Averaged out over a year, temperatures in greater urban areas, cities and megacities all across the world are now one to two Kelvin higher than in the surrounding rural areas. Depending on the size of the settlement, the temperature difference can be as much as ten Kelvin, which has given rise to the term “urban heat island.” One key factor affecting this urban heat balance is the dense concentration of buildings, which represents a considerable surface area for the transfer of heat on the one hand, and a massive thermal store on the other. Urban areas are also less adept at getting rid of heat through thermal radiation because they produce a greater volume of emissions that trap the heat. In addition, paving and buildings seal off the ground, preventing it from absorbing rainwater, which would in turn evaporate and improve the urban climate. Especially during prolonged periods of hot weather, this build-up of heat can lead to increased discomfort and even pose a risk to public health.
While there’s not much that can be done to existing buildings, certification processes for new buildings are changing. Several of these now include building surfaces that are designed to absorb less solar radiation and so don’t heat up as much when the sun shines. Another approach involves de-sealing roadways, but since these must remain functional this isn’t the most practicable solution.
As part of a collaborative project funded by the German Federal Ministry for Economic Affairs and Energy (BMWi), Fraunhofer IBP collaborated with the German Federal Institute for Materials Research and Testing (BAM) and industry partners to develop a novel paving stone concept.
The basic idea is for concrete paving stones to be able to store water. During the hot summer months, the water is transported as needed to the surface, where it evaporates and cools the stone surface in a process known as evaporation enthalpy. One effect of this integrated water store is to inhibit the concrete from absorbing and storing heat in the first place – it’s this absorbed heat that tends to prevent the concrete from cooling properly in the evening and at night. Another effect is to make being on or near paved surfaces significantly more comfortable. This is because the temperature difference to the fourth power of surfaces (in this case the ground and people walking on it) comes into play when energy is transferred as radiation.
Initial measurements indicated that under certain conditions, a temperature drop in the region of ten Kelvin is realistic. This is a difference comparable to a dry concrete surface and a stretch of grass in the midday sun.