AIR

120 The figure shows how temperature trends take on more or less value, unlike the maximum temperatures recorded for both tests. In fact, the data obtained from the simulation maintain values far higher than those recorded in the laboratory test.

To verify how much the two tests differ from each other, a percentage deviation has been made.

Figure 111 Percentage deviation made at the end of the comparative analysis

What emerges from the figure is what has been noted in advance, namely that the maximum temperatures of the two tests, never assume the same value.

While the percentage deviation relative to the average temperature has a different value of 0%

because in the initial phase of the tests, the two curves assume different values. Instead in the final phase, the two curves tend to re-join and therefore the percentage deviation tends to approach towards 0%.

The percentage deviation relative to the minimum temperatures is different from 0% but still maintains constant and low values, which means that the two curves obtained from the two tests tend to have temperatures very close to each other and with a constant temperature difference for the intact duration of the test.

Finally, the percentage deviation relative to the temperature of the insulation is almost constant and close to zero.

121 The last test performed in the scale component is the one without phase change material. This was mainly used to verify how the phase change technologies affect the thermal transmission phase.

Also in this case, the test was made with a duration approximately equal to the previous ones, so as to be able to verify the charge and discharge phase during the same time period.

In this case the test lasted 191055 seconds and, unlike the materials, only one data recording test was performed.

Figure 112 temperature trend obtained from the test in the laboratory

What emerges from the figure is how all the temperature profiles attributed to the material increase immediately, in the vicinity of the instant 0. In the subsequent phases the temperature remains constant until the start of the discharge phase.

When the electrical resistance is switched off, the air is suddenly cooled until, at the final moment of the test, it returns to values close to the environmental values.

This means that the air, not accumulating any amount of heat, cools quickly. Indeed, even water returns to the initial test temperatures, which means that all the accumulated heat has been released during the entire discharge phase of the test.

The graph shows that the profiles attributed to the average and minimum temperatures have more or less the same values, probably due to the close positioning of the thermocouples.

As for the charging phase, the simulation was carried out using the COMSOL Multiphysics software 4.3a.

122 The simulation was carried out in order to verify how the body behaves, from the most ideal point of view, if in contact with a thermal source.

The duration of the simulation is 18000 seconds and with a time interval of 600.

Figure 113 Temperature trend obtained from the simulation

The simulation produces very different results from the laboratory test. In fact, it is clear how the profiles attributed to the average and minimum temperatures have very different values.

Furthermore, the maximum temperature trend after the rapid reaching of high values does not maintain its stability but the temperature continues to increase.

The simulation carried out by the software allowed to check the temperature history by displaying the variation of the 3D volume colour reproduced by the software itself.

In this case, significant steps have been chosen, in order to be able to better comment on how the body behaves under the influence of a heat source.

From the follow figures, the main thing that you notice is how the first two steps identified, they differ from each other. This is due to the sudden increase in temperatures already from the initial instants of the test.

In the following phases, however, the temperature continues to increase but there is a much lower speed compared to the initial seconds of the simulation.

123

124 Moreover, it is possible to notice, in the initial step of the simulation, how the internal part of the cylinder has a very different colour to the remaining part of the body. This is due to the boundary conditions imposed in the software, as the inner part of the cylinder is in contact with a heat source.

A comparative analysis was performed to verify how much the two tests differ. This analysis was done only for the charging phase. As the test carried out in the laboratory has a longer duration, the comparison was based on the duration of the simulation, i.e. 18000 seconds.

Figure 114 Comparative analysis between the results of the two tests

What emerges from the figure is how the profile of the maximum temperatures have similar values only in the initial phase of the test while in the subsequent instants the temperature trend of the laboratory test remains constant while the data obtained from the simulation continues to increase.

125 On the other hand, the average temperature trend obtained from the simulation has more or less the same values as those obtained from the laboratory test. While the minimum temperatures have different values for the entire duration of the test.

To see in more detail how much the two tests differ, the percentage deviation for all the temperature profiles of the obtained temperatures has been calculated.

Figure 115 Percentage calculated from the comparative analysis

The percentage deviation relative to the minimum temperatures is the most interesting curve.

In fact, it tends to have an almost constant trend throughout the study but with very high values.

On the other hand, the percentage deviations of average temperatures and insulation temperature are those that tend to have a value close to 0%. While the percentage deviation relative to the maximum temperature has a percentage deviation close to 0% in the initial phases, as the two tests in these first moments tend to assume the same values, and then increase.