University of Tehran Journal of Solar Energy Research 2588-3097 10 4 2025 10 01 Upgrading Solar Thermal Performance with Engine Oil in Evacuated-Tube Collectors for Sustainable Energy Solutions 2691 2701 105220 10.22059/jser.2025.404409.1657 EN Estabraq Khudhair Abbas Ministry of Higher Education and Scientific Research/Scientific Research Authority, Iraq Khalil Alwan Hussien Ministry of Higher Education and Scientific Research/Scientific Research Authority, Iraq Mudhar A. Al-Obaidi Technical Instructor Training Institute, Middle Technical University, Baghdad 10074, Iraq Farhan Rashid Petroleum Engineering Department, College of Engineering, University of Kerbala, Karbala 56001, Iraq Hazim Abdulla Jasim Ministry of Higher Education and Scientific Research/Scientific Research Authority, Iraq Jabbar Ahmed Abdullah Ministry of Higher Education and Scientific Research/Scientific Research Authority, Iraq Moneer H. Tolephih Al-Naji University, Baghdad 10015, Iraq; Tanvir Hussein College of Administrative and Financial Science, Gulf University, Sanad, 26489, Kingdom of Bahrain Journal Article 2025 10 16 The high-demand for solar power systems has stimulated research efforts to find better heat transfer fluids for evacuated tube solar collectors (ETSCs). This research aims to appraise the thermal characteristics of an ETSC, which utilises engine oil as a heat transfer vehicle within a new efficient system beyond conventional flat-plate collectors. Temperature elevation under solar radiation becomes rapid because the evacuated tube contains engine oil with Grade 20-w50, which possesses a high-boiling point of >350 °C and heat transfer properties including 2.5 kJ/kg K heat capacity and 88 kg/m³ density. Experiments are conducted for different ranges of solar radiation intensity with different ranges of engine oil temperatures. According to the results, this system reacts swiftly to solar radiation changes and reaches a maximum temperature of 198 °C at a solar radiation of 800 w/m2, which makes water evaporation and superheating possible. More importantly, in comparison to traditional flat-plate collectors, this collector demonstrates a high conversion efficiency and quick response to the influencing parameters. The introduced approach not only enhances energy efficiency but also brings into line with the United Nations Sustainable Development Goal (SDG 7) to deliver reasonable and clean energy solutions to conflict climate change and improve sustainable industrialization. https://jser.ut.ac.ir/article_105220_277154781c3f6245ea6426cc7f4c2a0f.pdf