An investigation into how incident power density affected the optical efficiency of the collector was conducted using the solar simulator to provide the irradiance source. Various irradiance power densities were achieved by modifying the spatial distribution of the source lamps and the displacement between the

simulator and collector planes. The collector was sloped at 45° with respect to the horizontal plane, the mass flow was maintained at 0.07 kgs-1 ± 1% and the collector temperature was held at ~3 K above the ambient temperature. Two typical irradiance

maps can be seen in Figure 2 for 590 and 1150 Wm"’

Figure 2 — 2-D Maps for the Solar Simulator Irradiance at average power densities of 590 ± 50 Wm-2 and 1150 ± 50 Wm-2 with the collector at a slope of 45° ± 2° with respect to the horizontal plane (Contour lines are in units of Wm-2)

The optical efficiency of the direct-flow evacuated tube collector was monitored for irradiances in the range 200 to 1400 Wm-2. Observations shown in Figure 3 revealed that the optical efficiency remained quasi-constant with increasing power density of the irradiance source; ^0 averaged at a value of 0.82 ± 0.02 over this range. The
corresponding global heat losses from the evacuated collector were determined for these experimental conditions. Values of UL ranged from 0.05 to 5.31 Wm-2K‘1 for increasing power density.

A fit to the data illustrated with the dashed black line in Figure 3 indicates that the global heat loss follows a simple power series. The calculation method

of ULand the corresponding heat loss mechanisms will be discussed later.