# 2D Conduction Simulation of a PCM Storage Coupled with a Heat Pump in a Ventilation System
## 2 Experimental Facility
### PCM Properties  
The commercially available paraffin product, Microtek 37 [5] was chosen for the PCM. For the use as heating storage placed behind a heat pump (with air flow reaching about 45 \({}^{\circ}\)C), it was necessary to find a material with a melting temperature under 45 \({}^{\circ}\)C but higher than 25 \({}^{\circ}\)C, necessary for thermal comfort. Paraffin is also well adapted to a multiple phase changing without loose in performance. The thermal properties of the material were measured, and a summary can be found in Table 1.  
The heat of fusion was measured for a 405.05 \(\pm\) 0.05 mg sample, using the DSC method under a steady rate of heating and cooling of 0.15 \({}^{\circ}\)C\(\cdot\) min\({}^{-1}\). This rate was taken for the validation because it corresponds to a low rate made possible with the technique of the micro DSC (SETARAM Instrumentation, Caluire, France) that needs a higher mass for a better precision considering the heat flow resolution. Moreover, this rate corresponds to a close value of the temperature evolution in the process. The double peak that is possible to observe for very low temperature evolution of DSC is characteristic of the impure composition of the commercial product. For this same product, and for a rate of 1 \({}^{\circ}\)C\(\cdot\) min\({}^{-1}\), only one peak appears (see reference [3]). Phase change occurs when the temperature rises through the range 30 to 38 \({}^{\circ}\)C, with a major peak capacity observed at 37.2 \({}^{\circ}\)C. By the same method the heat of solidification was also measured, which was found to agree within 0.1% of the value obtained for melting heat, albeit with a peak at a lower temperature than when the sample is melting (major peak at 31.7 \({}^{\circ}\)C). The constituent paraffin of Microtek 37 belongs to the saturated hydrocarbon family. [PERSON] summarizes the advantages and disadvantages of these substances as paraffin for use in a PCM device [6]. Non-toxic and recyclable, paraffin offers high chemical stability, allowing it to undergo a large number of melting and solidification cycles without it separating. It also has a relatively high latent heat of fusion and, as it does not exhibit super cooling, nucleating agents are not necessary. Drawbacks include a rather low thermal conductivity of solid paraffin, and a significant hysteresis in curves of equivalent heat capacity when characterised using DSC. A further disadvantage is the reliance on fossil petroleum to produce commercially available paraffin [7]. In a study by the International Energy Agency [8] it has been shown that DSC measurements are sensitive to the rate of heating and cooling of the scan and the mass of the sample. Thus for faster tests (or smaller sample mass), the measured heat capacity curve is distorted and shifted towards higher temperatures in the heating case and lower temperatures in the cooling case. Since experimental curves obtained by DSC are the basis of simulations and dictate the rise and fall in temperature offset, systematic variations arising from the method with which they have been measured can have a considerable impact upon the predicted behaviour of a system. Indeed, a study addressing this issue has been presented in [9; 10]. In that work, an experimental procedure was implemented that entailed a series of temperature rises and descents in order to give a better indication of the macroscopic behaviour in the case of partial melting. It was then possible to deduce the preferable way to model the thermal behaviour of PCM using appropriate specific heat capacity curves obtained by DSC depending on the phase. Despite this improved method, some inaccuracies remain, and in particular the case of temperature changes near melting or solidification peaks. In addition, the rate of temperature variation in the material is variable, which is not the case for curves derived from DSC determined with a constant rate of temperature evolution and with a given mass.