Thermo-hydraulic modelling of direct steam generation in a solar parabolic trough collector system

Abstract

Direct steam generation using a solar parabolic trough collector for industrial process heat is gaining ground owing to the improved system performance and economic benefits. The major challenges in the design of a feasible parabolic trough collector system for the direct steam generation process are the large pressure losses in the evaporating section and the high thermal gradients in the absorber tube. The numerical results of a sensitivity study based on a thermo-hydraulic model for saturated steam generation in a small-sized solar parabolic trough collector are analyzed. In the present study, a solar parabolic trough collector system with aperture width of 0.84 m, focal length of 0.3 m and absorber tube with an inner diameter of 0.02 m is considered. For accuracy, the complex two-phase flow in the evaporating section of the collector field is analyzed using a flow pattern map integrated heat transfer and pressure drop models. The effect of inlet water temperature, pressure, mass flow rate and direct solar irradiance on pressure loss, flow pattern distribution, outlet temperature, and dryness fraction is investigated. For inlet pressure of 1 MPa, a significant pressure drop of 444.48 kPa (about 44.5% of inlet pressure) is found at direct solar irradiance of 900 W/m2 and mass flow rate of 0.06 kg/s. The transition from stratified-wavy to annular flow pattern on the left-hand side of the flow pattern map occurs at dryness fraction of 0.28, 0.33 and 0.39 for inlet pressure of 1.0, 1.5 and 2 MPa respectively. Lower inlet pressures are found favorable for an early transition to annular flow pattern. However, significant pressure drop at lower operating pressure may play a crucial role in the viability of using small-sized parabolic trough collector systems for saturated steam generation.