The thermal performances of four heat pipe ETCs are measured. They are ETC 1, 2, 3 and 4. ETC 1 and 2 have 8 tubes of tube diameter 100 mm. ETC 3 and 4 have 20 tubes of tube diameter 70 mm. The difference between ETC 1, 3 and ETC 2, 4 is that ETC 1 and 3 have a curved/semi-cylindrical fin while ETC 2 and 4 have a flat fin. As shown in Fig.4, the performance ratio of ETC 3/ETC 4 is in the range of 0.73-0.94 meaning that the ETC with a flat fin performs better than the ETC with a curved fin. For a mean collector fluid temperature of 63°C, the ETC with a flat fin has a thermal performance 12% higher than the ETC with a curved fin. With an increase of the mean collector fluid temperature to 75°C, the thermal performance of the ETC with a flat fin is increased to be 15% higher than the thermal performance of the ETC with the curved fin. The comparison of a collector with a curved fin and a flat fin with a tube diameter of 100 mm is given by the ratio of ETC 1/ETC 2. The advantage of the ETC with a flat fin tends to weaken with an increase of the tube diameter. However, the ETC with a flat fin is better than the ETC with a curved fin for most of the test period.

Fig. 4. Performance ratio of the differently designed ETCs.

Thermal performance of the ETCs in phase 1 is summarized in Fig. 5. The measurements were carried out half a year from winter to summer with the aim to get a better estimation of the yearly collector performance. The result shows that the collectors with flat fins perform relatively better than the collectors with curved fins. For a collector with a tube diameter of 70 mm, types 3 and 4, there is an

increase of 13% of collector performance if a flat fin is used instead of a curved fin, while for a collector with a tube diameter of 100 mm, the extra thermal performance of the collector with a flat fin compared to the collector

curved fin absorbs 15.9% more solar energy annually than the flat fin. The explanation is the larger heat loss from the curved fin compared to that of the flat fin. The surface area of the curved fin is approx. 40% larger than the surface area of the flat fin, resulting in a higher heat loss from the curved fin and a lower thermal performance. It shall be noted that the location of the collector and the fact that He’s investigations only considered solar radiation from the front side might influence the conclusion as well. The measurement presented in this paper was carried out for a latitude of 56°, while in He’s investigations [3], a latitude of 40° was used.

The performance ratio between the ETC 2 and ETC 4 is less varying throughout the measuring period. This can be explained by the similarity of the fin design. The mean solar collector fluid temperature has a slight influence on the ratio of the thermal performance. There is an increase of the performance ratio with a decrease of mean solar collector fluid temperature, indicating that ETC 2 has a higher heat loss coefficient than that of ETC 4.