On the Suitability of Test Method of GB/T 18708 for Water-in-Glass Evacuated Tube Solar Water Heaters

The thermal performance of water-in-glass evacuated tube solar water heaters is evaluated based on the daily radiation on the collector surface instead of the daily radiation actually received by a solar tube array, as described by test method of GB/T 18708 (eqv. ISO 9459-2). However, the daily radiation actually received by a solar tube array is quite different from that on the collector surface and strongly dependent on its structural and installing parameters. To assess the suitability of the said test standard, a sensitivity analysis on the ratio of daily beam radiation actually received by a south-north oriented solar tube array to that by the collector surface, R_b,t-s, was performed based on the radiation predicted under clear sky condition. Results show that R_b,t-s, is strongly dependent on the distance between two adjacent tubes, implying that the testing result of thermal performance based on the test standard of GB/T 18708 is not of comparability for water-in-glass evacuated tube solar water heaters with different tube distance, and a further modification is necessary to essure test results comparable. Results also indicate that R_b,t-s is less sensitive on the tilt and azimuth angles of the collector for the case of |β - λ| ≤ 10° and |φ| ≤ 20°.