Abstract:
A detailed analysis of the heat transfers and fluid flows within a
direct thermosyphonic solar-energy water-heater has been undertaken.
The collector energy equations when cast in a two-dimensional form
enabled heat transfer and thermal capacitance effects to be simulated
accurately at the small flow rates encountered commonly in such
systems. An investigation of thermocline relaxation processes within
the store indicated negligible mixing at the store inlet over a wide
range of Richardsons numbers (43,608 < Ri < 729,016). Thermal
relaxation under conditions of no flow was shown to be due
predominantly to axial conduction along the store wall. The use of an
appropriate non-isothermal friction factor correlation when calculating
frictional losses in the collector's riser pipes, produced predicted
steady-state flow rates which were corroborated experimentally to
within 2%. An indoor test facility, monitored and controlled by a
microcomputer, enabled "real" operating conditions to be simulated. The
predicted responses of the system to identical conditions showed good
agreement with the corresponding experimental observations, the
predicted heat delivery being within 2.8% of the measured value.
A technique for correlating the daily performances of thermosyphon
solar-energy water-heaters has been developed. The five dimensionless
groups which form the basis of the correlations and the functional
relationships between these groups were derived from an analytical
solution of a linear first-order differential transient heat balance
carried out on a generic system. Thermal performance data used in the
correlations was generated by the numerical simulation using
representative U. K. hourly weather data and operating conditions. The
minimum amount of data required to establish a characteristic curve for
an individual system was found to be thirty days. Using such a curve,
the total annual solar fraction agreed with that predicted by the high
level model to within 3%. Two universal curves were determined in which
the gradients of characteristic curves were correlated against the
derived dimensionless groups. The accuracy of the resulting two-stage
algorithm in determining annual solar fractions was established as
ranging from 5.5% for predominantly multiple-pass systems to a mean of
10.5% for single-pass systems.
