Sunscreen for lakes and dams: could a thin floating layer of nanomaterial shield water from evaporation?

Daniel Kwasi Kpeglo, University of South Africa

Water stored in dams and reservoirs for humans to use often evaporates, especially in hot regions like southern Africa. Rising temperatures speed evaporation up, leaving less water for people, farming and industry. Daniel Kwasi Kpeglo has just completed a PhD in physics. He explains his laboratory research into a new design – a thin nanomaterial floating cover that could be placed over lakes and dams like a blanket, reducing evaporation by limiting the amount of infrared heat from the sun that reaches the water. He also outlines what hurdles need to be cleared for the design to be tested in the real world.

Why is reducing water evaporation so important?

Reducing evaporation simply means wasting less of the water we already have. The idea behind my research was quite simple: if we can keep more of the water that we have already stored, we reduce the pressure to find new sources of water.

This is especially relevant for Africa, where water stored in dams and lakes can make an important contribution to water security, food production and resilience to drought.

How does the nanomaterial cover stop evaporation while letting sunlight through?

My research focused on a new nanotechnology-based floating cover for water. The idea is similar to putting a very carefully designed shade or cover over the water surface. I decided to see if a nanomaterial that already exists could be placed on water to act like a selective filter for sunlight. My research aimed to find out for the first time if a nanomaterial could solve the problem of evaporation by allowing much of the visible light through while blocking much of the heat-carrying infrared radiation.

This nanomaterial is made by putting an extremely thin layer of material called a transparent conducting oxide onto a lightweight, flexible plastic sheet.

This cover helps keep the water surface cooler. It’s necessary to allow some sunlight through because otherwise the aquatic plants and animals would die. My cover allows the useful light to get through, but protects the water from the heat.

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In our experiments, the cover allowed more than 60% of the visible light to pass through on average, while reflecting more than 80% of the infrared light back at the sun. As a result, the water underneath the cover remained noticeably cooler than the upper surface of the cover. The water was 10°C cooler during the sunniest part of the day, minimising evaporation.

There is another important advantage. Because this cover is lightweight and water-repellent (water does not cling to its surface), it can float on the water rather than requiring a heavy structure to hold it above the surface.

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In our laboratory experiments, the water level remained essentially unchanged under the coated cover during the test period, meaning very little was evaporating.

On the other hand, the water level decreased when the reservoir was left uncovered or covered only with the ordinary plastic sheet.

What else stops water evaporating, and why is this way better?

Floating or suspended covers, shade cloths, wind barriers, plants, and natural materials such as palm fronds have been used to prevent water from evaporating. Some scientists have looked into using chemicals to make a sunscreen for the water. But these tend to block all sunlight, whereas our research is only trying to block infrared radiation.

We believe our new design is better because nanomaterials are lightweight. The transparent conducting oxide is applied as a thin coating of a few hundred nanometres thick over a flexible and lightweight plastic sheet. To emphasise how very thin this coating is, one nanometre is one billionth of a metre. This makes our cover easier to handle than a heavy permanent structure.

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Second, because it allows much of the visible light through, it has the potential to reduce evaporation without completely cutting off sunlight from the water below. This is important for reservoirs that support aquatic plants and other forms of life.

Third, the coated surface is water-repellent and when we tested it, we found it was less likely to develop layers of algae or other natural growth. This could mean less cleaning and maintenance over time.

Although the nanomaterial contaminating the water is a potential concern, our design didn’t focus on placing the nanomaterial directly in the water. Rather it was used as a thin coating fixed onto a plastic sheet, not touching the water itself.

What needs to happen next?

We have only tested this in a laboratory-scale water reservoir using an artificial light source designed to mimic sunlight. We tested different light conditions and distances from the water surface, comparing an uncovered reservoir, one covered with ordinary plastic and one covered with the coated material. The results consistently showed much less water was lost when the coated cover was used.

The next important step is to move from the laboratory to the real world.

This is what would need to happen:

First, large-scale field trials are needed to determine how it compares with existing solutions in terms of cost, durability, maintenance and overall environmental impact.

A field test would help answer questions that laboratory experiments cannot. How does the material perform in real sunlight, wind, rain and changing temperatures? How long will the cover last? How will it behave on moving water? What would it cost to make and install over a large reservoir?

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Secondly, in a natural dam or reservoir, the cover would need to be attached to some kind of floating system so water and wind would not move the sheets out of place.

Thirdly, to use the nanomaterial on real dams, a scientific study or investigation needs to be carried out on durable, ultraviolet resistant materials, such as polycarbonate. This is because prolonged exposure to ultraviolet can degrade certain plastics, causing them to become brittle, crack and break down in little pieces. Before this cover can be launched, we need to make sure that the material it’s made of can withstand years of exposure to sunlight while maintaining its floating performance.

This article is republished from The Conversation under a Creative Commons license. Read the original article.