Keeping a School Cool
School elevation - architects Scott Brownrigg
Children at the new Ruislip High School to be built by the London Borough of Hillingdon are set to benefit from an innovative low-energy solution to the problem of keeping classrooms cool in summer, whilst sealing the external envelope.
In a typical classroom with 32 pupils, the heat gain from people, computers, lighting and sunshine can be as much as 6.5 kW. The school is close to an elevated railway and to Northolt and Heathrow airports, and the high external acoustic levels dictate that the windows have to remain closed.
Due to sustainability aspects and project cost constraints, the normal option of mechanical cooling was not feasible, and consultants Scott Wilson decided to adopt the innovative approach of “passive slab precooling”. This requires the usually concealed underside of the concrete floor slabs to be left exposed and for the upper pane of each classroom window to be opened at night to admit cool air which instils “coolth“ into the slab.
Air movement through the classrooms is achieved by inserting acoustic transfer grilles in the walls between each classroom and the adjacent corridor / atrium space, together with extract fans located in the upper part of the building. To ensure adequate ventilation rates are achieved in the daytime, the fans operate at a reduced duty and induce “trickle ventilation” via acoustic ventilation units located beneath each classroom window.
Scott Wilson commissioned Flowsolve Consultants to simulate the airflow in a typical classroom, to assess whether or not passive pre-cooling would be a viable solution. Transient simulations with a one-hour time step were continued for seven days and nights, by which time the temperatures settle into a periodic daily pattern. The CFD model shows that provided an adequate flow rate is maintained during the night, the reservoir of “coolth” stored within the concrete floor slabs achieves a reduction in peak daytime classroom temperature of 6oC for a typical day in early summer.
Without using mechanical cooling, the results satisfy the Building Bulletin Guide requirement that classroom temperatures should not exceed 28oC for more than 40 hours in a year.
Temperatures on room cross-section - air cools as it flows along the ceiling from right to left, then falls
