Comparative study on the annual performance between loop thermosyphon solar water heating system and conventional solar water heating system

摘要

Loop thermosyphon (LT) is usually introduced to overcome the freezing and corrosion problems associated with the conventional solar water heating (SWH) system. Compared with the conventional SWH system, the LT-SWH system possesses a lower nighttime heat loss because of the thermal diode property of loop thermpsyphon but bigger daytime heat loss because of the secondary heat exchange. However, the effect of above interaction to the system performance is rarely reported based on long-term running. In this study, based on the typical meteorological year data of Fuzhou city, annual performances of above two systems, including the effective number of supplying days, effective heat gain and nighttime heat loss, are comparatively analyzed under two different operational modes. Variations of above mentioned variables with the increment in the set temperature are discussed. The results indicate that, under the discontinuous heating mode, the effective numbers of supplying days of SWH system and LT-SWH system are 139 and 153, respectively. While the numbers of days are respectively 168 and 173 under the continuous heating mode. The SWH system possesses an expected bigger nighttime heat loss ratio with an average annual value of 15.07% corresponding to 6.15% for the LT-SWH system. Particularly, for the LT-SWH system, the different relative magnitudes of heat loss coefficients functioning at different times leads to a smaller temperature drop at night and also a smaller temperature rise at the subsequent day. It generates an unanticipated results that corresponds to the same month from November to April, the two systems have the approximate effective heat gain. The set temperature significantly influences the relative magnitudes of annual effective number of supplying days and annual effective heat gain, the superiority of LT-SWH system gradually diminishes and even reverses with the increment in the set temperature. The bigger daytime heat loss dominating the dominance is responsible for that transition. Combining with a longer static payback period, it is conditional to substitute the conventional SWH system with the LT-SWH system, especially when the water temperature on demand is high.