Thermal inertia: definition and materials

Thermal inertia: definition and materials

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Thermal inertia: together with conductivity, always thermal, it is very useful, especially when you want or need to evaluate theenergy efficiency of a structure. It may happen that you find yourself doing this operation, following the completion of works by thermal insulation. Or precisely because the need is seen.

It is a parameter, thethermal inertia, which measures the ability of a material to oppose the passage of heat flow and to accumulate part of it, maintaining a homogeneous, constant and comfortable internal temperature, despite the external temperatures going up and down.

Thermal inertia: definition

For thermal inertia, in thermodynamics, we mean the ability of a material or structure to vary its temperature more or less slowly in response to changes in external temperature or to a source of internal heat / cooling. It is the perfect analogue of the inertia of mechanical systems, just substitute mechanical energy for thermal.

Again theoretically, it can be evaluated in terms of temperature / time * temperature difference (s-1) but the most common unit of measurement is the one that derives from the formula power / time * temperature difference therefore W / s × K .
As greatness l'thermal inertia is directly proportional to the specific heat of materials and their mass, on the other hand, is inversely proportional to thermal conductivity and the temperature difference between inside and outside.

Thermal inertia: materials

For a building or walls the high materials thermal inertia I'm compact concrete and bricks, even natural stones are not bad: they must be able to slow down the flow of summer heat inwards and to store heat in the winter in order to be able to release it inwards.

More generally, a material with thermal inertia good must have both good insulating properties, but must also be able to accumulate heat on one side and not transfer it directly. Abrupt changes in temperature on the outside must not be reflected "as if nothing had happened" on the inside, time must pass. This time that makes the internal temperature change "deferred" compared to what happens in the rest of the world, it is said phase displacement.

In physics, the phase shift can be seen as the time it takes tothermal wave to flow from the outside to the inside through a building material. Going back to the materials, if they have a high specific heat they have one phase displacement greater: the more the material is able to absorb heat, the more it will be able to release it slowly.

Thermal inertia of the wall

Placed in front of a wall, to evaluate itsthermal inertia, we can say that it has a stabilizing function of the temperature if it is very insulating and at the same time it is able to store a lot of heat which is then transferred to the internal environments during the night.

It is difficult for this mix of thermal powers to be obtained with just one material: it is therefore better to resort to layered casings. The best composition as for thermal inertia would be the one in which there is a layer with a high thermal capacity inside the casing and an insulating one on the outside. The first keeps the internal temperature constant, the second protects the internal environment from changes in temperature.

The wall thus formed, or otherwise, must mitigate sudden changes in temperature that come from the outside, whether the outside is the actual external environment, or whether it is another room always inside the building in question. Solar radiation, people, appliances: there are many factors that can cause them changes in temperature but the wall with high thermal inertia it must be able to soften these thermal "shocks". In the summer season they are more frequent than in the winter.

If we want to describe the thermal inertia of one of our walls, we take into consideration two thermal properties of the same: the periodic thermal transmittance (Yie = W / mqK) and the periodic internal air heat capacity (Cip = kJ / mqK). The first represents both the degree of damping and the phase shift of the thermal variation that occurs outside and tells how the wall is able to "control" and mitigate it. The Internal Periodic Areic Thermal Capacity (Cip), on the other hand, is the capacity of a material or wall of accumulate heat or cold coming from the inside: the higher the CIP, the more surface temperatures are kept at acceptable levels, the more comfortable you are in your environment and the less you spend on summer air conditioning.

Thermal inertia of a building

L'thermal inertia of a building consists in its ability to retain the heat inside its walls over time once the heating system is turned off. High thermal inertia as already mentioned, it means low energy consumption, both when we want to cool and when we want to heat our indoor environments. And it also means having a good level of comfort.

When we think ofthermal inertia of a building, we often focus on the material with which the external walls are made, or on the layers that make it up, without thinking that their position is also important. Which goes inside and which goes outside? Excellent ingredients badly "mixed" give a bad result: it is therefore necessary to look at it with more respect and foresight constructive solution as a whole and not just material by material, the walls.

In a building there'thermal inertia of the perimeter walls is not the most important for the purposes of the inertial response of the building which is concentrated on average for about 70% in the internal structures while only 30% in the external-perimeter ones if they are not insulated.

In addition, thethermal inertia it is not strongly dependent on the thickness and / or weight of the brick blocks adopted while substantial improvements in terms of thermal inertia are obtained by placing a thermal insulation panel with good characteristics on the external side of the masonry. (eg. "external insulation"). Thus we concretely limit the escape of the heat accumulated or produced inside.

Thermal inertia heating and cooling

L'thermal inertia of a building or a single wall is also used as a contribution to heating and cooling. In the first case, just think that an envelope with athermal inertia high does not disperse internal heat and accumulates heat during the hottest hours of the day. An envelope like this also takes longer to release the accumulated heat. For heating, the ideal is when it happens that the heat accumulated during the day is transferred inside at night so as not to have to use systems that heat.

Let's turn the conversation around when we want to refresh the rooms in the summer. In this case, an interesting resource is water with its own thermal inertia capable of maintaining a low temperature and cooling adjacent rooms. Basins containing water are placed near the perimeter of the building, they cool down at night and remain protected from solar radiation through a system of shields. What happens? During the night, the water cools down during the night, then during the day, shielded from the sun, it slowly releases "its" coolness to the building, giving us relief. The following night the water will cool down again to give us more relief the following day.

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