Fog harp gets an upgrade

Researchers at Virginia Tech have upgraded the design of a device that harvests water from fog to make it significantly more effective.

A third of the world’s population struggles with water scarcity. In many of these areas, fog holds water that could provide a lifeline – if only it could be captured. Harvesting that water more efficiently has become the work of researchers from two colleges at Virginia Tech, who recently improved on their original fog harp design with a model that more closely resembles another musical instrument: a guitar.

Harvesting water from resources such as fog isn’t a new idea. Archaeologists have found evidence of ancient cultures doing some form of this practice in Israel and Egypt. The most popular method today uses nets that are mounted upright, catching fog droplets as they pass in the wind. The captured water trickles downward and is collected. A single harvesting net can capture several litres of water per day.

However, the crisscross design of a net presents difficulties. When the net’s holes are too small, they get clogged with water, which redirects the fog stream away from the harvester. Larger holes can avoid clogging, but then almost all of the microscopic fog droplets pass through without being captured.

In 2018, teams led by associate professors Jonathan Boreyko from mechanical engineering and Brook Kennedy from industrial design addressed those shortfalls with a new device called a fog harp, resembling its namesake musical instrument. They created an upright frame with parallel vertical fibres arranged close together for maximum harvesting without the horizontal crisscross wires that cause clogging.

The new harp design improved the efficiency of water collection from fog remarkably. The harps collected up to seven times the amount of water as nets. However, continued testing revealed that harps have an issue of their own.

‘Tangling,’ Boreyko said. ‘That became the issue. Without any cross-supports, the fog droplets tended to pull wires together by surface tension, just like when long hair gets wet. This opens big gaps between the clumped fibres that allow fog droplets to pass through without getting captured. We first realised the issue when testing full-size fog harps outdoors, instead of scale-model harps in the lab.’

Put simply, the harp was performing most poorly when water was most plentiful. Under those conditions, the team found that a heavily tangled harp didn’t perform much better than a clog-prone mesh.

Having discovered a significant issue, the team turned back to nets for a solution. Realising that too many horizontal fibres cause clogging but removing all of them causes tangling, the researchers tried creating various hybrid models. These hybrids still resembled fog harps but now included a small number of horizontal cross-supports to combat tangling.

‘If our first creation was a harp; our new hybrids resemble a guitar neck,’ Boreyko said. ‘Think of the vertical harp fibres as the guitar strings, with the occasional cross-support resembling the frets. This analogy is probably influenced by our research group going to the Metallica concert in Lane Stadium last month.’

To find the best solution, the researchers fabricated seven different ‘guitar neck’ harvesters with varying numbers of ‘frets’ crossing the harp fibres. The version with the most frets was simply a net, while the other harvesters continually decreased the number of frets until the final design option was simply a harp. Researchers, including Jimmy Kaindu (pictured above), then a doctoral student in mechanical engineering, and Lilly Olejnicki, a rising senior in industrial design, subjected all of the options to fog and measured which would capture the most water.

The hybrid designs in the middle of the spectrum found the right balance: they neither clogged nor tangled when exposed to heavy fog streams. By avoiding both issues, these new hybrid harvesters captured several times more water than any of their predecessors.

‘With our hybrid approach, we have demonstrated that scientifically informed design has a huge impact on the amount of water we collect,’ Kennedy said. ‘With this information, we can choose the best design for the benefit of communities suffering from water scarcity to provide new options for drinking, agriculture, sanitation and more. We hope to see our designs flourish in the real world at scale and facilitate their economic mass production.’

The research has been published in the Journal of Materials Chemistry A.