What is passive cooling?

How can we make the heat inside buildings more bearable? Air conditioning is often the first thing that comes to mind, but other approaches—with reduced or even zero energy consumption—can also be employed. This is known as passive cooling.

Passive cooling has been gaining traction for several years. The term refers to all solutions that lower a building’s temperature or limit its heating during summer or heat waves, while consuming little to no energy.

Heat waves, which are becoming longer and more frequent due to climate change, remind us that cooling our buildings is not just a matter of comfort, but also a public health issue for the decades to come.

This is particularly true in regions of the world where temperatures regularly exceed 40°C, or for vulnerable individuals such as the elderly, young children, or those who are ill. Cooling may also simply be necessary for reasons of thermal comfort.

Air conditioning: for better or for worse?

Currently, building cooling relies primarily on the use of air conditioners, which have the drawback of being highly energy-intensive.

Cooling a home in the summer can thus result in an increase in electricity consumption of around 15%. Admittedly, 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 building sector’s total electricity consumption in 2016, compared to 13% in 1990.

In addition to their high energy consumption, air conditioners also have the drawback of releasing heat outdoors, contributing to the well-known heat island effect in cities. For example, in Paris, their use during the summer could be responsible for an increase of up to 2 °C in outdoor temperatures in certain neighborhoods.

Key levers: ventilation, architecture, and urban planning

In this context, solutions that consume 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 early in the 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 easily implemented, even without specific devices.

The traditional houses on the island of Santorini, Greece, are an example of Cycladic architecture, featuring whitewashed buildings and a cross-ventilated design (windows on two opposite sides) to harness the wind for ventilation and cooling. Pexels/Paulo Veloso

Architectural design can also be optimized to improve natural ventilation: for example, by orienting buildings to minimize obstacles to wind flow, taking into account the local topography (mountainous terrain, plains, hills, etc.), or the layout of other surrounding buildings.

It is thus possible to opt for architectural designs that promote natural ventilation by drawing inspiration from traditional constructions, such as U-shaped buildings open to prevailing winds, or shaded interior courtyards equipped with shafts that function like chimneys to exhaust hot air.

Finally, direct exposure to the sun can be reduced, for example by designing narrow streets, as in Masdar in the United Arab Emirates.

The choice of materials and vegetation

Certain natural building materials, such as limestone (and, by extension, lime plaster) or mud, 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, creating a cooling effect during the day.

Greening urban areas also contributes to passive cooling and combats heat islands. Indeed, in addition to providing shade for facades, trees cool their surroundings through evapotranspiration. The water released during leaf transpiration and its evaporation thus limit the warming of the air surrounding the tree.

It is also possible to modify glazing by adjusting the optical properties of the glass. Certain insulating glasses filter specific wavelengths in the solar infrared spectrum while allowing the thermal infrared radiation emitted by the building to escape. So-called “active” glazing, capable of changing its tint with the seasons, can greatly limit the heating of buildings during the day.

Harnessing the coolness of the ground with geothermal energy

Another highly effective solution is to harness the coolness of the ground, in other words, the surface geothermal potential.

A few meters below the surface, in temperate latitudes, the ground temperature remains stable at around 12°C year-round. This coolness has been harnessed since ancient times using the climatic well technique, also known as a “Provençal well,” for cooling purposes.

Diagram of a ground-source heat well. Ademe-BRGM

The principle involves circulating outside air into the home through a pipe buried between 2 and 4 meters deep. In winter, the air is warmed by a few degrees (since the ground at this depth is warmer than the air, as it is less affected by day/night temperature fluctuations). In summer, the ground remains cooler: the air transported through the pipe can then cool the building.

Another passive cooling solution based on geothermal energy involves capturing groundwater at 12 to 15 °C from a vertical well several dozen meters deep, or installing a geothermal probe in a borehole, typically consisting of a U-shaped tube containing a heat transfer fluid. Using a heat exchanger, the coolness from the ground is then directly transferred to the building’s distribution system (cooling floor or ceiling, water-filled radiator, fan coil unit, air handling unit, etc.), without the need for a geothermal heat pump. This is known as geocooling, or geothermal cooling.

This method is particularly efficient, characterized by coefficients of performance ranging from 30 to 50. In other words, for every 1 kilowatt-hour (kWh) consumed by the circulation pump, 30 to 50 kWh of cooling can be delivered.

This article is published in partnership with the General Delegation for the French Language and the Languages of France of the Ministry of Culture.