Thermal Performance of Resource-Efficient Geopolymeric Mortars Containing Phase Change Materials

2.1.1. Mortars Design and Production

The mix design is shown in Table 3. A sodium hydroxide solution (12M) was mixed with sodium silicate to dissolve the silica and alumina of the fly ash particles, resulting in a homogenized gel lasting 1 min; next, all the solid materials were mixed by using standard mixer operating at the following speed I (65rpm) for 3 min. After the addition of the alkaline activator, the mixer was operated during 1min with speed I (65rpm) and plus another 1 min with speed II (90rpm). Afterward, the mixture was placed into moulds. The specimens were kept at room conditions with temperature around 25°C and relative humidity of 65%RH. After 4h the hardened specimens were demoulded and sealed with plastic wrap and then left in the same room till the testing age.

2.1.2. Testing

2.1.2.4. Design and Fabrication of the Pilot Prototypes

A total of 20 panels with dimensions of 30 cm×30 cm×3 cm were built. The mortar specimens with a diameter of 10 cm and thickness of 3 cm were placed at the geometrical center of the panels made of extruded polystyrene (XPS). Each panel was placed inside the hollow pilot prototype whose walls were made of XPS. The cross-section schematic diagram of prototype has outer dimensions of 60 cm×36 cm×36 cm (Fig. 3). The prototype was equipped with a heater in order to produce heat through the prototype. The heater system had dimensions of 25 cm×13 cm×22.5 cm, with a power rating of 1000W and was placed on the left side of the prototype. In each experiment, the control of the heating system was set at ON mode to produce the heat for 30 minutes and then OFF mode for 30 minutes. With regard to temperature monitoring, thermocouples tyke K, were positioned at two points at the interior ambient of the prototype, here termed as “X”. The two thermocouples were placed in the air ambient of the prototype: one on the left side to measure imposed temperature and another on the right side to measure interior air temperature. The thermocouples were connected to a data acquisition system (AGILENT 34970A) with a rate of one measuring per ten seconds during the whole period of testing. The prototype was placed inside a climatic chamber room with a constant temperature of 18°C and a relative humidity of 60% in order to ensure an initializing of the test setup at a temperature below the phase change transition of the PCM. A total of twenty experiments were conducted submitting the prototype to the environmental conditions, with each experiment lasting 60 minutes (a heating/cooling cycle).

3.1.1. Density

The results of density can be found in Table 4. Overall, it can be said that the percentage of PCM content is proportional to the densities of geopolymer mortars. The use of PCM with low density made the mortar mixture slightly lighter with lower density. Regarding the density difference between mortars termed as group A to E with different mass fraction of PCM, it is interesting to note that the density of mortar with incorporation of 10%PCM, 20%PCM and 30%PCM in group A is approximately 86%, 85% and 84% to that of reference mixture (mortar without PCM) in group A. Similar trends have been observed for different groups. Fig. (4) shows a linear correlation between density and PCM percentage. However, it is less pronounced for the mortars in groups of B, C and D due to the utilization of additives PC and CH in the mixtures. The reason may be the lower density of PCM when compared to the density of sand.

3.1.2. Thermal Conductivity

Table 5 shows the thermal conductivity results. They show that the addition of microencapsulated PCM decreases the thermal conductivity of the mortars. Mortars without PCM in the groups A to E have thermal conductivities of 0.77, 0.52, 0.94, 0.71 and 0.91, respectively. Fig. (5) shows that the addition of 10% PCM into the mortars in group A to E leads to a reduction of thermal conductivities approximately 5% to 12%, when compared with the mortars without PCM. When a PCM percentage of 20% was used (mortars in group A to E) reductions of approximately 11% to 20% in the thermal conductivities were observed. The reductions can be as higher as 60% for mortars with a PCM percentage of 30%. These findings are in the line with the previous studies on PCM mortars [29, 30].

3.1.3. Thermal Energy Storage

Table 6 shows the Weights of specimens used in DSC test. The steps for the test procedure were as follows: (i) initial isothermal period at 5°C for 5 min; (ii) dynamic heating up to 40°C with the rate of 5°C/min; (iii) stabilization at 40°C for 5 min; (iv) dynamic cooling to 5°C with the rate of 5°C/min. It should be mentioned that, the phase change and temperature were obtained from the DSC heat fluxes signal response by integration. The peak temperatures of heating/cooling cycle have been considered as the representative temperature of a phase change transition. The onset and end temperature for each phase change were determined according to the recommendation of the EN ISO 11357-1 [31]. Fig. (6) shows the DSC results for pure microencapsulated PCM. The heating process revealed a melting peak temperature coinciding with the reference value indicated by the supplier, with maximum deviation reaching 0.57°C. The measured specific enthalpies for the pure microencapsulated PCM were lower than that reported by the supplier by 15% and 5% for heating and cooling, respectively. Furthermore, the difference between the peak temperatures of heating and cooling processes was of 7.5 K. This is coherent with the reports of hysteretic behaviour of the phase change materials [32].The DSC curves for testing of mortars in group A to E at heating/cooling rate of 5°C/min are shown in Fig. (7). The peak temperatures in the thermograms of the heating processes are consistently being increased from ≈25°C to ≈26°C as the PCM percentage increases. Conversely, in the cooling processes (Figs. 7a-e), the peak temperature decreases (from ≈20.5 °C to ≈22 °C) with increased PCM percentage. The results suggested that the PCM peak temperature shifts in the direction of the imposed flux, thus showing higher peaks for mortars with high PCM percentage. This trend has also been reported by others [27, 33] and it can be explained by the increase in specific heat capacity. The representation of peak temperatures and the percentage of PCM in the mortars was made with two dashed lines that showed a clear linear relationship between them for both heating and cooling processes. The distance between phase change experienced from ∆≈1.5 K to ∆≈2.1 K which depends on the sample and on the internal thermal gradients, tends to lag or raise heat exchange from DSC test explaining the observed differences. In the tested PCM mortar samples, the phase change melting/freezing temperatures were coherent with that of the pure microencapsulated PCM. Overall differences between heating/cooling peaks in the thermograms for different mortars were observed to be smaller as the weight percentage of PCM decreased, being that such hysteretic behaviour is observed for all tested mortars with PCM. An example of such a case for the mortars at group A is presented in Fig. (8), where significant shifts are generally observed. It should be noted that the DSC thermograms have been zoomed to their peaks in order to facilitate the observation of the hysteretic behaviour. The difference between heating/cooling peaks (here termed as “D”) was 3.5K, 4 K, and 5 K for the mortars in group A with 10% PCM, 20% PCM and 30% PCM, respectively, and compared with the case with 100% PCM with the highest hysteretic behaviour of D≈7.5 K. It is worth noticing that when the purpose of DSC testing is to assist numerical simulation of PCM performance in building façade, it becomes apparent that the low heating/cooling rate is the one that mostly resembles real scenarios of daily temperature variation. It should be mentioned that the specific enthalpy of the PCM mortars for each phase transition is almost independent of the heating/cooling rate. Moreover, the impact of the heating/cooling rate depends on the hysteretic behaviour of the thermal response of mortars with PCM, in which, lower heating/cooling rate exhibits minimum to no apparent hysteresis. Figs. (9 and 10) show the volumetric heat capacity calculations and thermal inertia. These figures solely plot the results for mortars at group A upon heating process. The volumetric heat capacity (C_pv) of the materials is calculated by multiplying the density and specific heat capacity according to:

(1)

Where C_p is the specific heat capacity of the material and ρ is the density of the material. Such value ascribes the ability of the material in terms of energy storage in a certain volume while undergoing a given temperature change. The thermal inertia I is calculated according to the following equation [11] with respect to the thermal conductivity of the material λ:

(2)

The high thermal inertia describes a material that is characterized by high thermal mass and high thermal conductivity. In fact, higher thermal inertia means “faster materials” that are able to be thermally activated and more thermal load can be stored during the dynamic thermal process. Results in Fig. (9) show that the thermal inertia of PCM based mortars is higher than that of the plain mortar (particularly for the phase change range temperature), thus highlighting its higher potential for attenuating the effect of external environment temperatures on buildings [11]. The values of the specific enthalpy are shown in Fig. (11) for both heating and cooling processes. It can be observed that the specific enthalpy value is increased systematically for cooling processes regardless of the PCM mortar type. The results show that the specific enthalpies for mortars at different groups from A to E are almost the same regardless of PCM percentage. Even though mixtures with 10% PCM in the group D, with 20% PCM in group E and with 30%PCM in group A have shown lower or higher specific enthalpy than those already obtained for PCM mortars. It is further interesting to investigate whether the specific enthalpy values calculated from DSC experiment can be used to predict the energy performance of PCM mortars. Also, if a proportionality is assumed between latent heat capacity of the PCM mortar and the latent heat capacity of the pure microencapsulated PCM, according to its mass fraction of the PCM, the following formula can be considered:

(3)

Where H_T is the calculated latent heat for the mortar (J/g), H_pCM is the latent heat of the pure microencapsulated PCM, W_pCM% is the weight percentage of the PCM in the matrix. Fig. (12) shows the measured specific enthalpies of the PCM mortars, together with the expected specific enthalpies of the mortars calculated according to Eq. (3). The numerical extrapolated enthalpy values for all studied PCM mortars are quite close in both heating and cooling processes (with <0.5 J/g differences), except for the mortar with the formulation of 90FA_10CH_30PCM _12M_2.5S/H_0.8A/B within group A, with slightly higher differences between numerical extrapolated enthalpies and measured enthalpies. This indicates the feasibility of the extrapolation procedure for the estimation of the specific enthalpy of the PCM mortars.

3.1.4. Temperature Monitoring of the Pilot Prototypes

The monitored temperatures for several mixtures within group A under a heating/cooling conditions are shown in Fig. (13). It can be observed that the difference of temperature amplitude between the enclosed part (right side) and imposed heating load (left side), for case without PCM was of 3°C, whereas the cases with 10% PCMs, 20%PCM and 30%PCM were associated to smaller internal temperature difference amplitudes of 3.3°C, 3.5°C and 4°C. These internal temperature differences at laboratory scale are consistent with the concept of improved heat capacity of the PCM mortars which, in turn, is in line with other studies [33]. The time delay between the maximum temperatures registered at the left side and right side for the case with 30% PCM was of nearly 3 min, in opposition to a smaller delay of 2.7 min (for the case with 20% PCM), 2.5 min (for the case with 20% PCM) and smaller delay associated to the case without PCM (1.5 min). It is interesting to note that the achieved temperature variations for different case scenarios with PCM were close to the phase change transition of microencapsulated PCM (Fig. 7). The results for the cases at group A revealed that PCM acts by reducing inside temperature amplitudes during the heating period, leveling them and turning them closer to melting temperature of the PCM (around 26°C). It is also worth noticing that the differences in performance were not only caused by the thermal energy storage capacity. Indeed, the PCM mortars had a slightly smaller thermal conductivity that has also contributed to reduce heat fluxes inwards and outwards the panel within prototype. Nonetheless, the PCM-based mortars also had a lower density than the case of mortars without PCM, so its volumetric heat capacity could have been reduced accordingly. In general, during the heating process, for the mortar without PCM, the temperature changeon the right side of the panel depends on the heat capacity of the mortar, whereas for the PCM mortars, the latent heat of the PCMs provides much higher storage and thus shows a lower amplitude. It can be further stated that the incorporation of PCM allows the mortar to delay the temperature change. The overall observations for the mortars at group B to E were similar to those already reported above for mortar at group A. The incorporation of PCM leads to a significant attenuation of the amplitude of the effects of the heating temperature, with a reduction in peak temperatures and time delaying effect. Fig. (14) indicates that the mixture 90FA_10PC_30PCM_12M_2.5S/H_0.8A/B from group E with temperature difference of 5.2°C had the best performance in terms of temperature amplitude and the mortar with the formulation of 90FA_10CH_30PCM_12M_2.5S/H_0.8A/B from group A had shown the worst with 23% less performance (4°C).