Water for Humanity - Appropriate Technology
Fog Harvesting Technology
by Steve Herbert
In 2006, I made a Central American multi-country tour through
Roman, who is also a guide, took us on an awesome hike one Sunday. We started by busing to ‘La Ventosa’ (which means “windy place”) at about 11,000 feet. Frost was on the ground as we started up about sunrise. On a previous trip, Adam had attempted to initiate a project near here setting up a fog-harvesting array. This was meant to take advantage of the unique mountainous relief, altitude and fog conditions as a way to collect fresh water. At the beginning of our hike, we saw another fog-harvesting array established by a separate organization, which was of great interest to both Adam and I.
Ordinarily, the people living in valleys or on mountain slopes get their water from springs which find their way out of the mountainsides. But for the people who live on or near the peaks, there is little watershed above them and water resources is a problem. Therefore some have adapted by using the same conditions to their advantage to harvest water from that laden in the fog pushed up the slopes by prevailing winds.
The fog-harvesting array we saw consisted of two white-colored sheets, about four meters by eight meters each, strung up tautly between poles on either side. The sheets of nylon or plastic had a very loose weave, not unlike greenhouse shade clothe. These two nets were set up at only slightly different angles to each other, facing the downslope. Along the bottom edge of each sheet was a length of PVC pipe with about an inch slot, presumably to catch the water dripping off the net. Net and tubing together had a slight pitch to facilitate gravity drainage. Flexible tubing led from the lower ends of each of the two pipes to a black plastic cistern, sitting a bit downslope. Another tube then led to a much larger concrete cistern.
Roman explained that these nets were set up perpendicular to the prevailing winds, which push fog upslope into cooler climes where conditions are right for microscopic droplets to condense on the net material. These tiny drops then coalesce into bigger drops which then migrate down the net material by the force of gravity and drip into the slot of the PVC collection pipe. I learned later that water can also be collected in this way from stratocumulus clouds under the right conditions. The filaments of the net are typically 1mm wide and .1mm thick, with a triangular weave. The ratio of material to opening space is very important. It may vary some, but is usual to be about one third material to two thirds openings. One must also take into account the velocity of the wind in choosing the proper netting.
The prevalence of fog in any particular area may vary over the year or have a season. Generally it is thought that a fog season must be at least six months to make the system practical. One must also be aware that dust can collect on the nets as well as water, and one must take precautions that water collected in the cisterns does not become contaminated. However, the advantage is of course, that a surprising amount of water can be harvested in this way. Yield is typically estimated in terms of liters per square meter of netting surface per day. It is not unreasonable to expect as much as 5l/m3/day and there have been reports of yields almost triple that. Some systems set up under the most ideal conditions have even supplied the water needs of small communities.
In about two or three hours after our visit to the fog-harvesting array, we reached the peak of ‘La Torre’ (el. 3,837 meters or 12,588 feet)! This was physically demanding and quite a challenge in the thin air, but we made it and I nearly equaled my record of 13,100 feet trekking in the