Effects Of Structural And Installing Parameters Of The Water-in-glass Evacuated Tube Solar Water Heater On Its Operating Characteristics And Thermal Performance

In this work, a comprehensive study on thermal performance of water-in-glass evacuated tube solar waters (SWH, in short) was theoretically and experimentally performed with the aim at investigating the effects of structural and installing parameters on the performance of systems.First, a mathematical procedure to calculate the daily beam radiation actually collected by a south-north oriented solar tube collector and daily beam radiation received by the collector surface was developed. Results calculated based such mathematical procedure and radiation predicted by clear sky conditions shows that the ratio of daily beam radiation actually received by the solar tube collector to that by the collector surface, Rb,t-s, is strongly dependent on the distance between two adjacent tubes, implying that the testing result of thermal performance based on radiation on the collector surface is not of comparability for water-in-glass evacuated tube solar water heaters with different tube distance. Results also indicate that Rb,t-s is less sensitive on the tilt and azimuth angles of the collector for the case of|β-λ|≤10°and|φ|≤20°To further investigate the effect of tube distance between two adjacent tubes on the thermal performance of the SWH, a comparative study on SWHs with different tube distance was experimentally performed, and results showed that the daily efficiency of SWHs calculated based on the radiation actually received by the solar tube collector, ηt, was almost independent on tube distance, but the daily efficiency calculated based on the collector surface (ηs) or daily heat gain (Q17) was strongly dependent on the tube space, the smaller the tube distance, the larger the Q17.To investigate effects of all structural and installing parameters of the SWH its thermal performance, a theoretical model was suggested based on energy balance equations and one dimensional temperature distribution inside the water storage tank, and was verified by comparing the predicted water temperature inside the water tank with those actually measured. Simulation results indicated that the tube distance, tilt and azimuth angles of the collector had an insignificant effect on ηt but a significant effect on ηs or Q17, and the shorter the tube distance, the larger the ηs or Q17, and better the thermal performance based on the current test standard (GB/T18708) and technical specification (GB/T19141). However, in practice, the shorter the tube distance, the less the radiation actually received by the collector due to more incidence radiation being shaded by its adjacent tubes, thus less daily solar gain obtained and worse its thermal performance. This implies that the test results based on the current standard for SWH with different tube distance contradict the actual results, and the test standard GB/T18708should be modified or specify the tube distance in the technical specification (GB/T19141). It is advisable to use ηt as the indication of thermal performance of the SWH.To experimentally investigate the effect of tilt-angle on the thermal performance of solar water heaters, two sets of SWH were constructed and tested, the one was inclined at22°(SWH-22) and the other was inclined at46°from the horizon. Experimental results revealed that the collector tilt-angle of SWHs had no significant influence on the thermal performance of the SWH, both systems had almost the same daily solar thermal conversion efficiency but different daily solar and heat gains, and climatic conditions had a negligible effect on the daily thermal efficiency of systems due to less heat loss of the collector to the ambient air. These findings indicated that, to maximize the annual heat gain of such solar water heaters, the collector should be inclined at a tilt-angle for maximizing its annual collection of solar radiation. Experiments also showed that, for the SWH-22, the cold water from the storage tank circulated down to the sealed end of tubes along the lower wall of tubes and then returned to the storage tank along the upper wall of solar tubes with a clear water circulation loop; whereas for the SWH-46, the situation in the morning was the same as the SWH-22, but in the afternoon, the cold water from the storage tank on the way to the sealed end was partially or fully mixed with the hot water returning to the storage tank without a clear water circulation loop, furthermore, such mixing became more intense with the increase in the inlet water temperature of solar tubes. Thesefounding indicated that increase in the tilt-angle of the collector has not any positiveinfluence on the heat removal from solar tubes to the water tank, in turn, if the solartube is too long, and it would be possible to form an inactive region near the sealedend of solar tubes. This implied that all-glass evacuated solar tubes can not installedat near90ofrom the horizon in the practical application.Experimental measurements at nights showed that the reverse thermosyphoniccirculation in solar tubes was observed, the reverse flow-rate seemed to beindependent on climatic conditions but on dependent on the ambient temperature, andthe larger the tilt-angle, the larger the reverse flow-rate. Experimental results alsoshowed that the heat loss of SWH at nights mainly originated from the heat loss fromthe water tank to the surroundings and the heat loss due to reverse flow in solar tubeswas only taken about8-10%of the total heat loss of the SWH. As compared withsolar water heaters with flat-plate collectors, the heat loss of SWH at nights due toreverse flow was much less, indicating that the SWH performed better at nights.