Effect of the collector tilt angle on thermal efficiency and stratification of passive water in glass evacuated tube solar water heater

Passive solar water heaters are extensively studied and used around the world. Nevertheless, little is known about thermal stratification in their storage tank as most research on these devices reports averaged temperatures and energy conversion efficiency. Here we address the issue by analyzing the impact of the slope on thermal behavior of the device at low tilt angles, of particular interest for sub-tropical regions. In the present work the effect of the tilt angle on flow patterns, energy conversion efficiency and the stratification effect was studied with experimental setups and tridimensional numerical simulations as well. The study was performed using a commercial “water in glass evacuated tube solar water heater” (WGET-SWH) with nominal capacity of 40 L and 8 evacuated tubes. Experiments were carried during four weeks varying the tilt angle of the collector. The data gathered during the clearest days were used to validate the tridimensional Computational Fluid Dynamics (CFD) model. The numerical model was used to study the effect of several tilt angles (10°, 27° and 45°) with the same energy input. It was found that the tilt angle has significant effect on daily solar energy gain, flow patterns inside the storage tank and stratification. Nevertheless, it was found little impact on the thermal efficiency due to the low magnitude of heat losses though the vacuum tubes. Also, 10° tilt angle allowed to achieve significant higher temperatures and thermal stratification, along with a thermal inactive area at the bottom of the tank. Conversely, the content of the storage tank for the 45° tilt angle is fully mixed at the end of the heating process. This occurred mainly due to the relative position of the tubes' opening inside the tank and the mixing effect of the convective water flow. A non dimensional number to quantify the stratification effect is proposed and it was found an exponential dependence on the tilt angle. Some considerations regarding the use of WGET-SWHs on subtropical regions are made. No charge or discharge processes were considered in this work.