The continuous increase in the level of greenhouse gas (GHG) emissions and the increase in fuel prices are the main driving force to utilize various source of renewable energy. Among the clean energy technologies, solar energy is recognized as one of the most promising choice since it is free and provides clean and environmentally friendly energy. The objective of this paper is to improve the heating capabilities of evacuated tubes solar cookers that operates based on a closed loop thermosyphon action, and utilize thermal oil as the heating medium. Two identical evacuated tube solar cookers have been designed and built to study the influence of inserting metallic wires or nanographene particles on the thermal performance of the cooker. Also, the study was based on the thermal transfer of natural convection of the thermal oil movement within the evacuated tubes. The metallic wires and the nanographene particles are inserted inside the evacuated tube, which is filled with the heat exchange oil. One cooker is always tested without any modifications, i.e. taken as a reference point. The other cooker is the developed cooker, i.e. with wires or particles. Steel, aluminum and copper wires have been examined, and the number of wires has been varied between 5, 10 and 15. It has been found that the copper wires improve the rate of heat transfer compared with steel wires, aluminum wires and the nanographene particles. It has been also obtained that there is a critical number of wires, i.e. 10 wires, above that the rate of heat transfer by natural convection decreases and this is due to the increased friction which resists the natural convection currents. Finally, adding nanographene particles increases the viscosity of the oil, which increases the resistance to natural convection currents and consequently decreases the rate of heat transfer by natural convection. (C) 2019 Published by Elsevier Ltd.