Active Thermal Insulation – A Possible Option to reduce Thermal Inertia of Cavity Receivers

An Active Thermal Insulation (ATI) is proposed as suitable option to reduce the transient response of high-temperature cavity-type solar receivers. It consists of a straight tubes bundle, embedded in the insulating material, covering the entire receiver length through which the heat transfer fluid (HTF) is fed prior being injected into the cavity. The implications of adding the ATI to the reference absorbing gas solar receiver were evaluated through a series of 3D CFD simulations campaigns. Two key parameters on the ATI design (i.e., tubes number and radial tubes bundle radial position with respect to the cavity axis) were evaluated assuming the receiver operating at both design conditions (nominal incoming concentrated solar radiation) and part-load (reduced incoming input power). For all the CFD simulations performed, an HTF inlet temperature of 700 °C was assumed with a case-dependent mass flow rate tuned to reach 1’200 °C as HTF outflow temperature. The simulations campaign allowed to observe that a HTF pre-heating, between 4% and 6% with respect to the HTF inlet temperature within the ATI, was achieved for all the cases considered. However, despite this, for some cases the receiver efficiency remained unchanged. The ATI resulted to be beneficial on the receiver efficiency if it is operated at full-load and almost irrelevant in the case of part-load.