Experimental Demonstration of a mass flow determination in Concentrated solar systems via collector defocusing using Time-of-Flight method

Abstract

An optimized mass flow distribution of heat transfer fluid (HTF) and its understanding are essential for an effective solar field operation in a concentrated solar thermal (CST) system. Typically, the flow of HTF through a solar field of linear concentrating collectors is divided into different loops and the flow rate in different loops may differ. While the mass flow distribution aims for a constant and high outlet temperature from the solar field, valuable insights into the efficiency of individual loops can be deduced from the mass flow distribution itself. Nevertheless, a spatially high-resolution measurement of mass flow in the solar field is typically not yet implemented. This paper demonstrates that the mass flow of a loop can be determined by briefly focusing and defocusing individual collectors and thus without additional measurement equipment as needed for common measurements. For this purpose, a measurement campaign with over 100 individual experiments was conducted at the Evora Molten Salt Platform (EMSP) test facility. Based on the resulting thermal step response and its measurement considering various temperature sensors, the mass flow of the test loop was determined using the Time-of-Flight (ToF) method, yielding a mean deviation of less than 5% compared to the mass flow measured by mass flow meters and a high precision with a mean standard deviation of 0.3% for multiple measurements under identical conditions. Therefore, this method offers a high potential for enhanced early fault detection of loops or individual collectors with reduced efficiency as well as improved predictive maintenance.