Optimal Start-Up Policies for a Solar Thermal Power Plant

Abstract

Sustainability and depletion of fossil fuels have propelled the use of renewable energy sources to meet energy demands. Solar radiation is perhaps the most economical and widely available alternative energy source. Energy in the form of solar radiation can be recovered using either photovoltaic or thermal processes. Nowadays, both approaches can only capture a small fraction of the available solar radiation. In this work, we have addressed the dynamic optimal operation of thermal solar plants during start-up. During a normal operating day when solar radiation becomes available, power should be available as soon as possible to meet consumer demands. One of the major problems related to thermal solar plants is the lack of power when solar radiation is off. To overcome this problem, energy storage tanks are considered in the design of the thermal plant. We assume that a conventional Rankine cycle can be used for power generation from low-temperature energy sources. In this work, a dynamic optimization framework is deployed to identify optimal dynamic start-up policies in thermal solar plants. Since the dynamic plant model is composed of a set of partial and differential equations, we have deployed the method of lines and the direct transcription approach for spatial and time discretization, respectively. The results indicate that fast optimal control policies result in power production in a more efficient fashion than simple heuristic-based start-up policies.

Hierarchical control for the start-up procedure of solar thermal fields with direct storage

“…However, these rules can be inefficient since they do not explicitly take into consideration the operating constraints and the process disturbances. In facilities with two tanks with direct storage (see Fig 1-(c)), the start-up problem has been already addressed by Lopez-Alvarez et al (2018). In that work, an off-line dynamical-optimization problem was proposed for computing the optimal flow rate according to the states of the hot and cold tanks and irradiance conditions, trying to achieve full operation from shutdown as fast as possible.…”

“…The upper one includes a Model Predictive Control (MPC) strategy that is responsible for computing the setpoints for the lower layer with the objective of maximizing the temperature in the storage device as fast as possible. Note that this layer takes into account the operating conditions at each sampling time unlike the approach proposed in Lopez-Alvarez et al (2018). The lower layer includes Proportional Integral Derivative (PID) controllers which are in charge of tracking the setpoints provided by the upper layer by acting on the two actuators available in these kind of solar fields: flow rate and valve aperture (see Fig.…”

Optimal Start-up Policies for a Nanofluid-Based Solar Thermal Power Plant

“…While photovoltaic cells convert the energy present in the photons of sunlight into electricity through a photoelectric phenomenon encountered in certain types of semiconductor materials, 2 solar thermal plants reflect incident radiation onto a surface suitable for absorbing heat, which is transferred to a fluid pursuing energy recovery. 3 Even though photovoltaic cells tend to have reasonable capital costs and imply easier operation, their usage is mainly restricted to accommodations and offices because of their low efficiency (12−19% at the most promising conditions). 2 On the other hand, solar radiation capture at solar thermal plants fluctuates in a higher recovery range, meaning greater energy availability for power generation when deploying organic Rankine cycle (ORC) frameworks.…”

“…Despite of the fact that the online approach constitutes the way realistic start-up procedures should be implemented, its application becomes a very complicated task since control action must be estimated from optimization within a tight range of CPU time, and the optimal control problem needs to be solved between two successive sampling intervals. 3 The organization of this paper is as follows: Problem Definition covers the mode of operation of a solar thermal plant with energy storage tank facilities. The mathematical models describing the thermal plant, the storage tanks, and the organic Rankine cycle, along with formulation for optimum start-up, are presented in Problem Formulation.…”

“…Even though photovoltaic cells tend to have reasonable capital costs and imply easier operation, their usage is mainly restricted to accommodations and offices because of their low efficiency (12–19% at the most promising conditions) . On the other hand, solar radiation capture at solar thermal plants fluctuates in a higher recovery range, meaning greater energy availability for power generation when deploying organic Rankine cycle (ORC) frameworks. − …”