Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.
This paper presents a one dimensional thermal model for an evacuated tube under transient conditions. The first order differential equations derived are solved with a fully explicit scheme using a fourth order Runge-Kutta algorithm. The comparison between simulated and experimentally measured outlet air temperature showed a good agreement as a root mean square error on the outlet air temperature of about 0.52 °C and a mean bias difference of 0.20 °C were observed on a bright sunny day. Finally, the validated model applied for steady state heat transfer is used to conduct an analysis on different parameters. The airflow rate is the most important parameter acting on the defined performance indicators. Higher is the airflow, better is the efficiency and lower is the outlet air temperature. On the other side, a low airflow can lead to as much as 100 °C of temperature gain, but the efficiency is then reduced to values as low as 45 %.