Specific heat determination (DSC)
In thermodynamics, the amount of heat that is supplied or withdrawn from a system is expressed in isobaric processes by the enthalpy change of the system. The energy storage capacity of PCMs is normally defined by the sum of the sensible and latent enthalpies. Latent heat is related with phase changes, while outside of a phase transition, the enthalpy change is characterized by the specific heat capacity (or specific heat). This property is defined as the amount of energy required to increase, per unit mass, the temperature of a substance by one degree. For constant pressure processes, this property is referred to as constant pressure specific heat (Cp).
For PCMs, a proper characterization of the specific heat capacity is essential in order to determine the storage capacity of the material. This is normally performed by DSC. There are different procedures available for the specific heat determination, each with its own advantages and disadvantages. The most widely known routine is the sapphire method. Which offers a good compromise between accuracy and complexity.
In this procedure, The DSC signal of the sample is compared with the DSC signal of the calibration sample of known specific heat (sapphire). A blank curve correction is also required. A total of three measurements are made: blank (empty crucible), sapphire and the sample itself. The two sapphire and sample curves are blank corrected. The specific heat of the sample is obtained (as a function of temperature) from the heat flow signal obtained, as follows: Cp=(HFsample*msap*Cpsap)/(msample*HFsap).
The typical heating rates employed range from 10 to 20 K/min. This is because, compared to the phase change, the heat flow signal in the sensible area is relatively low; thus, high heating rates are required to achieve an acceptable accuracy. According to the literature, the typical uncertainty is within 2-3 %. Segments not larger than 100 °C to 200 °C should be measured. If a wider temperature range is of interest, then it is better to divide it up into segments of maximum 200 °C seperated by isothermal segments of 5 minutes.
Additional procedures for the specific heat determination by DSC are the direct determination and temperature Modulated DSC (TMDCS). Further information about them can be found in the literature.