OF THE PREVIOUS WORK A new application of the PCM in buildings was studied at the University of Lleida. An innovative concrete with commercial microencapsulated PCM (Micronal BASF PCM: melting temperature ºC, a phase change enthalpy of 1 kj/kg, microcapsules size: 5 µm) was used. First, the new building material was prepared mixing microencapsulated PCM with the concrete (it contains about 5% in weight of PCM mixed with the concrete). The next step was to construct a small house-sized cubicle (2.4x2.4x2.4 m) with this new PCM-concrete to simulate real conditions. PCM was included in West and South walls and on the roof. In order to compare the new material, a second cubicle with the same characteristics and orientation, but built with conventional concrete, was located next to the first as a reference system. More detailed information can be found in Cabeza (07b). The effect of ventilation was studied opening and closing windows at certain times of the day. Experiments were the same in both cubicles. First results in a typical continental weather climate such as the one in Lleida or Madrid (Spain) were very promising. Temperature oscillations were reduced up to 4 ºC between both cubicles and peak temperatures in the PCM cubicle were shifted to later hours during spring and autumn. From those experiments was also concluded that PCM will only have an effect if the whole cycle takes place, that is, if the PCM solidifies and melts every day and night ventilation can help this to occur. Because of the fact that in summer and winter the whole cycle does not take place PCM does not work properly. In late autumn and winter the outdoors ambient temperatures do not achieve the melting temperature of the PCM. Later on, a Trombe wall was added to solve the winter problem. The Trombe wall was built in front of the south wall in order to prove the feasibility of activating the PCM (melting) during late autumn and winter, when outdoors temperatures are lower than the melting point of the PCM. Experiments were performed during late autumn and winter 07. Results showed that it is possible to activate the PCM with the Trombe wall during this period. More detailed information can be found in Cabeza (07a). In summer, the high peak of outdoor temperatures (around ºC) and hot nights during summer prevent the solidification of the PCM. Therefore, the goal of this paper was to test the effect of adding awnings in the cubicles during summer to reduce the high temperatures of the walls and see the PCM effect. 3. METHODOLOGY One way to activate the solidification of PCM during summer time is to decrease the number of hours of direct radiation in the walls of the cubicle. To fulfill this, a new experimental set up to provide shadows in the South, West, and East walls and also the roof of both cubicles during June, July and August was designed. Shadows will be provided with an awning at the top of the cubicles, as shown in Figure 1 (left). To evaluate the dimensions of the awning the Sun trajectory in June, July, and August was assessed using the solar altitude (α) and the solar azimuth (γ). Results indicated that a 4.4

3 m x 4 m awning installed cm above the roof of the cubicles (3 m x 3 m) would be appropriate. Figure 1 (right) shows the experimental set up. North 3 m cm West 3 m 4 m East 1 m South Figure 1. Left: View of the cubicles with awning; July th, 08 at 13: pm. Right: A bird's eye view of the roof With this awning East wall will be partially covered during the morning (at am half covered).then, East and West walls will be totally covered from 1 pm to 3 pm. The West wall will be partially covered in the afternoon (at 5 pm half covered). Half of the South wall will be shadowed at am, being totally covered at noon, and partially covered again in the afternoon. The roof will be protected all day long. The experimental sequence conducted during 08 with the cubicles with awning was the same that was conducted in previous works presented by Cabeza (07a), which is summarized as follows: a) Open windows, the windows were opened all day (only the windows in the South wall could be opened). b) Closed windows, the windows stayed closed all day. c) Free cooling, windows remained opened during the night and closed during the day. 4. RESULTS AND DISCUSSION A comparison for the cubicles with and without awning is shown. First, results of the cubicles without the awning and open windows are presented in order to see the PCM effect. Figure 2 (left) shows the temperature evolution of the South wall and outdoor temperature during the last week of July 08. This illustrates four important points: The maximum temperature of the cubicle without PCM is 1.5 ºC higher than the one of the cubicle with PCM, and its minimum temperature is 2 ºC lower. At around ºC the phase change of the PCM can be observed. From July 27 th to 29 th the solidification of the PCM was not completed due to the higher outdoor temperatures. At ºC, the temperature of the wall containing PCM shows a delay of 2 hour than the corresponding wall without PCM: the thermal inertia of the PCM wall is higher (in other season the difference arrives up to 3 hours). A delay of 2 hours in the heat curve would mean a decrease in the electrical consumption due to air conditioning. Outdoor temperature is 4-6 ºC lower than temperatures of the South wall (with and without PCM). Second, the same experiment was done with the awning over the cubicles. Results for the South wall and for the open window experiment are presented next. South wall temperatures 4.4 m

4 and outdoor temperatures over the first week of August are shown in figure 2 (right). It can be observed that the difference of temperature between outdoor and the South wall decreases respect to figure 2 (left). Plus, in some days the outdoor temperature was higher than the one in the South wall because of the awning effect. 45 Open windows without awning- July Open windows with awning - August 08 Temperature [ºC] Temperature [ºC] outdoor T. 15 outdoor T. /07/08 23/07/08 /07/08 25/07/08 Date /07/08 27/07/08 /07/08 29/07/08 02/08/08 03/08/08 04/08/08 05/08/08 Date 06/08/08 07/08/08 Figure 2. Left: outdoor temperatures and South wall temperatures (open windows) without awning. Right: outdoor temperatures and South wall temperatures (open window) with awning. 08/08/08 In order to draw conclusions it is necessary to present and compare results of days with similar outdoor temperatures and solar radiation. Figure 3 shows solar radiation and outdoor temperature values for August 6 th, 7 th 08 (with awning) and July th, 27 th 08 (without awning); both cases were performed with open windows. Outdoors conditions were similar; maximum temperatures around 35- ºC and minimum around ºC. In both experiments the maximum solar radiation was around 900 W/m 2. Outdoors temperature [ºC] July th, 27 th 08 August 6 th, 7 th 08 Outdoors Temperature 00 Solar July th, August 6 th, radiation 27 th 08 7 th 08-0 :00 :00 :00 Figure 3. Different days: August 6 th, 7 th and July th, 27 th with similar solar radiation and similar outdoor temperature Results for the open windows experiment with and without awning and for the South, and West walls and for the roof are described next. Temperatures in the South wall of the standard cubicle and the cubicle with PCM with and without awning are shown in figure 4. It can be observed that when the PCM and no PCM cubicles are covered with the awning, the temperature of the wall decreases around 5 ºC to 6 ºC and therefore, the solidification in the PCM cubicle was completed. Due to the increased thermal inertia of the PCM-concrete, for example at ºC, the walls containing PCM show a delay of 1 hour (without awning) and 1.5 hours (with awning) than the corresponding wall without PCM Solar radiation [W/m 2 ]

5 Temperature [ºC] Open windows July 08 :00 :00 :00 Open windows with awning - August 08 Figure 4. Comparison between South wall temperatures: with awning and without awning. Experiment with open windows Temperatures for the West wall for open window and with and without awning are presented in figure 5. One can see the same awning effect in the cubicles with and without PCM. Looking at the maximum temperatures of the wall a reduction around 4 ºC is observed. When looking at the minimum temperatures the decrease is around 1.5 ºC. Comparing these results with the once depicted in figure 4 for the South wall it can be seen that in both cases there is a decrease of the wall temperatures when using awning. However, the decrease is lower in the West wall than in the South, where there were more hours with shadow and the decrease was around 5-6 ºC for both maximum and minimum temperatures. Temperature [ºC] Open windows July 08 :00 :00 :00 Opening windows with awning - August 08 Figure 5. Comparison between West wall temperatures: with awning and without awning. Experiment with open windows Finally, results for open window and with and without awning for the roof are presented in figure 6. Since the roof was covered with shadow all day long, reductions of temperature were very significant. For the maximum temperatures a reduction of 11 ºC was observed with the awning. Similar minimum temperatures were detected with and without awning during the night.

6 Temperature [ºC] Open windows July 08 :00 :00 :00 Opening windows with awning - August 08 Figure 6. Comparison between roof temperatures: with awning and without awning. Experiment with open windows Results for the second experimental sequence described in the methodology section are presented here. Figure 7 shows a comparison between South wall temperatures with and without awning of different days with similar ambient conditions with closed window. For the maximum temperatures a reduction of 5-6 ºC was observed with the awning. Looking at minimum temperatures, a reduction of 1.5 ºC was seen. Temperature [ºC] 42 Close windows August 07 Closed windows with awning - August 08 :00 :00 :00 Figure 7. Comparison between South wall temperatures: with awning and without awning. Experiment with closed windows Results of the free cooling experiments with and without awning are shown in figure 8. Since no data with similar outdoor temperature and solar radiation have been found yet, data for two different days with similar outdoor temperature is presented. For the maximum temperatures a reduction of 6 ºC and 1 ºC, at the first and second day respectively, was observed with the awning. Looking at minimum temperatures, a reduction of 1.5 ºC was seen in both days.

7 Temperature [ºC] 42 Free cooling July 06 Free cooling with awning - Sept 08 :00 :00 Figure 8. Comparison between South wall temperatures: with awning and without awning. Free cooling experiment 5. CONCLUSIONS This paper shows the work done to test the effect of adding awning in the cubicles during summer to reduce the high temperatures of the walls and see the PCM effect. In summer 08 the Sun trajectory was explored and an awning at the top of the cubicles was built in order to provide shadows to the East, West, and South walls as well as the roof, with the goal of activating the solidification of the PCM. With the designed awning (4 x 4.4 m) East wall will be partially covered during the morning (at am half covered).then, East and West walls will be totally covered from 1 pm to 3 pm. The West wall will be partially covered in the afternoon (at 5 pm half covered). Half of the South wall will be shadowed at am, being totally covered at noon, and partially covered again in the afternoon. The roof will be protected all day long. An experimental sequence: open windows, closed windows and free cooling were carried out. Results show a decrease in the difference between the outdoor temperature and the South wall temperature, when using the awning. Plus, in some days the outdoor temperature was higher than the South wall temperature because of the awning effect. To validate the set up comparisons with and without awning of different days with similar solar radiation and outdoors temperature were done. Results for the open windows experiments with and without awning and for the South and, West walls and also for the roof are presented. First, it can be observed that when both the PCM and no PCM cubicles are covered with the awning, the maximum and minimum temperature of the walls decreases around 5 ºC to 6 ºC in the South wall, 4 ºC and 1.5 ºC in the West wall, and 11 ºC and 1 ºC in the roof. Therefore, the solidification in the PCM cubicle was completed. Results for the closed windows experiment show a reduction of 5-6 ºC for the maximum temperatures, when using the awning. When looking at minimum temperatures, a reduction of 1.5 ºC is achieved. For the free cooling experiment, since no data with similar outdoors conditions have been found yet, a comparison between two different days with similar outdoor temperature is

8 presented. Looking at maximum and minimum temperatures a reduction of 6 ºC and 1 ºC is seen, respectively. From this work a very important conclusion can be drawn. The effect of the awnings in cubicles allows the use of PCMs for a longer time and therefore contributes to reduce the energy demand. ACKNOWLEDGMENTS The work was partially funded by the Spanish government (project ENE C02-01/CON). Dr. Marc Medrano would like to thank the Spanish Ministry of Education and Science for his Ramon y Cajal research appointment. REFERENCES Babich M.W., Benrashid R., Mounts R.D., DSC studies of energy storage materials. (1994) Part 3. Thermal and flammability studies. Thermochimica Acta Banu D., Feldman D., Haghighat F., Paris J., Hawes D., Energy-storing wallboard: flammability tests. (1998) J. Mater Civ Eng : Cabeza L. F., Medrano M., Castellón C., Castell A., Solé C., Roca J. and Nogués M. (07a), Thermal energy storage with phase change materials in buildings envelopes, Contribution to Science, 3 (4): Cabeza L. F., Castellón C., Nogués M., Medrano M., Leppers R., Zubillaga O. (07b), Use of microencapsulated PCM in concrete walls for energy savings. Energy and Buildings Hauer A., H. Mehling, P. Schossig, M. Yamaha, Cabeza L.F., Martin V., Setterwall F. (05), International Energy Agency Implementing Agreement on Energy Conservation through energy storage. Annex 17 Final Report. Khudair A. M., Farid M. M. (04), A review on energy conservation in building applications with thermal storage by latent heat using phase change materials. En Conv & Management 45: Salyer I.O., Sircar A. K., Chartoff R.P., Miller D.E. (1985), Advanced phase-change materials for passive solar storage applications. In: Proceedings of the th Intersociety Energy Conversion Engineering Conference. Warrendale, Pennsylvania, USA, pp Shapiro M., Feldman D., Hawes D., Banu D. (1987), PCM thermal storage in drywall using organic phase change material. Passive Solar J Zalba B., Marín J. M., Cabeza L. F., Mehling H. (03), Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Applied Thermal Engineering 23:251-3.