| Photovoltaic water pumps(PVP)share the advantages of simpler in structure and control,less maintenance and no use of battery for energy storage and conversion.PVPs are widely regarded as one of the best technical solutions to the water shortage in remote sparsely populated areas where the grid is not accessible.PVPs operate only when the radiation incident on PV panels is higher than the minimum value(I_c)required to start pumps.However,the collectible radiation on fixed PV panels assumes periodical variation in a day,lower in the early morning and late afternoon,and higher in hours near solar-noon,hence the utilization of collectible radiation for pumping water is very low.One of common solutions to improve the utilization of collectible radiation for water pumping is to track the sun.However,the continuous sun-tracking techniques often suffer from mechanical and control failures.In this work,the inclined north-south axis(INSA)multi-position sun-tracked photovoltaic system(MP-PV)and V-trough concentrating photovoltaic system(MP-VPV)are proposed with the aim at stabilizing the power output in a day,thereby improving the utilization of solar radiation for pumping water.The design of proposed systems is that,V-trough concentrating photovoltaic modules(VPV)or PV panels is fixed on an inclined north-south axis multiple positions sun-tracking platform,in the early morning,the azimuth of platform is adjusted towards east and then daily adjusted several times(M)from eastward in the morning to westward in the afternoon by rotating 2θ_a about INSA once when the projected incident angle of solar rays on the plane perpendicular to INSA(PIA)is equal to the specified angleθ_a to ensure the PIA is always withinθ_a in a day,thus make the power output stable in a day.V-trough concentrators are extremely easy in fabrication,irradiation on the base to which solar cells are attached is uniform thus especially suitable for photovoltaic water pumping(PVP)applications.The V-trough is a typical non-ideal concentrator,hence,the optical efficiency is low and the geometric concentration(C_g)is limited.In this study,INSA multiple-position sun-tracked V-trough with restricted reflections for photovoltaic applications(MP-VPV-k/θ_a)is suggested.For such V-trough photovoltaic module,all the incident solar rays withinθ_a are required to arrive on solar cells after less than k reflections,and the azimuth angle of V-trough is daily adjusted M times about INSA to ensure the PIA always withinθa in a day.Firstly,to investigate and compare the performance of different sun-tracked PV panels,a mathematical procedure is developed based on solar geometry and the theory of solar radiation,and the optimal design of INSA multiple-position sun-tracked PV panels(MP-PV)is addressed.Then,a mathematical procedure is suggested based on the imaging principle of mirrors,solar geometry,vector algebra and three-dimensional solar radiation transfer for investigating the performance and design of MP-VPV-k/θ_a optimized for maximizing annual electricity generation.Finally,the performance comparison of three-position sun-tracked,fixed and 2-axis sun-tracked PV panels based water pumping systems(termed as 3P-PVP,PVP and 2A-PVP,respectively)is experimentally performed.The results show that,with the increase of daily azimuth adjustment times(M),the annual power output(P_a)of PM-PV increases,but the increase rate decreases.Compared with fixed PV panels,the annual solar gain(S_a)of 3P,5P-and7P-PV is increased about 17%,19%and 20%,respectively,whereas the annual power output is increased about 24%,27%and 28%,respectively.The annual solar gain of 3P-,5P-,7P-PV is respectively about 93.5%,95.3%and 95.8%of that 2-axis sun-tracked PV(2A-PV)panels annually collected,and the P_a is respectively about 92.4%,94.6%and95.3%of that from 2A-PV.For MP-PV system with the tilt angle of INSA being yearly adjusted four times at three tilts(3T-MP-PV),optimalθ_a of 3P-,5P-,7P-PV for maximizing P_a is about 23~o,14.8 ~o and 11~o,respectively.In comparison with fixed PV panels,the S_a of 3P-,5P-and 7P-PV is respectively increased by 21.6%,23.5%,24%and the P_a is respectively increased by 29%,32%and 33%.The S_a of 3P-,5P-,7P-PV is respectively about 96.7%,98.3%and 98.7%of that collected by 2A-PV,and the P_a is about 96%,98%and 98.7%of that from 2A-PV,respectively.The results also show that,for MP-VPV-k/θ_a with tilt angle of INSA being yearly fixed(1T-MP-VPV-k/θ_a),the optimalθ_a of 3P-,5P-,7P-VPV-k/θ_a(k=1,2)for P_a is 21~o,13.5~o and 10~o,respectively,the opening angleφ_p is close toφ_g as the reflectivityρof side walls is larger than 0.8 and aboutφ_g+3 asρ<0.8.For 3T-MP-VPV-k/θ_a(k=1,2),the optimalθ_a is about 0.5~o less than that of similar 1T-MP-VPV-k/θ_a,and optimalφ_p is almost identical to that of similar 1T-MP-VPV-k/θ_a.Compared with 1T-MP-VPV-k/?_a,the Pa of 3T-MP-VPV-k/θa is 5-7%higher.As compared to similar fixed south-facing PV panels,the increase of P_a from MP-VPV-k/θa is even larger than the C_g of optimized MP-VPV-k/θa forρ>0.8 in sites with abundant solar resources.The experimental tests showed that,as compared to the pump system driven by fixed PV panels(PVP),the daily solar gain of INSA three-position sun-tracked PV pumping system(3P-PV)was increased by about 22%,and daily pumped water volume was increased by 55%.It was also found that the daily pumped water of 3P-PVP was about 97%of that by similar 2-axis sun-tracked PV pump(2A-PVP). |
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