A Vietnamese research team has developed a low-cost passive cooling system for PV modules based on hydrogel-coated paper that combines water flow and interfacial evaporation to reduce operating temperatures. Outdoor tests showed temperature reductions of up to 14 C and efficiency gains of up to 16.8%, achieving stable operation using both freshwater and natural seawater.
A research group in Vietnam has proposed a new passive cooling technique for PV modules that relies on hydrogel-coated paper as a cooling medium. The approach enables convective heat transfer through flowing water, while removing latent heat via interfacial evaporation. The system was tested under real outdoor conditions with a reference module without cooling.
“The novelty of our research lies in developing a simple and low-cost passive cooling strategy based on hydrogel-coated paper that combines water flow and interfacial evaporation,” said corresponding author Vanthan Nguyen. pv magazine. “Unlike conventional hydrogel systems, our design uses a thin, porous structure that minimizes thermal resistance while maintaining efficient water transport. This enables effective cooling under real outdoor conditions, including stable operation even with natural seawater.”
To produce the hydrogel-coated paper, the team first froze flattened air-laid paper for four hours at -20 C. Meanwhile, the hydrogel solution was prepared by dissolving polyvinyl alcohol (PVA) in water for three hours at 95 C, followed by the addition of glutaraldehyde, SDS and HCl. The paper was then covered on one side with the solution and frozen again for 20 hours at -20°C. Finally, it was thawed and washed several times.
The system was first evaluated in an indoor experiment with 5.5 cm x 6.0 cm epoxy-polycrystalline solar panels. The wet papers were attached to the panels and covered approximately two-thirds of the surface. The remaining edges of the paper were immersed in two water tanks, causing water transport from the top to the bottom tank through the paper. Experiments were conducted under single sun irradiation using both distilled water and natural seawater with a salinity of approximately 32 ppt.
For the outdoor experiments, two identical panels were installed on the roof of a building in Ho Chi Minh City. One 5.5 cm x 6.0 cm panel was left uncooled, while the other contained the hydrogel-coated paper system. The coated surface measured 4.0 cm x 5.5 cm, matching the indoor layout. Both panels were mounted at a tilt angle of 25° from the horizontal plane, facing south, and each was connected to a 35 Ω resistor.
The tests showed that when illuminated with one sun, the system reduced the temperature of the PV panel by 7 C when there was no wind and by 14 C when the wind speed was 1 m/s. These reductions translated into relative electrical efficiency gains of 12.8% and 16.8%, respectively. The hydrogel-coated paper also showed strong long-term stability when tested with natural seawater, both indoors and outdoors.
“One of the most surprising findings was the strong performance under real outdoor conditions,” said Nguyen. “We achieved a temperature reduction of up to 14°C, which resulted in a relative efficiency improvement of 16.8% and an increase of approximately 14.6% in energy generation without any external energy input. The system also operated stably with seawater, preventing salt build-up and allowing seawater concentration.”
The new technique was presented in “Passive photovoltaic cooling via water flow and interfacial evaporation using hydrogel-coated paper”, published in Solar energy. Researchers from Vietnam’s Van Lang University, Ngo Quyen University, Vietnam Institute of Tropical Technology and Environmental Protection, Vietnam German University, Dong Thap University and Phenikaa University participated in the study.
They are currently working on scaling up the system to improve its long-term durability for real-world use. “Future research will focus on optimizing materials for different climates, improving corrosion protection and integrating this approach into large-scale solar systems. We also want to further explore the combination of energy generation and water treatment on one platform,” said Nguyen.
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