Techniques for the Thermal Analysis of PCM

*Abdelwaheb Trigui*

### **Abstract**

Thermal Energy Storage (TES) technologies based on Phase Change Materials (PCMs) with small temperature differences have effectively promoted the development of clean and renewable energy. Today, accurate thermal characterization is needed to be able to create an optimal design for latent heat storage systems. The thermo-physical properties of PCMs, namely latent heat, phase-change temperatures, enthalpy and specific heat capacity are obtained by means of differential scanning calorimetry (DSC), which is one of the most widely used techniques to study reactions related to the transformation of a material subjected to temperature constraints. This method presents some limitations due, among other things, to the fact that only a very small quantity (less than 90 mg) of material can be tested. Indeed, the small mass samples, taken out of the large testing specimen and out of testing system, is not representative of the thermal behavior of a material on a large scale. The Transient Guarded Hot Plate Technique (TGHPT) presents several advantages when compared to the commercially available thermal analysis methods (DSC, DTA) to determine PCM thermophysical properties. The most significant are large sample amount, optimized measuring time and a simple and economical built up.

**Keywords:** thermal energy storage (TES), phase change materials (PCMs), differential scanning calorimetry (DSC), transient guarded hot plate technique (TGHPT)

#### **1. Introduction**

Phase change materials (PCMs) are thermal storage materials with a high storage density for small temperature range applications. The determination of the thermophysical properties is a key step not just for the application itself but also for the material selection to define the suitability of a material for use in TES. The choice of the suitable thermal analysis method for experimental data acquisition depends on the device outputs, measured values accuracy, experimental setup requirements as sample size, influence of heating/cooling rate maintenance and equipment price, the implementation, etc. There are different thermal analysis techniques to characterize materials and the convenience to use them relies on the properties that want to be determined. The non-steady-state or transient technique records a measurement during the heating/cooling process. The method determines the enthalpy change of the PCM as a function of temperature with high precision by means of transient sensors. Depending on the application, the amount of PCM used can vary from a few

(milli) grams (e.g., cooling of electronic equipment or clothes) up the kilograms (e.g., storage in a solar thermal power plant or in the walls of buildings in thermal management and occupant comfort). To determine the specific heat and latent heat of materials a number of thermal characterization techniques help to provide better results especially differential scanning calorimetry (DSC) are commonly used [1]. DSC is an effective method to characterize the thermal behavior of PCMs, and to determine their TES capacities, in terms of transition temperature, latent heat and specific heat capacity and its stability throughout the various melting and crystallization cycles. Using the data measured by the DSC method, it is also possible to represent the enthalpy change versus temperature and determine the amount of stored/released energy in a given temperature interval. In case of the heat capacity measurement for a sample which does not undergo phase change, the energy supplied is weak and generally not very variable. On the other hand, in the case of a fusion process, there is a rapid transient which require important heat rates from the DSC. The thermal imbalance between the two cells is then very important although the quantity of product remains low. DSC presents some problems in analyzing PCM due to their high enthalpy density per unit volume: the small sample amount (less than 90 mg), the sample size influence on its thermal behavior, the response dependence on the used heating rate, repeatability when undergoing huge number for solidification/melting cycle. For large size samples, the melting process occurs gradually through the material. The latter is then heterogeneous and the two phases may coexist over long periods of time before a complete fusion. Moreover, heat conduction in the solid and convection in the liquid occur [2]. This strongly influences the global (or apparent) behavior of the PCM. In practical application, the material volume is much more important, up the kilograms for instance, so testing using a noninvasive method for latent and specific heat determination is necessary. It was found that identification of thermophysical properties of PCMs over several cycles (solidification and fusion) requires the design of a new experimental device Transient Guarded Hot Plate Technique (TGHPT). The TGHPT being an alternative to DSC measurements allows one to determine the same thermophysical properties as DSC.

### **2. Differential scanning calorimetry (DSC)**

DSC is nowadays the most used technique to determine the melting/solidification points and the latent heat of phase change and specific heat capacity of PCM. It is also useful to observe other phenomena such as supercooling, hysteresis and glass transition [1]. The TA Instruments DSC is a "HEAT FLUX" type system where the differential heat flux between a reference (e.g., sealed empty Aluminum pan) and a sample (encapsulated in a similar pan) is measured. The measuring device operates in the temperature range from 150 to 700°C. To attain information on the heat flows corresponding to the temperature fluctuations the differential scanning calorimeter uses an operation analogue to Ohm's Law. In DSC analysis, the equipment has two melting crucibles, one empty and used as the reference, and the other with the substance to analyze (sample). Then it is programmed an ascendant ramp of temperature versus time. It was found that the difference between the two melting pots to obtain a signal. A schematic diagram of Heat Flux DSC is shown in **Figure 1**.

The sample and the reference are placed symmetrically in the furnace. The furnace is controlled under a temperature program and the temperature of the sample and the *Techniques for the Thermal Analysis of PCM DOI: http://dx.doi.org/10.5772/intechopen.105935*

**Figure 1.** *Schematic diagram of Heat Flux DSC.*

reference are changed. During this process, three situations can occur with respect to the sample:


The thermogram is a plot of heat flow against temperature profile. During thermal process reactions either liberate or absorb heat. Thus, when *ΔH* is positive (endothermic reaction), the sample heating device is energized and a positive signal is obtained; when *ΔH* is negative the reference heating device is energized and a negative signal is obtained. The peak areas in DSC are proportional to the amount of sample, the heat of reaction.

Commonly, calorimetric measurements on PCMs affect the shape, precision and accuracy of the experimental results are:
