聚光和非聚光光伏-热电耦合系统的优化
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摘要
基于能量平衡建立了光伏-热电(PV-TE)耦合发电系统的一维稳态导热模型,研究了在不同的光伏电池效率温度系数和热电优值系数条件下PV-TE耦合发电系统的性能.(1)对于非聚光PV-TE耦合系统,分析了热聚焦因子、热电引脚长度和TE模块的负载电阻与内阻之比对系统发电效率的影响.结果表明:在TE模块的优值系数Z=4×10~(-3)K~(-1),电池效率温度系数β_(ref)=0.001 K~(-1)的情况下,系统总效率相对较好.此时总效率随着热聚焦因子、热电引脚长度、TE模块的负载电阻与内阻之比的增加,均是先增大后减小.因此,可选择最佳的热聚焦因子、热电引脚长度、负载电阻的值,使系统总效率达到最大值.(2)对于聚光PV-TE耦合系统,研究了热电引脚对数和热扩散因子对系统性能的影响.结果表明在Z=2.545×10~(-3)K~(-1),β_(ref)=0.001 K~(-1);Z=4×10~(-3)K~(-1),β_(ref)=0.001 K~(-1);Z=4×10~(-3)K~(-1),β_(ref)=0.0015 K~(-1)情况下,总效率随着热扩散因子的增加而减小.当热扩散因子从0.32上升到5.12时,对于其他β_(ref)和Z的情况,总效率会显著增加.当热扩散因子继续增大时,由热扩散因子增加引起的效率变化非常平缓,原因在于热扩散因子足够大时,热传递非常快,PV电池和TE热侧的温度随热扩散因子变化很小.基于研究结果,可以发现当聚光系统的热扩散因子在3.0左右时,耦合系统具有相对高的效率.
The photovoltaic-thermoelectric coupled system has drawn widespread attention in recent years due to its potential of utilization of full-spectrum solar energy.The output power of thermoelectric(TE)is usually much less than that of photovoltaic(PV),because the temperature difference of TE module is usually very small.Thus,the optical concentrating method has been used to increase the temperature difference,but it increases complexity of the structure and extra cost for the sunlight tracking system.Another practical way is using the thermal concentrating method to increase the heat flux of the TE module.However,for there two PV-TE coupled systems i.e.optical and thermal concentrating systems,there exists a confrontation between the PV cell and the TE module.The efficiency of PV cell decreases with the increasing temperature,while the performance of the TE module increases with the increasing temperature difference.In some conditions,the total efficiency of the PV-TE coupled system may be even lower than that without TE module.Therefore,it is necessary to investigate the optimal combination mode of PV cell and TE module for the coupled system.In this paper,a thermodynamic model based on energy-balanced equation is established to investigate the structural optimization and efficiency improvement of PV-TE coupled power generation system,which considers the effects of the thermal concentration factor,the length of thermocouples,the loading resistance of TE module and thermal diffusion factor.The influence rules of these factors on power generation efficiency of PV-TE coupled system without and with concentrator are also theoretically analyzed.(1)For the PV-TE coupled system without concentrator,the influences of thermal concentration factor,the length of thermocouples and the ratio of loading resistance to the internal resistance of TE module on the total efficiency of PV-TEcoupled system are analyzed under four efficiency temperature coefficients of PV cells(β_(ref)=0.001,0.002,0.003,0.004 K~(-1)).It is found that,when the figure of merit of TE module Z is 4×10~(-3)K~(-1)andβ_(ref)is 0.001 K~(-1),the total efficiency increases initially and then decreases with the increases of the thermal concentration factor,the length of thermocouples and the ratio of loading resistance to the internal resistance of TE module.Therefore,there exist optimal values of thermal concentration factor,length of thermocouples and loading resistance of TE module to maximize the total efficiency.(2)For the PV-TE coupled system with concentrator,the influence of thermal diffusion factor on the total efficiency of PV-TE coupled system under five efficiency temperature coefficients of PV cells(β_(ref)=0.001,0.0015,0.002,0.0025,0.003 K~(-1))is studied for the figure of merit of TE module Z=2.545×10~(-3)K~(-1)and Z=4×10~(-3)K~(-1).The results shown that,for the cases of Z=2.545×10~(-3)K~(-1),β_(ref)=0.001 K~(-1);Z=4×10~(-3)K~(-1),β_(ref)=0.001 K~(-1);Z=4×10~(-3)K~(-1),β_(ref)=0.0015 K~(-1),the total efficiency decreases as the thermal diffusion factor increases.While for other cases,the total efficiency increases significantly as the thermal diffusion factor increases.It has also shown that the increases of efficiency caused by the increase in thermal diffusion factor becomes small when the thermal diffusion factor is over 5.12.The reason is that the heat transfer becomes fast when the thermal diffusion factor becomes larger,which leads to the increase of temperature of the PV cell and the hot side of the TE becomes small as the thermal diffusion factor is over 5.12.Based on the above results,it is suggested that an optimal thermal diffusion around 3.0 can have a better balance of both cost and total efficiency of the PV-TE coupled system with concentrator.
The photovoltaic-thermoelectric coupled system has drawn widespread attention in recent years due to its potential of utilization of full-spectrum solar energy.The output power of thermoelectric(TE)is usually much less than that of photovoltaic(PV),because the temperature difference of TE module is usually very small.Thus,the optical concentrating method has been used to increase the temperature difference,but it increases complexity of the structure and extra cost for the sunlight tracking system.Another practical way is using the thermal concentrating method to increase the heat flux of the TE module.However,for there two PV-TE coupled systems i.e.optical and thermal concentrating systems,there exists a confrontation between the PV cell and the TE module.The efficiency of PV cell decreases with the increasing temperature,while the performance of the TE module increases with the increasing temperature difference.In some conditions,the total efficiency of the PV-TE coupled system may be even lower than that without TE module.Therefore,it is necessary to investigate the optimal combination mode of PV cell and TE module for the coupled system.In this paper,a thermodynamic model based on energy-balanced equation is established to investigate the structural optimization and efficiency improvement of PV-TE coupled power generation system,which considers the effects of the thermal concentration factor,the length of thermocouples,the loading resistance of TE module and thermal diffusion factor.The influence rules of these factors on power generation efficiency of PV-TE coupled system without and with concentrator are also theoretically analyzed.(1)For the PV-TE coupled system without concentrator,the influences of thermal concentration factor,the length of thermocouples and the ratio of loading resistance to the internal resistance of TE module on the total efficiency of PV-TEcoupled system are analyzed under four efficiency temperature coefficients of PV cells(β_(ref)=0.001,0.002,0.003,0.004 K~(-1)).It is found that,when the figure of merit of TE module Z is 4×10~(-3)K~(-1)andβ_(ref)is 0.001 K~(-1),the total efficiency increases initially and then decreases with the increases of the thermal concentration factor,the length of thermocouples and the ratio of loading resistance to the internal resistance of TE module.Therefore,there exist optimal values of thermal concentration factor,length of thermocouples and loading resistance of TE module to maximize the total efficiency.(2)For the PV-TE coupled system with concentrator,the influence of thermal diffusion factor on the total efficiency of PV-TE coupled system under five efficiency temperature coefficients of PV cells(β_(ref)=0.001,0.0015,0.002,0.0025,0.003 K~(-1))is studied for the figure of merit of TE module Z=2.545×10~(-3)K~(-1)and Z=4×10~(-3)K~(-1).The results shown that,for the cases of Z=2.545×10~(-3)K~(-1),β_(ref)=0.001 K~(-1);Z=4×10~(-3)K~(-1),β_(ref)=0.001 K~(-1);Z=4×10~(-3)K~(-1),β_(ref)=0.0015 K~(-1),the total efficiency decreases as the thermal diffusion factor increases.While for other cases,the total efficiency increases significantly as the thermal diffusion factor increases.It has also shown that the increases of efficiency caused by the increase in thermal diffusion factor becomes small when the thermal diffusion factor is over 5.12.The reason is that the heat transfer becomes fast when the thermal diffusion factor becomes larger,which leads to the increase of temperature of the PV cell and the hot side of the TE becomes small as the thermal diffusion factor is over 5.12.Based on the above results,it is suggested that an optimal thermal diffusion around 3.0 can have a better balance of both cost and total efficiency of the PV-TE coupled system with concentrator.
引文
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17 Zhu W,Deng Y,Wang Y,et al.High-performance photovoltaic-thermoelectric hybrid power generation system with optimized thermal management.Energy,2016,100:91-101
18 Makki A,Omer S,Su Y,et al.Numerical investigation of heat pipe-based photovoltaic-thermoelectric generator(HP-PV/TEG)hybrid system.Energy Convers Manage,2016,112:274-287
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20 Bergman T L,Incropera F P,DeWitt D P,et al.Fundamentals of Heat and Mass Transfer.7th ed.New York:John Wiley&Sons,2011
21 Gomez M,Reid R,Ohara B,et al.Influence of electrical current variance and thermal resistances on optimum working conditions and geometry for thermoelectric energy harvesting.J Appl Phys,2013,113:174908
22 Yin E,Li Q,Xuan Y.Thermal resistance analysis and optimization of photovoltaic-thermoelectric hybrid system.Energy Convers Manage,2017,143:188-202
23 Mills A F.Heat Transfer.2th ed.New Jersey:Prentice Hall,1999
24 Skoplaki E,Palyvos J A.On the temperature dependence of photovoltaic module electrical performance:A review of efficiency/power correlations.Sol Energy,2009,83:614-624
25 Khattab N M,El Shenawy E T.Optimal operation of thermoelectric cooler driven by solar thermoelectric generator.Energy Convers Manage,2006,47:407-426
26 John A D,William A B.Solar Engineering of Thermal Processes.4th ed.New York:Wiley,2013
2 Vorobiev Y,González-Hernández J,Vorobiev P,et al.Thermal-photovoltaic solar hybrid system for efficient solar energy conversion.Sol Energy,2006,80:170-176
3 Chávez-Urbiola E A,Vorobiev Y V,Bulat L P.Solar hybrid systems with thermoelectric generators.Sol Energy,2012,86:369-378
4 Beeri O,Rotem O,Hazan E,et al.Hybrid photovoltaic thermoelectric system for concentrated solar energy conversion:Experimental realization and modeling.J Appl Phys,2015,118,doi:10.1063/1.4931428
5 Park K T,Shin S M,Tazebay A S,et al.Lossless hybridization between photovoltaic and thermoelectric devices.Sci Rep,2013,3:2123
6 Hashim H,Bomphrey J J,Min G.Model for geometry optimisation of thermoelectric devices in a hybrid PV/TE system.Renew Energy,2016,87:458-463
7 Dallan B S,Schumann J,Lesage F J.Performance evaluation of a photoelectric-thermoelectric cogeneration hybrid system.Sol Energy,2015,118:276-285
8 Ju X,Wang Z,Flamant G,et al.Numerical analysis and optimization of a spectrum splitting concentration photovoltaic-thermoelectric hybrid system.Sol Energy,2012,86:1941-1954
9 Hajji M,Labrim H,Benaissa M,et al.Photovoltaic and thermoelectric indirect coupling for maximum solar energy exploitation.Energy Convers Manage,2017,136:184-191
10 Kraemer D,Hu L,Muto A,et al.Photovoltaic-thermoelectric hybrid systems:A general optimization methodology.Appl Phys Lett,2008,92:S15
11 Elsarrag E,Pernau H,Heuer J,et al.Spectrum splitting for efficient utilization of solar radiation:A novel photovoltaic-thermoelectric power generation system.Renewables,2015,2:1-11
12 Vorobiev Y V,Gonzalez-Hernandez J,Kribus A.Analysis of potential conversion efficiency of a solar hybrid system with high-temperature stage.J Sol Energy Eng,2006,128:258-260
13 Zhang J,Xuan Y,Yang L.Performance estimation of photovoltaic-thermoelectric hybrid systems.Energy,2014,78:895-903
14 Liao T,Lin B,Yang Z.Performance characteristics of a low concentrated photovoltaic-thermoelectric hybrid power generation device.Int JThermal Sci,2014,77:158-164
15 Lamba R,Kaushik S C.Modeling and performance analysis of a concentrated photovoltaic-thermoelectric hybrid power generation system.Energy Convers Manage,2016,115:288-298
16 Da Y,Xuan Y,Li Q.From light trapping to solar energy utilization:A novel photovoltaic-thermoelectric hybrid system to fully utilize solar spectrum.Energy,2016,95:200-210
17 Zhu W,Deng Y,Wang Y,et al.High-performance photovoltaic-thermoelectric hybrid power generation system with optimized thermal management.Energy,2016,100:91-101
18 Makki A,Omer S,Su Y,et al.Numerical investigation of heat pipe-based photovoltaic-thermoelectric generator(HP-PV/TEG)hybrid system.Energy Convers Manage,2016,112:274-287
19 Yang D,Yin H.Energy conversion efficiency of a novel hybrid solar system for photovoltaic,thermoelectric,and heat utilization.IEEE Trans Energy Convers,2011,26:662-670
20 Bergman T L,Incropera F P,DeWitt D P,et al.Fundamentals of Heat and Mass Transfer.7th ed.New York:John Wiley&Sons,2011
21 Gomez M,Reid R,Ohara B,et al.Influence of electrical current variance and thermal resistances on optimum working conditions and geometry for thermoelectric energy harvesting.J Appl Phys,2013,113:174908
22 Yin E,Li Q,Xuan Y.Thermal resistance analysis and optimization of photovoltaic-thermoelectric hybrid system.Energy Convers Manage,2017,143:188-202
23 Mills A F.Heat Transfer.2th ed.New Jersey:Prentice Hall,1999
24 Skoplaki E,Palyvos J A.On the temperature dependence of photovoltaic module electrical performance:A review of efficiency/power correlations.Sol Energy,2009,83:614-624
25 Khattab N M,El Shenawy E T.Optimal operation of thermoelectric cooler driven by solar thermoelectric generator.Energy Convers Manage,2006,47:407-426
26 John A D,William A B.Solar Engineering of Thermal Processes.4th ed.New York:Wiley,2013