Pros and cons of natural ventilation energy efficiency

Energy efficiency is the keystone of the revolution in new buildings; one concept that has been suggested is natural ventilation in order to reduce the energy consumption (as opposed to mechanical ventilation). The main idea is to use the difference of temperature in the building to generate the air flow (without any fan). Thereby, natural ventilation is the most thrifty way to infuse air in a building.

However, a review of what happens in practice indicates that a lot of naturally ventilated buildings don’t save as much energy as hoped for.  Cambridge University and the M.I.T (Massachusetts Institute of Technology) have investigated this riddle in order to find out what is needed to be done to find the best process for a ventilation system.

It appears that for the most part, buildings which use natural ventilation have excessive heating bills because the hot air rises to be extracted from the building and it can’t be controlled, moreover, in the lower parts of the building, people can feel cold because of the inflow of fresh air.

The Cambridge University team found that the natural ventilation strategy in winter is fundamentally flawed and the resulting heating bills are a disaster. However, a pre mixing of fresh air with warm room air is usually achieved using low energy mixing fans immediately beneath high level vents. Then, a mixing ratio of 1:2 in wintertime allows the renewal of the air with a thermal comfort. In summer, fans can be turned off (without blocking the vent). This alternative solution looks like a good compromise between the full mechanical solution and natural ventilation to low energy consumption joined to thermal comfort.

This technique can also be used for atriums. If the incoming fresh air can be introduced to the building at high level in the atrium in winter, then the atrium can be used as a natural mixing chamber. In summer, a basic strategy can be used to introduce fresh air in each room and extract air from the atrium. This technique can be used to reduce the overall energy consumption of the building.