**3. Transient guarded hot plate technique (TGHPT)**

The Transient Guarded Hot Plate Technique (TGHPT) provides an alternative approach to characterize some of the thermo-physical of PCMs. Most of thermal analysis techniques, such as the DSC method are designed to test small, pure and homogeneous samples. During long-term performance testing, when undergoing huge number for freezing/melting cycles, large PCM specimens often are subject to settling or stratification and are typically not perfectly homogeneous. The TGHPT being an alternative to DSC measurements allows one to determine the phase change material (PCM) thermal properties (thermal conductivity *λ*, sensible and latent heat thermal energy storage, *CP* and *Lm* latent heat of fusion or melting enthalpy) in the

solid phase, during the solid–liquid transition and in the liquid phase and its thermochemical stability after thermal cycling test. The TGHPT eliminates the previously mentioned limitations of DSC measurements (i.e., mass influence, low thermal conductivity and thermal diffusivity of PCMs, non-equilibrium thermal gradients in the DSC pan and heating/cooling rate) and also allows for measurement of much larger samples than DSC (about 1000 times larger) [19]. The TGHPT has some advantages over other methods: it can be used, due to large sample size, for a wide variety of PCM (inorganic and organic, being encapsulated or composite), heating and cooling rates and temperatures ranges are variable and large enough to fit PCM for different applications. Therefore, for PCM characterization large samples are preferable. The TGHPT method has also the advantages of (i) a simple experimental device as it can be built up with basic laboratory devices (ii) lowest maintenance and lowest price. To clarify the effects on the heat transfer performance, an experimental device allows to determinate changes in enthalpy (latent heat) or the specific heat capacity (sensible heat), thermo-physical properties like thermal conductivity of an examined sample by Trigui et al. [6] (**Figure 5**). The proposed test bench for the parallelepiped-shape of composite (4, 5 4, 5 0.6 cm<sup>3</sup> ) allows the simultaneous measurements of the temperature and heat flux at the material edges. The PCM material is placed between two horizontal exchanging aluminum plates. The plates get tempered by fluid thermostats with a precision of about 0.1°C. Each side of composite is equipped with a heat flux sensors and thermocouples (type *T*). These heat flux sensors fixed at the sample surfaces to measure input and output heat flux have 0.2 mm of thickness, 202 μV/W/m<sup>2</sup> of sensitivity and 400 cm<sup>2</sup> surface areas. The sensors were connected to LabVIEW software to measure temperature fluctuations and heat flux exchanged during melting and solidification processes which allow the investigation of the thermal stability of the samples. The acquisition of the experimental data is recorded at regular and adjustable time steps (1–6 s). To ensure a one dimensional heat flow through the sample the lateral sides are insulated by 11 mm thickness of expanded polystyrene. The enthalpy–temperature curve of a sample is measured similar to DSC measurements by heating up and cooling two aluminum heat exchanger plates in

#### **Figure 5.**

*(a) Scheme of the Transient Guarded Hot Plate Technique (TGHPT) for testing PCM materials, (b) Picture of the whole apparatus.*

#### *Techniques for the Thermal Analysis of PCM DOI: http://dx.doi.org/10.5772/intechopen.105935*

parallel at a certain heating rate or in stepwise mode. The TGHPT was developed to test construction materials with PCM in large scale samples in different boundary conditions following:


With the TGHPT, the thermal performance of PCM such as the thermal conductivity, its temperature in liquid and solid state for PCM, the specific heat capacity and the latent heat can be determined. Furthermore, it is possible to investigate the thermal performance of PCM material such as the thermal stability and repeatability to store and release the thermal energy.

The TGHPT optimize measurement with shorter experimental time than DSC.

#### **3.1 Apparent thermal conductivity**

The experimental sample is placed in between two aluminum heat exchanger plates, and the position of the plates is adjusted to establish a full contact with the surface of the measured sample. To determine experimentally solid and liquid thermal apparent conductivities of the composite, the apparatus establishes a steady state one-dimensional heat flux through the test sample between the two plates. The plates are set a at constant but different temperatures to establish a thermal gradient through the sample. The temperature difference between the top and bottom plates is a user defined value to perform measurements at various averaged temperatures. The apparent thermal conductivity can be calculated using Fourier's law of heat conduction. The thermal conductivity of the test sample is given by Eq. (7) [3, 20, 21]:

$$
\lambda\_{s,l} = \frac{e.\sum \mathcal{Q}\_{s,l}}{2.\Delta T\_{s,l}} \tag{7}
$$

where *e* is the thickness of the specimen; Σ*φ<sup>s</sup>*,*<sup>l</sup>* is the sum of the measured heat fluxes of the solid or liquid states.

As an example, the composites materials (LDPE/Wax) were studied for thermal energy storage with a melting point around 26°C [6]. In order to characterize the apparent thermal conductivity of the solid phase, the TGHPT has been used with temperature variation only in a single face of the sample. The state of initial balance (*Tinit* ¼ 15°C , lower than the melting temperature (*Tm*)) is brought back towards another state of final balance (*Tend* ¼ 20°C), such that *Tend* <*Tm*) (fusion process)) where heat flux tightens towards a non null value corresponding to a temperature gradient generated by the two heat exchangers plates. For the liquid phase, the same experimental procedure was applied by choosing a temperature difference (40 and 50°C). **Figure 6** summarizes the measured thermal conductivities and associated uncertainties of the composites with different Wax contents.

**Figure 6.** *Thermal conductivities of LDPE/Wax composites [6].*

#### **3.2 Sensible heat and apparent heat capacity (in solid and in liquid states)**

At the beginning, the two aluminum heat exchanger plates and the composite were maintained at a constant temperature *Tinit*. By modifying the temperature set point of the thermo regulated bath, the composite evolved to a steady isothermal final temperature*Tend*. Between both states, the PCM composite stored an amount of sensible heat, which represents the internal variation of the system's energy [22]. The heat capacity of the material was determined for the solid and liquid phases by calculating the integral of the heat flux difference from the initial state ð Þ *tinit* and the final state *t*ð Þ *end* using the following expression Eq. (8) [22, 23].

$$\mathbf{Q}\_{sens} = \frac{\mathbf{1}}{\rho \text{ }\mathbf{c}} \int\_{init}^{end} \Delta \otimes \mathbf{dt} = \mathbf{C}\_p \cdot (T\_{end} - T\_{init}) \text{ [kJ/kg]} \tag{8}$$

Where *CP* is the apparent specific heat capacity of the composite, Δ∅ represents the difference heat flux measured at each time step during the acquisition *dt*, *e* is the composite's width and *ρ* is its density.

To evaluate the apparent specific heat of the PCM, Different tests were carried out:


**Figure 7** presents the evolution of the heat fluxes and temperatures on both sides of two samples during the determination of the specific heat capacities of the LDPE/ Wax composite in the solid and liquid states respectively. For the liquid and solid phases, the measured temperatures on the lower (*T*1) and the upper (*T*2) faces of the material evolve in an asymptotic way to the set point. Also, we can note that the flow *Techniques for the Thermal Analysis of PCM DOI: http://dx.doi.org/10.5772/intechopen.105935*

#### **Figure 7.**

*Heat flux and temperatures evolution of the solid phase (15–20°C) [6].*


#### **Table 2.**

*Quantity of heat stored and apparent heat capacity for (LDPE/Wax) composites [6].*

evolves very quickly at the beginning of recording and then to a steady state obtained at the end of the test. The results obtained for the specific heat capacity and the sensible heat stored for the solid and liquid states are given in **Table 2**.

#### **3.3 Storage and release of latent heat**

LHTES is the most promising method in this field due to excellent phase change behavior and high heat storage capacity. The amount of total energy storage/release can be calculated by a temperature variation from 15 to 50°C. Between these two positions, the composite stores and releases a sensible *Qsens* ð Þ heats and latent (*Lm*) heats. The Latent heat is calculated by subtracting the sensible heat from the total amount of heat. This quantity can also be expressed by Eq. (9) [24, 25]:

$$Q = Q\_{sens} + L\_m = \begin{pmatrix} C\_{ps} \ \Delta T\_s + C\_{pl} \ \Delta T\_l \end{pmatrix} + L\_m \begin{bmatrix} \text{kJ/kg} \end{bmatrix} \tag{9}$$

where *Cps* and *Cpl* are the heat capacities of composite at solid and liquid state, respectively, Δ*Ts* and Δ*Tl* are the temperature variations in solid and in liquid state and *Lm* is the latent heat par unit mass of melting.

The selected temperatures (15–50°C) are sufficiently far from the melting point to consider the material as being strictly in one state or the other. **Figures 8** and **9** show that the heat stored is much more important than sensible heat transfer influenced by

**Figure 8.** *Heat flux and temperatures evolution from solid to liquid (15–50°C) [6]*.

**Figure 9.** *Latent heat of LDPE/Wax composites [6].*

the paraffin wax addition. Hence, the addition of PCM significantly improves the thermal performance of composite in terms of saving energy.

**Figure 10** shows present the evolution of the heat fluxes and temperatures on both sides of the composite during the solidification process where the temperature evolves from 15–50°C. Initially, it can be observed a normal evolution of measured heat flux corresponding to the cooling of the liquid phase. At the end of this phase (*t* = 33 min), when the temperatures of surfaces are in the vicinity of 28°C, it can be seen the release of latent heat, and the appearance of a distinct inflection point in the heat flux curves. From this critical moment, the cooling of the sample continues, the material is solidified slowly and cools until it reaches the prescribed 15°C: after more than 1 h30 min, the sample reaches an equilibrium state. Notably, heat restitution was a very long process since at the start of the solidification. At the beginning of solidification, a layer solid Hexadecane was formed at the upper surface of the material which was in

**Figure 10.** *Heat flux and temperatures evolutions (50–15°C) [6].*

contact with the heat exchanger plate. This slight layer "insulates" the liquid PCM from the cooling source and reduce the effect of the convection mode. Then, the solidification continued slowly because of the low thermal conductivities of the solids SS-PCMs until the sample temperature reached 15°C at the end of the test. Furthermore, it can be noted that the period for solidification was much longer than the melting process.

### **4. Conclusion**

Phase change materials (PCMs) with its capacity of storing thermal energy as latent heat is a viable approach of the utilization of solar heat, a green source of energy, and the optimization of energy consumption in buildings. An accurate material characterization of thermal energy storage materials including energy storage capacity and phase change temperature, during several charging and discharging cycles is significant to develop an efficient thermal storage device or application. Conventionally, thermal characterization of materials is performed using thermal analysis techniques especially Differential Scanning Calorimetry (DSC). However, all these methods involve very small samples that can be significantly influenced by local heterogeneities, the response dependence on the used heating rate, etc. For investigation of PCMs in real conditions, it is essential to design an experimental device to provide robust insights on the steady-state parameters, the dynamic thermal responses, the heat storage capacity of PCM at large scale, to meet the design requirements of thermal energy storage applications and to prevent potential failures. This chapter reports on successful use of a specific experimental method to measure the temperatures and the heat fluxes in order to characterize the phase change effects. The Transient Guarded Hot Plate Technique (TGHPT) provides reliable information about thermal conductivity, fusion enthalpy, specific heat and thermal stability for large phase change materials samples. Heat flux measurements make it possible to highlight very specific behaviors of these products and are thus a very interesting experimental source of data which comes to complete the traditional measurement methods like calorimetric device (differential scanning calorimetry). The Transient

Guarded Hot Plate Technique (TGHPT) allows large samples analysis and entails less maintenance, equipments price and time evaluation than the DSC. Thus, it is the most suitable method for PCM characterization.
