Stack Thermo-Osmotic System for Low-Grade Thermal Energy Conversion

Abstract

Thermo-osmotic energy conversion (TOEC) technology, developed from membrane distillation, is an emerging method that has the potential of obtaining electricity efficiently from a low-grade heat source but faces the difficult problem of pump power loss. In this study, we build a novel TOEC system with a multistage architecture that can work without pump assistance. The experiment system, made of cheap commercial materials, can obtain a power density of 1.39 ± 0.25 W/m 2 , with a heating temperature of 80 °C, and its efficiency increased linearly with the total stage number. A theory calculation shows that a 30-stage system with a specific membrane and a working pressure of 5.0 MPa can obtain an efficiency of 2.72% with a power density of 14.0 W/m 2 . By a molecular dynamics simulation, it is shown that a high-performance membrane has the potential to work at 40 MPa. This study proves that TOEC technology is a practical and competitive approach to covert low-grade thermal energy into power efficiently.

Enhancing Thermo‐Osmotic Low‐Grade Heat Recovery by Applying a Negative Pressure to the Feed

“…Other parameters, such as evaporation efficiency α and temperature polarization coefficient θ, must be considered in addition to power density and power efficiency. [ 10 ] $$\[ \begin{array}{*{20}{c}}{\alpha = \frac{1}{n}\cdot\frac{{{J_{\rm{W}}}\Delta {H_{{\rm{vap}}}}}}{q}}\end{array} \]$$ $$\[ \begin{array}{*{20}{c}}{\theta = \frac{{\mathop \sum \nolimits_{i = 1}^N \left( {T_{{\rm{m}},{\rm{F}}}^i - T_{{\rm{m}},{\rm{P}}}^i} \right)}}{{{T_{\rm{h}}} - {T_{\rm{c}}}}}}\end{array} \]$$where J W is the transmembrane mass transfer flux and Δ H vap is the enthalpy of vaporization. i and n are the current and total working stage numbers, and q is the heat flux.…”

“…Theory Prediction Method: Based on previous studies on thermoosmosis and membrane distillation, [10,14,27] a theoretical prediction model for stack thermo-osmotic systems was established (Section S1, Supporting Information). Water was employed as the working fluid in the model, which was consistent with the working fluid used in the experiment.…”

“…For example, methylene chloride and chloroform may perform better due to better volatility and lower vaporization enthalpy, which needs further study. [ 10 ] In this study, if there is no other explanation, the membrane is hydrophobic, and the working fluid will likewise be water.…”

“…[28] Other parameters, such as evaporation efficiency α and temperature polarization coefficient θ, must be considered in addition to power density and power efficiency. [10] n…”

“…However, this system still has severe temperature polarization and would face the challenges of flow control and thermal insulation when applied. Li et al [10,19] proposed the stack TOEC system that utilizes thermal energy stage by stage for efficiency improvement. This system structure has advantages, including low-temperature polarization, less pump power loss, and good feasibility.…”