Detailed Thermal and Hydraulic Modeling of Parabolic Trough Collectors for Hybrid CSP Applications

Correct modeling of parabolic trough collectors is crucial for the design and optimization of stand-alone and hybrid concentrated solar power (CSP) systems. A detailed one-dimensional steady state thermal and hydraulic model for parabolic troughs developed for use in hybrid solar tower-trough power plant is presented in this paper. The thermal model takes into consideration all important heat transfer phenomena throughout the receiver tube, such as the absorption of the solar energy in the receiver tube, the convective and radiative heat loss from the glass envelope to the ambient temperature, the convective heat transfer to the heat transfer fluid (synthetic thermal oil), the radiative heat exchange between the glass envelope and the absorber tube, and the conduction through the walls of the tube. The receiver tube is divided into several small segments to capture the progressive heating of the fluid and the effect of thermophysical properties on the fluid's behavior as a function of temperature. Glass envelope temperature, absorber temperature and outlet temperature of the fluid are iteratively calculated in each segment using the energy balance equations. A parallel hydraulic model calculates the pressure loss and flow distribution in the collector loops, taking into account the pressure loss due to the pipes and fittings. All the model was validated with the data obtained during its operation at the Andasol power plant and correlated with the technical specifications of the Luz LS-3 collector, with the use of correlations for the oil properties (density, viscosity, specific heat and thermal conductivity) that were obtained as a function of temperature. Results show good predictive performance for the thermal efficiency of the collector, the temperature distribution along the collector line and the total heat gain for clear-sky conditions. The model shows that at high operating temperatures radiation losses control and convection within the annulus is minimized because of the vacuum. The trough model can be combined with a solar tower plant to determine the best heat introduction points to the Rankine cycle. This extensive modelling framework can serve as a powerful tool for the prediction of system performance, parametric analysis and optimization of parabolic trough systems for hybrid CSP applications. It provides a temperature range and enables more efficient hybrid plant designs between medium- and high-temperature solar technologies.