Solar collector flow rates and temperatures

The system employed a small capacity fan, 12 DC 5We PAPST that required an external energy supply. The power needed was supplied by a small PV-panel unit of 10 We. Thus the performance of the fan was not continuous and therefore the flow rate was mainly depended on the irradiance levels. Thus for the highest irradiance level, the collector delivered the maximum flow rate and for lower irradiance levels, there was a limit where the fan did not operate.

The study of the solar collector performance requires the analysis of the flow rate dependence with the irradiance but also the pneumatic system characteristics. The fan was powered by the PV-panel that pumped the flow through the system to cross the woodchip. In order to find the relationship between flow rate and irradiance, it was necessary to study the pneumatic characteristics of both fan and system, and also the electrical connection between fan and PV-panel separately.

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The electrical connection between PV-panel and fan can be analysed in the I-V curves that describes the electrical characteristics of both 10We PV-panel and 5We fan, Figure 5 shows operational voltages at two irradiances levels. It may be noted that not all the power generated by the PV-panel is supply to the fan as operational points don’t coincide with the maximum power curve. During the periods of high irradiance, the fan will be operated over capacity, although the fan motor showed no signs of fatigue or damage. [6]

Fig 5: PV module and fan electrical characteristics with systems operating voltage

The final drying flow depended on the pneumatic characteristics curves of the system. The flow blown by the fan needs to overcome the pressure drop associated to the air resistance to pass through the solar dryer. The head losses of the solar dryer were located in three differenced parts: the transpired plate, the flexible duct and the woodchip layer. Therefore the drying flow depended on the type of plate used and on the thickness of the woodchip layer. For all the tests, the thickness

Подпись: Figure 6 shows a plot of measured flow rate as a function of irradiance for a single day period. The test was taken on the 4th of August in 2008 when the sky was overcast and occasionally sunny. The flow rate increases reaching its maximum value for the higher irradiance, so for 1310 W/m2 the air flow was 248 m3/h. The connection of the PV-panel with the low power fan is reflected on a low threshold irradiance of 135 W/m2 that corresponds to a flow of 57 m3/h. This enhanced the system potential to work even on days with low light.

of the woodchip layer was 3cm that corresponds to the 3kg of product employed before in the drying tests.

Fig 6: Measured flow rate vs. irradiance

Figure 7 shows the collector outlet and collector temperature rise against the irradiance for the same day as previous. Considering that air flow rate increases with irradiance, the graph shows the tendency of temperatures to increase with irradiance as well. For low irradiances the average temperature rise was 10°C and for high irradiance levels it was 20°C. The degree of scatter in the temperature rise is mainly attributed to variances in the ambient temperature and fluctuations in the collector flow. Also the transitory time to the steady state and wind effect may affect in the collector air and absorber plate temperatures [11].