Purpose – The purpose of this paper is to use numerical simulations to investigate the energy conversion performance and the flow and temperature structures inside horizontal tubes connected to a vertical manifold channel. Design/methodology/approach – The simulations are performed for different flow rates and inlet temperatures using CFD. Findings – In both the “flowing wind mode” and “upwind mode,” the inlet velocity is not infinitely small under the influence of natural convection; however, such small inlet velocities cannot be achieved in practice and are of no practical significance. In the “flowing wind mode,” the appropriate velocity for achieving high efficiency is 0.01-0.02 m/s. In the “upwind mode,” the appropriate velocity for obtaining high efficiency is 0.1-0.2 m/s. A high inlet temperature can lead to high efficiency; therefore, a large temperature difference and a small flow can be used in actual designs. Originality/value – The energy conversion performance and flow structures inside evacuated tubular collectors were investigated using CFD for different operating conditions, notably in the “following wind mode” and the “upwind mode.”