The large indoor solar simulator used for some of the measurements in this work, is shown in figure 1. The simulator is further described in Hakansson (2003 a, b) and Hakansson (2001). Seven large parabolic reflectors are used to render the light nearly parallel, which is a rare quality for solar simulators. The simulator was originally designed for simulation of

daylight for evaluations of solar shadings. However, the parallel light is an important characteristic also for evaluation of the incidence angle dependence of concentrating collectors and it is desirable to be able to adequately perform this kind of evaluations. In order to achieve correct registered values of the irradiation on the collector surface during measurements, it is important that the total irradiation is known throughout the measurement, hence also when shifting solar altitude, i. e. lifting the simulator.

The good parallel light quality of the simulator has been achieved, to some extent, at the expense of a uniform area distribution. Fewer larger lamps generating nearly parallel light are used instead of smaller, but more numerous lamps generating more diverging but more evenly distributed light. The light intensity has been measured at several locations over a central test area, perpendicularly to the simulator, and the resulting intensity distribution diagram is shown in figure 2 (Hakansson, 2003 a, b).

The light intensity peaks emanates the rim of the reflectors surrounding the lamps (Hakansson, 2003 a). When the simulator is lifted to simulate different incidence angles,
the light intensity pattern tend to move over the test area, thus possibly changing the total irradiation on the object. As the irradiation normally is measured by a single pyranometer, the results could be misleading. Earlier results from evaluations of incidence angle dependence of concentrating collectors were initially found to be poorly corresponding to similar outdoors measurements, thus indicating that the effect of the lifted simulator and the consequently moving light intensity pattern significantly altered these results. (Gajbert et al., 2003)

method for non-uniform illumination

In order to compensate for the moving light pattern an array consisting of six photodiodes connected in parallel, has been created and placed on the glazed surface of the collectors at the time of the measurement. The placement of the photodiodes divides the front area of the solar collector into smaller areas of approximately equal size with one photodiode placed in the centre of each area. With this arrangement, the total current from the photodiode array should be a rather good representation of the total irradiance on the collector surface.

The non-uniform irradiance makes it difficult to get absolute values of optical efficiency, ^0, from indoor measurements. Therefore, the calculated optical efficiencies from indoor measurements were adjusted to match the value of ^0 from outdoor measurements.