Just like for direct night ventilation, in which airflow may be increased at night to bring fresh air into the building, one can also consider preceding systems with airflows more important than the strict minimum air-change value, as long as the ventilation temperature is lower than the building.

Basing an occupation of 2 people per office, the base airflow during occupation is set at 72 m3/h per office (1.3 ach), dropping at night to 6 m3/h (0.1 ach).

As an alternative, we will also consider following 3 options of controlled flow:

• Same nominal flow (72 m3/h), however also activated at night if the ventilation temperature is fresher than the building.

• Twice the base flow (144 m3/h), activated according to the same thermal condition (else reduced to the base flow).

• Four times the base flow (288 m3/h), activated according to the same thermal condition (else reduced to the base flow).

Such increased ventilation strategies imply adapting of the storage sizes, as well as adapted distribution systems (ducts and fans). They also imply electric overconsumption, which is to be kept as low as possible. While we will limit our study to the thermal contribution of these systems, we stress that the problem of electricity should eventually be studied carefully.

2.2. System integration and control

Since the investigated systems base on day/night charge/discharge, they must be irrigated 24/24 h. Control of the airflow injected into the building hence must take place by way of a valve at storage exit, with an upstream fan always running at nominal capacity.

At night, because of dampening or phase-shifting, temperature at storage output is warmer than ambient. Instead of single-mode operation, these storage devices may hence also be used in

alternative mode with direct night cooling from ambient, with a control strategy injecting air into the building from which ever cooler source. Because of continuous irrigation of the thermal storage, such a strategy however requires a second fan for direct night ventilation.

We shall finally explore the potential of evaporative cooling, as a complement to inertial or direct ventilation. Latter potential will be examined for a constant 50% efficiency (humidification up to 50% of the potential given by the differential between dry and wet bulb temperatures).

Latter configurations will finally be evaluated both for the case of a free-floating building, as for backup by auxiliary cooling during occupation (26.5°C set-point).

3. Simulation

Simulation over the summer period (May-September) is carried out in two steps, with an automated overall approach:

• For each of the four meteorological data sets, the storage systems are pre-simulated for constant airflow, by way of specific analytical models developed previously [4, 5].

• Control of the systems, humidification and building response are then simulated within Trnsys.