Membrane-Based Processes for Energy Production from Salinity Gradients and Low-Grade Hea

To meet increasing global energy needs and mitigate anthropogenic climate change, new technologies are needed that harvest energy from untapped or underutilized sources. Membrane processes are particularly well-suited for power generation since they can offer scalability, cost-effectiveness, and environmental sustainability. The objective of this dissertation research is to advance two membrane-based processes for sustainable power generation: pressure-retarded osmosis for power generation from salinity gradients and thermo-osmotic energy conversion of low-grade heat to electricity.;The energy released when two solutions of different concentrations spontaneously mix is a promising source of renewable energy. One of the largest potential sources of this salinity gradient energy is river water mixing with seawater. This research analyzes the energy extractable from river water and seawater using pressure-retarded osmosis (PRO), a membrane-based technology that utilizes a concentration difference to generate a pressurized flow of water. The maximum energy extractable is shown to be 0.26 kWh per cubic meter of river water and seawater mixed in the system, and it is determined that a realistic PRO system could obtain approximately 60% of this energy. Including energetic losses to the system from pumping and pretreatment, however, it is found that the energy extractable is meager, highlighting that it will only be possible to obtain energy from river water and seawater mixing with radically improved energy conversion technologies.;Alternative sources of salinity gradient energy with greater short-tenui viability are explored. In particular, this work demonstrates that utilizing high salinity brines, such as those available from hypersaline lakes or subsurface reservoirs, can increase the power output in PRO by more than an order of magnitude, vastly increasing the feasibility of energy production from salinity gradients. Systems incorporating PRO into reverse osmosis desalination systems were also shown to reduce the energy of desalination by up to 50% by recovering energy from the concentrated seawater brine discharge.;Based on the promise of high salinity resources, this dissertation research experimentally demonstrates PRO systems that efficiently operate with high concentrations. Specifically, systems are built to tolerate the high operating pressures (~50 bar) required for effective operation. These systems enabled dramatically increased power output, reaching power densities up to 60 W m -2. The performance of membranes is also extensively characterized to elucidate the impact of high salinities and pressures on membrane transport properties.;Low-grade heat (< 100 °C) is widely available from industrial facilities, geothermal reservoirs, and solar collectors. Current technologies are limited in their ability to extract energy from low-grade heat sources due to the small temperature difference available and temporal variability in heat output. In this dissertation, a new process is introduced to harvest energy from low-temperature heat sources using thermo-osmotic vapor flow through a hydrophobic, nanoporous membrane. The work experimentally demonstrates the thenno-osmotic energy conversion (TOEC) process for the first time using novel pressure-resistant hydrophobic membranes, and power outputs of up to 3.5 W m-2 with a 60 °C heat source and a 20 °C heat sink are shown.;The TOEC process for power generation from low-grade heat sources is further investigated to understand the expected efficiency and identify key parameters in the system. It is demonstrated that optimization of the process can lead to realistic heat-toelectricity energy conversion efficiencies around 4.1% (34% of the Carnot efficiency) with a 60 °C heat source and a 20 °C heat sink. The optimal membrane properties and system conditions are also defined, and promising areas for future research on the topic are proposed.;Overall, the dissertation work shows pioneering advancements for membrane-based power generation processes. In the field of salinity gradient energy, the most promising potential resources are identified, and new system designs are used to demonstrate unprecedented power output. A novel technology for power generation from low-grade heat is introduced with high power outputs. Design criteria for both salinity gradient energy and low-grade heat processes are introduced to shape ongoing research in the field.