## 5 Results and Discussion  
### Stage 1--Initial Assessment of the TES Properties at Lower Temperature  
Results from the first stage of the experiment are shown in Figure 4.  
In spite of the same size of the samples and the fact that they were left in the same thermal conditions, they showed different thermal characteristics. After the heating process, the specimens had  
Figure 4: A temperature drop of the materials during cooling on the first stage of the experiment; S: steel; AC: aerated concrete; SLB: sand-lime brick; CB: ceramic brick; FB: fireclay brick; CGA-PC: concrete with gravel aggregate and Portland cement; CGA-AC: concrete with gravel aggregate and alumina cement; CCA-PC: concrete with ceramic aggregate and Portland cement; CCA-AC: concrete with ceramic aggregate and alumina cement; G: granite.  
Figure 3: Thermal images of materials tested after 3 min of cooling.  
different temperatures. None of the materials, after 3 h of heating did not reach the set temperature. After 15 min of cooling, the highest temperature reached was the steel specimen (122.2 \({}^{\circ}\)C), while the lowest was the sample made of the aerated concrete (76.8 \({}^{\circ}\)C). The reason for the differences, while heating and cooling of these materials, is their structure and internal interactions between the material's molecules. Steel is a compact material which has the highest density, and the least amount of voids and free spaces inside its volume in comparison to the other materials tested. In addition, the high thermal conductivity of steel is due to the metallic bonding between molecules. An aerated concrete is radically different in terms of its structure: it has low density and high porosity; the appearance of air voids makes it an insulating material with a low heat transfer coefficient. The porosity of the surface of the aerated concrete gives it the largest contact surface with the cooler room air of all the materials tested. Such an effect unfavorably influences the emission time of the heat stored--the sample very quickly drops in temperature.  
The analysis of the temperature drops during further cooling confirms this pattern. The steel sample, despite the fact that after 15 min reached a temperature close to the concrete samples (steel: 122.2 \({}^{\circ}\)C; ordinary concrete: 117.2 \({}^{\circ}\)C), it remained warm the longest, and the temperature drops in the subsequent intervals were the lowest. The aerated concrete specimen cooled down in the fastest time.  
The analysis of the thermal behavior of other samples indicates the superiority of the concretes (including concrete with the ceramic aggregate) over the traditional ceramics in terms of the TES issues. Concrete samples as the more compact material with higher density, heat up at the same time to the higher temperatures under the same conditions (concrete samples: over 100 \({}^{\circ}\)C; ceramic samples: less than 100 \({}^{\circ}\)C).  
The calculation of the thermal power is shown in Table 7. During the analysis of the results, it is noted that the highest thermal power reached was in the steel (8.8 W), and the lowest were concretes (5.3\(-\)6.6 W). The lowest thermal power was found in the aerated concrete (1.616 W) and the fierclay brick (2.408 W).