LOOP THERMOSYPHON DESIGN FOR SOLAR THERMAL DESALINATION

Abstract

A two-phase loop thermosyphon solar collector is under development that can efficiently and passively transfer thermal energy from a concentrated solar collector to a thermal desalination process. The design of a loop thermosyphon is critical to ensure that the thermosyphon can transfer the required thermal energy through the passive circulation of a two-phase flow. This flow is driven by the difference in gravitational head between the liquid return line and the two-phase riser located between the evaporator and condenser. A numerical mass, energy, and pressure balance model was developed to provide insights into the behavior of the two-phase working fluid and predict system performance as geometric parameters (tube diameter, condenser height, etc.) are varied. This model is based on well-established empirical two-phase pressure drop and heat transfer correlations. The model accounts for gravitational head and frictional, minor, and acceleration pressure drops throughout and uses locally-calculated heat transfer coefficients to estimate heat losses and heat exchanger performance. A lab-scale loop thermosyphon prototype has been tested across a range of fluid charges, saturation temperatures, and evaporator powers. Modeling results for flow rate and pressure drop have been found to agree well with experimental data across the range of conditions examined. This model is a valuable tool to design and optimize a wide range of two-phase loop thermosyphons for solar thermal and other heat transfer applications.