An Evaluation to the Performance of Evacuated Tube Solar Heaters for Arid and Hot Areas

Solar energy applications could be the best alternative to the conventional fuels for the purposes of domestic,water and space heating and some industries in the sunny,arid,and hot areas.In the present study,the performance of an evacuated tube solar heater for water heating for months of February and March was experimentally investigated.This was performed in a hot and arid area(Nasiriya City,South of Iraq).A solar heater with ten evacuated tube solar collectors with a capacity of 100 liter was used in the experiments.Each evacuated tube had a length of 1.8m with an outside diameter of 8 cm.It was observed that for the two selected months,water temperature of the solar heater reached a maximum more than 70°C during sunny days with no heat extraction from the tank of the solar heater.Moreover,heat was extracted from the solar collector with four different flowrates 0.5,0.75,1,and 1.25 l/min,respectively.The results showed that temperature of the solar heater behaved differently from the static situation.When the heat extraction begun,there was a gradual and noticeable decrease in the water temperature of the heater.The observed decrease was slight with the lowest flowrate(0.25 l/m)and becomes sharp with the highest flowrate(1.25 l/min).However,water temperature of the solar heater remained higher than 40°C for the investigated flowrates except the case of 1.25 l/min.The results showed that evacuated tube solar heater can work efficiently in arid and hot areas in winter and spring seasons when the conditions of solar radiation are suitable.

The Effect of Injected Air Bubble Size on the Thermal Performance of a Vertical Shell and Helical Coiled Tube Heat Exchanger

In the present study,the effect of injecting air bubble size on the thermal performance of a vertical counter-current shell and coiled tube heat exchanger is experimentally investigated.The experiments were accomplished in a cylindrical shape heat exchanger with a 50 cm height and 15 cm outer diameter.Copper coil with 3.939 m equivalent length and 0.6 cm outer diameter was used to carry the hot fluid(water).Four different cold fluid(shell side)flow rates(Q_(s)=2;4;6 and 8 LPM)Þunder laminar flow conditions(316≤Re≤1223),constant hot(coil side)flow rate fluid rates(Q_(h)=1 LPM),four different injected air flow rates(Q_(a)=0:5;1;1:5 and 2 LPM),invariant temperature difference(ΔT=20°C),and constant bubble’s number(1400)were tested.To demonstrate the effect of bubble size,a sparger with orifice diameters of 0.1,0.8,and 1.5 mm was manufactured and used in the study.The overall heat transfer coefficient(U),NTU,effectiveness,and pressure loss were invested.The experimental results clearly showed that the heat exchanger’s thermal efficiency significantly improved with increasing the shell side flow rate and the injected air flow rate.The maximum improvement in U,NTU,and effectiveness was 153%,153%,and 68%,respectively.The thermal performance of the heat exchanger was shown to be improved with increasing the bubble size.Although the latter finding agrees with recent CFD published results,more studies need to be confirmed.

The Effect of a Liquid Cover on the Thermal Performance of a Salinity Gradient Solar Pond:An Experimental Study

SalinityGradient Solar Ponds(SGSPs)offer the potential to capture and store solar energy for use in a range of domestic and industrial activities in regions with high solar insolation.However,the evaporation of water from these ponds is a significant problem that must be overcome for them to be deployed successfully.Thus,two ponds were constructed in the city of Nasiriya,Iraq.The two ponds were cylindrical with a diameter of 1.4 m and a total depth of 1.4 m.The water body in the two ponds was constructed with layer depths of 0.5,0.75 and 0.1 m for the lower convective zone(LCZ),non-convective zone(NCZ)and the upper convective zone(UCZ)respectively.One of the two ponds was covered with a thin liquid paraffin layer(0.5 cm)to eliminate evaporation from the surface of the UCZ.The behavior of the standard SGSP and that of the covered pond with evaporation suppressed can be straightforwardly compared.The experimental units were run for six months from 1st of February to 31st of July 2019.It was shown in the first instance that by covering the pond with a thin layer of paraffin,that evaporation could be suppressed.The results showed that for the conventional SGSP,the temperature of the LCZ reached a maximum of ca.76℃ while in the covered pond the temperature of the LCZ was consistently lower than that in the uncovered pond by approximately 5-6℃.The results also indicated that the temperature of the UCZ in the covered pond was higher than that in the uncovered pond by about 10℃ in the second half of the study period.However,it was noted that on rainy days the paraffin layer was swept away from the surface;and this could hinder the implementation of thin liquid cover in the large SGSP.