We will now quantify the performances of the glass collector relative to its various functions.

The evaluation of the glass collector characteristics, presented hereafter, comes from an extrapolation of test results carried out in 2003, on a first version [1], [2].

3.1: Heat insulation function.

The purpose here is to determine the Ug heat exchange coefficient of the glass collector. a) The Modeling Selected

Int.

_43_±_21 _ JL 15 Ё_1 1 ^4> * ! ^—1 " Fvt U 1__ J L_ —w 1 1 1 : Ui

.1. I I

-4,

Figure 2 : Illustration of selected modeling. These coefficients are given by the following relations :

Ui =

f vi 11 11

h———— Nu35 + hr35————- Nu62 + hr62 he

U2 =

V e ( 1

e

1

1

1

h 3‘X 3‘X 3‘X ь —— Nu35 + hr35————- Nu56 + hr56————- Nu62 + hr62

V e

(1)

(2)

Because of the heterogeneous composition of the glass collector, we modeled heat exchange between inside environment and the outside, considering three distinct zones as indicated on the diagram, figure 2.

r

U3 =

1

— • NU23 + hr23 e

(3)

he )

V

1 hi

1

SHAPE * MERGEFORMAT

hrij = 4 — a — Fij

ТЛ3

2

and Fij = 7

1

1

+ -1 Є; є j

ij From these exchange coefficients, one can determine Ug, the overall exchange coefficient of the glass collector, taking into account the respective surface of the various zones previously defined. We obtain then: ІUk • Sk U = (4)

with : hrij : radiative exchange coefficient between parallel surfaces express by :

b) U value estimation

The estimation was based on the thermophysical properties of the various components of the glass collector, as described in paragraph 2.2.

The parameters which mostly influence the Ug value are the surfaces emissivities and the thermal conductivity of included gas.

The value of various surfaces emissivities intervening in calculation are known with a good approximation and increase slowly during the time. Calculations were thus carried out with an emissivity equal to 0.05 for the absorber, to 0.04 for the low emissivity layer, position 3, and equal to 0.85 for the external glass, position 2.

The Ug value exchange coefficient according to the thermal conductivity of included gas, has a significant variation as shown on the figure 3.

If Krypton is used, the Ug coefficient is nearly 0.55 W/m2K, which positions the glass collector on the same level as the best triple glazing.

If the filling is air, the glass collector has an Ug coefficient equivalent to the double glazing filling with argon.

Knowing that the glass collector is designed to be installed instead of walls, the current version is manufactured with Krypton gas filling. The nature of the filling gas also influences the inside glass temperature, position 4, and consequently the solar factor. In order to reduce these values, a Krypton filling gas is recommended.

Next in the article, the performances of the glass collector are defined on the basis of the emissivity value specified previously, for a filling with Krypton.