All glass ETCs
It can be seen from Figure 4 that the thermal performance of the all glass ETC 5 is always larger than that of the reference collector, ETC 4, simply due to the fact that ETC 5 has a larger transparent area. Despite of small fluctuations, it is clearly shown that the performance ratio increases from winter to summer and decreases from summer to winter meaning that the all-glass ETC 5 performs relatively better in summer compared to the winter. The reason could be the difference of the two collectors in
tube orientation and the distance between the tubes. The all glass ETC 5 has east-west oriented horizontal tubes while ETC 4 has south-north oriented tubes with a tilt of 67°. Since the solar azimuth variation is much larger than the solar altitude variation, especially in the summer, the shadowed tube area, caused by shadows from neighbouring tubes, is much larger for ETC 4 than for ETC 5 in large parts of the day. Furthermore the ratio of tube diameter to tube centre distance is 63-65% for ETC 5 and 75-81% for ETC 4. Therefore there is a relatively larger tube distance and thus less shadow from neighbouring tubes of the ETC 5 compared to ETC 4. The performance ratio of ETC 5 to ETC 4 is insignificantly influenced by the mean collector fluid temperature.
The energy output of ETC 5 in the three phases is summarized and presented in Figures 5, 6 and 7. It can be concluded that ETC 5 has the smallest thermal performance per m2 gross area while it has the second largest thermal performance m2 transparent area because ETC 5 has the smallest ratio of tube diameter to tube centre distance and thus has the largest distance between the tubes.
