Artificial turf gets cooler with a new evaporation-based system

It relies on rainwater that gets stored below a field of plastic ‘grass’

Artificial turf fields, such as this one, come with challenges. They can heat to dangerous temperatures. And when it rains, runoff carries harmful chemicals that can pollute waterways. A new system works to avoid both problems.

serjunco/iStock/Getty Images Plus

By Laura Allen

September 11, 2024 at 6:30 am

This is another in our series of stories identifying new technologies and actions that can slow climate change, reduce its impacts or help communities cope with a rapidly changing world.

When Gijsbert Cirkel coached his son’s soccer team, he remembers the kids laying in the shade after a game. They’d put their feet in the air and cry, “Our feet are hot!” Their games had been played on artificial turf. This heat-trapping plastic can get so hot it burns. A new water-storage system that Cirkel’s Dutch research team has just developed works to avoid that problem.

It cools through the evaporation of rainwater.

A hydrologist, Cirkel studies the water cycle. His team’s new system collects and stores rainwater about 8.5 centimeters (3.3 inches) below a playing field. Eventually, that water will evaporate. As it does, it will cool the playing surface. It’s like the way sweating works. There, the body releases moisture to carry heat away from our skin.

Natural grass stayed cooler than the new system — but not by much. In fact, the new fake-grass system worked really well in the sun. One hot June day, the surface of regular artificial grass soared to 62.2° Celsius (144° Fahrenheit)! That was 25 degrees C (45 degrees F) hotter than regular grass. The new fake-grass system was only 1.7 degrees C warmer than the real grass.

Cirkel’s group shared its new findings July 8 in Frontiers in Sustainable Cities.

Cirkel works at the KWR Water Research Institute. It’s in Nieuwegein, the Netherlands. For an earlier project, his team developed a system to store rainwater under a living roof — one with plants growing on it. The collected water later wicked up to slake the thirst of plant roots.

It’s the same process by which water travels up a paper towel when a part of the towel falls into water. Cirkel wondered if such a system could be adapted to cool artificial turf.

Fields of artificial turf are built of layers, like a sandwich. The fake grass sits on top. Some filler material, often made of rubber, lies below that. Cirkel’s team added a rainwater-storage area below the filler and a waterproof liner under that. The team also swapped in a fiber filler for the usual rubber.

Rigid plastic blocks in the water-storage zone have holes to let water enter. The blocks clip together a bit like thin Lego bricks. Linking the water-storage layer to the filler above is a cone of wicking fiber. It’s made from rock wool.

The team hoped — but wasn’t sure — the water would move up the layers to evaporate at the surface. So they were “pleasantly surprised that such a huge amount of water was evaporating,” recalls Marjolein van Huijgevoort. She’s another hydrologist on the team.

Those first tests were indoors. The team had crafted 33 small glass boxes and stacked them with different combinations of materials.

All the boxes had water storage at the bottom with a pad on top and a wick to move moisture between layers. The more water that wicked up from below, the better the system should cool.

The different types of artificial turf didn’t affect the evaporation rates. What did seem to matter was the filler layer.

Here, sand performed well. Water wicked up through it and evaporated at a rate of 2.7 millimeters (0.1 inch) per day. The only filler that didn’t move water well was the conventional rubber (the standard filler in fields topped by artificial turf). Only about 1 millimeter (0.04 inch) of the stored water evaporated from its surface each day.

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For their outdoor tests, the researchers created four small fields in the city of Amsterdam. Each was five meters (16 feet) on a side. Two plots topped the new rainwater-cooled system with artificial turf. One plot had real grass planted on top of that system. The last plot used standard artificial turf with no water storage below.

The new fake-grass system did a good job storing rainwater and preventing runoff. It stored up to six times as much rainwater as typical artificial turf. This water would later evaporate to cool the surface.

If many urban plots were covered with such a system, they might help limit a city’s warming from what’s known as the heat-island effect, says Cirkel.

Artificial turf has become more and more common. It holds up better than real grass to heavy use and wet conditions. But normal artificial grass doesn’t soak up rain. Instead, water runs off it, carrying with it turf-based pollutants.

The new alternative “is a good effort,” says Vasilis Vasilou. He’s an environmental toxicologist at the Yale School of Public Health in New Haven, Conn. He likes the new concept, even though it doesn’t solve all the issues with artificial turf. Fake grass contains harmful chemicals, he notes. These include “forever chemicals” called PFAS and hydrocarbons (the building blocks of most plastics). Players will still be exposed to these, Vasilou notes. These pollutants will also leach into the air and rainwater.

Cooling the fake grass and catching rainwater should cut back on the amount of pollution coming off these fields. And that’s a good step forward, Vasilou says. But in the end, he concludes, nothing beats natural grass.

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The Dutch team also wants to see more natural grass fields. In future work, they hope to make grass fields more robust — able to be played on longer before getting muddy or damaged. They’ll see if storing rainwater below these fields and improving their drainage might help.

The rain-cooled system was a big improvement over today’s fake grass. And in the outdoor tests, rain fell often enough to supply all the water the system needed. But in hotter, drier cities, the system may need irrigation, as real grass does.

Cost is another factor: This system is more expensive than real grass or fields of standard fake grass. But if cities can view sports fields as part of how they manage rainwater, the costs for both can be combined, Cirkel says.

His team’s study was done in partnership with a company that installs sports fields. Based on their encouraging field tests, they’ve now gone on to build full-sized systems in the Netherlands, United Kingdom and Japan.

chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

evaporate: To turn from liquid into vapor.

humidity: A measure of the amount of water vapor in the atmosphere. (Air with a lot of water vapor in it is known as humid.)

irrigation: Water delivered by artificial means (such as pipes, channels or sprinklers) to help the growth of plants during dry periods.

leach: (in geology and chemistry) The process by which water (often in the form of rain) removes soluble minerals or other chemicals from a solid, such as rock, or from sand, soil, bones, trash or ash.

pollutant: A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

rock wool: A fibrous material made from rock (such as basalt) that has been melted and then spun into fibers. Owing to its flame resistance, it is often used as a fire-retarding form of insulation for buildings, electronic equipment and ovens.

runoff: The rainwater that runs off of land into rivers, lakes and the seas. As that water travels through soils, it picks up bits of dirt and chemicals that it will later deposit as pollutants in streams, lakes and seas.

sustainable: (n. sustainability) An adjective to describe the use of resources in a such a way that they will continue to be available long into the future.

system: A network of parts that together work to achieve some function. For instance, the blood, vessels and heart are primary components of the human body's circulatory system. Similarly, trains, platforms, tracks, roadway signals and overpasses are among the potential components of a nation's railway system. System can even be applied to the processes or ideas that are part of some method or ordered set of procedures for getting a task done.

toxicologist: A scientist who investigates the potential harm posed by physical agents in the environment. These may include materials to which we may be intentionally exposed, such as chemicals, cigarette smoke and foods, or materials to which we are exposed without choice, such as air and water pollutants. Toxicologists may study the risks such exposures cause, how they produce harm or how they move throughout the environment.

United Kingdom: Land encompassing the four “countries” of England, Scotland, Wales and Northern Ireland. More than 80 percent of the United Kingdom’s inhabitants live in England. Many people — including U.K. residents — argue whether the United Kingdom is a country or instead a confederation of four separate countries. The United Nations and most foreign governments treat the United Kingdom as a single nation

Journal: M.H.J. van Huijgevoort, D.G. Cirkel and J.W.F. Voeten. Climate adaptive solution for artificial turf in cities: integrated rainwater storage and evaporative cooling. Frontiers in Sustainable Cities. July 8, 2024. doi: 10.3389/frsc.2024.1399858.

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