Passive cooling has been necessary for several years. The expression designates all the solutions making it possible to lower the temperature of a building or limit its heating during summer seasons or episodes of high heat, while having reduced or even zero energy consumption.
Heatwave episodes, increasingly long and frequent due to climate change, remind us that cooling our buildings is not only a comfort issue, but also a public health issue for decades to come.
This is particularly the case in regions of the world where temperatures regularly exceed 40°C, or for vulnerable people such as the elderly, young children or sick people. Cooling may also be simply necessary for reasons of thermal comfort.
Air conditioning, for better or for worse?
Currently, the cooling of buildings relies mainly on the use of air conditioners, which have the disadvantage of being very energy-intensive.
Summer air conditioning in a home can thus generate an excess electricity consumption of around 15%. Certainly, the carbon impact of this consumption remains relatively limited in France, thanks to low-carbon electricity production, but this is not the case in many countries. Globally, cooling accounted for 18.5% of the buildings sector’s total electricity consumption in 2016, up from 13% in 1990.
In addition to their high energy consumption, air conditioners also have the disadvantage of rejecting heat outside, contributing to the well-known heat island effect in cities. For example, in Paris, their summer use could be responsible for an increase of up to 2°C in outdoor temperatures in certain neighborhoods.
The main levers: ventilation, architecture and town planning
In this context, solutions consuming little or no energy (by definition, these are passive cooling solutions) are increasingly sought after. These include a wide range of practices and devices.
For example, there are very simple practices that promote natural ventilation, such as opening windows in the early morning to lower the indoor temperature by a few degrees, then closing shutters and windows during the day to limit heat gain. This first level can be operated easily, even in the absence of specific devices.
The architectural design can also be optimized in order to improve natural ventilation: for example, by orienting buildings in order to limit obstacles to wind circulation, or by taking into account the topography of the environment (relief, plain, hill, etc.) or the location of other surrounding buildings.
It is thus possible to opt for architecture that promotes natural ventilation by drawing inspiration from traditional constructions, such as U-shaped buildings, open to the prevailing winds, or shaded interior courtyards, equipped with wells functioning like chimneys evacuating hot air. Finally, direct exposure to sunlight can be reduced, for example by designing narrow streets, such as in Masdar, United Arab Emirates.
The choice of materials and vegetation
Certain natural building materials, such as limestone (and, by extension, lime plasters) or silt, a sediment composed of clay, effectively regulate temperatures. They absorb water during humid or cool periods (such as at night). This water evaporates during part of the day as temperatures rise, causing a daytime cooling effect.
The greening of urban areas also contributes to passive cooling and fights against heat islands. In fact, in addition to shading facades, trees cool their environment thanks to evapotranspiration. The water emitted during leaf transpiration and its evaporation thus limit the heating of the air surrounding the tree.
It is also possible to act on the glazing by working on the optical properties of the glasses. Some insulating glass filters specific wavelengths in solar infrared while allowing thermal infrared radiation emitted by the building to escape. So-called “active” glazing, capable of changing its color over the seasons, can greatly limit the heating of buildings during the day.
Take advantage of the coolness of the subsoil with geothermal energy
Another very effective solution is to harness the coolness of our subsoil, in other words the surface geothermal potential. At a depth of a few meters, in temperate latitudes, the subsoil temperature remains stable around 12°C all year round. This freshness has been exploited since Antiquity with the technique of the climatic well, also called “Provençal well”, when it comes to cooling.
The principle consists of circulating outside air towards the home in a pipe buried between 2 and 4 meters deep. In winter, the transported air warms up by a few degrees (the ground being, at this depth, warmer than the air, because it is less subject to day/night temperature differences). In summer, the ground remains cooler: the air transported by the pipe can then cool the building.
Another passive cooling solution based on geothermal energy consists of capturing groundwater at 12 to 15°C in a vertical well a few tens of meters deep, or installing a geothermal probe in a borehole, generally consisting of a U-shaped tube containing a heat transfer fluid.
Thanks to a heat exchanger, the coolness of the basement is then directly transmitted to the building’s emitting network (cooling floor or ceiling, water radiator, fan coil, air handling unit, etc.), without using a geothermal heat pump. We then speak of “geocooling”, or geo-refreshing.
This method is particularly efficient, characterized by performance coefficients of 30 to 50. In other words, for 1 kilowatt hour (kWh) consumed by the circulation pump, 30 to 50 kWh of freshness can be restored.
