Dynamic Performances Of Photovoltaic-thermal Systems With Functionally Thermal Fluids

As a kind of thermal radiant energy,solar energy needs to be converted into other forms of energy in time to be used.At present,there are two kinds of common way to utilize solar energy,one is photovoltaic technology and the other is solar thermal utilization.The commercial solar cells are currently less efficient while converting solar radiation into electricity,and during electric power convention,most of the absorbed energy is dissipated to the surroundings.In order to improve energy efficiency,plenty of researchers made efforts to investigate and develop hybrid photovoltaic and thermal collector systems.A photovoltaic-thermal(PV/T)system generates electric power and produces hot water or other forms of heat simultaneously,thus the overall efficiency increases.In this paper,Chapter One discusses the current situation of global energy and the energy status of China,analyzes the development and utilization of new energy and introduces the hybrid photovoltaic/thermal collector system,as well as its optimized direction.Two functional thermal fluids(nanofluids and microencapsulated phase change slurry(MPCS))are also introduced.Chapter Two compares the dynamic performances of the sheet-and-tube PV/T system using nanofluid as the coolant under different conditions,and studies the effects of nanoparticles,types,volume concentration,particle diameter and mass flow rate of the nanofluids on the collector.In Chapter Three,MPCS is introduced into the flat-plate PV/T system as coolant.The influences of mass fraction and mass flow rate of MPCS on the performances of system are studied,and optimization methods are proposed.In the simulation,Matlab software is applied to construct numerical models and conduct numerical simulations.At the end of the article,results of the simulation are summarized,and suggestions for the future of PV/T developments and the future work are made.The main conclusions of each chapter are as follows:1.Dynamic performances of sheet-and-tube PV/T systems with nanofluidsThe sheet-and-tube PV//T systems with different parameter settings have the same trend of PV panel temperature,photovoltaic conversion efficiency,electric power and thermal power in one day.Among the two types of nanofluids selected in the work,the Al2O3/water nanofluid performs better than the TiO2/water nanofluid,and the PV/T system has the highest energy output when the volume concentration of Al2O3/water nanofluid is set as 10%.In addition,increasing the mass flow rate of the nanofluid can improve the system performance.The electrical power of the system with a mass flow rate of 0.03 kg/s is much higher than the systems with mass flow rates of 0.01 kg/s,0.001 kg/s and 0.0005 kg/s.The increasing the diameter of pipes can also improve the electrical and thermal performances of the system.2.Dynamic performances of flat-plate PV/T systems with microencapsulated phase change slurryThermal efficiency,electrical efficiency and net efficiency of PV/T collectors using MPCS as working fluid are always higher than that of PV/T collectors using water.The net efficiency of PV/T collectors using MPCS can reach 67.39%at 12:00;the value is 4.16%higher than that of water.Using MPCS may result in better heat transfer performance by controlling the temperature of the working fluid.The phase change process influences thermal convection heat transfer strongly.When the MPCS mass fraction is 20 wt%,the MPCS mass flow rate is 0.005 kg/s,and the channel height is 0.010 m,the PV/T collector can achieve the highest net efficiency.As the MPCS mass fraction increases,the energy output increases more and more slowly,because the MPCS has not completed the melting process when reaching the exit area.In addition,the thermal efficiency of the system is simultaneously affected by solar radiation intensity and ambient temperature,while electrical efficiency is mainly dominated by solar radiation intensity.3.Maximizing the energy output of PV/T systemThe working fluid mass flow rate is the most important parameter to optimize the performance of PV/T collector,increasing the mass flow rate can increase the electrical and thermal efficiency of PV/T collector.Taking the plate-flat PV/T collector using MPCS as an example,when the ambient temperature is 34℃ and the solar radiation intensity is 1014 W/m2,the thermal power with 0.003kg/s,0.005kg/s and 0.008kg/s fluid flow rates are 277.21 W,299.46 W and 314.78 W when the MPCS mass fraction is 20 wt%.The net efficiency of the cases with 0.003kg/s,0.005kg/s and 0.008kg/s fluid flow rates are 61.22%,65.78%and 68.91%when the MPCS mass fraction is 20 wt%.In addition,moderate adjustment of the pipe height can also improve the electrical and thermal performances of the PV/T collector.The thermal power with 0.010 m,0.015 m and 0.020 m channel height are 307.31 W,299.46Wand 292.83 W when the MPCS mass fraction is 20 wt%and the net efficiency of the case with 0.010 m,0.015 m and 0.020 m channel height is 67.39%,65.78%and 64.42%.When the channel height decreases,the heat removed by the fluid increases.As a result,the PV panel temperature increases more slowly.Therefore,there must be a moderate channel height for a better electrical performance.