Some construction parameters of unglazed perforated solar dryers for medicinal plants

Erecting solar dryers for medicinal plants require the best design option to suit such plants.Investigations were carried out on some design parameters of the unglazed perforated solar dryers under the prevailing weather conditions of Ismailia,Egypt.Effects of different parameters on the heat exchange effectiveness were investigated.Parameters considered in this study including hole(perforation)diameters,dimensions apart,material being used and its thickness all were investigated to reveal the effects of suitable suction velocity,prevailing wind speed and its direction under the simulated incident solar radiation of the investigation site.Based on the obtained results for heat exchange effectiveness under controllable conditions,solar drier was built and investigated outdoors under the prevailing weather conditions of the investigation site.Holes diameter of 3 mm for all the investigated plates and under the different conditions resulted the highest obtained heat exchange effectiveness.Diamond shape for Holes distribution with dimensions of 29 mm×29 mm apart gave higher heat exchange effectiveness above the commonly used distribution(square shape).Temperature increase above ambient per unit of incident radiation was found as(0.0162±0.0007)℃m^(2)/W,when the best designed performance was investigated in the open environment.Using that perforated surface as an absorber in constructed dryer to dry Rosemary(Rosmarinus officinalis L.),reduced the moisture content from 65%(wet basis)to 38.1%on eight hours on the first drying day of the 18th April,2010 while the reduction in moisture content of the sample dried in the shade area was from 65%(wet basis)to 42.2%under the same prevailing weather conditions at the same time.

Thermal performance of solar air collector with slit-like perforations

To offer a potentially low-cost and high-efficiency option for once-through applications,the unglazed transpired solar collector was studied. The semi-empirical correlation of overall heat exchange effectivenessε for UTC with circular holes was proposed to verify whether it being fit for this kind of absorber with slit-like perforations. The results show that in certain parameters,the predicted values of ε are very close to measured values,and the root mean square difference in betweens about 0.10,so the modle is suitable. Based on experimental results,this kind of absorber is proved to be feasible,with highest value of ε,up to 0.81. In addition,with the increase of γ,overall heat exchange effectiveness ε will decrease,i.e.,the value of ε will decrease with the increase of suction velocity vs or the decrease of wind velocity uw. However,the actual value of solar radiation intensity G and ambient temperature T∞ have little influence on ε.