Study On Solar Spectral Beam Splitting Photovoltaic/Concentrated Solar Thermal System

Currently,the two relatively mature big scale solar-based power techniques are thermal and PV power generations.In conventional PV power systems,the solar energy has photons in full waveband,but only part of them can be used for generating power by PV cells,while the others will become heat and thus augment the cell temperature as well as decrease the photoelectric efficiency.By using the beam splitting（BS）,the inefficient photons can be split and applied in thermal utilization.That will reduce the cell temperature and increase the photoelectric efficiency.Focusing on the low solar utilization efficiency problem of PV systems,this article proposes a novel solar beam splitting PV/concentrated solar thermal（BS-PVCST）system to try to find a low-cost and high-efficiency solar utilization method.The main contents and conclusions of this article are:First,a new solar BS-PVCST system is proposed.The layout and design of the BS-PVCST system are given.This system consists of multi PV panels,BS device and thermal collector tube.The PV section uses no solar concentration.The BS device is utilized for the BS and concentrating sunlight for the thermal collector tube.The beam splitter is a spectrally selective film system,in which Nb₂O₃ and Na₃Al F₆ are the high refractivity materials,and Ge is the low refractivity material.For 250～2500 nm,the average reflection and transmission rates are 27.2%and 72.8%.Secondly,the MCRT is employed to simulate and analyze the sunlight concentration of the BS-PVCST system.The optical characteristics of the BS-PVCST system are revealed.The impacts of key geometric parameters on the optical parameters are calculated,and the results show that the concentration ratio of the thermal tube augments with the relative aperture increased when the tube diameter is fixed.When the relative aperture is unchanged,the concentration ratio of the thermal tube decreases with the tube diameter augmented.The influence of the thermal tube position on the optical efficiency of the thermal subsystem is evaluated by using optics simulation.The results show that the tube height should be in 978～1022 nm to keep the optical efficiency of the thermal subsystem to be higher than 95%.The effect of sun-tracking error is analyzed,and the results show that when the error is 1°,the optical efficiency of the BS-PVCST system is 93.8%.That means the BS-PVCST system has relatively good adaptive capacity to sun-tracking error.Finally,the thermodynamic parameters of the PV subsystem,thermal subsystem and BS-PVCST system are determined and analyzed.When the thermal subsystem is assumed to be used for heat engine power generation,the results show that the output powers of the PV and thermal subsystems are 1665 W and 413.3 W,and the PV efficiency and overall energy efficiency of the BS-PVCST are 26.7%and 24.2%.The impacts of incident solar DNI,PV cell temperature and thermal tube operating temperature on the thermodynamic performance of the BS-PVCST system are studied.The results show that the output power and overall efficiency of the system both augment when the solar DNI increases.The increasing of PV cell temperature will reduce the output power and overall efficiency of the system.With the thermal tube operating temperature increased,the output power augments first and then decreases.The operating temperature leading to the maximum output power is 274°C.