Phase change materials for thermal packaging of agricultural products

BOUCIGUEZ, A.C., LARA, M. A., VILLA, L. T.

La Plata

Jornada; Argentine-Germany: a century of scientific cooperation in Physics; 2011

FCE-UNLP

The agricultural products need adequate condition temperature and controlled atmosphere of storage and transport. Each product needs different temperature condition; it must be maintained into a certain range, which guarantees the conservation of its organoleptic characteristic during a certain period of time. All the biological products generate heat (respiration rate). When they are stored, little changes takes place, and the respiration rate produce heat, which must be taken into account when cooling and storing those products. Temperature is an important factor in any chemical reaction with heat generation. Fruits, vegetables, flowers, etc. produce high respiration rate when they are storage. Nearly all vegetables have a minimal and a maximum respiration rate for each temperature. To estimate the respiration rate, when the product is storage at equilibrium state, the lower value might be used to estimate the storage temperature; the higher value might be used for calculating the heat load for the first or two days. The best results in the packing are obtained when the temperature is approximately constant. Variations of one or two degree can produce a quickly ripening and increase the risk of putrefaction. A way of keeping the product at constant temperature is removing its internal heat generation. The use of phase change materials (PCM) is proposed for these purposes. It is placed in container specially designed, which are positioned along the wall with the agricultural products. The dimensions of the packing (width, length and height) are big in comparison to the wall thickness containers, so convective effects can be neglected, and the heat flux is practically one-dimensional. The PCM is initially solid at the melting point; when it is heated, the liquid phase appears and the solid initial phase stays at the fusion temperature. So, it is necessary to study the distribution temperature only in one phase: the liquid one. For this reason that mathematical situation is named: “One phase Stefan problem”. It is important to say that the interface position, which is a priory unknown, must be determined too. In such a way, the performance of PCM is studied in relation to know the interface position and the temperature distribution inside it to maintain the agricultural product in adequate condition of temperature during a certain period of time. For the PCM, a defined melting point, a high latent heat of fusion per unit mass, a high specific heat, a constant density in the operation range temperature, and a similar density in the liquid and solid phase, are required. Besides, they must be chemically stable, and not corrosive, nor flammable, nor toxic and nor explosive. Some animal fatty substances can be used for that purposes, with density around 1100 kg/m3, latent heat in the order of 150 kJ/kg, specific heat about 1500 J/kg ºC and thermal conductivity about 0.2 W/m ºC in the liquid phase and 0.4 W/m ºC in the solid one. Its melting point must be in the order of the temperature condition of the agricultural products. It is possible to obtain artificial substances which present phase change at different temperature.