Experimental and numerical investigations of passive air cooling of a residential flat-plate solar collector under stagnation conditions

A flat-plate solar collector can reach “stagnation” temperatures exceeding 170 °C particularly during power failures or periods when there is minimal energy demand. Excessive pressures may occur in a solar collector during stagnation conditions - a potentially danger for residents. In this work the authors propose experimental and numerical investigations of natural cooling of flat plate solar collectors to control their overheating under stagnation conditions. Experimental and numerical investigations have been conducted on a commercially available Enerworks Heat Safe (EHS) residential flat plate solar collector integrated with a back mounted air channel. Numerical investigations have been performed employing a three-dimensional finite-volume numerical model validated against experiments carried out by the authors. The validated numerical tool was applied to explore the dissipation of heat from the collector through natural convection and its dependence on collector geometrical parameters, tilt angle and operational conditions. It was found that a solar collector integrated with a well designed back mounted air cooling channel and a control valve at the outlet opening would be able to provide suitable heat transfer rates and keeps the maximum temperature of the absorber plate over a practical range of stagnation conditions. From the conducted investigations it has been evidenced that an optimal size of back-mounted collector air-channel can be found with suitable tilt angle, corresponding to which the overheating of collectors can be controlled passively under stagnation conditions.