Liquid droplet mops optimise droplet impact energy, reducing the amount of water needed for solar panel cleaning by up to 80%.

With the rapid expansion of the global solar energy industry, the number of solar panels has surged; however, dust and pollutants accumulating on panel surfaces can significantly reduce their energy conversion efficiency. Traditional cleaning methods are highly water-intensive: high‑pressure liquid jets are commonly used, requiring in excess of 12 billion gallons of water per year globally.

This water use is a major environmental concern. To reduce it, a number of inefficiencies in current cleaning methods have been identified: the cleaning water doesn’t always impart sufficient energy to the dust or pollutants, meaning that more massive or strongly adhering particles in particular get left behind, and larger water droplets are easily deflected from their intended cleaning path by surface vibrations, leaving parts of the surface dirty.

In response to these challenges, an international research team led by Professor Steven Wang, Associate Vice-President (Resources Planning) and Associate Professor in the Department of Mechanical Engineering and the School of Energy and Environment at City University of Hong Kong (CityUHK), in collaboration with Professor Omar Matar (Imperial College London) and Professor Chao Yang (Institute of Process Engineering, CAS, Beijing), has developed a new technology that enhances the cleaning efficiency to use significantly less water.

Their ‘liquid droplet mops’ concept leverages the physics of droplet spreading and recoiling upon impact with the panel surface, which generates targeted hydrodynamic forces that drag and lift pollutants away. Professor Wang explains that this draws inspiration from natural phenomena such as butterfly wings and plant leaves, which achieve self-cleaning through raindrop impacts.

Lo Wai-kin, a PhD student at CityUHK and co-author of the study, shares how the team tested droplets repeatedly using sand particles of various sizes to simulate actual desert conditions. Through this, they discovered that the cleaning efficiency depends on droplet energy in a non‑monotonic manner, meaning that a stronger impact of droplets on the panel surface does not yield better cleaning efficiency; optimal cleaning performance occurs at moderate energy levels rather than maximum force.

Using a typical range of sand particle sizes deposited on solar panels, droplets impacting with moderate energy effectively were found to coalesce and remove the sand particles. By avoiding maximum force, each droplet’s impact energy could be more efficiently used to carry away contaminants, resulting in a highly effective and stable cleaning mechanism.

Based on this key discovery, the researchers engineered their ‘liquid droplet mops’ method to precisely control droplet impact strength, removing contaminants from superhydrophobic-coated solar panel surfaces with a particle removal efficiency of up to 99.9%.

Their new technology also successfully tackles the challenge of high-density dust, including heavy particles with densities 6–10 times greater than water.

Compared to conventional methods, this approach reduces water usage by over 80%, potentially lowering global annual water consumption for solar panel cleaning from over 12 billion gallons to just 2 billion gallons.

“We hope this groundbreaking technology will help conserve global water resources, improve the efficiency of renewable energy utilisation, and ultimately generate significant economic and environmental benefits,” says Professor Wang. He adds that this is a simple yet highly impactful research outcome: it not only validates a new cleaning mechanism but also provides important insights for future engineering design of solar panel cleaning devices.

The ‘liquid droplet mops’ method is particularly promising for large-scale solar farms in arid desert regions, alleviating water stress and enhancing the sustainability and viability of renewable energy deployment worldwide.

Adapted with permission; original text by City University of Hong Kong. Original reference: Wai-Kin Lo et al., Liquid droplet mops. Nature Sustainability (2026). DOI: 10.1038/s41893-026-01804-z