The power of the collectors during the day is studied to investigate the transient thermal performance of the ETCs. Figure 2 shows the collector power in an autumn day. In the morning, the direct flow ETC 7 starts up first, followed by the heat pipe ETC 4, ETC 2 and the double glass ETC with heat pipe. There is a sharp increase of the power of the heat pipe ETCs which is most likely caused by the late start-up of evaporation in the heat pipe causing “overheated” absorber temperatures. A possible explanation is that the upper part of the collector is heated up first, but the heat pipe will not be able to work until the bottom part of the heat pipe is heated up to the evaporation temperature. After 10:00 and before 15:00, the power of the ETC 6 levels out while the power of the other collectors increases in the morning and decreases in the afternoon. That can be explained by the cylindrical shape of the absorber of ETC 6. When there is almost no shadow on the tubes between 10:00 and 15:00, the irradiated surface area of the cylindrical absorber does not change significantly, therefore there is insignificant change of the collector power. The heat pipe ETC 2 and 4 and the direct flow ETC have a flat absorber, so the collector power will increase in the morning due to a decrease of incidence angle and an increase of irradiated surface area. In the afternoon the power will decrease due to increased incidence angle and reduced irradiated surface area.

The direct flow ETC 7 performs almost the same as ETC 4 but in the early morning and the late afternoon ETC 7 performs a bit better than ETC 4. The all-glass ETC 5 on the other hand starts up slowly and stops almost 1 hour later than the other collectors. This is due to its large thermal capacity since a large quantity of collector fluid is stored in the glass tubes.

Figure 3 shows power of the collectors in a summer day. In the morning ETC 4, 6 and 7 almost start up at the same time. The power of the collectors increases gradually and smoothly. The power of ETC 6 is higher than ETC 4 and 7 in the morning and in the afternoon. That is because the cylindrical absorber of ETC 6 has a larger irradiated surface than a flat absorber in the morning and in the afternoon.

3.1. Long term thermal performance

The thermal performances of the seven ETCs are compared. Figure 4 shows relative thermal performances of the differently designed ETCs. The performance ratio is defined as the ratio between the weekly thermal performance of the collector in question and the weekly thermal performance of the reference collector shown in the figure. The mean solar collector fluid temperature during operation is given at the bottom of the figure.