The thermal performance of a Thermosyphon Domestic Solar Water Heater (DSWH) with a vertical storage tank is investigated experimentally. The system is installed on a roof - top of a four person family house and its thermal characteristics is evaluated by means of carefully measuring the temperature distribution of water inside the storage tank, solar collector flow rate and its inlet and outlet temperatures as well as load/consumption outlet and inlet temperatures and the corresponding water flow rate under a realistic operating conditions. The measurements are conducted every hour starting from morning until late night on a daily basis and continued for about 120 days during August until November 2004. It is seen that thermal stratification is well established inside the tank from 11 AM until 10 PM especially during August to September enabling the tank to provide the necessary amount of hot water at an acceptable temperature. However, thermal stratification is observed to start degrading from mid-night until morning when there is no hot water supply from the collector and due to the diffusion of heat from the top hot water layers to the bottom cold region and conduction through tank’s wall. The thermal behavior of the storage tank is also assessed based on both energy and exergy analysis and its first and second law efficiencies are calculated. It is observed that the storage tank under study has an average first law efficiency of 47.8% and is able to supply the required amount of hot water at a proper temperature. The average second law efficiency of the storage tank is observed to be 28.7% and, although is less than its first low efficiency, but is high enough to ensure that the quality of the hot water supply is well preserved. The proper level of second law efficiency is due to the preservation of the thermal stratification inside the storage tank, leading to supply of hot water at highest possible temperature and hence highest possible energy potential. Experiments are also done for no-load conditions when the storage tank only interacts with the collector, without hot water withdrawal from the tank. It is seen that for no-load condition, thermal stratification continuously develops from morning until around 16 PM after which no noticeable changes in the temperature distribution inside the tank is observed.
Domestic hot water consumption vs. solar thermal energy storage: The optimum size of the storage tank
