Investigating the effects of geometry in solar thermal absorber plates with micro-channels

Experimental studies were carried out to investigate the effects of micro-channel geometry on the thermal and hydraulic performance of absorber plates for compact (thin and light-weight) solar thermal collectors. Three plates with channel depths 0.25 mm, 0.5 mm and 1 mm were studied. Each plate had sixty channels which were 270 mm long and 2 mm wide. Experiments were run at typical operating conditions for flat plate solar collectors. The results showed a Reynolds number dependent Nusselt number; this was due to axial thermal conduction. The Nusselt number was observed to increase as the aspect ratio approached unity. Measured friction factors were similar in trend to the predictions for rectangular channels, although the overall rise in fluid temperature resulted in slightly lower friction factors. The plate with 0.25 mm deep channels was found to have best thermo-hydraulic performance; thermo-hydraulic performance reduced slightly with increase in hydraulic diameter. The results showed that thermal improvement can be achieved by increasing the fluid velocity, however, pumping the thermal fluid above a pump power per plate area of 0.3 W/m2 resulted in marginal improvement. The results are beneficial for the design of micro-channel absorber plates.