FAT

116 One of the materials used as a case study for thermal storage with phase change material is animal fat.

This fat has been obtained from a previous cooking phase of the meat itself and has served to understand if an organic waste material can easily be reused for a possible thermal storage phase.

In the next results that will be illustrated, it will be possible to see how this material takes on characteristics that can be taken into account.

The first part of the analysis coincides with a recording of the data by a laboratory test.

This phase lasted for 195289 seconds and, having carried out two registration tests, the choice for the following discussion fell on the second.

Figure 108 Temperature trend obtained from the test in the laboratory

As can be seen from the figure 108, the maximum temperature trend reaches the peak with respect to the other temperatures attributed to the material taken into consideration.

Immediately after that it reaches its thermal stability, in which the melting phase of the material corresponds, up to the beginning of the discharge phase of the material.

As for the other temperatures, during the charging phase, the minimum temperature trend reaches higher values than those recorded by the T4 thermocouple, for the average temperature.

This particularity will be due to the positioning of the thermocouples but that, however, having

117 acceptable values we have considered them for the analysis. Moreover in the following phases the average temperature returns to have higher values than the minimum.

As regards the discharge phase, the curves tend to have higher values than the ambient temperature. This determines that the fat cools much more slowly, keeping the water warmer and dispersing as little thermal energy as possible.

In fact, at the end of the test it is possible to check the final water temperature, which in this case corresponds to 26 °C. Therefore the final temperature of the water has a value far higher than that of the external environment and above all of the final temperatures recorded in the other materials previously examined.

For a more in-depth analysis, a simulation was performed using the COMSOL Multiphysics 4.3a software. For this part of the test, only the charge phase to which the material is subjected, with a duration of 18000 seconds and a time interval of 600 seconds, is considered.

Figure 109 Temperature profiles obtained from the simulation

From the simulation, it has been shown that the maximum temperature of the material rises very quickly and then, after reaching a peak, the maximum temperature increases much more slowly than the initial phases of the simulation.

The average and minimum temperature increase but gradually and without peaks within the curve. On the contrary, the temperature related to the insulation remains almost low and close to the ambient temperature.

118 To highlight how these temperatures vary within the material, the body was studied by observing 3D reproduction. Some significant phases of the simulation have been chosen to describe the temperature trend into the body.

119 The substantial difference that can be seen from the beginning is how from the first to the second step the colouring is totally different. This is due to the fact that the material heats up quickly already in the early stages of the simulation while in the subsequent phases the colouring progresses gradually.

The first step of the simulation shows how the colouring is highlighted by the boundary conditions inserted by the software settings.

In fact the inner part of the cylinder has a very different coloration with respect to the different parts of the body since the inner part of the material is in contact with a heat source.

The final phase was to determine a comparative analysis between the data recorded in the laboratory and those obtained from the simulation.

In this case, only the charging phases of both tests were taken into consideration and based on the duration of the simulation, of 18000 seconds, since the duration of the laboratory test is much longer than the simulation, therefore the comparative analysis is been made on a temporal phase.

Figure 110 Comparative analysis between results of test in the laboratory and the simulation

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.