Inertial Elements Integration on Thermal Solar Collectors

Transcription

1 Inertial Elements Integration on Thermal Solar Collectors A. Bejan1, C. Croitoru1, F. Bode1,2, I. Nastase1, M. Sandu1, A. Labihi1.3 1CAMBI, Building Services Department UTCB University, 66 Pache Protopopescu Av., , Bucharest, Romania 2 Department of Mechanical Engineering Technical University of Cluj-Napoca Romania 3 LP2M2E laboratory, Department of applied physics UCA University, Av. Abdelkrim Khattabi, B.P , Marrakesh, Morocco SUMMARY The study emphasis the concept of a ventilated solar façade for air preheating with thermal inertia elements integration. A Transpired Solar Collector (TSC) is made of metal cladding with perforations, installed at a certain distance from a building wall, thus creating a cavity through which the air is circulating. The metal cladding is heated by the solar radiation from the Sun and ventilation fans create negative pressure in the cavity, extracting the solar heated air through the perforated panel. The heat transfer between the fluid and the metal is intensified depending on the flow's characteristics and other external parameters like the special geometry of the perforation used in this case. Studies have shown that the change of these perforations geometry can be an important factor and leads to heat transfer increase. Thermal behaviour of TSC with latent heat storage was experimentally investigated and the results show that thermal storage contributes to higher stability of the air temperature at the outlet of the collector and to better overall efficiency of the collector with thermal storage. INTRODUCTION The current world energetic context contains a very wide framework of regulations, directives and conditions that will have to be met by the states and especially by the members of the European Union regarding the energy efficiency of buildings and greenhouse gases emissions. According to COP21 Conference (Gütschow 2015, Rajgor 2016) all the members of The United Nations must contribute in order to limit the global warming to 2 C above preindustrial levels by Also, all the members of the European Union must accomplish a 20% reduction of CO2 emissions by 2020 and, moreover, a 40% reduction of CO2 emissions by 2030 (EU 2014). Considering the Energy Performance of Buildings Directive (EU 2010) from 2020 all the new buildings must be nzeb (nearly zero energy building) and a part of the energy consumption must be covered by systems using renewable energy sources because the buildings are responsible for 40% of the energy consumptions worldwide (UNEP 2009) and more than 52% of these consumptions are because of the HVAC systems (Hajdukiewicz and Goggins 2014). Therefore, according to the above mentioned, it is very important to achieve the indoor comfort in the buildings with minimum energy consumption. These goals can be met only by using high performance materials, cost-effective energy efficient systems and systems based on renewable energy sources. According to the literature, the solar air collectors acting as a part of the building envelope are a promising strategy that could be used in order to reduce the energy consumption for heating the buildings or for the fresh air needed in the buildings, for drying systems or to improve the efficiency of HVAC systems (Dymond and Kutscher 1997, Alkilani, Sopian et al. 2011, Nkwetta and Haghighat 2014, Zhang, Tan et al. 2016). Transpired solar air collectors (TSC) are systems recommended because of their efficiency and reduced implementation/operating costs (Wang, Lei et al. 2017). Also, the transpired solar air collectors are more efficient than the plane or corrugated collectors (Chan, Zhu et al. 2014). Many studies conducted by researchers are emphasizing that solar collectors could reach a rise in temperature (between inlet and outlet) from 3 C to 35 C and a collector efficiency from 25% to 75% depending of the air flow, solar radiation, absorber plate, orifices type, pitch etc.(wang, Lei et al. 2017) (Leon and Kumar 2007, Cordeau and Barrington 2011, Croitoru, Nastase et al. 2016, Croitoru, Nastase et al. 2016). Usually, the transpired solar collectors are without thermal inertia materials integrated. According to different studies, the integration of thermal storage is essential in order to increase the operation time and efficiency of a solar air collector (Kisilewicz 2009, Hami, Draoui et al. 2012) and also to stabilize the outlet temperature. The energy could be stored during the day and released during the night (Goyal, Tiwari et al. 1998). Thermal energy storage materials are classified as materials with: sensible heat storage, latent heat storage and chemical heat storage (Khadiran, Hussein et al. 2016). Materials with sensible heat storage are classical materials used on a large scale (concrete, water, bricks, granite etc.), materials with chemical heat storage are not used usually in the buildings due to their instability and materials with latent heat storage, also called Phase Changing Materials (PCM), have a higher storage capacity and can store 5 to 14 times more energy than classical materials (Kuznik, Virgone et al. 2011, Soares, Costa et al. 2013, Kylili and Fokaides 2016). PCM materials are implemented in passive systems (external walls, internal walls, floors, windows etc.) and in active systems (solar collectors, boilers, fan coils etc.) (Heier, Bales et al. 2015) and they have the potential to: reduce energy consumptions for heating (especially in low inertia buildings), reduce energy consumptions for cooling, reduce thermal loads and improve the efficiency of active systems. page 552

2 The thermal energy storage (TES) is possible by a simple increase in temperature of a material. The energy is then stored in the form of sensible heat. However, to store a important amount of energy, this method needs high temperature levels. This is because the specific heat of most materials is relatively low. Another way of storing thermal energy consists in using a PCM. The energy is then stored in the form of latent heat through the melting of these materials and then released through the solidification process. Usually PCM as thermal storage are implemented in the glazed solar air collectors such as Trombe walls in order to replace classic masonry (Tyagi and Buddhi 2007) to improve the efficiency and outlet temperature stability (Li 2013, P. Charvat, O.Pech. et al. 2013) but, according to the literature studied, we didn t find a TSC with PCM integrated which acts like a solar wall (Alkilani, Sopian et al. 2011, Shukla, Nkwetta et al. 2012, Nkwetta and Haghighat 2014, Zhang, Tan et al. 2016). This paper s objective is to evaluate the thermal behavior of an innovative TSC with and without PCM integrated on the back of the TSC system. EPERIMENTAL SETUP Experimental setup consists mainly of an air cavity, a perforated plate, insulation material, thermocouples, a fan and a data acquisition system from ALMEMO. A hole with the diameter of 15cm has been cut on the back of the wall in order to connect our solar collector to a suction fan via a tube. This will draw in the exterior air into the collector cavity through the metal absorber orifices and allow it to warm up. The warm air will mix inside the cavity. In order to ensure homogeneity of the air discharged by the system, we have installed a downstream mixing box that includes the fan allowing the airflow we also used a DOMOVENT 100X1 circulator of 14W allowing 2300 rotations/min. The airflow circulated is 63.4m3/h per 1m2 of solar collector, in agreement with previous results (Croitoru, Nastase et al. 2016, Croitoru, Nastase et al. 2016). The comparison was made in laboratory conditions, where the thermal effect over the outlet air was studied. For the same solar collector type (a metal plate with x and + perforations- lobed) it was studied the influence of phase changing materials integration. Six aluminum rectangular bars (figure 2), painted in black were filled with PCM RT35 from Rubitherm, a long-life product, with high thermal energy storage capacity and with stable performance through the phase change cycles, chosen for the melting range close to our scenario. RT35 is an organic material (paraffin) and its key specifications are presented in table 1. The bars contain approximate 0.83 liter of PCM each, since the bars have the following dimensions 20x60x800 mm with 2 mm thickness of the walls. In this study, the enclosure cavity has 1000mm x 500mm x 150mm, as interior geometric dimensions. It was heated across a vertical plate with special lobed orifices in order to introduce the air inside. The working principle of the transpired solar collector is used in this study is presented in figure 1. The opposite facade was insulated and presents a hole of 15 cm as diameter which relates to a fan. The remaining side walls were thermally insulated. The solar collector was heated by solar radiation, while the air is aspirated through this collector, exchanging heat by forced convection with the ambient temperature. Two K-type thermocouples with accuracy of 0.2 C were assembled in the middle of the plate perforated and in the middle of the cavity and two others were placed in the center of the outlet and in ambient air, protected from radiation. Temperature was constantly monitored for loading data s every 60s via data acquisition system. Figure 2. Aluminum bar filled with PCM, 80 cm long (right image: the bar painted in black) Table 1. RT35 technical specifications Specification Value Unit Melting area C Solidification area C Heat storage capacity 160 kj/kg Specific heat capacity 2 kj/kgk Density in solid phase (15 C) 0.86 kg/l Density in liquid phase (45 C) 0.77 kg/l Heat conductivity 0.2 W/mK The 800 mm long bars were vertically placed on the back of the box, as seen in Figure 3. The tests were conducted in the laboratory by simulating solar radiation using six halogen lamps of 500W power each. The lamps are positioned on a metal frame at a distance of 15cm (Figure 4) and they generate 500W/m2 solar radiation on the metal plate at 80 cm distance from the solar collector. Figure 1. Solar collector with lobed perforations and inertial elements integration: the concept page 553

3 with lamps turned on and off. For each step, a period of 3 hours was used in order to obtain the permanent regime. The system without PCM entered quickly in permanent regime, after 30 minutes, as seen in the Figure 6. We can observe that the system has stabilized rapidly and approximately constant values were measured for all 4 temperatures. 6 PCM bars After 3 hours with the lamps on, we have turned the lamps off and let it another 3 hours, in order to simulate a sunny period and a cloudy period or a period where the sun radiation is absent. At the end of the interval, all 4 temperatures have approximately the same value. The outlet temperature has a value of 64 C while the ambient temperature has a value of 33 C, indicating a temperature increase of 31 C. After the lights were off, in about 20 minutes the thermal effect of the radiation cannot be observed anymore. Figure 3. Solar collector without and with PCM bars Figure 4. Solar lamps simulator (left) and the transpired solar collector under simulated solar radiation (right) Four sensors were positioned strategically in order to evaluate the thermal effects. Tamb is the temperature of the ambient air (exterior), Tplate is the temperature of the perforated absorber (metal plate), Tout is the temperature of the air exhausted from TSC (outlet) and Tcavity is a sensor placed in the center of the box. The experimental setup as described before is fully represented in figure 5. Figure 6. The temperature evaluation for all 4 sensors (case without PCM) This is why we have decided to carry out a measurement campaign comparing the case with and without phase changing materials (PCM) implemented on the back side of the solar collector, to see if the PCM can induce a longer period of heating after the radiation is off. We can observe in Figure 7 that the perforated collector has heated less since a radiative transfer had taken place between the PCM bars and the metal collector. We also observe that after a stable temperature increase period (I), another increase in temperature occurs at minute 105 (II). An explanation might be that during the phase I, the PCM starts to melt, accumulating heat, from the heated air and the metal plate. Figure 5. Experimental setup RESULTS AND DISCUSSIONS The first measurement phase consisted in collecting data for the system without phase changing materials implemented Figure 7. The temperature evaluation for all 4 sensors (case with PCM) page 554

4 When the PCM is completely melted, the heat is entirely transferred to the metal plate and the air. For the phase I, the outlet temperature has a value of 59 C while the ambient temperature has a value of 30 C, indicating a temperature increase of 29 C. In the phase II, the outlet temperature has a value of 62 C while ambient temperature is at 31 C, indicating a temperature increase of 31 C. During the period when the lights are off, the difference between the 2 cases can be seen on the temperature of the outlet which will be higher for a longer period (60 minutes instead of 20 minutes for the case without PCM). In case of using phase changing materials in the transpired solar collector, when the radiation is on, the temperature on the metal plate is lower than in the case without PCM which means that the heat transfer between the metal absorber and the air from the cavity/back layer with PCM is enhanced (figure 8). after the complete melting occurs and probably the accumulated heat is now transferred to air (II), leading to a higher temperature difference in phase II with 3 C. After the radiation is off, we can see that the temperature difference between ambient air and outlet air is larger with up to 5 C for approximately 80 minutes, indicating the possible advantage of using PCM in such systems by lagging the heating transfer from the moments when the solar radiation is available to the periods when there is a lack of solar radiation. Furthermore, as it can be observed in figure 9, the outlet air temperature is lower in the stage (I) of the experimental study in case of using PCM resulting a value of temperature which could improve the indoor thermal comfort. In stage (II) of the experimental setup, the rise in temperature is bigger in case of using phase changing material within the cavity of TSC with up to 10% more. Also, in stage (III) of the experimental setup the thermal effect of the radiation is four times longer in the case of the TSC with PCM implemented. In order to understand better and to analyse the phase changing cycle, respectively the melting and solidification phases in time, longer heating and cooling periods are needed. A longer heating period could increase the rise in temperature in the phase II, could also increase the period when the outlet air temperature is higher than the ambient air temperature in the phase III and finally could increase the global efficiency of the transpired solar collector. Moreover, longer heating and cooling periods corroborated with a bigger quantity of phase changing materials implemented within the solar collector could determine a higher outlet temperature during the periods when the solar radiation is not sufficient (e.g. during night-time). Figure 8. Temperature difference between the metal plate and ambient air for the 2 cases If the influence of the PCM cannot be observed for the metal plate temperature after the radiation is off (figure 8), the real interest in using phase changing materials is underlined when analyzing the difference between the outlet temperature and the ambient air (Figure 9). CONCLUSIONS We have chosen to add the PCM RT35 to enhance the thermal inertia inside a transpired solar wall to store the surplus energy. The tests were done for a heating period of three hours with halogen lamps, while after we turn off the heating and let the system cool for a same duration. The results indicate the interest in using PCMs when coupling with solar systems to overcome more the Sun radiation instability. For different cases, we have observed that the PCM elements are able to gain more heat and then to release it later when the solar radiation is not available (during cloudy period or during the night-time). By integrating organic PCMs in rectangular aluminium bars placed on the back side of a transpired solar collector we have obtained very interesting results. First of all the rise in temperature is bigger in case of using phase changing materials, with about 3 C which means almost 10% more. Also, when the radiation is off, the PCMs are slowly releasing the energy and the outlet air temperature is 5 C higher than the ambient air for more than 80 minutes in the case with PCMs. Future studies will engage larger periods of heating, in order to observe better the melting/solidification phases. Moreover, the efficiency of the solar collector will be assessed. Different types and quantities of PCM will also be investigated. Figure 9. Temperature difference between the metal plate and ambient air for the 2 cases We can see that if in a first phase (I), the case without PCM has a larger temperature difference, the situation is changed ACKNOWLEDGMENT This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS UEFISCDI, project number PN-III-P2-2.1-PED page 555

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