does not fully melt and shows less latent heat release. In summary, paraffin filled tubes show better performance in normal operation mode compared with Erythritol filled
tubes.
Fig. 10b presents the inner tube temperatures recorded
during the cooling process after the stagnation mode. To mimic the stagnation mode on the second system, no water
flow is initiated through the manifold for an extended time until the PCMs in the tubes completely melt, then the systern (collectors) were covered completely from sun and water circulation starts by switching the pump on. It is observed that the initial temperature inside the standard
tubes reach 200 °C, for paraffin tubes temperature is around 155°C and for Erythritol tubes around 130°C.
The PCM tubes initial temperatures are above their related melting point, therefore phase change (melting) process has occurred for the majority of the paraffin and Erythritol material. After switching the pump on, energy transfers
to the water through the heat pipe and the manifold. The
paraffin tube temperature curve shows a plateau around
70 °C, and Erythritol tube shows a plateau around the
transition temperature of 100 °C.
As observed, during stagnation operation (on-demand),
Erythritol filled tubes show an improved performance with
higher storage capacity compared with paraffin filled tubes.
It can be seen that, as in the stagnation operation the temperature inside the tubes can reach to a higher value compare to the normal operation, so it allows the Erythritol to
be fully melted this time, so the Erythritol would have
higher rate of heat storage capacity and consequently results in the slower cooling rate inside the tubes. The
delayed cooling of PCM collectors can extend operation of SWH systems when solar radiation is not enough (after sunset).