  Rubitherm RT 27 is a paraffin-based phase-change material (PCM) in which a
change between a solid and liquid phase is used to store/release thermal
energy. Its enthalpy and heat capacity, as measured in a quasistatic regime by
adiabatic scanning calorimetry, has a single distinct jump and peak,
respectively, at about 27.3 C. We present a microscopic development from which
the jump and peak can be accurately fitted and that could be analogously
applied even to other PCMs. It enables us to determine the baseline and excess
part of the heat capacity and thus the latent heat associated with the phase
change. It is shown to be about 84 % of the total enthalpy change that occurs
within 5 C from the peak maximum position. The development is based on the
observation that PCMs often have polycrystalline structure, being composed of
many single-crystalline grains. The enthalpy and heat capacity measured in
experiments are therefore interpreted as superpositions of many contributions
that come from the individual grains.
