1.1. Salt/graphite composite building

The structuring of the salt/graphite composites produced by cold compression method or infiltration method [3], exposed fundamental problems, which define the thermal capacity and thermal stability of the modified PCM composite for heat storage. On Table 1 are presented scanning electron microscopy (SEM) images of the (Na/K)NO3 eutectic/graphite composite structures.

Table 1. SEM structural images of (Na/K)NO3eutectics/graphite composites made by two elaboration

methods.

1.2. Thermal stability of the modified PCM: (Na/K)NO3 eutectics/graphite composite

Optimisation of the modified PCM according the specific technology requirements was based on correlation between structural and thermal properties of the composite. Thermophysical characteristics, including thermal storage capacity and thermal stability tests at thermal cycling of heating/cooling were collected on a first step from Differential scanning calorimetry (DSC) measurements. Although the small amount of the sample material (up to 20 mg) the DSC data provided with a clear illustration of PCM composite thermal behaviour. The DSC curves, plotted for different modified PCM (Fig.1) are well informative for particular

(Na/K)NO3eutectic/graphite composite thermal properties. Composites show repeatable kinetics and thermal stability of micro samples tested at multiple DSC thermal cycling, while macro sample thermal stability needs to be controlled and improved further. Temperature hysteresis, calculated from peaks of DSC melting/ crystallization curves, varies from 2.69 to 4.09 degrees, depending on a composite type. The NaNO3/KNO3eutectics/graphite composite, elaborated with 20wt% of a ground expanded natural graphite of 500pm particle size (GFG500-20wt%), named Composite 3, shows temperature hysteresis of 3.5 degrees, while the hysteresis of NaNO3/KNO3eutectics/natural graphite (NG), named Composite 1, is 2.69 degrees. The sub­cooling characteristics, measured from the onsets of DSC heating/cooling curves are in limits of

3.2

— 4 degrees.

Analysis of the well-defined single peaks on melting and solidification DSC curve of the Composite 1, verify its proper thermal performance of heat absorption/heat released during thermal cycling. On Table 2 are summarized typical DSC data of the optimal modified PCM: the salt/graphite Composite 1.

Thermophysical characteristics correspond to a better DSC thermal behaviour of the Composite 1 in comparison to the Composite 2 or Composite 3 in this typical case. Maximum thermal capacity achieved is in the range 85 -87 kJ/kg.