J.D. Álvarez

Bumpless switching in control - A comparative study

Industrial processes are characterized by changing dynamics associated to different operating points. Control systems usually face this behavior by adapting the control parameters or switching among controllers, in order to achieve high performance of the system in the whole process operation range. Different methods have been developed to avoid undesirable transients in the closed loop output signal when changes in the control system structure occur. This paper presents a comparative study of some of these methods showing simulation results.

Hybrid Modeling of a Solar Cooling System

Abstract Solar systems are deeply affected by weather conditions and their main energy source, irradiation, cannot be manipulated. This paper describes a solar cooling system located at the University of Almeria, Spain. The system is composed by a solar collector, a gas heather, and two serial connected tanks to storage hot water. They are used as energy sources for an absorption machine whose objective is to obtain chilled water for the fan-coil system. All these components are included or excluded from the cooling system by means of switching valves and activating or deactivating signals. The switching among different configuration modes in real time increases the system challenge to be modeled or controlled, due to the mix of both discrete signals and continuous dynamics. In this paper, a model for each energy source is developed and afterwards they are integrated into one state-space model used to obtain a hybrid model. This hybrid model is able to simulate the different configuration modes of the solar cooling system. Some real tests are presented.

Hybrid modeling of central receiver solar power plants

Abstract With the forecast exhaustion of fossil fuels, governments are promoting renewable power plants as a viable alternative for electricity generation. One of the disadvantages of this kind of power generation facilities is the need to have a backup fuel, because the primary renewable resource is uncontrollable and discontinuous. From a modeling point of view, a power station with more than one source of supply changes its dynamical behavior according to Boolean events, thus mixing continuous and discrete dynamics in the system. The main contribution of this work is the modeling of a renewable power plant with two energy sources using a Mixed Logical Dynamical (MLD) representation, which is a modeling tool for the representation of hybrid systems. The advantages of using this modeling approach are discussed in the paper.

Learning Switching Control: A Tank Level-Control Exercise

A key topic in multicontroller strategies is the mechanism for switching between controllers, depending on the current operating point. The objective of the switching mechanism is to keep the control action coherent. To help students understand the switching strategy involved in multicontroller schema and the relationship between the system dynamics and the switching structure, this paper proposes a student practical where basic linear control knowledge and a simple switching strategy are applied to an educational nonlinear control problem, that of controlling the level in a tank. Evaluation provided by the students is also included.

Técnicas de Control del Confort en Edificios

Most of the time, people perform their daily activities within buildings, making it necessary to monitor the conditions of comfort (thermal, visual and air quality) within them, trying to minimize energy consumption simultaneously. A review of techniques for comfort control in building is presented in this work. The paper also shows tesults obtained in the implementation of a hierarchical control system, which provides adequate thermal comfort inside the CDdI-CIESOL-ARFRISOL (Research Container Demonstrator – Centre for Solar Energy Research – Strategic Singular Project ARFRISOL) of the University of Almeria.

Dynamic models for hydrogen peroxide control in solar photo-fenton systems

Simulations and real decomposition experiments with hydrogen peroxide induced by dissolved iron and solar illumination were performed in a solar pilot plant with compound parabolic collectors designed for photo-Fenton wastewater treatment. The structure of a gray-box linear model aimed at reproducing system dynamics with parameters dependent on operating conditions when operating around prescribed set-points is proposed. This simple model relates the changes in hydrogen peroxide concentration due to changes in hydrogen peroxide injection, dissolved iron, and solar illumination. Based on this model, control of the hydrogen peroxide concentration at set points between 200–900 mg/L±10 mg/L under dynamic conditions (simultaneous decomposition and addition of hydrogen peroxide by a dosage pump), was achieved. Different basic approaches for controller actuation upon the frequency of the dosage pump were tested, ranging from a simple PI controller to a PI controller plus antiwindup action and feedforward control. From these basic approaches, conclusions can be drawn about the processs behavior under closed-loop control

Comfort optimization in a solar energy research center

Abstract Most of the time, people usually do their daily activities inside a building. Thus, it is necessary to reach comfort conditions for the people. This paper presents a hierarchical control system which allows to obtain a high comfort level, optimizing the use of HVAC (Heating, Ventilation and Air Conditioning) systems and actuators while minimizing energy consumption. Specifically, in this paper an analysis about the relation between comfort (thermal comfort, visual comfort and indoor-air quality) and different degrees of freedom to control inside a research center is done. The main objective is to obtain a cost function which allows to determine optimal conditions of humidity, temperature, illuminance and CO 2 concentration minimizing energy consumption.

ITCRI: An Interactive Software Tool for Control-Relevant Identification Education

Abstract This paper describes the theoretical basis, features, and functionality of an I nteractive T ool for C ontrol R elevant I dentification (ITCRI). ITCRI captures the control-relevant identification process, from input design to closed-loop control, depicting these stages simultaneously and interactively in one screen. Control-relevance in ITCRI is accomplished primarily through prefiltering, which is evaluated using single-pass and two-step algorithms. By displaying both open- and closed-loop responses of the estimated models and important control-relevant validation criteria, ITCRI enables the user to readily assess how design variable choices, control performance requirements and model error can impact the achievable closed-loop performance from a restricted complexity model estimated under noisy conditions.

A parallel Teaching---Learning-Based Optimization procedure for automatic heliostat aiming

The flux distribution generated by the heliostat field of solar central receiver system (SCRS) over the receiver needs to be carefully controlled. It is necessary to avoid dangerous radiation peaks and temperature distributions to maximize the efficiency and keep the system in a safe state. These tasks imply both selecting the subset of heliostats to be activated and assigning each one to a certain aiming point at the receiver. The heliostat field is usually under human control and supervision, what is a potential limiting factor. Thus, there is an active research line to define automatic aiming procedures. In fact, a general and autonomous methodology is being developed by the authors of this work. However, the mathematical modeling leads to face a complex large-scale optimization problem. In this work, applying Teaching–Learning-Based Optimization (TLBO), a population-based large-scale optimizer, is considered. It is intended to serve to perform large explorations of the search-space to finally deploy further local optimizers over the most promising results. Considering the computational cost of the objective function, a parallel version of TLBO has been developed. It significantly accelerates the procedure, and the possibility of being included in a more complex process remains viable. Additionally, the parallel version of TLBO is also linked as a generic open-source library.

High performance computing for the heliostat field layout evaluation

In Solar Central Receiver Systems (SCRS), the heliostat field is generally the most important subsystem in terms of initial investment and energy losses. Therefore, heliostat field layout needs to be carefully designed and optimized when deploying this kind of power facilities. This optimization procedure can be focused on multiple and heterogeneous criteria depending on particular factors that lead to define different optimization problems based on specific objective functions. However, objective functions defined for this problem are, in general terms, computationally very expensive. This fact may make an exhaustive optimization process infeasible, specially depending on the available resources, and forces particular simplifications at some steps of the process. Fortunately, some of the objective functions defined can benefit from parallelization, even though this idea is not usually pointed out or discussed, and then, become affordable in better conditions. In this paper, the heliostat field optical efficiency, which is a common objective function in this area, is analyzed to be parallelized by three different approaches.