Temperature control of a solar tower receiver based on the Lyapunov method

This article describes an approach to design a temperature controller for a solar tower receiver that receives concentrated solar energy from a field of heliostats and is employed to transfers energy to a heat transfer fluid (HTF), such as molten salt. During the operation of solar receiver, its temperature and the temperature of the HTF fluid must be controlled to obtain the maximum economical yield constrained by operational safety margins and by the designed life span of the receiver. To attain this purpose, fluid flow and the positions of the heliostats are used as manipulated variables to control the error between the temperature reference and the temperature of the fluid at the receiver outlet. The presence of disturbances is an important factor to consider during the receiver operation. They include solar energy fluctuations caused by weather conditions, presence of moving clouds, the apparent movement of the sun, and wind that can change energy losses. The controller is designed in two steps. In the first step, the thermal stationary operating point of the solar receiver is defined and is used to build a nonlinear dynamical equation of the temperature error. In the second step, the stability of the nonlinear error dynamics, that depends on the speed of the fluid and on the concentrating power of the heliostats, is analyzed using the Lyapunov method. From this analysis the control system is defined, resulting in a nonlinear feed-forward term used to compensate the solar energy fluctuations, and a statefeedback controller that adjusts the the fluid flow to compensate temperature deviations from the temperature reference.