Displacement ventilation versus mixing ventilation - comparison of energy use.

There are basically two ventilation principles that can be utilised in a room: mixing ventilation (denoted MV) and displacement ventilation (denoted DV). In MV, air with high velocity is supplied outside the zone of occupancy, which ideally gives uniform temperature and concentration in the room. In DV, cool air with low velocity is supplied in the lower part of the room. Contaminated air and heat is transported towards the ceiling by the convection currents set up by heat sources, where is is extracted. This ventilation principle has the potential to provide clean air to the occupants, give energy efficient cooling and lower noise and turbulence level due to the low air supply velocity. Although DV is believed to be better than mixing ventilation regarding air quality and thermal comfort, few studies have tried to quantify the energy savings using DV instead of MV. In this paper tow different types of rooms, an office cell and a classroom, have been simulated to estimate how much energy use can be reduced by applying a DV system instead of a MV system. The simulated rooms are exposed to the climate of Oslo, Norway. A basic requirement is that the DV system has to give at least the same air quality in the occupation zone as the MV system. Results from the simulations show that the net annual energy use is reduced from 190 kWh/m2 to 128 kWh/m2 (33% reduction) in the office cell, by using DV instead of MV. For the classroom the reduction is from 223 kWhm2 to 137 kWh/m2, a reduction in energy use by 38%. The results indicate that a significant reduction in energy use can be achieved by applying DV system instead of MV systems in rooms similar to those simulated.