The efficiency of solar collector was evaluated by comparing the total radiant heat energy flux to the solar wall with the change in energy of the transpired air after passing through the collector. The estimates of efficiency are only presented for when irradiance level was 300W/m2, suction airflow rate (Vs) was 20~133.34m3/h/m2 and different wind speeds (U) were 0m/sec, 1.6m/sec, and 3.1m/sec.

The purpose of this work was to evaluate the practical performance of the solar collector as a function of wind speed. The efficiency as defined in Eq.2 was used since it measures the conversion rate of solar energy to useful energy in the transpired air stream. It was observed
that the most important factor in determining efficiency was the irradiance level and plenum temperature. Following this we compared the system efficiency for different wind speeds under different suction airflow rates.

Table 2 shows that peak efficiencies do not occur at high wind speed. The data seem to suggest that the collector operate at peak efficiency when the wind speed is 0 m/sec. It is clear from the results (see Fig.4) that the efficiency of solar collector increases proportionally with the increase of airflow rate and decreases with the increase of wind-speed.

Results show that at low airflow rate, the efficiency of the collector is low and at high suction velocity the efficiency of the solar collector is high. The efficiency of solar collector reaches a maximum value of 67% (at Vs =133.34m3/h/m2 & U=0m/sec) and a minimum value 17% (at Vs =20m3/h/m2 & U=3.1m/sec) being reduced by 50%. This reduction in efficiency is only due to the decrease in airflow rate. And reverse trend in efficiency has been observed for different wind speeds. Results show that different wind speeds effect the efficiency of solar absorber i. e. the efficiency decreases with the increase of wind speed.

The main reason of these variations in the efficiency is that the solar collector, which was installed perpendicularly in front of the artificial solar light source, absorbs the radiations and the energy absorbed by the collector bed increases the plenum temperature (see Table.3). The fan then propels the heat collected in the plenum to the duct. As we increase the airflow rate more heat is transferred to the duct, where the T-type thermocouple measured the temperature.

Table. 3. Plenum and ambient temperatures at different airflow rates under different wind — speed conditions.

The reason in decreased efficiency with the increase of wind speed is that, when wind deflector is in operational form, it deflects the air parallel to the face of the collector. When this wind with ambient temperature of 21oC ~ 22oC enter into the tiny holes of the collector bed it causes to decrease the plenum temperature. Also continuous sucking of hot air out of plenum area causes to decrease the efficiency.