The air is tempered by the water tanks to provide cooling in summer and to raise the temperature of external air in winter. The logic behind the water-based ground-coupled ventilation is based on ground temperature, at 2 m depth, holding at a near constant 15°C all year round.
"The thermal capacity of water is over twice that of clay soil and three times that of concrete," said project architect Neville Cowland of Now Architecture. "That's where we started. 1000 litres per 10 m2 will give us enough heat exchange to provide supply air at 15°C in summer."
The water storage is claimed to provide a four-fold increase in heat transfer capacity compared with dry ground.
The physics of all this was checked out by running various water tank and duct configurations through computational fluid dynamics (CFD) modelling. The Flovent software tool helped the designers to determine laminar flows and air volumes and to optimise the balance between duct size and length.
"Poor ventilation in schools is a major problem in Australia," said Judith North of Now Architecture. "The major objective here was to get air moving through the school without energy penalty."
Outlets of 150 mm diameter will therefore discharge tempered air to the occupied spaces by the displacement principal, each outlet serving around 12 m2 of floor area. Air supply will vary as ambient external temperature increases, air movement induced and modulated by varying the opening of louvre windows and skylights at high level.
Taking 35°C as an example external ambient summer condition, the modelling confirmed that the ventilation system could deliver approximately 30 litres/s of tempered air from each outlet to approximately at close to 15°C.
In heating seasons, the supply air will be heated by a conventional hydronic system serving finned tube radiators located in ventilation channel recesses run in the floor. This is regarded as the most effective and efficient method of heating: retaining, as it does, the heat at low level without loss to ventilation systems.
The heating will be zoned by using separate small on-demand gas hot water units to supply heat to the school modules, with emitter capacity between 8-10 W/m2 depending on design load. Each zone will be controlled by individual thermostats and timers.
The school's environmental systems were run through Australia's environmental rating tool, Greenstar, the Australian equivalent of BREEAM and LEED. Meinhardt (Vic) Pty Ltd undertook the energy modelling and estimated the energy consumption as per the Greenstar Education Energy Calculator Guide.