Thermal balance of large scale parabolic trough plants: A case study

Abstract

The main control objective in commercial solar parabolic plants is to track the average temperature of all the loops around a reference set by the operator, by manipulating the flow of a synthetic oil. Due to the large number of loops existing in current solar plants and the vast extension that they cover, obtaining a precise knowledge of every loops efficiency becomes a very challenging task. Parameters such as loops reflectivity, tube absorptance, optical efficiency may exhibit a great disparity due to dirt accumulated and atmospheric conditions. Because of this, the most efficient loops achieve higher temperatures and the collectors may have to be set out of focus in many cases.This paper presents a case study on a simulation model of a large scale parabolic plant, 50 MW, in which a control of the aperture of the solar field loops inlet valves is applied through a nonlinear optimization problem. The goal is to reduce the imbalance of loops temperature caused by the difference in efficiencies. The idea is opening the input valves of the most efficient loops to increase the incoming flow-rate and reducing the flow in the less efficient loops. This will prevent, in many cases, the activation of the defocus control avoiding energy losses and minimizing the deterioration of actuators.The optimization problem will need estimations of the loops states and parameters. An Unscented Kalman Filter is used to estimate the loops temperatures states and a concentrated parameter model is used to estimate the loops efficiencies. A loop clustering is implemented to avoid high computation times. Simulation results showing thermal balance of the field minimizing the defocusing actions are presented.