For these types of 2D concentrating collectors, the incidence angle dependence of the optical efficiency is composed by the angular dependence of both the transversal and the longitudinal plane, as illustrated in figure 5.

A propsed biaxial model, based on separate measurements of the effect on the optical efficiency of the reflector and the glazing, is shown in equation (1).

K (0L,0T) = RT (вт )fL (0t)

Here, fL (0) gives the influence of the glazing and is obtained by measuring the angular optical efficiency in the longitudinal plane at a given Or, when the reflector gives a constant contribution. RT (вт) gives the influence of the reflector and is obtained by measuring the angular optical efficiency of the collector in the transversal plane when 6L = 0. If glazing is used, the function K(0L= 0, вт) gets contribution from both the glazing and the reflector according to equation (1). Then R. T(eT) will be obtained from equation (2) according to equation (3).

K(0L=O, 0т)= Rt (вт)* fL (вт). (2)

This means that the ratio between the measured dependency in the 0T and 0L directions gives the influence of the reflector only.

The following complete expression for the angular dependency is obtained from equation (4).

In the longitudinal plane, the angular dependence consists only of the dependence of the glazing, f(0L). This means Km in the longitudinal plane can be described as flat plate collector by equation (5), where b0 is an incidence angle modifier coefficient, characteristic for the glazing (Duffie and Beckman, 1991).