2.1. Day/night storage systems

As an alternative or in conjunction with direct night ventilation, we will consider two types of passive cooling systems based on thermal storage of the meteorological day/night oscillation that is carried by ventilation (fig. 2):

• The so-called air-soil heat exchanger, in which the air passes through an array of pipes buried under or next to the building, for the meteorological day/night oscillation to be dampened by charge/discharge in the soil. The daily heat wave propagation extends on approximately 15-20 cm around the pipes, so that latter can be arranged in a compact geometry, with inter-axial distance of approximately 50 cm, immediately under the building, and if necessary in multi­layer.

In the case of our study, we choose pipes with 12 cm diameter, for a specific flow of 100 m3/h per pipe (2.5 m/s). With such a configuration 10 m of pipes make it possible to reduce the

day/night amplitude to 41%, and 20 m of pipe to 17% (exponential damping), for a phase-shift which remains lower than an hour.

• The thermal phase-shifting device, in which the storage material is homogeneously distributed within the ventilating duct, in order to increase the heat-transfer surface and to decrease the penetration distance to thermal mass. Providing a homogeneous airflow and a good convective exchange, it then becomes possible to delay the day/night oscillation almost without dampening, for the night cooling peak to be available in the middle of the day.

In the case of our study we choose a storage material consisting of 13/16 mm diameter PVC tubes that are filled with water, piled up perpendicular to the airflow, with a 2 mm spacing between tubes. With a duct cross-section of 50 x 50 cm subject to a specific flow of 100 m3/h (0.39 m/s average interstitial velocity between tubes), the system enables an 8 h phase-shift with

1.6 m length, respectively a 12 h phase-shift with 2.4 m (linear phase-shifting), for a residual amplitude higher than 80%. This system hence not only differs from the buried pipes in terms of thermal behavior, but also in terms of an almost 10 times inferior storage volume.

First of these systems was subject of several case studies and theoretical analysis [2, 3], whereas second arises from a theoretical work that gave rise to recent lab developments [5]. They have both been object of theoretical developments, in particular in term of well validated analytical models [4, 5], which are used in this study.