5.4. Effect of collector inclination
Fig. 8 shows the effect of varying collector inclination
for a 220 L, 30 tube single-tank system with 2.9 m2 collector
in Sydney. When the collector inclination is increased to
45, the annual saving during winter months is 12% higher
than when the collector is inclined at a standard roof inclination
of 22. When the collector is inclined at a smaller
angle to horizontal, the portion of the cold volume below
the level of the evacuated tube opening increases and the
solar portion in the tank is reduced. For a flat-plate collector,
it is important that the solar portion in the tank is big
enough to avoid circulation of the boost volume into the
collector. Experimental and numerical studies by
Budihardjo et al. (2005) showed that the circulation rate
through a 21 evacuated tube array was of the order of 3–
4 tank volume exchanges per hour, hence the circulation
of boost volume into the collector cannot be avoided. This
factor is not critical to the performance of evacuated tube
collectors since the heat loss due to high temperature collector
operation is small. The annual solar fraction of the
water-in-glass system with 45 collector inclination is
1.5% higher than the solar fraction with a standard 22 collector
inclination.
5.