Simple thermosyphon solar water heaters (SWHs) which are integrated with solar collectors and storage tanks were widely used in 1970-1980s in Japan. According to the afterward economic development, however, they are rapidly shrinking even though its environmental benefit. In order to rejuvenate the solar thermal systems, this study aims to evaluate its correct performance to show potential users and to propose new ideas to increase its efficiency by manufacturers.
In part1, a long-term field test of three SWHs was conducted in Tsukuba, Japan. Each SWH was connected to one of three secondary distribution systems and switched to the other systems in rotation over the tests. In the secondary distribution systems, there are S1-system in which solar hot water is sent to a bathtub and a shower only, S2-system in which users can select hot water from SWH or an auxiliary gas boiler by switching a valve depending on the solar water temperature, and S3-system in which the auxiliary gas boiler works automatically by the device named the connection unit. The demand hot water temperature is 40 degC and the flow rate is periodically changed to one of the standard flow patterns matching to their distribution systems.
The collected data from 1st Apr., 2017 to 28th Feb., 2019 was sorted to the three seasons, summer, spring and autumn, and winter, after eliminating rainy days. Next, the average performance in each combination of the season and the secondary system were analyzed. Then the average value of three seasons was deemed as the annual average performance. As the results, S1 showed the highest annual SCOP as 2.10-3.50 though S2 and S3 showed 1.25-1.62. There are two reasons of the good performance in S1. First, the heat loss in the secondary pipes is highly eliminated in S1 because of the less frequent of hot water supply, which is relating to the fact that the Japanese traditional SWHs has no insulation on their secondary pipes. Second, there is no additional electric consumption from controllers and a pressure device. From the viewpoint of the net solar energy efficiency, S1 show the 30.1-37.1% which compares significantly well with PV systems.
From the multiple temperature data in the tanks, important knowledge was gained as follows. First, the tank is estimated as fully mixed in the collecting period in all SWHs. Second, SWH1 showed the large heat loss in the storage period, which made the tank performance low. Third, all SWHs kept good temperature stratification while hot water is supplying. Finally, the higher part temperature in SWH3 rose again soon after finishing supply which is expected that the highest layer still remains solar heat regardless of the last supply.
In conclusion, S1 shows the excellent performance but S2 and S3 need to save electrical consumption and to reduce the heat loss. Some SWHs need to add the insulation of the tanks or optimize internal tank flow. Part2 is going to report the detailed computer analysis which simulate these SWHs and the secondary distribution systems correctly.