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Session 3 Paper 3, CIBSE ASHRAE Technical Symposium, Imperial College, London, UK
18th and 19th April 2012
The use of air conditioning and mechanical ventilation to provide cooling during hot summers creates a high demand for electricity. This demand can be mitigated by targeted use of night air to cool the building fabric. This effectively creates a heat sink which regulates daytime temperatures and thereby improves comfort levels.
Night ventilation has traditionally relied on mixing ventilation at high air change rates to achieve cooling. The required flow rates are often higher than can be maintained through the use of natural ventilation, and so energy is consumed driving a mechanical ventilation system, offsetting the daytime energy savings.
Of the natural ventilation methods, night purging by displacement ventilation has been shown to remove warm air more effectively than mixing ventilation, and potentially allows for successful night ventilation without resorting to mechanical systems.
For a single room, the fluid mechanics associated with displacement night purging are now understood. However, when considering multiple rooms on different storeys ventilated via a common space, such as an atrium, the interaction of the flow paths causes complex, non-linear behaviours. In Lynch and Hunt (Building & Environment, 2011) we developed a mathematical model to predict the purging of warm air from such a building, and discovered that a range of different flow types were possible, each with consequences for the effective clearing of excess heat from the building.
From this model we identify herein two ‘optimal’ purges that purge the heat effectively and present a set of design curves that can be directly applied by architects and designers to achieve these optimal night purges.