Event-based MPC for defocusing and power production of a parabolic trough plant under power limitation

Abstract

Optimal operation of a solar plant is generally understood as a tracking of the optimal working temperatures which maximize the net electric power. However, a commercial solar plant may receive a limitation from the Transmission System Operator due to saturation of the electrical grid. In these situations the plant moves to an operation mode in which the objective is not maximum production but compliance with the orders of the Transmission System Operator.The paper proposes an Event-Based Gain Scheduling Generalized Predictive Control strategy for electric power production reference tracking when power limitations are imposed by the Transmission System Operator. Gain Scheduling Generalized Predictive Controllers are proposed to control fourth and third collector defocus in order to prevent heating fluid temperature from exceeding the limits of the manufacturer and therefore, avoid oil degradation. A 50 M W parabolic solar trough plant model has been used to design and validate the strategy. Simulation results are presented showing the advantages of using the proposed strategy.

Hierarchical set-point optimization and feedforward strategy for collector defocusing of a solar plant

“…In previous works, (Sánchez et al, 2018(Sánchez et al, , 2019a, different MPC strategies were presented to control the outlet temperature of the solar field of a 50 MW plant by defocusing the third and fourth collectors. However, collector defocusing is a safety mechanism that should only be applied in cases where the plant flow is not capable of controlling the outlet temperature (maximum flow rate reached) or when the plant is under power limitations.…”

“…The GS-GPC controller strategy design that is applied for collector defocusing can be found in Sánchez et al (2018). This controller was designed based on the nonlinear defocus curve shown in Fig.…”

“…For diphenyl oxide (DPO) and biphenyl mixture fluids such as Therminol VP1 or similar, this temperature is around 400 • C. In commercial plants, this security strategy is based on partial or total collector defocusing, mainly in the fourth collector and based on temperature hysteresis, which, in general, causes oscillations in the loop outlet temperature and a large number of actions and number of degrees traveled by the collector. Model based predictive control (MPC) strategies, Gain Scheduling Generalized Model Predictive Control (GS-GPC) and state space MPC, were presented in Sánchez et al (2018Sánchez et al ( , 2019a to control the outlet temperature of the third and fourth collectors of 50 MW solar plants by defocusing. In Sánchez et al (2020) a comparative analysis was carried out on the use of GS-GPC strategy for defocus control in collectors 3 and 4 with respect to the use of the same type of controller in the 4 collectors, showing that on certain occasions it will be necessary to defocus the four collectors in order to maintain the outlet temperature below the maximum allowed.…”

“…231 However, this study could also be carried out using other 232 controllers such as MPC in the state space, although as 233 already mentioned, it would require a more complex im-234 plementation when programming in a PLC due to the need 235 of using state observers and adaptation of all the parame-236 ters of the matrices of the systems of each of the loops. This section briefly describes the 50 MW plant used, 239 (Sánchez et al, 2018(Sánchez et al, , 2019b. Two mathematical models, 240 a distributed parameter model and a concentrated param-241 eter model, are used for simulation purposes and controller 242 design.…”

Mathematical Modeling of the Mojave Solar Plants

“…Obtaining the heat transmission coefficient is a bit trickier. It is composed of two parts, one depending on the fluid temperature and the other on the flow-rate [34]. Obtaining the expression of this coefficient involves using complex convection heat transmission formulas [35].…”