In phases 2 and 3, the thermal performance of the ETC 6 was measured. It is can be seen from Fig. 4 that the performance ratio of ETC 6/ETC 4 decreases from summer to winter and increases from winter to summer. In summer the ETC 6 has a thermal performance maximum 28% higher than ETC 4 while in winter ETC 4 performs up to 8% better than ETC 6. This is due to the larger tube distance of ETC 6 and thus less shadow from neighbouring tubes. ETC 6 has a tube diameter to tube centre distance ratio of 0.69-0.73 which is a bit smaller than that of ETC 4, 0.75-0.81. It is reasonable that ETC 6 performs better than ETC 4 based on the thermal performance per m2 transparent area, while ETC 4 performs better than ETC 6 based on the thermal performance per m2 gross area, see Fig, 6, 7.