Due to its ability to absorb and store heat, concrete can be used to passively cool buildings, reducing the energy consumed by air conditioning and/or reducing the risk of overheating.
The use of air conditioning to cool buildings is one of the fastest growing uses of energy, according to the International Energy Agency, with energy demand for space cooling expected to more than triple by 2050 unless action is taken to address energy efficiency.
Using the thermal mass of concrete to provide passive cooling is one potential solution to this challenge and one of the most significant ways that the material can be used to enhance the CO2 performance of buildings. Concrete that is left exposed (or painted) inside a building will soak up excess heat during the day, providing thermal inertia against temperature fluctuations, reducing or eliminating the need for mechanical cooling. Heat retained by the concrete is removed overnight by ventilating the building with cool night air.
The process is often likened to a thermal flywheel, whereby much of the cooling load is shifted from day to night, when it is more easily dealt with by passive means. In practical terms, the concrete typically takes the form of an exposed soffit, providing a large surface area for heat transfer and the ability to store a significant amount of heat within the slab.
The CO2 savings that can be realised by avoiding or minimising the need for mechanical cooling is project and country/location specific. However, a study by Arup for The Concrete Centre in the UK made a simple estimate based on the generic characteristics of an exposed concrete soffit and those of an average air-conditioning chiller. This found that the annual CO2 saving that could be achieved using passive cooling was enough to completely offset the embodied CO2 in the concrete frame and ground works of a typical office building in around 50 years. The study assumed the building could be cooled wholly using thermal mass, instead of adopting a fully air-conditioned solution. In buildings with higher cooling loads, requiring some air conditioning, the use of passive cooling based on the thermal mass of concrete would still reduce annual emissions.
It is worth noting that, just as all materials have mass and some are very heavy and some are very light, all materials also have thermal mass. However, brick, stone, and concrete have far more than other materials, such as timber and steel, because of their relative specific heat capacity and conductivity.
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