Optimal operation of a Solar Membrane Distillation pilot plant via Nonlinear Model Predictive Control

Abstract

Solar Membrane Distillation (SMD) is an under-investigation desalination process suitable for developing self-sufficient small scale applications. The use of solar energy considerably reduces the operating costs, however, its intermittent nature requires a non-stationary optimal operation that can be achieved by means of advanced control strategies. In this paper, a hierarchical control system composed by two layers is used for optimizing the operation of a SMD pilot plant, in terms of thermal efficiency, distillate production and cost savings. The upper layer is formed by a Nonlinear Model Predictive Control (NMPC) scheme, that allows us to obtain the optimal operation by optimizing the solar energy use. The lower layer includes a direct control system, in charge of attaining the variable references provided by the upper layer. Simulation and experimental tests are included and commented in order to demonstrate the benefits of the developed control system.

A Multivariable Controller for the Start-up Procedure of a Solar Membrane Distillation Facility

“…The use of solar energy allows membrane distillation technology to be an efficient and a sustainable solution to mitigate the lack of water in places with good solar irradiance conditions and access to brackish or sea water. However, due to its unpredictable nature, it requires the use of specific energy storage systems and suitable discontinuous operation techniques, which can be obtained by means of adequate control strategies (Gil et al, 2018a).…”

“…Another example is the one presented in (Lin et al, 2011), in which two control loops aimed at controlling the solar field temperature were tested in simulation. Nevertheless, as suggested in (Gil et al, 2018a), the key element for the successful combination of membrane distillation processes with solar energy is the use of low…”

“…In the previous work (Gil et al, 2018a), a hierarchical control system composed by two layer was proposed for the real time optimization of the plant operation, in terms of distillate production, thermal efficiency and cost savings. This paper presents an improvement of the control approach proposed in (Gil et al, 2018a), by developing a new start-up procedure that allows to minimize the starting time of the membrane distillation module. This startup procedure includes a multivariable TITO controller complemented with a real time optimizer-based reference governor.…”

“…This startup procedure includes a multivariable TITO controller complemented with a real time optimizer-based reference governor. On the one hand, the reference governor generates the optimal setpoints for the multivariable controller at each sample time, using a static model of the SMD facility (Gil et al, 2018a). On the other hand, the multivariable control structure is formed by two PID-based controllers with feedforward and decouplers, which are in charge of tracking the optimal references calculated by the reference governor.…”

“…Finally, the buffer system is connected directly with the thermal solar field, which is composed by flat-plate collectors. In (Zaragoza et al, 2014;Gil et al, 2018a) a complete description of the SMD plant was presented. It should be noted that other heating and cooling systems included in the facility are not considered in this work.…”