the single-ended water-in-glass collector tubes; the details
of which have been presented in Budihardjo et al. (2003,
2005). The long-term performance of water-in-glass evacuated
tube systems is predicted for a range of system configurations
and operating conditions.
2. Modelling water-in-glass solar water heaters in TRNSYS
The primary difference in modelling a thermosyphon
solar water heater and a pumped circulation system is the
coupling between the heat transfer and the natural circulation
flow rate computations. The natural circulation flow
rate through the collector loop depends on the solar input
and fluid temperature hence varies continuously across the
day. In conventional thermosyphon solar water heaters,
the circulation flow rate can be computed by equating the
pressure head due to the density gradients in the loop
and the pressure drop due to friction and the plumbing
arrangement. The analysis of water-in-glass evacuated tube
systems is more complex due to the unique relationship
between the heat transfer and circulation flow rate through
a single-ended tube. The difference in modelling a conventional
thermosyphon solar water heater and the water-inglass
evacuated tube system in TRNSYS is outlined in this
section.