Modeling, design and control of dish-stirling solar-thermal power generating system

• Main Author: Li, Yang
• Other Authors: Wang Youyi
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Electric power
• Online Access: https://hdl.handle.net/10356/64896
• Institution: Nanyang Technological University

Dish-Stirling (DS) solar-thermal generating system is one type of renewable energy technology in which a parabolic dish-like reflector is used to concentrate sunlight to a small area located at the focal point of a number of mirrors. The high temperature achieved at the focal point is used as a heat source for a Stirling engine, which is a type of closed-cycle external heat engine with high thermal efficiency and no emissions. Unfortunately, the intermittent and uncontrollable nature of the solar irradiance makes the control of the harnessed energy most challenging. Quality and reliability of the supply could be degraded to unacceptable level if the harnessed energy is fed directly into an electrical power grid in significant amount. Various types of technical and economic issues for the DS system must be solved in order to compete with the other type of renewable technologies in the dynamic market. Hence, appropriate system design and operations of the DS power plant are called for. The existing models of the most important component of the DS system – the Stirling Engine, are not suitable for the studies of the DS solar-thermal power generation, in terms of the compatibility of the computer simulation step size and control system design. Proper average-value models for the four-cylinder double-acting kinematic Stirling engine, including a general model for variable-speed operation and a specific model for constant-speed operation are developed based on thermodynamic analysis for the studies of the DS system. With the derived models, a suitable model for such a generating system is developed. Linearized model of the constant-speed DS system is derived for temperature control system design. New temperature control schemes with transient droop compensation and feedforward compensation are proposed to overcome the problems caused by the non-minimum phase and nonlinear characteristic of the system model. The potential of the maximum energy harness of the DS system is analyzed and discussed, and the results show that variable-speed operation of the DS system is a viable method to extract considerable more energy. The overall configuration of the variable-speed DS system using doubly-fed induction generator (DFIG) is proposed and the model of the entire system will be derived especially for the design of temperature control while achieving maximum power harnessing. The computer simulation results are given to show the performance of the proposed temperature controllers to overcome the problems introduced by the continuous engine/generator speed variation which is not seen in the case of the conventional constant-speed DS system. A DS-DFIG simulator has been developed in the author’s laboratory with the view to study the maximum energy harness ability of the DS-DFIG system. The developed system is based on the use of a separately-excited dc motor to generate the equivalent mechanical torque from the DS under specific insolation level and engine/motor speed. The dc motor therefore emulates the torque vs. speed behavior of the DS system, and the produced torque is to drive a coupled DFIG in the laboratory. In order to meet the grid code requirements for the potential integration of the large-scale DS system, the ability to provide frequency support from the DS-DFIG system is investigated. An overall system configuration for the DS-DFIG plant is proposed with the objectives to enhance the dynamic performance and to reduce the entire cost of the plant. Based on the model and the control system derived previously, various methods to provide inertia and frequency support from the DS-DFIG system are proposed and discussed. Numerical examples are included to show the availability of the proposed schemes.