A semi-empirical long-term performance correlation for a solar thermosyphon water heater was derived. The derivation used the simulated results from a well-tested computer program developed previously. It was shown that the mean-day simulation results agree very well with the hour-by-hour simulation for the long-term performance of solar thermosyphon water heaters and hence were adopted for the derivation of the performance correlation. The correlations for the yearly- and monthly-mean efficiency in terms of eleven parameters which characterize the performance of solar thermosyphon water heaters were derived and shown to accurately fit the simulation results. Several design rules for the thermosyphon water heater were deduced from the present study. The total incident global radiation is not present in the correlation function of the yearly efficiency. For the performance evaluation on a monthly basis, the monthly efficiency of thermosyphon water heater was shown to vary linearly with the monthly-mean daily total incident global radiation. This result can be useful in developing a rating method for the thermal performance of thermosyphon water heaters.

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