Fig. 1: Concept of thermodiffusive desalination and unit design.
a Concept figure showing a laminar flow of saline water passing through a thermodiffusive separation channel. The temperature difference ΔT can be established with low-grade thermal energy. For temperatures above the inversion temperature, the thermophobic ions in the solution migrate towards the cold side as the saline flow progresses along the channel. This results in an upper stream having a lower salinity than the feedwater. The hydrodynamic and thermal entrance lengths, Lv and LT, are both less than 1% of the total channel length, Lcha. b The Reynolds number, Ra, indicates that Lv and LT are negligible compared to Lcha. Thus, the flow in the thermodiffusive separation channel can be approximated as a laminar, fully-developed planar Poiseuille flow with a positive quasi-linear temperature profile. x is the saline water flow direction and x = 0 corresponds to the channel inlet. The Péclet number, Pe, indicates an advection-dominant mass transport in the x direction and diffusion-dominant transport in the y direction. At the inlet, the solution is homogeneous, whereas at the outlet it becomes heterogeneous due to thermodiffusion. c Thermodiffusive desalination unit (TDU) design. The volumetric flow rate Q of the saline mixture in the TDU is controlled by a peristaltic pump between 1 to 16 mL min−1. Feedwater is degassed before entering the channel. The fluid path for the saline water is indicated by yellow lines. The channel is 500 mm long, 20 mm wide, and 1 mm high. At the exit of the channel, the saline water is separated into two streams by a spacer, as shown in the inset. After bifurcation, the two streams are collected in bottles at the same height, ensuring equal pressure and hence equal flow rate. Two hollow copper blocks with water circulation create the ΔT. Thermocouples were embedded in top and bottom channel walls to monitor the temperature (Supplementary Fig. 7).
