Pressure-retarded membrane distillation for low-grade heat recovery: The critical roles of pressure-induced membrane deformation

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

A newly developed technology, thermo-osmotic energy conversion (TOEC), is supposed to convert low-grade heat into power. However, the performance of existing TOEC experiments is deficient. This paper discusses the feasibility of strengthening TOEC by applying negative pressure to the feed liquid, which can reduce air pressure in the membrane pores and molecular diffusion resistance. Theoretical calculation shows that when the cooling and heating temperatures are 40 and 80 °C, respectively, and the transmembrane pressure difference is 5.0 MPa, the TOEC system with a negative pressure of 0.5 bar at the feed side can approach an efficiency of 3.01% and a power density of 16.85 W m-2, which increases by 20% and 27% compared with no negative pressure, respectively. Given the nonuniformity in the real system, computational fluid dynamics simulation is used to obtain the correction factor, which is then used to revise the theory prediction results for the first time. Moreover, a lab-scale experiment also proves that a negative pressure at the feed benefits the performance of the TOEC device. Overall, this research presents a feasible method to enhance a TOEC system, which may promote the development of a more-efficiently TOEC system for low-grade heat utilization.

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

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/m², 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/m². 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.

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

Nano Heat Pump Based on Reverse Thermo-osmosis Effect

Heat pumps are widely used in domestic applications, agriculture, and industry. Here, we report a novel heat pump based on the reverse thermo-osmosis (RTO) effect in a nanoporous graphene (NPG) membrane. Through classical molecular dynamics (MD) simulation, we prove that the heat pump can transport mass and heat efficiently. The heat and mass fluxes are increased linearly with the hydraulic pressure provided. Ultrahigh heat fluxes of 6.2 ± 1.0 kW/cm² and coefficient of performance (COP) of 20.2 are obtained with a temperature increment of 5 K and a working pressure of 80 MPa. It is interesting that water molecules on the NPG membrane can evaporate in a cluster state, and the cluster evaporations reduce the vaporization enthalpy of the processes.

Fluid–Structure Interaction and Flow Redistribution in Membrane-Bounded Channels

The hydrodynamics of electrodialysis and reverse electrodialysis is commonly studied by neglecting membrane deformation caused by transmembrane pressure (TMP). However, large frictional pressure drops and differences in fluid velocity or physical properties in adjacent channels may lead to significant TMP values. In previous works, we conducted one-way coupled structural-CFD simulations at the scale of one periodic unit of a profiled membrane/channel assembly and computed its deformation and frictional characteristics as functions of TMP. In this work, a novel fluid–structure interaction model is presented, which predicts, at the channel pair scale, the changes in flow distribution associated with membrane deformations. The continuity and Darcy equations are solved in two adjacent channels by treating them as porous media and using the previous CFD results to express their hydraulic permeability as a function of the local TMP. Results are presented for square stacks of 0.6-m sides in cross and counter flow at superficial velocities of 1 to 10 cm/s. At low velocities, the corresponding low TMP does not significantly affect the flow distribution. As the velocity increases, the larger membrane deformation causes significant fluid redistribution. In the cross flow, the departure of the local superficial velocity from a mean value of 10 cm/s ranges between −27% and +39%.