Engineers have long dreamed of squeezing electricity from the quiet, invisible energy released when river water meets the sea. A new modeling study now proposes a way to make that salinity gradient power both stronger and steadier by pairing it with two other abundant resources: fluctuating sunlight and industrial waste heat. The work, published in the journal Carbon Neutral Systems, presents a detailed mathematical simulation of an integrated system in which a dish-type solar engine drives reverse osmosis to concentrate salt water, and low-grade heat warms the solutions before they pass through a pressure retarded osmosis, or PRO, membrane unit. Under the study’s assumptions, the combined enhancements raise the theoretical power output by roughly 91 percent compared with generation from untreated natural salt and fresh water alone.
The central problem the researchers set out to address is instability. Solar power, however clean, rises and falls with cloud cover and the sun’s daily arc, and that variability makes it awkward to feed directly into a grid or a factory. Salinity gradient energy, by contrast, is inherently stable: the concentration difference between fresh and salt water barely changes between day and night. But on its own, the osmotic pressure difference between natural river water and seawater is modest, which keeps the power density of PRO membranes low. The new proposal treats these two weaknesses as complementary. Instead of storing solar energy in batteries, molten salt, or pumped water, the system converts it into chemical potential energy, stored as an enlarged salinity gradient between two reservoirs of water.
In the proposed configuration, a dish solar Stirling engine, chosen for its compactness and relatively high solar-to-electric conversion efficiency, converts captured sunshine into mechanical power. That power drives a pump pressurizing salt water in a reverse osmosis module, pushing fresh water through the membrane and leaving a more concentrated brine behind. The energy is thereby stored as additional Gibbs free energy, the thermodynamic currency of the salinity gradient. Pressure exchangers recover residual pressure from the concentrated effluent, and both the concentrated salt water and the diluted fresh water are parked in reservoirs, ready to release their stored energy on demand through the PRO unit at any hour of the day or night.
A key design decision examined in the study is whether the loop should be closed, recycling a single salt solution, or open, drawing real fresh and salt water from the environment. Using representative concentrations of 0.6 M for salt water, roughly seawater, and 0.005 M for fresh water, the modelers compared the two arrangements at identical stored energy. With the same 20 kW of pressurization power input, both configurations stored about 4.31 kW of Gibbs free energy, but the open-loop scheme achieved an energy storage efficiency of 74 percent against roughly 41 percent for the closed loop. The advantage arises because the closed loop must permeate more water to store the same energy, leaving less recoverable pressure in its concentrated effluent.
The team also derived an analytical condition for the reverse osmosis pressure that maximizes storage efficiency, solving it with a bisection procedure. For their specified inlet conditions, the optimum came out at 3.90 megapascals, yielding a maximum reverse osmosis storage efficiency of 75 percent. Because the membrane area was assumed sufficient and the operating pressure held constant, the effluent concentrations stayed essentially flat through the daylight hours: the salt water emerged at about 0.8 M, enriched from 0.6 M, while the fresh water was diluted from 0.005 M to about 0.004 M. What fluctuated instead was flow rate, peaking at midday, when solar input was strongest, and ebbing toward the edges of the 8-to-16-hour operating window.
Simulating a full day with a bell-shaped solar flux curve, the modelers found that solar power output from the engine ranged from 5 to 14 kW, with solar-to-mechanical efficiency between 18 and 26 percent depending on wind speed, since wind cools the receiver. Higher wind shaved the daily volume of concentrated solution from around 222 cubic meters under still air to 209 cubic meters at 10 meters per second of wind. Overall, the theoretical efficiency of converting captured sunlight into stored Gibbs free energy was estimated at 14 to 19 percent, a figure the authors explicitly flag as a system-level modeling result that real-world losses such as hydraulic friction, non-ideal components, and membrane degradation would erode.
The release stage is where the enhancements pay off. Feeding the natural, untreated solutions into the PRO model produced a maximum power output of 2.89 kW. With the solar-enhanced concentrations, that figure climbed to 4.10 kW, a 41 percent increase under no-wind conditions, driven by two reinforcing effects: the optimal PRO operating pressure rose from 1.08 to 1.44 megapascals, and the stronger concentration difference pulled more water across the membrane. Adding heat pushed the performance further. When both inlet streams were warmed to 60 degrees Celsius, the inlet osmotic pressure difference grew from 2.90 to 4.41 megapascals, the transmembrane water flow increased from 2.84 to 3.26 liters per second, and power output reached 5.51 kW, with the PRO energy conversion efficiency improving from about 44 to nearly 52 percent.
The accounting of contributions is telling. Of the total 5.51 kW output, solar-driven concentration enhancement added 1.22 kW and low-grade heat added 1.41 kW, corresponding to shares of 22 and 25 percent of the total. The heat route is notably inefficient at the thermodynamic level: because water has a high heat capacity and density, converting waste heat into extra Gibbs free energy at the PRO inlet achieved only 0.75 to 2.27 percent efficiency depending on regenerator effectiveness. Yet the authors argue this hardly matters when the heat is essentially free, since sub-80-degree-Celsius thermal energy is widely available as industrial waste heat, geothermal warmth, or surplus solar heat. The improved PRO sub-model was validated against published experimental data from Touati and colleagues covering 20 to 60 degrees Celsius, showing consistent trends and a relative error of around 5 percent, though it slightly overestimated power density.
The authors are careful to frame the work as a fundamental technical analysis rather than a blueprint for an immediate product. The membrane areas in both units were assumed unlimited to isolate the system-level energy coupling, and the simulations rest on idealized membrane behavior, negligible salt flux, and stable performance over time. In practice, membrane fouling, concentration polarization, aging, heat exchanger losses, and the coordinated control of pumps, pressure exchangers, and turbines would all chip away at the predicted numbers. The system is also mechanically complex, integrating a solar engine, two membrane processes, pressure exchangers, heat exchangers, and a turbine-generator set, which raises capital cost and scalability questions the authors defer to future techno-economic studies. As an open-loop scheme, it would additionally need site-specific assessment of water intake and brine discharge against local environmental rules.
Even with those caveats, the study sketches an appealing vision: a power plant whose fuel is sunshine, waste heat, and the humble mixing of river and sea. The concept is best suited, the authors note, to regions with strong solar irradiance, access to both fresh and salt water, and a nearby source of low-grade heat, particularly where grids are weak or fluctuating renewables need stabilizing. Whether the modeled 19 percent solar-to-stored-energy efficiency and the 91 percent power boost survive contact with real membranes, real seawater, and real economics remains to be tested. But the underlying idea, storing fickle solar energy as a saltier gradient and topping it up with heat that would otherwise be thrown away, offers a genuinely different route to round-the-clock clean electricity.
Subject of Research: A hybrid solar, low-grade heat, and salinity gradient power generation system modeled for stable clean electricity from salt and fresh water.
Article Title: Synergetic clean energy production: solar power and low-grade heat assisted osmotic power generation from salt and fresh water
Article References: Lai, X., Cui, H., Wei, Z., Xu, D., Lou, R., Yang, X., Yue, Y., Wang, Q., Guo, Y., Mao, J., Dou, B., & Liu, P. (2026). Synergetic clean energy production: solar power and low-grade heat assisted osmotic power generation from salt and fresh water. Carbon Neutral Systems, 2(1), Article 5. https://doi.org/10.1007/s44438-026-00026-2
Image Credits: AI Generated
DOI: 10.1007/s44438-026-00026-2
Keywords: osmotic power, pressure retarded osmosis, reverse osmosis, salinity gradient energy, solar Stirling engine, low-grade heat, energy storage, Gibbs free energy, membrane modeling, renewable energy, desalination, hybrid energy system
News Source: Faith Mcneil. (October 29, 2026). Solar Power and Waste Heat Boost Osmotic Energy from Salt and Fresh Water. Scienmag.
