| Concentrating photovoltaic technology uses concentrator to improve surface heat flux of PV module, which is an effective way to improve system efficiency and reduce system costs. However, photovoltaic efficiency decreased rapidly with the increase of module temperature and module structure will be damaged permanently as the long-term effect of thermal stress. Concerning the problem of uneven distribution of heat flux in concentrating photovoltaic system and overtemperature of PV module, a spiral cooling structure using nanofluid as cooling fluid is proposed and optimized in this paper. The main contents of this paper include the following aspects:Firstly,Stable Si O2-water nanofluids(mass fraction 5%, 3%, 1%) are produced by the M-110 P Microfluidizer Processor. Distribution of nanoparticles, thermal conductivity and radiant characteristics of nanofluids are measured and analyzed in the experiment.Mainly for the heat transfer characteristics of Si O2-water nanofluid flowing inside the collector tube, the temperature field and velocity field distribution of distilled water and nanofluid are simulated by the finite element software ANSYS. For the experimental study, the distilled water and nanofluid are chosen as working fluid of solar-collector vacuum tubes in the insolation experiment, respectively. Results show that,thermal conductivity of Si O2-water nanofluids increases with the increase of mass fraction and temperature. Selective permeability for solar radiation exists in Si O2-water nanofluids and mass fraction of nanofluid is one of the important influence factors.Heat transfer characteristics of Si O2-water nanofluid is higher than distilled water’s and increases as its mass fraction increasing. The longer time of nanofluid is placed, the more obviously nanofluid reuniting, the lower of its heat transfer characteristics becomes.Secondly,a spiral cooling structure in high concentration module is proposed based on the microchannel cooling technology. Influence of PCB plates’ area, length of microchannel and velocity of cooling fluid to the heat transfer characteristics of cooling structure are simulated by Fluent, meanwhile, the cooling structure is optimized with multifactor using thermal enhancement factor. Results show that, thermal enhancement factor decreaces with the extension of microchannel and the decrease of velocity. On this basis, the model of 4 turns of microchannel and entrance velocity is 5.8m/s which applys heat to membrane distillation is obtained.Thirdly,using two phase model, heat transfer characteristics of nanofluids flowing in the microchannel is simulated in respects of temperature field, velocity field distribution and change of Nusselt number. What’s more, influence of mass fraction, particle size and types of nanofluids to the heat transfer mechanism are analyzed, respectively. With the same Re, heat transfer characteristics of Al2O3-water nanofluid is higher than that of Si O2-water nanofluid, and increase as nanoparticle size decreasing. Heat transfer characteristics of nanofluid is results from the interaction of high thermal conductivity of nanoparticles and the flow resistance caused by viscosity.Fourthly,non-uniform heat flux distribution on the battery calculated by the Tracepro, and a intermediate program forms a data file of the solar energy flux distribution that can be read by the FLUENT as a boundary condition. In this way, heat transfer characteristics of cooling structure with the non-uniform heat flux distribution is analyzed. Results show that, reaching the best heat transfer characteristics of cooling structure, entrance velocity of cooling fluid should be increased with the increase of direct solar irradiance. Cooling structure heat transfer efficiency increases as the development of evenness degree of the heat flux distribution on battery. Therefore, it is necessary to govern the entrance velocity match to the actual direct solar irradiance. |
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