Howe Dell School, Hatfield

The architect confirmed that steel was chosen for the structural frame for both its sustainability features as well as practical and financial reasons. These included flexibility; quality control; effect on the programme and re-use and recycling opportunities. The steel frame supports façade build-ups which allow low u-values to be realised with 0.15 Wm-2K-1 achieved for both the wall and roof. High thermal performance windows were also specified with U-values down to 1.1 Wm-2K-1.

The design team has pioneered the use of a new low-carbon technique for heating and cooling called inter-seasonal heat transfer (IHT). IHT makes use of the largest source of thermal mass in the school, the ground, as a storage facility for heat and coolth. As the sun beats down on the black tarmac of the playground, its heat is absorbed by a series of pipes buried beneath it that contain a mixture of water and anti-freeze. This collector absorbs the sun’s heat and transfers it to the ground below the building where it is stored in the earth virtue of the resistance of heat transfer to the surrounding soil.

When needed this heat is then extracted through a series of underground pipes and “upgraded” to a useful temperature by heat pumps and transferred to the building through heat exchangers to the underfloor heating and ThermoDeck system.

Conversely in the summer when comfort cooling may be required, the system can be effectively reversed, heat being extracted from the building and stored in the thermal “battery”.