The outdoor experiments are conducted from June 01 to June 10. The temperature data is recorded using J-type thermocouples. Note that \({T}_{amb}\) represents the ambient temperature (°C).\({T}_{ext}\) refers to the temperature at the exterior surface of the specimen (°C), \({T}_{int, Ref}\) indicates the interior temperature of the normal RCC unit (°C), \({T}_{int, Exp}\) represents the interior surface temperature of the PCM integrated biaxial void slab unit (°C), respectively.
3.1. Temperature Reduction
Figure 7 compares temperature profiles of \({T_{ext}}\), \({T_{\operatorname{int} , \operatorname{Re} f}}\), \({T_{\operatorname{int} , Exp}}\), \({T_{amb}}\)and from June 01 to June10. The figure also includes the corresponding variation in solar radiation. As observed, on several days, \({T_{ext}}\)tends to be higher than \({T_{\operatorname{int} , Exp}}\) during the peak sunny hours, whereas it falls below \({T_{\operatorname{int} , Exp}}\)during non-sunny hours. This suggests a decrease in temperature during sunny hours, which can be attributed to the absorption of heat by PCM. On the contrary, it is observed that during non-sunny hours, there is an increase in temperature as the PCM releases the stored heat. Thus, macroencapsulated PCM offers valuable thermal regulation to the roof, which is an essential consideration in ensuring the thermal comfort of the occupants. It is worth mentioning that, on several days, PCM transitions from a solid to a liquid phase during the daytime and from a liquid to a solid phase during the nighttime. Therefore, it is evident that the biaxial voided slab unit can absorb thermal energy during the daytime and then release it during the nighttime.
During the testing period, a maximum temperature difference of 4°C is observed between \({T_{ext}}\) and \({T_{\operatorname{int} , Exp}}\)during the sunny hours specifically on June 07. Additionally, the mean temperature difference is observed to be 2.9°C. These observations indicate that deploying the biaxial voided slab unit reduced the interior temperature by a maximum of 4°C and an average reduction of 2.9°C. The reduction in temperature is caused by the PCM absorbing thermal energy as latent heat during its transition from a solid to a liquid state. The latent heat storage in PCM is intricately connected to its phase change characteristics. During melting, PCM endures a semi-transient state in regions undergoing the initial phase change. In this state, the PCM is neither fully solid nor liquid but rather mushy. As the heat influx rises, the liquid fraction within the mushy zone gradually increases until it transitions into a fully liquid state. According to Fig. 7, \({T_{\operatorname{int} , Exp}}\)consistently exceeds \({T_{\operatorname{int} , \operatorname{Re} f}}\)during non-sunny hours on multiple days. Although it indicates latent heat release into the interior, it is undesirable if the temperature exceeds the thermal comfort threshold. In such adverse situations, measures must be taken to discharge this heat into the atmosphere. \({T}_{int, Exp}\)is observed to be higher than \({T_{\operatorname{int} , \operatorname{Re} f}}\)by upto 1.7°C. This implies interior temperature rises by upto 1.7°C, which can be attributed to the release of stored heat by the Exp–SU.
A comparison of temperature differences between Exp–SU and Ref–SU testing units can also be observed in Fig. 8. During sunny hours, the temperature difference between \({T_{\operatorname{int} , Exp}}\) and \({T_{\operatorname{int} , \operatorname{Re} f}}\)can be noted to reach a maximum of 3.8°C. This implies that the Exp–SU results in a 3.8°C reduction in interior temperature compared to the Ref–SU. One notable thermal characteristic observed is the sustained increase in temperature in the Exp–SU compared to the Ref–SU. As shown in Fig. 8, the temperature in the Ref–SU rises relatively faster, typically peaking during the late afternoon hours. The temperature increases in the Exp–SU occurs over a more extended period, typically peaking well into the evening hours. This indicates that the Exp–SU also provides sustained thermal regulation and insulation to the interior throughout the phase transition. Note that sustained thermal energy changes are generally more favorable for thermal comfort than rapid changes, which the Exp–SU could provide. During the non-sunny hours, \({T_{\operatorname{int} , Exp}}\)is observed to be higher than \({T_{\operatorname{int} , \operatorname{Re} f}}\)by up to2.3°C. This implies interior temperature rises by up to2.3°C, which can be attributed to the release of stored heat by the Exp–SU. The increase in thermal energy within the interior generally provides thermal comfort to occupants and reduces the energy required for space heating.
3.2. Heat transfer
Figure 9 depicts the temporal distribution of heat flux(W/m2), for all specimens during the test period. Negative heat flux values in the plot indicate that the specimens are releasing heat, while positive heat flux values indicate heat absorption. All values in the early morning and late evening hours are almost temporally synchronous. This observation suggests that the thermal heat flux remains consistent during the specified hours of the day. However, heat flux profiles of Exp–SU(\({q_{Exp}}\)) deviate from the Ref–SU (\({q_{\operatorname{Re} f}}\))when they gain heat. This transition signifies enhanced heat storage capabilities of PCM specimen. The standard Ref–SU exhibits a significantly higher heat gain, up to 60.6%, compared to the Exp–SU, which experiences a corresponding 60.6% reduction in heat gain. This discrepancy in heat flux is ascribed to the thermal inertia of the PCM contained within the void former.
A daily heat flux profile reveals two distinct transition points. The first transition occurs as the ascending \({q_{Exp}}\) profile transitions from \({q_{\operatorname{Re} f}}\)during the early morning hours, with a transition heat flux value averaging about 1.8 W/m^2. The second transition occurs in the late afternoon hours as the descending \({q_{\operatorname{Re} f}}\)profile transitions from\({q_{Exp}}\), also within the same range of heat flux values. These transition points and their corresponding values signify the heat flux levels that trigger the phase change of the PCM, either from solid to liquid or from liquid to solid. Furthermore, it is noted that the \({q_{\operatorname{Re} f}}\)profile maintains a consistent slope at transition points, indicating a steady heat flux rate within the Ref–SU.
During periods of non-sunny, the heat flux within the Ref–SU (\({q_{\operatorname{Re} f}}\)) experiences a three-fold increase compared to\({q_{Exp}}\). Furthermore, during nighttime hours, \({q_{\operatorname{Re} f}}\) rises by up to 2.5 times more than \({q_{Exp}}\). It is also observed that \({q_{Exp}}\) maintains a daily average of -2.1 W/m2, while \({q_{\operatorname{Re} f}}\) exhibits a daily average of -6.7 W/m2, indicating a substantial 3.2-fold increase. Consequently, the utilization of macroencapsulated PCM within the Exp–SU leads to a notable reduction in heat release to the interior in comparison to the Ref–SU. Moreover, the Exp–SU demonstrates superior heat storage capacity relative to the Ref–SU, thereby mitigating cooling demands and discharge rates.
3.3. Maximum heat gain reduction
The calculation of the maximum heat gain reduction (\(HG{R_{\hbox{max} }}\)) involves assessing the maximum heat gain in each element of the PCM room and its corresponding element in the reference room. This is carried out according to Eq. (5) [63], as expressed below:
$$HG{R_{\hbox{max} }} = \frac{{H{G_{\operatorname{Re} f - SU}} - H{G_{Exp - SU}}}}{{H{G_{\operatorname{Re} f - SU}}}} \times 100\%$$
5
Figure 10 shows the \(HG{R_{\hbox{max} }}\) in each unit considering the heat gain of PCM and conventional slab units. In the context of \(HG{R_{\hbox{max} }}\)analysis, the PCM slab unit exhibited \(HG{R_{\hbox{max} }}\)values of 37.56% and 62.87%, respectively, during space cooling and space heating periods. However, in general, space cooling reductions are lower than space heating reductions. There is a reduction of approximately 40.3%. These observations are attributed to the enhanced latent heat storage capacity of the PCM favored by the local climactic conditions. The data indicates both units reduce the transfer of thermal energy from solar radiation on the roof's outer surface to the interior environment. Thus, using macroencapsulated PCM in the slab unit resulted in a notable reduction in heat gain to the interior compared to the conventional slab unit. Lower \(HG{R_{\hbox{max} }}\)values during space cooling indicate the release of significant heat to the interior, which is unfavorable. This percentage aligns realistically with findings from various locations worldwide. For example, a study conducted in the United States reported \({HGR}_{max}\) ranging from 3.5–47.2%, while under the climate conditions of Saudi Arabia, \(HG{R_{\hbox{max} }}\)reached up to 35%. Additionally, an optimization study for buildings incorporating insulation and PCM demonstrated a 33.5% \(HG{R_{\hbox{max} }}\) under the hot climate conditions of India.
3.4. Thermal loading
Based on heat flux estimates, cooling and heating thermal load through the testing units during the two testing periods can be calculated. To calculate thermal loads, the heat flux values are integrated over time:
$${q_d} = \int {\dot {q} dt}$$
6
where \(\dot {q}\)represents heat flux (W/m2) and \({q_d}\)represents thermal load (Wh/m2). Figure 11 illustrates the cooling and heating thermal load distribution between the two units. As observed, thermal inertia causes an increase in thermal load during sunny hours and decreases during non-sunny hours. The data indicates both units reduce the transfer of thermal energy from solar radiation on the roof's outer surface to the interior environment. The cooling load of the Exp–SU is approximately 49.8% lower than that of the Ref–SU. According to the data presented in Fig. 19, it is evident that the heating loads for the Exp–SU are considerably lower compared to the Ref–SU. Specifically, the heating loads for the Exp–SU are approximately 2.9 times lower. Additionally, for a given test period, the total thermal load in the Exp–SU is about 59.4% lower than that of the normal Ref–SU. These findings indicate using the macroencapsulated PCM integrated slab unit significantly improves comfort by reducing operative temperature and enhancing thermal load management. Further, Fig. 11 illustrates an apparent difference in cooling and heating loads in the same testing unit during a specific testing period. The thermal load difference in the Exp–SU is significantly smaller, with factor of 1.08 compared to 1.57 in the normal Ref–SU, during the testing period. The Exp–SU has lower factors than the Ref–SU, suggesting it experiences significantly reduced temperature fluctuations.
3.5. Electricity cost savings
Figure 12 shows the distribution of electricity cost savings for peak cooling and heating purposes during the test periods. Note that the cost savings estimates are based on the assumption that comparable electrical energy meets the energy needs for space cooling through air conditioning and space heating through heating systems. According to Fig. 12, the savings in space cooling are greater than in space heating. This is due to the ability of the PCM-integrated voided slab unit to store latent heat, thereby reducing indoor space cooling needs. The cost savings associated with using voided slab units for space heating appear to be smaller, possibly due to the impact of local climatic conditions. Passive houses are designed to have minimal heating requirements and a shorter heating period, so space heating may not be required. Therefore, their ability to adjust their space heating energy is limited to the coldest hours of the day rather than during transition period.
Table 4 summarizes the electricity cost savings over the test periods. The \({U_{PE}}\)is taken as USD 0.09/kWh. Daily savings on space cooling are significant, with a maximum of 0.04 USD/kWh/m2 and an average of 0.03. Space heating savings are slightly better, averaging 0.02 USD/kWh/m2, compared to 0.03 USD/kWh/m2. It is worth noting that significant peaks in space cooling savings (e.g. 0.0368 USD/kWh/m2) are observed on June2, especially on the day after less sunlight. Therefore, it is emphasized that weather conditions allowing complete discharge of PCM in the voided slab unit are beneficial for charging and result in improved cost savings.
Table 4
Summary of electricity cost savings
Testing period | Thermal cycle | Cost savings per day (USD/kWh/m2) |
Maximum | Average | Minimum |
01–10 June | Cooling | 0.0368 | 0.0356 | 0.0338 |
Heating | 0.0220 | 0.0190 | 0.0133 |
3.6. Payback period
The cost payback period is another criterion commonly employed to assess the viability of an investment. Payback period is a metric used to determine the time it takes to recover the initial investment cost. Payback period in this study refers to the time required to recoup the initial investment cost of integrating macroencapsulated PCM in a roof. Recovery depends on the annual energy savings from reducing cooling and heating loads. The simple payback period is a commonly used method of calculating the time it takes to recover the initial investment cost through annual cost recovery. However, it is essential to note that this method fails to consider factors such as inflation, the time value of money, and uneven cash flows over time [77]. In the current study, the initial investment cost includes the aluminium void former and PCM costs. The price for one unit of void former is 170 Rupees/2 USD. Nine units are required to cover an area of one square meter, resulting in a total cost of 1530 Rupees/18 USD. OM37 costs 450 Rupees/Kg or 5.4 USD/Kg. Each void former is integrated with 600 g of PCM. Thus, the PCM cost per square meter is 2430 Rupees/29 USD. Hence, the total Annual cost recovery is affected by the amount of electricity saved, which depends on the thermal performance of the PCM throughout the year. Note that the daily average savings, including space heating and cooling, from April 15 to June 9 was 4.7 Rupees/day, as given in Table 6. The potential PCM working days can be estimated using the temperature profile of the study area, city of Rupnagar, as test results for an entire year are not available. According to the temperature profile (Ref. Figure 1), there are about 148 days with temperatures above 35°C, which suggests these days are suitable for PCM functioning. These days are 23 between March and April, 28 between April and May, 30 between May and June, 21 between June and July, 18 between July and August, 22 between August and September, and 6 between September and October. It is noteworthy that these 148 days are not in need of day space heating. To estimate the payback period, PCM is assumed to function for 148 days resulting in a daily average electricity savings of 4.7 Rupees. Thus, the simple payback period per square meter of roof area is estimated at 3960 Rupees/4.7×148 days, which is 5.7 years. This means initial investment cost in employing macroencapsulated PCM-integrated void formers in the roof can be recouped in about 5.7 years. Thus, it can be stated that the proposed PCM integrated biaxial voided roof is promising and commercially viable.
3.7. Carbon emissions savings
Figure 13 illustrates the fluctuation in CO2 emissions savings for space cooling throughout the testing period. It is evident that electricity generation from lignite results in the highest savings, whereas natural gas exhibits the lowest. This implies that utilizing PCM integrated voided slab technology can lead to CO2 emissions savings of up to 4.8 kgCO2/kWh for lignite-powered electricity generation plants and 1.5 kgCO2/kWh for those fuelled by natural gas. Coal-powered electricity generation plants demonstrate similar savings of 4.3 kgCO2/kWh, respectively.
Figure 13 also depicts the variation in CO2 emissions savings for space heating during the testing period. The trends in savings for space heating align with those observed for space cooling. However, in general, space heating savings are comparatively lower than space cooling savings. Notably, there is a reduction of approximately 44.24% during the period. In the Indian context, space heating demands are relatively minor compared to space cooling, and with the increasing effects of global warming, it is expected that space heating requirements will further diminish. The implications of CO2 emissions savings include the reduction of direct emissions from fossil fuel combustion or consumption, indirect emissions from electricity generation, and the mitigation of global warming and climate change by controlling carbon emission intensity.
The cumulative carbon emissions (\({C_s}\)) for space cooling utilizing various fuels are calculated to be 14.94 kgCO2/kWh, respectively. Correspondingly, the calculated values for space heating are 8.33 kgCO2/kWh. It is discernible that under the prevailing weather conditions, space heating offers a notably higher total CO2 emissions savings compared to space cooling. The regulation of carbon emissions plays a critical role in mitigating global warming and climate change by curbing emissions stemming from fossil fuel combustion and power generation. Consequently, the utilization of HRS macroencapsulated with PCM facilitates increased CO2 emission savings.