Solar efficiency (Eq. 6) and solar fraction (Eq. 7) will be used to characterize the thermal behaviour of the facades:

QLOAD

4 s — і

Where 7 stands for the incident solar radiation.

c_ QLOAD " ° S QLOAD + Qaux

Prototype description

A double skin envelope constituted by an external glass, an air channel and an indoor layer contanining the integral collector-accumulator has been experimentally and numeri­cally tested. The solar collector is formed by a glass layer, transparent insulation and the accumulation zone. This zone is a water tank, whose external surface is black-painted and its internal surface is covered by thermal insulation. Therefore, the facade external appear­ance does not present any particular feature, it looks like a completely glazed area, whereas

Figure 1: Schematic design of the implementation analyzed in a double skin facade, in this case for a space heating application

the internal surface (facing indoor room), looks like an opaque conventional wall, it may presents a rebound if it is combined with a top glazed area, as may be observed in Figure 1. Air channel has been considered closed in the results shown in this paper. Geometry and thermo-physical properties considered are shown in Table 3, where the prototype is described from outdoors to indoors.

In Table 3, a stands for the solar absorptivity, r is the solar transmissivity, є represents the thermal emissivity, p is the density, cp is the specific heat and Л is the thermal conductivity.