where a0 represents the optical efficiency at normal incidence and a1, a2, the thermal losses of the collector.

We will show here the specificities of the glass collector in its function of solar collector, namely the evolution of the efficiency according to the angle of incidence (IAM) and the expression of the thermal losses.

a) Optical efficiency — evolution according to the angle of incidence

The results given here come mainly from the tests carried out by the ITW of Stuttgart [3], in accordance with the EN 12975-2 standard.

The optical efficiency, related to the entering surface, for the current version of the glass collector is estimated at 0.595. This value is logically lower than the optical efficiency of a flat plate solar collector (0.8) while the surface of absorber accounts only for approximately 65% of entering surface.

The incident angular modifier factors (IAM) according to the angle of incidence are represented on the figure 7:

Contrary to the flat plate solar collectors, the corrective coefficient relating to the transversal component of the solar radiation is higher than the unit. This is due to the effect of the inner glass reflectors, position 3.

b) Glass collector efficiency

A specificity of the glass collector, in its solar collector function, is to have one of its sides in contact with the inner of the building. The losses in this case, cannot be expressed any more as in the equation (6). It is proposed then to represent the efficiency of the glass collector under its solar collector function, by the following equation:

Here one distinguishes the losses to the interior which are gains for the room presented in the equation 5 and losses to the exterior. Frame losses are here not taking into account. This model was implemented in TRNSYS like an alternative of type 1 [5]. From this implementation we carried out various simulations in order to quantify the energetic benefits of the glass collector.

2) Example.

In this paragraph we present a dynamic simulation of the energy efficiency of the glass collector [5].

First of all we considered a reference case in a low energy house of 120 m2 including a southern zone (zone 1) equipped with 10 m2 windows surface (Ug = 1.1 W/m2K).

From this reference, we simulated the replacement of 12.4 m2 opaque walls (Ug = 0.2 W/m2K) on the southern facade, by 4 modules of glass collector (height: 2.059 m, width: 1.51 m). The results obtained are summarized in the table 1, for a climate of the North­East of France.

The replacement of the walls by glass collectors gives nearly the same thermal behavior in zone 1. The heating load is slightly weaker during the winter months because of the passive gains. The passive gains are lower in the middle season, due to the shading effect of the glass collector, and tend to increase the heating load.

In regard to the summer comfort, simulations showed that the cumulated curves of the ambient temperatures of the zone 1 are similar in the reference case and the alternative with glass collector. Thus, the replacement of walls by glass collectors does not affect the summer comfort.

The efficiency of the solar collector function is illustrated by the values of the fsave factor, for each month of the year. This relative ratio compares the energy saved by the solar collectors with the heating and domestic hot water load. The installation of 12.4 m2 of glass collectors in facades leads to a fsave annual value of 39%, so completely equivalent to one obtained with a similar area of a facade flat plate solar collectors [6].