Fig. 7 shows the variation of the efficiency and the outlet air temperature for different mass flow rates.
The thermal efficiency of the collector increases with the air mass flow: from 0.015 kg/s to 0.025 kg/s the
increase is more pronounced, while from 0.025 kg/s to 0.26 kg/s the there is a slow increase of the
efficiency with the air mass flow, reaching a value of around 80%. The curve representing the outlet air
temperature decreases asymptotically down to 18ºC (the inlet temperature). In the monitored collector,
the measured average air mass flow was 0.020 kg/s (air velocity of 0.8 m/s), marked in the figure with
orange dotted lines. This working point is far away from the optimum, thus, increasing the air mass flow
is a good strategy to improve the efficiency of the collector. A conservative value for the air mass flow in
0.045 kg/s, allows counterbalancing the improvement of the efficiency against higher electric
consumption, higher pressure drops and noise generation and lower outlet termperatures. To this air mass
flow, the corresponding mean outlet air temperature is 46ºC and the thermal efficiency is around 60%
(green dotted lines).