EVALUATIONS OF THERMOSYPHON SOLAR THERMAL WATER HEATERS PART 2: SIMULATION ANALYSIS ON THE PERFORMANCE OF OPEN CIRCUIT SOLAR WATER HEATERS

Abstract

 In part1, the long-term field test of thermosyphon solar water heaters (SWHs) was conducted. In the test, S1-system in which solar hot water is sent only to a bathtub and shower, 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 called connection unit were given as the secondary distribution systems. Due to the various test conditions, however, straightforward comparisons of each SWH and distribution systems were impossible. Therefore, in this paper, we aimed to analyze the annual performance by computer simulations in an objective manner.

 In the first half, two SWHs used in the field test were modeled by Type45 in TRNSYS ver. 18. The collector efficiency parameters were estimated theoretically from the sectional diagrams contributed by the manufacturer. The insulation performance parameters of tank were given from design information with fine-adjustment. In order to confirm the validity of calculation method, each system was simulated with the solar radiation, the ambient temperature, the main water temperature and the flow rate from the field test data. Over 50days results were compared to the field test performance to show an uncertainty of 5% for S1 and S3 systems and 10% for S2-system.

 In the second half, annual simulations were executed with data of 1st Apr., 2017 to 31st Mar., 2018. There were three cases of hot water demand pattern corresponding to Japanese family of four, three, and two. S3-system showed the largest solar contribution in all three distribution systems and SCOP in the case of family of four was 1.26-1.34 and the primary energy reduction rate was 28.1-32.1%. S2-system consistently showed the poorer performance than the others because the solar water temperature lower than 40oC was not available. SCOP in the case of family of four was 1.10-1.17 and the primary energy reduction rate was 17.7-22.4%. Though S1-system has smaller hot water demand, its energy performance was close to that in S3 system because of little primary energy consumption derived from electricity. Its SCOP in the case of family of four was 1.23-1.25 and the primary energy reduction rate was 26.0-27.2%. From a viewpoint of conversion efficiency of solar radiation, selection of relatively small sized SWH is suitable. Solar efficiency of S3-system was 30.5-34.2% in the case of family of four and 24.9-27.4% in the case of family of two.