Automatic control strategies for hybrid solar-fossil fuel power plants

Solar electrical generating systems are a class of solar energy systems which use parabolic trough collectors (PTC) to produce electricity from sunlight. In order to provide power production, one of the major challenges is to held the collector outlet temperature or steam temperature around of a specified set point by adjusting the flow rate of the heat transfer fluid (HTF) within upper and lower bounds. In some cases, an auxiliary heater can be used to provide heat in absence of solar radiation or during cloudy days. This paper presents a comprehensive study of three control schemes proposed to keep the steam temperature around its set point by adjusting the fuel (propane) and air mass flow rate of the auxiliary fossil fuel-fired heater. A non-linear dynamic model was developed in SIMULINK^® to study the performance of each control scheme. Variation of controlled and manipulated variables along with the valve signals is presented for a period of a cloudy day. The results showed that the combination of feedforward and three level cascade control is the best alternative to track the temperature set point. It was also found that a single three level cascade control without feedforward had less oscillations and low fuel consumption compared to the others control strategies.