When Gijsbert Cirkel coached his son’s soccer team, he remembers the kids lying in the shade after games. They’d lift their legs in the air and yell, “Our legs are hot!” Their games were played on artificial turf. This heat-trapping plastic can get so hot it can burn. Cirkel’s Dutch research team recently developed a new water-storage system that works to avoid this 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) beneath the playing field. Eventually, that water will evaporate. As it evaporates, it will cool the playing surface. It’s similar to the way sweat works. There, the body releases moisture to carry heat away from our skin.
The natural grass stayed cooler than the new system — but not by much. In fact, the new fake grass system works really well in the sun. On a hot June day, the regular artificial grass surface rose to 62.2 degrees Celsius (144 degrees Fahrenheit)! That was 25 degrees Celsius (45 degrees Fahrenheit) hotter than normal grass. The new fake grass system was only 1.7 degrees Celsius hotter than real grass.
The structure of the rainwater cooling system
Cerkel works at the KWR Institute for Water Research in Nieuwegein, the Netherlands. For an earlier project, his team developed a system in which rainwater was stored under a living roof – a roof on which plants grew. The collected water would later rise to quench the thirst of the plants’ roots.
This is the same process by which water rises up a paper towel when part of the towel is dipped in water. Cirkel wondered if such a system could be adapted to cool artificial grass.
Artificial grass fields are made of layers, like a sandwich. Fake grass is on top. Some filler material, often made of rubber, is underneath it. The Circle team added a rainwater-harvesting area below the fill and a waterproof liner below that. The team also changed the fiber filler for normal rubber.
The hard plastic blocks in the water-harvesting area have holes to let water in. The blocks are glued to each other like thin Lego bricks. There is a cone of wicking fiber to connect the water-harvesting layer to the top filler. This is made of rock wool.
The team expected — but wasn’t sure — that water would go over the layers and evaporate on the surface. So they were “pleasantly surprised that such a large amount of water was evaporating,” recalls Marjolein van Huijvoort. She’s another hydrologist on the team.
Those first tests were done indoors. The team had built 33 small glass boxes and placed different combinations of materials in them.
All the boxes had water storage on the bottom, a pad on the top and a wick to transfer moisture between the layers. The more water moved from the bottom to the top, the better the system cooled.
The different types of artificial turf did not affect evaporation rates. What mattered was the filler layer.
Here, sand performed well. Water moved upward through it and evaporated at a rate of 2.7 millimeters (0.1 inches) per day. The only filler that did not transfer water well was conventional rubber (the standard filler in fields covered with artificial turf). Only about 1 millimeter (0.04 inches) of the stored water evaporated from its surface each day.
Field tests show a resounding success
For their outdoor tests, the researchers built four small fields in the city of Amsterdam. Each had a side of five meters (16 feet). Two plots were with artificial turf on top of the new rainwater-cooling system. One plot had real grass planted on top of that system. The last plot used standard artificial turf, with no water storage underneath.
The new fake grass system did a good job at storing rainwater and preventing runoff. It stored six times more rainwater than normal artificial turf. This water was later evaporated to cool the surface.
If many urban plots were covered with such a system, they could help limit citywide temperatures, known as the heat-island effect, Circle says.
Even cooler artificial turf is not free from problems
Artificial turf has become more and more common. It holds up better than real grass under heavy use and wet conditions. But normal artificial grass doesn’t soak up rain. Instead, the water runs off it, taking turf-based pollutants with it.
The new option is “a good effort,” says Vassilis Vassiliou. He’s an environmental toxicologist at the Yale School of Public Health in New Haven, Connecticut. He likes the new concept, even though it doesn’t solve all the issues with artificial turf. He said fake grass contains harmful chemicals. These include “forever chemicals” called PFAS and hydrocarbons (the building blocks of most plastics). Players will still be exposed to these, Vassilo said. These pollutants will also leach into the air and rainwater.