| Photovoltaic(PV)power generation is a technology that can directly convert solar energy into electricity,which has been widely investigated and applied.As the restriction of semiconductor's band gap,thermalization effect and combination effect,the photovoltaic cell can only convert part of the received solar energy into electricity,while the rest part will be transformed into heat.This part of converted thermal energy cannot be used by the photovoltaic cell but will increase the operating temperature of the photovoltaic cell and decrease its efficiency.Typically,the heat generated by the PV cell will be dissipated into the environment,which is a waste of sources since most of the solar energy is not used.In order to utilize the waste heat of the PV cell and improve the solar power generation efficiency,some researchers have provided that employing the thermoelectric(TE)generator to absorb the converted heat by the photovoltaic cell and to generate an additional electricity based on the Seebeck effect,which is the so-called photovoltaic-thermoelectric(PV-TE)hybrid system.The part of solar energy that can be used by the photovoltaic cell will be directly used for photovoltaic power generation,while the part of solar energy that cannot be used by the photovoltaic cell will be transformed into heat and then be delivered to the thermoelectric module.The thermoelectric module can use this part of thermal energy to generate extra electricity,which achieves the full-spectrum solar energy utilization.However,the PV-TE hybrid system still has several problems that severely limit its practical application.The most important issue of the PV-TE hybrid system is the operating temperature matching of the PV cell and the TE module: the efficiency of the PV cell decreases with the rise of its operating temperature,while the efficiency of the TE module increases as its temperature difference grows.Therefore,determining the optimal operating temperature distribution and optimizing the system structure is the key to improve the performance of the PV-TE hybrid system.Moreover,there also exists some difficulties that need to be solved when designing the PV-TE hybrid system,such as the extraordinarily many types of coupling devices,the complex energy transfer and conversion processes within the hybrid system,the numerous influencing factors and the intense fluctuation of solar radiation.Thus,this paper focuses on the key optimization issue of the PV-TE hybrid system and provides a comprehensive optimization process based on the temperature matching the PV cell and the TE module,considering the effects of thermal resistance,coupling devices capabilities,solar irradiance,and other factors.The established optimization process includes the selection principle of the coupling devices,the design method of the system structure and the optimization of the outdoor operation condition,which can provide important guidance for the practical application of the PV-TE hybrid system.The main contents are as follows:1.Thermal resistance analysis of photovoltaic-thermoelectric hybrid systemAs there exist many influencing factors and extremely complex energy transfer and conversion processes within the PV-TE hybrid system,determining the key influencing factors of the hybrid system is the first step of optimizing the hybrid performance.Both the theoretical and experimental thermal resistance analysis methods are employed to evaluate the impact of various factors on the hybrid performance and to determine the key factor that can be the most effective parameter for adjusting the performance of the PV-TE hybrid system.The results can be used as the basis for establishing the optimal design method of the PV-TE hybrid system.2.Selection principle of coupling devices in photovoltaic-thermoelectric hybrid systemThe performance matching of the PV cell and the TE module is important for the PV-TE hybrid system since it decides the superiority of hybrid utilization compared to the pure PV system.A standard,which is the minimum figure of merit of the thermoelectric generator that enables the efficiency of the PV-TE hybrid system to be larger than that of the pure PV system,is provided and regarded as the reference of evaluating the feasibility and selecting the coupling devices.The effect of optical concentration ratio,thermal resistances,thermoelectric structure and cooling system on the device selection of the PV-TE hybrid system are discussed.A basic method for selecting coupling devices of the PV-TE hybrid system is established.Then,the steady-state experimental platform of the PV-TE hybrid system is built.The performance of the PV-TE hybrid system and the pure PV system with various types of PV cells are compared under concentrating and non-concentrating conditions to verify the accuracy of the established selection method of the coupling devices.3.A novel optimal design method for photovoltaic-thermoelectric hybrid systemFirstly,based on the idea of temperature matching of the PV cell and the TE module,a novel optimal design method for the PV-TE hybrid system,which can directly obtain the optimal temperature distribution of the hybrid system,is provided.The optimal design process and some design examples of the new optimal design method are presented.The effect of the photovoltaic reference efficiency,photovoltaic efficiency temperature coefficient,thermoelectric figure of merit,and cooling system on the optimal design results are revealed.Then,the feasibility of adjusting the hybrid performance and achieving the optimal temperature distribution by changing the thermoelectric structure is experimentally proved.The accuracies of the model and the results of the new optimal design method are verified by comparing the theoretical and experimental results.Finally,the idea of the optimal design method for the PV-TE hybrid system is extended to the spectrum splitting photovoltaic–thermoelectric hybrid system and a novel optimal design method of the spectrum splitting hybrid system is also provided.4.Investigations on the unsteady-state performance of photovoltaic-thermoelectric hybrid systemFirstly,the operating characteristics of the PV-TE hybrid system under unsteady-state conditions are revealed.Subsequently,the effects of weather and season on the unsteady-state performance of the hybrid system are studied.Then,the unsteady-state performance of the pure PV system,the PV-TE hybrid system,and the photovoltaic-phase change materialsthermoelectric(PV-PCM-TE)system are experimentally compared.Finally,the performance of the PV-PCM-TE system is experimentally optimized.The results show that the performance of the PV-TE hybrid system is always better than the pure PV system,while the operating temperature of the PV-TE hybrid system is much higher than that of the pure PV system.Moreover,the temperature fluctuation of the PV-TE hybrid system caused by the solar irradiance varying is larger than that of the pure PV system,and the PV-TE hybrid system has poorer system stability.Introducing the phase change materials into the PV-TE hybrid system can well maintain the operating temperature at a suitable value and improve the stability of the hybrid system under varying solar irradiance.5.A novel photovoltaic-thermoelectric-thermal cogeneration systemSince the PV-TE hybrid system has poor stability and security,it can hardly be applied in practice.Moreover,most of the received energy of the PV-TE hybrid system cannot be used and will be converted into waste heat,which is a massive waste of energy.In order to improve the stability and the security of the PV-TE hybrid system and increase the solar energy utilization efficiency,a novel photovoltaic-thermoelectric-thermal(PV-TE-T)cogeneration system is provided.The energy transfer and conversion processes within the PV-TE-T cogeneration system are analyzed,and the theoretical model is established.The performance of the PV-TE-T cogeneration system and the existing systems in different weather and seasons are first compared to demonstrate the superiority of the new structure.The PV-TE-T cogeneration system is then optimized by investigating the effect of controlled temperature and thermoelectric structure on the system operating temperature,energy,and exergy efficiency to improve solar energy utilization.The results indicate that when the solar irradiance is high,the new structure can well mitigate the temperature fluctuation caused by the varying solar irradiance and keep the PV cell operating at a safe temperature.The PV-TE-T cogeneration system can provide more exergy output than the existing systems because it produces both electricity and high-grade thermal output.Compared with the conventional PV-TE hybrid system,the new PV-TE-T cogeneration system achieves the improvement in the energy efficiency of 14.39% and exergy efficiency of0.62%.When the solar irradiance is low,the PV-TE-T cogeneration system behaviors similar to the PV-TE hybrid system,but it also has a lower temperature fluctuation than the PV-TE hybrid system. |
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