Korean Solar Desalination Device Reaches 3.4 Liters per Square Meter per Hour Without Electricity

Sunlight and a paper wick turn seawater into fresh water

A solar desalination prototype developed at the Ulsan National Institute of Science and Technology (UNIST) in South Korea has reached an evaporation rate of 3.40 kilograms per square meter per hour under standard test sunlight. That is roughly 3.4 liters of water per square meter each hour, using sunlight without an external electricity supply.

The research, led by Professor Ji Hyun Jang of UNIST's School of Energy and Chemical Engineering, combines a paper structure that draws up seawater with a material that converts light into heat. Its other central feature addresses a persistent problem: salt collecting on the evaporation surface and gradually blocking the sunlight needed to keep the device working.

The prototype directs salt toward its edges, leaving the hot central area exposed. In laboratory durability tests, it operated stably for two weeks in solutions containing 20 percent salt, compared with the approximately 3.5 percent salinity of typical seawater.

The work, published in Advanced Energy Materials, names Dr. Saurav Chaule as lead author. The researchers also see a role for the design in recovering resources, because salt can crystallize at the margins for collection instead of remaining in concentrated liquid brine discharged into the sea.

For islands, remote villages and places recovering from disasters, the attraction is straightforward: turn seawater into usable fresh water without relying on grid electricity or fuel for the evaporation process. Translating the laboratory result into a dependable water supply will require durable equipment and an effective way to capture the vapor and turn it back into liquid.

How water moves through the device

The UNIST evaporator has an inverse L shape built around water absorbing paper. The paper acts as a wick, drawing seawater upward by capillary action, the same process that lets an absorbent strip pull liquid into its small spaces. The researchers describe its absorbing behavior as similar to litmus paper.

A layer at the top of the vertical section absorbs sunlight and turns it into heat. When seawater reaches that hot surface, the water evaporates while the dissolved salt stays behind. The vapor then moves to a cooler surface, where it condenses into liquid fresh water for collection.

The arrangement concentrates heating where evaporation takes place. Its geometry also controls the direction in which water moves across the hot surface, helping transport dissolved salt away from the area receiving the most useful sunlight.

This combination of a wick, a light absorbing surface and a controlled flow path reduces the need for pumps, membranes and frequent manual cleaning. The device relies on the movement of liquid through the paper and the heat supplied by sunlight, rather than pressure driven machinery.

Why the light absorbing material matters

The photothermal layer uses La0.7Sr0.3MnO3, often shortened to LSMO. Photothermal means that a material converts light into heat. LSMO is an oxide perovskite, part of a class of materials whose optical and electrical properties can be adjusted.

Its ingredients include lanthanum, strontium and manganese. Unlike the lead based halide perovskites commonly associated with solar cells, LSMO does not rely on volatile or toxic lead. That makes the material attractive for water applications, where stability and environmental safety matter.

The explanation for its heat production lies in how it handles energy from sunlight. Light can push electrons into higher energy states. The material contains what are called trap states within its electronic structure, which affect how those excited electrons release their energy.

Many of the excited charge carriers recombine without emitting light, releasing energy directly as heat instead. This process, described as nonradiative recombination or thermalization, turns a high fraction of the incoming solar energy into localized surface heating.

For the evaporator, the practical benefit is a surface that heats quickly and drives faster evaporation for a given amount of sunlight. LSMO's thermal stability is also useful when considering how a device might withstand repeated operation outside a laboratory.

Moving salt away from the hot center

Salt buildup is a major obstacle for solar desalination. Each time water evaporates, the liquid left behind becomes saltier. Eventually, dissolved salts form crystals on the hot surface, where they scatter light and interfere with heat transfer.

Earlier systems have often worked well for hours before needing cleaning or a pause to flush away accumulated salt. The UNIST design tackles this through its shape and controlled liquid movement, rather than relying on complex coatings or moving parts.

The inverse L geometry creates flow in one direction and a salt concentration gradient across the evaporation surface. Dissolved ions move toward regions near the edges where water activity is lower. Water activity describes how available water is within a solution.

Crystals begin to form and grow at those edges, away from the central area that needs to absorb sunlight. Operators can collect the crystals at the margins, while the main photothermal zone remains open to light. Keeping that surface clear helps maintain both light absorption and heat generation over time.

The two weeks of stable operation in a solution containing 20 percent salt are especially relevant to concentrated brines. The original research description identifies that concentration as typical of brines from industrial processes. It is also far above the 3.5 percent salinity of ordinary seawater, suggesting a useful margin for seawater treatment and possible applications in brine concentration.

What the performance figures mean

The reported rate of 3.40 kilograms per square meter per hour was measured under one sun. This is a standard test level of sunlight equal to about one kilowatt of incoming solar power per square meter.

Many passive solar stills produce around 0.3 to 0.4 kilograms per square meter per hour under similar conditions. Against that range, the UNIST prototype's reported evaporation rate is approximately eight to ten times higher.

Daily output depends on how much sunlight reaches the equipment throughout the day and how efficiently its condenser collects the vapor as liquid. Under strong sunshine for several hours, the reported rate suggests that a module covering one square meter could yield on the order of several tens of liters a day. Multiple modules could increase that output.

The laboratory figure measures water leaving the hot surface as vapor. Delivering drinking water also requires that vapor to reach a clean collection system, cool and become liquid. A practical installation therefore needs to capture nearly all of the vapor while limiting heat loss.

Condenser design, insulation and the reuse of heat all affect how closely collected water output can approach the evaporator's measured rate. The salt management system contributes to sustained performance by keeping the surface that absorbs sunlight clear during extended operation.

How it compares with reverse osmosis

Reverse osmosis dominates modern desalination because it can produce large amounts of fresh water for cities and industry. It forces seawater through fine membranes at high pressure, separating much of the dissolved salt from the water.

That process consumes several kilowatt hours of electricity for each cubic meter of water produced. It also needs pretreatment to protect the membranes and leaves concentrated brine that requires responsible management. Plants require substantial investment and usually depend on the electricity grid or a dedicated power source.

The UNIST concept takes a different approach. A passive solar evaporator avoids the pressure equipment, pumps and high electricity demand associated with reverse osmosis. It produces less water per unit of time and area, but can operate away from the grid, quietly and with simpler maintenance.

Directing salt crystallization to the edges can also reduce the volume of liquid brine. In a suitable climate, groups of modular evaporators could serve households, clinics or farms, while larger installations could support community needs.

For cities, solar evaporation is more likely to complement large desalination plants than replace them. Its potential roles include providing a backup water supply and serving remote neighborhoods at the edge of the electricity network.

Where modular systems could be useful

Island communities, fishing villages and coastal farms are among the settings targeted by the design. Each can have access to seawater while facing limited grid electricity or a shortage of people able to maintain complex machinery.

Humanitarian operations after storms, earthquakes or conflicts present another possible use. When water infrastructure fails, equipment that depends on sunlight rather than a functioning power network could help fill gaps in supply.

The concept is modular, allowing individual panels to serve smaller needs or clusters to provide more water. Low noise and the absence of high pressure equipment could also make it suitable for schools and clinics, where safety and dependable operation matter.

Billions of people lack steady access to safely managed drinking water. Some coastal communities depend on desalination systems powered by diesel, with the running costs and brine production that accompany them. The UNIST approach targets part of that need through simple materials, controlled geometry and sunlight instead of large plant infrastructure.

Collecting salt instead of discharging it

The crystals at the evaporator's edges offer a second possible product alongside fresh water. Operators could collect dry salt and other crystallized minerals, giving the device a role in resource recovery as well as water production.

Recovering solid material can reduce concentrated liquid discharges into nearby coastal waters. Where suitable, collected salt could be reused in industrial processes or safely returned to salt pans. Resource recovery could also create small revenue streams.

The team has suggested combining many inverse L shaped evaporators into larger modules. Such an arrangement offers a route toward equipment that is easier to install and maintain in the field, while retaining access to the margins where salt collects.

Brine management remains part of the practical design. The value of directing crystals to accessible edges is that water production and salt collection can be considered together, rather than treating concentrated waste only after fresh water has been made.

What needs to work beyond the laboratory

The paper wick must survive months of repeated wetting and drying, ultraviolet exposure and biofouling, the buildup of marine organisms on a surface. The photothermal coating must also retain its ability to absorb sunlight and release heat with minimal degradation.

Manufacturing methods will influence cost and consistency. Techniques capable of coating large areas uniformly at low temperatures could reduce expense and make modules more repeatable. LSMO's thermal stability helps, but producing dependable equipment involves more than selecting a heat resistant material.

Condensation is another central engineering task. Each kilogram of water vapor carries a large amount of latent heat, the energy absorbed when liquid water becomes vapor. A condenser that transfers some of that heat to incoming seawater could recycle energy and improve total output.

Enclosures need to limit heat loss and vapor leakage without becoming difficult to clean. Dust, sea spray and collected salt crystals will still require simple cleaning routines. These practical details will affect how much water reaches a storage tank and how much attention an installation needs.

The two weeks of testing at high salinity are encouraging, while trials lasting several months in coastal conditions would help establish longer term durability and maintenance requirements. Partnerships with aid organizations and local utilities could support such trials and guide designs for different climates. Field yields, production costs and maintenance needs remain questions for deployment.

Energy savings and environmental costs

For locations outside the electricity grid, solar desalination could replace energy from diesel generators with sunlight. Avoiding fuel consumption during operation offers climate benefits, while removing high pressure pumps also reduces machinery noise around coastal communities and wildlife.

The equipment still has environmental costs over its lifetime. Those include sourcing lanthanum and strontium and manufacturing the absorbers and housings. Durable materials and simple assembly could extend service life, spreading the carbon associated with production over more years of water supply.

Salt recovery could also make desalination more compatible with marine ecosystems by reducing concentrated brine discharges near the shore. The approach connects with global development goals for clean water and sanitation, alongside the need to manage the waste created when seawater becomes fresh water.

As climate change brings more intense droughts and heat waves, communities will need practical ways to improve water security. The UNIST prototype offers a specific route: combine efficient heating, a simple wick and controlled salt crystallization. Its usefulness in daily life will depend on turning those laboratory strengths into durable modules that reliably collect water under local conditions.

What to Know

• UNIST researchers reported evaporation of 3.40 kilograms, roughly 3.4 liters, per square meter per hour under one sun, without external electricity.
• Professor Ji Hyun Jang led the research, published in Advanced Energy Materials, with Dr. Saurav Chaule as lead author.
• A paper wick and inverse L shape feed seawater to a photothermal layer made from La0.7Sr0.3MnO3, or LSMO.
• Controlled liquid movement directs salt toward the edges, helping keep the central light absorbing surface clear.
• The device operated stably for two weeks in solutions containing 20 percent salt, compared with typical seawater salinity of about 3.5 percent.
• The evaporation rate is approximately eight to ten times the 0.3 to 0.4 kilograms per square meter per hour reported for many passive solar stills.
• Collecting the vapor as fresh water requires effective condensation, insulation and a clean reservoir.
• Potential uses include islands, villages, farms, clinics, schools and disaster relief, alongside salt and mineral recovery.
• Field trials, manufacturing methods and condenser performance will determine practical output, costs and maintenance needs.