What Is Pressure Retarded Osmosis? A Complete Guide to PRO Technology

Introduction: The Untapped Power of Salinity Gradients

Wherever freshwater meets saltwater, an enormous reserve of energy goes unharvested. Rivers emptying into oceans, brine discharge mixing with lower-salinity streams, and industrial wastewater blending with receiving waters all represent points where the chemical potential difference between two solutions can be captured and converted into usable power. Pressure Retarded Osmosis, commonly known as PRO, is the membrane-based technology designed to do exactly that.

PRO belongs to a family of processes collectively referred to as salinity gradient energy or blue energy technologies. Unlike solar or wind power, salinity gradient energy is available continuously. That characteristic alone makes PRO one of the most promising candidates for baseload renewable power generation. By some estimates, the global theoretical potential of osmotic power exceeds 2 terawatts.

This guide provides a comprehensive introduction to pressure retarded osmosis: how it works at the membrane level, what distinguishes it from related processes such as forward osmosis and reverse osmosis, where the technology stands in terms of commercial readiness, and how researchers and engineers can evaluate PRO membrane performance using purpose-built laboratory equipment.

How Pressure Retarded Osmosis Works

The Thermodynamic Basis

At its core, PRO exploits the natural tendency of water to move across a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration. This movement, osmosis, is driven by the chemical potential difference between the two solutions, commonly expressed as osmotic pressure. For seawater at standard conditions, the osmotic pressure is roughly 27 bar; for concentrated brines from desalination plants, it can exceed 70 bar.

In a PRO system, the low-salinity feed solution is placed on one side of a semi-permeable membrane, while a pressurized high-salinity draw solution occupies the other side. The applied hydraulic pressure on the draw side is set lower than the osmotic pressure difference. Water still permeates from the feed into the draw against the applied pressure, hence the term pressure retarded. The pressurized volumetric flow can be directed through a hydro-turbine or pressure exchanger to generate electricity.

Key Process Parameters

Water flux (Jw) describes the rate at which water permeates through the membrane. Reverse salt flux (Js) quantifies the undesirable diffusion of solute from the draw solution back through the membrane. Power density (W) is the amount of energy generated per unit of membrane area, usually expressed in watts per square meter. The widely cited target for commercially viable PRO is 5 W/m2. Specific energy (SE) refers to the energy generated per unit volume of permeate consumed.

PRO vs. Forward Osmosis vs. Reverse Osmosis

Reverse osmosis (RO) applies hydraulic pressure exceeding the osmotic pressure to force water through a membrane, producing purified permeate. It is an energy-consuming desalination process. Forward osmosis (FO) allows water to move naturally from a low-salinity feed to a higher-salinity draw without applied pressure. Pressure retarded osmosis sits between the two. Like FO, it uses the osmotic pressure difference as the primary driving force. But unlike FO, the draw side is pressurized enough to generate useful work. PRO is an energy-generating process.

The Role of Membranes in PRO Performance

The membrane is the heart of any PRO system. PRO membranes need high intrinsic water permeability (A value) to maximize flux, low salt permeability (B value) to minimize reverse salt flux, and a support layer structure that minimizes internal concentration polarization (ICP). ICP is the single largest barrier to achieving high PRO power densities, sometimes reducing the effective osmotic pressure difference by more than half.

Flat-sheet membranes are the most common configuration for laboratory-scale testing. Hollow-fiber membranes offer a much higher surface-area-to-volume ratio, advantageous for large-scale deployment. Both configurations are studied at Tech Inc.'s PRO testing facilities with purpose-built PRO systems.

Current State of PRO Technology

PRO has progressed from a theoretical concept proposed by Sidney Loeb in the 1970s to demonstrated pilot-scale performance. Statkraft's prototype osmotic power plant in Tofte, Norway, operated from 2009 to 2013. Since then, laboratory-scale power densities exceeding 10 W/m2 have been reported using optimized thin-film composite membranes and concentrated draw solutions.

Why PRO Research Matters Now

The expansion of seawater desalination capacity worldwide is generating vast volumes of concentrated brine. Pairing PRO with desalination plants offers a dual benefit: recovering energy from the brine while reducing its salinity before discharge. Climate commitments are driving demand for novel renewable energy sources that can provide baseload power. Advances in membrane materials science require robust testing infrastructure with automatic digital data acquisition capabilities.

Getting Started with PRO Research

For research groups entering the field, the first step is acquiring a testing platform that provides precision and reproducibility. Key features include adjustable hydraulic pressure up to 80 bar, variable-speed pumps, real-time monitoring of water flux and reverse salt flux, temperature control, and integrated data acquisition. Tech Inc. manufactures PRO testing systems with corrosion-resistant wetted components (SS316L, PVDF, PTFE) and compatibility with both flat-sheet and hollow-fiber membrane configurations.

Frequently Asked Questions

What is Pressure Retarded Osmosis (PRO)?

Pressure Retarded Osmosis is a membrane-based process that generates energy by harnessing the osmotic pressure difference between two solutions of different salinities.

How does PRO differ from reverse osmosis?

Reverse osmosis consumes energy for desalination. PRO generates energy by allowing water to flow along the osmotic gradient into a pressurized draw solution. They are thermodynamically opposite processes.

What is blue energy?

Blue energy, also called salinity gradient energy or osmotic power, is the energy available from mixing two solutions of different salt concentrations. PRO is one of the leading technologies for capturing blue energy, alongside reverse electrodialysis (RED).

Where can I obtain PRO testing equipment?

Tech Inc. manufactures specialized Pressure Retarded Osmosis testing systems for laboratory and pilot-scale research. Visit techincresearch.com for specifications.