With the increasing cost of petroleum and the announced consequences of climatic changes, there is a huge pressure for a rational use of energy and to explore energy sources that do not contribute to the greenhouse effect, to the acid rain problem, or to other environmental problem. Exploration of solar energy is thus been considered in several ways, with passive and active solar heating, use of heliostat fields to produce electrical energy with solar thermal engines or solar photovoltaic cells. One possibility, dating from the seventies, but currently attracting a renewed interest is the use of distributed collector solar fields. Distributed collector solar fields are large structures that aim at collecting and storing energy from solar radiation. They are made up from mirrors, whose elevation is varied by a sun tracking controller, that concentrate direct incident sun light in a pipe where an oil able to accumulate thermal energy flows. The oil is extracted at low temperature from the bottom of a storage tank, passed through the field where it is heated by solar radiation and returns to the tank, where it is injected at the top. Inside the storage tank the oil forms layers at different temperatures that do not mix, a fact making possible energy storage. From the top of this tank (hot zone inside the tank) the oil may be extracted for usage, e.g. in a desalination or steam generation plant. Once the energy used, the cool oil is re-injected at the bottom of the storage tank. The main control objective consists of making the average of the loop outlet oil temperatures to track a reference value by manipulating the oil flow in the presence of fast acting disturbances caused by passing clouds. Other main disturbances are changes in radiation due to atmosphere scattered moisture, in the temperature of the inlet oil coming from the bottom of the storage tank and in ambient temperature. Dust deposition and other factors such as wind that changes collectors shape, also act as disturbances because they alter mirror reflectivity but may be better described as inducing parameter changes. The main difficulties in designing controllers for these type of plants are the uncertainty in dynamic knowledge and its distributed and nonlinear character (dynamics depend on space as well as on time, being described by an hyperbolic partial differential equation where the manipulated variable multiplies the derivative of temperature with respect to space). The problem of temperature control in distributed collector solar fields attracted much attention, not only for its scientific interest but also for its major social impact. Therefore, a wealth of techniques have, and are currently, been developed. A number of these are concisely reviewed in section 1. This chapter explains how to design an adaptive predictive controller that takes advantage of the structure of the equations describing plant dynamics in order to yield an algorithm that allows fast set-point changes with reduced overshoot. The key issue is the use of a time varying sampling that allows an exact compensation of the nonlinearity. The approach is illustrated by experimental results obtained in the ACUREX field of Plataforma Solar de Almeria (Spain).
|Title of host publication||Power Plant Application of Advanced Control Techniques|
|Place of Publication||VIENNA / Austria|
|Publisher||ProcessEng Engineering GmbH|
|Publication status||Published - 1 Jan 2010|