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Materials for energy

Materials for energy

Hydrogel helps make self-cooling solar panels

12 Jun 2020 Isabelle Dumé
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Gel designed to cool solar panels. Courtesy: © 2020 KAUST; Youssef A. Khalil

Moisture harvested from the atmosphere at night by a hydrogel can be used to cool down solar panels during the day, boosting their efficiency. So say researchers at the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia and the Hong Kong Polytechnic University (PolyU) who made the hydrogel from a mix of carbon nanotubes in polymers with a hygroscopic calcium chloride salt. The technology could be an environmentally friendly way to increase photovoltaic electricity generation and also cool down other devices.

Solar photovoltaic (PV) panels currently produce more than 600 GW of the world’s power, and this figure is expected to increase to 1500 GW by 2025 and 3000 GW by 2030. While solar energy is an abundant, inexhaustible and very clean energy resource, commercial silicon-based PV cells can only convert between 6–25% of absorbed sunlight into electric current. The rest is transformed into waste heat, which increases the temperature of a solar panel by up to 40 °C. This makes the cells less efficient, and it can also damage them – especially in hot climates, where the problem is even more serious than in more temperate areas.

Current technologies to cool PV panels include refrigeration or air conditioning, but these can be energy-hungry. Water-cooling systems also exist, but they require abundant water supplies, as well as storage tanks and a complicated network of pipes and pumps.

A gel-like material with a high affinity for water molecules

A team of researchers led by Peng Wang of KAUST’s Water Desalination and Reuse Center and the Department of Civil and Environmental Engineering at PolyU recently developed an alternative cooling method. Their technique is based on a gel-like material that comprises heat-absorbing carbon nanotubes (CNTs) embedded in a cross-linked polyacrylamide (PAM) and calcium chloride (CaCl2). This gel has a high affinity for water molecules and can therefore take up large quantities of water vapour from ambient air. According to lead author Renyuan Li, the gel also has the ability to self-adhere to numerous surfaces, including solar panels, through strong hydrogen bonding.

In their experiments, the researchers pressed a 1-cm-thick layer of the hydrogel against the underside of a standard silicon solar panel. When the temperature drops in the evening and overnight, the water absorbed by the material condenses to form liquid water, explains Wang. During the daytime, as the temperature increases, the heat from the PV panel causes the water to evaporate – a process that not only removes heat from the panel, but also regenerates the vapour sorbent so that the atmospheric water harvester (AWH) is ready for the next night-day cycle.

Experiments by the KAUST-PolyU team on PV panels in the laboratory showed that a fully-engorged gel could free enough water to produce a cooling power of 295 W/m2 under 1000 W/m2 solar radiation and reduce the temperature of the panel by 10 °C. This reduced temperature improves the panel’s efficiency enough to boost the amount of electric current produce by an average of 15%. When the team tested the system on an outdoor prototype during winter and summer months on the KAUST campus, this figure increased to 19% – probably because wind enhanced the cooling effect, the researchers say.

With global PV capacity expected to reach 1500 GW by 2025, the researchers calculate that cooling all these panels using their approach would generate more than 150 GW of additional power. This boost to the electricity supply would translate into a reduction of 8.52 × 107 metric tonnes of coal consumed per year, and a drop in CO2 emissions of more than 1.48× 108 metric tonnes per year (assuming 20% solar PV electricity generation efficiency).

Improving water vapour sorption-desorption kinetics

Wang and colleagues, who report their work in Nature Sustainability, are now working to improve the corrosion resistance of their atmospheric water harvester (AWH) and increase its water vapour sorption-desorption kinetics (and thus its capacity). They are also investigating better ways of dealing with dust on PV panels – a particular problem in arid and semi-arid regions, where particles deposited by frequent dust storms can reduce the power output of a solar panel by as much 20% if not removed. One design option for the AWH cooling here, Wang says, would be to use the system to trap and condense water after it has evaporated from the hydrogel. This water could not only be used to remove any dust build up on the PV panels, it might even be clean enough to drink.

On another positive note, the technology could easily be adapted to different scales, Li adds. “It could be made as small as several millimetres for cooling down electronic devices, hundreds of square metres for a building, or even larger for passive cooling of industrial-sized PV farms.”

The researchers say they will now be testing the long-term stability of their AWH. “Our long-term goal is to demonstrate its commercial value and make it market-competitive,” Wang tells Physics World.

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