Benefits and Limits of Free Cooling in Non-Residential Buildings.

In urban non-residential buildings air-conditioning systems are generally required to achieve acceptable air quality. To reduce the energy demand of HVAC-plants free cooling is proposed. The present study deals with free cooling by outdoor air (untreated or additionally cooled by evaporation) during the night. Therefore a sufficient building mass (about 600 to 800 kg/m²) is necessary which stores the heat produced in daytime and which is cooled down at night. In most conventional non-residential buildings, however, the building mass is at about 400 to 600 kg/m². A reference room which provides optimum conditions for night cooling has been investigated using a dynamic building simulation program. Three variants have been examined: Night cooling by untreated outdoor air and increased heat transfer to cool down the ceiling of the reference room, night cooling combined with evaporative cooling and ventilation through a false floor. The simulation results show that room air temperatures below 28 °C can be achieved exclusively by combining night cooling and evaporative cooling as long as the total load does not exceed 55 W/m². For higher loads night cooling combined with mechanical cooling provides a 15 to 20 % reduction in the energy demand compared to mechanical cooling during the occupied period exclusively. As the ventilation energy demand has a strong impact on the total energy demand decentralized systems with low pressure losses are required. Without mechanical cooling less comfort and poorer air quality are inevitable, as dehumidification of supply air is a presupposition for high comfort and air quality. Night cooling should be realized without mechanical systems, e.q. by using buoyancy forces of an atrium. .This requires a close cooperation between the architect and the HVAC-engineer .