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Lower-energy design

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Totally passive operation was not an option for the offices, given the lack of thermal mass and the cellular planning. Occupants can open floor- to-ceiling glazing, which is the inner skin of a double facade. Opening these switches off chilled ceilings but allows mechanical ventilation, with its air entering above the windows and exhausted via the corridors and lobbies, then entrance piazza and underground car park.

The outer leaf of the double facade is offset 1m, with maintenance walkways at each floor. This outer leaf consists of motorised glass louvres, controlled in one-floor-deep by 6m-wide modules to respond to their changing orientation and exposure, so providing solar shading and a light shelf. The inside finish is black dots, and is surprisingly easy on the eye when looking out. The outside will probably be a white frit to give the building a soft appearance when the louvres are closed. Full closure of louvres will give some small thermal benefit, and around 1dB of extra sound insulation.

The main aim has been to reduce solar gain, hopefully to about 12 per cent. The accompanying loss of daylight is less of a problem. With one whole office wall fully glazed and a shallow plan, the average daylighting factor is around 6 per cent, with reasonable uniformity. A rising internal blind is also planned for occupants to reduce glare.

The double facade is partly an aesthetic choice, partly due to the building's shape and orientation, and partly the result of the lack of thermal capacity, which rules out some simpler passive options.

There are chilled ceiling planks for supplementary cooling. The temperature design standard is 20-23degreesC for an external temperature range of -10degreesC to 28degreesC. Then for every two degrees the outdoor temperature rises above 28degreesC, the indoor temperature is expected to rise by 1degreesC. So, for example, if it is 34degreesC outside it will be 26degreesC inside. (That is just for the worst case of those facing the sun, of course.)

Winter heating will be provided by a 2mw chp plant, which will also supply 40 per cent of the peak electricity load. In these relatively small offices with floor-to-ceiling low-E glazing and thus modest U-value, it will be important for room heating to counter the tendency for strong downdraughts at the windows. In winter there will be heated ceilings.

There will be no hot water at washbasins, on the grounds that people have usually finished using them before the water runs hot. This is common Belgian practice.

Where artificial lighting is needed, dimmer switches will be adequate, and the planned presence detectors in each office should help save energy.

The overall target for building energy consumption is 100kWh/m2/y.

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