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Experimental assessment of Phase Change Material (PCM) embedded bricks for passive conditioning in buildings
Renewable Energy 149 (2020) 587e599
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental assessment of Phase Change Material (PCM) embedded bricks for passive conditioning in buildings Rajat Saxena, Dibakar Rakshit*, S.C. Kaushik Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2019 Received in revised form 19 November 2019 Accepted 17 December 2019 Available online 20 December 2019
This study aims at providing a formidable solution to rapid increasing building energy demands. It projects Phase Change Material (PCM) incorporated bricks as a passive solution for cooling load abatement. The PCMs for this research are selected based on their thermal characteristics through Differential Scanning Calorimeter (DSC) and climatic conditions of the place. In this study, the experimental testing of PCM bricks under actual conditions, followed by, assessing the impact of various PCM configurations is carried out. The experiments are carried out for peak summer conditions, with ambient temperature above 40 C, during the day. The temperature reduction of 4 Ce9.5 C is observed across single and dual PCM layer bricks, compared to the conventional ones. The heat transfer reduction between 40% and 60% is observed, during the day. These bricks are also used to determine the effect of increasing the PCM thickness and using it in combination with fins, to assess the impact in terms of temperature and heat transfer to the inside surface. However, the results showed that using fins has a detrimental impact on temperature and heat flow. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Energy conservation Passive conditioning Phase change materials Differential scanning calorimeter Characterization Macro-encapsulation
1. Introduction Energy exists in many forms of which thermal energy is most researched field worldwide, as storing this low-grade energy helps in minimizing the losses and it can be stored over longer durations. This study however deals in heat storage during day and disseminate it during the night. The specific application lies in buildings, incorporated with Phase Change Materials (PCMs). The PCM gets charged during the day, and it must release the stored heat, during night, to absorb heat on the following day [1]. There are studies, that use PCMs, to assist in reducing the heating load during the night [2e4] and show significant impact in terms of temperature fluctuation and energy savings. This study specifically deals in assessing the cooling load reduction for peak hours of the day, during summer. When sun is overhead, the temperature within any building rises at a brisk pace. The incorporation of PCM increases the heat capacity of the walls/roof, thus, restricting the temperature rise and amplitude of temperature variation [5].
Abbreviations: PCM, phase change material; SHS, sensible heat storage; PCT, phase change temperature; LHS, latent heat storage; DSC, differential scanning calorimeter; CFD, computational fluid dynamics. * Corresponding author. E-mail address: [email protected] (D. Rakshit). https://doi.org/10.1016/j.renene.2019.12.081 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
There are studies [6e15] which discuss different PCMs, their properties and various applications. PCM in buildings have been studied for heating load reduction in Netherlands [16], East Tennessee [17], and Montreal [3]. These studies postulate that PCM incorporation results in energy savings in terms of heating load up to 30% for cold climates. These studies clearly indicate that PCM implementation is a feasible solution, to reduce, building heating load. The PCMs selected in all these studies had melting range between 18 C and 24 C (close to comfort temperature). These studies lack discussion on PCM selection and their benchmarking. PCM application for reducing the temperature rise in buildings for hot conditions, have also been analysed in few studies. PCM incorporation increases thermal mass of the buildings, due to their high storage density; thus, a large amount of heat is absorbed for a corresponding, small rise in temperature. A dynamic modelling for PCM embedded building to evaluate the PCM performance in conserving energy, during summer [18], has been carried out. The study shows that an energy saving up to 75% is possible, for the PCM melting temperature, close to ambient conditions. Thermal testing of a test cell to assess the temperature variation through artificial heating, has also been carried out [19]. This study showed significant temperature reduction with PCM incorporation over the test surfaces. All these studies use different PCMs, based on the location. Thus, unlike cold places, PCM selection and
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Nomenclature Ig, Io hi, ho k ts, ti Ksky Cp l, b, d
r a ε
ao DR t
horizontal global and extra-terrestrial irradiance (W/m2) Inside and outer surface convective heat transfer coefficient (W/m2∙ C) thermal conductivity (W/m∙ C) outer and inner brick surface temperature ( C) sky clearance index specific heat at constant pressure (J/kg∙ C) Length, breadth and thickness (m) Density (kg/m3) absorptivity emissivity thermal diffusivity (m2/s) radiation exchange between sky and ambient air temperature (W/m2) time (s)
implementation, is location specific, for hot and temperate conditions. For Indian conditions, simulation study has been carried out to compare between PCM and insulation [20]. The study recommended the use of PCM over insulation, due to increased cooling load reduction tendency. Pasupathy et al. [21]. numerically analysed a PCM incorporated roof for hot and humid conditions of Chennai (India), however the experimental validation was carried out for January month (winter). CFD modelling of PCM incorporated bricks for Rae Bareli location for March 15th (spring season) in India [22] has also been performed. The temperature at three instances, during the day at different sections of the brick, is simulated. This study however lacked suitability assessment of PCM application and its analysis during peak summer conditions. PCMs have low thermal conductivity, which tends to reduce the dispatchability of, PCM based, thermal storage systems. Thus, there are many studies on thermal enhancement of PCMs for reducing their melting and solidification time through different measures, such as, use of fins, adding nanoparticles etc. Bondareva et al. [23]. provided with a 2-d model, for assessing the heat transfer enhancement of PCMs with alumina nanoparticles. Teng et al. [24]. experimentally compared different metal oxide nanoparticles within PCMs and suggested TiO2 to show best results in terms of melting and solidification time. Bondareva et al. [25,26]. discussed numerical modelling for PCM heat transfer enhancement in combination with nanoparticles and fins with inclination. Similarly, Singh et al. [27,28]. discussed the impact of adding graphene along with fins, to mark a melting time reduction by 68% and solidification time by 49%. It is important to note here, that use of nanoparticles for heat exchanger or for circuit cooling may be feasible, however, for building implementation; it may not be economically viable. The present study investigates experimentally, the impact of increasing the heat transfer within bricks, using fins. This study also aims at demonstrating PCM bricks as a feasible solution for passive cooling of buildings. So far, there are studies, which check the impact of PCMs, through simulation for Indian conditions; however, there is a need for studies on selecting a particular PCM for a specific climatic condition followed by assessment, within building elements for different PCM configurations. The basis of PCM selection followed by their impact on temperature reduction has been assessed in this study. First, PCMs are selected, within the appropriate temperature range, followed by experimental characterization through DSC. These PCMs are then macro-encapsulated
and placed within the developed modified bricks and tested for their thermal performance, for different configurations. Fig. 1 shows a schematic plan of how PCM bricks can be prepared and incorporated to form LHS in buildings to enhance their thermal mass. This study overcomes the shortcomings of the earlier studies followed by real-time experiments for PCM incorporated bricks, directly exposed to sunlight during peak summer for composite climate of Delhi. These PCM bricks are used to estimate the impact of single and multilayers of PCMs followed by assessing the impact of using PCM in combination with fins within the bricks. The detailed design of experiments is as shown in Table 1. 2. PCM selection PCM embedded bricks are a sustainable passive solution for building cooling load reduction. PCM incorporation increases the thermal mass of the building elements thereby increasing its thermal inertia [30]. This results in significant lowering in heat flow across the brick. During day, PCM stores most of the solar radiation falling on the surface, allowing only a part of it to flow through, to the inside. During the night, PCM temperature is greater than the ambient thus, the heat flow direction is reversed. The energy stored is discharged, to both, ambient and inside. To ensure PCM charging and discharging daily, it is important to carefully choose the PCM. This is referred to as PCM mapping [31] and benchmarking [32] in which implications of their thermophysical properties, cyclic behaviour, stability, environmental conditions, heat transfer reduction and cost etc. have been discussed. Thus, a detailed analysis is required to find out an optimum PCM, best suited for Indian climatic conditions. Saxena et al. [31]. carried out a systematic numerical analysis to short-list PCMs based on their melting temperature. It is important to mark here that PCMs close to the thermal comfort temperature may not behave suitably. This is because the average temperature of Delhi (composite) is above 32 C during summer, thus, PCMs may not solidify (by discharging heat) during the night. The numerical modelling showed that melting temperature of around 34 C is suitable for application in Delhi, provided they show less sub cooling (i.e. the difference in PCM solidification and melting temperature; solidification starts below the melting temperature). PCMs are characterized using DSC for testing their appropriateness with respect to their solidification/ melting temperatures, and to assess its thermophysical properties such as specific and latent heat capacities. The charging and discharging of PCM depends upon the inherent PCM properties and the temperature variation of a place, which is governed by its climatic condition. The climatic condition is characterised by mean temperature, wind velocity, air humidity, sky condition, precipitation and solar radiation for a location. The radiation on a surface depends on its orientation, location, time of the year, time of the day, etc. These parameters together govern the temperature over any surface (solar air temperature) which is accountable for the heat transfer taking place. The mean maximum and minimum temperature for Delhi is given in Fig. 2. The temperature here in summers can reach up to 46 C and in winters, can fall to 6 C. During summer particularly, the temperature varies from around 42 C during day time and around 27 C during the night, causing huge temperature fluctuation within the building. PCM incorporation within bricks increases the overall thermal mass, which corresponds to increased thermal inertia. To ensure PCM to charge and discharge on a daily basis it is necessary that its PCT lies within the maximum and minimum temperature of the location or else PCM would not behave as LHS material. The PCM selected in Ref. [33] showed melting temperature of 35 C, however, it showed sub cooling of 7 C, when tested through DSC, making it inappropriate for building application.
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Fig. 1. Schematic plan of PCM incorporation within buildings.
Two PCMs, OM35 and Eicosane, are selected based on their temperature of melting as given in the literature. However, there is a lack of consistency observed between the literature cited PCM properties because of non-availability of proper PCM characterisation standards [34]. Thus, to ensure PCM properties and assess their sub cooling, DSC characterisation has been performed. 2.1. DSC experiment. The heat flow analysis for OM35 and Eicosane is performed using DSC-Q2000 (TA, New Castle, USA), having high precision (±0.05%) and thermal accuracy (±0.01 C). The heat absorbed or released by the PCM is given by the difference in the heat flow between PCM sample and the reference. PCMs are melted and placed within Tzero pan and lids. These were weighed on a semi-micro balance (GR-202, A&D, Japan), having an accuracy of 0.1 mg. During the experiment, pure nitrogen, flow rate of 50 ml/min is supplied. PCM is tested for temperature 10 Ce60 C. RCS90 (refrigerated cooling system) is used for controlled rate of heating and cooling. The ramp rate is taken to be 1 C/min in the present study. The complete pre-processing before the DSC experiment and accessories are depicted in Fig. 3.
The DSC thermograms of Eicosane (Fig. 4) and OM35 (Fig. 5), for three cycles have been shown. It is observed that Eicosane and OM35 starts melting at 35.2 C and 33 C, respectively. PCM melting is completed at 37 C for both these PCMs. During the cooling cycle solidification starts at 35 C and 33.6 C and completed at 31 C and 30 C, respectively. They showed relatively low sub cooling effect, of around 2 C, which is significantly lower compared to salts hydrates. At 30 C, solidification of both the PCMs is completed. The latent heats for Eicosane and OM35 are 247 J/g and 159 J/g, respectively. It is seen that melting and solidification temperature for both these PCMs fall within the daily temperature variation in Delhi, therefore, these two PCMs are selected to be embedded in bricks for their thermal testing. The PCM properties for these selected PCMs and their properties are investigated and tabulated in Table 2.
2.1. Sky condition determination based on sky clearance index The condition of the sky is defined by sky clearance index (Ksky), which is as given in Table 3 [35]. It is calculated using Eq. (1).
Table 1 Design of experiments. S. No.
Set 1 [29]
Combinations
Without PCM
Objective 1
Temperature at all the interfaces is recorded for PCM embedded bricks to assess the impact of PCM incorporation within bricks. This study quantifies the energy saving that can be achieved on PCM incorporation within bricks and also compares two PCMs
Objective 2
Set 2 Eicosane (1.2 cm)
OM35 (1.3 cm)
Without PCM
Set 3 Eicosane/OM35 multiple layer (1.1 cm each)
Without PCM
OM35 (finned 2.2 cm)
OM35 (2.2 cm)
OM35 (1.1 cm)
Temperature difference for multiple PCM layers is assessed.
PCM thickness variation is studied along with the finned structure to increase the heat transfer and assist in PCM discharging.
Comparison between with and without PCM bricks and multiple PCM layers is assessed in terms of heat transfer
Assessment of impact on the heat flow with and without fin within the PCM layer
Configuration
Studies Carried ➢ out ➢ ➢ ➢ ➢ ➢
OM35 (1.1 cm)
PCM selection Assessment of PCM impact in terms of temperature difference achieved Comparison of multiple PCM layer with single layer within the brick Impact of increasing the PCM thickness is assessed Impact of finned configuration on PCM charging and discharging rates Temperature fluctuation and heat transfer is calculated for all the cases
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Fig. 2. Mean maximum and minimum temperature of Delhi round the year.
3. Thermal testing of PCM embedded bricks
Ksky ¼ Ig
.
Io
(1)
where, value of Ig is measured by weather station (Ingen tech., India) as shown in Fig. 6. Both the experimental setup and the weather station are installed at rooftop of block V, IIT Delhi, 40 m apart. The solar radiation falling on the setup and weather station, along with the wind velocity, are assumed to be same. Io is calculated from Eq. (2).
Io ¼ Isc sinðas Þ
(2)
The PCM charging and discharging is greatly affected by the sub cooling effect, which is taken into account for accurate assessment of heat flow across the brick in this study. The reduction in heat flow and temperature with PCM incorporation within building components is investigated under peak summer conditions. To perform the experiment, mud bricks with single and double slots are prepared using conventional Indian brick making process, as it is well established and could be scaled for future applications. The slot in the brick is achieved by modifying the mould design. The process is as explained in Fig. 7. The bricks are first prepared using the modified mould and are dried under the Sun for duration of 7e10 days. The slot is then inserted with fuel wood to retai1n its
Fig. 3. (a) Melting of PCM (b) DSC pan and dies (accessories) (c) Crimping machine (d) DSC cell (e) Complete DSC system with RCS90 as cooling system.
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Fig. 4. DSC thermogram for Eicosane (PCM 1).
original shape and size. This is followed by baking of these bricks within a conventional kiln to get the final slotted bricks. Both single and double slot bricks along with the conventional ones are produced using this method for the experimentation. 3.1. Brick geometry and setup development Fig. 8 (a), 8 (d). show the modified bricks with single and double slots. Fig. 8 (b), 8 (e) show galvanized steel encapsulations without
and with fins. Brick outer dimensions are 22.5 cm 12 cm 10 cm and slot of 1.7 cm (thickness) and 16 cm (length), respectively. An ASTM A525 steel (galvanized), thickness 0.4 mm, having dimensions 15.5 cm 9 cm 1.5 cm (Fig. 7 b), is used for PCM encapsulation. The selection of the optimum thickness of PCM is made based on the available literature. The thickness of 1 cm as per Kuznik et al., 2008 [36] assists in increasing the thermal inertia of the wallboard by two times. Similarly, the parametric study carried out by Liu et al., 2017 [37] suggested PCM thickness of
Fig. 5. DSC thermogram for OM35 (PCM 2).
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Table 2 PCM properties. PCM(s)
Cp (liq.) Cp (kJ/kg (solid) (kJ/kg C) C)
Eicosane (PCM 2.46 1) (99% Pure) OM35 (PCM 2) 2.71
Melting Temperature ( C)
Thermal Conductivity (liq.) (W/m C)
Thermal Conductivity (solid) (W/m C)
Latent Heat (kJ/ Density (liq.) (kg/ Density (solid) (kg/ kg) m3) m3)
1.92
36e38
0.15
0.39
247.3
780
815
2.31
35
0.16
0.2
160
870
900
Table 3 Sky Condition relation with Ksky. Sky Clearness Index (Ksky)
Ksky < 0.2
0.2 Ksky 0.65
Ksky 0.65
Sky Condition
Overcast
Intermediate
Clear
of around 1.2 cm is considered for set 1 and set 2 configurations. In set 1 configuration the impact of different PCMs being incorporated within the brick with respect to the conventional brick without PCM, was compared. Set 2 configuration tests the thermal impact of single and multiple PCM layer within the brick. Set 3 configuration is used to test the impact of increasing the PCM thickness from 1.2 cm to 2 cm and also assess the impact of having fins within the macro-encapsulation. To minimize the losses from other sides of PCM bricks (except the exposed surface), they are packed with thick layers of polystyrene (insulation). The gaps are carefully filled with glass wool, to avoid heat leakages, from or to, any other but the top surface having dimensions 22.5 cm 10 cm. This surface is directly exposed to the sunlight, as depicted in the schematic diagram of the test setup (Fig. 9). The one-dimensional heat flow is assumed, first through brick section, then encapsulation cover followed by air gap, PCM, encapsulation and then through the remaining brick. ‘T’ type thermocouples are used for measuring the temperature of different surfaces. These thermocouples were calibrated using ice and boiling water in the laboratory. The temperature accuracy is measured to be ±0.2 C conforming to the accuracy as provided by the supplier. A 16-channel datalogger (NI, India) is used to store the data in excel format using LabVIEW 2018. The temperature is recorded at a time step of 10 min.
Fig. 6. (a) Weather Station control module (b) Weather station installation at rooftop, IIT Delhi.
12 mme30 mm to be suitable for building incorporation. For Indian context, the simulation study by Saikia et al., 2018 [38] also considered 12 mm PCM thickness to assess the PCM impact in different directions within a building. As thickness of PCM increases, the cost also increases proportionally. Thus, PCM thickness
3.2. Heat transfer calculations The assumptions made to calculate heat flow across the brick section are as follows: i. Uni-directional heat transfer, along x, as marked in Fig. 11.
Fig. 7. Steps followed for brick preparation.
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Fig. 8. (a) Single-slot brick (b) Sheet-metal casing for PCM encapsulation (c) Macro-encapsulated PCM (d) Double slot brick (e) Sheet-metal casing with fins (f) PCM incorporated brick with twin slots (g) Installation of experimental setup on the roof top (h) Final setup.
ii. The heat transfer takes place only from the top surface of the brick being exposed to the environment. All other sides are assumed to be insulated adiabatically. iii. Irregularities in dimensions of encapsulation and bricks are neglected.
The solar radiation influx on the surface of the brick increases its temperature higher than the ambient, this is known as the solar air effect [39]. The difference in surface temperature on the outside and on the inside is responsible for heat transfer to occur.
Fig. 9. Schematic view of the test facility for thermal test of bricks.
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vtbrick v2 tbrick ¼a vt vx2
(3)
The heat transfer is determined using heat conduction equation (Eq. (3)) which is simplified to Eq. (4) as the boundary conditions are applied. This is the generalised equation used for calculating the heat transfer [41].
Q ¼ Ueff A ðts ti Þ
(4)
1 1 d d 2 d3 d4 dn 1 …: þ þ ¼ þ 1þ 2þ þ Ueff ho k1 k2 k3 k4 kn hi
(5)
To account for the heat stored within different layers, a capacitance (for an equivalent electrical analogy) has been taken. The circuit of a composite brick is designed with a combination of thermal capacitance and thermal resistances, as shown in Table 4 [40] and illustrated in Fig. 10 which shows the equivalent electrical circuit for a composite brick. This equivalent analogy can be used for calculating the interfacial temperatures, which, however in this case, has been experimentally determined. Most of the incoming heat (Q1) is absorbed by PCM, a part of it is stored by other construction elements, leaving behind only a part of it (Q2), which flows through, to the inside and is represented as shown in Eq. (6). The heat stored in different layers of brick (Qs,i) is given in Eq. (7).
Q2 ¼ Q1 Qs; i Qs; i ¼ m:Ci :
dt dt
(6)
i ¼ 1…:n ðdifferent brick layersÞ (7)
Where, Ci is heat capacity of different layers of brick. All the calculations have been done first without the application of PCM, and then by incorporating the selected PCMs in their respective configurations. Heat flow to the inside is directly calculated using the temperature difference and the resistance between the last two interfaces. Uncertainty Analysis: The uncertainty of heat transferred across the PCM incorporated brick has been computed to verify the accuracy of heat transfer and the estimated effectiveness of the brick. The heat transfer across the brick is a function of area of cross section (A), effective thermal conductivity of the brick (Ueff) and the temperature difference across the thickness of the brick (DT), given by the following equation:
Table 4 Thermal resistances (R) and thermal capacitances (C) for equivalent electrical network model. Interface
1
R
1 h0 Abrick rbrick.Abrick.Cs,1.Dx1
C
2 …. n-1
Dx i kAbrick rbrick.Abrick.Cs,i.Dxi
n 1 hi Abrick Mair. Cs,air
4. Results and discussion Present study is an extension of Saxena et al., 2019 [29] in which the assessment of PCM embedded bricks with respect to conventional bricks has been reported. The temperature reduction up to 6 C for OM35, has been observed and is stated here in this study as set 1 configuration. It was concluded that OM35 was better compared to Eicosane in terms of cost and the heat transfer rate, was comparable in both the cases. This study compares the thermal impact of double slot PCM brick with respect to the single slot PCM and conventional brick. This is classified as Set 2 configuration for experiments as shown in Fig. 11 (a) Brick 1 (B1) is a conventional brick of equivalent size followed by twin slot brick marked as Brick 2 (B2). Two PCMs, Eicosane (PCM 1), thickness 1.1 cm, and OM35 (PCM 2), thickness 0.9 cm, are incorporated in these slots within a sheet-metal encapsulation. Owing to higher melting temperature of PCM 1 it is installed in the outer slot and PCM 2 is kept in the inner slot. Brick 3 (B3) is a single slot brick which is incorporated with macro-encapsulated OM35, thickness 1.1 cm. The comparison of temperature profile of different surfaces for the three bricks, from May 31st to June 5th, 2018, is shown in Fig. 11 (b) The sky condition during the day is mostly intermediate and clear. The results show a temperature reduction up to 9.5 C for double slot PCM brick, whereas a temperature reduction up to 6 C for single slot PCM brick, as also envisaged in Set 1 experiment [29]. The most critical parameter as observed is the temperature during the night, as PCM discharges its heat and becomes available for charging on the subsequent day. It is observed that temperature at night for 3rd June and 5th June was around 30 C, thus, PCM discharging was relatively less. The impact of this is observed on temperature rise on the next day. It is seen when temperature at night falls to around 27 C, which is the mean minimum temperature of Delhi during summer, both these combinations show convincing results. The temperature difference of around 9 C and 6 C on the inside for double and single slot PCM bricks, is observed on 31st May, 1st, 2nd and 4th June. It is also observed that the interface temperatures across the PCMs, falls below the melting temperature during off-sunshine
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 2 2 2 vUQ vUQ vUQ vUQ vUQ vUQ vUQ UQ ¼ Udbrick þ Udair þ UdPCM þ Ulbrick þ Ubbrick þ UTouter þ UTinner vdbrick vdair vdPCM vlbrick vbbrick vTouter vTinner (8)
Where uncertainty of a variable ‘x’ is given by. Ux ¼ maximum error of variable }x} magnitude of the variable ’x’
The maximum possible error in the dimensions of the brick (l, b and d) are ±0.1 mm (least count of Vernier Callipers). The maximum error in temperature measurement using T-type thermocouples through NI, data logger is measured to be ±0.3 C.
hours and rises above the melting temperature during the afternoon hours. This temperature variation obtained, clearly indicates that the PCM undergoes melting and solidification on daily basis which has been the aim for effective utilization of PCMs as discussed under Section 2. The temperature fluctuation within the brick is shown in Fig. 11 (c), which clearly shows that it reduces by 10 C, which is significantly high. Fig. 12 shows the heat transfer-taking place, which clearly
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Fig. 10. Equivalent electrical circuit for a composite brick.
indicates that heat flow to the inside, is significantly lower in case of PCM bricks. Particularly during the day (sunshine hours), the heat flow to the inside is reduced by around 60% for double slot and around 40% for the single slot PCM brick. Over a complete 24 h cycle energy savings between 16% and 20% is observed on all the days for both the cases. As PCM discharges heat during the night, the inside surface temperature of PCM bricks, is observed to be higher. This is due to insulations being provided on the inside. Under more realistic scenarios, the ambient air which is at lower temperature can be utilised for conditioning of the inner space, thus lowering the temperature during the night. Set 3 configuration is used to assess the impact of increasing the heat transfer of PCM on the interface temperatures and heat transfer for July 9th to July 12th (morning), 2018. This configuration also tests the impact of varying the PCM thickness. This experiment uses four bricks as shown in Fig. 13(a) Brick 1 is the conventional
brick followed by Brick 2 with OM35, thickness 1.9 cm, within finned sheet-metal encapsulation. Brick 3 consists of OM35, thickness 2 cm within sheet-metal encapsulation but without fins. Brick 4 is a single slot PCM brick with thickness of OM35 equal to 1.1 cm. The sky condition is predominantly intermediate. With incorporation of PCM, temperature reduction on the inside is observed (Fig. 13 (b)). The temperature fluctuation amplitude is shown in Fig. 13 (c). The reduction in temperature varies from 4 C to 6 C for Brick 4. However, similar impact is not observed in the other two cases. For Brick 3, the temperature reduction is found to be 7 C, which is maximum on Day 1 but on other days, it did not follow the same trend. This is due to higher night temperature (around 29 C) which caused low heat rejection rate during the night. Additionally, due to increased PCM thickness its discharging rate is reduced which inhibits PCM solidification. Therefore, it may be concluded that
Fig. 11(a). Configuration for Set 2 experiment showing location of the thermocouples.
Fig. 11(b). Temperature distribution for Set 2 configuration.
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Fig. 11 (c). Amplitude of temperature variation for Set 2.
increasing the PCM thickness may not be effective unless there is a separate arrangement for PCM discharging. The finned PCM configuration in Brick 2 also do not show favourable results. This is due to the fact that it increases the effective heat transfer through
the PCM, thereby increasing the heat flow rate during the day, thus increasing the temperature more rapidly. During the night, the temperature difference is not sufficiently high for PCMs to get discharged on Day 2 and Day 3. Thus, PCM did not solidify and
Fig. 12. Heat transfer for Set 2 configuration.
Fig. 13(a). Configuration for Set 3 experiment showing location of the thermocouples.
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Fig. 13(b). Temperature distribution for Set 3 configuration.
Fig. 13(c). Amplitude of temperature variation for Set 3.
behaved only as a sensible heat storage on these days. Thus, inside temperature of Brick 2 attained the highest temperature among all the brick on these days. Therefore, increasing the heat transfer of the PCMs may not be a well thought solution, in the case of buildings. Thus, increasing the PCM thickness and its thermal
conductivity, may not be a suitable as in both cases PCM discharging becomes difficult and PCM storage capacity is underutilised. Heat transfer rate is reduced by around 40% during the day (Fig. 14). During night, the heat rejection rate of PCMs is observed to be low due to very less temperature difference
Fig. 14. Heat transfer for Set 3 configuration.
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between the inside and the ambient temperature. About the cost of these PCM bricks, it must be mentioned that major share of the cost is due to the PCM. OM35 (INR 500/kg) is preferred during experiments over Eicosane as its cost is thirty times lower as compared to the Eicosane (INR 2200/kg). Other costs include cost of encapsulation. The PCM encapsulations made up of polyethylene can be a lucrative solution however, during the test runs it was found that leak which was a major issue in them. Polyethylene cannot sustain the abrasiveness of the brick surface and are susceptible to leakage. About the modified bricks, they show good thermal characteristics in terms of temperature reduction and heat transfer to the inside during the day. However, these are still to be tested for their mechanical strength and vapour absorptivity, in order to be available for mass production for building applications in India. With Eicosane as PCM the brick cost was around 2800 INR/kg (all inclusive) which has come down to INR 80 ($ 1.2) with use of OM35 as the PCM. It is important to mention here that with the increase in demand the cost could come down, substantially. 5. Conclusions This study is an effort towards testing of PCM incorporation, as passive storage, within the building elements like bricks. The study illustrates a systematic and scalable method of producing modified bricks for PCM incorporation. The modified bricks are prepared to assess the impact of multiple PCM layers and thickness variation within the bricks. The impact of increasing the effective PCM thermal conductivity on temperature and heat transfer has also been adjudged. The inferences drawn are as follows: 1.) A temperature reduction up to 9.5 C for dual PCM layer within the brick and a temperature reduction of 6 C is achieved for single layered PCM brick. 2.) The heat gain reduction up to 60% is observed for dual PCM layer brick and around 40% for single layer PCM brick, during the day. This however is not the case during the night, when PCMs are rejecting heat. The overall heat reduction between 16% and 20% is observed over the period of 24 h. 3.) The use of fins within the PCM encapsulation affects adversely as the heat transfer during the day increases substantially. The inside brick temperature for these bricks, rises higher, than in case of conventional bricks and PCMs could not get discharged sufficiently, during the night. 4.) Increasing the PCM thickness may not be a viable solution unless the heat rejection during the night is ensured through any auxiliary means apart from using finned configuration, for a building space conditioning. A PCM thickness of from 1 cm to 1.3 cm has been tested experimentally and found suitable in achieving a peak temperature reduction of around 6 C. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study is funded by Department of Science and Technology, India under project no. TMD/CERI/BEE/2016/084(G). Authors also acknowledge Head, CES for his continuous support and help during this research.
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Experimental assessment of Phase Change Material (PCM) embedded bricks for passive conditioning in buildings Rajat Saxena, Dibakar Rakshit*, S.C. Kaushik Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2019 Received in revised form 19 November 2019 Accepted 17 December 2019 Available online 20 December 2019
This study aims at providing a formidable solution to rapid increasing building energy demands. It projects Phase Change Material (PCM) incorporated bricks as a passive solution for cooling load abatement. The PCMs for this research are selected based on their thermal characteristics through Differential Scanning Calorimeter (DSC) and climatic conditions of the place. In this study, the experimental testing of PCM bricks under actual conditions, followed by, assessing the impact of various PCM configurations is carried out. The experiments are carried out for peak summer conditions, with ambient temperature above 40 C, during the day. The temperature reduction of 4 Ce9.5 C is observed across single and dual PCM layer bricks, compared to the conventional ones. The heat transfer reduction between 40% and 60% is observed, during the day. These bricks are also used to determine the effect of increasing the PCM thickness and using it in combination with fins, to assess the impact in terms of temperature and heat transfer to the inside surface. However, the results showed that using fins has a detrimental impact on temperature and heat flow. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Energy conservation Passive conditioning Phase change materials Differential scanning calorimeter Characterization Macro-encapsulation
1. Introduction Energy exists in many forms of which thermal energy is most researched field worldwide, as storing this low-grade energy helps in minimizing the losses and it can be stored over longer durations. This study however deals in heat storage during day and disseminate it during the night. The specific application lies in buildings, incorporated with Phase Change Materials (PCMs). The PCM gets charged during the day, and it must release the stored heat, during night, to absorb heat on the following day [1]. There are studies, that use PCMs, to assist in reducing the heating load during the night [2e4] and show significant impact in terms of temperature fluctuation and energy savings. This study specifically deals in assessing the cooling load reduction for peak hours of the day, during summer. When sun is overhead, the temperature within any building rises at a brisk pace. The incorporation of PCM increases the heat capacity of the walls/roof, thus, restricting the temperature rise and amplitude of temperature variation [5].
Abbreviations: PCM, phase change material; SHS, sensible heat storage; PCT, phase change temperature; LHS, latent heat storage; DSC, differential scanning calorimeter; CFD, computational fluid dynamics. * Corresponding author. E-mail address: [email protected] (D. Rakshit). https://doi.org/10.1016/j.renene.2019.12.081 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
There are studies [6e15] which discuss different PCMs, their properties and various applications. PCM in buildings have been studied for heating load reduction in Netherlands [16], East Tennessee [17], and Montreal [3]. These studies postulate that PCM incorporation results in energy savings in terms of heating load up to 30% for cold climates. These studies clearly indicate that PCM implementation is a feasible solution, to reduce, building heating load. The PCMs selected in all these studies had melting range between 18 C and 24 C (close to comfort temperature). These studies lack discussion on PCM selection and their benchmarking. PCM application for reducing the temperature rise in buildings for hot conditions, have also been analysed in few studies. PCM incorporation increases thermal mass of the buildings, due to their high storage density; thus, a large amount of heat is absorbed for a corresponding, small rise in temperature. A dynamic modelling for PCM embedded building to evaluate the PCM performance in conserving energy, during summer [18], has been carried out. The study shows that an energy saving up to 75% is possible, for the PCM melting temperature, close to ambient conditions. Thermal testing of a test cell to assess the temperature variation through artificial heating, has also been carried out [19]. This study showed significant temperature reduction with PCM incorporation over the test surfaces. All these studies use different PCMs, based on the location. Thus, unlike cold places, PCM selection and
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Nomenclature Ig, Io hi, ho k ts, ti Ksky Cp l, b, d
r a ε
ao DR t
horizontal global and extra-terrestrial irradiance (W/m2) Inside and outer surface convective heat transfer coefficient (W/m2∙ C) thermal conductivity (W/m∙ C) outer and inner brick surface temperature ( C) sky clearance index specific heat at constant pressure (J/kg∙ C) Length, breadth and thickness (m) Density (kg/m3) absorptivity emissivity thermal diffusivity (m2/s) radiation exchange between sky and ambient air temperature (W/m2) time (s)
implementation, is location specific, for hot and temperate conditions. For Indian conditions, simulation study has been carried out to compare between PCM and insulation [20]. The study recommended the use of PCM over insulation, due to increased cooling load reduction tendency. Pasupathy et al. [21]. numerically analysed a PCM incorporated roof for hot and humid conditions of Chennai (India), however the experimental validation was carried out for January month (winter). CFD modelling of PCM incorporated bricks for Rae Bareli location for March 15th (spring season) in India [22] has also been performed. The temperature at three instances, during the day at different sections of the brick, is simulated. This study however lacked suitability assessment of PCM application and its analysis during peak summer conditions. PCMs have low thermal conductivity, which tends to reduce the dispatchability of, PCM based, thermal storage systems. Thus, there are many studies on thermal enhancement of PCMs for reducing their melting and solidification time through different measures, such as, use of fins, adding nanoparticles etc. Bondareva et al. [23]. provided with a 2-d model, for assessing the heat transfer enhancement of PCMs with alumina nanoparticles. Teng et al. [24]. experimentally compared different metal oxide nanoparticles within PCMs and suggested TiO2 to show best results in terms of melting and solidification time. Bondareva et al. [25,26]. discussed numerical modelling for PCM heat transfer enhancement in combination with nanoparticles and fins with inclination. Similarly, Singh et al. [27,28]. discussed the impact of adding graphene along with fins, to mark a melting time reduction by 68% and solidification time by 49%. It is important to note here, that use of nanoparticles for heat exchanger or for circuit cooling may be feasible, however, for building implementation; it may not be economically viable. The present study investigates experimentally, the impact of increasing the heat transfer within bricks, using fins. This study also aims at demonstrating PCM bricks as a feasible solution for passive cooling of buildings. So far, there are studies, which check the impact of PCMs, through simulation for Indian conditions; however, there is a need for studies on selecting a particular PCM for a specific climatic condition followed by assessment, within building elements for different PCM configurations. The basis of PCM selection followed by their impact on temperature reduction has been assessed in this study. First, PCMs are selected, within the appropriate temperature range, followed by experimental characterization through DSC. These PCMs are then macro-encapsulated
and placed within the developed modified bricks and tested for their thermal performance, for different configurations. Fig. 1 shows a schematic plan of how PCM bricks can be prepared and incorporated to form LHS in buildings to enhance their thermal mass. This study overcomes the shortcomings of the earlier studies followed by real-time experiments for PCM incorporated bricks, directly exposed to sunlight during peak summer for composite climate of Delhi. These PCM bricks are used to estimate the impact of single and multilayers of PCMs followed by assessing the impact of using PCM in combination with fins within the bricks. The detailed design of experiments is as shown in Table 1. 2. PCM selection PCM embedded bricks are a sustainable passive solution for building cooling load reduction. PCM incorporation increases the thermal mass of the building elements thereby increasing its thermal inertia [30]. This results in significant lowering in heat flow across the brick. During day, PCM stores most of the solar radiation falling on the surface, allowing only a part of it to flow through, to the inside. During the night, PCM temperature is greater than the ambient thus, the heat flow direction is reversed. The energy stored is discharged, to both, ambient and inside. To ensure PCM charging and discharging daily, it is important to carefully choose the PCM. This is referred to as PCM mapping [31] and benchmarking [32] in which implications of their thermophysical properties, cyclic behaviour, stability, environmental conditions, heat transfer reduction and cost etc. have been discussed. Thus, a detailed analysis is required to find out an optimum PCM, best suited for Indian climatic conditions. Saxena et al. [31]. carried out a systematic numerical analysis to short-list PCMs based on their melting temperature. It is important to mark here that PCMs close to the thermal comfort temperature may not behave suitably. This is because the average temperature of Delhi (composite) is above 32 C during summer, thus, PCMs may not solidify (by discharging heat) during the night. The numerical modelling showed that melting temperature of around 34 C is suitable for application in Delhi, provided they show less sub cooling (i.e. the difference in PCM solidification and melting temperature; solidification starts below the melting temperature). PCMs are characterized using DSC for testing their appropriateness with respect to their solidification/ melting temperatures, and to assess its thermophysical properties such as specific and latent heat capacities. The charging and discharging of PCM depends upon the inherent PCM properties and the temperature variation of a place, which is governed by its climatic condition. The climatic condition is characterised by mean temperature, wind velocity, air humidity, sky condition, precipitation and solar radiation for a location. The radiation on a surface depends on its orientation, location, time of the year, time of the day, etc. These parameters together govern the temperature over any surface (solar air temperature) which is accountable for the heat transfer taking place. The mean maximum and minimum temperature for Delhi is given in Fig. 2. The temperature here in summers can reach up to 46 C and in winters, can fall to 6 C. During summer particularly, the temperature varies from around 42 C during day time and around 27 C during the night, causing huge temperature fluctuation within the building. PCM incorporation within bricks increases the overall thermal mass, which corresponds to increased thermal inertia. To ensure PCM to charge and discharge on a daily basis it is necessary that its PCT lies within the maximum and minimum temperature of the location or else PCM would not behave as LHS material. The PCM selected in Ref. [33] showed melting temperature of 35 C, however, it showed sub cooling of 7 C, when tested through DSC, making it inappropriate for building application.
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Fig. 1. Schematic plan of PCM incorporation within buildings.
Two PCMs, OM35 and Eicosane, are selected based on their temperature of melting as given in the literature. However, there is a lack of consistency observed between the literature cited PCM properties because of non-availability of proper PCM characterisation standards [34]. Thus, to ensure PCM properties and assess their sub cooling, DSC characterisation has been performed. 2.1. DSC experiment. The heat flow analysis for OM35 and Eicosane is performed using DSC-Q2000 (TA, New Castle, USA), having high precision (±0.05%) and thermal accuracy (±0.01 C). The heat absorbed or released by the PCM is given by the difference in the heat flow between PCM sample and the reference. PCMs are melted and placed within Tzero pan and lids. These were weighed on a semi-micro balance (GR-202, A&D, Japan), having an accuracy of 0.1 mg. During the experiment, pure nitrogen, flow rate of 50 ml/min is supplied. PCM is tested for temperature 10 Ce60 C. RCS90 (refrigerated cooling system) is used for controlled rate of heating and cooling. The ramp rate is taken to be 1 C/min in the present study. The complete pre-processing before the DSC experiment and accessories are depicted in Fig. 3.
The DSC thermograms of Eicosane (Fig. 4) and OM35 (Fig. 5), for three cycles have been shown. It is observed that Eicosane and OM35 starts melting at 35.2 C and 33 C, respectively. PCM melting is completed at 37 C for both these PCMs. During the cooling cycle solidification starts at 35 C and 33.6 C and completed at 31 C and 30 C, respectively. They showed relatively low sub cooling effect, of around 2 C, which is significantly lower compared to salts hydrates. At 30 C, solidification of both the PCMs is completed. The latent heats for Eicosane and OM35 are 247 J/g and 159 J/g, respectively. It is seen that melting and solidification temperature for both these PCMs fall within the daily temperature variation in Delhi, therefore, these two PCMs are selected to be embedded in bricks for their thermal testing. The PCM properties for these selected PCMs and their properties are investigated and tabulated in Table 2.
2.1. Sky condition determination based on sky clearance index The condition of the sky is defined by sky clearance index (Ksky), which is as given in Table 3 [35]. It is calculated using Eq. (1).
Table 1 Design of experiments. S. No.
Set 1 [29]
Combinations
Without PCM
Objective 1
Temperature at all the interfaces is recorded for PCM embedded bricks to assess the impact of PCM incorporation within bricks. This study quantifies the energy saving that can be achieved on PCM incorporation within bricks and also compares two PCMs
Objective 2
Set 2 Eicosane (1.2 cm)
OM35 (1.3 cm)
Without PCM
Set 3 Eicosane/OM35 multiple layer (1.1 cm each)
Without PCM
OM35 (finned 2.2 cm)
OM35 (2.2 cm)
OM35 (1.1 cm)
Temperature difference for multiple PCM layers is assessed.
PCM thickness variation is studied along with the finned structure to increase the heat transfer and assist in PCM discharging.
Comparison between with and without PCM bricks and multiple PCM layers is assessed in terms of heat transfer
Assessment of impact on the heat flow with and without fin within the PCM layer
Configuration
Studies Carried ➢ out ➢ ➢ ➢ ➢ ➢
OM35 (1.1 cm)
PCM selection Assessment of PCM impact in terms of temperature difference achieved Comparison of multiple PCM layer with single layer within the brick Impact of increasing the PCM thickness is assessed Impact of finned configuration on PCM charging and discharging rates Temperature fluctuation and heat transfer is calculated for all the cases
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Fig. 2. Mean maximum and minimum temperature of Delhi round the year.
3. Thermal testing of PCM embedded bricks
Ksky ¼ Ig
.
Io
(1)
where, value of Ig is measured by weather station (Ingen tech., India) as shown in Fig. 6. Both the experimental setup and the weather station are installed at rooftop of block V, IIT Delhi, 40 m apart. The solar radiation falling on the setup and weather station, along with the wind velocity, are assumed to be same. Io is calculated from Eq. (2).
Io ¼ Isc sinðas Þ
(2)
The PCM charging and discharging is greatly affected by the sub cooling effect, which is taken into account for accurate assessment of heat flow across the brick in this study. The reduction in heat flow and temperature with PCM incorporation within building components is investigated under peak summer conditions. To perform the experiment, mud bricks with single and double slots are prepared using conventional Indian brick making process, as it is well established and could be scaled for future applications. The slot in the brick is achieved by modifying the mould design. The process is as explained in Fig. 7. The bricks are first prepared using the modified mould and are dried under the Sun for duration of 7e10 days. The slot is then inserted with fuel wood to retai1n its
Fig. 3. (a) Melting of PCM (b) DSC pan and dies (accessories) (c) Crimping machine (d) DSC cell (e) Complete DSC system with RCS90 as cooling system.
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591
Fig. 4. DSC thermogram for Eicosane (PCM 1).
original shape and size. This is followed by baking of these bricks within a conventional kiln to get the final slotted bricks. Both single and double slot bricks along with the conventional ones are produced using this method for the experimentation. 3.1. Brick geometry and setup development Fig. 8 (a), 8 (d). show the modified bricks with single and double slots. Fig. 8 (b), 8 (e) show galvanized steel encapsulations without
and with fins. Brick outer dimensions are 22.5 cm 12 cm 10 cm and slot of 1.7 cm (thickness) and 16 cm (length), respectively. An ASTM A525 steel (galvanized), thickness 0.4 mm, having dimensions 15.5 cm 9 cm 1.5 cm (Fig. 7 b), is used for PCM encapsulation. The selection of the optimum thickness of PCM is made based on the available literature. The thickness of 1 cm as per Kuznik et al., 2008 [36] assists in increasing the thermal inertia of the wallboard by two times. Similarly, the parametric study carried out by Liu et al., 2017 [37] suggested PCM thickness of
Fig. 5. DSC thermogram for OM35 (PCM 2).
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Table 2 PCM properties. PCM(s)
Cp (liq.) Cp (kJ/kg (solid) (kJ/kg C) C)
Eicosane (PCM 2.46 1) (99% Pure) OM35 (PCM 2) 2.71
Melting Temperature ( C)
Thermal Conductivity (liq.) (W/m C)
Thermal Conductivity (solid) (W/m C)
Latent Heat (kJ/ Density (liq.) (kg/ Density (solid) (kg/ kg) m3) m3)
1.92
36e38
0.15
0.39
247.3
780
815
2.31
35
0.16
0.2
160
870
900
Table 3 Sky Condition relation with Ksky. Sky Clearness Index (Ksky)
Ksky < 0.2
0.2 Ksky 0.65
Ksky 0.65
Sky Condition
Overcast
Intermediate
Clear
of around 1.2 cm is considered for set 1 and set 2 configurations. In set 1 configuration the impact of different PCMs being incorporated within the brick with respect to the conventional brick without PCM, was compared. Set 2 configuration tests the thermal impact of single and multiple PCM layer within the brick. Set 3 configuration is used to test the impact of increasing the PCM thickness from 1.2 cm to 2 cm and also assess the impact of having fins within the macro-encapsulation. To minimize the losses from other sides of PCM bricks (except the exposed surface), they are packed with thick layers of polystyrene (insulation). The gaps are carefully filled with glass wool, to avoid heat leakages, from or to, any other but the top surface having dimensions 22.5 cm 10 cm. This surface is directly exposed to the sunlight, as depicted in the schematic diagram of the test setup (Fig. 9). The one-dimensional heat flow is assumed, first through brick section, then encapsulation cover followed by air gap, PCM, encapsulation and then through the remaining brick. ‘T’ type thermocouples are used for measuring the temperature of different surfaces. These thermocouples were calibrated using ice and boiling water in the laboratory. The temperature accuracy is measured to be ±0.2 C conforming to the accuracy as provided by the supplier. A 16-channel datalogger (NI, India) is used to store the data in excel format using LabVIEW 2018. The temperature is recorded at a time step of 10 min.
Fig. 6. (a) Weather Station control module (b) Weather station installation at rooftop, IIT Delhi.
12 mme30 mm to be suitable for building incorporation. For Indian context, the simulation study by Saikia et al., 2018 [38] also considered 12 mm PCM thickness to assess the PCM impact in different directions within a building. As thickness of PCM increases, the cost also increases proportionally. Thus, PCM thickness
3.2. Heat transfer calculations The assumptions made to calculate heat flow across the brick section are as follows: i. Uni-directional heat transfer, along x, as marked in Fig. 11.
Fig. 7. Steps followed for brick preparation.
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Fig. 8. (a) Single-slot brick (b) Sheet-metal casing for PCM encapsulation (c) Macro-encapsulated PCM (d) Double slot brick (e) Sheet-metal casing with fins (f) PCM incorporated brick with twin slots (g) Installation of experimental setup on the roof top (h) Final setup.
ii. The heat transfer takes place only from the top surface of the brick being exposed to the environment. All other sides are assumed to be insulated adiabatically. iii. Irregularities in dimensions of encapsulation and bricks are neglected.
The solar radiation influx on the surface of the brick increases its temperature higher than the ambient, this is known as the solar air effect [39]. The difference in surface temperature on the outside and on the inside is responsible for heat transfer to occur.
Fig. 9. Schematic view of the test facility for thermal test of bricks.
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vtbrick v2 tbrick ¼a vt vx2
(3)
The heat transfer is determined using heat conduction equation (Eq. (3)) which is simplified to Eq. (4) as the boundary conditions are applied. This is the generalised equation used for calculating the heat transfer [41].
Q ¼ Ueff A ðts ti Þ
(4)
1 1 d d 2 d3 d4 dn 1 …: þ þ ¼ þ 1þ 2þ þ Ueff ho k1 k2 k3 k4 kn hi
(5)
To account for the heat stored within different layers, a capacitance (for an equivalent electrical analogy) has been taken. The circuit of a composite brick is designed with a combination of thermal capacitance and thermal resistances, as shown in Table 4 [40] and illustrated in Fig. 10 which shows the equivalent electrical circuit for a composite brick. This equivalent analogy can be used for calculating the interfacial temperatures, which, however in this case, has been experimentally determined. Most of the incoming heat (Q1) is absorbed by PCM, a part of it is stored by other construction elements, leaving behind only a part of it (Q2), which flows through, to the inside and is represented as shown in Eq. (6). The heat stored in different layers of brick (Qs,i) is given in Eq. (7).
Q2 ¼ Q1 Qs; i Qs; i ¼ m:Ci :
dt dt
(6)
i ¼ 1…:n ðdifferent brick layersÞ (7)
Where, Ci is heat capacity of different layers of brick. All the calculations have been done first without the application of PCM, and then by incorporating the selected PCMs in their respective configurations. Heat flow to the inside is directly calculated using the temperature difference and the resistance between the last two interfaces. Uncertainty Analysis: The uncertainty of heat transferred across the PCM incorporated brick has been computed to verify the accuracy of heat transfer and the estimated effectiveness of the brick. The heat transfer across the brick is a function of area of cross section (A), effective thermal conductivity of the brick (Ueff) and the temperature difference across the thickness of the brick (DT), given by the following equation:
Table 4 Thermal resistances (R) and thermal capacitances (C) for equivalent electrical network model. Interface
1
R
1 h0 Abrick rbrick.Abrick.Cs,1.Dx1
C
2 …. n-1
Dx i kAbrick rbrick.Abrick.Cs,i.Dxi
n 1 hi Abrick Mair. Cs,air
4. Results and discussion Present study is an extension of Saxena et al., 2019 [29] in which the assessment of PCM embedded bricks with respect to conventional bricks has been reported. The temperature reduction up to 6 C for OM35, has been observed and is stated here in this study as set 1 configuration. It was concluded that OM35 was better compared to Eicosane in terms of cost and the heat transfer rate, was comparable in both the cases. This study compares the thermal impact of double slot PCM brick with respect to the single slot PCM and conventional brick. This is classified as Set 2 configuration for experiments as shown in Fig. 11 (a) Brick 1 (B1) is a conventional brick of equivalent size followed by twin slot brick marked as Brick 2 (B2). Two PCMs, Eicosane (PCM 1), thickness 1.1 cm, and OM35 (PCM 2), thickness 0.9 cm, are incorporated in these slots within a sheet-metal encapsulation. Owing to higher melting temperature of PCM 1 it is installed in the outer slot and PCM 2 is kept in the inner slot. Brick 3 (B3) is a single slot brick which is incorporated with macro-encapsulated OM35, thickness 1.1 cm. The comparison of temperature profile of different surfaces for the three bricks, from May 31st to June 5th, 2018, is shown in Fig. 11 (b) The sky condition during the day is mostly intermediate and clear. The results show a temperature reduction up to 9.5 C for double slot PCM brick, whereas a temperature reduction up to 6 C for single slot PCM brick, as also envisaged in Set 1 experiment [29]. The most critical parameter as observed is the temperature during the night, as PCM discharges its heat and becomes available for charging on the subsequent day. It is observed that temperature at night for 3rd June and 5th June was around 30 C, thus, PCM discharging was relatively less. The impact of this is observed on temperature rise on the next day. It is seen when temperature at night falls to around 27 C, which is the mean minimum temperature of Delhi during summer, both these combinations show convincing results. The temperature difference of around 9 C and 6 C on the inside for double and single slot PCM bricks, is observed on 31st May, 1st, 2nd and 4th June. It is also observed that the interface temperatures across the PCMs, falls below the melting temperature during off-sunshine
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 2 2 2 vUQ vUQ vUQ vUQ vUQ vUQ vUQ UQ ¼ Udbrick þ Udair þ UdPCM þ Ulbrick þ Ubbrick þ UTouter þ UTinner vdbrick vdair vdPCM vlbrick vbbrick vTouter vTinner (8)
Where uncertainty of a variable ‘x’ is given by. Ux ¼ maximum error of variable }x} magnitude of the variable ’x’
The maximum possible error in the dimensions of the brick (l, b and d) are ±0.1 mm (least count of Vernier Callipers). The maximum error in temperature measurement using T-type thermocouples through NI, data logger is measured to be ±0.3 C.
hours and rises above the melting temperature during the afternoon hours. This temperature variation obtained, clearly indicates that the PCM undergoes melting and solidification on daily basis which has been the aim for effective utilization of PCMs as discussed under Section 2. The temperature fluctuation within the brick is shown in Fig. 11 (c), which clearly shows that it reduces by 10 C, which is significantly high. Fig. 12 shows the heat transfer-taking place, which clearly
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Fig. 10. Equivalent electrical circuit for a composite brick.
indicates that heat flow to the inside, is significantly lower in case of PCM bricks. Particularly during the day (sunshine hours), the heat flow to the inside is reduced by around 60% for double slot and around 40% for the single slot PCM brick. Over a complete 24 h cycle energy savings between 16% and 20% is observed on all the days for both the cases. As PCM discharges heat during the night, the inside surface temperature of PCM bricks, is observed to be higher. This is due to insulations being provided on the inside. Under more realistic scenarios, the ambient air which is at lower temperature can be utilised for conditioning of the inner space, thus lowering the temperature during the night. Set 3 configuration is used to assess the impact of increasing the heat transfer of PCM on the interface temperatures and heat transfer for July 9th to July 12th (morning), 2018. This configuration also tests the impact of varying the PCM thickness. This experiment uses four bricks as shown in Fig. 13(a) Brick 1 is the conventional
brick followed by Brick 2 with OM35, thickness 1.9 cm, within finned sheet-metal encapsulation. Brick 3 consists of OM35, thickness 2 cm within sheet-metal encapsulation but without fins. Brick 4 is a single slot PCM brick with thickness of OM35 equal to 1.1 cm. The sky condition is predominantly intermediate. With incorporation of PCM, temperature reduction on the inside is observed (Fig. 13 (b)). The temperature fluctuation amplitude is shown in Fig. 13 (c). The reduction in temperature varies from 4 C to 6 C for Brick 4. However, similar impact is not observed in the other two cases. For Brick 3, the temperature reduction is found to be 7 C, which is maximum on Day 1 but on other days, it did not follow the same trend. This is due to higher night temperature (around 29 C) which caused low heat rejection rate during the night. Additionally, due to increased PCM thickness its discharging rate is reduced which inhibits PCM solidification. Therefore, it may be concluded that
Fig. 11(a). Configuration for Set 2 experiment showing location of the thermocouples.
Fig. 11(b). Temperature distribution for Set 2 configuration.
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Fig. 11 (c). Amplitude of temperature variation for Set 2.
increasing the PCM thickness may not be effective unless there is a separate arrangement for PCM discharging. The finned PCM configuration in Brick 2 also do not show favourable results. This is due to the fact that it increases the effective heat transfer through
the PCM, thereby increasing the heat flow rate during the day, thus increasing the temperature more rapidly. During the night, the temperature difference is not sufficiently high for PCMs to get discharged on Day 2 and Day 3. Thus, PCM did not solidify and
Fig. 12. Heat transfer for Set 2 configuration.
Fig. 13(a). Configuration for Set 3 experiment showing location of the thermocouples.
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Fig. 13(b). Temperature distribution for Set 3 configuration.
Fig. 13(c). Amplitude of temperature variation for Set 3.
behaved only as a sensible heat storage on these days. Thus, inside temperature of Brick 2 attained the highest temperature among all the brick on these days. Therefore, increasing the heat transfer of the PCMs may not be a well thought solution, in the case of buildings. Thus, increasing the PCM thickness and its thermal
conductivity, may not be a suitable as in both cases PCM discharging becomes difficult and PCM storage capacity is underutilised. Heat transfer rate is reduced by around 40% during the day (Fig. 14). During night, the heat rejection rate of PCMs is observed to be low due to very less temperature difference
Fig. 14. Heat transfer for Set 3 configuration.
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between the inside and the ambient temperature. About the cost of these PCM bricks, it must be mentioned that major share of the cost is due to the PCM. OM35 (INR 500/kg) is preferred during experiments over Eicosane as its cost is thirty times lower as compared to the Eicosane (INR 2200/kg). Other costs include cost of encapsulation. The PCM encapsulations made up of polyethylene can be a lucrative solution however, during the test runs it was found that leak which was a major issue in them. Polyethylene cannot sustain the abrasiveness of the brick surface and are susceptible to leakage. About the modified bricks, they show good thermal characteristics in terms of temperature reduction and heat transfer to the inside during the day. However, these are still to be tested for their mechanical strength and vapour absorptivity, in order to be available for mass production for building applications in India. With Eicosane as PCM the brick cost was around 2800 INR/kg (all inclusive) which has come down to INR 80 ($ 1.2) with use of OM35 as the PCM. It is important to mention here that with the increase in demand the cost could come down, substantially. 5. Conclusions This study is an effort towards testing of PCM incorporation, as passive storage, within the building elements like bricks. The study illustrates a systematic and scalable method of producing modified bricks for PCM incorporation. The modified bricks are prepared to assess the impact of multiple PCM layers and thickness variation within the bricks. The impact of increasing the effective PCM thermal conductivity on temperature and heat transfer has also been adjudged. The inferences drawn are as follows: 1.) A temperature reduction up to 9.5 C for dual PCM layer within the brick and a temperature reduction of 6 C is achieved for single layered PCM brick. 2.) The heat gain reduction up to 60% is observed for dual PCM layer brick and around 40% for single layer PCM brick, during the day. This however is not the case during the night, when PCMs are rejecting heat. The overall heat reduction between 16% and 20% is observed over the period of 24 h. 3.) The use of fins within the PCM encapsulation affects adversely as the heat transfer during the day increases substantially. The inside brick temperature for these bricks, rises higher, than in case of conventional bricks and PCMs could not get discharged sufficiently, during the night. 4.) Increasing the PCM thickness may not be a viable solution unless the heat rejection during the night is ensured through any auxiliary means apart from using finned configuration, for a building space conditioning. A PCM thickness of from 1 cm to 1.3 cm has been tested experimentally and found suitable in achieving a peak temperature reduction of around 6 C. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study is funded by Department of Science and Technology, India under project no. TMD/CERI/BEE/2016/084(G). Authors also acknowledge Head, CES for his continuous support and help during this research.
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