3.1. PCM Oudoor Characterization
Two measurement campaigns were carried out, each including three full days of data acquisition. The first campaign was carried out during the warm season, from the 26th to the 29th of September 2016. Two boxes were exposed outdoors, one with a PCM board layered on the box floor (PCM box, see Figure 3
b) and the other one without PCM (REF box). Air temperature inside both boxes was measured together with outdoor air temperature, relative humidity, wind speed and direction, and global irradiance on a vertical plane. Climatic conditions during the test are presented in Figure 5
a where air temperature and solar irradiance measured on a vertical plane are showed. Good weather conditions with high temperatures were characteristic of the time period (maximum peak at 29 °C) and there was a significant thermal range between day and night (12–13 °C). Solar irradiance reached peaks of approximately 800 W/m2
. Figure 5
b shows the air temperature trends inside the reference box (REF) and the PCM box (PCM) during the test. A significant decrease in maximum temperature was observed in the PCM box with respect to the REF box, due to the PCM melting in the temperature range 22–23 °C. An average decrease of the temperature peaks of approximately 10 °C was observed during the day, while at night an opposite behavior occurred. Indeed, air temperature inside the PCM box was higher than in the REF box due to PCM solidification and thermal mass.
No evident shift of the temperature trend due to PCM heat capacity was observed; this was because the amount of material into the box was not enough to produce this effect.
A second measurement campaign was carried on later, between the 5th and the 9th of December 2016 during the cold season. Nice weather was experienced during the last two days of test while the first day was overcast but not rainy, as evidenced by Figure 6
a. Outdoor air temperature, in this case, was lower than the first campaign, with a maximum of approximately 21 °C and a minimum of approximately 2 °C with a thermal range of 15 °C in the clear days. Solar irradiance reached values as high as 930 W/m2
. This is because in this period, sun elevation was low, so a vertical surface would receive higher irradiance than a horizontal one. Figure 6
b shows the temperature trends inside REF and PCM boxes.
Also in this case, a temperature peak damping of approximately 10 °C caused by the PCM was observed during clear days, while for the overcast day (6th of December), no effect on the material was observed due to the low temperatures experienced inside the box (well below melting point).
PCM solidification occurred earlier in the day (around 7 p.m.) than in the first study (around midnight). Apart from this shift, a change in the curvature of the decreasing temperature trend with respect to the first study was observed. This was probably due to the behavior of the solid phase at temperatures well below the solidification temperature.
a,b show the heat fluxes of PCM and of incoming solar radiation, together with the temperature experienced by PCM simulated for the two monitoring campaigns.
The panel removed approximately 15 W peak during the day compared to an incoming solar flux, with peaks of around 40 W (around 40% of heat reduction) lowering the box air temperature peaks by approximately 10 °C.
During the night, it released heat (4–5 W) due to solidification. While during the warm season campaign it stayed in the liquid phase throughout the day, and in solid phase during the night, in the cold season studies, it was mainly in the solid phase due to low outside temperatures, melting occurring only between 11:00 a.m. and 6:00 p.m.
3.3. PCM Energy Saving Potential Assessment
Three different PCM panels were positioned on the floor of each office room of the STD building, as sketched in Figure 1
. Table 7
depicts the melting temperature ranges of the three panels. The other thermal properties are the same of SP21E as shown in Table 1
. Only the heat storage capacity of SP24E and SP26E is 180 kJ/kg instead of 160 kJ/kg. Figure 9
shows the total delivered energy of the reference buildings (STD and NZEB) compared to that delivered for the STD building modified with the SP21E, SP24E and SP26E, for the three cities of interest (Trento, Rome and Palermo). Even if the use of PCM reduced the energy request, this reduction was not enough to reach NZEB conditions.
This was particularly true for Trento, while for Rome, the goal was almost reached. Table 8
shows the percentage differences of energy saving (D) with respect to STD and NZEB buildings for the three PCM. In bold are the maximum savings obtained in Rome with SP21E (17%). In Trento and Palermo, SP21E outperformed STD with an improvement of 7% and 8%, respectively. A negative percentage means that the reference outperformed the other cases, as it was for NZEB for all PCM in the three cities. The lowest difference with respect to NZEB was in Rome with SP21E (−9%).
It is interesting to look at the monthly values of delivered energy for the three locations to better understand the real behavior of PCM. From the energy analysis, it emerged that heating was the primary load for Trento, while in Rome and Palermo, cooling load was the most demanding. Table 9
shows the monthly delivered energy (heating and cooling) for Trento, Rome and Palermo. The STD performance was compared with the three PCM for each considered month. In the tables, green cells indicate the best performing PCM of the month, while bold numbers pertain to PCM subjected to phase change during the month. It can be noted how SP21E was the best for all months in Trento, apart from February where STD slightly outperformed PCM. Moreover this material was in phase change for most of the time, owing to the low outside temperatures experienced at the location. During the coldest months, SP21E was mainly in the solid phase (no phase change observed). Also, SP24E experienced phase changes in June, July, September and October, while in August this was seen in SP26E. The maximum percentage difference in energy saving was observed during summer months where the energy consumption was the lowest.
In Rome, SP21E was always the best performing PCM apart from May and June, where SP24E and SP26E performed the best, respectively. During winter months SP21E was in phase change, while SP24E and SP26E were in phase change in May and June. From July to October the same materials were in the temperature range near the phase change, but surprisingly they were not the best performing with respect to energy saving. The best performance of PCM in terms of energy saving was observed for the months of the intermediate seasons. SP24E and SP26E performed the best during summer months in Palermo, due to the higher outside temperatures. However, the improvement in energy saving during summer months was not significant, while it was more effective for intermediate seasons and winter months.
The temperature of PCM was compared to the temperature of the surface floor without PCM (STD building) to better understand the different performances among the various PCM and to explain the PCM gains. As an example, we showed the temperature trends for the month of June in Rome, where SP26E gave the best performance, as shown in Table 9
shows the temperature trends experienced inside an office room positioned in the central part of the office building and provided with SP21E (a), SP24E (b) and SP26E (c). Melting and solidification temperature ranges are evidenced with yellow and light blue lines, respectively. Temperature of the various PCM (black line) is compared with temperature of the floor of STD building, without PCM, (red line), also outside air temperature is presented (Tair
). It can be noted that the capacity of PCM of storing energy in sensible or latent heat can be identified by observing its temperature trend. Indeed when heat is stored, the trend showed maximum and minimum temperature peaks with a similar behavior to the floor temperature of STD (Figure 10
On the contrary, when latent heat is stored, these peaks disappeared, since the temperature was fixed to the phase change temperature. This was observed for SP26E (Figure 10
c), since the material was working perfectly in its phase change range for most of the time, guaranteeing the best performance with respect to STD. The temperature of SP21E, instead, always ranged outside its phase change temperature interval, indicating that the material was mainly working in mono phase state (liquid). In this case, PCM acted like a thermal mass and the heat storage was mainly due to heat. The same thing occurred for SP24E, even if for a certain amount of time it reached the melting phase.
From the observation of the PCM temperature trends as the ones in Figure 10
, for all months and all cities, some conclusions could be drawn. The performance of PCM not only depends on the phase transition of the material but it also seemed to depend on the extent of the temperature range experienced by the material during the day/night cycle, and also on the absolute values of the maximum and minimum daily temperature peaks. These variables influenced the results differently between winter and summer. For this reason, PCM was effective both as latent and sensible heat storage systems. In the case of Figure 10
(summer period, June in Rome) the small temperature range variation of SP26E due to its phase change was beneficial to energy consumption, since this floor temperature induced a reduction in cooling demand. In August in Rome, instead, all PCM were far from phase change due to high temperatures (Figure 11
). In these conditions the best performing material was SP21E because it has less thermal capacity than the other PCMs and can reach lower temperatures in the liquid phase during the night, reducing the heating effect that is typical of high capacity materials in summer. In winter, considering Rome as an example, the daily temperature range was suitable for phase change in SP21E so that its temperature was locked around 21 °C for most of the time, maintaining a floor configuration that helped to save heating.
Nevertheless, in November (Figure 12
) thermal conditions were suitable for the SP24E phase change so the material temperature was locked around 24 °C. However SP21E, for some days of the months, was in the liquid phase and reached higher temperatures than SP24E during the day, acting as a more effective heat source than SP24E.
Moreover, during the night, SP21E minimum temperature was similar to SP24E. This explained why SP21E prevailed as an energy saving material on an annual basis.
The PCMs worked both in winter and summer, as mentioned while discussing the monthly energy performance behavior. The STD building heat balance of one office room was compared to the case of a building implemented with SP21E, to better explain how PCM works during the winter in Trento, as an example (Figure 13
). In this case, the material works as sensible heat storage.
It can be seen from Figure 13
that the power request for heating (HVAC) in the STD building was mainly during the night, since the inside air temperature fell below 12 °C (lower set point for heating outside office hours).
When SP21E was placed into the building [HVAC (SP21E)], the heating power requirement was reduced with respect to STD, since PCM released the heat absorbed during the day, increasing the inside air temperature during night.
For example, on the 12th of January, the release of approximately 200 W by PCM produces almost an annulment of the heating power.
On the contrary, a certain amount of heating is needed for PCM at the end of the day, because the material was absorbing heat from both solar radiation, and the HVAC system so that more power is requested to guarantee the set point temperature in the room. Indeed, as soon as solar radiation is not available, and in the likely case of low solar gains if compared to other thermal losses, the higher heat capacity of the room would require an additional heat source (as clearly visible in Figure 13
). Nevertheless, the balance between heating reduction (at night and early morning) and heating request (at the end of the day) summarizes to a PCM monthly power request that is lower than STD.
a reports, as an example, the heat balance for a week of June in Rome to explain the PCM behavior for the cooling period.
In this case, SP26E produced the best performance (Table 9
) and was always in phase transition. In the figure, the air temperature reached inside the STD office room and STD office room with SP26E are also plotted (Figure 14
The reduction of cooling power due to PCM was clearly visible during the days of the week, while during the weekend, no cooling was requested for both STD and SP26E, since the set point temperature was set to 28 °C, and outside temperature decreased in those days.
The increase in temperature of SP26E during the night (almost 3 °C) was due to heat release during solidification, while during the day, SP26E absorbed heat, reducing the cooling power needed (both temperatures stayed at set point).
During the weekend, as showed in Figure 14
b, inside air temperature reached by SP26E was higher than STD. This is because PCM kept its phase change temperature, acting like a heat source inside the room.
In the present analysis, air infiltration of 0.6 ACH was taken into consideration. Night ventilation during the cooling period could improve the performance of PCM that in several cases was not able to discard completely the heat stored during the day. Indeed this solution proved to be effective for energy saving improvements [21
In the cited case, however, night ventilation was applied only to the building provided with PCM and not to the reference. But it should be considered that night ventilation could be a valid means to reduce heat during the warm season even if the PCM material was not applied. To verify these assumptions, night ventilation was applied for the city of Rome. Windows were opened during the night from midnight till 7 a.m. for all cases (STD, NZEB, SP21E, SP24E and SP26E) and the calculations were made for the months from June till September. Figure 15
shows the delivered energy for cooling with night ventilation (NV) and without night ventilation (NO NV).
A considerable reduction of delivered energy was observed for the NV case for all configurations. SP21E was confirmed to be the best performing material and in this case it also outperformed the NZEB reference building. Indeed, SP21 reduced cooling demand by 39% with respect to STD, and by 2% with respect to NZEB.