Study on carnauba wax as phase-change material integrated in evacuated-tube collector for solar-thermal heat production

The evacuated-tube collector (ETC) for solar heat production is one of the most efficient systems to convert Sun energy into usable energy. Unfortunately, some technological barriers are still present in solar-thermal heat systems that limit the use of these technologies to decarbonize processes. An important obstacle is the requirement for heat at stable temperatures, which is not always achievable by solar fields, e.g. variable sky. For that reason, in this work, we studied the phase-change materials embedded in the ETC to keep stable the water temperature for a short period (minutes, hours). In this study, carnauba wax was used as a sustainable phase-change material (PCM), to avoid the use of PCM based on hydrocarbon waxes. The PCM has been packaged using a polyethylene bag inserted into an ETC with heat-pipe technologies and tested. The collector has been tested outdoors under solar irradiation and under shading conditions. The experimental results show that is possible to detect a sensible effect of the PCM on the temperature using 4 kg of carnauba wax while there is an important effect on the temperature stabilization using 9 kg of PCM. Using 9 kg of the PCM, we observed a stability of the outlet water temperature at 65°C for 30 minutes under shading conditions.