## 6 Conclusions and Outlooks  
This study presents an experimental comparison of three characterization methods (DSC, STEP, FM) for two PCM (paraffin RT58 and polymer PEG6000) suitable for DHW storage. The two PCM studied have very different thermal behaviors, as PEG6000 experiences supercooling, whereas RT58 has a similar behavior between melting and solidification processes. The following findings are recommended to perform an experimental characterization of a PCM:  
* Results from the DSC method are sufficient where a high accuracy is not required and when the PCM does not experience supercooling. The accuracy of the characterization might be improved using a calorimeter with the STEP method, but the experiments will be longer and it will be more complex to configure the settings of the calorimeter;
* A complete and accurate characterization of a PCM, with observations and quantification of local and global thermal behaviors, require experiments under various heat loads, with both heat flux and temperature measurements. Only the FM method can conduct this complete characterization. However, compared with DSC and STEP methods, no commercial device exists, the experimental set-up is complex and the experiments are time-consuming;
* Large samples enable a closer resemblance to the thermal behavior of a real-scale system, especially concerning supercooling, which is strongly influenced by the size of the sample. The FM method enables a much closer resemblance to the PCM mass of a storage, than DSC and STEP methods. Figure 12: Comparison of the results obtained by the three methods for PEG6000. Table 2 summarizes the advantages and disadvantages of the three characterization methods used in this study. Concerning the comparison between RT58 and PEG6000 for DHW storage, the experimental results show:  
* An experimental identification of specific heat, latent heat and phase change temperature consistent with the literature for the two PCM;
* Similar values of specific heat and latent heat for RT58 and PEG6000. This means an equivalent energy density for the two PCM. However, a greater amount of energy is stored inside a same volume for PEG6000, as the mass density is more important;
* The RT58 shows a melting and solidification behavior over a wider temperature range than PEG6000. Therefore, for RT58, the full potential of latent heat might not be fully exploited under the operating conditions of DHW applications. PEG6000 offers a narrower temperature range for the changes of state but, when tap water is drawn off, the PCM temperature must decrease below the supercooling breakthrough temperature to release the latent heat. However, once the solidification process has started, the heat peak is significant;
* Selection of the PCM must be made according to the selected application considering the complete system including the heat exchangers. An accurate modelling of the system is therefore required as PEG6000 and RT58 show different thermal behaviors, especially during the cooling process;
* Because of supercooling, PEG6000 seems more suited to a DHW storage operating with full charging and discharging cycles;
* For a DHW frequently operating with partial phase change cycles, if the PCM temperature does not drop below 52 \({}^{\circ}\)C, latent heat will not be released for PEG6000 and the selection of RT58 is recommended, where most of the latent heat is released between 59 \({}^{\circ}\)C and 53 \({}^{\circ}\)C.  
For future research on PCM characterization, it is recommended to investigate the thermal behavior of PCM for partial cycles, with an interruption of the heating and cooling loads during the phase change process. Concerning the experimental comparison of PCM suitable for DHW storage, the investigation might be extended to other PCM, and especially bio-based materials to improve the environmental impact of the storage.