Starting-up strategies for solar thermal fields attending to time and economic criteria: Application of hierarchical control

Abstract

Solar thermal fields are usually coupled to storage tanks to improve the energy dispatchability. In direct configuration and without an auxiliary energy source, the state of the storage tank at the beginning of the operation is very relevant. If the storage device is stratified or discharged in terms of energy, the start-up phase until reaching the desired operating point can take a long time and reduce the benefits. Consequently, this paper presents a two-layer hierarchical controller aimed at reducing the costs and time spent in this operating phase. The upper layer is based on a Nonlinear Model Predictive Control (NMPC) strategy, and the lower one is composed of Proportional Integral Derivative (PID) controllers. The proposed technique was applied to a real facility located at Plataforma Solar de Almería (southest of Spain). Moreover, a comparative simulation study with manual and previous approaches proposed in literature was performed to evidence the important economic and time savings achieved by the application of the developed technique.