Thermal model of a parabolic trough solar field with a closed-loop operation during sunrise period

This work presents a numerical simulation of a thermal model for a solar loop with parabolic trough collectors (PTC) considering fluid recirculation at closed-loop operation during sunrise. At the beginning of the day, the heat transfer fluid is recirculated in a closed-loop in order to obtain the inlet loop operating temperature without resorting to additional preheating energy. Energy balances are carried out on the heat transfer fluid (HTF), the absorber tube and the glass envelope as a function of optical and thermo-physical parameters of the heat collector element (HCE). A system of second-order differential equations was established and mathematical model resolved by finite difference and Newton-Raphson methods for solution. This model has been well validated by comparing the results with existing experimental and numerical data. Three typical days of winter, spring and summer were simulated for the solar loop operation considering a closed-loop (CL) fluid recirculation at start-up conditions. Results show a more flexible closed-loop operation at relatively large flow rates compared to the open-loop (OL) operation, which requires substantial preheating energy at the same conditions; the start-up solar field using a closed loop recirculation allows us both operational autonomy and significant energy savings. Solar loop thermal and optical powers gained and lost are plotted for the typical days considered; we observe that maximum thermal efficiency of 66.53 % is achieved at 2.27 p.m. for the summer day.