Nov. 26, 2012
-- Natural ventilation is a means of reducing the energy consumption in buildings, the savings coming from the removal of the fan power supplying the mechanical ventilation. However, a review of what happens in practise indicates that many naturally ventilated buildings do not save as much energy as they should. A BP-sponsored research programme was undertaken at Cambridge University and the Massachusetts Institute of Technology (MIT) to investigate and to find a way to create a new generation of low energy buildings. The team found that naturally ventilated buildings invariably had lower electricity consumption than building ventilated mechanically – if you don’t have an electricity hungry fan system supplying an extract air, you won’t have the same electricity consumption. However, the team discovered that heating bills were much higher in naturally ventilated buildings. It is reasonable to expect that the heating consumption were higher compared to a mechanically ventilated building using a heat exchanger (in winter the outflow air can transfer its heat to the incoming fresh cold air). However, the team was intrigued to understand why the heating bills in naturally ventilated buildings were often higher than the design had intended.
The design for heating requirements of naturally ventilated buildings is often based on the assertion that the amount of heating needed is simply that which is not provided by the internal heat gains. This is a laudable intent but not usually achieved in practise because of the following:
Hot air rises, so it seem reasonable to design a natural ventilation strategy which involves a low level vent for inflow and a high level one for outlet. The problem with this strategy is that once the external temperature falls below 15-18 degrees Celsius cold draughts may occur at the low level inflow. The solution used has been to locate a heating device in front of the low level inlet. However, if the rate of supply of cold fresh air is reduced to the minimum of 10 L/s/pers. to comply with the Building Regulations (which would be sensible in order to minimise the amount of heating of the heating device) then the internal heat gains result in a overheating problem in the room away from the inlet point. Heat gains in modern buildings are significant with ranges of 150-500 W/pers. and the only way of addressing the overheating problem in winter is to increase the fresh air rate and thus leads to more heating.
The team realised that in winter if the incoming cold fresh air was introduced at high level and mixed with enough room air to mitigate cold draughts then heating would not be needed until the exterior temperature feel below 6-10 degrees Celsius (depending of the internal heat gains).
In 2006 Cambridge University licensed the intellectual property exclusively to Breathing Buildings. The company has a range of “e-stack” labelled products which are derived from the patent for wintertime ventilation. The pre-mixing of cold fresh air with warm room air is usually achieved using low energy mixings fans immediately beneath high levels vents. In warmer weather the system typically operates with higher ventilation flow rates in order to minimize the risk of overheating.
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