A coaxial evacuated solar tube has been analyzed. The tube is included in a small-scale concentrated solar power (CSP) system, which runs a cogeneration Stirling engine unit. The engine provides electricity and at the same time generates hot water for heating and sanitary purposes, by cooling down the compression cylinder. The present work is focused on the thermodynamic characterization for a forced-flow in the coaxial evacuated tube, which can heat thermal oil up to 300 °C, when coupled with a parabolic trough collector. The single coaxial tube is 2 m long, it has one glass penetration, it is provided with a glass–metal seal and it has an absorber tube in the focal point with a diameter of 12 mm. A model based on heat transfer analysis coupled with fluid dynamic is presented and discussed. The model is then used to investigate spatial temperature profiles and thermal behaviors for the whole solar collector. It improves previous works in the field of concentrating solar collectors and covers the research in small-size concentrating system using thermal oil as heat transfer fluid.